Papers by Jean Carlos Ramirez Mendez
Search for supersymmetry in hadronic final states using M T2 in pp collisions at √ s = 7 TeV
Results from the first study of isolated-photon+jet correlations in relativistic heavy ion collis... more Results from the first study of isolated-photon+jet correlations in relativistic heavy ion collisions are reported. The analysis uses data from PbPb collisions at a centre-ofmass energy of 2.76 TeV per nucleon pair corresponding to an integrated luminosity of 150 µb −1 recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum p γ T > 60 GeV/c and an associated jet with p Jet T > 30 GeV/c, the photon+jet p T imbalance is studied as a function of collision centrality and compared to pp data and PYTHIA calculations at the same collision energy. Using the p γ T of the isolated photon as an estimate of the momentum of the associated parton at production, this measurement allows a characterisation of the in-medium parton energy loss. For more central PbPb collisions, a significant decrease in the ratio p Jet T /p γ T relative to that in the PYTHIA reference is observed. Furthermore, significantly more p
A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two sa... more A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two same-sign charged leptons of the same flavour and two jets. The data correspond to an integrated luminosity of 4.98 fb −1 of pp collisions at a centre-ofmass energy of 7 TeV collected with the CMS detector at the Large Hadron Collider. No excess of events is observed beyond the expected standard model background and therefore upper limits are set on the square of the mixing parameter, |V N | 2 , for = e, µ, as a function of heavy Majorana-neutrino mass. These are the first direct upper limits on the heavy Majorana-neutrino mixing for m N > 90 GeV.
The results of a search for pair production of a heavy, top-like quark, t , in the decay mode t t... more The results of a search for pair production of a heavy, top-like quark, t , in the decay mode t t → bW + bW − → b + ν b − ν are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0 fb −1 in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t t production is found. Upper limits on the production cross section as a function of t mass are presented, and t masses below 557 GeV/c 2 are excluded at the 95% confidence level.
A search is presented for physics beyond the standard model (BSM) in final states with a pair of ... more A search is presented for physics beyond the standard model (BSM) in final states with a pair of opposite-sign isolated leptons accompanied by jets and missing transverse energy. The search uses LHC data recorded at a center-of-mass energy √ s = 7 TeV with the CMS detector, corresponding to an integrated luminosity of approximately 5 fb −1 . Two complementary search strategies are employed. The first probes models with a specific dilepton production mechanism that leads to a characteristic kinematic edge in the dilepton mass distribution. The second strategy probes models of dilepton production with heavy, colored objects that decay to final states including invisible particles, leading to very large hadronic activity and missing transverse energy. No evidence for an event yield in excess of the standard model expectations is found. Upper limits on the BSM contributions to the signal regions are deduced from the results, which are used to exclude a region of the parameter space of the constrained minimal supersymmetric extension of the standard model. Additional information related to detector efficiencies and response is provided to allow testing specific models of BSM physics not considered in this paper. T and hadronic energy, while the counting experiments do not as-2 3 Event Selection sume a specific dilepton production mechanism and are also sensitive to BSM scenarios that produce lepton pairs with uncorrelated flavor.
The first measurement of the cross section for top-quark pair production in pp collisions at the ... more The first measurement of the cross section for top-quark pair production in pp collisions at the LHC at center-of-mass energy √ s = 7 TeV has been performed using 3.1 ± 0.3 pb −1 of data recorded by the CMS detector. This result utilizes the final state with two isolated, highly energetic charged leptons, large missing transverse energy, and two or more jets. Backgrounds from Drell-Yan and non-W/Z boson production are estimated from data. Eleven events are observed in the data with 2.1 ± 1.0 events expected from background. The measured cross section is 194 ± 72(stat.) ± 24(syst.) ± 21(lumi.) pb, consistent with next-to-leading order predictions.
A search is performed in pp collisions at √ s = 7 TeV for exotic particles decaying via WZ to fin... more A search is performed in pp collisions at √ s = 7 TeV for exotic particles decaying via WZ to final states with electrons and muons. The data sample corresponds to an integrated luminosity of approximately 5 fb −1 . No significant excess is observed in the data above the expected standard model background. Upper bounds at 95% confidence level are set on the production cross section of the W boson described by the sequential standard model, and on the W WZ coupling. W bosons with masses below 1143 GeV are excluded. Limits are also set in the context of low-scale technicolor models, under a range of assumptions concerning the model parameters.
A search for anomalous production of events with three or more isolated leptons in pp collisions ... more A search for anomalous production of events with three or more isolated leptons in pp collisions at √ s = 7 TeV is presented. The data, corresponding to an integrated luminosity of 4.98 fb −1 , were collected by the CMS experiment at the LHC during the 2011 run. The search is applicable to any model of new physics that enhances multiple lepton production. The observed multilepton events are categorized into exclusive search channels based on the identity and kinematics of the objects in the events. An estimate of the standard-model background rates from data is emphasized, but simulation is also used to estimate some of the background rates. The search results are interpreted in the context of supersymmetry, including both R-parity-conserving and R-parity-violating models. We derive exclusion limits as a function of squark, gluino, and chargino masses.
Physical Review Letters, 2011
Measurements of dijet azimuthal decorrelations in pp collisions at √ s = 7 TeV using the CMS dete... more Measurements of dijet azimuthal decorrelations in pp collisions at √ s = 7 TeV using the CMS detector at the CERN LHC are presented. The analysis is based on an inclusive dijet event sample corresponding to an integrated luminosity of 2.9 pb −1 . The results are compared to predictions from perturbative QCD calculations and various Monte Carlo event generators. The dijet azimuthal distributions are found to be sensitive to initial-state gluon radiation.
A search is presented for a massive particle, generically referred to as a Z , decaying into a tt... more A search is presented for a massive particle, generically referred to as a Z , decaying into a tt pair. The search focuses on Z resonances that are sufficiently massive to produce highly Lorentz-boosted top quarks, which yield collimated decay products that are partially or fully merged into single jets. The analysis uses new methods to analyze jet substructure, providing suppression of the non-top multijet backgrounds. The analysis is based on a data sample of proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 5 fb −1 . Upper limits in the range of 1 pb are set on the product of the production cross section and branching fraction for a topcolor Z modeled for several widths, as well as for a Randall-Sundrum Kaluza-Klein gluon. In addition, the results constrain any enhancement in tt production beyond expectations of the standard model for tt invariant mass larger than 1 TeV/c 2 .
The cross section for dijet production in pp collisions at √ s = 7 TeV is presented as a function... more The cross section for dijet production in pp collisions at √ s = 7 TeV is presented as a function of ξ, a variable that approximates the fractional momentum loss of the scattered proton in single-diffractive events. The analysis is based on an integrated luminosity of 2.7 nb −1 collected with the CMS detector at the LHC at low instantaneous luminosities, and uses events with jet transverse momentum of at least 20 GeV. The dijet cross section results are compared to the predictions of diffractive and nondiffractive models. The lowξ data show a significant contribution from diffractive dijet production, observed for the first time at the LHC. The associated rapidity gap survival probability is estimated.
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Papers by Jean Carlos Ramirez Mendez