Energy Sqrt Research Articles

Search for excited Bc+ states

A search is performed in the invariant mass spectrum of the Bc+π+π− system for the excited Bc+ states Bc(21S0)+ and Bc(23S1)+ using a data sample of pp collisions collected by the LHCb experiment at the centre-of-mass energy of sqrt{s}=8 TeV, corresponding to an integrated luminosity of 2 fb−1. No evidence is seen for either state. Upper limits on the ratios of the production cross-sections of the Bc(21S0)+ and Bc(23S1)+ states times the branching fractions of Bc(21S0)+ → Bc+π+π− and Bc(23S1)+ → Bc*+π+π− over the production cross-section of the Bc+ state are given as a function of their masses. They are found to be between 0.02 and 0.14 at 95% confidence level for Bc(21S0)+ and Bc(23S1)+ in the mass ranges [6830, 6890] MeV/c2 and [6795, 6890] MeV/c2, respectively.

Pseudorapidity distributions of charged hadrons in proton-lead collisions at sqrt{s_{mathrm{NN}}}=5.02 and 8.16 TeV

The pseudorapidity distributions of charged hadrons in proton-lead collisions at nucleon-nucleon center-of-mass energies sqrt{s_{mathrm{NN}}}=5.02 and 8.16 TeV are presented. The measurements are based on data samples collected by the CMS experiment at the LHC. The number of primary charged hadrons produced in non-single-diffractive proton-lead collisions is determined in the pseudorapidity range |ηlab| < 2.4. The charged-hadron multiplicity distributions are compared to the predictions from theoretical calculations and Monte Carlo event generators. In the center-of-mass pseudorapidity range |ηcm| < 0.5, the average charged-hadron multiplicity densities 〈dNch/dηcm〉|ηcm| < 0.5 are 17.31 ± 0.01 (stat) ± 0.59 (syst) and 20.10 ± 0.01 (stat) ± 0.85(syst) at sqrt{s_{mathrm{NN}}}=5.02 and 8.16 TeV, respectively. The particle densities per participant nucleon are compared to similar measurements in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Observation of Top Quark Production in Proton-Nucleus Collisions.

The first observation of top quark production in proton-nucleus collisions is reported using proton-lead data collected by the CMS experiment at the CERN LHC at a nucleon-nucleon center-of-mass energy of sqrt[s_{NN}]=8.16 TeV. The measurement is performed using events with exactly one isolated electron or muon candidate and at least four jets. The data sample corresponds to an integrated luminosity of 174 nb^{-1}. The significance of the tt[over ¯] signal against the background-only hypothesis is above 5 standard deviations. The measured cross section is σ_{tt[over ¯]}=45±8 nb, consistent with predictions from perturbative quantum chromodynamics.

Measurement of the Drell-Yan triple-differential cross section in pp collisions at sqrt{s}=8 TeV

This paper presents a measurement of the triple-differential cross section for the Drell-Yan process Z/γ* → ℓ+ℓ− where ℓ is an electron or a muon. The measurement is performed for invariant masses of the lepton pairs, mℓℓ, between 46 and 200 GeV using a sample of 20.2 fb−1 of pp collisions data at a centre-of-mass energy of sqrt{s}=8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented in bins of invariant mass, absolute dilepton rapidity, |yℓℓ|, and the angular variable cos θ* between the outgoing lepton and the incoming quark in the Collins-Soper frame. The measurements are performed in the range |yℓℓ| < 2.4 in the muon channel, and extended to |yℓℓ| < 3.6 in the electron channel. The cross sections are used to determine the Z boson forward-backward asymmetry as a function of |yℓℓ| and mℓℓ. The measurements achieve high-precision, below the percent level in the pole region, excluding the uncertainty in the integrated luminosity, and are in agreement with predictions. These precision data are sensitive to the parton distribution functions and the effective weak mixing angle.

Measurement of the Y(nS) polarizations in pp collisions at sqrt{s}=7 and 8 TeV

The polarization of the Y(1S), Y(2S) and Y(3S) mesons, produced in pp collisions at centre-of-mass energies sqrt{s}=7 and 8 TeV, is measured using data samples collected by the LHCb experiment, corresponding to integrated luminosities of 1 and 2 fb−1, respectively. The measurements are performed in three polarization frames, using Y → μμ decays in the kinematic region of the transverse momentum pTΥ < 30 GeV/c and rapidity 2.2 < yY < 4.5. No large polarization is observed.

Measurement of the toverline{t}gamma production cross section in proton-proton collisions at sqrt{s}=8 TeV with the ATLAS detector

The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV with 20.2 fb−1 of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum pT > 15 GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a b-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be 139 ± 7 (stat.) ± 17 (syst.) fb, in good agreement with the theoretical prediction at next-to-leading order of 151 ± 24 fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at sqrt{s}=13 TeV using the ATLAS detector

Measurements of differential cross-sections of top-quark pair production in fiducial phase-spaces are presented as a function of top-quark and toverline{t} system kinematic observables in proton-proton collisions at a centre-of-mass energy of sqrt{s}=13 TeV. The data set corresponds to an integrated luminosity of 3.2 fb−1, recorded in 2015 with the ATLAS detector at the CERN Large Hadron Collider. Events with exactly one electron or muon and at least two jets in the final state are used for the measurement. Two separate selections are applied that each focus on different top-quark momentum regions, referred to as resolved and boosted topologies of the toverline{t} final state. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations by means of calculated χ2 and p-values.

Measurement of CP observables in B\xb1 \u2192 DK*\xb1 decays using two- and four-body D final states

Measurements of CP observables in B± → DK*± decays are presented, where D denotes a superposition of D0 and {overline{D}}^0 meson states. Decays of the D meson to K−π+, K−K+, π−π+, K−π+π−π+ and π−π+π−π+ are used and the K*± meson is reconstructed in the KS0π± final state. This analysis uses a data sample of pp collisions collected with the LHCb experiment, corresponding to integrated luminosities of 1 fb−1, 2 fb−1 and 1.8 fb−1 at centre-of-mass energies sqrt{s}=7 TeV, 8 TeV and 13 TeV, respectively. The sensitivity of the results to the CKM angle γ is discussed.

Measurement of the B^{±} Meson Nuclear Modification Factor in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

The differential production cross sections of B^{±} mesons are measured via the exclusive decay channels B^{±}→J/ψK^{±}→μ^{+}μ^{-}K^{±} as a function of transverse momentum in pp and Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=5.02 TeV per nucleon pair with the CMS detector at the LHC. The pp(Pb-Pb) data set used for this analysis corresponds to an integrated luminosity of 28.0 pb^{-1} (351 μb^{-1}). The measurement is performed in the B^{±} meson transverse momentum range of 7 to 50 GeV/c, in the rapidity interval |y|<2.4. In this kinematic range, a strong suppression of the production cross section by about a factor of 2 is observed in the Pb-Pb system in comparison to the expectation from pp reference data. These results are found to be roughly compatible with theoretical calculations incorporating beauty quark diffusion and energy loss in a quark-gluon plasma.

Study of prompt D0 meson production in pPb collisions at sqrt{s_{mathrm{N};mathrm{N}}}=5 TeV

Production of prompt D0 mesons is studied in proton-lead and lead-proton collisions recorded at the LHCb detector at the LHC. The data sample corresponds to an integrated luminosity of 1.58±0.02 nb−1 recorded at a nucleon-nucleon centre-of-mass energy of sqrt{s_{mathrm{NN}}}=5 TeV. Measurements of the differential cross-section, the forward-backward production ratio and the nuclear modification factor are reported using D0 candidates with transverse momenta less than 10 GeV/c and rapidities in the ranges 1.5 < y∗< 4.0 and −5.0 < y∗< −2.5 in the nucleon-nucleon centre-of-mass system.

Study of WWgamma and WZgamma production in pp collisions at sqrt{s} = {8} ,{text {TeV}} and search for anomalous quartic gauge couplings with the ATLAS experiment

This paper presents a study of WWgamma and WZgamma triboson production using events from proton–proton collisions at a centre-of-mass energy of sqrt{s} = text{8},text{TeV} recorded with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 20.2 fb^{-1}. The WWgamma production cross-section is determined using a final state containing an electron, a muon, a photon, and neutrinos (enu mu nu gamma ). Upper limits on the production cross-section of the enu mu nu gamma final state and the WWgamma and WZgamma final states containing an electron or a muon, two jets, a photon, and a neutrino (enu jjgamma or mu nu jjgamma ) are also derived. The results are compared to the cross-sections predicted by the Standard Model at next-to-leading order in the strong-coupling constant. In addition, upper limits on the production cross-sections are derived in a fiducial region optimised for a search for new physics beyond the Standard Model. The results are interpreted in the context of anomalous quartic gauge couplings using an effective field theory. Confidence intervals at 95% confidence level are derived for the 14 coupling coefficients to which WWgamma and WZgamma production are sensitive.

Top-quark mass measurement in the all-hadronic toverline{t} decay channel at sqrt{s}=8 TeV with the ATLAS detector

The top-quark mass is measured in the all-hadronic top-antitop quark decay channel using proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV with the ATLAS detector at the CERN Large Hadron Collider. The data set used in the analysis corresponds to an integrated luminosity of 20.2 fb−1. The large multi-jet background is modelled using a data-driven method. The top-quark mass is obtained from template fits to the ratio of the three-jet to the dijet mass. The three-jet mass is obtained from the three jets assigned to the top quark decay. From these three jets the dijet mass is obtained using the two jets assigned to the W boson decay. The top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV.

Measurement of the inclusive jet cross-sections in proton-proton collisions at sqrt{s}=8 TeV with the ATLAS detector

Inclusive jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV recorded by the ATLAS experiment at the Large Hadron Collider at CERN. The total integrated luminosity of the analysed data set amounts to 20.2 fb−1. Double-differential cross-sections are measured for jets defined by the anti-kt jet clustering algorithm with radius parameters of R = 0.4 and R = 0.6 and are presented as a function of the jet transverse momentum, in the range between 70 GeV and 2.5 TeV and in six bins of the absolute jet rapidity, between 0 and 3.0. The measured cross-sections are compared to predictions of quantum chromodynamics, calculated at next-to-leading order in perturbation theory, and corrected for non-perturbative and electroweak effects. The level of agreement with predictions, using a selection of different parton distribution functions for the proton, is quantified. Tensions between the data and the theory predictions are observed.

Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in sqrt{s}=13 TeV pp collisions with the ATLAS detector

The results of a search for vector-like top quarks using events with exactly one lepton, at least four jets, and large missing transverse momentum are reported. The search is optimised for pair production of vector-like top quarks in the Z(→νν) t + X decay channel. LHC pp collision data at a centre-of-mass energy of sqrt{s}=13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess over the Standard Model expectation is seen and upper limits on the production cross-section of a vector-like T quark pair as a function of the T quark mass are derived. The observed (expected) 95% CL lower limits on the T mass are 870 GeV (890 GeV) for the weak-isospin singlet model, 1.05 TeV (1.06 TeV) for the weak-isospin doublet model and 1.16 TeV (1.17 TeV) for the pure Zt decay mode. Limits are also set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV.

Measurement of the kt splitting scales in Z \u2192 \u2113\u2113 events in pp collisions at sqrt{s}=8 TeV with the ATLAS detector

A measurement of the splitting scales occuring in the kt jet-clustering algorithm is presented for final states containing a Z boson. The measurement is done using 20.2 fb−1 of proton-proton collision data collected at a centre-of-mass energy of sqrt{s}=8 TeV by the ATLAS experiment at the LHC in 2012. The measurement is based on chargedparticle track information, which is measured with excellent precision in the pT region relevant for the transition between the perturbative and the non-perturbative regimes. The data distributions are corrected for detector effects, and are found to deviate from state-of-the-art predictions in various regions of the observables.

Measurement of D-meson production at mid-rapidity in pp collisions at {sqrt{s}=7}\xa0TeV

The production cross sections for prompt charmed mesons mathrm{D^0}, mathrm{D^+}, mathrm{D^{*+}} and mathrm{D_s^+} were measured at mid-rapidity in proton–proton collisions at a centre-of-mass energy sqrt{s}=7~{mathrm {TeV}} with the ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays mathrm{D}^0 rightarrow mathrm{K}^-pi ^+, mathrm{D}^+rightarrow mathrm{K}^-pi ^+pi ^+, mathrm{D}^{*+} rightarrow mathrm{D}^0 pi ^+, mathrm{D_s^{+}rightarrow phi pi ^+rightarrow K^-K^+pi ^+}, and their charge conjugates.With respect to previous measurements in the same rapidity region, the coverage in transverse momentum (p_mathrm{T}) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total mathrm{c}{overline{mathrm{c}}} production cross section is likewise improved. The measured p_mathrm{T}-differential cross sections are compared with the results of three perturbative QCD calculations.

Studies of Z\u03b3 production in association with a high-mass dijet system in pp collisions at sqrt{s}=8 TeV with the ATLAS detector

The production of a Z boson and a photon in association with a high-mass dijet system is studied using 20.2 fb−1 of proton-proton collision data at a centre-of-mass energy of sqrt{s}=8 TeV recorded with the ATLAS detector in 2012 at the Large Hadron Collider. Final states with a photon and a Z boson decaying into a pair of either electrons, muons, or neutrinos are analysed. Electroweak and total pp → Zγjj cross-sections are extracted in two fiducial regions with different sensitivities to electroweak production processes. Quartic couplings of vector bosons are studied in regions of phase space with an enhanced contribution from pure electroweak production, sensitive to vector-boson scattering processes VV → Zγ. No deviations from Standard Model predictions are observed and constraints are placed on anomalous couplings parameterized by higher-dimensional operators using effective field theory.

Measurement of the J/\u03c8 pair production cross-section in pp collisions at sqrt{s}=13 TeV

The production cross-section of J/ψ pairs is measured using a data sample of pp collisions collected by the LHCb experiment at a centre-of-mass energy of sqrt{s}=13 TeV, corresponding to an integrated luminosity of 279 ±11 pb−1. The measurement is performed for J/ψ mesons with a transverse momentum of less than 10 GeV/c in the rapidity range 2.0 < y < 4.5. The production cross-section is measured to be 15.2 ± 1.0 ± 0.9 nb. The first uncertainty is statistical, and the second is systematic. The differential cross-sections as functions of several kinematic variables of the J/ψ pair are measured and compared to theoretical predictions.

Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton\u2013proton collisions at the LHC

This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of sqrt{s} = 8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30;upmu {text {m}} is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20;upmu {text {m}} and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.

Prompt neutrino fluxes in the atmosphere with PROSA parton distribution functions

Effects on atmospheric prompt neutrino fluxes of present uncertainties affecting the nucleon composition are studied by using the PROSA fit to parton distribution functions (PDFs). The PROSA fit extends the precision of the PDFs to low x, which is the kinematic region of relevance for high-energy neutrino production, by taking into account LHCb data on charm and bottom hadroproduction collected at the center-of-mass energy of sqrt{s}=7 TeV. In the range of neutrino energies explored by present Very Large Volume Neutrino Telescopes, it is found that PDF uncertainties are far smaller with respect to those due to renormalization and factorization scale variation and to assumptions on the cosmic ray composition, which at present dominate and limit our knowledge of prompt neutrino fluxes. A discussion is presented on how these uncertainties affect the expected number of atmospheric prompt neutrino events in the analysis of high-energy events characterized by interaction vertices fully contained within the instrumented volume of the detector, performed by the IceCube collaboration.