ETD system

Electronic theses and dissertations repository


Tesi etd-04132015-114959

Thesis type
Tesi di laurea magistrale
Search for the Standard Model Higgs boson produced in vector boson fusion and decaying to a pair of b-quarks with the CMS experiment
Corso di studi
relatore Dott. Azzurri, Paolo
Parole chiave
  • invariant mass
  • b-quarks
  • standard model
  • energy regression
  • higgs boson
  • trigger
Data inizio appello
Riassunto analitico
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.

[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.