Search for the Standard Model Higgs boson produced by vector boson fusion and decaying to bottom quarks

The search for the Standard Model (SM) Higgs boson (H) produced through the Vector Boson Fusion (VBF) mechanism and decaying to a pair of bottom quarks is reported. The used data have been collected with the CMS detector and correspond to an integrated luminosity of 19.8 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=8$ TeV at the CERN LHC. Parked data have been exploited as well. This search resulted in an observed (expected) significance in these data samples for a H $\rightarrow$ b$\bar{{b}}$ signal at a mass of 125 GeV of 2.2 (0.8) standard deviations. The cited signal strength, $\mu=\sigma/\sigma_{\textrm{SM}}$, was measured to be 2.8$^{+1.6}_{-1.4}$. This result has been combined with other CMS searches for the SM Higgs boson decaying in a pair of bottom quarks exploiting other Higgs production mechanisms. The obtained combined signal strength is 1.0 $\pm$ 0.4, corresponding to an observed signal significance of 2.6 standard deviations for a Higgs boson mass of 125 GeV.Comment: 9 pages, 3 figures. To be published in the LHCP 2015 proceeding

Search for the Standard Model Higgs boson produced in vector boson fusion and decaying to a pair of b-quarks with the CMS experiment

In the Standard Model (SM) the Brout-Englert-Higgs mechanism explains the electroweak symmetry breaking and allows electroweak gauge bosons to acquire mass. This theory predicted the existence of a scalar boson, the Higgs boson, and its observation was one of the main goals of the LHC physics program. Recently, the ATLAS and CMS Collaborations have reported the discovery of a new boson, with a mass of 125 GeV and with properties compatible with those of the standard model Higgs boson. In this thesis work the data collected with the CMS detector, which is one of the experiment taking place at CERN’s LHC, have been used. The LHC operated at 3.5 TeV per beam in 2010 and 2011 and at 4 TeV in 2012. After the end of the 2010-2013 run, which is called the Run 1 period, the LHC went into shutdown for upgrades to increase beam energy to 6.5 TeV and protons began to circulate again in April 2015, officially starting the Run 2. At the LHC, a standard model Higgs boson can be produced through a variety of different mechanisms. As a function of the Higgs boson mass, the Standard Model predicts that after the gluon-fusion (GF) mechanism, the second largest Higgs boson production cross section is the vector boson fusion (VBF) mechanism. Furthermore, for a SM Higgs boson with a mass lower than 135 GeV, the expected dominant decay mode is to a b-quark pair. While the inclusive observation of the SM Higgs boson decaying to b b pairs is not viable in proton collisions, the observation of the bb channel in the VBF production context can be pursued thanks to the kinematic properties of the VBF process. This thesis presents the search for a SM Higgs boson in the VBF production mode, followed by a bb decay. This analysis was performed with Run 1 at 8 TeV and the corresponding integrated luminosity used was of 19.8 inverse femtobarn. The prominent feature of the searched signal is the presence of four energetic hadronic jets. Two jets are expected to originate from light-flavored quarks, that are typically two valence quarks from each of the colliding protons scattered away from the beam line and inside the detector acceptance by the VBF process. These VBF “tagging” jets (qq) are therefore roughly expected in the forward and backward direction with respect to the beam line while two additional jets are expected from the Higgs boson decay to a b-quark pair, in more central regions of the detector. Preliminary results of this search have been produced [1] and, subsequently, the analysis has been upgraded using parked data and final results produced [2]. This thesis focuses both on the analysis performed with the data collected during Run 1 at 8 TeV and on the preparation for the analysis which will be using the data of Run 2 at 13 TeV. Following a first part where the theoretical and experimental background of the analysis is explained, the thesis structure is articulated in two parts: the former describes the work carried out with the Run 1 data, while the latter explains preliminary studies looking forward to the Run 2 data-taking period. More in details, in the first part the SM theory is presented, along with the explaination of the Brout-Englert-Higgs mechanism. Additionally, the search for the Higgs boson is recalled, starting from the studies performed at LEP, passing through Tevatron and finally arriving to the discovery achieved at LHC in 2012. In the second part, the searches for the Higgs boson decaying to bottom-quarks with the CMS experiment and using the data collected during Run 1 are described. The analysis exploiting the associated production of an Higgs boson with a vector boson (VH) or with a pair of top-quarks (ttH) are introduced, while more details are given about the VBF search. This analysis is particularly challenging as, despite a relatively larger production cross section, the QCD background rate is very considerable. In order to discriminate between signal and background, some tools have been specifically developed. In particular, since the most important variable is the invariant mass between the two b-tagged jets, an important tool is the jet transverse momentum (pT) regression technique, which provides a corrective factor to the b-jet pT and thus results in a better resolution for the invariant mass of the two b-jets. Hence, I validated the regression using events characterized by the decay of the Z boson in two electrons or muons and the presence of one or two b-tagged jets. Within this study, which essentially exploits the pT-balance between leptons and b-jets, a good agreement between data and Monte Carlo is obtained and an overall improvement (~10/15%) in the resolution is found, both when considering one b-jet or when considering two of them. Concerning the preparation for the Run 2 data-taking period at 13 TeV, the first step to accomplish was on the trigger design. Since the VBF Hbb channel has a particular total hadronic final state, it is necessary to specifically design and optimize a new trigger, both at hardware level (Level 1) and at software level (High Level Trigger). For the former, I first used the same logic used in the Run 1 analysis, focusing on the search for new optimized thresholds, in order to maximize the efficiency with a fixed background bandiwidth. Some changes have been made and a general purpose L1 trigger, requiring three jets with transverse momentum above optimized thresholds, has been implemented. As regards the High Level Trigger, in addition to the optimization of the thresholds to use, I also implemented a new sorting algorithm useful to identify on-line the final state VBF-tagging jets and b-jets, which is more efficient respect to the one used during the Run 1 analysis. Multiple trigger paths were implemented with my work and have been adopted for the Run 2 data-taking period Menu. Additionally, the VBF Hbb signal properties expected at 13 TeV have been investigated at generator level, focusing on the four jets final state kinematics, and a comparison with the 8 TeV distributions is performed. It appears that the VBF tagging jets will be more energetic and with a larger pseudorapidity opening, while the b-tagged jets will not change. Finally, looking forward to the Run 2 data-taking period, projections on the possible sensitivity of the VBF Hbb analysis for several integrated luminosity scenarios have been evaluated, assuming the same shapes in the m(bb) distribution and scaling signal and background yields according to the expected Run 2 rates. Combining the expected results at 13 TeV with 100 inverse femtobarn and the results obtained at the end of the Run 1, the analysis will achieve an expected 95% confidence level upper limits of 0.51 and an expected significance of 2.25. It appeared that, already with an amount of collected data of about 15 inverse femtobarn, the same Run 1 results will be achieved and considering that additional dedicated strategies will be exploited for the updated analysis, further improvements in the analysis are envisaged. Even if these expected results would probably not allow to discover or neither to observe an evidence of the Higgs boson produced in vector boson fusion and decaying to bottom quarks, this analysis will howewer gain even more importance in the next future since it may be combined with other search in the Hbb channel, giving a fundamental contribute. References: [1] Higgs to bb in the VBF channel. CMS Physics Analysis Summary CMS-PAS-HIG-13-011, CERN, Geneva, 2013. [2] Search for the standard model Higgs boson produced by vector-boson fusion and decaying to bottom quarks. CMS Physics Analysis Summary CMS-HIG-14-004, CERN, Geneva, 2015

Search for supersymmetry in pp collisions at s\sqrt{s} = 13 tev in the single-lepton final state using the sum of masses of large-radius jets

Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single lepton, multiple jets, including at least one b-tagged jet, and large missing transverse momentum. The search uses a sample of proton-proton collision data at $\sqrt{s}$ = 13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9  $fb^{−1}$. The observed event yields in the signal regions are consistent with those expected from standard model backgrounds. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production, with gluino decay into either on- or off-mass-shell top squarks. Assuming that the top squarks decay into a top quark plus a stable, weakly interacting neutralino, scenarios with gluino masses up to about 1.9 TeV are excluded at 95% confidence level for neutralino masses up to about 1 TeV

Search for dark matter produced in association with heavy-flavor quark pairs in proton-proton collisions at s\sqrt{s} = 13 TeV

A search is presented for an excess of events with heavy-flavor quark pairs ($t\overline{t}$ and $b\overline{b}$) and a large imbalance in transverse momentum in data from proton–proton collisions at a center-of-mass energy of 13TeV. The data correspond to an integrated luminosity of 2.2 $fb^{−1}$ collected with the CMS detector at the CERN LHC. No deviations are observed with respect to standard model predictions. The results are used in the first interpretation of dark matter production in $t\overline{t}$ and $b\overline{b}$ final states in a simplified model. This analysis is also the first to perform a statistical combination of searches for dark matter produced with different heavy-flavor final states. The combination provides exclusions that are stronger than those achieved with individual heavy-flavor final states

Combination of searches for heavy resonances decaying to WW, WZ, ZZ, WH, and ZH boson pairs in proton–proton collisions at s\sqrt{s} = 8 and 13 TeV

A statistical combination of searches is presented for massive resonances decaying to WW, WZ, ZZ, WH, and ZH boson pairs in proton–proton collision data collected by the CMS experiment at the LHC. The data were taken at centre-of-mass energies of 8 and 13 TeV, corresponding to respective integrated luminosities of 19.7 and up to 2.7 $fb^{−1}$. The results are interpreted in the context of heavy vector triplet and singlet models that mimic properties of composite-Higgs models predicting W′ and Z′ bosons decaying to WZ, WW, WH, and ZH bosons. A model with a bulk graviton that decays into WW and ZZ is also considered. This is the first combined search for WW, WZ, WH, and ZH resonances and yields lower limits on masses at 95% confidence level for W′ and Z′ singlets at 2.3 TeV, and for a triplet at 2.4 TeV. The limits on the production cross section of a narrow bulk graviton resonance with the curvature scale of the warped extra dimension $\tilde{k}=0.5$, in the mass range of 0.6 to 4.0 TeV, are the most stringent published to date

The CMS trigger system

This paper describes the CMS trigger system and its performance during Run 1 of the LHC. The trigger system consists of two levels designed to select events of potential physics interest from a GHz (MHz) interaction rate of proton-proton (heavy ion) collisions. The first level of the trigger is implemented in hardware, and selects events containing detector signals consistent with an electron, photon, muon, τ lepton, jet, or missing transverse energy. A programmable menu of up to 128 object-based algorithms is used to select events for subsequent processing. The trigger thresholds are adjusted to the LHC instantaneous luminosity during data taking in order to restrict the output rate to 100 kHz, the upper limit imposed by the CMS readout electronics. The second level, implemented in software, further refines the purity of the output stream, selecting an average rate of 400 Hz for offline event storage. The objectives, strategy and performance of the trigger system during the LHC Run 1 are described

Search for dark matter produced with an energetic jet or a hadronically decaying W or Z boson at s=13\sqrt{s}=13 TeV

A search for dark matter particles is performed using events with large missing transverse momentum, at least one energetic jet, and no leptons, in proton-proton collisions at $\sqrt{s}=13$ TeV collected with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 12.9 $fb^{−1}$. The search includes events with jets from the hadronic decays of a W or Z boson. The data are found to be in agreement with the predicted background contributions from standard model processes. The results are presented in terms of simplified models in which dark matter particles are produced through interactions involving a vector, axial-vector, scalar, or pseudoscalar mediator. Vector and axial-vector mediator particles with masses up to 1.95 TeV, and scalar and pseudoscalar mediator particles with masses up to 100 and 430 GeV respectively, are excluded at 95% confidence level. The results are also interpreted in terms of the invisible decays of the Higgs boson, yielding an observed (expected) 95% confidence level upper limit of 0.44 (0.56) on the corresponding branching fraction. The results of this search provide the strongest constraints on the dark matter pair production cross section through vector and axial-vector mediators at a particle collider. When compared to the direct detection experiments, the limits obtained from this search provide stronger constraints for dark matter masses less than 5, 9, and 550 GeV, assuming vector, scalar, and axial-vector mediators, respectively. The search yields stronger constraints for dark matter masses less than 200 GeV, assuming a pseudoscalar mediator, when compared to the indirect detection results from Fermi-LAT

Search for a light pseudoscalar Higgs boson produced in association with bottom quarks in pp collisions at s\sqrt{s} = 8 TeV

A search for a light pseudoscalar Higgs boson (A) produced in association with bottom quarks and decaying into a muon pair is reported. The search uses 19.7 $fb{−1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV, collected by the CMS experiment. No signal is observed in the dimuon mass range from 25 to 60 GeV. Upper limits on the cross section times branching fraction, $\sigma (pp \to b\overline{b}A)\mathcal{B}(A \to \mu \mu)$, are set

Measurement of charged pion, kaon, and proton production in proton-proton collisions at s\sqrt{s} = 13 TeV

Transverse momentum spectra of charged pions, kaons, and protons are measured in proton-proton collisions at $\sqrt{s}$ = 13  TeV with the CMS detector at the LHC. The particles, identified via their energy loss in the silicon tracker, are measured in the transverse momentum range of $p_T \approx 0.1–1.7  GeV/c$ and rapidities $\vert y \vert < 1$. The $p_T$ spectra and integrated yields are compared to previous results at smaller $\sqrt{s}$ and to predictions of Monte Carlo event generators. The average $p_T$ increases with particle mass and charged particle multiplicity of the event. Comparisons with previous CMS results at $\sqrt{s}$ = 0.9, 2.76, and 7 TeV show that the average $p_T$ and the ratios of hadron yields feature very similar dependences on the particle multiplicity in the event, independently of the center-of-mass energy of the pp collision