Measurement of the Top Quark Mass in the Dilepton Final State using the Matrix Element Method

The top quark, discovered in 1995 by the CDF and D0 experiments at the Fermilab Tevatron Collider, is the heaviest known fundamental particle. The precise knowledge of its mass yields important constraints on the mass of the yet-unobserved Higgs boson and allows to probe for physics beyond the Standard Model. The first measurement of the top quark mass in the dilepton channel with the Matrix Element method at the D0 experiment is presented. After a short description of the experimental environment and the reconstruction chain from hits in the detector to physical objects, a detailed review of the Matrix Element method is given. The Matrix Element method is based on the likelihood to observe a given event under the assumption of the quantity to be measured, e.g. the mass of the top quark. The method has undergone significant modifications and improvements compared to previous measurements in the lepton+jets channel: the two undetected neutrinos require a new reconstruction scheme for the four-momenta of the final state particles, the small event sample demands the modeling of additional jets in the signal likelihood, and a new likelihood is designed to account for the main source of background containing tauonic Z decay. The Matrix Element method is validated on Monte Carlo simulated events at the generator level. For the measurement, calibration curves are derived from events that are run through the full D0 detector simulation. The analysis makes use of the Run II data set recorded between April 2002 and May 2008 corresponding to an integrated luminosity of 2.8 fb{sup -1}. A total of 107 t{bar t} candidate events with one electron and one muon in the final state are selected. Applying the Matrix Element method to this data set, the top quark mass is measured to be m{sub top}{sup Run IIa} = 170.6 {+-} 6.1(stat.){sub -1.5}{sup +2.1}(syst.)GeV; m{sub top}{sup Run IIb} = 174.1 {+-} 4.4(stat.){sub -1.8}{sup +2.5}(syst.)GeV; m{sub top}{sup comb} = 172.9 {+-} 3.6(stat.) {+-} 2.3(syst.)GeV. Systematic uncertainties are discussed, and the results are interpreted within the Standard Model of particle physics. As the main systematic uncertainty on the top quark mass comes from the knowledge of the absolute jet energy scale, studies for a simultaneous measurement of the top quark mass and the b jet energy scale are presented. The prospects that such a simultaneous determination offer for future measurements of the top quark mass are outlined

Dark Higgs bosons at colliders

The Large Hadron Collider (LHC) has confirmed the Higgs mechanism to generate mass in the Standard Model (SM), making it attractive also to consider spontaneous symmetry breaking as the origin of mass for new particles in a dark sector extension of the SM. Such a dark Higgs mechanism may in particular give mass to a dark matter candidate and to the gauge boson mediating its interactions (called dark photon). In this review, we summarize the phenomenology of the resulting dark Higgs boson and discuss the corresponding search strategies with a focus on collider experiments. We consider both the case that the dark Higgs boson is heavier than the SM Higgs boson, in which case leading constraints come from direct searches for new Higgs bosons as well missing-energy searches at the LHC, and the case that the dark Higgs boson is (potentially much) lighter than the SM Higgs boson, such that the maximum sensitivity comes from electron–positron colliders and fixed-target experiments. Of particular experimental interest for both cases is the associated production of a dark Higgs boson with a dark photon, which subsequently decays into SM fermions, dark matter particles or long-lived dark sector states. We also discuss the important role of exotic decays of the SM-like Higgs boson and complementary constraints arising from early-universe cosmology, astrophysics, and direct searches for dark matter in laboratory experiments

Precision measurements of the top quark mass and width with the D0 detector

Since the discovery of the top quark in 1995 at the Fermliab Tevatron Collider, top quark properties have been measured with ever higher precision. In this article, recent measurements of the top quark mass and its width using up to 3.6 fb{sup -1} of D0 data are summarized. Different techniques and final states have been examined and no deviations within these measurements have been observed. In addition to the direct measurements, a measurement of the top quark mass from its production cross section and a measurement of the top-antitop quark mass difference are discussed. With a mass of 173.3 {+-} 1.1 GeV, the top quark is the heaviest of all known fundamental particles. Due to the high mass, its Yukawa coupling is close to unity suggesting that it may play a special role in electroweak symmetry breaking. Precise measurements of both, the W boson and the top quark mass, constrain the mass of the yet unobserved Higgs boson and allow to restrict certain extensions of the Standard Model. At the Tevatron collider with a center-of-mass energy of 1.96 TeV, 85% of the top quark pairs are produced in quark-antiquark annihilation; 15% originate from gluon fusion. Top quarks are predicted to decay almost exclusively to a W boson and a bottom quark. According to the number of hadronic W decays, top events are classified into all-jets, lepton+jets and dilepton events. The lepton+jets channel is characterized by four jets, one isolated, energetic charged lepton and missing transverse energy. With 30%, the branching fraction of the lepton+jets channel is about seven times larger than the one of the dilepton channel whereas the signal to background ratio is about three times smaller. The main background in this final state comes from W +jets events. Instrumental background arises from events in which a jet is misidentified as an electron and events with heavy hadrons that decay into leptons which pass the isolation requirements. The topology of the dilepton channel is described by two jets, two isolated, energetic charged leptons and significant missing transverse energy from the undetected neutrinos. The main background are Z + jets and diboson events (WW/WZ/ZZ+jets) as well as instrumental background as characterized above. At the D0 experiment, different techniques are used to measure the top quark mass. They are summarized in the following sections together with the first measurement of the top anti-top quark mass difference and the first precise determination of the top quark width

A new LHC search for dark matter produced via heavy Higgs bosons using simplified models

Searches for dark matter produced via scalar resonances in final states consisting of Standard Model (SM) particles and missing transverse momentum are of high relevance at the LHC. Motivated by dark-matter portal models, most existing searches are optimized for unbalanced decay topologies for which the missing momentum recoils against the visible SM particles. In this work, we show that existing searches are also sensitive to a wider class of models, which we characterize by a recently presented simplified model framework. We point out that searches for models with a balanced decay topology can be further improved with more dedicated analysis strategies. For this study, we investigate the feasibility of a new search for bottom-quark associated neutral Higgs production with a $b \bar b Z + p_\text{T}^\text{miss}$ final state and perform a detailed collider analysis. Our projected results in the different simplified model topologies investigated here can be easily reinterpreted in a wide range of models of physics beyond the SM, which we explicitly demonstrate for the example of the Two-Higgs-Doublet model with an additional pseudoscalar Higgs boson.Comment: 27 pages, 8 figures, 4 table

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum