The CDMS II Experiment（冷暗物质探测实验）
本帖最后由 positron 于 2010-1-17 21:26 编辑CDMS 官网上的文章，感兴趣的可以译一下。
http://cdms.berkeley.edu/experiment.html
CDMS II Overview Using state-of-the-art cryogenic germanium and silicon detectors, the CDMSII collaboration is searching for weakly-interacting massive particles, or WIMPS, whose discovery could resolve the dark matter problem, revolutionizing particle physics and cosmology. This WIMP direct detection experiment is similar to CDMSI, which was located in a tunnel under the Stanford campus. But CDMSII is located deep underground in the Soudan mine in Minnesota, USA. This location provides vastly improved shielding from cosmogenic events which will reduce interference of known backgrounds particles. This will increase the chances of positively identifying a WIMP signal or will allow better limits to be placed on the interaction cross section of the hypothesized particles. The experiment is now running in the Soudan facility.
WIMP Dark Matter
Expectations for non-baryonic dark matter are founded principally in Big Bang nucleosynthesis calculations, which indicate that the missing mass of the universe is not likely to be baryonic. The supersymmetric standard model (SUSY) offers a promising framework for expectations of particle species which could satisfy the observed properties of dark matter. The most likely SUSY candidate for a dark matter particle is the lightest supersymmetric particle (LSP), the neutralino X01, which is a superposition of the fermionic superpartners of the Higgs and neutral gauge bosons. In order to be consistent with an early-universe annihilation rate leaving proper relic abundances, such a particle should have a small but measureable interaction cross section with ordinary matter. Specifically, a cross section for interaction between a neutralino and a nucleon in ordinary matter of the order of the electro-weak scale would be consistent with a meaningful cosmological role for the particle. This expectation of a weak interaction together with the expected mass range of the neutralino, 10 to 1000 GeV, produce the acronym "WIMP": Weakly Interacting Massive Particle. By virtue of their weak-scale interaction, WIMPs should be able to be observed by directly detecting their interactions with ordinary matter.
Direct Detection of WIMPs
According to models of cosmological structure formation, the luminous matter of galaxies is gravitationally bound to a more massive, sprawling halo of dark matter. Should the dark matter of the universe consist of unidentified particles, our solar system and our planet would be passing through a flux of these dark matter particles which constitute the dark halo of the Milky Way galaxy. WIMP dark matter could then be detected directly as the Earth (and some detection apparatus beneath its surface) pass through our galaxy's DM halo. Given the expected weak interaction scale of the neutralino-nucleon scattering, galactic WIMPs should deposit a measurable amount of energy in an appropriately sensitive detector apparatus. This can occur through elastic scattering between an incident WIMP and a nucleus in the fiducial volume of some monitored detector material. The CDMS experiments aim to measure the recoil energy imparted to detector nuclei through neutralino-nucleon collisions by employing sensitive phonon detection equipment coupled to arrays of cryogenic germanium and silicon crystals. The phonon signals generated within the crystal detectors can be processed and interpreted with information about known background rates. If found, a confidently identified above-background event rate would be analyzed to determine the nature of the responsible interaction -- perhaps enabling an identification of WIMPs. Conversely, a null WIMP-nucleon scattering find can be used to improve greatly current limits on the possible neutralino interaction cross section.
CDMSII Detectors
CDMSII detectors are designed with the primary functionality of detecting the minute phonon signals generated within a detector crystal by elastic collisions between detector nuclei and WIMPs. The energy deposited in a detector by an incident WIMP through the weak interaction may be as low as a few tens of keV. Event detection at such energy levels requires a sensitive experimental apparatus. The foremost requirement is that the detector be maintained at a very low temperature to distinguish the deposited energy from the thermal energy of the detector's nuclei. The CDMSII project and associated test facilities employ helium3-helium4 dilution refrigerator techniques which, with the appropriate cryostat apparatus, are able to achieve detector base temperatures as low as 10mK.
The detectors themselves, known as ZIP detectors, feature state-of-the-art thin film superconducting technology. Each 250g germanium or 100g silicon crystal provides two sets of information about interactions with incident particles. On one surface of the detectors are charge-collection plates which record the amount of electrical charge displaced within the detector's body by the incident particle. On the opposite detector surface is an array of tiny superconducting transition edge sensors (TES) consisting of micro strips of tungsten coupled to aluminum "fins" which collect phonon energy from the crystal.
This array works as follows. An incident particle, perhaps a WIMP, collides with a nucleus in the detector generating vibrations in its crystal lattice. These vibrations are called "phonons". These phonons propagate through the crystal and some reach the surface. There, they are absorbed by the aluminum collector fins. In the aluminum, the phonons convert their energy into "quasi-particles", which are basically just electrons which had been in a superconducting "Cooper pair". The incident phonon energy breaks these Cooper pairs and gives energy to the electrons. These "quasi-particle" electrons migrate (or "diffuse") to the tiny strip of tungsten which is attached to each aluminum fin. This is the important step. The tungsten strips are "biased" with some electrical energy already which pushes them right near the brink of going through a transition from being a superconductor to being "normal". When the tungsten strips receive the energy from the "quasi-particles" which were made in the aluminum by the phonons, they go through the transition. This means their electrical resistance changes dramatically with the addition of a very small amount of energy (funneled to it from the aluminum). The tungsten strips are thus called "transition edge sensors" since we exploit their transition from superconducting to normal as a way to sense a small input of energy. This change in electrical resistance caused by the transition is amplified first by a SQUID circuit down within the cryostat itself and then by a sophisticated series of amplifiers at room temperature. This amplified change in resistance makes the "pulse" which we observe.
The tungsten TES array (and accompanying aluminum fins) on phonon-detecting side of the detector is divided into 4 "channels" (like 4 slices of pie), each containing over 1000 tungsten sensors.
Physics Goals
The CDMSII experiment program seeks to combine high fiducial detector mass and extensive run time with reduced backgrounds to advance WIMP dark matter knowledge significantly. CDMSI results are apparently in conflict with the results by the DAMA collaboration in which an annual modulation in signal was reported. CDMSII is expected to probe WIMP-nucleon cross section regimes at least an order of magnitude lower than those of current exclusion limits within less than a year of commissioning.
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