Francesco Broggi | Istituto Nazionale di Fisica Nucleare (original) (raw)
Papers by Francesco Broggi
IEEE Transactions on Applied Superconductivity, 2020
Physical Review D - Particles, Fields, Gravitation and Cosmology, 2012
We report a study of final states containing a W boson and hadronic jets, produced in protonproto... more We report a study of final states containing a W boson and hadronic jets, produced in protonproton collisions at a center-of-mass energy of 7 TeV. The data were collected with the ATLAS detector at the CERN LHC and comprise the full 2010 data sample of 36 pb −1. Cross sections are determined using both the electron and muon decay modes of the W boson and are presented as a function of inclusive jet multiplicity, Njet, for up to five jets. At each multiplicity, cross sections are presented as a function of jet transverse momentum, the scalar sum of the transverse momenta of the charged lepton, missing transverse momentum, and all jets, the invariant mass spectra of jets, and the rapidity distributions of various combinations of leptons and final-state jets. The results, corrected for all detector effects and for all backgrounds such as diboson and top quark pair production, are compared with particle-level predictions from perturbative QCD. Leading-order multiparton event generators, normalized to the NNLO total cross section for inclusive W-boson production, describe the data reasonably well for all measured inclusive jet multiplicities. Next-toleading-order calculations from MCFM, studied here for Njet ≤ 2, and BlackHat-Sherpa, studied here for Njet ≤ 4, are found to be mostly in good agreement with the data.
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 2012
The results of a search for pair production of the lighter scalar partners of top quarks (t 1) in... more The results of a search for pair production of the lighter scalar partners of top quarks (t 1) in 2.05 fb −1 of pp collisions at √ s = 7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, µ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking 'natural' scenarios, where the neutralino (χ 0 1) is the next-tolightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV < mχ0 1 < 230 GeV at 95% confidence level, reaching an exclusion of mt 1 < 330 GeV for mχ0 1 = 190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of mχ0 1 > m Z .
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 2013
A search is presented for production of a heavy up-type quark (t ′) together with its antiparticl... more A search is presented for production of a heavy up-type quark (t ′) together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb −1 of pp collisions at √ s = 7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton+jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t ′t′ signal when m t ′ 400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t ′ → W b) = 1, a fourth-generation t ′ quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass ∼ 126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t ′ → W b) versus BR(t ′ → Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.
Journal of High Energy Physics, 2013
A search is presented for new particles decaying to large numbers (7 or more) of jets, with missi... more A search is presented for new particles decaying to large numbers (7 or more) of jets, with missing transverse momentum and no isolated electrons or muons. This analysis uses 20.3 fb−1 of pp collision data at $ \sqrt{s}=8 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. The sensitivity of the search is enhanced by considering the number of b-tagged jets and the scalar sum of masses of large-radius jets in an event. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of various simplified supersymmetry-inspired models where gluinos are pair produced, as well as an mSUGRA/CMSSM model.
Journal of High Energy Physics, 2012
The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 ... more The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 of proton-proton collision data at $ \sqrt {s} = 7\;TeV $ collected by the ATLAS experiment at the Large Hadron Collider, using top quark pair events with a τ lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching ratio of the charged Higgs boson to a τ lepton and a neutrino is 100 %, this leads to upper limits on the branching ratio of top quark decays to abquarkandachargedHiggsbosonbetween5%and1%forchargedHiggsbosonmasses ranging from 90 GeV to 160 GeV, respectively. In the context of the $ m_h^{\max } $ scenario of the MSSM, tan β above 12-26, as well as between 1 and 2-6, can be excluded for charged Higgs boson masses between 90 GeV and 150 GeV.
Journal of High Energy Physics, 2013
A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is p... more A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is performed in pp collisions at $ \sqrt{s}=7\;\mathrm{TeV} $ using 4.7 fb−1 of data collected with the ATLAS experiment at the LHC. In these models, the lightest chargino is predicted to have a lifetime long enough to be detected in the tracking detectors of collider experiments. This analysis explores such models by searching for chargino decays that result in tracks with few associated hits in the outer region of the tracking system. The transverse-momentum spectrum of candidate tracks is found to be consistent with the expectation from the Standard Model background processes and constraints on chargino properties are obtained.
Physics Letters B, 2012
A search is presented for a narrow resonance decaying to a pair of Z bosons using data correspond... more A search is presented for a narrow resonance decaying to a pair of Z bosons using data corresponding to 1.02 fb −1 of integrated luminosity collected by the ATLAS experiment from pp collisions at √ s = 7 TeV. Events containing either four charged leptons () or two charged leptons and two jets (jj) are analyzed and found to be consistent with the Standard Model background expectation. Lower limits on a resonance mass are set using the Randall-Sundrum (RS1) graviton model as a benchmark. Using both and jj events, an RS1 graviton with k/m pl = 0.1 and mass between 325 and 845 GeV is excluded at 95% confidence level. In addition, the events are used to set a modelindependent fiducial cross section limit of σ fid (pp → X → ZZ) < 0.92 pb at 95% confidence level for any new sources of ZZ production with m ZZ greater than 300 GeV.
Physics Letters B, 2012
A search for the Standard Model Higgs boson in the H → W W () → ν ν (= e, μ) decay mode is presen... more A search for the Standard Model Higgs boson in the H → W W () → ν ν (= e, μ) decay mode is presented. The search is performed using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb −1 at a centre-of-mass energy of 7 TeV collected during 2011 with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed. An upper bound is placed on the Higgs boson production cross section as a function of its mass. A Standard Model Higgs boson with mass in the range between 133 GeV and 261 GeV is excluded at 95% confidence level, while the expected exclusion range is from 127 GeV to 233 GeV.
Physics Letters B, 2012
This Letter presents evidence for the associated production of a W boson and a top quark using 2.... more This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb −1 of pp collision data at √ s = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σ Wt = 16.8 ± 2.9 (stat) ± 4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |V tb | = 1.03 +0.16 −0.19 is derived assuming that the Wt production through |V ts | and |V td | is small.
Physics Letters B, 2012
The production of events with multiple high transverse momentum particles including charged lepto... more The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 1.04 fb −1 of proton-proton collision data recorded by the ATLAS detector during the first half of 2011 at √ s = 7 TeV. No excess beyond Standard Model expectations is observed, and upper limits on the fiducial cross sections for non-Standard Model production of these final states are set. Using models for string ball and black hole production and decay, exclusion contours are determined as a function of mass threshold and the fundamental gravity scale.
Physics Letters B, 2012
A measurement is presented of the inclusive cross-section for b-jet production in association wit... more A measurement is presented of the inclusive cross-section for b-jet production in association with a Z boson in pp collisions at a centre-of-mass energy of √ s = 7 TeV. The analysis uses the data sample collected by the ATLAS experiment in 2010, corresponding to an integrated luminosity of approximately 36 pb −1. The event selection requires a Z boson decaying into high p T electrons or muons, and at least one b-jet, identified by its displaced vertex, with transverse momentum p T > 25 GeV and rapidity |y| < 2.1. After subtraction of background processes, the yield is extracted from the vertex mass distribution of the candidate b-jets. The ratio of this cross-section to the inclusive Z cross-section (the average number of b-jets per Z event) is also measured. Both results are found to be in good agreement with perturbative QCD predictions at next-to-leading order.
Physics Letters B, 2011
CERN-PH-EP-2011-090 A search for new phenomena in events featuring a high energy jet and large mi... more CERN-PH-EP-2011-090 A search for new phenomena in events featuring a high energy jet and large missing transverse momentum in proton-proton collisions at √ s = 7 TeV is presented using a dataset corresponding to an integrated luminosity of 33 pb −1 recorded with the ATLAS detector at the Large Hadron Collider. The number of observed events is consistent with the Standard Model prediction. This result is interpreted in terms of limits on a model of Large Extra Dimensions.
Physics Letters B, 2013
This Letter presents a search for high-mass resonances decaying into τ + τ − final states using p... more This Letter presents a search for high-mass resonances decaying into τ + τ − final states using protonproton collisions at √ s = 7 TeV produced by the Large Hadron Collider. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 4.6 fb −1. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Z resonances decaying into τ + τ − pairs as a function of the resonance mass. As a result, Z bosons of the Sequential Standard Model with masses less than 1.40 TeV are excluded at 95% credibility.
Physics Letters B, 2012
A search for a heavy Standard Model Higgs boson decaying via H → Z Z → + − qq, where = e, μ, is p... more A search for a heavy Standard Model Higgs boson decaying via H → Z Z → + − qq, where = e, μ, is presented. The search is performed using a data set of pp collisions at √ s = 7 TeV, corresponding to an integrated luminosity of 1.04 fb −1 collected in 2011 by the ATLAS detector at the CERN LHC collider. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section (relative to that expected from the Standard Model) of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. Within this mass range, there is at present insufficient sensitivity to exclude a Standard Model Higgs boson. For a Higgs boson with a mass of 360 GeV, where the sensitivity is maximal, the observed and expected cross section upper limits are factors of 1.7 and 2.7, respectively, larger than the Standard Model prediction.
Physics Letters B, 2012
A measurement is presented of the cross section for the production of a W boson with one or two j... more A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at √ s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb −1 of data collected with the ATLAS detector at the LHC. The W+b-jet cross section is defined for jets reconstructed with the anti-k t clustering algorithm with transverse momentum above 25 GeV and rapidity within ±2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 ± 1.9 (stat) ± 2.6 (syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.
Physics Letters B, 2012
Lepton + τ A measurement of the cross section of top quark pair production in proton-proton colli... more Lepton + τ A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb −1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σ tt = 186 ± 13 (stat.) ± 20 (syst.) ± 7 (lumi.) pb, is in good agreement with the Standard Model prediction.
Physics Letters B, 2011
A search for squarks and gluinos in final states containing jets, missing transverse momentum and... more A search for squarks and gluinos in final states containing jets, missing transverse momentum and no electrons or muons is presented. The data were recorded by the ATLAS experiment in √ s = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 35 pb −1 of analysed data. Gluino masses below 500 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino. The exclusion increases to 870 GeV for equal mass squarks and gluinos. In MSUGRA/CMSSM models with tan β = 3, A 0 = 0 and μ > 0, squarks and gluinos of equal mass are excluded below 775 GeV. These are the most stringent limits to date.
![Research paper thumbnail of Search for New Phenomena in tt[over ¯] Events with Large Missing Transverse Momentum in Proton-Proton Collisions at sqrt[s]=7 TeV with the ATLAS Detector](https://attachments.academia-assets.com/96700284/thumbnails/1.jpg)
](Physical Review Letters, 2012
A search for new phenomena in t " t events with large missing transverse momentum in proton-proto... more A search for new phenomena in t " t events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1:04 fb À1 of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, toppartner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.
Physical Review Letters, 2011
A search for a heavy standard model Higgs boson decaying via H ! ZZ ! ' þ ' À " , where ' ¼ e, , ... more A search for a heavy standard model Higgs boson decaying via H ! ZZ ! ' þ ' À " , where ' ¼ e, , is presented. It is based on proton-proton collision data at ffiffi ffi s p ¼ 7 TeV, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of 1:04 fb À1. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region 340 < m H < 450 GeV at the 95% confidence level.
IEEE Transactions on Applied Superconductivity, 2020
Physical Review D - Particles, Fields, Gravitation and Cosmology, 2012
We report a study of final states containing a W boson and hadronic jets, produced in protonproto... more We report a study of final states containing a W boson and hadronic jets, produced in protonproton collisions at a center-of-mass energy of 7 TeV. The data were collected with the ATLAS detector at the CERN LHC and comprise the full 2010 data sample of 36 pb −1. Cross sections are determined using both the electron and muon decay modes of the W boson and are presented as a function of inclusive jet multiplicity, Njet, for up to five jets. At each multiplicity, cross sections are presented as a function of jet transverse momentum, the scalar sum of the transverse momenta of the charged lepton, missing transverse momentum, and all jets, the invariant mass spectra of jets, and the rapidity distributions of various combinations of leptons and final-state jets. The results, corrected for all detector effects and for all backgrounds such as diboson and top quark pair production, are compared with particle-level predictions from perturbative QCD. Leading-order multiparton event generators, normalized to the NNLO total cross section for inclusive W-boson production, describe the data reasonably well for all measured inclusive jet multiplicities. Next-toleading-order calculations from MCFM, studied here for Njet ≤ 2, and BlackHat-Sherpa, studied here for Njet ≤ 4, are found to be mostly in good agreement with the data.
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 2012
The results of a search for pair production of the lighter scalar partners of top quarks (t 1) in... more The results of a search for pair production of the lighter scalar partners of top quarks (t 1) in 2.05 fb −1 of pp collisions at √ s = 7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, µ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking 'natural' scenarios, where the neutralino (χ 0 1) is the next-tolightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV < mχ0 1 < 230 GeV at 95% confidence level, reaching an exclusion of mt 1 < 330 GeV for mχ0 1 = 190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of mχ0 1 > m Z .
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics, 2013
A search is presented for production of a heavy up-type quark (t ′) together with its antiparticl... more A search is presented for production of a heavy up-type quark (t ′) together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb −1 of pp collisions at √ s = 7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton+jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t ′t′ signal when m t ′ 400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t ′ → W b) = 1, a fourth-generation t ′ quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass ∼ 126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t ′ → W b) versus BR(t ′ → Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.
Journal of High Energy Physics, 2013
A search is presented for new particles decaying to large numbers (7 or more) of jets, with missi... more A search is presented for new particles decaying to large numbers (7 or more) of jets, with missing transverse momentum and no isolated electrons or muons. This analysis uses 20.3 fb−1 of pp collision data at $ \sqrt{s}=8 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. The sensitivity of the search is enhanced by considering the number of b-tagged jets and the scalar sum of masses of large-radius jets in an event. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of various simplified supersymmetry-inspired models where gluinos are pair produced, as well as an mSUGRA/CMSSM model.
Journal of High Energy Physics, 2012
The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 ... more The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 of proton-proton collision data at $ \sqrt {s} = 7\;TeV $ collected by the ATLAS experiment at the Large Hadron Collider, using top quark pair events with a τ lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching ratio of the charged Higgs boson to a τ lepton and a neutrino is 100 %, this leads to upper limits on the branching ratio of top quark decays to abquarkandachargedHiggsbosonbetween5%and1%forchargedHiggsbosonmasses ranging from 90 GeV to 160 GeV, respectively. In the context of the $ m_h^{\max } $ scenario of the MSSM, tan β above 12-26, as well as between 1 and 2-6, can be excluded for charged Higgs boson masses between 90 GeV and 150 GeV.
Journal of High Energy Physics, 2013
A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is p... more A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is performed in pp collisions at $ \sqrt{s}=7\;\mathrm{TeV} $ using 4.7 fb−1 of data collected with the ATLAS experiment at the LHC. In these models, the lightest chargino is predicted to have a lifetime long enough to be detected in the tracking detectors of collider experiments. This analysis explores such models by searching for chargino decays that result in tracks with few associated hits in the outer region of the tracking system. The transverse-momentum spectrum of candidate tracks is found to be consistent with the expectation from the Standard Model background processes and constraints on chargino properties are obtained.
Physics Letters B, 2012
A search is presented for a narrow resonance decaying to a pair of Z bosons using data correspond... more A search is presented for a narrow resonance decaying to a pair of Z bosons using data corresponding to 1.02 fb −1 of integrated luminosity collected by the ATLAS experiment from pp collisions at √ s = 7 TeV. Events containing either four charged leptons () or two charged leptons and two jets (jj) are analyzed and found to be consistent with the Standard Model background expectation. Lower limits on a resonance mass are set using the Randall-Sundrum (RS1) graviton model as a benchmark. Using both and jj events, an RS1 graviton with k/m pl = 0.1 and mass between 325 and 845 GeV is excluded at 95% confidence level. In addition, the events are used to set a modelindependent fiducial cross section limit of σ fid (pp → X → ZZ) < 0.92 pb at 95% confidence level for any new sources of ZZ production with m ZZ greater than 300 GeV.
Physics Letters B, 2012
A search for the Standard Model Higgs boson in the H → W W () → ν ν (= e, μ) decay mode is presen... more A search for the Standard Model Higgs boson in the H → W W () → ν ν (= e, μ) decay mode is presented. The search is performed using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb −1 at a centre-of-mass energy of 7 TeV collected during 2011 with the ATLAS detector at the Large Hadron Collider. No significant excess of events over the expected background is observed. An upper bound is placed on the Higgs boson production cross section as a function of its mass. A Standard Model Higgs boson with mass in the range between 133 GeV and 261 GeV is excluded at 95% confidence level, while the expected exclusion range is from 127 GeV to 233 GeV.
Physics Letters B, 2012
This Letter presents evidence for the associated production of a W boson and a top quark using 2.... more This Letter presents evidence for the associated production of a W boson and a top quark using 2.05 fb −1 of pp collision data at √ s = 7 TeV accumulated with the ATLAS detector at the LHC. The analysis is based on the selection of the dileptonic final states with events featuring two isolated leptons, electron or muon, with significant transverse missing momentum and at least one jet. An approach based on boosted decision trees has been developed to improve the discrimination of single top-quark Wt events from background. A template fit to the final classifier distributions is performed to determine the cross-section. The result is incompatible with the background-only hypothesis at the 3.3σ level, the expected sensitivity assuming the Standard Model production rate being 3.4σ. The corresponding cross-section is determined and found to be σ Wt = 16.8 ± 2.9 (stat) ± 4.9 (syst) pb, in good agreement with the Standard Model expectation. From this result the CKM matrix element |V tb | = 1.03 +0.16 −0.19 is derived assuming that the Wt production through |V ts | and |V td | is small.
Physics Letters B, 2012
The production of events with multiple high transverse momentum particles including charged lepto... more The production of events with multiple high transverse momentum particles including charged leptons and jets is measured, using 1.04 fb −1 of proton-proton collision data recorded by the ATLAS detector during the first half of 2011 at √ s = 7 TeV. No excess beyond Standard Model expectations is observed, and upper limits on the fiducial cross sections for non-Standard Model production of these final states are set. Using models for string ball and black hole production and decay, exclusion contours are determined as a function of mass threshold and the fundamental gravity scale.
Physics Letters B, 2012
A measurement is presented of the inclusive cross-section for b-jet production in association wit... more A measurement is presented of the inclusive cross-section for b-jet production in association with a Z boson in pp collisions at a centre-of-mass energy of √ s = 7 TeV. The analysis uses the data sample collected by the ATLAS experiment in 2010, corresponding to an integrated luminosity of approximately 36 pb −1. The event selection requires a Z boson decaying into high p T electrons or muons, and at least one b-jet, identified by its displaced vertex, with transverse momentum p T > 25 GeV and rapidity |y| < 2.1. After subtraction of background processes, the yield is extracted from the vertex mass distribution of the candidate b-jets. The ratio of this cross-section to the inclusive Z cross-section (the average number of b-jets per Z event) is also measured. Both results are found to be in good agreement with perturbative QCD predictions at next-to-leading order.
Physics Letters B, 2011
CERN-PH-EP-2011-090 A search for new phenomena in events featuring a high energy jet and large mi... more CERN-PH-EP-2011-090 A search for new phenomena in events featuring a high energy jet and large missing transverse momentum in proton-proton collisions at √ s = 7 TeV is presented using a dataset corresponding to an integrated luminosity of 33 pb −1 recorded with the ATLAS detector at the Large Hadron Collider. The number of observed events is consistent with the Standard Model prediction. This result is interpreted in terms of limits on a model of Large Extra Dimensions.
Physics Letters B, 2013
This Letter presents a search for high-mass resonances decaying into τ + τ − final states using p... more This Letter presents a search for high-mass resonances decaying into τ + τ − final states using protonproton collisions at √ s = 7 TeV produced by the Large Hadron Collider. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 4.6 fb −1. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Z resonances decaying into τ + τ − pairs as a function of the resonance mass. As a result, Z bosons of the Sequential Standard Model with masses less than 1.40 TeV are excluded at 95% credibility.
Physics Letters B, 2012
A search for a heavy Standard Model Higgs boson decaying via H → Z Z → + − qq, where = e, μ, is p... more A search for a heavy Standard Model Higgs boson decaying via H → Z Z → + − qq, where = e, μ, is presented. The search is performed using a data set of pp collisions at √ s = 7 TeV, corresponding to an integrated luminosity of 1.04 fb −1 collected in 2011 by the ATLAS detector at the CERN LHC collider. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section (relative to that expected from the Standard Model) of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. Within this mass range, there is at present insufficient sensitivity to exclude a Standard Model Higgs boson. For a Higgs boson with a mass of 360 GeV, where the sensitivity is maximal, the observed and expected cross section upper limits are factors of 1.7 and 2.7, respectively, larger than the Standard Model prediction.
Physics Letters B, 2012
A measurement is presented of the cross section for the production of a W boson with one or two j... more A measurement is presented of the cross section for the production of a W boson with one or two jets, of which at least one must be a b-jet, in pp collisions at √ s = 7 TeV. Production via top decay is not included in the signal definition. The measurement is based on 35 pb −1 of data collected with the ATLAS detector at the LHC. The W+b-jet cross section is defined for jets reconstructed with the anti-k t clustering algorithm with transverse momentum above 25 GeV and rapidity within ±2.1. The b-jets are identified by reconstructing secondary vertices. The fiducial cross section is measured both for the electron and muon decay channel of the W boson and is found to be 10.2 ± 1.9 (stat) ± 2.6 (syst) pb for one lepton flavour. The results are compared with next-to-leading order QCD calculations, which predict a cross section smaller than, though consistent with, the measured value.
Physics Letters B, 2012
Lepton + τ A measurement of the cross section of top quark pair production in proton-proton colli... more Lepton + τ A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb −1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σ tt = 186 ± 13 (stat.) ± 20 (syst.) ± 7 (lumi.) pb, is in good agreement with the Standard Model prediction.
Physics Letters B, 2011
A search for squarks and gluinos in final states containing jets, missing transverse momentum and... more A search for squarks and gluinos in final states containing jets, missing transverse momentum and no electrons or muons is presented. The data were recorded by the ATLAS experiment in √ s = 7 TeV proton-proton collisions at the Large Hadron Collider. No excess above the Standard Model background expectation was observed in 35 pb −1 of analysed data. Gluino masses below 500 GeV are excluded at the 95% confidence level in simplified models containing only squarks of the first two generations, a gluino octet and a massless neutralino. The exclusion increases to 870 GeV for equal mass squarks and gluinos. In MSUGRA/CMSSM models with tan β = 3, A 0 = 0 and μ > 0, squarks and gluinos of equal mass are excluded below 775 GeV. These are the most stringent limits to date.
Physical Review Letters, 2012
A search for new phenomena in t " t events with large missing transverse momentum in proton-proto... more A search for new phenomena in t " t events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1:04 fb À1 of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, toppartner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.
Physical Review Letters, 2011
A search for a heavy standard model Higgs boson decaying via H ! ZZ ! ' þ ' À " , where ' ¼ e, , ... more A search for a heavy standard model Higgs boson decaying via H ! ZZ ! ' þ ' À " , where ' ¼ e, , is presented. It is based on proton-proton collision data at ffiffi ffi s p ¼ 7 TeV, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of 1:04 fb À1. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region 340 < m H < 450 GeV at the 95% confidence level.