Search for higgsinos with compressed mass spectra using low-momentum mildly-displaced tracks with the ATLAS detector

The Standard Model is an effective theory that accurately describes nature and has successfully explained most of the experimental results. However, some questions remain unresolved in the Standard Model, such as the existence of the dark matter and the quadratic divergence of the Higgs boson mass. Supersymmetry, a symmetry between fermions and bosons, is a theoretical extension of the Standard Model that may resolve these problems by introducing a new superpartner for each Standard Model particle. In particular, it can solve the hierarchy problem when the mass of the higgsinos, superpartners of the Higgs boson, is close to the electroweak scale. Collider experiments have attempted searches for light higgsinos in the region where the mass difference between charged and neutral higgsinos is $\mathcal{O}(100)\;\mathrm{MeV}$, but a sensitivity gap remains. The mass difference between higgsinos depends strongly on the masses of wino and bino, which are superpartners of the electroweak gauge bosons. The sensitivity gap corresponds to wino and bino masses between 10 and 100 TeV and is difficult to probe by existing collider searches. This thesis describes a search for higgsinos with compressed mass spectra of O(100) MeV using 140 $\mathrm{fb}^{−1}$ of $\sqrt{s} = 13\;\mathrm{TeV}$ proton-proton collision data collected by the ATLAS detector. In the compressed mass region, the charged higgsino ("chargino") has the largest branching ratio for decaying to a single pion and the lightest neutral higgsino ("neutralino"). Neutralinos do not interact with the detector and are identified as missing transverse momentum. Tracks produced by charged particles are characterized by low momentum. However, they are produced at a small distance from the interaction point due to the relatively long lifetime of the chargino in the compressed mass region. Therefore, requiring tracks with significant impact parameters can reduce background events and significantly increase sensitivity. A dedicated event selection is applied to define two regions, maximizing the signal-to-noise ratio to conduct the counting experiment. The number of data observed in the two signal regions, with 35 and 15 events, and the number predicted from the Standard Model were consistent within the uncertainty. This search sets a 95% CL limit on the higgsino mass as a function of the mass difference. In the mass difference region of 0.3 – 1.0 GeV, the higgsino mass was excluded up to approximately 170 GeV, extending the mass reach significantly beyond the LEP results of approximately 90 GeV. As a result, this analysis excluded a range of wino and bino masses between 10 and 100 TeV, with a higgsino mass of 100 GeV.