Experiment: Search for the Higgs Boson

Top PDF Experiment: Search for the Higgs Boson:

Higgs Boson search at the CMS experiment

Higgs Boson search at the CMS experiment

in this channel would appear as a small and narrow peak above a large and smooth prompt di-photon background. A search is performed for a Higgs boson decaying into two photons. The analysis is done using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of- mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 fb −1 . To improve the sensitivity of the search, selected diphoton events are subdivided into classes according to indicators of mass resolution and signal-to-background ratio. Five mutually exclusive event classes are defined as shown in Figure 1: four in terms of the pseudorapidity and the shower shapes of the photons, and a fifth class into which are put all events containing a pair of jets passing selection requirements which are designed to select Higgs bosons produced by the vector boson fusion process. Two photon classifiers are used: the minimum value of a variable R 9 of the two photons, R min 9 , and the maximum
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Ruckert, Benjamin
  

(2009):


	Search for a Higgs Boson Produced in Association with a W Boson at ATLAS.


Dissertation, LMU München: Fakultät für Physik

Ruckert, Benjamin (2009): Search for a Higgs Boson Produced in Association with a W Boson at ATLAS. Dissertation, LMU München: Fakultät für Physik

Modern experiments in particle physics have to cope with huge data rates. For example, in the last century bubble chambers were widely used for particle identi- fication: the tracks of particles have been photographed and then analysed. With experiments getting larger, the data taking procedure became not only more so- phisticated but also more complex. The ATLAS experiment will face data of several Petabytes per year, which has to be stored and processed. The recon- struction of the raw physics events is also very time consuming, i.e. on modern CPUs the reconstruction of one single event takes approximately 15 minutes. Clearly, at event rates of 1 GHz, it is impossible to consider every event. Still, after triggering and selecting only interesting events, a rate of 200 Hz remains (this has been explained in chapter 4.2). At design luminosity 10 9 events will be
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Beyond Standard Model Higgs boson physics with the ATLAS experiment at the LHC

Beyond Standard Model Higgs boson physics with the ATLAS experiment at the LHC

neutral Higgs boson, h, and W bosons: H → W − H + → W − W + h, see Fig. 1(a). The light neutral Higgs boson decays to bb and it is assumed to have mass 125 GeV. The first of the W bosons decays leptonically and provides a way to trigger the events whereas the second one decays hadronically maximizing the branching ratio. The final state shares the same topology with tt events with similar W boson decays, which is the main background process for the search. A multivariate discriminant is employed to exploit the kinematic differences between signal and tt events. Upper limits on the production cross section of heavy neutral Higgs boson, H , times the branching ratio BR(H → W ∓ H ± → W ∓ W ± h → W ∓ W ± bb) are derived as a function of its mass and the charged scalar mass, as shown in Fig. 1(b).
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Search for the Higgs boson produced in association with top quarks

Search for the Higgs boson produced in association with top quarks

During the three years of data taking, the CMS experiment [1] has performed extremely well, collect- ing an integrated luminosity of ∼ 6 fb −1 , at a center of mass energy of 7 TeV and more than 20 f b −1 at an energy of 8 TeV. With this amount of data, CMS has discovered a new boson, compatible with the Standard Model Higgs boson, with ∼125 GeV mass [2][3].

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Search for the SM Higgs Boson in Di-τ Final States at CMS

Search for the SM Higgs Boson in Di-τ Final States at CMS

On July 4, 2012, the discovery of a new boson, with mass around 125 GeV and with properties compatible with those of a standard-model Higgs boson, was announced at CERN by the ATLAS and CMS collaboration [1, 2]. The reported excess is most significant in the SM Higgs searches using the decay modes into γγ and ZZ. The re- sults in the ττ decay mode showed no excess of observed events in the mass range near 125 GeV, still compatible with both, a downward fluctuation from a background- only or background plus SM Higgs boson hypothesis. In this document a search for the SM Higgs boson is reported using final states with pairs of τ leptons in proton-proton collisions at √ s =7 and 8 TeV at the LHC using the data that have been collected in 2011 and 2012 correspond- ing to an integrated luminosity of 17 fb − 1 recorded by the CMS experiment. This luminosity splits in 4.9 fb − 1 of data taken at 7 TeV center-of-mass energy and 12.1 fb − 1 at 8
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Setting limits and application to Higgs boson search

Setting limits and application to Higgs boson search

incorrect confidence level (i.e.: coverage). In some cases, experiment searching for a rare signal make the chose, while quoting their result, to switch from a central interval to an upper limit depending on the outcome of the measurement. A typical choice is to quote an upper limit if the significance of the observed signal is smaller than 3σ, and a central value otherwise. This problem is sometimes referred to in literature as flip-flopping, and can be illustrated in a simple example. Imagine a model where a random variable x obeys a Gaussian distribution with a fixed and known r.m.s., for simplicity we can take σ = 1, and an unknown average μ which is bound to be greater or equal to zero (this is the case of a signal yield). The quoted central value must always be greater than or equal to zero, given the assumed constraint. Assume we decide to quote zero if the significance is less than 3σ:
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Search for the Standard Model Higgs boson at CMS in the 4-lepton channel

Search for the Standard Model Higgs boson at CMS in the 4-lepton channel

Abstract. One of the main targets of the CMS experiment is to search for the Standard Model Higgs boson. The 4-lepton channel (from the decay H → ZZ → 4l, l = e, μ) is one of the most promising. The analysis is based on the identification of two opposite-sign, same-flavor lepton pairs: leptons are required to be isolated and to come from the same primary vertex. The Higgs would be statistically revealed by the presence of a resonance peak in the 4-lepton invariant mass distribution. The Higgs mass is a free parameter of the Standard Model, and the 4-lepton channel search is sensitive almost in the entire mass range. With data collected in 2010 and 2011 (4.7 fb − 1 at √ s = 7 TeV) the Higgs boson
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Search for a Standard Model Higgs Boson Decaying to Two Photons in CMS at the LHC

Search for a Standard Model Higgs Boson Decaying to Two Photons in CMS at the LHC

In the Standard Model, there are three generations of leptons and quarks, each of which consists of four particles and their antiparticles. Table 1.1 lists the names and symbols of particles in each generation. All leptons and quarks have spin 1/2, and thus belong to the fermions. Each particle in a generation has a larger mass than the corresponding one of lower generations. The charged particles in the first generation are stable, and all other charged particle in the second and third generations decay. All generations of neutrinos do not decay. Each particle has its own corresponding antiparticle. All antiparticles, with the exception of neutrinos, have the opposite electric charge to its corresponding particle, while all other quantum numbers are identical. The unique feature of quarks is that they carry color charge, in analogy with electric charge. Therefore they interact with each other via gluons that mediate the strong interaction. Quarks also carry electric charge and weak isospin, thus they also interact with other fermions via both the electromagnetic and the weak interaction. The neutrinos carry neither color charge nor electric charge, thus they only interact with other particles by weak interactions. This unique feature makes neutrinos very difficult to be detected directly. In a collider experiment, the presence of neutrino is inferred from an apparent imbalance of energies measured in the detector, resulting in “missing energy”.
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Search for dark matter produced in association with a Higgs boson decaying to two bottom quarks in pp collisions at root s=8 TeV with the ATLAS detector

Search for dark matter produced in association with a Higgs boson decaying to two bottom quarks in pp collisions at root s=8 TeV with the ATLAS detector

Two approaches are commonly used to model generic processes yielding a final state with a particle X recoiling against a system of noninteracting particles. One option is to use nonrenormalizable operators in an effective field theory (EFT) framework [17], where particles that mediate the interactions between DM and SM particles are too heavy to be produced directly in the experiment and are described by contact operators. Alternatively, simplified models that are characterized by a minimal number of renormalizable interactions, and hence explicitly include the particles at higher masses, can be used [18]. The EFT approach is more model independent, but is not valid when a typical momentum transfer of the process approaches the energy scale of the contact operators that describe the interaction. Simplified models do not suffer from these concerns, but include more assumptions by design and are therefore less generic. The two approaches are thus complementary and both are included in this analysis.
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Discovery of the Higgs Boson, Measurements of its Production, and a Search for Higgs Boson Pair Production

Discovery of the Higgs Boson, Measurements of its Production, and a Search for Higgs Boson Pair Production

The LHC itself may be divided into eight arcs, each of which has a long straight section of 528 m and two bending regions at either end. Each straight region serves as an insertion point either for an experiment or for a beam utility. Adjacent to the ATLAS [66] experiment are experimental sites for LHCb [67] and ALICE [68], while CMS [69] is installed on the opposite side of the ring. Of the four remaining insertion points, two are taken up by collimators for the beam, one is reserved for the beam dump, and the last holds the radio frequency (RF) acceleration cavities. The total energy achievable by the LHC is limited by its circumference and the field of its bending magnets. The LHC contains 1232 bending dipoles with nominal field 8.33 T, for √ s = 14 TeV. Quadrupole, sextupole, and octopoles are used to focus the beam and reduce aberrations. It takes around 20 minutes to ramp the beam energy from the 450 GeV injection energy to the full energy. Due to persistent concerns over the catastrophic magnet failure mentioned in Chapter 1, the LHC was operated at √ s = 7 TeV in 2011 and √ s = 8 TeV in 2012, instead of at its design energy of √ s = 14 TeV.
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A Search For Nothing: Dark Matter And Invisible Decays Of The Higgs Boson At The Atlas Detector

A Search For Nothing: Dark Matter And Invisible Decays Of The Higgs Boson At The Atlas Detector

running on a large (tens of thousands of cores) processor farm rather than FPGAs and has more information to work with. This allows it to run much more complex algorithms, but it still has finite time and resources. It needs to keep up with the maximum L1 output rate of 100 kHz, and each event typically takes one HLT core 200-300 ms to process, depending on the pileup conditions. Ultimately, about 1 kHz of events are written to permanent storage via the “main physics” stream (i.e. that which is used for analysis). The HLT also sends a small number of events to other output streams for technical purposes such as calibration measurements and debugging abnormal events. To save resources, not all of the information for each event is always saved for these secondary streams. This results in much smaller event sizes, allowing for much higher trigger rates in these cases. In some cases, physics analyses are able to make use of even this limited information, such as ATLAS’s trigger-level dijet resonance search [67].
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Final results of the searches for neutral Higgs bosons in e+e- collisions at sqrt(s) up to 209GeV

Final results of the searches for neutral Higgs bosons in e+e- collisions at sqrt(s) up to 209GeV

the beam axis, in a sample of events from randomly-triggered beam crossings. Only clusters with E > 3 GeV are shown. The candidate event b is indicated by the cross in the upper-left corner. The plane of the collider is defined by φ = 0 and π. (b) The distribution of R for the events selected by the cut-based four-jet search, for the expected SM background with (shaded histogram) and without (hatched histogram) contamination from beam-related background, and for the data (dots with error bars).

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The search for the Standard Model Higgs boson produced in association with a vector boson and decaying to bottom quarks with the ATLAS detector

The search for the Standard Model Higgs boson produced in association with a vector boson and decaying to bottom quarks with the ATLAS detector

or muons, targeting the three Higgs boson decay channels mentioned above. The 0-lepton channel is most sensitive to ZH production, with a small although not insignificant contribution from W H production. The sensitivity of the 1-lepton channel is dominated by W H production with a small contribution from ZH, whereas the 2-lepton chan- nel is only sensitive to ZH production. The channels are split into further categories, depending on the vector boson transverse momentum, and the number of jets.

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Performance of the LHC, ATLAS and CMS in 2011

Performance of the LHC, ATLAS and CMS in 2011

Efficient B-tagging is a key to top physics, to some Higgs channels,... Already commissioned with 2010 data, “ad- vanced tagging methods” were validated with the 2011 data set. Among them ATLAS uses a combination of the track impact parameter in 3D (IP3D) and of a fit of secondary vertices (SV1). At 60% efficiency, this combined approach has a rejection 4 times larger than the early “SV0” algo- rithm [12]. Fig. 14 illustrates this performance by showing the fraction of jets satisfying the b-tagging cut at 60% ef- ficiency, compared to Monte-Carlo simulation. The agree- ment is satisfactory, and shows that, around 100 GeV, 50% of the events passing the cut are genuine b-jets while 60% of the remaining ones are actually charmed jets.
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Search for the standard model Higgs boson decaying to two photons with CMS at the LHC

Search for the standard model Higgs boson decaying to two photons with CMS at the LHC

The success of the search of the Higgs boson in the two- photon channel decay relies on four main points: high invariant-mass resolution, vital to distinguish a tiny peak on a huge background; e ff ective photon identification, essential to separate prompt photons from photons sec- ondary to neutral meson decays produced in jets; event categorization, e ff ective with probing di ff erent signal-to- background or di ff erent mass resolution signal categories; and finally the background modeling, which after photon identification is mostly ( ∼ 70%) irreducible, due to gen- uine isolated two-photon QCD events. The inter-channel calibration of the electromagnetic calorimeter (ECAL) as well as the corrections for the crystal transparency loss due to radiation, are the fundamental components to in- sure good energy resolution. Energy corrections are fur- ther applied to clustered electromagnetic energy in order to insure complete shower and converted photon contain- ment as well as to correct the dependence on the pile-up event rate. The energy corrections are derived with a mul- tivariate energy regression trained on simulated Higgs bo- DOI: 10.1051 /
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Higgs Boson Physics at CMS

Higgs Boson Physics at CMS

H → γγ [4] represents one of the Higgs boson discovery channels, characterized by high resolution with a very small uncertainty on di-photon mass, a clean final state, and a small branching ratio. Boosted decision tree is used for the photon identification, selection of di-photon vertices, and se- lection of di-photon events. Boosted Decision Tree (BDT) distribution has been flattened according to the total γγ signal [4], which is composed of the four Higgs boson production mode components. In this report, the Fig. 5 (left) of [4] has been presented. The distribution of the transformed score of the di-photon multivariate (BDT) classifier for events with two photons satisfying the preselection requirements in data has been compared to the distribution of the simulated signal and simulated back- ground stacked together [4]. The correspondence between the data distribution and the distribution of the total signal plus background stack has been achieved. The score of the BDT classifier is used to select and divide the events into four ”untagged” categories according to the di-photon mass resolution and predicted signal over background ratio. The category of the events with the lowest BDT score has been discarded from the analysis. The signal is selected according to the highest transverse energy photons and the background is composed of the irreducible γγ component, and the reducible fakes that originate from γ+jet and di-jet events. In Fig. 1 (left), a di-photon mass distribution is presented, for the total signal and background (upper row in Fig. 1, left) and for the background component subtracted (bottom row in Fig. 1, left). The best fit mass is found at m H = 125.4 GeV with statistical uncertainty of approximately 0.15 GeV and the systematic uncertainties preliminarily estimated to be between 0.2 GeV and 0.3 GeV (still under study) [4]. In Fig. 1 (right), the results of the signal strength measurements in the H → γγ are presented. The signal strength measurement for the four categories combined provide the value of 1.16 +0.15
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On The Nature Of The Higgs Boson

On The Nature Of The Higgs Boson

The spin-2 gravifakeon is necessary to make the quantization of gravity consistent. In other sectors of high-energy physics, like the standard model in flat space, as well as its extensions, there might be no need of fake particles. However, if one fakeon exists in nature, it might not be the only one. Are there any other fakeons, maybe in the realm of the standard model? In this paper we provide enough arguments to exclude this possibility for most particles, but the cases of the Higgs boson and a few other particles remain unresolved.

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Search for Higgs and Z Boson Decays to phi gamma with the ATLAS Detector

Search for Higgs and Z Boson Decays to phi gamma with the ATLAS Detector

Rare decays of the 125 GeV Higgs boson [1, 2] H to a light meson and a photon γ have been suggested to present one viable probe of the Yukawa coupling of the Higgs boson to light (u, d, s) quarks [3–5]. While the Standard Model (SM) predicts these couplings to be small, substantial modifications are pre- dicted in several scenarios beyond the SM, which include the Minimal Flavor Violation framework [6], the Froggatt-Nielsen mechanism [7], the Higgs-dependent Yukawa couplings model [8], the Randall- Sundrum family of models [9], and the possibility of the Higgs boson being a composite pseudo-Goldstone boson [10]. The light-quark Yukawa couplings are almost entirely unconstrained by existing data and the large multijet background at the Large Hadron Collider (LHC) severely inhibits the study of such coup- lings with inclusive H → q q ¯ decays. The decay of the Higgs boson to a φ meson and a photon would give access to the strange-quark Yukawa coupling and to potential deviations from the SM prediction. The expected SM branching fraction is B (H → φ γ) = (2.3 ± 0.1) × 10 −6 [4], and no direct experimental information about this decay mode currently exists. The analogous rare decays of the Higgs boson to a heavy quarkonium state and a photon o ff er sensitivity to the charm- and bottom-quark Yukawa coup- lings [11–13]. The Higgs boson decays to J/ψ γ and Υ γ have already been searched for by the ATLAS collaboration [14]. The former decay mode has also been searched for by the CMS collaboration [15]. The corresponding decay of the Z boson has also been considered from a theoretical perspective [16, 17], as it offers a precision test of the SM and the predictions of the factorization approach in quantum chromodynamics [17]. Owing to the large Z boson production cross section at the LHC, rare Z boson decays can be probed at branching fractions much smaller than for Higgs boson decays to the same final state. The most precise prediction for the SM branching fraction is B (Z → φ γ) = (1.17±0.08)×10 −8 [16]. The decay Z → φ γ has not yet been observed and is not well constrained by existing measurements of Z boson decays.
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Measurement of properties of the Higgs boson in the diphoton channel using the ATLAS detector

Measurement of properties of the Higgs boson in the diphoton channel using the ATLAS detector

Abstract. Results for the properties of the Higgs boson measured in the diphoton decay channel by the ATLAS experiment at the LHC are presented, which include coupling, spin, and differential cross section measurements. Searches for t tH ¯ production and Flavor Changing Neutral Current t → cH decay in this channel are also briefly discussed. The results are based on either the entire data sample collected with the ATLAS detector in 2011 at √ s = 7 TeV and 2012 at √ s = 8 TeV, or only the entire 8 TeV data sample. Within the experimental and theoretical uncertainties, no significant deviation from the Standard Model expectation is observed.
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Beyond the discovery: Higgs results from CMS

Beyond the discovery: Higgs results from CMS

The observation of a Higgs boson at a mass of about 125 GeV, in the following referred to as H(125), by the ATLAS and CMS experiments [1–3] has been a fundamental step forward for particle physics. Since this discovery, further Higgs research focuses on two essential questions. The first concerns the detailed properties of the new state, and whether they are precisely in accord with the expectations of the Standard Model (SM). The second question is whether there are additional Higgs bosons. Both kinds of studies have the potential to reveal physics beyond the SM. A huge number of new results have been obtained by the CMS collaboration, and selected highlights are discussed in this article.
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