Physicists discovered that all the known matter in the universe accounts for only about 5% of the total mass-energy of the universe. About 25% of the unknown matter, called dark matter, is missing. The remaining 70% is called dark energy. One of the most popular candidates for the dark matter is the Weakly Interacting Massive Particle (WIMP). WIMPs are theoretical particles that interact via the weak-nuclear and the gravitational forces. Its mass is in the order of 100 GeV/c2.
Many detection technologies have been developed to search for WIMPs. The Large Underground Xenon experiment (LUX) uses a dual-phase xenon detector to search for WIMPs. The dual-phase xenon detector uses xenon in liquid and gaseous phases to detect interactions between WIMPs and xenon atoms. An event from a WIMP-xenon interaction consists of two light signals, called S1 and S2 signals. Successful data taking and subsequent analyses rely on an ability to correctly identify and record the S1 and S2 signals.
LUX uses a trigger system developed at the University of Rochester for event selection. It is crucial to understand the performance of the trigger system and its impact on the data collected, so that the WIMP search results can be correctly interpreted. Many studies were carried to ensure that the performance and the functionality of the trigger system are understood. In the WIMP analysis, not only the WIMP-xenon interaction events are important, but also the background events. For WIMPs, which do not interact via the electromagnetic force, the interactions occur with the nucleus of the xenon atoms. This process is called nuclear recoil (NR). For background events, which are dominated by γ-rays and electrons from beta-decays, the interactions occur mostly with the orbital electrons of the xenon atoms. This type of interaction is called electron recoil (ER).
The trigger efficiency, which is defined as the probability that an event of interest is selected for offline analysis, is studied using raw data obtained from both ER and NR calibrations. The measured efficiency exceeds 98% at a pulse area of 90 detected photons, which is well below the WIMP analysis threshold on the S2 pulse area of 165 phd. The efficiency also exceeds 98% at recoil energies of 1.3 keV and above, for both ER and NR. The measured trigger efficiency varies between 99% and 100% over the fiducial volume of the detector. These efficiencies are sufficiently high for WIMP search analyses.
The LUX experiment has been completed. The results from LUX yield no discovery of WIMPs and LUX has put a strong limit on the possible properties of WIMPs. The LUX-ZEPLIN experiment (LZ), which is a successor of LUX, is currently under construction. LZ will continue searching for WIMP and probe the WIMP property regions where LUX could not reach before.
|Advisor:||Wolfs, Frank L.H.|
|Commitee:||Das, Ashok, Manly, Steven, Schroeder, Wolf-Udo|
|School:||University of Rochester|
|Department:||Arts and Sciences|
|School Location:||United States -- New York|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Subjects:||Astrophysics, Physics, Particle physics|
|Keywords:||Dark matter detectors, Liquid xenon, Trigger efficiency, WIMPs|
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