Top PDF Search for Heavy Neutral Resonances in the Dielectron Channel with the CMS Detector at the LHC

Search for Heavy Neutral Resonances in the Dielectron Channel with the CMS Detector at the LHC

Search for Heavy Neutral Resonances in the Dielectron Channel with the CMS Detector at the LHC

Chapter 6 Electron Identification Electron objects are reconstructed in CMS as a combination of a track in the sili- con tracker and a supercluster (cluster of clusters) in the ECAL, as summarized in Chapter 5. Electron reconstruction is designed to incorporate a characteristic sig- nature that electrons leave in the CMS detector. However similar signatures can sometimes be produced by charged hadrons, hadron jets, or photon conversions, as well as secondary electrons. In order to distinguish “real” electrons from such pos- sible contributions a further selection has to be applied to the quantities that carry discriminating power against objects that can mimic electrons.
Show more

173 Read more

Search for heavy resonances with the ATLAS detector

Search for heavy resonances with the ATLAS detector

p T regions. The dominant systematics are due to the fitting uncertainty whereas the most important experimental uncertainty arises due to in situ techniques used for the jet energy scale. Since no evidence for new physics can be observed, frequentistic CLs limits are set on a fiducial cross-section (defined as the part of the total cross-section within kinematic restrictions) 3 as well as on TeV-scale gravity benchmark models using a profile likelihood ratio as test statistic. Figure 8 demonstrates a stronger exclusion for an increasing number of extra dimensions due to larger sig- nal cross-sections, as well as for non-rotating systems because of the increasing number of Hawking emissions. The limits provided are the strongest published ATLAS exclusions and represent, together with those placed by CMS, the most stringent bounds on the considered models.
Show more

10 Read more

Search for Heavy Resonances in Leptonic Final States with CMS

Search for Heavy Resonances in Leptonic Final States with CMS

Abstract. Recent searches are presented for heavy resonances, such as Z 0 , W 0 , and leptoquarks, in leptonic final states. LHC collisions at √ s = 8 TeV are analyzed with the CMS detector. No deviations from Standard Model expectations are observed. Cross section and mass limits are established for a variety of new physics scenarios, which in many cases represent the most stringent limits in existence.

5 Read more

Search for new neutral gauge bosons with the CMS Experiment at the LHC

Search for new neutral gauge bosons with the CMS Experiment at the LHC

The search for new particles is carried out using the CMS detector (Compact Muon Solenoid), one of the four main LHC detectors [14, 15, 17]. It has an overall length of 22 m, a diameter of 15 m, and weighs 14 000 tonnes. The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter, and a brass and scintilla- tor hadron calorimeter, each composed of a barrel and two endcap sections. Muons are detected in gas-ionization chambers embedded in the steel flux-return yoke outside the solenoid, in the pseudora- pidity range | η | < 2.4, with detection planes made using three technologies: drift tubes, cathode strip chambers, and resistive plate chambers. Matching muons to tracks measured in the silicon tracker results in a relative transverse momentum resolution for muons with 20 < p T < 100 GeV of 1.3–2.0%
Show more

5 Read more

Search for heavy resonances, and resonant diboson production with the ATLAS detector

Search for heavy resonances, and resonant diboson production with the ATLAS detector

On behalf of the ATLAS collaboration Abstract. Heavy resonances decaying into a pair of fundamental particles such as jj, ℓ + ℓ − , γγ, and ℓν, are among the most common features to search for phenomena beyond the standard model (SM). Electroweak boson pair production, such as W W or ZZ with subsequent decays to ℓνℓ ′ ν ′ and ℓℓjj respectively, is a powerful test of the spontaneously broken gauge symmetry of the SM and can be also used to search for phenomena beyond the SM. There is a wide spectrum of theoretical models predicting these kinds of resonant signatures. This note covers several searches for these new phenomena conducted within ATLAS in 2011 and 2012 for the LHC 7 and 8 TeV center of mass energies respectively. No significant deviations from the SM have been observed and therefore, limits are set on the characteristic parameters of several new physics models.
Show more

5 Read more

Search for heavy Higgs-like resonances in the H→ZZ→1⁺1⁻qq final state in pp collisions in the CMS experimented at the LHC|

Search for heavy Higgs-like resonances in the H→ZZ→1⁺1⁻qq final state in pp collisions in the CMS experimented at the LHC|

This thesis has first introduced the theoretical background of the mass mechanism, to continue with the description of the LHC and CMS designs. In chapter 6 the studies performed on the DT chambers with the first data from collisions collected by CMS have been presented. DT chambers are one of the responsibilities of the CIEMAT group in the CMS Collaboration, and a key component of the CMS detector. A mapping of all cells has been done and very high efficiency have been observed throughout the DT muon system, with an average efficiency of 97.7% for single hit detection. Resolution of individual cells has also been measured, although the lack of statistics and the chang- ing initial conditions of the CMS setup produced a result less satisfactory than the actual detector behaviour. The impact of background signals in the muon system have been evaluated. A strong and linear dependency have been observed with the instant luminosity of the LHC, and in particular with the beam cross through the detector. A mapping of the detector and the regions suffering more noise have been achieved, and results have been extrapolated to higher luminosity conditions, like the ones reached during 2012 and that are expected when the LHC starts running again. The conclusion (confirmed in 2012) was that the background rates would be under control and would not imply any trouble in the muon reconstruction and the Physics involved.
Show more

197 Read more

Search for High Energy Electrons from New Neutral Massive Gauge Boson Decay in the CMS Detector at the LHC Using Monte Carlo Simulation

Search for High Energy Electrons from New Neutral Massive Gauge Boson Decay in the CMS Detector at the LHC Using Monte Carlo Simulation

ABSTRACT The existence of new heavy neutral massive boson Z′ is a feature of many extensions of Standard Model models as the two-Higgs-doublet model (2HDM), the Hidden Abelian Higgs Model (HAHM), Left-Right Symmetric Model (LRSM), Sequential Standard Model (SSM) and Baryon number minus Lepton number Model (B-L). In the present work we search for two high energy electrons produced from decaying Z B L   heavy neutral massive boson in the events pro- duced in proton-proton collisions at LHC and can be detected by CMS detector. We used the data which is produced from proton-proton collisions by Monte Carlo events generator for different energies at LHC, then we use the angular distribution, invariant mass, combined transverse momentum and combined rapidity distributions for the two high en- ergy electrons produced from Z B L   decay channel to detect the Z B L   signal. B-L extension of the SM model predicts the existence of a Z B L   heavy neutral massive boson at high energies. From our results which we had simulated using MC programs for Z B L   in the B-L extension of standard model, we predict a possible existence of new gauge Z B L   at LHC in the mass range 1 TeV to 1.5 TeV via electrons identification of the two high energy electrons by CMS detector.
Show more

9 Read more

Search for heavy resonances in vector boson fusion

Search for heavy resonances in vector boson fusion

s = 13 TeV recorded by the ATLAS detector at the LHC. No excess above the SM background expectation is observed. First sets of limits are obtained on the production cross section times branching ratio of five types of new resonances with di ff erent spins and isospins using K-matrix unitarization of the vector boson scattering process. The observed cross section limits are found to be in the range of 460−220 fb for resonance masses from 200 to 500 GeV. For an assumed coupling of 2.5 to longitudinal vector bosons, ρ and f particles with a mass below 230 GeV and 300 GeV respectively are excluded at 95% C.L.
Show more

10 Read more

Search for a heavy top quark partner in final states with two leptons with the ATLAS detector at the LHC

Search for a heavy top quark partner in final states with two leptons with the ATLAS detector at the LHC

The fake lepton background consists of semi-leptonic t ¯ t, s-channel and t-channel single top, W +jets and light- and heavy-flavour jet production. The contribution from this back- ground is small (less than 10% of the total background). It is estimated from data with a method similar to that described in refs. [71, 72]. Two types of lepton identification criteria are defined for this evaluation: “tight”, corresponding to the full set of identification cri- teria described above, and “loose” corresponding to preselected electrons and muons. The method counts the number of observed events containing loose-loose, loose-tight, tight- loose and tight-tight lepton pairs in the SR. The probability for real leptons passing the loose selection criteria to also pass the tight selection is measured using a Z → `` sam- ple. The equivalent probability for fake leptons is measured from multijet-enriched control samples. From these probabilities the number of events containing a contribution from one or two fake leptons is calculated.
Show more

36 Read more

Search for Heavy Neutral Higgs Bosons in the tau+tau- Final State in LHC Proton-Proton Collisions at sqrt{s}=13 TeV with the ATLAS Detector

Search for Heavy Neutral Higgs Bosons in the tau+tau- Final State in LHC Proton-Proton Collisions at sqrt{s}=13 TeV with the ATLAS Detector

4.1 Low-Level Objects Most high-level object reconstructions are based on tracks, primary and decay vertices, and calorimeter clusters. To reconstruct the trajectories of charged particles, the Inner Detector hits, caused by energy losses along the flight path of particles, are associated to form tracks. In ATLAS, the tracks of primary charged particles are built using inside-out track finding [145, 146]. This algorithm is seeded using information of the innermost layers of the Pixel detector and then follows the general direction to the outer layers of the Inner Detector to build track candidates. Afterwards, a Kalman filter is applied iteratively to all hits associated to the track candidate and outlier hits are rejected. The amount of misidentified trajectories and tracks with shared hits is reduced using dedicated ambiguity solving algorithms. In a last step the track is extended to the TRT and fitted together with the track from the silicon detectors. The resulting track has five parameters, which consist of the location on a plane, as well as two angles and the curvature of the trajectory. Additionally, outside-in track finding is performed, which is seeded in the TRT and directed towards the silicon detectors. In particular, this recovers tracks from decays of long-lived particles that decay beyond the silicon detectors. Based on track quality criteria like the number of silicon hits, tracks are usually filtered further for the reconstruction of higher level physics objects. The commonly used “loose” track quality selection results in reconstruction efficiencies over 90 %, with dependencies on the pseudorapidity and track transverse momentum, p T
Show more

171 Read more

Search for heavy neutral MSSM Higgs bosons in the di-tau final state with the ATLAS detector

Search for heavy neutral MSSM Higgs bosons in the di-tau final state with the ATLAS detector

1.5 Proton-Proton Collisions In hadron colliders, and in the LHC particularly, the colliding particles are protons which are not elementary but composite states consisting of quarks and gluons, collectively called partons. This makes the initial state rather complicated and all of the pp collision events are collective interactions of multiple partons mostly inter- acting at relatively low energies. But every now and then there is a hard scattering event in which two initial partons, carrying relatively high fraction of proton’s en- ergy and momentum, interact with each other. During such hard collisions the partons undergo high momentum transfer which means that the interaction time is rather short and therefore the initial state partons are behaving as if they are free particles. The interaction strength at high momentum transfer is relatively small due to asymptotic freedom, and cross sections of such collisions can be calculated in perturbative QCD. It is impossible to know which partons have undergone the interaction and with what initial energy thus only a probabilistic description of partons inside the proton can be obtained. The probability to find a parton of a certain flavor and energy is described with parton distribution functions (PDF). It is not strictly a distribution of probability but rather a distribution of an average number of each type of parton that carries a fraction of the proton’s total four- momentum. The reason why the number of partons in a proton is not constant is because the proton is a very active and dynamical place where virtual gluons, quarks and anti-quarks are perpetually popping in the existence and subsequently disappearing. The virtual quarks inside a proton are also called sea-quarks, in con- trast to the three valence quarks which make up the proton and are always present.
Show more

183 Read more

Multi channel search for squarks and gluinos in √s=7 TeV pp collisions with the ATLAS detector at the LHC

Multi channel search for squarks and gluinos in √s=7 TeV pp collisions with the ATLAS detector at the LHC

The systematic effects included as nuisance parameters in the fit are: the jet energy scale and resolution uncertainties; b-tagging uncertainties; uncertainty on the MC simulation modelling of the JVF; the additional cross-section uncer- tainty on the production of heavy flavour in association with a vector boson; the uncertainties on trigger efficiency and matching and reconstruction efficiency; a systematic uncer- tainty on the re-weighting of the W -boson p T ; uncertainties on the missing transverse momentum pile-up dependence and the calibration of energy not associated with an object in the event; the matrix method statistical and systematic un- certainties; the charge flip systematic uncertainties; the dibo- son W W shape systematic uncertainty taken from compar- ing H ERWIG to A LPGEN . Where the systematic uncertain- ties affect object definitions, corrections are propagated to the missing transverse momentum and razor variable calcu- lations. The effects of other uncertainties on the final results are negligible. These uncertainties affect the signal yield and shape in the signal regions, as well as the allowed variation in signal-region background estimates after the control re- gion constraints. In most signal regions, the jet energy scale uncertainty is the dominant experimental uncertainty (from 10 % to 25 %).
Show more

34 Read more

Search for neutral MSSM Higgs bosons with CMS at LHC: a comparison between a cut-based analysis and a Machine Learning approach

Search for neutral MSSM Higgs bosons with CMS at LHC: a comparison between a cut-based analysis and a Machine Learning approach

In the following sections, the LHC experiments are briefly introduced;for a major focus on the CMS experiment see sec. 1.2 . A.1 ALICE ALICE [24, 25] is a detector specialized in heavy ions collisions. It is designed to study the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma forms. At these conditions, similar to those just after the Big Bang, quark confinement no longer applies: studying the quark-gluon plasma as it expands and cools allows to gain insight on the origin of the Universe. Some ALICE specifications are illustrated in Table A.1 .
Show more

67 Read more

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at √s=8 TeV with the ATLAS detector

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at √s=8 TeV with the ATLAS detector

Acknowledgments We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Aus- tralia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia;
Show more

39 Read more

Search for heavy resonances decaying to a Z boson and a photon in pp collisions at root s=13 TeV with the ATLAS detector

Search for heavy resonances decaying to a Z boson and a photon in pp collisions at root s=13 TeV with the ATLAS detector

Acknowledgements We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated e ffi ciently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF and DNSRC, Denmark; IN2P3-CNRS, CEA- DSM/IRFU, France; GNSF, Georgia; BMBF, HGF, and MPG, Germany; GSRT, Greece; RGC, Hong Kong SAR, China; ISF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan;
Show more

36 Read more

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at s=8 TeV with the ATLAS detector

Search for heavy lepton resonances decaying to a Z boson and a lepton in pp collisions at s=8 TeV with the ATLAS detector

Acknowledgments We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Aus- tralia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia;
Show more

38 Read more

Analysis of Standard Model Higgs Boson Decays to Tau Pairs with the CMS Detector at the LHC

Analysis of Standard Model Higgs Boson Decays to Tau Pairs with the CMS Detector at the LHC

s = 7 TeV and 19.7 fb −1 at 8 TeV, respectively, has been analysed. In the scope of all di-τ decay modes of the H → τ τ analysis, the same-flavour fully-leptonic channels µµ and ee are particularly challenging. Compared to other channels, the H → τ τ → µµ channel suffers from the huge additional background from Z → µµ events, which accounts for more than 99 % of all background events after the inclusive event selection. In addition, the four neutrinos in the final state lead to a reduced di-τ mass resolution. Multivariate methods are necessary to sufficiently suppress the backgrounds and to bring the sensitivity of this channel to a level that is comparable to the eµ channel, which gains from an intrinsic suppression of the Z → `` background. In the combination of all fully leptonic di-τ states (eµ, µµ, ee), the two same-flavour channels contribute 20 % to the sensitivity expressed in terms of expected upper limits on the Higgs boson production cross section. Both same- flavour channels follow the signal extraction method that has been developed in the µµ channel in the scope of this thesis. The sensitivity of the search was improved by 18 % at the Higgs boson mass hypothesis of 125 GeV with respect to the preliminary analysis.
Show more

169 Read more

Search for Stop Pair Production at the LHC using the CMS Detector

Search for Stop Pair Production at the LHC using the CMS Detector

s = 8 TeV is performed in a sample with a single isolated electron or muon, jets, large missing transverse energy and large transverse mass. The data sample used corresponds to an integrated luminosity of 19.5 fb −1 collected by the CMS experiment. No significant excess in data is observed above the expectation from standard model processes. In the context of supersymmetric models with pair production of top squarks decaying to either a top quark and the lightest supersymmetric particle (LSP) or a bottom quark and a chargino that subsequently decays to a W boson and the LSP, the results probe top squarks with masses up to about 650 GeV for LSP masses up to about 250 GeV.
Show more

5 Read more

Search for heavy resonances in the W/Z-tagged dijet mass spectrum at CMS

Search for heavy resonances in the W/Z-tagged dijet mass spectrum at CMS

• Scaling of parton luminosity with collision energy gives huge increase in discovery potential for the energy increase.. - For 2 TeV Bulk Graviton, increasing 
 energy by factor less [r]

17 Read more

Search for R-parity Violating Supersymmetry in the Dielectron Channel

Search for R-parity Violating Supersymmetry in the Dielectron Channel

In conclusion, we have searched for events containing at least two electrons and four or more jets. Such events would be characteristic of processes involving the pair production of SUSY particles with the decay of the LSP through a R-parity-violating coupling. Finding no excess of events beyond the prediction of the standard model, we interpret this result within the mSUGRA framework as an excluded region in the 共 m 0 , m 1 兾 2 兲 plane for fixed values of A 0 and sign of m and for several values of tanb. This is the first result reported from Tevatron on a search for R-parity-violating SUSY involving several l 0 couplings in the mSUGRA framework. The Tevatron will continue to provide unique opportunities for search- ing for R-parity-violating SUSY in a larger range of pa- rameter space with the improved data anticipated from the next run [12].
Show more

6 Read more

Show all 10000 documents...