Tag Archives: WIMPs

SuperCDMS Collaboration Possible Detection of Dark Matter

Hard on the heels of the Alpha Magnetic Spectrometer (AMS) positron excess and possible dark matter report, we now have a hint of direct dark matter detection from the SuperCDMS Collaboration this month. A recent blog here on darkmatterdarkenergy.com discusses the detection of excess positron flux seen in the AMS-02 experiment on board the Space Shuttle. The two main hypotheses for the source of excess positrons are either a nearby pulsar or dark matter in our Milky Way galaxy and halo.


Photo: CDMS-II silicon detector, Credit: SuperCMDS Collaboration

CDMS-II (CDMS stands for Cryogenic Dark Matter Search) is a direct dark matter detection experiment based in the Soudan mine in Minnesota. The deep underground location shields the experiment from most of the cosmic rays impinging on the Earth’s surface. Previously they had reported a null result based on their germanium detector and ruled out a detection. Now they have more completely analyzed data from their silicon detector, which has higher sensitivity for lower possible dark matter masses, and they have detected 3 events which might be due to dark matter and report as follows.

“Monte Carlo simulations have shown that the probability that a statistical fluctuation of our known backgrounds could produce three or more events anywhere in our signal region is 5.4%. However, they would rarely produce a similar energy distribution. A likelihood analysis that includes the measured recoil energies of the three events gives a 0.19% probability for a model including only known background when tested against a model that also includes a WIMP contribution.”

So essentially they are reporting a possible detection with something ranging from 95% to 99.8% likelihood. This is a hint, but cannot be considered a firm detection as it rises to the level of perhaps 3 standard deviations (3 sigma) of statistical significance. Normally one looks to see a 5 sigma significance for a detection to be well confirmed. If the 3 events are real they suggest a relatively low dark matter particle mass of around 8 or 9 GeV/c² (the proton mass is a little under 1 GeV/c², and the Higgs boson around 126 GeV/c²).


Figure: Error ellipses for CDMS-II and CoGeNT, assuming a dark matter WIMP explanation. Blue ellipses are the 68% (dark blue) and 90% (cyan) confidence levels for the CDMS-II experiment. The purple ellipse is the 90% confidence level for CoGeNT.

The figure shows the plane of dark matter (WIMP, or weakly interacting massive particle) cross-section on the y-axis vs. the WIMP mass on the x-axis. Note this is a log-log plot, so the uncertainties are large. The dark blue region is the 1 sigma error ellipse for the CDMS experiment and the light blue region is the 90% confidence error ellipse. The best fit is marked by an asterisk located at mass of 8.6 GeV/c² and with a cross-section a bit under 2 x 10^-41 cm². But the mass could range from less than 6 to as much as 20 or more GeV/c². And the cross-section uncertainty is over two orders of magnitude.

However, this is quite interesting as the error ellipse for the mass and interaction cross-section from this CDMS-II putative result overlaps well with the (smaller) error ellipse of the CoGeNT results. The CoGeNT experiment is a germanium detector run by a different consortium, but based in the same Soudan Underground Laboratory as the CDMS-II experiment! COGENT sees a possible signal with around 2.8 sigma significance as an annual modulated WIMP wind, with the modulation in the signal due to the Earth’s motion around the Sun and thus relative to the galactic center. The purple colored region in the figure is the CoGeNT 90% confidence error ellipse, and it includes the CDMS-II best fit point and suggests also a mass of roughly 10 GeV/c².

The DAMA/LIBRA experiment in Italy has for years been claiming a highly significant 9 sigma detection of a WIMP (dark matter) wind, but with very large uncertainties in the particle mass and cross-section. However both the COGENT results and this CDMS-II possible result are quite consistent with the centroid of the DAMA/LIBRA error regions.

And both the CoGeNT and DAMA experiments are consistent with an annual modulation peak occurring sometime between late April and the end of May, as is expected based on the Earth’s orbit combined with the Sun’s movement relative to the galactic center.

What we can say at this point is the hottest region to hunt in is around 6 to 10 GeV/c² and with a cross section roughly 10^-41 cm². Physicists may be closing in on the target area for a confirmed weakly interacting dark matter particle detection. We await further results, but the pace of progress seems to be increasing.


https://darkmatterdarkenergy.com/2013/04/07/ams-positron-excess-due-to-dark-matter-or-not/ – Recent first results from AMS for positron excess

http://cdms.berkeley.edu/ – SuperCDMS Collaboration web site

http://arxiv.org/abs/1304.4279 – “Dark Matter Search Results Using the Silicon Detectors of CDMS II”

http://cdms.berkeley.edu/APS_CDMS_Si_2013_McCarthy.pdf – Kevin McCarthy’s presentation at the American Physical Society, April 15, 2013

http://cogent.pnnl.gov/ – CoGeNT website

https://darkmatterdarkenergy.com/2011/06/16/do-we-have-a-cogent-direct-detection-of-dark-matter/ – Discussion of CoGeNT 2011 results

http://arxiv.org/pdf/1301.6243v1.pdf  – DAMA/LIBRA results summary, 2013

Supersymmetry in Trouble?


There’s a major particle physics symposium going on this week in Kyoto, Japan – Hadron Collider Physics 2012. A paper from the LHCb collaboration, with 619 authors, was presented on the opening day, here is the title and abstract:

First evidence for the decay Bs -> mu+ mu-

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.

In other words, the LHCb consortium claim to have observed the quite rare decay channel from B-mesons to muons (each B-meson decaying to two muons), representing about 3 occurrences out of each 1 billion decays of the Bs type of the B-meson. Their detection has marginal statistical significance of 3.5 standard deviations (one would prefer 5 deviations), so needs further confirmation.

What’s a B-meson? It’s a particle that consists of a quark and an anti-quark. Quarks are the underlying constituents of protons and neutrons, but they are composed of 3 quarks each, whereas B-mesons have just two each. The particle is called B-meson because one of the quarks is a bottom quark (there are 6 types of quarks: up, down, top, bottom, charge, strange plus the corresponding anti-particles). A Bs-meson consists of a strange quark and an anti-bottom quark (the antiparticle of the bottom quark). Its mass is between 5 and 6 times that of a proton.

What’s a muon? It’s a heavy electron, basically, around 200 times heavier.

What’s important about this proposed result is that the decay ratio (branching fraction) that they have measured is fully consistent with the Standard Model of particle physics, without adding supersymmetry. Supersymmetry relates known particles with integer multiple spin to as-yet-undetected particles with half-integer spin (and known particles of half-integer spin to as-yet-undetected particles with integer spin). So each of the existing Standard Model particles has a “superpartner”.

Yet the very existence of what appears to be a Higgs Boson at around 125 GeV as announced at the LHC in July of this year is highly suggestive of the existence of supersymmetry of some type. Supersymmetry is one way to get the Higgs to have a “reasonable” mass such as what has been found. And there are many other outstanding issues with the Standard Model that supersymmetric theories could help to resolve.

Now this has implications for the interpretation of dark matter as well. One of the favored explanations for dark matter, if it is composed of some fundamental particle, is that it is one type of supersymmetric particle. Since dark matter persists throughout the history of the universe, nearly 14 billion years, it must be highly stable. Now the least massive particle in supersymmetry theories is stable, i.e. does not decay since there is no lighter supersymmetric particle into which it can decay. And this so called LSP for lightest supersymmetric particle is the favored candidate for dark matter.

So if there is no supersymmetry then there needs to be another explanation for dark matter.

Dark Matter Powered Stars

Gamma Ray Burster 070125

GRB (gamma ray burster) 070125. Credit: B. Cenko, et al. and the W. M. Keck Observatory.

So what is a “dark star”? It is not a Newtonian black hole as suggested by John Michell in the 18th century who used the term while postulating that gravity could prevent light escaping from a very massive, compact star. It is not a “dark energy star”, which is related to a black hole, but rather than having a singularity at the center, quantum effects cause infalling matter to be converted to vacuum state energy, dark energy. It is not a comic book, science fiction comedy film, or Grateful Dead song.

In this blog entry we are writing about dark matter powered stars. These would be the very first stars, formed within the first few hundred million years of the universe’s existence. The working assumption is that dark matter consists of WIMPs – weakly interacting massive particles, in particular the favored candidate is the neutralino. The neutralino is the lightest particle among the postulated supersymmetric companions to the Standard Model suite of particles. As such it would be stable, would not ordinarily decay and is being searched for with XENON, CDMS, DAMA, AMS-02 and many other experiments.

The first stars are thought by astrophysicists to have been formed from clouds of ordinary hydrogen and helium as well as dark matter, with dark matter accounting for 5/6 of the total mass. These clouds, called “dark matter halos” are considered to have contained from about one million to 100 million times the Sun’s mass. The ordinary matter would settle towards the center as it cooled via radiation processes and the dark matter (which does not radiate) would be more diffuse. The stars forming at the center would be overwhelmingly composed of ordinary matter (hydrogen and helium nuclei and electrons).

Without any dark matter at all, ordinary matter stars up to about 120 to 150 solar masses could form; above this limit they would have very hot surfaces and their own radiation would inhibit further growth by infall of matter from the halo. But if as little as one part in a thousand of the protostar’s mass was in the form of dark matter this limitation goes away. The reason is that the neutralino WIMPs will, from time to time, meet one another inside the star and mutually annihilate since the neutralino is its own anti-particle. The major fraction of the energy produced in the annihilation remains inside the star, but some escapes in the form of neutrinos (not neutralinos).

Annihilation of these neutralinos is a very efficient heating mechanism throughout the volume of the star, creating a great amount of heat and pressure support, basically puffing up the star to a very large size. The stellar surface is, as a result, much cooler than in the no dark matter case, radiation pressure is insignificant, and accretion of significantly more material onto the star can occur. Stars could grow to be 1000 solar masses, or 10,000 solar masses, potentially even up to one million solar masses. Their sizes would be enormous while they were in the dark matter powered phase. Even the relatively small 1000 solar mass star, if placed at the Sun’s location, would extend through much of our Solar System, beyond the orbit of Saturn.

We have mentioned the neutralino meets neutralino annihilation mechanism. A second mechanism for heating the interior of the star would be direct impact of neutralinos onto protons and helium nuclei. This second mechanism could help sustain the duration as a dark matter powered star potentially even beyond a billion years.

Eventually the dark matter fuel would be exhausted, and the heat and pressure support from this source lost. The star would then collapse until the core was hot enough for nuclear fusion burning. Stars of 1000 solar masses would burn hydrogen, and later helium, and evolve extremely rapidly because of the high density and temperature in their cores. After their hydrogen and helium fusion cycles completed there would be no source of sufficient pressure support and they would collapse to black holes (or maybe dark energy stars).

It is calculated with detailed simulations that the dark star mechanism allows for much more massive stars than could be formed otherwise, and this provides a potentially natural explanation for the creation of massive black holes. Our own Milky Way has a black hole around 3 million solar masses at its center, and it appears a majority of galaxies have large black holes. The image at the top of this blog is of a gamma ray burst detection that may have come from a large black hole formation event.



Freese, K. et al 2008, Dark Stars: the First Stars in the Universe may be powered by Dark Matter Heating, http://arxiv.org/pdf/0812.4844v1

Freese, K. et al 2010, Supermassive Dark Stars, http://arxiv.org/abs/1002.2233


Direct Search for Dark Matter: XENON100

The direct detection of putative dark matter particles, as opposed to measuring their collective gravitational effects, remains a significant challenge. A number of experiments are actively searching for WIMPs (= Weakly Interacting Massive Particles) as the currently favored candidates for dark matter. Particle physics models with supersymmetric extensions to the Standard Model suggest that the most abundant particle of dark matter would have a mass significantly greater than the proton. The mass is expected to lie somewhere in the range of under 10 times the mass of a proton to possibly as much as 10,000 times the mass of a proton (around 10 to 10,000 GeV/c^2 where GeV is a billion electron volts of energy and we divide by the square of the speed of light to convert to mass). The WIMP name reflects that these particles would only interact with other matter via the weak nuclear force and via gravity. They do not react via either the strong nuclear force or electromagnetism.

It is believed that WIMPs are produced in the Big Bang as a decay mode from the massive release of energy during the inflation phase. The currently most favored candidate WIMP is the proposed least massive supersymmetric particle (LSP), which is expected to be stable. Supersymmetric particles are considered to have large masses and would have the same quantum numbers (properties) as corresponding Standard Model particles, except for their spins, that would differ by 1/2 from their partners. The local density of dark matter is estimated to be about .3 GeV / cc (GeV per cubic centimeter). If the WIMP mass is 100 GeV/ c^2 there would be about 3 particles per liter.

Two major techniques are being employed to search for cosmic WIMPs. One of these is to detect the direct impact of WIMPs with atomic nuclei (via elastic scattering) in underground laboratories here on Earth. These would be very rare events, so large detectors are required and experiments must gather data for a long time. Such an impact leaves products from the interaction and it is these products that are actually detected in an experiment. A second technique is to look for gamma rays, which are produced in the galactic halo of the Milky Way or also the Sun’s interior, when dark matter (WIMP) collisions with ordinary matter occur at those locations. The gamma rays produced in this way can in principle be detected with satellites in Earth orbit.

Beyond these two general techniques to detect WIMPs there is the hope of actually creating these dark matter particles via high energy collisions at the Large Hadron Collider.

One recent set of results is from the XENON collaboration, which is funded by the US government and 6 European nations. The XENON100 experiment is located underground in Italy, in the Gran Sasso National Laboratory. The heart of the detector consists of cooled Xenon of quantity 65 kilograms. The target is in both the liquid and gas phases. When a WIMP strikes a Xenon atom directly, electrons are either knocked out of the Xenon atom or boosted to higher energy orbital levels in the atom. Both scintillation light, due to subsequent decay of the electron orbital, and ionization electrons, are thus generated. The 100 days of exposure of XENON100 analyzed to date have yielded 3 events, but one expects 2 events from background neutrons producing similar signatures, so there is statistically no detection. This result does allow the placement of upper limits on the WIMP cross-section for interaction as a function of mass.

The result appears to be in conflict with another experiment, also located at the same Gran Sasso laboratory, run by the DAMA team. The DAMA/Libra experiment claims a statistically significant detection of an annually modulated “WIMP wind” which reflects the variation in the Earth’s orbital direction with respect to the diffuse background of WIMP particles. The intensity is well above XENON100 limits for certain possible mass ranges of the WIMP major constituent particle.

The race is on to secure the direct detection of dark matter particles, beyond their extensive apparent gravitational effects. Rapid progress in enhancing the sensitivity of detection methods, typically including the use of larger detectors, will increase the probability of better WIMP detection and mass determination in the future.


M. Drees, G. Gerbier and the Particle Data Group, 2010. “Dark Matter” Journal of Physics G37(7A) pp 255-260

J. Feng, 2010. Ann. Rev. Astron. Astrophys. 48: 495, “Dark Matter Candidates from Particle Physics and Methods of Detection” (also available at: arxiv.org/pdf/1003.0904)

S. Perrenod, 2011. Dark Matter, Dark Energy, Dark Gravity, chapter 4, BookBrewer Publishing


XENON Dark Matter Project

Dark Matter

Dark matter is like the hidden part of an iceberg found below the water line. The hidden part is the dominant portion of the mass and supports the structure apparent from the visible portion above. Dark matter couples to ordinary matter through the gravitational force. The ordinary visible matter, which we detect through light from galaxies and stars, is analogous to the portion of an iceberg above the water line.

Why is dark matter important? It dominates the mass-energy density of the universe during the early part of its lifetime. Just after the epoch of the cosmic microwave background (CMB) the universe is composed of mostly dark matter (dark energy comes to dominate much later, during the most recent 5 billion years). But also there is the ordinary matter, which at that time is a highly uniform gas of hydrogen and helium atoms, with slightly overdense and slightly underdense regions.

The existence of a large amount of dark matter promotes much more efficient gravitational collapse of the overdense regions. This is a self-gravitational process in which regions that are slightly denser than the critical mass density (which is also the average mass density of the universe) at a given time will collapse away from the overall expansion that continues around them. Both the dark and ordinary matter in such a region collapse together, but it is the ordinary matter that forms the first stars since it interacts via various physical processes (think friction, radiation, etc.) to a much greater degree allowing for collapse. The dark matter, interacting only via gravity and perhaps the weak nuclear force, but not through electromagnetism, remains more spread out, more diffuse. The dark matter promotes the collapse process though, through increasing the self-gravity of a given region and this results in more efficient formation of stars and galaxies. There are many more stars and galaxies formed at early times than would be the case in the absence of substantial dark matter.

From cosmological observations including the CMB and high redshift (distant) supernovae, we find dark matter is about 1/4 of the mass-energy density of the universe. Dark matter is composed of either faint ordinary matter or, more likely, exotic matter that interacts through the weak and gravitational forces only. Dark matter is clearly detected by its gravitational effects on galaxy rotation curves, and is inferred from the kinematics of clusters of galaxies and the temperature measures of X-ray emission from very hot gas between galaxies in these same clusters. Dark matter is also detected through gravitational lensing effects in our galactic halo and in very large-scale cosmological structures. The abundance of deuterium, which is produced from Big Bang nucleosynthesis, in conjunction with the universe’s now well-known expansion rate, severely constrains the density of baryons (amount of ordinary nucleonic matter, i.e. protons and neutrons) in the universe and leads to the conclusion that over 80% of matter is nonbaryonic.

The MACHO (MAssive Compact Halo Objects) alternative, which refers to potential ordinary matter, is thus limited. The WIMP (Weakly Interacting Massive Particles) contribution is dominant. Hot WIMPS (e.g. neutrinos) are ruled out because they inhibit clumpiness and galaxy formation in the early universe. Cold nonbaryonic dark matter is the best candidate, with the primary candidate being an hypothesized, undiscovered particle. Neutralinos are thought by many particle physicists to be the best candidate for this, and have an expected mass of order 50 to 250 times the mass of the proton. It must be emphasized that no neutralino or similar particles (known as supersymmetric particles) have ever been detected directly. It is hoped that the Large Hadron Collider newly operational at CERN near Geneva may do this.

There may be direct detection of an annual modulation of the WIMP wind in a large scintillation array. There is also a possible indirect detection which manifests as an excess of 1 GeV gamma rays in our galactic halo. Significantly more sensitive detectors are needed to find these elusive particles and to provide a stronger foundation for supersymmetric physics, which postulates many new and heavy particles. An important experiment AMS-02 (Alpha Magnetic Spectrometer, 2nd generation) is scheduled to be carried to the International Space Station on the last Endeavour Shuttle mission scheduled for April 19, 2011. See http://www.ams02.org and http://cosmiclog.msnbc.msn.com/_news/2011/04/04/6403905-will-space-jam-delay-shuttle-launch

The next decade should allow us to shed new light on dark matter. Whatever it is made of, without the existence of substantial amounts of dark matter, there wouldn’t be nearly as many stars and galaxies in the universe, and we very likely wouldn’t be here.