998 resultados para ATLAS, particle physics, SM, ZZ, aTGC
Resumo:
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb1 of s=8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT>120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between EmissT>150 GeV and EmissT>700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with large extra spatial dimensions, pair production of weakly interacting dark matter candidates, and production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presented.
Resumo:
The ATLAS detector at the Large Hadron Collider at CERN is used to search for the decay of a scalar boson to a pair of long-lived particles, neutral under the Standard Model gauge group, in 20.3 fb1 of data collected in proton--proton collisions at s = 8 TeV. This search is sensitive to long-lived particles that decay to Standard Model particles producing jets at the outer edge of the ATLAS electromagnetic calorimeter or inside the hadronic calorimeter. No significant excess of events is observed. Limits are reported on the product of the scalar boson production cross section times branching ratio into long-lived neutral particles as a function of the proper lifetime of the particles. Limits are reported for boson masses from 100 GeV to 900 GeV, and a long-lived neutral particle mass from 10 GeV to 150 GeV.
Resumo:
This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy s = 8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb1. An uncertainty on the offline reconstructed tau energy scale of 2% to 4%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5% for hadronically decaying tau leptons with one associated track, and of 4% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2% to 8%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton--proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.
Resumo:
The paper presents studies of Bose--Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range pT> 100 MeV and ||< 2.5 in proton--proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 b1, 190 b1 and 12.4 nb1 for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.
Resumo:
A search is performed for narrow resonances decaying into WW, WZ, or ZZ boson pairs using 20.3 fb1 of proton--proton collision data at a centre-of-mass energy of s = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted W or Z boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the WZ channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the WZ final state of a new heavy gauge boson, W, and for the WW and ZZ final states of Kaluza--Klein excitations of the graviton in a bulk Randall--Sundrum model, as a function of the resonance mass. W bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level.
Resumo:
The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb1 of pp collision data at s=7 TeV and 20.3 fb1 at s=8 TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the , ZZ, W W , Z, bb, , and decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the and ZZ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of m A > 370 GeV in the hMSSM simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with W/Z (Z , W/Z jj) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.
Resumo:
A summary of the constraints from the ATLAS experiment on R-parity-conserving supersymmetry is presented. Results from 22 separate ATLAS searches are considered, each based on analysis of up to 20.3 fb1 of proton-proton collision data at centre-of-mass energies of s=7 and 8 TeV at the Large Hadron Collider. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, in which the lightest supersymmetric particle is a neutralino, taking into account constraints from previous precision electroweak and flavour measurements as well as from dark matter related measurements. The results are presented in terms of constraints on supersymmetric particle masses and are compared to limits from simplified models. The impact of ATLAS searches on parameters such as the dark matter relic density, the couplings of the observed Higgs boson, and the degree of electroweak fine-tuning is also shown. Spectra for surviving supersymmetry model points with low fine-tunings are presented.
Resumo:
High transverse momentum jets produced in pp collisions at a centre of mass energy of 7 TeV are used to measure the transverse energy--energy correlation function and its associated azimuthal asymmetry. The data were recorded with the ATLAS detector at the LHC in the year 2011 and correspond to an integrated luminosity of 158 pb1. The selection criteria demand the average transverse momentum of the two leading jets in an event to be larger than 250 GeV. The data at detector level are well described by Monte Carlo event generators. They are unfolded to the particle level and compared with theoretical calculations at next-to-leading-order accuracy. The agreement between data and theory is good and provides a precision test of perturbative Quantum Chromodynamics at large momentum transfers. From this comparison, the strong coupling constant given at the Z boson mass is determined to be s(mZ)=0.11730.0010 (exp.) +0.00650.0026 (theo.).
Resumo:
Studies of the spin, parity and tensor couplings of the Higgs boson in the HZZ4 , HWWe and H decay processes at the LHC are presented. The investigations are based on 25 fb1 of pp collision data collected by the ATLAS experiment at s=7 TeV and s=8 TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers JP=0+, is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9% confidence level. Using the HZZ4 and HWWe decays, the tensor structure of the HVV interaction in the spin-0 hypothesis is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.
Resumo:
Les dtecteurs ATLAS-MPX sont des dtecteurs Medipix2-USB recouverts de convertisseurs de uorure de lithium et de polythylne pour augmenter lefcacit de dtection des neutrons lents et des neutrons rapides respectivement. Un rseau de quinze dtecteurs ATLAS-MPX a t mis en opration dans le dtecteur ATLAS au LHC du CERN. Deux dtecteurs ATLAS-MPX de rfrence ont t exposs des sources de neutrons rapides 252 Cf et 241 AmBe ainsi quaux neutrons rapides produits par la raction 7Li(p, xn) pour ltude de la rponse du dtecteur ces neutrons. Les neutrons rapides sont principalement dtects partir des protons de recul des collisions lastiques entre les neutrons et lhydrogne dans le polythylne. Des ractions nuclaires entre les neutrons et le silicium produisent des particules-. Une tude de lefcacit de reconnaissance des traces des protons et des particules- dans le dtecteur Medipix2-USB a t faite en fonction de lnergie cintique incidente et de langle dincidence. Lefcacit de dtection des neutrons rapides a t value deux seuils dnergie (8 keV et 230 keV) dans les dtecteurs ATLAS-MPX. Lefcacit de dtection des neutrons rapides dans la rgion du dtecteur couverte avec le polythylne augmente en fonction de lnergie des neutrons : (0.0346 0.0004) %, (0.0862 0.0018) % et (0.1044 0.0026) % pour des neutrons rapides de 2.13 MeV, 4.08 MeV et 27 MeV respectivement. Ltude pour dterminer lnergie des neutrons permet donc destimer le ux des neutrons quand le dtecteur ATLAS-MPX est dans un champ de radiation inconnu comme cest le cas dans le dtecteur ATLAS au LHC.
Resumo:
Les seize dtecteurs MPX constituant le rseau ATLAS-MPX ont t placs diffrentes positions dans le dtecteur ATLAS et sa averne au CERN dans le but de mesurer en emps rel les champs de radiation produits ar des particules primaires (protons des faisceaux) et des particules secondaires (kaons, pions, g, protons) issues des collisions proton-proton. Des films de polythylne (PE) et de fluorure de lithium (6LiF) recouvrent les dtecteurs afin daugmenter leur sensibilit aux neutrons produits par les particules primaires et secondaires interagissant avec les matriaux prsents dans lenvironnement dATLAS. La reconnaissance des traces laisses par les particules dans un dtecteur ATLAS-MPX se fait partir des algorithmes du logiciel MAFalda (Medipix Analysis Framework) bas sur les librairies et le logiciel danalyse de donnes ROOT. Une tude sur le taux didentifications errones et le chevauchement damas a t faite en reconstruisant les activits des sources 106Ru et 137Cs. Lefficacit de dtection des neutrons rapides a t mesure laide des sources 252Cf et 241AmBe (neutrons dnergie moyenne de 2.13 et 4.08 MeV respectivement). La moyenne des efficacits de dtection mesures pour les neutrons produits par les sources 252C f et 241AmBe a t calcule pour les convertisseurs 6LiF et PE et donnent (0.8580 0.1490)% et (0.0254 0.0031)% pour LiF et (0.0510 0.0061)% et (0.0591 0.0063)% pour PE bas et haut seuil dnergie respectivement. Une simulation du calcul de lefficacit de dtection des neutrons dans le dtecteur MPX a t ralise avec le logiciel GEANT4. Des donnes MPX correspondant aux collisions proton-proton 2.4 TeV et 7 TeV dans le centre de masse ont t analyses. Les flux dtects dlectrons et de photons sont particulirement levs dans les dtecteurs MPX01 et MPX14 car ils sont plus prs du point de collision. Des flux de neutrons ont t estims en utilisant les efficacits de dtection mesures. Une corrlation avec la luminosit du LHC a t tablie et on prdit que pour les collisions 14 TeV dans le centre de masse et avec une luminosit de 10^34 cm-1*s-1 il y aura environ 5.1x10^8 1.5x10^7 et 1.6x10^9 6.3x10^7 particules dtectes par les dtecteurs MPX01 et MPX14 respectivement.
Resumo:
Les collisions proton-proton produites par le LHC imposent un environnement radiatif hostile au dtecteur ATLAS. Afin de quantifier les effets de cet environnement sur la performance du dtecteur et la scurit du personnel, plusieurs simulations Monte Carlo ont t ralises. Toutefois, la mesure directe est indispensable pour suivre les taux de radiation dans ATLAS et aussi pour vrifier les prdictions des simulations. cette fin, seize dtecteurs ATLAS-MPX ont t installs diffrents endroits dans les zones exprimentale et technique d'ATLAS. Ils sont composs d'un dtecteur au silicium pixels appel MPX dont la surface active est partiellement recouverte de convertisseurs de neutrons thermiques, lents et rapides. Les dtecteurs ATLAS-MPX mesurent en temps rel les champs de radiation en enregistrant les traces des particules dtectes sous forme d'images matricielles. L'analyse des images acquises permet d'identifier les types des particules dtectes partir des formes de leurs traces. Dans ce but, un logiciel de reconnaissance de formes appel MAFalda a t conu. tant donn que les traces des particules fortement ionisantes sont influences par le partage de charge entre pixels adjacents, un modle semi-empirique dcrivant cet effet a t dvelopp. Grce ce modle, l'nergie des particules fortement ionisantes peut tre estime partir de la taille de leurs traces. Les convertisseurs de neutrons qui couvrent chaque dtecteur ATLAS-MPX forment six rgions diffrentes. L'efficacit de chaque rgion dtecter les neutrons thermiques, lents et rapides a t dtermine par des mesures d'talonnage avec des sources connues. L'tude de la rponse des dtecteurs ATLAS-MPX la radiation produite par les collisions frontales de protons 7TeV dans le centre de masse a montr que le nombre de traces enregistres est proportionnel la luminosit du LHC. Ce rsultat permet d'utiliser les dtecteurs ATLAS-MPX comme moniteurs de luminosit. La mthode propose pour mesurer et talonner la luminosit absolue avec ces dtecteurs est celle de van der Meer qui est base sur les paramtres des faisceaux du LHC. Vu la corrlation entre la rponse des dtecteurs ATLAS-MPX et la luminosit, les taux de radiation mesurs sont exprims en termes de fluences de diffrents types de particules par unit de luminosit intgre. Un cart significatif a t obtenu en comparant ces fluences avec celles prdites par GCALOR qui est l'une des simulations Monte Carlo du dtecteur ATLAS. Par ailleurs, les mesures effectues aprs l'arrt des collisions proton-proton ont montr que les dtecteurs ATLAS-MPX permettent d'observer la dsintgration des isotopes radioactifs gnrs au cours des collisions. L'activation rsiduelle des matriaux d'ATLAS peut tre mesure avec ces dtecteurs grce un talonnage en quivalent de dose ambiant.
Resumo:
We present a study of ee and events using 1109 (1009) pb-1 of data in the electron (muon) channel, respectively. These data were collected with the D detector at the Fermilab Tevatron p over(p, ) collider at sqrt(s) = 1.96 TeV. Having observed 453 (515) candidates in the ee () final state, we measure the Z production cross section for a photon with transverse energy ET > 7 GeV, separation between the photon and leptons R > 0.7, and invariant mass of the di-lepton pair M > 30 GeV / c2, to be 4.96 0.30 (stat . + syst .) 0.30 (lumi .) pb, in agreement with the Standard Model prediction of 4.74 0.22 pb. This is the most precise Z cross section measurement at a hadron collider. We set limits on anomalous trilinear Z and ZZ gauge boson couplings of - 0.085 < h30 < 0.084, - 0.0053 < h40 < 0.0054 and - 0.083 < h30 Z < 0.082, - 0.0053 < h40 Z < 0.0054 at the 95% C.L. for the form-factor scale = 1.2 TeV.
Resumo:
One of the main targets of the CMS experiment is to search for the Standard Model Higgs boson. The 4-lepton channel (from the Higgs decay h->ZZ->4l, l = e,mu) 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 4-lepton analysis at CMS is presented, spanning on its most important aspects: lepton identification, variables of isolation, impact parameter, kinematics, event selection, background control and statistical analysis of results. The search leads to an evidence for a signal presence with a statistical significance of more than four standard deviations. The excess of data, with respect to the background-only predictions, indicates the presence of a new boson, with a mass of about 126 GeV/c2 , decaying to two Z bosons, whose characteristics are compatible with the SM Higgs ones.
Resumo:
The main work of this thesis concerns the measurement of the production cross section using LHC 2011 data collected at a center-of-mass energy equal to 7 TeV by the ATLAS detector and resulting in a total integrated luminosity of 4.6 inverse fb. The ZZ total cross section is finally compared with the NLO prediction calculated with modern Monte Carlo generators. In addition, the three differential distributions ((l,l), ZpT and M4l) are shown unfolded back to the underlying distributions using a Bayesian iterative algorithm. Finally, the transverse momentum of the leading Z is used to provide limits on anoumalus triple gauge couplings forbidden in the Standard Model.