Search for new long-lived particles with the CMS experiment at √s = 13TeV

Abstract

Long-lived particles are an interesting extension of the standard model of particle physics. This thesis presents a search for such new long-lived particles with lifetimes of the order of a few millimetres produced in Higgs boson decays that then themselves decay into b quark pairs. The long-lived particles are predicted by, for example, Twin Higgs models. These theories solve the so-called little hierarchy problem, increasing the validity of the standard model. The search is performed with proton-proton collisions at a centre-of-mass energy of 13TeV at the LHC corresponding to an integrated luminosity of L = 36.3 fb−1 recorded with the CMS experiment.

The search is split into two analyses, depending on the mass of the long-lived particles, because the decay kinematics differ. One analysis is designed for light long-lived particles (m = 15 GeV), and the other analysis is for heavier long-lived particles (m = 40 GeV and m = 55 GeV). The final state of the signal events is expected to contain up to four b quarks. Therefore, events with at least four jets of which at least three have to be b-tagged jets are selected. Two production modes for the signal are investigated: gluon-gluon fusion and vector-boson fusion Higgs boson production modes. Possible signal events are identified by a requirement on the invariant mass of the signal candidate jets. Signal events are further separated from background events using a neural network based jet tagger. The network is parametrised as a function of the masses and lifetimes of the signal prediction. This makes it possible to train the network only once and simultaneously use it for the analyses of different masses and lifetimes. The main background consists of QCD multijet production and is estimated using a data-driven method. Upper limits on the signal strength modifier are calculated at 95% CL as a function of the lifetimes for all investigated masses of the long-lived particles. No evidence for such long-lived particles is observed.