Fermilab Today, Frontier Science Result: Searching for particles with fractional charges

Some extensions to the Standard Model of particle physics predict the existence of fractionally charged particles. Such particles that come to us from the cosmos interact with the electrons in our Earth-bound particle detectors. The probability of that interaction depends on the amount of charge it has — the more it has, the more likely it is to interact with the detector's electrons.

While the Cryogenic Dark Matter Search (CDMS-II) experiment was designed to focus on dark matter (as its name states), the experiment's ability to detect smaller energy depositions gives it sensitivity to other types of new physics as well. CDMS is the first experiment to probe for particles arriving from outer space with fractional charges less than 1/6 of that of an electron.

Read full feature at Fermilab Today.


The SuperCDMS SNOLAB Experiment

Released on July 11, 2014:
The DOE Office of High Energy Physics and the NSF Physics Division have jointly selected a portfolio of projects for the “second generation” of direct detection dark matter experiments. experiment is among the selected experiments which included searched targeted at low and high mass WIMPs as well as axions.

The DOE/NSF announcement

APS NEWS: Physicists Widen the Search for Dark Matter Particles.

Released on June, 2014:
Researchers at Cold Dark Matter Search (CDMS) search for dark matter candidates. After years of null results in the search for dark matter, physicists have started pursuing more exotic and expansive theories about what makes up most of the mass of the universe. For many physicists, the idea of a solitary dark matter particle seems outmoded, and they have ramped up their intellectual and experimental searches accordingly.

"It may be a little naïve to believe that the dark sector is only one particle," said Blas Cabrera of Stanford University, one of the researchers in the large Cold Dark Matter Search (CDMS) collaboration. "We're covering all the bases rather than focusing on the one that seemed attractive a decade ago."

Read full feature at APS NEWS June 2014, Vol. 24, No. 6.

Symmetry Magazine: CDMS result covers new ground in search for dark matter.

Released on February 28, 2014:
Scientists looking for dark matter face a serious challenge: No one knows what dark matter particles look like. So their search covers a wide range of possible traits-different masses, different probabilities of interacting with regular matter.

Today, scientists on the Cryogenic Dark Matter Search experiment, or CDMS, announced they have shifted the border of this search down to a dark-matter particle mass and rate of interaction that has never been probed.

"We're pushing CDMS to as low mass as we can," says Fermilab physicist Dan Bauer, the project manager for CDMS. "We're proving the particle detector technology here." Read full feature at Symmetry Magazine.
L. Hsu's presentation at Dark Matter 2014.


First Low-Mass Dark Matter Search Results using the CDMSlite Technique

Released on December 20, 2013:
A novel approach to search for low-mass dark matter has been successfully explored by the SuperCDMS collaboration. In the CDMS low ionization threshold experiment, high voltage was applied to ultra-cold germanium bolometers to produce a signal amplification of x 24. With this, CDMSlite achieved one of the lowest ionization thresholds in current generation of dark matter experiments. This low 170 electron-volt threshold has allowed the SuperCDMS collaboration to probe and constrain the parameter space of low mass WIMPs.

CDMSlite: A Search for Low-Mass WIMPs using Voltage-Assisted Calorimetric Ionization Detection in the SuperCDMS Experiment.
arXiv ins-det, (2013), arXiv:1309.3259

Use of Ancient Lead in Cryogenic Dark Matter Search (CDMS) Experiment

Released on December 16, 2013:
Searches for rare events such as the interaction of dark matter particles with normal matter require that natural radioactivity be suppressed well below common environmental levels. Lead is useful for doing so, but it has its own radioactivity due to Pb-210. Thus, rare-event search experiments try to use ultra-low-activity lead. Ancient lead is one such source.

In the search for dark matter in the form of weakly interacting massive particles or WIMPs, CDMS-I used ultra-low-activity lead in the shallow-site installation Stanford Underground Laboratory (SUF), and CDMS-II used additional ultra-low-activity lead at the deeper site in the Soudan mine in northern Minnesota. This shielding continues to be in use for the SuperCDMS Soudan upgrade.

All of the ultra-low-activity lead was purchased from the French company Lemer Pax and similar ultra-low-activity lead is still available from them today. As seen from this reference, standard commercial lead has a Pb-210 background of ~100 Bq/kg and the ultra-low-activity lead is more than a factor of 1000 lower activity which is very helpful for our rare-event experiments.

We support the need to protect ancient artifacts for archeological investigations, and we had assumed that the supply company had obtained proper authorization and had followed French law. Years ago, we did receive a phone call from French Customs officials in charge of an investigation about the ancient lead. Our purchases were not questioned, and we have not received any other inquires until the recent inquiries from the press. We assume that the ultra-low activity lead available today has all been properly authorized, and it is important that this be verified.

Dark Matter Search Results from CDMS-II Silicon Detectors

Released on April 15, 2013:
On Saturday April 13, Kevin McCarthy, from the MIT group on behalf of the SuperCDMS Collaboration, has presented the blind analysis results of the largest exposure with silicon detectors during CDMS-II operation. The collaboration has previously published the results of the entire germanium detector exposure [Science 327, 1619 (2010)] which resulted in 2 events in the signal region and an estimated background of 0.9 events. Afterward, our likelihood analysis concluded that these were more likely leakage surface electrons rather than nuclear recoils.

Increased interest in the low mass WIMP region motivated us to complete the analysis of the silicon detector exposure which is less sensitive than germanium for WIMP masses above 15 GeV/c², but more sensitive for lower masses. The analysis resulted in 3 events and the estimated background is 0.7 events.

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. This ~3-sigma confidence level does not rise to the status of a discovery, but does call for further investigation.

If the result is interpreted as spin-independent scattering of WIMPs, a mass around 8.6 GeV/c² and a WIMP-nucleon cross section of 1.9E-41 cm² are favored. For the simplest theories of WIMP interactions and using the standard dark matter halo model, the allowed region is in tension with exclusion limits from the XENON collaboration. A paper has been submitted to the arXiv and to PRL.

We will probe this WIMP sector with our operating germanium detectors in the SuperCDMS Soudan experiment, and we are considering using silicon detectors in future experiments.

CDMS-II was funded by DOE and NSF in the US and by NSERC in Canada.

Dark Matter Search Results Using the Silicon Detectors of CDMS II. arXiv astro-ph.CO, (2013), arXiv:1304.4279 Silicon Detector Results from the First Five-Tower Run of CDMS II. arXiv astro-ph.CO, (2013), arXiv:1304.3706 E. Figueroa-Feliciano's presentation at Light Dark Matter 2013. K. McCarthy's presentation at APS. B. Cabrera's Panofsky Prize presentation at APS. B. Sadoulet's Panofsky Prize presentation at APS. Fermilab Today and SLAC Today features. Symmetry Magazine feature. Upper Limits and Contour Curves from CDMS II Si http://lanl.arxiv.org/abs/1304.4279


Latest results of CDMS-II, December 17, 2009

Summary of the results (pdf) Science article 2/12/2010
Astrophysical observations indicate that dark matter constitutes most of the mass in our universe, but its nature remains unknown. Over the past decade, the Cryogenic Dark Matter Search (CDMS II) experiment has provided world-leading sensitivity for the direct detection of weakly interacting massive particle (WIMP) dark matter. The final exposure of our low-temperature germanium particle detectors at the Soudan Underground Laboratory yielded two candidate events, with an expected background of 0.9 ± 0.2 events. This is not statistically significant evidence for a WIMP signal. The combined CDMS II data place the strongest constraints on the WIMP-nucleon spin-independent scattering cross section for a wide range of WIMP masses and exclude new parameter space in inelastic dark matter models. (Abstract) Click for: Printed report (pdf) and Supplementary Online Material (pdf)

Announcement talks December 12, 2009

Scaling up the search for dark matter

Karl van Bibber; Published January 5, 2009 (APS Physics)
The dynamical evidence for dark matter in the universe— of anomalously large velocities of stars within galaxies, and galaxies within clusters—goes back three-quarters of a century [1]. Within the past decade, precision measurements of cosmological parameters have pinned down the partitioning of the energy density of the universe rather neatly: dark energy accounts for roughly three-fourths, and matter of all forms makes up only a quarter. Of the latter, nonbaryonic dark matter accounts for about five parts in six [2]. What this elusive invisible matter actually is represents one of the pre-eminent questions in all of science. In Physical Review Letters, Ahmed et al. [3] report results from one of the collaborations involved in the hunt for dark matter that place new upper bounds on the extent of any possible interaction between certain kinds of dark-matter particles and normal matter.... Read more at APS Physics

This work is supported by the National Science Foundation and the Department of Energy