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133-402: There are six types of leptons, known as flavours , grouped in three generations . The first-generation leptons, also called electronic leptons , comprise the electron ( e ) and the electron neutrino ( ν e ); the second are the muonic leptons , comprising the muon ( μ ) and the muon neutrino ( ν μ ); and

266-400: A {\displaystyle a} , and τ p {\displaystyle \tau _{\mathrm {p} }} decreases with increasing a {\displaystyle a} . Acceleration gives rise to a non-vanishing probability for the transition p → n + e + ν e . This was a matter of concern in

399-404: A charged lepton while a neutral lepton is called a neutrino . For example, the first generation consists of the electron e with a negative electric charge and the electrically neutral electron neutrino ν e . In the language of quantum field theory, the electromagnetic interaction of the charged leptons is expressed by the fact that the particles interact with

532-418: A lepton number L = 1 . In addition, leptons carry weak isospin , T 3 , which is − ⁠ 1 / 2 ⁠ for the three charged leptons (i.e. electron , muon and tau ) and + ⁠ 1 / 2 ⁠ for the three associated neutrinos . Each doublet of a charged lepton and a neutrino consisting of opposite T 3 are said to constitute one generation of leptons. In addition, one defines

665-401: A muonic number of L μ = 1 , while tau particles and tau neutrinos have a tauonic number of L τ = 1 . The antileptons have their respective generation's leptonic numbers of −1. Conservation of the leptonic numbers means that the number of leptons of the same type remains the same, when particles interact. This implies that leptons and antileptons must be created in pairs of

798-427: A zinc sulfide screen produced at a distance well beyond the distance of alpha-particle range of travel but instead corresponding to the range of travel of hydrogen atoms (protons). After experimentation, Rutherford traced the reaction to the nitrogen in air and found that when alpha particles were introduced into pure nitrogen gas, the effect was larger. In 1919, Rutherford assumed that the alpha particle merely knocked

931-449: A bare nucleus, consisting of a proton (and 0 neutrons for the most abundant isotope protium 1 H ). The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with the electron cloud of any available molecule. In aqueous solution, it forms

1064-613: A candidate to be a fundamental or elementary particle , and hence a building block of nitrogen and all other heavier atomic nuclei. Although protons were originally considered to be elementary particles, in the modern Standard Model of particle physics , protons are known to be composite particles, containing three valence quarks , and together with neutrons are now classified as hadrons . Protons are composed of two up quarks of charge + ⁠ 2 / 3 ⁠ e each, and one down quark of charge − ⁠ 1 / 3 ⁠ e . The rest masses of quarks contribute only about 1% of

1197-471: A capital R subscript (e.g. a positron e R ). Right-handed neutrinos and left-handed anti-neutrinos have no possible interaction with other particles (see Sterile neutrino ) and so are not a functional part of the Standard Model, although their exclusion is not a strict requirement; they are sometimes listed in particle tables to emphasize that they would have no active role if included in

1330-427: A charged meson has the same sign as its charge. Quarks have the following flavour quantum numbers: These five quantum numbers, together with baryon number (which is not a flavour quantum number), completely specify numbers of all 6 quark flavours separately (as n q − n q̅ , i.e. an antiquark is counted with the minus sign). They are conserved by both the electromagnetic and strong interactions (but not

1463-416: A decay involving a charmed quark or antiquark either as the incident particle or as a decay byproduct, Δ C = ±1  ; likewise, for a decay involving a bottom quark or antiquark Δ B′ = ±1 . Since first-order processes are more common than second-order processes (involving two quark decays), this can be used as an approximate " selection rule " for weak decays. A special mixture of quark flavours

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1596-416: A form-factor related to the two-dimensional parton diameter of the proton. A value from before 2010 is based on scattering electrons from protons followed by complex calculation involving scattering cross section based on Rosenbluth equation for momentum-transfer cross section ), and based on studies of the atomic energy levels of hydrogen and deuterium. In 2010 an international research team published

1729-488: A meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particles to be proposed. The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay . It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956. The muon neutrino

1862-484: A neutral hydrogen atom , which is chemically a free radical . Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H 2 ), which are the most common molecular component of molecular clouds in interstellar space . Free protons are routinely used for accelerators for proton therapy or various particle physics experiments, with

1995-543: A number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons . Free protons of high energy and velocity make up 90% of cosmic rays , which propagate through the interstellar medium . Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay . Protons also result (along with electrons and antineutrinos ) from

2128-465: A proton charge radius measurement via the Lamb shift in muonic hydrogen (an exotic atom made of a proton and a negatively charged muon ). As a muon is 200 times heavier than an electron, resulting in a smaller atomic orbital , it is much more sensitive to the proton's charge radius and thus allows a more precise measurement. Subsequent improved scattering and electron-spectroscopy measurements agree with

2261-418: A proton out of nitrogen, turning it into carbon. After observing Blackett's cloud chamber images in 1925, Rutherford realized that the alpha particle was absorbed. If the alpha particle were not absorbed, then it would knock a proton off of nitrogen creating 3 charged particles (a negatively charged carbon, a proton, and an alpha particle). It can be shown that the 3 charged particles would create three tracks in

2394-697: A proton's mass. The remainder of a proton's mass is due to quantum chromodynamics binding energy , which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. The root mean square charge radius of a proton is about 0.84–0.87  fm ( 1 fm = 10  m ). In 2019, two different studies, using different techniques, found this radius to be 0.833 fm, with an uncertainty of ±0.010 fm. Free protons occur occasionally on Earth: thunderstorms can produce protons with energies of up to several tens of MeV . At sufficiently low temperatures and kinetic energies, free protons will bind to electrons . However,

2527-646: A quantum number called weak hypercharge , Y W , which is −1 for all left-handed leptons. Weak isospin and weak hypercharge are gauged in the Standard Model . Leptons may be assigned the six flavour quantum numbers: electron number, muon number, tau number, and corresponding numbers for the neutrinos ( electron neutrino , muon neutrino and tau neutrino ). These are conserved in strong and electromagnetic interactions, but violated by weak interactions. Therefore, such flavour quantum numbers are not of great use. A separate quantum number for each generation

2660-547: A result, they become so-called Brønsted acids . For example, a proton captured by a water molecule in water becomes hydronium , the aqueous cation H 3 O . In chemistry , the number of protons in the nucleus of an atom is known as the atomic number , which determines the chemical element to which the atom belongs. For example, the atomic number of chlorine is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by

2793-455: A simplistic interpretation of early values of atomic weights (see Prout's hypothesis ), which was disproved when more accurate values were measured. In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio ( q / m ), they could not be identified with

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2926-469: A single generation. For example, the following processes are allowed under conservation of leptonic numbers: but none of these: However, neutrino oscillations are known to violate the conservation of the individual leptonic numbers. Such a violation is considered to be smoking gun evidence for physics beyond the Standard Model . A much stronger conservation law is the conservation of the total number of leptons ( L with no subscript ), conserved even in

3059-431: A single particle, unlike the negative electrons discovered by J. J. Thomson . Wilhelm Wien in 1898 identified the hydrogen ion as the particle with the highest charge-to-mass ratio in ionized gases. Following the discovery of the atomic nucleus by Ernest Rutherford in 1911, Antonius van den Broek proposed that the place of each element in the periodic table (its atomic number) is equal to its nuclear charge. This

3192-553: A subgroup. The larger symmetry was named the Eightfold Way by Murray Gell-Mann , and was promptly recognized to correspond to the adjoint representation of SU(3) . To better understand the origin of this symmetry, Gell-Mann proposed the existence of up, down and strange quarks which would belong to the fundamental representation of the SU(3) flavor symmetry. To explain the observed absence of flavor-changing neutral currents ,

3325-512: Is a form of explicit symmetry breaking . The strength of explicit symmetry breaking is controlled by the current quark masses in QCD. Even if quarks are massless, chiral flavour symmetry can be spontaneously broken if the vacuum of the theory contains a chiral condensate (as it does in low-energy QCD). This gives rise to an effective mass for the quarks, often identified with the valence quark mass in QCD. Analysis of experiments indicate that

3458-487: Is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons. The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on

3591-491: Is a stable subatomic particle , symbol p , H , or H with a positive electric charge of +1  e ( elementary charge ). Its mass is slightly less than the mass of a neutron and approximately 1836 times the mass of an electron (the proton-to-electron mass ratio ). Protons and neutrons, each with a mass of approximately one atomic mass unit , are jointly referred to as nucleons (particles present in atomic nuclei). One or more protons are present in

3724-460: Is a technical property, defined through transformation behaviour under the Poincaré group , that does not change with reference frame. It is contrived to agree with helicity for massless particles, and is still well defined for particles with mass. In many quantum field theories , such as quantum electrodynamics and quantum chromodynamics , left- and right-handed fermions are identical. However,

3857-441: Is a unique chemical species, being a bare nucleus. As a consequence it has no independent existence in the condensed state and is invariably found bound by a pair of electrons to another atom. Ross Stewart, The Proton: Application to Organic Chemistry (1985, p. 1) In chemistry, the term proton refers to the hydrogen ion, H . Since the atomic number of hydrogen is 1, a hydrogen ion has no electrons and corresponds to

3990-566: Is an eigenstate of the weak interaction part of the Hamiltonian , so will interact in a particularly simple way with the W bosons (charged weak interactions violate flavour). On the other hand, a fermion of a fixed mass (an eigenstate of the kinetic and strong interaction parts of the Hamiltonian) is an eigenstate of flavour. The transformation from the former basis to the flavour-eigenstate/mass-eigenstate basis for quarks underlies

4123-435: Is any 2 × 2 unitary matrix with a unit determinant . Such matrices form a Lie group called SU(2) (see special unitary group ). This is an example of flavour symmetry. In quantum chromodynamics , flavour is a conserved global symmetry . In the electroweak theory , on the other hand, this symmetry is broken, and flavour changing processes exist, such as quark decay or neutrino oscillations . All leptons carry

Lepton - Misplaced Pages Continue

4256-458: Is consistent with three generations of leptons, some particle physicists are searching for a fourth generation. The current lower limit on the mass of such a fourth charged lepton is 100.8  GeV/ c , while its associated neutrino would have a mass of at least 45.0  GeV/ c . Leptons are spin   ⁠ 1 / 2 ⁠ particles. The spin-statistics theorem thus implies that they are fermions and thus that they are subject to

4389-414: Is more useful: electronic lepton number (+1 for electrons and electron neutrinos), muonic lepton number (+1 for muons and muon neutrinos), and tauonic lepton number (+1 for tau leptons and tau neutrinos). However, even these numbers are not absolutely conserved, as neutrinos of different generations can mix ; that is, a neutrino of one flavour can transform into another flavour . The strength of such mixings

4522-425: Is no mixing of the different generations of charged leptons as there is for quarks . The zero mass of neutrino is in close agreement with current direct experimental observations of the mass. However, it is known from indirect experiments—most prominently from observed neutrino oscillations —that neutrinos have to have a nonzero mass, probably less than 2  eV/ c . This implies the existence of physics beyond

4655-424: Is not currently known whether this is the case. The first charged lepton, the electron, was theorized in the mid-19th century by several scientists and was discovered in 1897 by J. J. Thomson . The next lepton to be observed was the muon , discovered by Carl D. Anderson in 1936, which was classified as a meson at the time. After investigation, it was realized that the muon did not have the expected properties of

4788-576: Is related to the decay rate by where B ( x → y ) {\displaystyle \;{\mathcal {B}}(x\rightarrow y)\;} denotes the branching ratios and Γ ( x → y ) {\displaystyle \;\Gamma (x\rightarrow y)\;} denotes the resonance width of the process x → y   , {\displaystyle \;x\rightarrow y~,} with x and y replaced by two different particles from " e " or " μ " or " τ ". The ratio of tau and muon lifetime

4921-512: Is reversible; neutrons can convert back to protons through beta decay , a common form of radioactive decay . In fact, a free neutron decays this way, with a mean lifetime of about 15 minutes. A proton can also transform into a neutron through beta plus decay (β+ decay). According to quantum field theory , the mean proper lifetime of protons τ p {\displaystyle \tau _{\mathrm {p} }} becomes finite when they are accelerating with proper acceleration

5054-512: Is some constant, and G F is the Fermi coupling constant . The decay rate of tau particles through the process τ → e + ν e + ν τ is given by an expression of the same form where K 3 is some other constant. Muon–tauon universality implies that K 2 ≈ K 3 . On the other hand, electron–muon universality implies The branching ratios for

5187-547: Is specified by a matrix called the Pontecorvo–Maki–Nakagawa–Sakata matrix (PMNS matrix). All quarks carry a baryon number B = ⁠+ + 1 / 3 ⁠ , and all anti-quarks have B = ⁠− + 1 / 3 ⁠ . They also all carry weak isospin , T 3 = ⁠± + 1 / 2 ⁠ . The positively charged quarks (up, charm, and top quarks) are called up-type quarks and have T 3 = ⁠+ + 1 / 2 ⁠  ;

5320-452: Is the basis of the classification in the quark model . The relations between the hypercharge, electric charge and other flavour quantum numbers hold for hadrons as well as quarks. The flavour problem (also known as the flavour puzzle) is the inability of current Standard Model flavour physics to explain why the free parameters of particles in the Standard Model have the values they have, and why there are specified values for mixing angles in

5453-455: Is thus given by Using values from the 2008 Review of Particle Physics for the branching ratios of the muon and tau yields a lifetime ratio of ~  1.29 × 10 , comparable to the measured lifetime ratio of ~  1.32 × 10 . The difference is due to K 2 and K 3 not actually being constants: They depend slightly on the mass of leptons involved. Recent tests of lepton universality in B meson decays, performed by

Lepton - Misplaced Pages Continue

5586-600: The Cabibbo–Kobayashi–Maskawa matrix (CKM matrix). This matrix is analogous to the PMNS matrix for neutrinos, and quantifies flavour changes under charged weak interactions of quarks. The CKM matrix allows for CP violation if there are at least three generations. Flavour quantum numbers are additive. Hence antiparticles have flavour equal in magnitude to the particle but opposite in sign. Hadrons inherit their flavour quantum number from their valence quarks : this

5719-747: The GIM mechanism was proposed in 1970, which introduced the charm quark and predicted the J/psi meson . The J/psi meson was indeed found in 1974, which confirmed the existence of charm quarks. This discovery is known as the November Revolution . The flavor quantum number associated with the charm quark became known as charm . The bottom and top quarks were predicted in 1973 in order to explain CP violation , which also implied two new flavor quantum numbers: bottomness and topness . Proton A proton

5852-498: The LHCb , BaBar , and Belle experiments, have shown consistent deviations from the Standard Model predictions. However the combined statistical and systematic significance is not yet high enough to claim an observation of new physics . In July 2021 results on lepton flavour universality have been published testing W decays, previous measurements by the LEP had given a slight imbalance but

5985-587: The Morris water maze . Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study. There are many more studies that pertain to space travel, including galactic cosmic rays and their possible health effects , and solar proton event exposure. The American Biostack and Soviet Biorack space travel experiments have demonstrated the severity of molecular damage induced by heavy ions on microorganisms including Artemia cysts. CPT-symmetry puts strong constraints on

6118-519: The PMNS and CKM matrices. These free parameters - the fermion masses and their mixing angles - appear to be specifically tuned. Understanding the reason for such tuning would be the solution to the flavor puzzle. There are very fundamental questions involved in this puzzle such as why there are three generations of quarks (up-down, charm-strange, and top-bottom quarks) and leptons (electron, muon and tau neutrino), as well as how and why

6251-497: The Pauli exclusion principle : no two leptons of the same species can be in the same state at the same time. Furthermore, it means that a lepton can have only two possible spin states, namely up or down. A closely related property is chirality , which in turn is closely related to a more easily visualized property called helicity . The helicity of a particle is the direction of its spin relative to its momentum ; particles with spin in

6384-477: The Standard Model . Electrons are one of the components of atoms , alongside protons and neutrons . Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium . The name lepton comes from the Greek λεπτός leptós , "fine, small, thin" ( neuter nominative/accusative singular form: λεπτόν leptón ); the earliest attested form of

6517-504: The Stanford Linear Collider (SLC) and Large Electron–Positron Collider (LEP) experiments. The decay rate ( Γ {\displaystyle \Gamma } ) of muons through the process μ → e + ν e + ν μ is approximately given by an expression of the form (see muon decay for more details) where K 2

6650-594: The constituent quark model, which were popular in the 1980s, and the SVZ sum rules , which allow for rough approximate mass calculations. These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet. The CODATA recommended value of a proton's charge radius is 8.4075(64) × 10  m . The radius of the proton is defined by a formula that can be calculated by quantum electrodynamics and be derived from either atomic spectroscopy or by electron–proton scattering. The formula involves

6783-454: The doublet (the spin- 1 ⁄ 2 , 2 , or fundamental representation ) of SU(2), with the proton and neutron being then associated with different isospin projections I 3 = + + 1 ⁄ 2 and − + 1 ⁄ 2 respectively. The pions are assigned to the triplet (the spin-1, 3 , or adjoint representation ) of SU(2). Though there is a difference from the theory of spin: The group action does not preserve flavor (in fact,

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6916-462: The electron cloud in a normal atom. However, in such an association with an electron, the character of the bound proton is not changed, and it remains a proton. The attraction of low-energy free protons to any electrons present in normal matter (such as the electrons in normal atoms) causes free protons to stop and to form a new chemical bond with an atom. Such a bond happens at any sufficiently "cold" temperature (that is, comparable to temperatures at

7049-604: The electron shell in which it resides, which determines the energy level of the whole atom. Analogously, the five flavour quantum numbers ( isospin , strangeness , charm , bottomness or topness ) can characterize the quantum state of quarks, by the degree to which it exhibits six distinct flavours (u, d, c, s, t, b). Composite particles can be created from multiple quarks, forming hadrons , such as mesons and baryons , each possessing unique aggregate characteristics, such as different masses, electric charges, and decay modes. A hadron 's overall flavour quantum numbers depend on

7182-403: The family symmetries proposed for the quark-lepton generations. In classical mechanics, a force acting on a point-like particle can only alter the particle 's dynamical state, i.e., its momentum , angular momentum, etc. Quantum field theory , however, allows interactions that can alter other facets of a particle's nature described by non dynamical, discrete quantum numbers. In particular,

7315-451: The grand unified theory , the individual baryon and lepton number conservation can be violated, if the difference between them ( B − L ) is conserved (see Chiral anomaly ). Strong interactions conserve all flavours, but all flavour quantum numbers are violated (changed, non-conserved) by electroweak interactions . If there are two or more particles which have identical interactions, then they may be interchanged without affecting

7448-414: The hydronium ion , H 3 O , which in turn is further solvated by water molecules in clusters such as [H 5 O 2 ] and [H 9 O 4 ] . The transfer of H in an acid–base reaction is usually referred to as "proton transfer". The acid is referred to as a proton donor and the base as a proton acceptor. Likewise, biochemical terms such as proton pump and proton channel refer to

7581-455: The kaon led to a new quantum number that was conserved by the strong interaction: strangeness (or equivalently hypercharge). The Gell-Mann–Nishijima formula was identified in 1953, which relates strangeness and hypercharge with isospin and electric charge. Once the kaons and their property of strangeness became better understood, it started to become clear that these, too, seemed to be a part of an enlarged symmetry that contained isospin as

7714-714: The mean lifetime of a proton for various assumed decay products. Experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of 6.6 × 10  years for decay to an antimuon and a neutral pion , and 8.2 × 10  years for decay to a positron and a neutral pion. Another experiment at the Sudbury Neutrino Observatory in Canada searched for gamma rays resulting from residual nuclei resulting from

7847-537: The muon neutrino, which earned them the 1988 Nobel Prize , although by then the different flavours of neutrino had already been theorized. The tau was first detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl with his colleagues at the SLAC LBL group . Like the electron and the muon, it too was expected to have an associated neutrino. The first evidence for tau neutrinos came from

7980-405: The nucleus of every atom . They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z ). Since each element is identified by the number of protons in its nucleus, each element has its own atomic number, which determines

8113-423: The radioactive decay of free neutrons , which are unstable. The spontaneous decay of free protons has never been observed, and protons are therefore considered stable particles according to the Standard Model. However, some grand unified theories (GUTs) of particle physics predict that proton decay should take place with lifetimes between 10 and 10 years. Experimental searches have established lower bounds on

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8246-504: The spinor representation ( T = ⁠ 1  / 2 ⁠ ) of the weak isospin SU(2) gauge symmetry. This means that these particles are eigenstates of the isospin projection T 3 with eigenvalues ⁠+ +  1  / 2 ⁠ and ⁠− +  1  / 2 ⁠ respectively. In the meantime, the right-handed charged lepton transforms as a weak isospin scalar ( T = 0 ) and thus does not participate in

8379-460: The weak interaction , while there is no evidence that a right-handed neutrino exists at all. The Higgs mechanism recombines the gauge fields of the weak isospin SU(2) and the weak hypercharge U(1) symmetries to three massive vector bosons ( W , W , Z ) mediating the weak interaction , and one massless vector boson, the photon (γ), responsible for

8512-526: The Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the magnetosheath , where the Earth's magnetic field affects the solar wind, but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During

8645-521: The Standard Model . The currently most favoured extension is the so-called seesaw mechanism , which would explain both why the left-handed neutrinos are so light compared to the corresponding charged leptons, and why we have not yet seen any right-handed neutrinos. The members of each generation's weak isospin doublet are assigned leptonic numbers that are conserved under the Standard Model. Electrons and electron neutrinos have an electronic number of L e = 1 , while muons and muon neutrinos have

8778-431: The Standard Model's weak interaction treats left-handed and right-handed fermions differently: only left-handed fermions (and right-handed anti-fermions) participate in the weak interaction. This is an example of parity violation explicitly written into the model. In the literature, left-handed fields are often denoted by a capital L subscript (e.g. the normal electron e L ) and right-handed fields are denoted by

8911-435: The Standard Model. The theoretical and measured values for the electron anomalous magnetic dipole moment are within agreement within eight significant figures. The results for the muon , however, are problematic , hinting at a small, persistent discrepancy between the Standard Model and experiment. In the Standard Model, the left-handed charged lepton and the left-handed neutrino are arranged in doublet that transforms in

9044-421: The action of the weak force is such that it allows the conversion of quantum numbers describing mass and electric charge of both quarks and leptons from one discrete type to another. This is known as a flavour change, or flavour transmutation. Due to their quantum description, flavour states may also undergo quantum superposition . In atomic physics the principal quantum number of an electron specifies

9177-465: The case of neutrino oscillations, but even it is still violated by a tiny amount by the chiral anomaly . The coupling of leptons to all types of gauge boson are flavour-independent: The interaction between leptons and a gauge boson measures the same for each lepton. This property is called lepton universality and has been tested in measurements of the muon and tau lifetimes and of Z boson partial decay widths , particularly at

9310-407: The character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the electron cloud of an atom. The result is a diatomic or polyatomic ion containing hydrogen. In a vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming

9443-399: The cloud chamber, but instead only 2 tracks in the cloud chamber were observed. The alpha particle is absorbed by the nitrogen atom. After capture of the alpha particle, a hydrogen nucleus is ejected, creating a net result of 2 charged particles (a proton and a positively charged oxygen) which make 2 tracks in the cloud chamber. Heavy oxygen ( O), not carbon or fluorine, is the product. This was

9576-480: The coaccelerated frame there is a thermal bath due to Fulling–Davies–Unruh effect , an intrinsic effect of quantum field theory. In this thermal bath, experienced by the proton, there are electrons and antineutrinos with which the proton may interact according to the processes: Adding the contributions of each of these processes, one should obtain τ p {\displaystyle \tau _{\mathrm {p} }} . In quantum chromodynamics ,

9709-535: The current quark masses of the lighter flavours of quarks are much smaller than the QCD scale , Λ QCD , hence chiral flavour symmetry is a good approximation to QCD for the up, down and strange quarks. The success of chiral perturbation theory and the even more naive chiral models spring from this fact. The valence quark masses extracted from the quark model are much larger than the current quark mass. This indicates that QCD has spontaneous chiral symmetry breaking with

9842-438: The decay of a proton from oxygen-16. This experiment was designed to detect decay to any product, and established a lower limit to a proton lifetime of 2.1 × 10  years . However, protons are known to transform into neutrons through the process of electron capture (also called inverse beta decay ). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is: The process

9975-500: The electromagnetic interaction. The electric charge Q can be calculated from the isospin projection T 3 and weak hypercharge Y W through the Gell-Mann–Nishijima formula , To recover the observed electric charges for all particles, the left-handed weak isospin doublet (ν eL , e L ) must thus have Y W = −1 , while the right-handed isospin scalar e R must have Y W = −2 . The interaction of

10108-460: The electronic mode (17.82%) and muonic (17.39%) mode of tau decay are not equal due to the mass difference of the final state leptons. Universality also accounts for the ratio of muon and tau lifetimes. The lifetime T ℓ {\displaystyle \mathrm {T} _{\ell }} of a lepton ℓ {\displaystyle \ell } (with ℓ {\displaystyle \ell } = " μ " or " τ ")

10241-441: The first reported nuclear reaction , N + α → O + p . Rutherford at first thought of our modern "p" in this equation as a hydrogen ion, H . Depending on one's perspective, either 1919 (when it was seen experimentally as derived from another source than hydrogen) or 1920 (when it was recognized and proposed as an elementary particle) may be regarded as the moment when the proton was 'discovered'. Rutherford knew hydrogen to be

10374-453: The formation of a chiral condensate . Other phases of QCD may break the chiral flavour symmetries in other ways. Isospin, strangeness and hypercharge predate the quark model. The first of those quantum numbers, Isospin, was introduced as a concept in 1932 by Werner Heisenberg , to explain symmetries of the then newly discovered neutron (symbol n): Protons and neutrons were grouped together as nucleons and treated as different states of

10507-418: The gluons, and transitory pairs of sea quarks . Protons have a positive charge distribution, which decays approximately exponentially, with a root mean square charge radius of about 0.8 fm. Protons and neutrons are both nucleons , which may be bound together by the nuclear force to form atomic nuclei . The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol "H")

10640-463: The group action is specifically an exchange of flavour). When constructing a physical theory of nuclear forces , one could simply assume that it does not depend on isospin, although the total isospin should be conserved. The concept of isospin proved useful in classifying hadrons discovered in the 1950s and 1960s (see particle zoo ), where particles with similar mass are assigned an SU(2) isospin multiplet . The discovery of strange particles like

10773-429: The later 1990s because τ p {\displaystyle \tau _{\mathrm {p} }} is a scalar that can be measured by the inertial and coaccelerated observers . In the inertial frame , the accelerating proton should decay according to the formula above. However, according to the coaccelerated observer the proton is at rest and hence should not decay. This puzzle is solved by realizing that in

10906-435: The left- and right-handed parts of each quark field. This approximate description of the flavour symmetry is described by a chiral group SU L ( N f ) × SU R ( N f ) . If all quarks had non-zero but equal masses, then this chiral symmetry is broken to the vector symmetry of the "diagonal flavour group" SU( N f ) , which applies the same transformation to both helicities of the quarks. This reduction of symmetry

11039-477: The lepton. First-order quantum mechanical approximation predicts that the g  factor is 2 for all leptons. However, higher-order quantum effects caused by loops in Feynman diagrams introduce corrections to this value. These corrections, referred to as the anomalous magnetic dipole moment , are very sensitive to the details of a quantum field theory model, and thus provide the opportunity for precision tests of

11172-456: The leptons with the massive weak interaction vector bosons is shown in the figure on the right. In the Standard Model , each lepton starts out with no intrinsic mass. The charged leptons (i.e. the electron, muon, and tau) obtain an effective mass through interaction with the Higgs field , but the neutrinos remain massless. For technical reasons, the masslessness of the neutrinos implies that there

11305-450: The lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured. Protons also have extrasolar origin from galactic cosmic rays , where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, and their intensity is far more uniform and less variable than protons coming from the Sun,

11438-453: The mass and mixing hierarchy arises among different flavours of these fermions. Quantum chromodynamics (QCD) contains six flavours of quarks . However, their masses differ and as a result they are not strictly interchangeable with each other. The up and down flavours are close to having equal masses, and the theory of these two quarks possesses an approximate SU(2) symmetry ( isospin symmetry). Under some circumstances (for instance when

11571-402: The mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark. These masses typically have very different values. The kinetic energy of the quarks that is a consequence of confinement is a contribution (see Mass in special relativity ). Using lattice QCD calculations, the contributions to

11704-2129: The mass of the proton are the quark condensate (~9%, comprising the up and down quarks and a sea of virtual strange quarks), the quark kinetic energy (~32%), the gluon kinetic energy (~37%), and the anomalous gluonic contribution (~23%, comprising contributions from condensates of all quark flavors). The constituent quark model wavefunction for the proton is | p ↑ ⟩ = 1 18 ( 2 | u ↑ d ↓ u ↑ ⟩ + 2 | u ↑ u ↑ d ↓ ⟩ + 2 | d ↓ u ↑ u ↑ ⟩ − | u ↑ u ↓ d ↑ ⟩ − | u ↑ d ↑ u ↓ ⟩ − | u ↓ d ↑ u ↑ ⟩ − | d ↑ u ↓ u ↑ ⟩ − | d ↑ u ↑ u ↓ ⟩ − | u ↓ u ↑ d ↑ ⟩ ) . {\displaystyle \mathrm {|p_{\uparrow }\rangle ={\tfrac {1}{\sqrt {18}}}\left(2|u_{\uparrow }d_{\downarrow }u_{\uparrow }\rangle +2|u_{\uparrow }u_{\uparrow }d_{\downarrow }\rangle +2|d_{\downarrow }u_{\uparrow }u_{\uparrow }\rangle -|u_{\uparrow }u_{\downarrow }d_{\uparrow }\rangle -|u_{\uparrow }d_{\uparrow }u_{\downarrow }\rangle -|u_{\downarrow }d_{\uparrow }u_{\uparrow }\rangle -|d_{\uparrow }u_{\downarrow }u_{\uparrow }\rangle -|d_{\uparrow }u_{\uparrow }u_{\downarrow }\rangle -|u_{\downarrow }u_{\uparrow }d_{\uparrow }\rangle \right)} .} The internal dynamics of protons are complicated, because they are determined by

11837-472: The mid-1970s) ( 1777 MeV/ c ) is nearly twice that of the proton and 3477 ‍ times that of the electron. The first lepton identified was the electron, discovered by J.J. Thomson and his team of British physicists in 1897. Then in 1930, Wolfgang Pauli postulated the electron neutrino to preserve conservation of energy , conservation of momentum , and conservation of angular momentum in beta decay . Pauli theorized that an undetected particle

11970-410: The model. Even though electrically charged right-handed particles (electron, muon, or tau) do not engage in the weak interaction specifically, they can still interact electrically, and hence still participate in the combined electroweak force , although with different strengths ( Y W ). One of the most prominent properties of leptons is their electric charge , Q . The electric charge determines

12103-432: The modern theory of the nuclear force, most of the mass of protons and neutrons is explained by special relativity . The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valence quarks , while the gluons have zero rest mass. The extra energy of the quarks and gluons in a proton, as compared to the rest energy of the quarks alone in the QCD vacuum , accounts for almost 99% of

12236-410: The most common charged lepton in the universe , whereas muons and taus can only be produced in high-energy collisions (such as those involving cosmic rays and those carried out in particle accelerators ). Leptons have various intrinsic properties , including electric charge , spin , and mass . Unlike quarks , however, leptons are not subject to the strong interaction , but they are subject to

12369-484: The most powerful example being the Large Hadron Collider . Protons are spin- ⁠ 1 / 2 ⁠ fermions and are composed of three valence quarks, making them baryons (a sub-type of hadrons ). The two up quarks and one down quark of a proton are held together by the strong force , mediated by gluons . A modern perspective has a proton composed of the valence quarks (up, up, down),

12502-451: The most precise ratio so far testing the lepton flavour universality. Flavour (particle physics) In particle physics , flavour or flavor refers to the species of an elementary particle . The Standard Model counts six flavours of quarks and six flavours of leptons . They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles . They can also be described by some of

12635-440: The movement of hydrated H ions. The ion produced by removing the electron from a deuterium atom is known as a deuteron , not a proton. Likewise, removing an electron from a tritium atom produces a triton . Also in chemistry, the term proton NMR refers to the observation of hydrogen-1 nuclei in (mostly organic ) molecules by nuclear magnetic resonance . This method uses the quantized spin magnetic moment of

12768-416: The muon, initially classified as a meson, was reclassified as a lepton in the 1950s. The masses of those particles are small compared to nucleons—the mass of an electron ( 0.511  MeV/ c ) and the mass of a muon (with a value of 105.7 MeV/ c ) are fractions of the mass of the "heavy" proton ( 938.3 MeV/ c ), and the mass of a neutrino is nearly zero. However, the mass of the tau (discovered in

12901-421: The negatively charged quarks (down, strange, and bottom quarks) are called down-type quarks and have T 3 = ⁠− + 1 / 2 ⁠ . Each doublet of up and down type quarks constitutes one generation of quarks. For all the quark flavour quantum numbers listed below, the convention is that the flavour charge and the electric charge of a quark have the same sign . Thus any flavour carried by

13034-438: The neutral hydrogen atom. He initially suggested both proton and prouton (after Prout). Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle". The first use of the word "proton" in the scientific literature appeared in 1920. One or more bound protons are present in the nucleus of every atom. Free protons are found naturally in

13167-611: The new measurement by the ATLAS collaboration have twice the precision and give a ratio of R W τ / μ = B ( W → τ ν τ ) / B ( W → μ ν μ ) = 0.992 ± 0.013 {\displaystyle R_{W}^{\tau /\mu }={\mathcal {B}}(W\rightarrow \tau \nu _{\tau })/{\mathcal {B}}(W\rightarrow \mu \nu _{\mu })=0.992\pm 0.013} , which agrees with

13300-403: The new small radius. Work continues to refine and check this new value. Since the proton is composed of quarks confined by gluons, an equivalent pressure that acts on the quarks can be defined. The size of that pressure and other details about it are controversial. In 2018 this pressure was reported to be on the order 10  Pa, which is greater than the pressure inside a neutron star . It

13433-416: The nucleon structure is still missing because ... long-distance behavior requires a nonperturbative and/or numerical treatment ..." More conceptual approaches to the structure of protons are: the topological soliton approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a string theory of gluons, various QCD-inspired models like the bag model and

13566-598: The nucleus the proton , after the neuter singular of the Greek word for "first", πρῶτον . However, Rutherford also had in mind the word protyle as used by Prout. Rutherford spoke at the British Association for the Advancement of Science at its Cardiff meeting beginning 24 August 1920. At the meeting, he was asked by Oliver Lodge for a new name for the positive hydrogen nucleus to avoid confusion with

13699-681: The number of (negatively charged) electrons , which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl anion has 17 protons and 18 electrons for a total charge of −1. All atoms of a given element are not necessarily identical, however. The number of neutrons may vary to form different isotopes , and energy levels may differ, resulting in different nuclear isomers . For example, there are two stable isotopes of chlorine : 17 Cl with 35 − 17 = 18 neutrons and 17 Cl with 37 − 17 = 20 neutrons. The proton

13832-418: The number of atomic electrons and consequently the chemical characteristics of the element. The word proton is Greek for "first", and the name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus (known to be the lightest nucleus) could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore

13965-484: The numbers of constituent quarks of each particular flavour. All of the various charges discussed above are conserved by the fact that the corresponding charge operators can be understood as generators of symmetries that commute with the Hamiltonian. Thus, the eigenvalues of the various charge operators are conserved. Absolutely conserved quantum numbers in the Standard Model are: In some theories, such as

14098-509: The observation of "missing" energy and momentum in tau decay, analogous to the "missing" energy and momentum in beta decay leading to the discovery of the electron neutrino. The first detection of tau neutrino interactions was announced in 2000 by the DONUT collaboration at Fermilab , making it the second-to-latest particle of the Standard Model to have been directly observed, with Higgs boson being discovered in 2012. Although all present data

14231-497: The other three fundamental interactions : gravitation , the weak interaction , and to electromagnetism , of which the latter is proportional to charge, and is thus zero for the electrically neutral neutrinos. For every lepton flavor, there is a corresponding type of antiparticle , known as an antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign . According to certain theories, neutrinos may be their own antiparticle . It

14364-719: The particles in the solar wind are electrons and protons, in approximately equal numbers. Because the Solar Wind Spectrometer made continuous measurements, it was possible to measure how the Earth's magnetic field affects arriving solar wind particles. For about two-thirds of each orbit, the Moon is outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month,

14497-467: The physics. All (complex) linear combinations of these two particles give the same physics, as long as the combinations are orthogonal , or perpendicular, to each other. In other words, the theory possesses symmetry transformations such as M ( u d ) {\displaystyle M\left({u \atop d}\right)} , where u and d are the two fields (representing the various generations of leptons and quarks, see below), and M

14630-675: The pressure profile shape by selection of the model. The radius of the hydrated proton appears in the Born equation for calculating the hydration enthalpy of hydronium . Although protons have affinity for oppositely charged electrons, this is a relatively low-energy interaction and so free protons must lose sufficient velocity (and kinetic energy ) in order to become closely associated and bound to electrons. High energy protons, in traversing ordinary matter, lose energy by collisions with atomic nuclei , and by ionization of atoms (removing electrons) until they are slowed sufficiently to be captured by

14763-662: The production of which is heavily affected by solar proton events such as coronal mass ejections . Research has been performed on the dose-rate effects of protons, as typically found in space travel , on human health. To be more specific, there are hopes to identify what specific chromosomes are damaged, and to define the damage, during cancer development from proton exposure. Another study looks into determining "the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including dopaminergic functioning, amphetamine -induced conditioned taste aversion learning, and spatial learning and memory as measured by

14896-400: The proton's mass. The rest mass of a proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles confined to a system is still measured as part of the rest mass of the system. Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to

15029-479: The proton, which is due to its angular momentum (or spin ), which in turn has a magnitude of one-half the reduced Planck constant . ( ℏ / 2 {\displaystyle \hbar /2} ). The name refers to examination of protons as they occur in protium (hydrogen-1 atoms) in compounds, and does not imply that free protons exist in the compound being studied. The Apollo Lunar Surface Experiments Packages (ALSEP) determined that more than 95% of

15162-399: The quantum of the electromagnetic field, the photon . The Feynman diagram of the electron–photon interaction is shown on the right. Because leptons possess an intrinsic rotation in the form of their spin, charged leptons generate a magnetic field. The size of their magnetic dipole moment μ is given by where m is the mass of the lepton and g is the so-called " g  factor" for

15295-405: The quark masses are much smaller than the chiral symmetry breaking scale of 250 MeV), the masses of quarks do not substantially contribute to the system's behavior, and to zeroth approximation the masses of the lightest quarks can be ignored for most purposes, as if they had zero mass. The simplified behavior of flavour transformations can then be successfully modeled as acting independently on

15428-517: The quarks' exchanging gluons, and interacting with various vacuum condensates. Lattice QCD provides a way of calculating the mass of a proton directly from the theory to any accuracy, in principle. The most recent calculations claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürr et al. ). These claims are still controversial, because the calculations cannot yet be done with quarks as light as they are in

15561-444: The real world. This means that the predictions are found by a process of extrapolation , which can introduce systematic errors. It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the hadrons , which are known in advance. These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: "a detailed description of

15694-581: The relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of a proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10 . The equality of their masses has also been tested to better than one part in 10 . By holding antiprotons in a Penning trap , the equality of the charge-to-mass ratio of protons and antiprotons has been tested to one part in 6 × 10 . The magnetic moment of antiprotons has been measured with an error of 8 × 10 nuclear Bohr magnetons , and

15827-401: The same direction as their momentum are called right-handed and they are otherwise called left-handed . When a particle is massless, the direction of its momentum relative to its spin is the same in every reference frame, whereas for massive particles it is possible to 'overtake' the particle by choosing a faster-moving reference frame ; in the faster frame, the helicity is reversed. Chirality

15960-402: The same particle, because they both have nearly the same mass and interact in nearly the same way, if the (much weaker) electromagnetic interaction is neglected. Heisenberg noted that the mathematical formulation of this symmetry was in certain respects similar to the mathematical formulation of non-relativistic spin , whence the name "isospin" derives. The neutron and the proton are assigned to

16093-443: The simplest and lightest element and was influenced by Prout's hypothesis that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in other nuclei as an elementary particle led Rutherford to give the hydrogen nucleus H a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles. He named this new fundamental building block of

16226-574: The standard-model prediction of unity. In 2024 a preprint by the ATLAS collaboration has published a new value of R W μ / e = B ( W → μ ν μ ) / B ( W → e ν e ) = 0.9995 ± 0.0045 {\displaystyle R_{W}^{\mu /e}={\mathcal {B}}(W\rightarrow \mu \nu _{\mu })/{\mathcal {B}}(W\rightarrow e\nu _{e})=0.9995\pm 0.0045}

16359-421: The strength of their electromagnetic interactions . It determines the strength of the electric field generated by the particle (see Coulomb's law ) and how strongly the particle reacts to an external electric or magnetic field (see Lorentz force ). Each generation contains one lepton with Q = −1 e and one lepton with zero electric charge. The lepton with electric charge is commonly simply referred to as

16492-485: The surface of the Sun) and with any type of atom. Thus, in interaction with any type of normal (non-plasma) matter, low-velocity free protons do not remain free but are attracted to electrons in any atom or molecule with which they come into contact, causing the proton and molecule to combine. Such molecules are then said to be " protonated ", and chemically they are simply compounds of hydrogen, often positively charged. Often, as

16625-409: The third are the tauonic leptons , comprising the tau ( τ ) and the tau neutrino ( ν τ ). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons and neutrinos through a process of particle decay : the transformation from a higher mass state to a lower mass state. Thus electrons are stable and

16758-406: The weak interaction). From them can be built the derived quantum numbers: The terms "strange" and "strangeness" predate the discovery of the quark, but continued to be used after its discovery for the sake of continuity (i.e. the strangeness of each type of hadron remained the same); strangeness of anti-particles being referred to as +1, and particles as −1 as per the original definition. Strangeness

16891-562: The word is the Mycenaean Greek 𐀩𐀡𐀵 , re-po-to , written in Linear B syllabic script. Lepton was first used by physicist Léon Rosenfeld in 1948: Following a suggestion of Prof. C. Møller , I adopt—as a pendant to "nucleon"—the denomination "lepton" (from λεπτός, small, thin, delicate) to denote a particle of small mass. Rosenfeld chose the name as the common name for electrons and (then hypothesized) neutrinos. Additionally,

17024-417: Was carrying away the difference between the energy , momentum , and angular momentum of the initial and observed final particles. The electron neutrino was simply called the neutrino, as it was not yet known that neutrinos came in different flavours (or different "generations"). Nearly 40 years after the discovery of the electron, the muon was discovered by Carl D. Anderson in 1936. Due to its mass, it

17157-529: Was confirmed experimentally by Henry Moseley in 1913 using X-ray spectra (More details in Atomic number under Moseley's 1913 experiment). In 1917, Rutherford performed experiments (reported in 1919 and 1925) which proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons. These experiments began after Rutherford observed that when alpha particles would strike air, Rutherford could detect scintillation on

17290-572: Was discovered in 1962 by Leon M. Lederman , Melvin Schwartz , and Jack Steinberger , and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory . The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery. Leptons are an important part of

17423-495: Was initially categorized as a meson rather than a lepton. It later became clear that the muon was much more similar to the electron than to mesons, as muons do not undergo the strong interaction , and thus the muon was reclassified: electrons, muons, and the (electron) neutrino were grouped into a new group of particles—the leptons. In 1962, Leon M. Lederman , Melvin Schwartz , and Jack Steinberger showed that more than one type of neutrino exists by first detecting interactions of

17556-459: Was introduced to explain the rate of decay of newly discovered particles, such as the kaon, and was used in the Eightfold Way classification of hadrons and in subsequent quark models . These quantum numbers are preserved under strong and electromagnetic interactions , but not under weak interactions . For first-order weak decays, that is processes involving only one quark decay, these quantum numbers (e.g. charm) can only vary by 1, that is, for

17689-436: Was said to be maximum at the centre, positive (repulsive) to a radial distance of about 0.6 fm, negative (attractive) at greater distances, and very weak beyond about 2 fm. These numbers were derived by a combination of a theoretical model and experimental Compton scattering of high-energy electrons. However, these results have been challenged as also being consistent with zero pressure and as effectively providing

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