Nome
Participation in the LUX-ZEPLIN experiment

Código
PTDC/FIS-PAR/2831/2020

Entidade Beneficiária

LIP - Laboratório de Instrumentação e Física Experimental de Partículas

Sumário do Projecto

The main goal of this project is to make key contributions to the direct detection of dark matter in the framework of LUX-ZEPLIN (LZ) experiment. We were one of its proponent members in 2012 and have ever since played a key role in LZ through successive projects funded by FCT.  The nature of dark matter is one of the foremost open questions in physics today. Evidence for dark matter comes from a rich set of observations over a wide range of scales that allows to estimate that dark matter accounts for about 27% of the mass-energy budget of the Universe [1]. Yet, all the observational evidence is essentially of gravitational nature, and thus does not inform on the nature of dark matter. Weakly Interacting Massive Particles (WIMPs) constitute an important class of dark matter candidates [1].  The direct detection of galactic WIMPs relies on searching for nuclear recoil signals produced by WIMP-nucleon elastic scattering in the active volume of detectors deployed deep underground [2]. This is very challenging as these events are extremely rare (the upper limit of the WIMP-nucleon cross-section is currently set at 4.1×10-47 cm2 at 30 GeV mass WIMP [3]) and their energy spectrum is exponential, requiring a detection threshold lower than about 10 keV. Furthermore, these events have to be disentangled from the much more abundant background events due to radioactive contaminants in detector materials and the active detector material, as well as coming from the walls of the cavern where the detector is deployed [2].  Different detector designs and techniques have been explored for the WIMP search. The two-phase Time Projection Chamber (TPC) based on xenon or argon is the detector technology that proved to have higher sensitivity to WIMPs with masses from a few GeV to 10 TeV [3,5,9]. LUX, in which we have also been deeply involved, was the most sensitive direct detection experiment from 2013 until 2018 (it was decommissioned in 2016), having set the upper limit of the elastic scattering WIMP-nucleon cross section at 1.1e-46 cm2 for WIMP mass of 50 GeV [5].  Resulting from the merger of the LUX and ZEPLIN-III collaborations, LZ will be the most sensitive dark matter experiment for WIMPs with masses from 10 GeV to 10 TeV, with a predicted sensitivity of 1.4×10^-48 cm2 for WIMP mass of 40 GeV in a 1000 live-days run and 5.6 ton fiducial mass [7], surpassing XENONnT [8] and PANDA-X 4T [9].  Due to its extremely low background, LZ can also be used for other studies as, for instance, the search of other dark matter candidates and Xe rare and forbidden decay modes, as well as neutrino physics studies [10].  Therefore, the second major goal of this project is the search for rare and Standard Model forbidden Xe decays. In particular, neutrinoless decay modes require physics beyond the standard model. They would prove the Majorana nature of the neutrinos and inform on their mass hierarchy. Moreover, the resulting violation of the lepton number conservation law would show that leptons play a part in the observed matter/antimatter asymmetry of the Universe [11].  The first LZ science run is expected to start in Spring 2021 and last for about 6 months (140 live-days), enough for LZ to improve the present limit obtained by XENON1T [3]. This project includes the following major contributions:  1) Physics studies: i) improvement of the current implementation of the Profile Likelihood Ratio (PLR) for the WIMP search data analysis; ii) search for rare and forbidden Xe isotope  decays, in particular those requiring Physics Beyond the Standard Model; iii) observation, for the first time, of the Migdal effect, which is theoretically predicted but was never  confirmed experimentally [23]. This effect allows to extend the sensitivity of LZ to the sub-GeV mass region.  2) Transversal contributions, critical for all the LZ physics studies: refinement of the position reconstruction method, which is a vital part of the data analysis chain of LZ and its use  for monitoring the detector performance; ii) background modelling and mitigation which is of paramount importance in any rare event search experiment.  3) Responsible for the Control System (CS) and the Underground Performance Monitor (UPM) which are critical for running the experiment and ensure the quality of data.  This project is ambitious but the composition and size of the team, as well as the high dedication percentage of its members to the project, ensure its viability. Moreover, the team is deeply involved in LZ, having multiple responsibilities, with several members occupying key coordination positions. We also have a long history of participation in previous WIMP detection experiments (ZEPLIN-II, ZEPLIN-III and LUX). Finally, all but two of the project tasks are the follow-up of work carried out in previous FCT funded projects.  

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