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In this thesis the author has conducted a search for the Higgs boson in the H → ZZ → l +l - qq Decay Channel at CMS with fb−1 of data.
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The impact parameter of a reconstructed charged particle track is its distance of closest approach to the primary vertex. The magnitude of the impact parameter can be used to discriminate between the tracks of particles which originated in the interaction and those of particles produced from the decay of a relatively long-lived product of the interaction for example a b-hadron even if it is not possible to reconstruct a secondary vertex.

Leptons and photons originating from a hard interaction, or from the decay chain of a particle, such as a Z boson or Higgs boson, directly produced in a hard interaction are not except by chance closely accompanied by other particles — they are isolated. Many lepton and photon candidates reconstructed in the detector do not originate directly from the hard interaction, but come from the constituents of the jets of particles formed by the hadronization of gluons or quarks. These candidates tend not to be isolated — they are surrounded by other particles from the jet.

A measure of how many particles, or how much energy, surrounds a candidate lepton or photon can be used to determine whether it is probable that it emerged directly from the hard interaction, or more likely that it originated in a jet fragment. Events selected so as to have negligible probability of containing signal events constitute a control sample , and the selection requirements define a control region in parameter space.

The control sample in the control region contains only background events and can be used to estimate the number of background events expected in the region of the signal, provided the effect of whatever distinguishes the control region from the signal region is sufficiently well understood. For both experiments, an excess of events is present in the mass region around GeV.

Higgs boson observed decaying to b quarks – at last! | ATLAS Experiment at CERN

The corresponding numbers for the CMS experiment in the mass window More details about the observed and expected numbers of events, their uncertainties and the split in the various four-lepton channels are given in Table 1 for the ATLAS and in Table 2 for the CMS experiment. In order to quantify these excesses, the probabilities for the background-only hypotheses were calculated. The local p-value is defined to give the probability of a background fluctuation. The probability that an excess of events observed anywhere within an extended search range is due to a background fluctuation is termed the global p-value , and is larger than the local p -value — this fact is often referred to as the look-elsewhere effect.

The search for the Standard Model Higgs boson decaying to two photons involves detecting a narrow peak above a background diphoton invariant mass spectrum. The Higgs boson decay to two photons proceeds via loop diagrams containing charged particles. The W boson loop and the top quark loop diagrams dominate the decay amplitude, but contribute with opposite sign.

The Standard Model branching fraction is small, having a maximum value of 0. Events containing diphotons are collected by both experiments using diphoton triggers. Further selection requirements are applied to suppress the reducible background contribution from photons originating in jets. Requirements are made on the shapes of the showers in the calorimeter cells or crystals, and on the isolation of the photons from other activity in the detectors. There are many interactions for each bunch crossing, whose longitudinal position along the beam axis has a RMS spread of about 5 cm.

The diphoton interaction vertex is assigned using multiple sources of information combined in a global likelihood ATLAS or using a boosted decision tree BDT Hoecker et al. The analysis of the ATLAS experiment additionally considers the directions of the photon showers reconstructed in the calorimeter using its longitudinal granularity.

That of the CMS experiment additionally examines the correlation between the kinematic properties of the charged particle tracks associated with the reconstructed vertices, with those of the diphoton system.


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Both ATLAS and CMS enhance the sensitivity of their analyses by subdividing the selected diphoton events into mutually exclusive categories, where the categorization is based upon criteria sensitive to the diphoton mass resolution, and to the probability that an event is signal rather than background. The ATLAS classification is based on the location of the photons in the calorimeter, whether they are tagged as having converted in the material in front of the calorimeter, by the presence of a reconstructed electron track, and p Tt , the component of the diphoton transverse momentum that is orthogonal to the axis defined by the difference between the photon momenta.

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The CMS classification is based on the output of a BDT which is trained to give a high value for signal-like events and for events with good diphoton invariant mass resolution. Events in which a dijet is present, in addition to the diphoton, and satisfies selection criteria chosen to be consistent with the characteristics of signal events produced by the vector-boson fusion process, are placed in dijet categories. For the analysis of the 8 TeV dataset CMS uses two dijet categories with differing levels of selection stringency. Both analyses published tables listing the categories and indicating the numbers of events expected from a Standard Model Higgs boson signal in each category, the mass resolution expected in each category, and information about the numbers of events present in the data ATLAS Collaboration , CMS Collaboration Some important numbers from these tables are reproduced in Table 3 and Table 4.

For the statistical analysis of the data, the sum of a signal mass peak and a background distribution is fitted to the diphoton invariant mass distribution.

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The shape of the invariant mass distribution of the signals is obtained from detailed simulation for each of the categories. The background in the different categories is modelled by parametric functions. Various tests are made to determine the potential bias resulting from the choice of background fit functions. These tests involve either pseudo-experiments, or large samples of simulated events complemented by data-driven estimates.

CMS uses polynomials in the Bernstein basis with degree ranging from 3 to 5 depending on category. ATLAS uses Bernstein polynomials of degree 4, exponentials of a polynomial of degree 2, and plain exponential functions, depending on the category. Based on searches in this channel, mass regions could be excluded by both the Tevatron and the LHC experiments already in summer The presence of neutrinos makes the reconstruction of a narrow mass peak impossible, and evidence for a signal must be extracted from an excess of events above the expected backgrounds.

Usually, the WW transverse mass m T , computed from the leptons and the missing transverse momentum,. Typical selection requirements are the presence of two isolated high- p T leptons with a small azimuthal angular separation and a significant missing transverse energy. The weak decays of the W-bosons imply a correlation between the directions of the charged leptons, which can be exploited to reject the WW background.

The different jet categories are sensitive to different Higgs boson production mechanisms, and show very different background compositions. The distributions of two discriminating variables, the transverse mass and the invariant mass of the two leptons, as measured in July , after the application of all final selection criteria, are shown in Figure Excesses of events above the expectations from Standard Model processes excluding Higgs boson production are visible.

Details about the observed and expected numbers of events, their uncertainties and the split in the various categories are given in Table 5 for the ATLAS and in Table 6 for the CMS experiment. A shallow minimum of the local p -value appears at These decay channels require a larger data sample to produce statistically significant measurements. The background is estimated mainly through the use of control samples in data. The results in the individual categories are combined to give the final result. This restriction greatly improves the signal-to-background ratio.

The b-quark jets are identified by the presence of secondary vertices, and the requirement of a high p T on the dijet system significantly reduces the background. The dominant backgrounds are estimated from data, and the remaining backgrounds from simulation studies. In the Higgs boson discovery analyses no statistically significant enhancement over the expected background was observed in either of the two fermionic decay channels.

The significances of the signals seen by the two experiments are summarized in Table 7 and Table 8. The observed local p -value from the combination of all channels in the ATLAS experiment is shown as a function of m H in Figure 12 for the low mass range. A compatible picture was observed in the CMS experiment with an observed expected local significance of 5.

It was obtained by a combination of all channels.

Search for the Standard Model Higgs Boson in the H → ZZ → l + l - qq Decay Channel at CMS

The measured signal strength was found to be consistent with the one expected for the production of the Higgs boson of the Standard Model. The decays to pairs of vector bosons with a net electric charge of zero identified the new particle as a neutral boson. The observation in the diphoton channel excluded the spin-1 hypothesis for the production of an on-shell resonance by virtue of the Landau-Yang theorem.

Although these results were compatible with the interpretation in terms of the Standard Model Higgs boson, a wide range of other interpretations was possible as well. At the end of the first LHC operation period — about 2. Using this data, evidence for the Higgs boson was seen in many individual production and decay modes, including the Standard Model predicted decays into fermions. Extensive searches were made for deviations from the Standard Model predictions, but in general the measured signal strengths were broadly consistent with expectations, and tests of angular distributions indicated that the particle discovered is a scalar, as expected.

Books on Data Analysis and Statistical Methods. Karl Jakobs and Chris Seez , Scholarpedia, 10 9 Jump to: navigation , search. Post-publication activity Curator: Chris Seez Contributors:. Eric Kajfasz. Sponsored by: Dr. Categories : Physics Experimental high energy physics Experimental collider physics. Namespaces Page Discussion. Views Read View source View history. Pileup An event recorded by a detector may contain signals from more than one proton-proton collision and from several bunch crossings. Terminology used in the data analysis The reconstructed location of the proton-proton interaction is referred to as the primary vertex — charged particle tracks reconstructed in the tracking devices converge at this point.

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**Simulation** of a heavy non-Standard-Model Higgs boson produced at 13 TeV

The theoretical foundations of the Standard Model of elementary particles relies on the existence of the Higgs boson, a particle which has been revealed for the first time by the experiments run at the Large Hadron Collider LHC in As the Higgs boson is an unstable particle, its search strategies were based on its decay products. The presence of jets in the final state poses a series of challenges to the experimenter: both from a technical point of view, as jets are complex objects and necessitate of ad-hoc reconstruction techniques, and from an analytical one, as backgrounds with jets are copious at hadron colliders, therefore analyses must obtain high degrees of background rejection in order to achieve competitive sensitivity.

This is accomplished by following two directives: the use of an angular likelihood discriminant, capable of discriminating events likely to originate from the decay of a scalar boson from non-resonant backgrounds, and by using jet parton flavor tagging, selecting jets compatible with quark hadronization and discarding jets more likely to be initiated by gluons. The events passing the selection requirements in 4. The thesis describes the statistical tools and the results of this analysis. This work is a paradigm for studies of the Higgs boson with final states with jets. The non-expert physicists will enjoy a complete and eminently readable description of a proton-proton collider analysis.

At the same time, the expert reader will learn the details of the searches done with jets at CMS. Help Centre. My Wishlist Sign In Join. Be the first to write a review. Add to Wishlist.