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132-489: Observations of the large-scale structure of the universe show that matter is aggregated into very large structures that have not had time to form under the force of their own self-gravitation. It is generally believed that some form of missing mass is responsible for increasing the gravitational force at these scales, although this mass has not been directly observed. This is a problem; normal matter in space will heat up until it gives off light, so if this missing mass exists, it

264-424: A FET amplifier. CDMS detectors also provide data on the phonon pulse shape which is crucial in rejecting near-surface background events. Bolometric detection of neutrinos with semiconductors at low temperature was first proposed by Blas Cabrera , Lawrence M. Krauss , and Frank Wilczek , and a similar method was proposed for WIMP detection by Mark Goodman and Edward Witten . CDMS I collected WIMP search data in

396-425: A fermion with intrinsic angular momentum equal to ⁠ 1 / 2 ⁠   ħ , where ħ is the reduced Planck constant . For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin  ⁠ 1 / 2 ⁠ Dirac particle , the possibility that the neutron was a spin  ⁠ 3 / 2 ⁠ particle lingered. The interactions of

528-408: A nuclear chain reaction . These events and findings led to the first self-sustaining nuclear reactor ( Chicago Pile-1 , 1942) and the first nuclear weapon ( Trinity , 1945). Dedicated neutron sources like neutron generators , research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. A free neutron spontaneously decays to

660-511: A quasar and an observer. In this case, the galaxy cluster will lens the quasar. Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including Abell 1689 . By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the weak regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining

792-401: A bottle, while the "beam" method employs energetic neutrons in a particle beam. The measurements by the two methods have not been converging with time. The lifetime from the bottle method is presently 877.75 s which is 10 seconds below the value from the beam method of 887.7 s A small fraction (about one per thousand) of free neutrons decay with the same products, but add an extra particle in

924-456: A cascade known as a nuclear chain reaction . For a given mass of fissile material, such nuclear reactions release energy that is approximately ten million times that from an equivalent mass of a conventional chemical explosive . Ultimately, the ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible; most of

1056-544: A deuteron is formed by a proton capturing a neutron (this is exothermic and happens with zero-energy neutrons). The small recoil kinetic energy ( E r d {\displaystyle E_{rd}} ) of the deuteron (about 0.06% of the total energy) must also be accounted for. The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, et al. These give

1188-544: A great majority of them – may be dark bodies. In 1906, Poincaré used the French term [ matière obscure ] ("dark matter") in discussing Kelvin's work. He found that the amount of dark matter would need to be less than that of visible matter, incorrectly, it turns out. The second to suggest the existence of dark matter using stellar velocities was Dutch astronomer Jacobus Kapteyn in 1922. A publication from 1930 by Swedish astronomer Knut Lundmark points to him being

1320-488: A magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin  ⁠ 1 / 2 ⁠ particle. As a fermion, the neutron is subject to the Pauli exclusion principle ; two neutrons cannot have the same quantum numbers. This is the source of the degeneracy pressure which counteracts gravity in neutron stars and prevents them from forming black holes. Even though

1452-417: A mass spectrometer, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus the binding energy of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy ( B d {\displaystyle B_{d}} ) of the single 2.224 MeV gamma photon emitted when

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1584-478: A mass-to-light ratio of 50; in 1940, Oort discovered and wrote about the large non-visible halo of NGC 3115 . Early radio astronomy observations, performed by Seth Shostak , later SETI Institute Senior Astronomer, showed a half-dozen galaxies spun too fast in their outer regions, pointing to the existence of dark matter as a means of creating the gravitational pull needed to keep the stars in their orbits. The hypothesis of dark matter largely took root in

1716-416: A mean-square radius of about 0.8 × 10   m , or 0.8  fm , and it is a spin-½ fermion . The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields , whereas the neutron is unaffected by electric fields. The neutron has a magnetic moment , however, so it is influenced by magnetic fields . The specific properties of

1848-404: A neutron by some heavy nuclides (such as uranium-235 ) can cause the nuclide to become unstable and break into lighter nuclides and additional neutrons. The positively charged light nuclides, or "fission fragments", then repel, releasing electromagnetic potential energy . If this reaction occurs within a mass of fissile material , the additional neutrons cause additional fission events, inducing

1980-448: A neutron mass of: The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of Da to MeV/ c : Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured. The neutron is a spin  ⁠ 1 / 2 ⁠ particle, that is, it is

2112-543: A nucleon. The discrepancy stems from the complexity of the Standard Model for nucleons, where most of their mass originates in the gluon fields, virtual particles, and their associated energy that are essential aspects of the strong force . Furthermore, the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment. But the nucleon magnetic moment has been successfully computed numerically from first principles , including all of

2244-489: A nucleus. The observed properties of atoms and molecules were inconsistent with the nuclear spin expected from the proton–electron hypothesis. Protons and electrons both carry an intrinsic spin of ⁠ 1 / 2 ⁠ ħ , and the isotopes of the same species were found to have either integer or fractional spin. By the hypothesis, isotopes would be composed of the same number of protons, but differing numbers of neutral bound proton+electron "particles". This physical picture

2376-417: A pair of protons, one with spin up, another with spin down. When all available proton states are filled, the Pauli exclusion principle disallows the decay of a neutron to a proton. The situation is similar to electrons of an atom, where electrons that occupy distinct atomic orbitals are prevented by the exclusion principle from decaying to lower, already-occupied, energy states. The stability of matter

2508-423: A proton, an electron , and an antineutrino , with a mean lifetime of about 15 minutes. Free neutrons do not directly ionize atoms, but they do indirectly cause ionizing radiation , so they can be a biological hazard, depending on dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers , and by the natural radioactivity of spontaneously fissionable elements in

2640-419: A rare isotope carbon-13 with 7 neutrons. Some elements occur in nature with only one stable isotope , such as fluorine . Other elements occur with many stable isotopes, such as tin with ten stable isotopes, or with no stable isotope, such as technetium . The properties of an atomic nucleus depend on both atomic and neutron numbers. With their positive charge, the protons within the nucleus are repelled by

2772-481: A series of experiments that showed that the new radiation consisted of uncharged particles with about the same mass as the proton. These properties matched Rutherford's hypothesized neutron. Chadwick won the 1935 Nobel Prize in Physics for this discovery. Models for an atomic nucleus consisting of protons and neutrons were quickly developed by Werner Heisenberg and others. The proton–neutron model explained

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2904-708: A shallow underground site (called SUF, Stanford Underground Facility) at Stanford University 1998–2002. CDMS II operated (with collaboration from the University of Minnesota ) in the Soudan Mine from 2003 to 2009 (data taking 2006–2008). The newest experiment, SuperCDMS (or SuperCDMS Soudan), with interleaved electrodes, more mass, and even better background rejection was taking data at Soudan 2011–2015. The series of experiments continue with SuperCDMS SNOLAB, currently (2018) under construction in SNOLAB and to be completed in

3036-682: A significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the Voyager ;1 spacecraft. Tiny black holes are theorized to emit Hawking radiation . However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter. Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling, and

3168-491: A similar inference. Zwicky applied the virial theorem to the Coma Cluster and obtained evidence of unseen mass he called dunkle Materie ('dark matter'). Zwicky estimated its mass based on the motions of galaxies near its edge and compared that to an estimate based on its brightness and number of galaxies. He estimated the cluster had about 400 times more mass than was visually observable. The gravity effect of

3300-404: A simple nonrelativistic , quantum mechanical wavefunction for baryons composed of three quarks. A straightforward calculation gives fairly accurate estimates for the magnetic moments of neutrons, protons, and other baryons. For a neutron, the result of this calculation is that the magnetic moment of the neutron is given by μ n = 4/3 μ d − 1/3 μ u , where μ d and μ u are

3432-424: A smaller fraction, using greater values for luminous mass. Nonetheless, Zwicky did correctly conclude from his calculation that most of the gravitational matter present was dark. However unlike modern theories, Zwicky considered "dark matter" to be non-luminous ordinary matter. Further indications of mass-to-light ratio anomalies came from measurements of galaxy rotation curves . In 1939, H.W. Babcock reported

3564-572: A thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter. However, that study assumed a monochromatic distribution to represent the LIGO/Virgo mass range, which is inapplicable to the broadly platykurtic mass distribution suggested by subsequent James Webb Space Telescope observations. The possibility that atom-sized primordial black holes account for

3696-452: Is a hypothetical form of matter that does not interact with light or other electromagnetic radiation . Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies , gravitational lensing , the observable universe 's current structure, mass position in galactic collisions ,

3828-680: Is a 0.19% chance that these are anomalous background noise, giving the result a 99.8% (3 sigmas) confidence level. Whilst not conclusive evidence for WIMPs this provides strong weight to the theories. This signal was observed by the CDMS II-experiment and it is called the CDMS Si-signal (sometimes the experiment is also called CDMS Si) because it was observed by the silicon detectors. SuperCDMS search results from October 2012 to June 2013 were published in June 2014, finding 11 events in

3960-413: Is a consequence of these constraints. The decay of a neutron within a nuclide is illustrated by the decay of the carbon isotope carbon-14 , which has 6 protons and 8 neutrons. With its excess of neutrons, this isotope decays by beta decay to nitrogen-14 (7 protons, 7 neutrons), a process with a half-life of about 5,730 years . Nitrogen-14 is stable. "Beta decay" reactions can also occur by

4092-502: Is able to determine the probability of interactions being caused by neutrons. CDMS detectors are disks of germanium or silicon, cooled to millikelvin temperatures by a dilution refrigerator . The extremely low temperatures are needed to limit thermal noise which would otherwise obscure the phonon signals of particle interactions. Phonon detection is accomplished with superconduction transition edge sensors (TESs) read out by SQUID amplifiers, while ionization signals are read out using

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4224-482: Is classified as "cold", "warm", or "hot" according to velocity (more precisely, its free streaming length). Recent models have favored a cold dark matter scenario, in which structures emerge by the gradual accumulation of particles. Although the astrophysics community generally accepts the existence of dark matter, a minority of astrophysicists, intrigued by specific observations that are not well explained by ordinary dark matter, argue for various modifications of

4356-403: Is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations . With the discovery of nuclear fission in 1938, it was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as

4488-673: Is estimated that such noise would produce two or more events 25% of the time. Polythene absorbers were fitted to reduce any neutron background. A 2011 analysis with lower energy thresholds, looked for evidence for low-mass WIMPs (M < 9 GeV). Their limits rule out hints claimed by a new germanium experiment called CoGeNT and the long-standing DAMA/NaI , DAMA/LIBRA annual modulation result. Further analysis of data in Physical Review Letters May 2013, revealed 3 WIMP detections with an expected background of 0.7, with masses expected from WIMPs, including neutralinos. There

4620-448: Is for one of the neutron's quarks to change flavour (through a Cabibbo–Kobayashi–Maskawa matrix ) via the weak interaction . The decay of one of the neutron's down quarks into a lighter up quark can be achieved by the emission of a W boson . By this process, the Standard Model description of beta decay, the neutron decays into a proton (which contains one down and two up quarks), an electron, and an electron antineutrino . The decay of

4752-502: Is generally assumed to be in a form that is not commonly observed on earth. A number of proposed candidates for the missing mass have been put forward over time. Early candidates included heavy baryons that would have had to be created in the Big Bang , but more recent work on nucleosynthesis seems to have ruled most of these out. Another candidate are new types of particles known as weakly interacting massive particles , or "WIMP"s. As

4884-427: Is not known to interact with ordinary baryonic matter and radiation except through gravity, making it difficult to detect in the laboratory. The most prevalent explanation is that dark matter is some as-yet-undiscovered subatomic particle , such as either weakly interacting massive particles (WIMPs) or axions . The other main possibility is that dark matter is composed of primordial black holes . Dark matter

5016-536: Is of particular note because the location of the center of mass as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain. Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to

5148-474: Is revealed only via its gravitational effects, or weak lensing . In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection). In 2015, the idea that dense dark matter was composed of primordial black holes made a comeback following results of gravitational wave measurements which detected

5280-399: Is the gravitational lens . Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a cluster of galaxies lying between a more distant source such as

5412-420: Is the nuclear magneton . The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution. In the quark model for hadrons , the neutron is composed of one up quark (charge +2/3  e ) and two down quarks (charge −1/3  e ). The magnetic moment of

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5544-434: Is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below. Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter , such as protons or neutrons. Most of

5676-504: Is well fitted by the lambda-CDM model , but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND). Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed

5808-580: The 2dF Galaxy Redshift Survey . Results are in agreement with the lambda-CDM model . In astronomical spectroscopy , the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars . Lyman-alpha forest observations can also constrain cosmological models. These constraints agree with those obtained from WMAP data. The identity of dark matter

5940-713: The Chicago Pile-1 at the University of Chicago in 1942, the first self-sustaining nuclear reactor . Just three years later the Manhattan Project was able to test the first atomic bomb , the Trinity nuclear test in July 1945. The mass of a neutron cannot be directly determined by mass spectrometry since it has no electric charge. But since the masses of a proton and of a deuteron can be measured with

6072-603: The Earth's crust . An atomic nucleus is formed by a number of protons, Z (the atomic number ), and a number of neutrons, N (the neutron number ), bound together by the nuclear force . Protons and neutrons each have a mass of approximately one dalton . The atomic number determines the chemical properties of the atom, and the neutron number determines the isotope or nuclide . The terms isotope and nuclide are often used synonymously , but they refer to chemical and nuclear properties, respectively. Isotopes are nuclides with

6204-689: The ionization and phonons produced by every particle interaction in their germanium and silicon crystal substrates. These two measurements determine the energy deposited in the crystal in each interaction, but also give information about what kind of particle caused the event. The ratio of ionization signal to phonon signal differs for particle interactions with atomic electrons ("electron recoils") and atomic nuclei ("nuclear recoils"). The vast majority of background particle interactions are electron recoils, while WIMPs (and neutrons ) are expected to produce nuclear recoils. This allows WIMP-scattering events to be identified even though they are rare compared to

6336-486: The 1920s, physicists assumed that the atomic nucleus was composed of protons and "nuclear electrons", but this raised obvious problems. It was difficult to reconcile the proton–electron model of the nucleus with the Heisenberg uncertainty relation of quantum mechanics. The Klein paradox , discovered by Oskar Klein in 1928, presented further quantum mechanical objections to the notion of an electron confined within

6468-512: The 1944 Nobel Prize in Chemistry "for his discovery of the fission of heavy atomic nuclei". The discovery of nuclear fission would lead to the development of nuclear power and the atomic bomb by the end of World War II. It was quickly realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, in a cascade known as a nuclear chain reaction. These events and findings led Fermi to construct

6600-547: The 1970s. Several different observations were synthesized to argue that galaxies should be surrounded by halos of unseen matter. In two papers that appeared in 1974, this conclusion was drawn in tandem by independent groups: in Princeton, New Jersey, U.S.A., by Jeremiah Ostriker , Jim Peebles , and Amos Yahil, and in Tartu, Estonia, by Jaan Einasto , Enn Saar, and Ants Kaasik. One of the observations that served as evidence for

6732-422: The 21 cm line of atomic hydrogen in nearby galaxies. The radial distribution of interstellar atomic hydrogen ( H ) often extends to much greater galactic distances than can be observed as collective starlight, expanding the sampled distances for rotation curves – and thus of the total mass distribution – to a new dynamical regime. Early mapping of Andromeda with the 300 foot telescope at Green Bank and

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6864-434: The 250 foot dish at Jodrell Bank already showed the H rotation curve did not trace the decline expected from Keplerian orbits. As more sensitive receivers became available, Roberts & Whitehurst (1975) were able to trace the rotational velocity of Andromeda to 30 kpc, much beyond the optical measurements. Illustrating the advantage of tracing the gas disk at large radii; that paper's Figure 16 combines

6996-558: The American chemist W. D. Harkins first named the hypothetical particle a "neutron". The name derives from the Latin root for neutralis (neuter) and the Greek suffix -on (a suffix used in the names of subatomic particles, i.e. electron and proton ). References to the word neutron in connection with the atom can be found in the literature as early as 1899, however. Throughout

7128-489: The CDMS-experiment detectors and different data-sets thus collected are sometimes given names like CDMS Ge, CDMS Si, CDMS II Si et cetera. On December 17, 2009, the collaboration announced the possible detection of two candidate WIMPs, one on August 8, 2007, and the other on October 27, 2007. Due to the low number of events, the team could exclude false positives from background noise such as neutron collisions. It

7260-632: The CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe. The results support the Lambda-CDM model. Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts ;

7392-466: The CMB. The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters. The CMB anisotropy

7524-494: The Nobel Prize in Physics "for his demonstrations of the existence of new radioactive elements produced by neutron irradiation, and for his related discovery of nuclear reactions brought about by slow neutrons". In December 1938 Otto Hahn , Lise Meitner , and Fritz Strassmann discovered nuclear fission , or the fractionation of uranium nuclei into lighter elements, induced by neutron bombardment. In 1945 Hahn received

7656-488: The SNOLAB "Cryopit" location. Increasing the detector mass only makes the detector more sensitive if the unwanted background detections do not increase as well, thus each generation must be cleaner and better shielded than the one before. The purpose of building in ten-fold stages like this is to develop the necessary shielding techniques before finalizing the GEODM design. Missing mass In astronomy , dark matter

7788-474: The Solar System. This is not observed. Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases. If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there is a lot of non-luminous matter (dark matter) in the outskirts of

7920-403: The apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements. Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter

8052-452: The atom's heavy nucleus. The electron configuration is determined by the charge of the nucleus, which is determined by the number of protons, or atomic number . The number of neutrons is the neutron number . Neutrons do not affect the electron configuration. Atoms of a chemical element that differ only in neutron number are called isotopes . For example, carbon , with atomic number 6, has an abundant isotope carbon-12 with 6 neutrons and

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8184-406: The beta decay process. The neutrons and protons in a nucleus form a quantum mechanical system according to the nuclear shell model . Protons and neutrons of a nuclide are organized into discrete hierarchical energy levels with unique quantum numbers . Nucleon decay within a nucleus can occur if allowed by basic energy conservation and quantum mechanical constraints. The decay products, that is,

8316-400: The capture of a lepton by the nucleon. The transformation of a proton to a neutron inside of a nucleus is possible through electron capture : A rarer reaction, inverse beta decay , involves the capture of a neutrino by a nucleon. Rarer still, positron capture by neutrons can occur in the high-temperature environment of stars. Three types of beta decay in competition are illustrated by

8448-438: The common chemical element lead , Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Even though it is not a chemical element, the neutron is included in this table. Protons and neutrons behave almost identically under the influence of the nuclear force within the nucleus. They are therefore both referred to collectively as nucleons . The concept of isospin , in which

8580-441: The complex behavior of quarks to be subtracted out between models, and merely exploring what the effects would be of differing quark charges (or quark type). Such calculations are enough to show that the interior of neutrons is very much like that of protons, save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton. The neutron magnetic moment can be roughly computed by assuming

8712-429: The dark matter separating from the visible gas, producing the separate lensing peak as observed. Type Ia supernovae can be used as standard candles to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past. Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy . Since observations indicate

8844-403: The dark matter. However, multiple lines of evidence suggest the majority of dark matter is not baryonic: There are two main candidates for non-baryonic dark matter: new hypothetical particles and primordial black holes . Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the elements in the early universe ( Big Bang nucleosynthesis ) and so its presence

8976-420: The density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination,

9108-436: The diameter of the observable Universe via cosmic expansion , the scale, a , has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled); the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved. The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of

9240-552: The difference in mass represents the mass equivalent to nuclear binding energy, the energy which would need to be added to take the nucleus apart. The nucleus of the most common isotope of the hydrogen atom (with the chemical symbol H) is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium (D or H) and tritium (T or H) 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 most common nuclide of

9372-504: The early 2020s. The series of experiments also includes the CDMSlite experiment which used SuperCDMS detectors at Soudan in an operating mode (called CDMSlite-mode) that was meant to be sensitive specifically to low-mass WIMPs. As the CDMS-experiment has multiple different detector technologies in use, in particular, 2 types of detectors based on germanium or silicon, respectively, the experiments derived from some specific configuration of

9504-516: The effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (≈1 percent) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130–160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey . Combining

9636-459: The electron fails to gain the 13.6  eV necessary energy to escape the proton (the ionization energy of hydrogen ), and therefore simply remains bound to it, forming a neutral hydrogen atom (one of the "two bodies"). In this type of free neutron decay, almost all of the neutron decay energy is carried off by the antineutrino (the other "body"). (The hydrogen atom recoils with a speed of only about (decay energy)/(hydrogen rest energy) times

9768-405: The emitted particles, carry away the energy excess as a nucleon falls from one quantum state to one with less energy, while the neutron (or proton) changes to a proton (or neutron). For a neutron to decay, the resulting proton requires an available state at lower energy than the initial neutron state. In stable nuclei the possible lower energy states are all filled, meaning each state is occupied by

9900-593: The energy released from fission is the kinetic energy of the fission fragments. Neutrons and protons within a nucleus behave similarly and can exchange their identities by similar reactions. These reactions are a form of radioactive decay known as beta decay . Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force , and it requires the emission or absorption of electrons and neutrinos, or their antiparticles. The neutron and proton decay reactions are: where p , e , and ν e denote

10032-408: The existence of galactic halos of dark matter was the shape of galaxy rotation curves . These observations were done in optical and radio astronomy. In optical astronomy, Vera Rubin and Kent Ford worked with a new spectrograph to measure the velocity curve of edge-on spiral galaxies with greater accuracy. At the same time, radio astronomers were making use of new radio telescopes to map

10164-505: The first to realise that the universe must contain much more mass than can be observed. Dutch radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932. Oort was studying stellar motions in the galactic neighborhood and found the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be incorrect. In 1933, Swiss astrophysicist Fritz Zwicky studied galaxy clusters while working at Cal Tech and made

10296-445: The form of an emitted gamma ray: Called a "radiative decay mode" of the neutron, the gamma ray may be thought of as resulting from an "internal bremsstrahlung " that arises from the electromagnetic interaction of the emitted beta particle with the proton. A smaller fraction (about four per million) of free neutrons decay in so-called "two-body (neutron) decays", in which a proton, electron and antineutrino are produced as usual, but

10428-473: The galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it, similar to the Solar System . From Kepler's Third Law , it is expected that the rotation velocities will decrease with distance from the center, similar to

10560-429: The galaxies and clusters currently seen. Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process. The Bullet Cluster is the result of a recent collision of two galaxy clusters. It

10692-499: The galaxy. Stars in bound systems must obey the virial theorem . The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits. As with galaxy rotation curves,

10824-400: The homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure. If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into

10956-433: The late 1970s the existence of dark matter halos around galaxies was widely recognized as real, and became a major unsolved problem in astronomy. A stream of observations in the 1980–1990s supported the presence of dark matter. Persic, Salucci & Stel (1996) is notable for the investigation of 967 spirals. The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters ,

11088-474: The long-range electromagnetic force , but the much stronger, but short-range, nuclear force binds the nucleons closely together. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen nucleus. Neutrons are produced copiously in nuclear fission and fusion . They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes. The neutron

11220-437: The magnetic moments for the down and up quarks, respectively. This result combines the intrinsic magnetic moments of the quarks with their orbital magnetic moments, and assumes the three quarks are in a particular, dominant quantum state. The results of this calculation are encouraging, but the masses of the up or down quarks were assumed to be 1/3 the mass of a nucleon. The masses of the quarks are actually only about 1% that of

11352-469: The merger of intermediate-mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate-mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. A later survey of about

11484-445: The motion of galaxies within galaxy clusters , and cosmic microwave background anisotropies . In the standard lambda-CDM model of cosmology , the mass–energy content of the universe is 5% ordinary matter, 26.8% dark matter, and 68.2% a form of energy known as dark energy . Thus, dark matter constitutes 85% of the total mass, while dark energy and dark matter constitute 95% of the total mass–energy content. Dark matter

11616-476: The name implies, WIMPs interact weakly with normal matter, which explains why they are not easily visible. Detecting WIMPs thus presents a problem; if the WIMPs are very weakly interacting, detecting them will be extremely difficult. Detectors like CDMS and similar experiments measure huge numbers of interactions within their detector volume in order to find the extremely rare WIMP events. The CDMS detectors measure

11748-406: The neutron and its magnetic moment both indicate that the neutron is a composite , rather than elementary , particle. The quarks of the neutron are held together by the strong force , mediated by gluons . The nuclear force results from secondary effects of the more fundamental strong force . The only possible decay mode for the neutron that obeys the conservation law for the baryon number

11880-401: The neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. In the 1911 Rutherford model , the atom consisted of a small positively charged massive nucleus surrounded by a much larger cloud of negatively charged electrons. In 1920, Ernest Rutherford suggested that

12012-461: The neutron are described below in the Intrinsic properties section . Outside the nucleus, free neutrons undergo beta decay with a mean lifetime of about 14 minutes, 38 seconds, corresponding to a half-life of about 10 minutes, 11 s. The mass of the neutron is greater than that of the proton by 1.293 32   MeV/ c , hence the neutron's mass provides energy sufficient for the creation of

12144-401: The neutron can be modeled as a sum of the magnetic moments of the constituent quarks. The calculation assumes that the quarks behave like point-like Dirac particles, each having their own magnetic moment. Simplistically, the magnetic moment of the neutron can be viewed as resulting from the vector sum of the three quark magnetic moments, plus the orbital magnetic moments caused by the movement of

12276-434: The neutron is a neutral particle, the magnetic moment of a neutron is not zero. The neutron is not affected by electric fields, but it is affected by magnetic fields. The value for the neutron's magnetic moment was first directly measured by Luis Alvarez and Felix Bloch at Berkeley, California , in 1940. Alvarez and Bloch determined the magnetic moment of the neutron to be μ n = −1.93(2)  μ N , where μ N

12408-431: The neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron. In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin  ⁠ 1 / 2 ⁠ . In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a Stern–Gerlach experiment that used

12540-431: The nucleus consisted of positive protons and neutrally charged particles, suggested to be a proton and an electron bound in some way. Electrons were assumed to reside within the nucleus because it was known that beta radiation consisted of electrons emitted from the nucleus. About the time Rutherford suggested the neutral proton-electron composite, several other publications appeared making similar suggestions, and in 1921

12672-420: The nucleus via the nuclear force , effectively moderating the repulsive forces between the protons and stabilizing the nucleus. Heavy nuclei carry a large positive charge, hence they require "extra" neutrons to be stable. While a free neutron is unstable and a free proton is stable, within nuclei neutrons are often stable and protons are sometimes unstable. When bound within a nucleus, nucleons can decay by

12804-408: The nucleus, they are both referred to as nucleons . Nucleons have a mass of approximately one atomic mass unit, or dalton (symbol: Da). Their properties and interactions are described by nuclear physics . Protons and neutrons are not elementary particles ; each is composed of three quarks . The chemical properties of an atom are mostly determined by the configuration of electrons that orbit

12936-408: The obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter. Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways: Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1. One of the consequences of general relativity

13068-464: The optical data (the cluster of points at radii of less than 15 kpc with a single point further out) with the H data between 20 and 30 kpc, exhibiting the flatness of the outer galaxy rotation curve; the solid curve peaking at the center is the optical surface density, while the other curve shows the cumulative mass, still rising linearly at the outermost measurement. In parallel, the use of interferometric arrays for extragalactic H spectroscopy

13200-513: The ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category. A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost. These massive objects that are hard to detect are collectively known as MACHOs . Some scientists initially hoped that baryonic MACHOs could account for and explain all

13332-446: The original particle is not composed of the product particles; rather, the product particles are created at the instant of the reaction. "Free" neutrons or protons are nucleons that exist independently, free of any nucleus. The free neutron has a mass of 939 565 413 .3  eV/ c , or 939.565 4133   MeV/ c . This mass is equal to 1.674 927 471 × 10   kg , or 1.008 664 915 88   Da . The neutron has

13464-585: The potential number of stars around the Sun in the appendices of a book based on a series of lectures given in 1884 in Baltimore. He inferred their density using the observed velocity dispersion of the stars near the Sun, assuming that the Sun was 20–100 million years old. He posed what would happen if there were a thousand million stars within 1  kiloparsec of the Sun (at which distance their parallax would be 1  milli-arcsecond ). Kelvin concluded Many of our supposed thousand million stars – perhaps

13596-471: The project has suffered repeated delays (earlier plans hoped for construction to begin in 2014 and 2016), it remains active, with space allocated in SNOLAB and a scheduled construction start in early 2018. The construction of SuperCDMS at SNOLAB started in 2018 with beginning of operations in early 2020s. The project budget at the time was US$ 34 million. In May 2021, the SuperCDMS SNOLAB detector

13728-400: The proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the nuclear energy binding nucleons is many orders of magnitude greater than the electromagnetic energy binding electrons in atoms. In nuclear fission , the absorption of

13860-406: The proton to a neutron occurs similarly through the weak force. The decay of one of the proton's up quarks into a down quark can be achieved by the emission of a W boson. The proton decays into a neutron, a positron, and an electron neutrino. This reaction can only occur within an atomic nucleus which has a quantum state at lower energy available for the created neutron. The story of the discovery of

13992-409: The proton, electron and electron anti- neutrino decay products, and where n , e , and ν e denote the neutron, positron and electron neutrino decay products. The electron and positron produced in these reactions are historically known as beta particles , denoted β or β respectively, lending the name to the decay process. In these reactions,

14124-464: The proton, electron, and anti-neutrino. In the decay process, the proton, electron, and electron anti-neutrino conserve the energy, charge, and lepton number of the neutron. The electron can acquire a kinetic energy up to 0.782 ± 0.013 MeV . Still unexplained, different experimental methods for measuring the neutron's lifetime, the "bottle" and "beam" methods, produce different values for it. The "bottle" method employs "cold" neutrons trapped in

14256-519: The puzzle of nuclear spins. The origins of beta radiation were explained by Enrico Fermi in 1934 by the process of beta decay , in which the neutron decays to a proton by creating an electron and a (at the time undiscovered) neutrino. In 1935, Chadwick and his doctoral student Maurice Goldhaber reported the first accurate measurement of the mass of the neutron. By 1934, Fermi had bombarded heavier elements with neutrons to induce radioactivity in elements of high atomic number. In 1938, Fermi received

14388-452: The question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist. Neutron The neutron is a subatomic particle , symbol n or n , that has no electric charge, and a mass slightly greater than that of a proton . Protons and neutrons constitute the nuclei of atoms . Since protons and neutrons behave similarly within

14520-461: The redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind

14652-531: The rotation curve for the Andromeda nebula (now called the Andromeda Galaxy ), which suggested the mass-to-luminosity ratio increases radially. He attributed it to either light absorption within the galaxy or modified dynamics in the outer portions of the spiral, rather than to unseen matter. Following Babcock's 1939 report of unexpectedly rapid rotation in the outskirts of the Andromeda Galaxy and

14784-429: The same atomic number, but different neutron number. Nuclides with the same neutron number, but different atomic number, are called isotones . The atomic mass number , A , is equal to the sum of atomic and neutron numbers. Nuclides with the same atomic mass number, but different atomic and neutron numbers, are called isobars . The mass of a nucleus is always slightly less than the sum of its proton and neutron masses:

14916-493: The signal region for WIMP mass less than 30 GeV, and set an upper limit for spin-independent cross section disfavoring a recent CoGeNT low mass signal. A second generation of SuperCDMS is planned for SNOLAB. This is expanded from SuperCDMS Soudan in every way: The increase in detector mass is not quite as large, because about 25% of the detectors will be made of silicon, which only weights 44% as much. Filling all 31 towers at this ratio would result in about 222 kg Although

15048-469: The single isotope copper-64 (29 protons, 35 neutrons), which has a half-life of about 12.7 hours. This isotope has one unpaired proton and one unpaired neutron, so either the proton or the neutron can decay. This particular nuclide is almost equally likely to undergo proton decay (by positron emission , 18% or by electron capture , 43%; both forming Ni ) or neutron decay (by electron emission, 39%; forming Zn ). Within

15180-429: The speed of light, or 250  km/s .) Neutrons are a necessary constituent of any atomic nucleus that contains more than one proton. As a result of their positive charges, interacting protons have a mutual electromagnetic repulsion that is stronger than their attractive nuclear interaction , so proton-only nuclei are unstable (see diproton and neutron–proton ratio ). Neutrons bind with protons and one another in

15312-465: The standard laws of general relativity. These include modified Newtonian dynamics , tensor–vector–scalar gravity , or entropic gravity . So far none of the proposed modified gravity theories can describe every piece of observational evidence at the same time, suggesting that even if gravity has to be modified, some form of dark matter will still be required. The hypothesis of dark matter has an elaborate history. Wm. Thomson, Lord Kelvin, discussed

15444-454: The supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by

15576-470: The temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background . According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized subatomic particle . The search for this particle, by a variety of means, is one of the major efforts in particle physics . In standard cosmological calculations, "matter" means any constituent of

15708-417: The theoretical framework of the Standard Model for particle physics, a neutron comprises two down quarks with charge − ⁠ 1 / 3 ⁠ e and one up quark with charge + ⁠ 2 / 3 ⁠ e . The neutron is therefore a composite particle classified as a hadron . The neutron is also classified as a baryon , because it is composed of three valence quarks . The finite size of

15840-435: The three charged quarks within the neutron. In one of the early successes of the Standard Model, in 1964 Mirza A.B. Beg, Benjamin W. Lee , and Abraham Pais calculated the ratio of proton to neutron magnetic moments to be −3/2 (or a ratio of −1.5), which agrees with the experimental value to within 3%. The measured value for this ratio is −1.459 898 05 (34) . The above treatment compares neutrons with protons, allowing

15972-523: The universe is almost flat, it is expected the total energy density of everything in the universe should sum to 1 ( Ω tot ≈ 1 ). The measured dark energy density is Ω Λ ≈ 0.690 ; the observed ordinary (baryonic) matter energy density is Ω b ≈ 0.0482 and the energy density of radiation is negligible. This leaves a missing Ω dm ≈ 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter. Baryon acoustic oscillations (BAO) are fluctuations in

16104-439: The universe whose energy density scales with the inverse cube of the scale factor , i.e., ρ ∝ a . This is in contrast to "radiation" , which scales as the inverse fourth power of the scale factor ρ ∝ a , and a cosmological constant , which does not change with respect to a ( ρ ∝ a ). The different scaling factors for matter and radiation are a consequence of radiation redshift . For example, after doubling

16236-540: The vast majority of unwanted background interactions. From supersymmetry , the probability of a spin-independent interaction between a WIMP and a nucleus would be related to the number of nucleons in the nucleus. Thus, a WIMP would be more likely to interact with a germanium detector than a silicon detector, since germanium is a much heavier element. Neutrons would be able to interact with both silicon and germanium detectors with similar probability. By comparing rates of interactions between silicon and germanium detectors, CDMS

16368-404: The visible galaxies was far too small for such fast orbits, thus mass must be hidden from view. Based on these conclusions, Zwicky inferred some unseen matter provided the mass and associated gravitational attraction to hold the cluster together. Zwicky's estimates were off by more than an order of magnitude, mainly due to an obsolete value of the Hubble constant ; the same calculation today shows

16500-399: The volume under consideration. In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding " missing baryons ". Context will usually indicate which meaning is intended. The arms of spiral galaxies rotate around

16632-585: Was gamma radiation . The following year Irène Joliot-Curie and Frédéric Joliot-Curie in Paris showed that if this "gamma" radiation fell on paraffin , or any other hydrogen -containing compound, it ejected protons of very high energy. Neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge were convinced by the gamma ray interpretation. Chadwick quickly performed

16764-418: Was a contradiction, since there is no way to arrange the spins of an electron and a proton in a bound state to get a fractional spin. In 1931, Walther Bothe and Herbert Becker found that if alpha particle radiation from polonium fell on beryllium , boron , or lithium , an unusually penetrating radiation was produced. The radiation was not influenced by an electric field, so Bothe and Becker assumed it

16896-585: Was being developed. Rogstad & Shostak (1972) published H rotation curves of five spirals mapped with the Owens Valley interferometer; the rotation curves of all five were very flat, suggesting very large values of mass-to-light ratio in the outer parts of their extended H  disks. In 1978, Albert Bosma showed further evidence of flat rotation curves using data from the Westerbork Synthesis Radio Telescope . By

17028-556: Was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003–2012, and even more precisely by the Planck spacecraft in 2013–2015. The results support the Lambda-CDM model. The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure

17160-479: Was in progress, with a plan to start commissioning run in 2023. First science run with full detector payload in early 2024 and first result in early 2025. In June 2023, SuperCDMS SNOLAB installation was in full swing. Commissioning was expected to start in 2024. A third generation of SuperCDMS is envisioned, although still in the early planning phase. GEODM ( GErmanium Observatory for Dark Matter ), with roughly 1500 kg of detector mass, has expressed interest in

17292-423: Was ionized and interacted strongly with radiation via Thomson scattering . Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on

17424-445: Was under construction, with early science (or prototyping, or preliminary studies) ongoing with prototype/testing hardware, both at the SNOLAB location and at other locations. The full detector was expected ready for science data taking at the end of 2023, and the science operations to last 4 years (with two separate runs) 2023-2027, with possible extensions and developments beyond 2027. In May 2022, SuperCDMS SNOLAB detector installation

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