Search for vector-like T quarks decaying to top quarks and Higgs bosons in the all-hadronic channel using jet substructure

CMS Collaboration, Vardan Khachatryan, Albert M. Sirunyan, Armen Tumasyan, Wolfgang Adam, Thomas Bergauer, Marko Dragicevic, Janos Erö, Markus Friedl, Rudolf Fruehwirth, Vasile Mihai Ghete, Christian Hartl (+2127 others)
2015
A search is performed for a vector-like heavy T quark that is produced in pairs and that decays to a top quark and a Higgs boson. The data analysed correspond to an integrated luminosity of 19.7 fb −1 collected with the CMS detector in proton-proton collisions at √ s = 8 TeV. For T quarks with large mass values the top quarks and Higgs bosons can have significant Lorentz boosts, so that their individual decay products often overlap and merge. Methods are applied to resolve the substructure of
more » ... ch merged jets. Upper limits on the production cross section of a T quark with mass between 500 and 1000 GeV/c 2 are derived. If the T quark decays exclusively to tH, the observed (expected) lower limit on the mass of the T quark is 745 (773) GeV/c 2 at 95% confidence level. For the first time an algorithm is used for tagging boosted Higgs bosons that is based on a combination of jet substructure information and b tagging. The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter. Within the superconducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. Extensive forward calorimetry complements the coverage provided by the barrel and endcap detectors. The energy resolution for photons with E T ≈60 GeV varies between 1.1 and 2.6% over the solid angle of the ECAL barrel, and from 2.2 to 5% in the endcaps. The HCAL, when combined with the ECAL, measures jets with a resolution In the region |η| < 1.74, the HCAL cells have widths of 0.087 in η and 0.087 in azimuth (φ). In the η-φ plane, and for |η| < 1.48, the HCAL cells map on to 5 × 5 ECAL crystal arrays to form calorimeter towers projecting radially outwards from close to the nominal interaction point. At larger values of |η|, the size of the towers increases and the matching ECAL arrays contain fewer crystals. Within each tower, the energy deposits in ECAL and HCAL cells are summed to define the calorimeter tower energies, subsequently used to provide the energies and directions of hadronic jets. The silicon tracker measures charged particles within the pseudorapidity range |η| < 2.5. It consists of 1440 silicon pixel and 15 148 silicon strip detector modules and is located in the 3.8 T field of the superconducting solenoid. For nonisolated particles of 1 < p T < 10 GeV/c and |η| < 1.4, the track resolutions are typically 1.5% in p T and 25-90 (45-150) µm in the transverse (longitudinal) impact parameter [24]. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in ref. [25] . Event samples The data used for this analysis were collected by the CMS experiment using pp collisions provided by the CERN LHC with a centre-of-mass energy of 8 TeV, and correspond to an integrated luminosity of 19.7 fb −1 . Events are selected online by a trigger algorithm that requires H T , the scalar sum of the transverse momenta of reconstructed jets in the detector, to be greater than 750 GeV/c. The online H T is calculated from calorimeter jets with p T > 40 GeV/c. Calorimeter jets are reconstructed from the energy deposits in the calorimeter towers, clustered by the anti-k T algorithm [26, 27] with a size parameter of 0.5. Simulated samples are used to determine signal selection efficiencies as well as the background contribution from tt plus jets, ttH, and hadronically decaying W/Z plus b jet production. The background from QCD multijet production is derived from data. Events from T quark decays are generated for mass hypotheses between 500 and 1000 GeV/c 2 in steps of 100 GeV/c 2 . The inclusive cross sections for the signal samples and tt samples are calculated at next-to-next-to-leading order (NNLO) for the reaction gg → tt + X. The fixed order calculations are supplemented with soft-gluon resummation with next-to-next-to-leading logarithmic accuracy [28] . The tt cross sections are computed based on the Top++ v2.0 implementation using the MSTW2008nnlo68cl parton distribution functions (PDF) and the 5.9.0 version of LHAPDF [28, 29] . The evaluated -3 -
doi:10.18154/rwth-2016-04848 fatcat:wguhqec5bfellcyyf25ve2hjsy