Papers by Paolo Giacomelli
Physics Letters B, 2015
Measurements of the five most significant angular coefficients, A 0 through A 4 , for Z bosons pr... more Measurements of the five most significant angular coefficients, A 0 through A 4 , for Z bosons produced in pp collisions at √ s=8 TeV and decaying to µ + µ are presented as a function of the transverse momentum and rapidity of the Z boson. The integrated luminosity of the dataset collected with the CMS detector at the LHC corresponds to 19.7 f b 1 . These measurements provide comprehensive information about the Z boson production mechanisms, and are compared to the QCD predictions at leading order, next-to-leading order, and next-to-next-to-leading order in perturbation theory.
Physics Letters B, 2015
A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two mu... more A search is performed for heavy Majorana neutrinos (N) using an event signature defined by two muons of the same charge and two jets (µ ± µ ± jj). The data correspond to an integrated luminosity of 19.7 fb -1 of proton-proton collisions at a centerof-mass energy of 8 TeV, collected with the CMS detector at the CERN LHC. No excess of events is observed beyond the expected standard model background and upper limits are set on |V µN | 2 as a function of Majorana neutrino mass m N for masses in the range of 40-500 GeV, where V µN is the mixing element of the heavy neutrino with the standard model muon neutrino. The limits obtained are |V µN | 2 < 0.00470 for m N = 90 GeV, |V µN | 2 < 0.0123 for m N = 200 GeV, and |V µN | 2 < 0.583 for m N = 500 GeV. These results extend considerably the regions excluded by previous direct searches.
Physics Letters B, 2015
We present a measurement of the Z boson differential cross section in rapidity and transverse mom... more We present a measurement of the Z boson differential cross section in rapidity and transverse momentum using a data sample of pp collision events at a centre-of-mass energy √ s = 8 TeV, corresponding to an integrated luminosity of 19.7 fb -1 . The Z boson is identified via its decay to a pair of muons. The measurement provides a precision test of quantum chromodynamics over a large region of phase space. In addition, due to the small experimental uncertainties in the measurement the data has the potential to constrain the gluon parton distribution function in the kinematic regime important for Higgs boson production via gluon fusion. The results agree with the next-to-next-to-leading-order predictions computed with the FEWZ program. The results are also compared to the commonly used leading-order MADGRAPH and next-to-leading-order POWHEG generators.
The European Physical Journal C, 2015
Measurements of the differential and doubledifferential Drell-Yan cross sections in the dielectro... more Measurements of the differential and doubledifferential Drell-Yan cross sections in the dielectron and dimuon channels are presented. They are based on protonproton collision data at √ s = 8 TeV recorded with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 fb -1 . The measured inclusive cross section in the Z peak region (60-120 GeV), obtained from the combination of the dielectron and dimuon channels, is 1138 ± 8 (exp) ± 25 (theo) ± 30 (lumi) pb, where the statistical uncertainty is negligible. The differential cross section dσ/dm in the dilepton mass range 15-2000 GeV is measured and corrected to the full phase space. The double-differential cross section d 2 σ/dm d|y| is also measured over the mass range 20 to 1500 GeV and absolute dilepton rapidity from 0 to 2.4. In addition, the ratios of the normalized differential cross sections measured at √ s = 7 and 8 TeV are presented. These measurements are compared to the predictions of perturbative QCD at next-to-leading and next-to-next-toleading (NNLO) orders using various sets of parton distribution functions (PDFs). The results agree with the NNLO theoretical predictions computed with fewz 3.1 using the CT10 NNLO and NNPDF2.1 NNLO PDFs. The measured double-differential cross section and ratio of normalized differential cross sections are sufficiently precise to constrain the proton PDFs.
Physical Review Letters, 2015
JHEP, 2014
A search is reported for massive resonances decaying into a quark and a vector boson (W or Z), or... more 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.
Eur.Phys.J., 2014
A search for invisible decays of Higgs bosons is performed using the vector boson fusion and asso... more A search for invisible decays of Higgs bosons is performed using the vector boson fusion and associated ZH production modes. In the ZH mode, the Z boson is required to decay to a pair of charged leptons or a bb quark pair. The searches use the 8 TeV pp collision dataset collected by the CMS detector at the LHC, corresponding to an integrated luminosity of up to 19.7 fb -1 . Certain channels include data from 7 TeV collisions corresponding to an integrated luminosity of 4.9 fb -1 . The searches are sensitive to non-standardmodel invisible decays of the recently observed Higgs boson, as well as additional Higgs bosons with similar production modes and large invisible branching fractions. In all channels, the observed data are consistent with the expected standard model backgrounds. Limits are set on the production cross section times invisible branching fraction, as a function of the Higgs boson mass, for the vector boson fusion and ZH production modes. By combining all channels, and assuming standard model Higgs boson cross sections and acceptances, the observed (expected) upper limit on the invisible branching fraction at m H = 125 GeV is found to be 0.58 (0.44) at 95 % confidence level. We interpret this limit in terms of a Higgs-portal model of dark matter interactions.
Phys.Rev., 2014
A search for WVγ triple vector boson production is presented based on events containing a W boson... more A search for WVγ triple vector boson production is presented based on events containing a W boson decaying to a muon or an electron and a neutrino, a second V (W or Z) boson, and a photon. The data correspond to an integrated luminosity of 19.3 fb -1 collected in 2012 with the CMS detector at the LHC in pp collisions at √ s = 8 TeV. An upper limit of 311 fb on the cross section for the WVγ production process is obtained at 95% confidence level for photons with a transverse energy above 30 GeV and with an absolute value of pseudorapidity of less than 1.44. This limit is approximately a factor of 3.4 larger than the standard model predictions that are based on next-toleading order QCD calculations. Since no evidence of anomalous WWγγ or WWZγ quartic gauge boson couplings is found, this paper presents the first experimental limits on the dimension-8 parameter f T,0 and the CP-conserving WWZγ parameters κ W 0 and κ W C . Limits are also obtained for the WWγγ parameters a W 0 and a W C .
Phys.Lett., 2014
Constraints are presented on the total width of the recently discovered Higgs boson, Γ H , using ... more Constraints are presented on the total width of the recently discovered Higgs boson, Γ H , using its relative on-shell and off-shell production and decay rates to a pair of Z bosons, where one Z boson decays to an electron or muon pair, and the other to an electron, muon, or neutrino pair. The analysis is based on the data collected by the CMS experiment at the LHC in 2011 and 2012, corresponding to integrated luminosities of 5.1 fb -1 at a centre-of-mass energy √ s = 7 TeV and 19.7 fb -1 at √ s = 8 TeV. A simultaneous maximum likelihood fit to the measured kinematic distributions near the resonance peak and above the Z-boson pair production threshold leads to an upper limit on the Higgs boson width of Γ H < 22 MeV at a 95% confidence level, which is 5.4 times the expected value in the standard model at the measured mass of m H = 125.6 GeV.
JINST, 2014
View the article online for updates and enhancements. Commissioning and performance of the CMS si... more View the article online for updates and enhancements. Commissioning and performance of the CMS silicon strip tracker with cosmic ray muons CMS Collaboration -Commissioning and performance of the CMS pixel tracker with cosmic ray muons CMS Collaboration -Operation and performance of the ATLAS semiconductor tracker The ATLAS collaboration -
Journal of High Energy Physics, 2014
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 N N = 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-by-event fluctuating initial conditions is also presented.
Journal of High Energy Physics, 2013
The underlying event activity in proton-proton collisions at forward pseudo-rapidity (−6.6 < η... more The underlying event activity in proton-proton collisions at forward pseudo-rapidity (−6.6 < η < −5.2) is studied with the CMS detector at the LHC, using a novel observable: the ratio of the forward energy density, dE/dη, for events with a charged-particle jet produced at central pseudorapidity (|η jet| < 2) to the forward energy density for inclusive events. This forward energy density ratio is measured as a function of the central jet transverse momentum, p T, at three different pp centre-of-mass energies ( $ \sqrt{s}=0.9 $ , 2.76, and 7 TeV). In addition, the $ \sqrt{s} $ evolution of the forward energy density is studied in inclusive events and in events with a central jet. The results are compared to those of Monte Carlo event generators for pp collisions and are discussed in terms of the underlying event. Whereas the dependence of the forward energy density ratio on jet p T at each $ \sqrt{s} $ separately can be well reproduced by some models, all models fail to simul...
Journal of High Energy Physics, 2013
A study of proton-proton collisions in which two b hadrons are produced in association with a Z b... more A study of proton-proton collisions in which two b hadrons are produced in association with a Z boson is reported. The collisions were recorded at a centre-of-mass energy of 7 TeVwith the CMS detector at the LHC, for an integrated luminosity of 5.2 fb−1. The b hadrons are identified by means of displaced secondary vertices, without the use of reconstructed jets, permitting the study of b-hadron pair production at small angular separation. Differential cross sections are presented as a function of the angular separation of the b hadrons and the Z boson. In addition, inclusive measurements are presented. For both the inclusive and differential studies, different ranges of Z boson momentum are considered, and each measurement is compared to the predictions from different event generators at leading-order and next-to-leading-order accuracy.
Journal of High Energy Physics, 2012
Yields of prompt and non-prompt J/ψ, as well as $ \Upsilon $ (1S) mesons, are measured by the CMS... more Yields of prompt and non-prompt J/ψ, as well as $ \Upsilon $ (1S) mesons, are measured by the CMS experiment via their μ + μ − decays in PbPb and pp collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV for quarkonium rapidity |y| < 2.4. Differential cross sections and nuclear modification factors are reported as functions of y and transverse momentum p T, as well as collision centrality. For prompt J/ψ with relatively high p T (6.5 < p T < 30 GeV/c), a strong, centrality-dependent suppression is observed in PbPb collisions, compared to the yield in pp collisions scaled by the number of inelastic nucleon-nucleon collisions. In the same kinematic range, a suppression of non-prompt J/ψ, which is sensitive to the in-medium b-quark energy loss, is measured for the first time. Also the low-p T $ \Upsilon $ (1S) mesons are suppressed in PbPb collisions.
Journal of High Energy Physics, 2012
A search for the standard model Higgs boson decaying into two Z bosons with subsequent decay into... more A search for the standard model Higgs boson decaying into two Z bosons with subsequent decay into a final state containing two quark jets and two leptons, H → ZZ(*) → q $ \overline {\text{q}} $ ℓ − ℓ + is presented. Results are based on data corresponding to an integrated luminosity of 4.6 fb−1 of proton-proton collisions at $ \sqrt {s} = 7 $ TeV, collected with the CMS detector at the LHC. In order to discriminate between signal and background events, kinematic and topological quantities, including the angular spin correlations of the decay products, are employed. Events are further classified according to the probability of the jets to originate from quarks of light or heavy flavor or from gluons. No evidence for the Higgs boson is found, and upper limits on its production cross section are determined for a Higgs boson of mass between 130 and 600 GeV.
Physics Letters B, 2013
The results of a search for the bottomonium counterpart, denoted as X b , of the exotic charmoniu... more The results of a search for the bottomonium counterpart, denoted as X b , of the exotic charmonium state X(3872) is presented. The analysis is based on a sample of pp collisions at √ s = 8 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 20.7 fb -1 . The search looks for the exclusive decay channel X b → Υ(1S)π + π -followed by Υ(1S) → µ + µ -. No evidence for an X b signal is observed. Upper limits are set at the 95% confidence level on the ratio of the inclusive production cross sections times the branching fractions to Υ(1S)π + π -of the X b and the Υ(2S). The upper limits on the ratio are in the range 0.9-5.4% for X b masses between 10 and 11 GeV. These are the first upper limits on the production of a possible X b at a hadron collider.
Physics Letters B, 2014
The first measurement of jet shapes, defined as the fractional transverse momentum radial distrib... more The first measurement of jet shapes, defined as the fractional transverse momentum radial distribution, for inclusive jets produced in heavy-ion collisions is presented. Data samples of PbPb and pp collisions, corresponding to integrated luminosities of 150 µb -1 and 5.3 pb -1 respectively, were collected at a nucleon-nucleon centre-ofmass energy of √ s NN = 2.76 TeV with the CMS detector at the LHC. The jets are reconstructed with the anti-k T algorithm with a distance parameter R = 0.3, and the jet shapes are measured for charged particles with transverse momentum p T > 1 GeV/c. The jet shapes measured in PbPb collisions in different collision centralities are compared to reference distributions based on the pp data. A centrality-dependent modification of the jet shapes is observed in the more central PbPb collisions, indicating a redistribution of the energy inside the jet cone. This measurement provides information about the parton shower mechanism in the hot and dense medium produced in heavy-ion collisions.
Physics Letters B, 2014
A search for baryon number violation (BNV) in top-quark decays is performed using pp collisions p... more A search for baryon number violation (BNV) in top-quark decays is performed using pp collisions produced by the LHC at √ s = 8 TeV. The top-quark decay considered in this search results in one light lepton (muon or electron), two jets, but no neutrino in the final state. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of 19.5 fb -1 . The event selection is optimized for top quarks produced in pairs, with one undergoing the BNV decay and the other the standard model hadronic decay to three jets. No significant excess of events over the expected yield from standard model processes is observed. The upper limits at 95% confidence level on the branching fraction of the BNV top-quark decay are calculated to be 0.0016 and 0.0017 for the muon and the electron channels, respectively. Assuming lepton universality, an upper limit of 0.0015 results from the combination of the two channels. These limits are the first that have been obtained on a BNV process involving the top quark.
Physics Letters B, 2012
The t t charge asymmetry is measured in events containing a charged lepton (electron or muon) and... more The t t charge asymmetry is measured in events containing a charged lepton (electron or muon) and at least four jets, one of which is identified as originating from b-quark hadronization. The analyzed dataset corresponds to an integrated luminosity of 5.0 fb -1 collected with the CMS detector at the LHC. An inclusive and three differential measurements of the t t charge asymmetry as a function of rapidity, transverse momentum, and invariant mass of the t t system are presented. The measured inclusive t t charge asymmetry is A C = 0.004 ± 0.010 (stat.) ± 0.011 (syst.). This result and the three differential measurements are consistent with zero asymmetry as well as with the predictions of the standard model.
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Papers by Paolo Giacomelli