Być może odnaleziono pierwszy ślad ciemnej materii

Być może odnaleziono pierwszy ślad ciemnej materiiOriginal Press Release
Obserwacje wykonane przez teleskopy, satelity oraz pomiary promieniowania reliktowego doprowadziły astronomów do zaskakującego wniosku, że większość materii we Wszechświecie nie pochłania ani nie emituje promieniowania tak jak czyni to zwykła materia.

Niektóre teorie fizyki cząstek elementarnych sugerują iż ciemna materia może być zbudowana z hipotetycznych słabo oddziałujących masywnych cząstek – WIMPów (Weakly Interacting Massive Particles). Naukowcy sądzą, że cząstki te mogą mieć masę podobną, lub być nawet cięższe od jąder atomowych. Choć takie WIMPy z rzadka oddziałują z normalną materią od czasu do czasu zderzałyby się z jądrami normalnej materii na podobieństwo kul bilardowych przekazując im nieznaczne ilości energii, która w odpowiednich warunkach byłaby możliwa do wykrycia.

Eksperyment poszukiwania ciemnej materii w warunkach kriogenicznych – CDMS (Cryogenic Dark Matter Search) – wykorzystuje 30 detektorów zbudowanych z krzemu i germanu umieszczonych w kopalni Soudan ponad pół kilometra pod powierzchnią stanu Minnesota. Detektory schłodzone do temperatury bliskiej zera absolutnego mają za zadanie wykrycie właśnie takich zderzeń. Oddziaływania cząstek pozostawiają w krystalicznych detektorach energię w postaci ciepła i ładunku, który można przemieszczać za pomocą przyłożonego pola elektromagnetycznego. Specjalne czujniki mają wykryć te sygnały, wzmocnić je i zapisać do analizy. Porównanie wartości i momentu rejestracji obu sygnałów pozwala badaczom określić czy sygnał został wygenerowany przez interakcję ze znanymi cząstkami pochodzącymi z rozpadu radioaktywnego lub promieni kosmicznych, czy przez WIMP-a. Sygnały tła należy wytłumić, tak by możliwe było dostrzeżenie WIMPów. Warstwy osłaniające w połączeniu z ponad półkilometrową warstwą skał stanowią filtr tłumiący.

Eksperyment CDMS poszukuje śladów ciemnej materii od 2003 roku, jednak jak dotąd nie wykryto śladów WIMPów co dostarcza pewności, że promieniowanie tła zostało skutecznie wytłumione do poziomu, w którym możliwa jest rejestracja jednej interakcji z WIMP-em na rok.

Naukowcy zaprezentowali wyniki z badań w latach 2007-2008, w których odnaleziono dwa zdarzenia o charakterystyce spodziewanej dla interakcji z WIMPem. Dane są jeszcze zbyt ubogie aby dało się jednoznacznie i z całą pewnością wykluczyć inne pochodzenie sygnału, natomiast pozwalają wprowadzić wartości graniczne dla teorii próbujących opisać ciemną materię.

Źródła:

Latest Results in the Search for Dark Matter

Astronomical observations from telescopes, satellites and measurements of the cosmicmicrowave background have led scientists to believe that most of the matter in the universeneither emits nor absorbs light. This dark matter would have provided the gravitationalscaffolding that caused normal matter to coalesce into the galaxies we see today. In particular,we think our own galaxy is embedded within an enormous cloud of dark matter. As our solarsystem rotates around the galaxy, it moves through this cloud.

Particle physics theories suggest that dark matter may be composed of Weakly InteractingMassive Particles (WIMPs). Scientists expect these particles to have masses comparable to, orperhaps heavier than, atomic nuclei. Although such WIMPs would rarely interact with normalmatter, they may occasionally scatter from an atomic nucleus like billiard balls, leaving a smallamount of energy that might be detectable under the right conditions.

The Cryogenic Dark Matter Search (CDMS) experiment, located a half-mile underground at theSoudan mine in northern Minnesota, uses 30 detectors made of germanium and silicon in anattempt to detect such WIMP scatters. The detectors are cooled to temperatures very nearabsolute zero. Particle interactions in the crystalline detectors deposit energy in the form of heat,and in the form of charges that move in an applied electric field. Special sensors detect thesesignals, which are then amplified and recorded in computers for later study. A comparison of thesize and relative timing of these two signals can allow the experimenters to distinguish whetherthe particle that interacted in the crystal was a WIMP or one of the numerous known particlesthat come from radioactive decays, or from space in the form of cosmic rays. These backgroundparticles must be highly suppressed if we are to see a WIMP signal. Layers of shieldingmaterials, as well as the half-mile of rock above the experiment, are used to provide suchsuppression.

The CDMS experiment has been searching for dark matter at Soudan since 2003. Previous datahave not yielded evidence for WIMPs, but have provided assurance that the backgrounds havebeen suppressed to the level where as few as 1 WIMP interaction per year could have beendetected.

We are now reporting on a new data set taken in 2007- 2008, which approximately doubles thesum of all past data sets. With each new data set, we must carefully evaluate the performance ofeach of the detectors, excluding periods when they were not operating properly. Detectoroperation is assessed by frequent exposure to sources of two types of radiation: gamma rays andneutrons. Gamma rays are the principal source of normal matter background in the experiment.Neutrons are the only type of normal matter particles that will interact with germanium nuclei inthe billiard ball style that WIMPs would, although neutrons frequently scatter in more than oneof our detectors. This calibration data is carefully studied to see how well a WIMP-like signal(produced by neutrons) can be seen over a background (produced by gamma rays). Theexpectation is that no more than 1 background event would be expected to be visible in theregion of the data where WIMPs should appear. Since background and signal regions overlapsomewhat, achievement of this background level required us to throw out roughly 2/3 of the datathat might contain WIMPs, because these data would contain too many background events.All of the data analysis is done without looking at the data region that might contain WIMPevents. This standard scientific technique, sometimes referred to as ‘blinding’, is used to avoidthe unintentional bias that might lead one to keep events having some of the characteristics ofWIMP interactions but that are really from background sources. After all of the data selectioncriteria have been completed, and detailed estimates of background ‘leakage’ into the WIMPsignal region are made, we ‘open the box’ and see if there are any WIMP events present.In this new data set there are indeed 2 events seen with characteristics consistent with thoseexpected from WIMPs. However, there is also a chance that both events could be due tobackground particles. Scientists have a strict set of criteria for determining whether a newdiscovery has been made, in essence that the ratio of signal to background events must be largeenough that there is no reasonable doubt. Typically there must be less than one chance in athousand of the signal being due to background. In this case, a signal of about 5 events wouldhave met those criteria. We estimate that there is about a one in four chance to have seen twobackgrounds events, so we can make no claim to have discovered WIMPs. Instead we say thatthe rate of WIMP interactions with nuclei must be less than a particular value that depends on themass of the WIMP. The numerical values obtained for these interaction rates from this data setare more stringent than those obtained from previous data for most WIMP masses predicted bytheories. Such upper limits are still quite valuable in eliminating a number of theories that mightexplain dark matter.

What comes next? While the same set of detectors could be operated at Soudan for many moreyears to see if more WIMP events appear, this would not take advantage of new detectordevelopments and would try the patience of even the most stalwart experimenters (not tomention theorists). A better way to increase our sensitivity to WIMPs is to increase the number(or mass) of detectors that might see them, while still maintaining our ability to keepbackgrounds under control. This is precisely what CDMS experimenters (and many othercollaborations worldwide) are now in the process of doing. By summer of 2010, we hope to haveabout three times more Germanium nuclei sitting near absolute zero at Soudan, patiently waitingfor WIMPs to come along and provide the perfect billiard ball shots that will offer compellingevidence for the direct detection of dark matter in the laboratory.

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