Observation of upsilon suppression, search for long-lived particles, and observation of B0s → micro + micro-- at the LHC with the CMS experiment

Hu, Zhen Ph.D., Purdue University, December 2014. Observation of Upsilon Suppression, Search for Long-lived Particles, and Observation of B s → μ+μ− at the LHC with the CMS Experiment. Major Professor: Ian P. Shipsey. The LHC centre-of-mass energy allows abundant Υ production in lead-lead (PbPb) collisions. A detailed measurement of the bottomonium production will help to characterize the dense matter produced in heavy-ion collisions. The full spectroscopy of quarkonium states has been proposed as a possible thermometer for the QGP [1]. The measurement reported in Chapter 3 is performed with data recorded by CMS during the first PbPb run at 2010 and the proton-proton (pp) run at 2011, both at √ sNN = 2.76 TeV. The integrated luminosity corresponds to 7.28/μb for PbPb and 225/nb for pp collisions. Using muons of transverse momentum (pT) above 4 GeV/c and pseudorapidity (η) below 2.4, the double ratio of the Υ(2S+3S) excited states to the Υ(1S) ground state in PbPb and pp collisions, [Υ(2S+3S)/Υ(1S)]PbPb/[Υ(2S+3S)/Υ(1S)]pp, is found to be 0.31 −0.15(stat.)±0.03(syst.) [2]. The probability to obtain such a measured value, if the real double ratio is 1, is calculated to be less than 1% [2]. Υ(1S) suppression at √ sNN = 2.76 TeV PbPb collisions is also reported in Chapter 3. As a follow up study, the sequential suppression of the three individual Υ states in PbPb collisions with respect to their yields in pp collisions has been measured [3] in Chapter 4. The pp and PbPb datasets used in this chapter correspond to integrated luminosities of 230/nb and 150/μb, respectively, collected at 2.76 TeV in 2011 [4]. The relative suppression of the excited Υ states with respect to the Υ(1S) ground state has been measured as a double ratio [Υ(nS)/Υ(1S)]PbPb/[Υ(nS)/Υ(1S)]pp. The absolute suppression of the Υ(nS) yields in PbPb relative to the yields in pp scaled by the number of nucleon-nucleon collisions, RAA, is measured as a function of the xxxviii collision centrality [3]. Integrated over centrality, the RAA values for Υ(1S), Υ(2S), and the upper limit of Υ(3S) RAA are reported in this chapter, which demonstrate the sequential suppression of the Υ(nS) states in PbPb collisions at LHC energies [3]. In Chapter 5, a search is performed for long-lived particles decaying to a final state that includes a pair of leptons (μ+μ− or e+e−). The experimental signature is a distinctive topology consisting of a pair of charged leptons originating from a displaced secondary vertex [5]. Events were collected with the CMS detector at the CERN LHC in pp collisions at √ s = 8TeV, and selected from data corresponding to an integrated luminosity of 20.5 (19.6) fb−1 in the muon (electron) channel. No significant excess is observed beyond the standard model expectations. Upper limits are set at 95% confidence level on the product of the cross section and branching fraction of such a signal, as a function of the lifetime of the long-lived particle, in the context of two specific models. In the first, Higgs bosons decay to a pair of long-lived neutral bosons, each of which decays to dileptons. In the second, events contain a pair of squarks that each decay to a long-lived neutralino, each of which in turn decays to dileptons and a neutrino. We performed the search with the CMS tracker and muon system separately and combined the limits obtained. A search for B s rare decays in pp collisions at √ s = 8 TeV is performed by CMS. An excess of B s → μ+μ− events with respect to background is observed with a significance of 4.3 σ [6]. The result is in agreement with the standard model expectation [6]. In Chapter 6, we present an independent analysis with samples corresponding to an integrated luminosity of 20 fb−1, collected by the CMS experiment at the LHC. It follows closely the method of the initial analysis and includes direct comparisons. It identified incorrect procedures in the initial analysis. After these had been corrected the two analyses were in good agreement. This agreement was crucial to bring to publication the CMS observation of B s → μ+μ− and the joint LHCb-CMS article submitted to Nature [7] in 2014.