Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s... more Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s NN = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 µb −1 . The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v 2 to v 6 , are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T -averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-byevent fluctuating initial conditions is also presented. Submitted to the Journal of High Energy Physics c 2013 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license arXiv:1312.1845v1 [nucl-ex]
Calibration of the relative response of the individual channels of the barrel electromagnetic cal... more Calibration of the relative response of the individual channels of the barrel electromagnetic calorimeter of the CMS detector was accomplished, before installation, with cosmic ray muons and test beams. One fourth of the calorimeter was exposed to a beam of high energy electrons and the relative calibration of the channels, the intercalibration, was found to be reproducible to a precision of about 0.3%. Additionally, data were collected with cosmic rays for the entire ECAL barrel during the commissioning phase. By comparing the intercalibration constants obtained with the electron beam data with those from the cosmic ray data, it is demonstrated that the latter provide an intercalibration precision of 1.5% over most of the barrel ECAL. The best intercalibration precision is expected to come from the analysis of events collected in situ during the LHC operation. Using data collected with both electrons and pion beams, several aspects of the intercalibration procedures based on electrons or neutral pions were investigated.
Ensuring the radiation hardness of PbWO 4 crystals was one of the main priorities during the cons... more Ensuring the radiation hardness of PbWO 4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered.
Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s... more Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s NN = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 µb −1 . The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v 2 to v 6 , are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T -averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-byevent fluctuating initial conditions is also presented. Submitted to the Journal of High Energy Physics c 2013 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license arXiv:1312.1845v1 [nucl-ex]
Measurement of four-jet production in proton-proton collisions at root s=7 TeV
Physical Review D, 2014
ABSTRACT Measurements of the differential cross sections for the production of exactly four jets ... more ABSTRACT Measurements of the differential cross sections for the production of exactly four jets in proton-proton collisions are presented as a function of the transverse momentum p[subscript T] and pseudorapidity η, together with the correlations in azimuthal angle and the p[subscript T] balance among the jets. The data sample was collected in 2010 at a center-of-mass energy of 7 TeV with the CMS detector at the LHC, with an integrated luminosity of 36 pb[superscript −1]. The cross section for exactly four jets, with two hard jets of p[subscript T] > 50 GeV each, together with two jets of p[subscript T] > 20 GeV each, within |η| < 4.7 is measured to be σ = 330 ± 5(stat.) ± 45(syst.) nb. It is found that fixed-order matrix element calculations including parton showers describe the measured differential cross sections in some regions of phase space only, and that adding contributions from double parton scattering brings the Monte Carlo predictions closer to the data.
Measurement of the production cross sections for a Z boson and one or more b jets in pp collision... more Measurement of the production cross sections for a Z boson and one or more b jets in pp collisions at √ s = 7 TeV
A search is performed for an excess of events, over the standard model expectations, with a photo... more A search is performed for an excess of events, over the standard model expectations, with a photon, a lepton, and large missing transverse energy in pp collisions at √ s = 7 TeV. Such events are expected in many new physics models, in particular a supersymmetric theory that is broken via a gauge-mediated mechanism, when the lightest charged and neutral gauginos are mass degenerate. The data sample used in this search corresponds to an integrated luminosity of 35 pb −1 collected with the CMS detector at the LHC. No evidence of such an excess above the standard model backgrounds, dominated by Wγ production, is found. The results are presented as 95% confidence level upper limits on the cross section for a benchmark gauge-mediated scenario, and are then converted into exclusion limits on the squark, gluino, and wino masses.
Search for resonances in the dilepton mass distribution in pp collisions at $ \sqrt {s} = 7 $ TeV
Journal of High Energy Physics, 2011
A search for narrow resonances at high mass in the dimuon and dielectron channels has been perfor... more A search for narrow resonances at high mass in the dimuon and dielectron channels has been performed by the CMS experiment at the CERN LHC, using pp collision data recorded at \( \sqrt {s} = 7 \) TeV. The event samples correspond to integrated luminosities of 40 pb−1 in the dimuon channel and 35 pb−1 in the dielectron channel. Heavy dilepton resonances are predicted in theoretical models with extra gauge bosons (Z′) or as Kaluza-Klein graviton excitations (GKK) in the Randall-Sundrum model. Upper limits on the inclusive cross section of Z′(GKK) → ℓ + ℓ − relative to Z → ℓ + ℓ − are presented. These limits exclude at 95% confidence level a Z′ with standard-model-like couplings below 1140GeV, the superstring-inspired Z ψ ′ below 887 GeV, and, for values of the coupling parameter \( {{k} \left/ {{{{\overline M }_{\text{Pl}}}}} \right.} \) of 0.05 (0.1), Kaluza-Klein gravitons below 855 (1079) GeV.
The W + W − and ZZ production cross sections are measured in proton-proton collisions at √ s = 8 ... more The W + W − and ZZ production cross sections are measured in proton-proton collisions at √ s = 8 TeV with the CMS experiment at the LHC in data samples corresponding to an integrated luminosity of up to 5.3 fb −1 . The measurements are performed in the leptonic decay modes W + W − → ν ν and ZZ → 2 2 , where = e, µ and ( ) = e, µ, τ. The measured cross sections σ(pp → W + W − ) = 69.9 ± 2.8 (stat.) ± 5.6 (syst.) ± 3.1 (lum.) pb and σ(pp → ZZ) = 8.4 ± 1.0 (stat.) ± 0.7 (syst.) ± 0.4 (lum.) pb, for both Z bosons produced in the mass region 60 < m Z < 120 GeV, are consistent with standard model predictions. These are the first measurements of the diboson production cross sections at √ s = 8 TeV.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2002
Tests of a prototype for the Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid Expe... more Tests of a prototype for the Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid Experiment (CMS) at the Large Hadron Collider are described. The basic unit for the endcap ECAL in CMS is a "supercrystal" of 25 lead tungstate crystals. Results are presented from tests of the first full-sized supercrystal in electron beams and in a 3T magnetic field. The supercrystal was exposed to electron beams with energies from 25-180GeV. An energy resolution (σ E /E) of (0.48±0.01)% was measured at 180GeV.
Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s... more Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s NN = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 µb −1 . The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v 2 to v 6 , are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T -averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-byevent fluctuating initial conditions is also presented. Submitted to the Journal of High Energy Physics c 2013 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license arXiv:1312.1845v1 [nucl-ex]
Calibration of the relative response of the individual channels of the barrel electromagnetic cal... more Calibration of the relative response of the individual channels of the barrel electromagnetic calorimeter of the CMS detector was accomplished, before installation, with cosmic ray muons and test beams. One fourth of the calorimeter was exposed to a beam of high energy electrons and the relative calibration of the channels, the intercalibration, was found to be reproducible to a precision of about 0.3%. Additionally, data were collected with cosmic rays for the entire ECAL barrel during the commissioning phase. By comparing the intercalibration constants obtained with the electron beam data with those from the cosmic ray data, it is demonstrated that the latter provide an intercalibration precision of 1.5% over most of the barrel ECAL. The best intercalibration precision is expected to come from the analysis of events collected in situ during the LHC operation. Using data collected with both electrons and pion beams, several aspects of the intercalibration procedures based on electrons or neutral pions were investigated.
Ensuring the radiation hardness of PbWO 4 crystals was one of the main priorities during the cons... more Ensuring the radiation hardness of PbWO 4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered.
Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s... more Azimuthal dihadron correlations of charged particles have been measured in PbPb collisions at √ s NN = 2.76 TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 µb −1 . The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v 2 to v 6 , are extracted as a function of particle transverse momentum. At higher transverse momentum, the v 2 harmonic becomes significantly smaller than the higher-order v n (n ≥ 3). The p T -averaged v 2 and v 3 are found to be equal within 2%, while higher-order v n decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-byevent fluctuating initial conditions is also presented. Submitted to the Journal of High Energy Physics c 2013 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license arXiv:1312.1845v1 [nucl-ex]
Measurement of four-jet production in proton-proton collisions at root s=7 TeV
Physical Review D, 2014
ABSTRACT Measurements of the differential cross sections for the production of exactly four jets ... more ABSTRACT Measurements of the differential cross sections for the production of exactly four jets in proton-proton collisions are presented as a function of the transverse momentum p[subscript T] and pseudorapidity η, together with the correlations in azimuthal angle and the p[subscript T] balance among the jets. The data sample was collected in 2010 at a center-of-mass energy of 7 TeV with the CMS detector at the LHC, with an integrated luminosity of 36 pb[superscript −1]. The cross section for exactly four jets, with two hard jets of p[subscript T] &amp;amp;amp;amp;amp;gt; 50 GeV each, together with two jets of p[subscript T] &amp;amp;amp;amp;amp;gt; 20 GeV each, within |η| &amp;amp;amp;amp;amp;lt; 4.7 is measured to be σ = 330 ± 5(stat.) ± 45(syst.) nb. It is found that fixed-order matrix element calculations including parton showers describe the measured differential cross sections in some regions of phase space only, and that adding contributions from double parton scattering brings the Monte Carlo predictions closer to the data.
Measurement of the production cross sections for a Z boson and one or more b jets in pp collision... more Measurement of the production cross sections for a Z boson and one or more b jets in pp collisions at √ s = 7 TeV
A search is performed for an excess of events, over the standard model expectations, with a photo... more A search is performed for an excess of events, over the standard model expectations, with a photon, a lepton, and large missing transverse energy in pp collisions at √ s = 7 TeV. Such events are expected in many new physics models, in particular a supersymmetric theory that is broken via a gauge-mediated mechanism, when the lightest charged and neutral gauginos are mass degenerate. The data sample used in this search corresponds to an integrated luminosity of 35 pb −1 collected with the CMS detector at the LHC. No evidence of such an excess above the standard model backgrounds, dominated by Wγ production, is found. The results are presented as 95% confidence level upper limits on the cross section for a benchmark gauge-mediated scenario, and are then converted into exclusion limits on the squark, gluino, and wino masses.
Search for resonances in the dilepton mass distribution in pp collisions at $ \sqrt {s} = 7 $ TeV
Journal of High Energy Physics, 2011
A search for narrow resonances at high mass in the dimuon and dielectron channels has been perfor... more A search for narrow resonances at high mass in the dimuon and dielectron channels has been performed by the CMS experiment at the CERN LHC, using pp collision data recorded at \( \sqrt {s} = 7 \) TeV. The event samples correspond to integrated luminosities of 40 pb−1 in the dimuon channel and 35 pb−1 in the dielectron channel. Heavy dilepton resonances are predicted in theoretical models with extra gauge bosons (Z′) or as Kaluza-Klein graviton excitations (GKK) in the Randall-Sundrum model. Upper limits on the inclusive cross section of Z′(GKK) → ℓ + ℓ − relative to Z → ℓ + ℓ − are presented. These limits exclude at 95% confidence level a Z′ with standard-model-like couplings below 1140GeV, the superstring-inspired Z ψ ′ below 887 GeV, and, for values of the coupling parameter \( {{k} \left/ {{{{\overline M }_{\text{Pl}}}}} \right.} \) of 0.05 (0.1), Kaluza-Klein gravitons below 855 (1079) GeV.
The W + W − and ZZ production cross sections are measured in proton-proton collisions at √ s = 8 ... more The W + W − and ZZ production cross sections are measured in proton-proton collisions at √ s = 8 TeV with the CMS experiment at the LHC in data samples corresponding to an integrated luminosity of up to 5.3 fb −1 . The measurements are performed in the leptonic decay modes W + W − → ν ν and ZZ → 2 2 , where = e, µ and ( ) = e, µ, τ. The measured cross sections σ(pp → W + W − ) = 69.9 ± 2.8 (stat.) ± 5.6 (syst.) ± 3.1 (lum.) pb and σ(pp → ZZ) = 8.4 ± 1.0 (stat.) ± 0.7 (syst.) ± 0.4 (lum.) pb, for both Z bosons produced in the mass region 60 < m Z < 120 GeV, are consistent with standard model predictions. These are the first measurements of the diboson production cross sections at √ s = 8 TeV.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2002
Tests of a prototype for the Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid Expe... more Tests of a prototype for the Electromagnetic Calorimeter (ECAL) of the Compact Muon Solenoid Experiment (CMS) at the Large Hadron Collider are described. The basic unit for the endcap ECAL in CMS is a "supercrystal" of 25 lead tungstate crystals. Results are presented from tests of the first full-sized supercrystal in electron beams and in a 3T magnetic field. The supercrystal was exposed to electron beams with energies from 25-180GeV. An energy resolution (σ E /E) of (0.48±0.01)% was measured at 180GeV.
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