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  • 1
    Language: English
    In: Journal of High Energy Physics, 2015, Vol.2015(11), pp.1-33
    Description: We present a measurement of inclusive J/ ψ production in p-Pb collisions at s N N = 5.02 $$ \sqrt{s_{\mathrm{NN}}}=5.02 $$ TeV as a function of the centrality of the collision, as estimated from the energy deposited in the Zero Degree Calorimeters. The measurement is performed with the ALICE detector down to zero transverse momentum, p T , in the backward (−4 . 46 〈 y cms 〈 −2 . 96) and forward (2 . 03 〈 y cms 〈 3 . 53) rapidity intervals in the dimuon decay channel and in the mid-rapidity region (−1 . 37 〈 y cms 〈 0 . 43) in the dielectron decay channel. The backward and forward rapidity intervals correspond to the Pb-going and p-going direction, respectively. The p T -differential J /ψ production cross section at backward and forward rapidity is measured for several centrality classes, together with the corresponding average p T and p T2 values. The nuclear modification factor is presented as a function of centrality for the three rapidity intervals, and as a function of p T for several centrality classes at backward and forward rapidity. At mid- and forward rapidity, the J /ψ yield is suppressed up to 40% compared to that in pp interactions scaled by the number of binary collisions. The degree of suppression increases towards central p-Pb collisions at forward rapidity, and with decreasing p T of the J /ψ . At backward rapidity, the nuclear modification factor is compatible with unity within the total uncertainties, with an increasing trend from peripheral to central p-Pb collisions.
    Keywords: Quark gluon plasma ; QCD ; Heavy Ions
    ISSN: 11266708
    E-ISSN: 1029-8479
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  • 2
    Language: English
    In: The European Physical Journal C, 2015, Vol.75(5), pp.1-23
    Description: The measurement of primary $$\pi ^{\pm }$$ π ± , $$K^{\pm }$$ K ± , $$p$$ p and $${\overline{{p}}}$$ p ¯ production at mid-rapidity ( $$|y| 〈$$ | y | 〈 0.5) in proton–proton collisions at $$\sqrt{s}$$ s $$=$$ = 7 TeV performed with a large ion collider experiment at the large hadron collider (LHC) is reported. Particle identification is performed using the specific ionisation energy-loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/ $$c$$ c for pions, from 0.2 up to 6 GeV/ $$c$$ c for kaons and from 0.3 up to 6 GeV/ $$c$$ c for protons. The measured spectra and particle ratios are compared with quantum chromodynamics-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Furthermore, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.
    Keywords: Regular Article - Experimental Physics;
    ISSN: 1434-6044
    E-ISSN: 1434-6052
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  • 3
    Language: English
    In: Journal of High Energy Physics, 2015, Vol.2015(11), pp.1-24
    Description: The nuclear modification factor, R AA , of the prompt charmed mesons D 0 , D + and D ∗+ , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy s N N = 2.76 $$ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=2.76 $$ TeV in two transverse momentum intervals, 5 〈 p T 〈 8 GeV /c and 8 〈 p T 〈 16 GeV /c , and in six collision centrality classes. The R AA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the R AA of non-prompt J /ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.
    Keywords: Charm physics ; Heavy Ions ; Heavy-ion collision
    ISSN: 11266708
    E-ISSN: 1029-8479
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  • 4
    Language: English
    In: Journal of Instrumentation, 2017, Vol.12(01), p.P01020
    Description: This paper describes the CMS trigger system and its performance during Run 1 of the LHC. The trigger system consists of two levels designed to select events of potential physics interest from a GHz (MHz) interaction rate of proton-proton (heavy ion) collisions. The first level of the trigger is implemented in hardware, and selects events containing detector signals consistent with an electron, photon, muon, lepton, jet, or missing transverse energy. A programmable menu of up to 128 object-based algorithms is used to select events for subsequent processing. The trigger thresholds are adjusted to the LHC instantaneous luminosity during data taking in order to restrict the output rate to 100 kHz, the upper limit imposed by the CMS readout electronics. The second level, implemented in software, further refines the purity of the output stream, selecting an average rate of 400 Hz for offline event storage. The objectives, strategy and performance of the trigger system during the LHC Run 1 are described.
    Keywords: Physics - Instrumentation And Detectors ; High Energy Physics - Experiment;
    E-ISSN: 1748-0221
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  • 5
    Language: English
    In: The European Physical Journal C, 2015, Vol.75(5), pp.1-25
    Description: Results are presented from a search for particle dark matter (DM), extra dimensions, and unparticles using events containing a jet and an imbalance in transverse momentum. The data were collected by the CMS detector in proton–proton collisions at the LHC and correspond to an integrated luminosity of 19.7 $$\,\text {fb}^\text {-1}$$ fb -1 at a centre-of-mass energy of 8 $$\,{\mathrm{TeV}}\,$$ TeV . The number of observed events is found to be consistent with the standard model prediction. Limits are placed on the DM-nucleon scattering cross section as a function of the DM particle mass for spin-dependent and spin-independent interactions. Limits are also placed on the scale parameter $$M_\mathrm {D}$$ M D in the Arkani-Hamed, Dimopoulos, and Dvali (ADD) model of large extra dimensions, and on the unparticle model parameter $${{\Lambda _\mathrm{U}}} $$ Λ U . The constraints on ADD models and unparticles are the most stringent limits in this channel and those on the DM-nucleon scattering cross section are an improvement over previous collider results.
    Keywords: Regular Article - Experimental Physics;
    ISBN: 10 0520153451
    ISSN: 1434-6044
    E-ISSN: 1434-6052
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  • 6
    Language: English
    In: Journal of High Energy Physics, 2014, Vol.2014(8), pp.1-37
    Description: A search is reported for massive resonances decaying into a quark and a vector boson (W or Z), or two vector bosons (WW, WZ, or ZZ). The analysis is performed on an inclusive sample of multijet events corresponding to an integrated luminosity of 19.7 fb −1 , collected in proton-proton collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC. The search uses novel jet-substructure identification techniques that provide sensitivity to the presence of highly boosted vector bosons decaying into a pair of quarks. Exclusion limits are set at a confidence level of 95% on the production of: (i) excited quark resonances q * decaying to qW and qZ for masses less than 3.2 TeV and 2.9 TeV, respectively, (ii) a Randall-Sundrum graviton G RS decaying into WW for masses below 1.2 TeV, and (iii) a heavy partner of the W boson W ′ decaying into WZ for masses less than 1.7 TeV. For the first time mass limits are set on W ′ → WZ and G RS → WW in the all-jets final state. The mass limits on q * → qW, q * → qZ, W ′ → WZ, G RS → WW are the most stringent to date. A model with a “bulk” graviton G bulk that decays into WW or ZZ bosons is also studied.
    Keywords: Jet substructure ; Jets ; Jet physics ; Hadron-Hadron Scattering ; Particle and resonance production
    ISBN: 0003417633000
    E-ISSN: 1029-8479
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  • 7
    Language: English
    In: Journal of High Energy Physics, 2015, Vol.2015(1), pp.1-43
    Description: A search is presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. Disappearing tracks are identified as those with little or no associated calorimeter energy deposits and with missing hits in the outer layers of the tracker. The search uses proton-proton collision data recorded at s = 8 $$ \sqrt{s}=8 $$ TeV that corresponds to an integrated luminosity of 19.5 fb −1 . The results of the search are interpreted in the context of the anomaly-mediated supersymmetry breaking (AMSB) model. The number of observed events is in agreement with the background expectation, and limits are set on the cross section of direct electroweak chargino production in terms of the chargino mass and mean proper lifetime. At 95% confidence level, AMSB models with a chargino mass less than 260 GeV, corresponding to a mean proper lifetime of 0.2 ns, are excluded.
    Keywords: Hadron-Hadron Scattering ; Supersymmetry ; Exotics
    ISSN: 15507998
    E-ISSN: 1029-8479
    E-ISSN: 15502368
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  • 8
    Language: English
    In: Journal of High Energy Physics, 2014, Vol.2014(1), pp.1-86
    Description: A search for the standard model Higgs boson decaying to a W-boson pair at the LHC is reported. The event sample corresponds to an integrated luminosity of 4.9 fb −1 and 19.4 fb −1 collected with the CMS detector in pp collisions at $ \sqrt{s} $ = 7 and 8 TeV, respectively. The Higgs boson candidates are selected in events with two or three charged leptons. An excess of events above background is observed, consistent with the expectation from the standard model Higgs boson with a mass of around 125 GeV. The probability to observe an excess equal or larger than the one seen, under the background-only hypothesis, corresponds to a significance of 4.3 standard deviations for m H = 125.6 GeV. The observed signal cross section times the branching fraction to WW for m H = 125.6 GeV is $ 0.72_{-0.18}^{+0.20 } $ times the standard model expectation. The spin-parity J P = 0 + hypothesis is favored against a narrow resonance with J P = 2 + or J P = 0 − that decays to a W-boson pair. This result provides strong evidence for a Higgs-like boson decaying to a W-boson pair.
    Keywords: Hadron-Hadron Scattering ; Higgs physics
    E-ISSN: 1029-8479
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  • 9
    Language: English
    In: Journal of High Energy Physics, 2014, Vol.2014(2), pp.1-63
    Description: Measurements are presented of the associated production of a W boson and a charm-quark jet (W + c) in pp collisions at a center-of-mass energy of 7 TeV. The analysis is conducted with a data sample corresponding to a total integrated luminosity of 5 fb −1 , collected by the CMS detector at the LHC. W boson candidates are identified by their decay into a charged lepton (muon or electron) and a neutrino. The W + c measurements are performed for charm-quark jets in the kinematic region $ p_{\mathrm{T}}^{\mathrm{jet}} $ 〉 25 GeV, |η jet | 〈 2 . 5, for two different thresholds for the transverse momentum of the lepton from the W-boson decay, and in the pseudorapidity range |η ℓ | 〈 2 . 1. Hadronic and inclusive semileptonic decays of charm hadrons are used to measure the following total cross sections: σ(pp → W + c + X) × $ \mathcal{B} $ (W → ℓν) = 107 . 7 ± 3 . 3 (stat . ) ± 6 . 9 (syst . ) pb ( $ p_{\mathrm{T}}^{\ell } $ 〉 25 GeV) and σ(pp → W + c + X)× $ \mathcal{B} $ (W → ℓν) = 84 . 1 ± 2 . 0 (stat . ) ± 4 . 9 (syst . ) pb ( $ p_{\mathrm{T}}^{\ell } $ 〉 35 GeV), and the cross section ratios σ(pp → W + + $ \overline{c} $ + X) / σ(pp → W − + c + X) = 0 . 954 ± 0 . 025 (stat . ) ± 0 . 004 (syst . ) ( $ p_{\mathrm{T}}^{\ell } $ 〉 25 GeV) and σ(pp → W + + $ \overline{\mathrm{c}} $ + X) / σ(pp → W − + c + X) = 0 . 938 ± 0 . 019 (stat . ) ± 0 . 006 (syst . ) ( $ p_{\mathrm{T}}^{\ell } $ 〉 35 GeV). Cross sections and cross section ratios are also measured differentially with respect to the absolute value of the pseudorapidity of the lepton from the W-boson decay. These are the first measurements from the LHC directly sensitive to the strange quark and antiquark content of the proton. Results are compared with theoretical predictions and are consistent with the predictions based on global fits of parton distribution functions.
    Keywords: Hadron-Hadron Scattering
    E-ISSN: 1029-8479
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  • 10
    Language: English
    In: The European Physical Journal C, 2014, Vol.74(9), pp.1-26
    Description: A measurement of the cross section for the production of top quark–antiquark pairs ( $${\mathrm {t}}\overline{{\mathrm {t}}}$$ t t ¯ ) in association with a vector boson V (W or Z) in proton-proton collisions at $$\sqrt{s} = 8$$ s = 8 $$\,\text {TeV}$$ TeV is presented. The results are based on a dataset corresponding to an integrated luminosity of 19.5 fb $$^{-1}$$ - 1 recorded with the CMS detector at the LHC. The measurement is performed in three leptonic (e and $$\upmu $$ μ ) channels: a same-sign dilepton analysis targeting $${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W} $$ t t ¯ W events, and trilepton and four-lepton analyses designed for $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}} $$ t t ¯ Z events. In the same-sign dilepton channel, the $${\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W} $$ t t ¯ W cross section is measured as $$\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} \mathrm {W}} = 170 ^{+90}_{-80}\,\text {(stat)} \pm 70\,\text {(syst)} \, \text {fb} $$ σ t t ¯ W = 170 - 80 + 90 (stat) ± 70 (syst) fb , corresponding to a significance of 1.6 standard deviations over the background-only hypothesis. Combining the trilepton and four-lepton channels, a direct measurement of the $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}} $$ t t ¯ Z cross section, $$\sigma _{{\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {Z}}} = 200 ^{+80}_{-70}\,\text {(stat)} ^{+40}_{-30}\,\text {(syst)} \mathrm{fb}^{-1} $$ σ t t ¯ Z = 200 - 70 + 80 (stat) - 30 + 40 (syst) fb - 1 , is obtained with a significance of 3.1 standard deviations. The measured cross sections are compatible with standard model predictions within their experimental uncertainties. The inclusive $${\mathrm {t}}\overline{{\mathrm {t}}} {\mathrm {V}} $$ t t ¯ V process is observed with a significance of 3.7 standard deviations from the combination of all three leptonic channels.
    Keywords: Regular Article - Experimental Physics;
    ISSN: 1434-6044
    E-ISSN: 1434-6052
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