Cryogenic Dark Matter Search Research Articles

APS Honors Physics Contributions

APS Honors Physics Contributions

Exclusion limits on the WIMP-nucleon cross section from the cryogenic dark matter search.

The Cryogenic Dark Matter Search (CDMS) employs Ge and Si detectors to search for weakly interacting massive particles (WIMPs) via their elastic-scattering interactions with nuclei while discriminating against interactions of background particles. CDMS data, accounting for the neutron background, give limits on the spin-independent WIMP-nucleon elastic-scattering cross section that exclude unexplored parameter space above 10 GeV/c2 WIMP mass and, at >75% C.L., the entire 3sigma allowed region for the WIMP signal reported by the DAMA experiment.

Cryogenic detectors based on superconducting transition-edge sensors for time-energy-resolved single-photon counters and for dark matter searches

We present the recent progress using transition-edge sensors (TES) for cryogenic particle detectors. First, by directly absorbing photons in tungsten TES devices, an instrument has been made which time stamps ( 0.1 μs ) and energy resolves (0.15 eV FWHM) each photon at rates up to 10 kHz. Observations of the Crab pulsar are the first broad spectrum infrared through full optical and time resolved on any astronomical object. Second, in the CDMS (cryogenic dark matter search) experiment looking for WIMPs, large crystals of silicon and germanium are instrumented with QET (quasiparticle-trap-assisted electrothermal-feedback transition-edge sensors) phonon sensors which provide the recoil energy and location in x, y and z for each event. Together with an ionization readout, these detectors provide powerful discrimination capabilities against known backgrounds and they are now probing new regions for WIMP dark matter.

Design of QET phonon sensors for the CDMS ZIP detectors

The Cryogenic Dark Matter Search (CDMS) ZIP detectors utilize quasiparticle trapping as the mechanism for coupling the energy of a particle interaction in the Ge (or Si) absorber into a tungsten (W) transition edge sensor (TES). Consequently, the dynamics of quasiparticle propagation and loss significantly impact the energy collection and resolution of the detector. This paper describes the considerations necessary in optimizing the detector surface geometry in order to have maximal quasiparticle collection.

Exclusion limits on the WIMP-nucleon scattering cross-section from the Cryogenic Dark Matter Search

The Cryogenic Dark Matter Search (CDMS) employs massive ionization- and phonon-mediated detectors to search for WIMPs via their elastic scattering interactions with nuclei while discriminating against interactions by other background particles. Limits on the WIMP-nucleon scattering cross-section, based on 3.1 kg d of exposure, exclude new parameter space in the 10–30 GeV WIMP mass region and also a portion of the region allowed by the DAMA annual modulation search (Bernabei, Phys. Lett. 450 (1999) 448).

The CDMS II Z-sensitive ionization and phonon germanium detector

The last year saw major progress in the development of the nuclear-recoil dark matter detector for the Cryogenic Dark Matter Search (CDMS II) experiment. The basic γ-ray background discrimination is based on ionization yield. However, as shown in separate experiments, surface events and especially electrons also results in low ionization yield due to incomplete charge collection. The two-fold strategy to reduce these unwanted backgrounds was to improve charge collection at the detector surface and to employ fast phonon sensors on Ge. A 250 g Ge detector with an Al Schottky contact on an amorphous Si blocking layer and a Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensor has been characterized. The phonon collection efficiency of the new detector is similar to that of the latest 100 g Si Fast Large Ionization and Phonon (FLIP) detector, which has the same phonon sensor design. The fall times of the phonon pulses are longer and consistent with simulations. The charge collection at the surface is high and very similar to that of smaller test devices and the last revision of Berkeley Large Ionization and Phonon (BLIP) detectors. The four fast non-thermal phonon sensors yield x, y position resolution similar to the Si FLIP detector. A rise time surface effect was demonstrated on both sides of the detector. The effect results in a much more effective rejection of surface events than the rejection based on charge yield alone and therefore improves the sensitivity for a dark matter search.

Preliminary limits on the WIMP-nucleon cross section from the cryogenic dark matter search (CDMS)

We are conducting an experiment to search for WIMPs, or weakly-interacting massive particles, in the galactic halo using terrestrial detectors. This generic class of hypothetical particles, whose properties are similar to those predicted by extensions of the standard model of particle physics, could comprise the cold component of nonbaryonic dark matter. We described our experiment, which is based on cooled germanium and silicon detectors in a shielded low-background cryostat. The detectors achieve a high degree of background rejection through the simultaneous measurement of the energy in phonons and ionization. Using exposures on the order of one kilogram-day from initial runs of our experiment, we have achieved (preliminary) upper limits on the WIMP-nucleon cross section that are comparable to much longer runs of other experiments.

Results and status of the Cryogenic Dark Matter Search (CDMS)

The Cryogenic Dark Matter Search experiment uses cooled germanium and silicon detectors for a direct search for weakly interacting massive particles in our Galaxy. The novel detectors allow a high degree of background rejection by discriminating between electron and nuclear recoils through the simultaneous measurement of the energy deposited in phonons and ionization. Exposures on the order of one kilogram-day from initial runs of our experiment yield (preliminary) upper limits on the WIMP-nucleon cross section that are comparable to much longer runs of other experiments. Current and future runs promise significant improvement, primarily due to improved detectors and reduced surface-electron backgrounds.

Direct searches for dark matter: Recent results

There is abundant evidence for large amounts of unseen matter in the universe. This dark matter, by its very nature, couples feebly to ordinary matter and is correspondingly difficult to detect. Nonetheless, several experiments are now underway with the sensitivity required to detect directly galactic halo dark matter through their interactions with matter and radiation. These experiments divide into two broad classes: searches for weakly interacting massive particles (WIMPs) and searches for axions. There exists a very strong theoretical bias for supposing that supersymmetry (SUSY) is a correct description of nature. WIMPs are predicted by this SUSY theory and have the required properties to be dark matter. These WIMPs are detected from the byproducts of their occasional recoil against nucleons. There are efforts around the world to detect these rare recoils. The WIMP part of this overview focuses on the cryogenic dark matter search (CDMS) underway in California. Axions, another favored dark matter candidate, are predicted to arise from a minimal extension of the standard model that explains the absence of the expected large CP violating effects in strong interactions. Axions can, in the presence of a large magnetic field, turn into microwave photons. It is the slight excess of photons above noise that signals the axion. Axion searches are underway in California and Japan. The axion part of this overview focuses on the California effort. Brevity does not allow me to discuss other WIMP and axion searches, likewise for accelerator and satellite based searches; I apologize for their omission.

SQUID based W-Al quasiparticle-trap assisted superconducting transition edge sensor with position resolution

We have demonstrated a new type of phonon sensor for cryogenic particle detectors with high-bandwidth SQUID readout. Our Quasiparticle-trap assisted Electrothermal feedback Transition edge sensor (QET) utilizes aluminum quasiparticle traps attached to a tungsten superconducting transition edge sensor patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. Phonons from particle interactions in the silicon deposit energy into the creating quasiparticles. The quasiparticles are trapped into the tungsten and cause its electrical resistance to increase. The resulting decrease in current through the sensor is measured with a DC SQUID array. We have been able to demonstrate xy-position resolution of /spl sim/0.3 mm for 6 keV X-rays from an /sup 55/Fe source in a 2 cm/spl times/2 cm/spl times/4 mm (4 g) detector. We describe our optimizations of the sensor design for improved energy and position. By simultaneously measuring the ionization yield we have demonstrated discrimination between electron and nuclear recoil events in a 100 g Si detector for the CDMS (Cryogenic Dark Matter Search) experiment.

Searching for WIMPs

While the article “To catch a WIMP” by Andrew Watson (Research News, 21 Mar., p. 1736) provides a useful overview of experimental ideas for searching for weakly interacting massive particles (WIMPs), it could give a misleading impression of the relative merits of different techniques, as only a small minority of approximately 20 experiments described are likely to be capable of making a positive identification of the most favored particle candidate. Currently, the only theory that predicts a new weakly interacting massive particle and provides estimates of its range of mass and interaction rates is supersymmetry. The experimental challenge is to differentiate—to a precision of 0.1% to 1%—between low energy nuclear-recoil events from dark matter interactions and background electron-recoil events from gamma and beta decay. Only two of the techniques mentioned in the article appear to have such capability. One is the Ge detector of the Cryogenic Dark Matter Search (CDMS) collaboration, which simultaneously measures both thermal energy and ionization. The other is the planned liquid-xenon detector (the U.K. Dark Matter Collaboration—a University of California, Los Angeles-U.K. collaboration), which will simultaneously measure both scintillation and ionization. It is likely that larger detectors with even more powerful discrimination will be needed to prove conclusively the existence of WIMP dark matter. While this is a daunting task, the discovery of supersymmetric particles in the galactic dark matter could bring particle physics back to the realm of “small” experiments—analogous to the birth of particle physics in the early cosmic-ray studies.

Progress of the cryogenic dark matter search (CDMS) experiment

We report on progress of the CDMS experiment, which seeks to detect WIMP dark matter through its interactions in a particle detector operated in a low radioactivity environment. We have developed novel cryogenic detectors which discriminate between nuclear recoils from WIMPs and electron recoils from background photons. We describe the experiment and discuss recent progress, including first operation of a cryogenic detector in the low radioactivity cryostat in June of 1996.

Installation of the cryogenic dark matter search (CDMS)

We discuss the status of a cryogenic dark matter search beginning operation in the Stanford Underground Facility. The detectors will be cooled in a specially designed cryostat connected to a modified side access Oxford 400 dilution refrigerator. We discuss two detector designs and performance, the cryostat construction and operation, and the multi-level shield surrounding the cryostat. Finally, we will examine the limits which we will be able to set on WIMP dark matter with this experiment.

Design of kilogram mass scale TES for the Cryogenic Dark Matter Search

Recently there has been much interest in the direct detection of the dark matter candidates known as WIMPs. We are developing very sensitive detectors based on phonon detection with transition edge sensors on silicon substrates. These detectors will be deploy ed as part of the Cryogenic Dark Matter Search in collaboration with the Center for Particle Astrophysics. As we extend this technology to practical WIMP searches we will need much higher mass scale detectors. We have demonstrated detectors on 500 µm substrates. To reach the kilogram mass scales we need to pattern wafers that are an order of magnitude thicker with a detector that is at least two orders of magnitude more sensitive. Progress is reported on both these areas and a detector design is discussed.