Resistive Multiplexed Micromegas Detectors to Search for Dark Sector Physics and Test the Weak Equivalence Principle for Anti-Matter at CERN

Abstract

The experiments NA64 and GBAR aiming to explore the still unanswered questions in physics of the existence of dark matter and the matter- antimatter asymmetry, respectively, are presented in the scope of this thesis. NA64 is an experiment at CERN searching for a new U0(1) gauge boson, A0 (dark photon) which may mediate the interaction of dark matter with ordinary matter via a very weak force. The experiment is sensitive to the still unexplored area of gamma-A0 mixing strength 10^-5 < e < 10^-3 and masses MA0 <= 100 MeV. The results from the first beam run are reported and new limits were set on the gamma-A0 mixing strength and the results exclude the invisibly decaying A0 with a mass <= 100 MeV as an explanation for the (g-2)µ anomaly. GBAR is an experiment set up at the AD hall at CERN aiming to measure the free-fall of anti-hydrogen with a relative precision of 1 % in the fi rst phase for a direct test of the equivalence principle for anti-matter. The experiment plans to use the ELENA anti-proton beam to produce H(bar)+ ions from its interaction with positronium, cool the ions down to 10 µK temperature and eventually detect the free-fall of H(bar) after photo-detaching the excess positron from the ion. The signal of detection is given by its annihilation producing pion tracks. The experiment is being set up at CERN and is expected to start a commissioning run in 2018. For both experiments a tracker is essential - NA64 requires precise tracking of the incoming particles to reconstruct their momentum and suppress background from the low energy beam tail. In GBAR tracking is required to track the pion tracks and reconstruct the vertex of the H(bar) annihilation and reject cosmic ray background. Multiplexed XY Resistive Micromegas modules chosen for the tracking requirements of both the experiments are presented. The use of multiplexed modules in high intensity environments was not explored so far, due to the e ffect of ambiguities in the reconstruction of the hit point caused by the multiplexing feature. The first performance results of multiplexed modules tested at the CERN SPS 100 GeV/c electron beam at intensities up to 3.3 x10^5 e-/sec/cm2 is reported. At these rates, a factor 5 multiplexing introduces more than 50 % level of ambiguity. The results prove that by using the additional information of cluster size and integrated charge of the induced XY signal clusters the ambiguities can be reduced to a level below 2%. The expected performances of the GBAR Micromegas tracker is also summarized from the simulation of H(bar) annihilation done with Geant4, taking into account the initial parameters of the atom, geometric acceptance and intrinsic resolution of tracker modules. The resolution of vertex reconstruction and estimation of the background rejection is also presented.