Heavy Quark Production Research Articles

Heavy quark production and gluon saturation in double parton scattering at the LHC

The production of $c\bar{c}c\bar{c}$, $b\bar{b}b\bar{b}$ and $c\bar{c}b\bar{b}$ pairs considering double parton scattering at LHC energies is investigated. We estimate the contribution of saturation effects to the different final states and predict the energy dependence of the cross sections. Moreover, we estimate the ratio between the double and single parton scattering cross sections for the full rapidity range of the LHC and for the rapidity range of the LHCb experiment. For the full rapidity range we confirm a previous prediction, namely that for charm production the double parton scattering contribution becomes comparable with the single parton scattering one at LHC energies. We also demonstrate that this result remains valid when one introduces saturation effects in the calculations. Finally we show that the production of $c\bar{c}b\bar{b}$ contributes significantly to bottom production. For the LHCb kinematical range the ratio is strongly reduced.

Search for exotic same-sign dilepton signatures (b' quark, T5/3 and four top quarks production) in 4.7 fb−1 of pp collisions at = 7 TeV with the ATLAS detector

We present a search for pair production of down-type heavy quarks (b'b̄'), single and pair production of heavy top quark partners (T5/3qq̄' and T5/3T̄5/3, respectively), and production of events containing four top quarks (ttt̄t) in pp collisions at = 7 TeV recorded with the ATLAS detector at the LHC. For an integrated luminosity of 4.7 fb−1, the corresponding lower limit on both the b' and T5/3 mass is 0.67 TeV at 95% confidence level, when produced in pairs. When including the single production mechanism, limits on T5/3 production are 0.68 TeV and 0.70 TeV for a coupling constant of the tWT5/3 vertex equal to 1 and 3, respectively. The upper limit on the four top quarks production cross section is 61 fb at 95% confidence level.

Search for Single Production of Vector-like Quarks Coupling to Light Generations in 4.64 fb−1 of ATLAS Data at = 7 TeV

This analysis presents a search for singly-produced vector-like quarks (VLQs) coupling to light quark generations in events with a W or Z boson produced in association with two or more high transverse momentum jets. The vector boson produced in the VLQ decay is reconstructed in the W → ℓν or Z → ℓℓ modes, where ℓ is an electron or a muon. No evidence of VLQs is observed above the expected background in 4.64 fb−1 of data taken in 2011 by the ATLAS experiment at the LHC at a centre-of-mass energy of 7 TeV. A fit to the distribution of the invariant mass of the system composed of the vector boson and the highest transverse momentum jet is used to set limits on the heavy quark production cross section times branching ratio, and mass bounds are obtained for three flavors of VLQs.

Open heavy flavour production via semi-leptonic decay muons in Pb + Pb collisions at sNN=2.76 TeV with the ATLAS detector at the LHC

Measurements of heavy quark production and suppression in ultra-relativistic nuclear collisions probe the interactions of heavy quarks with the hot, dense medium created in the collisions. ATLAS has measured heavy quark production in $\sqrtsnn = 2.76$ \TeV \PbPb collisions via semi-leptonic decays of open heavy flavour hadrons to muons. Results are presented for the per-event muon yield as a function of muon transverse momentum, \pT, over the range of $4 < \pT < 14$ \GeV. The centrality dependence of the muon yields is characterized by the "central to peripheral" ratio, \Rcp. Muon production is suppressed by approximately a factor of two in central collisions relative to peripheral collisions. Within the experimental errors, the observed suppression is independent of muon \pT for all centralities.

The measurement of non-photonic electrons in STAR

The measurements of non-photonic electrons (NPE), mainly produced by semileptonic decays of D and B mesons, provide information on heavy quarks production as well as properties of nuclear matter produced in heavy ion collisons. In order to interpret the NPE measurements it is important to determine the relative charm and bottom contribution to the NPE spectrum. In this proceedings we present the measurements of NPE spectra and NPE-hadron azimuthal correlations in p + p collisions at s=200 GeV and at s=500 GeV. NPE-hadron correlations allow extraction of the B decay contribution to the NPE. The B decay contribution is comparable to the contribution from the D meson decay at s=200 GeV at pT higher than 5 GeV/c, and is about 60% at s=500 GeV at pT higher than 5 GeV/c. STAR measured NPE spectrum in p + p collisions as well as relative constribution of bottom decays to the spectrum is consistent with FONLL pQCD calculations. The preliminary results of NPE spectra and NPE-hadron correlations in Au + Au collisions at s=200 GeV is shown.

Hard Probes 2012: Experimental Summary

Abstract The 5th international Conference on Hard and Electromagnetic Probes in High-Energy Nuclear Collisions was held in May 2012 in Cagliari, Italy. This contribution summarises some of the experimental highlights presented at the meeting, concentrating on new results from LHC and RHIC on parton energy loss (‘jet-quenching’) and heavy quark meson production (‘quarkonia suppression’).

Heavy flavor production in pp and AA collisions at the LHC

Abstract A refined version of a multi-step calculation of heavy-flavor observables in pp and AA collisions has been developed, based on pQCD at NLO accuracy followed by parton shower evolution to describe heavy-quark production and on the relativistic Langevin equation to describe their stochastic evolution in the QCD plasma. Then, hadronization is modeled through an implementation of fragmentation functions based on pQCD and constrained by e + e − collider data. Results of our calculations can be compared with recent measurements performed at the LHC in Pb–Pb collisions at s N N = 2.76 TeV : nuclear modification factor R A A of the p T spectra at mid-rapidity of heavy-flavor decay electrons and of exclusively reconstructed open-charm mesons at different centralities, as well as their elliptic-flow v 2 ( p T ) in semi-central collisions. To test the validity of our setup for such studies, its predictions are also checked against the p T spectra measured in pp collisions at s = 7 TeV and 2.76 TeV.

CHARMONIUM PRODUCTION IN Pb–Pb COLLISIONS AT ALICE: FROM SUPPRESSION TO REGENERATION?

Heavy quarkonium states are considered as one of the key observables for the study of the phase transition from a system made of hadrons towards a Quark–Gluon Plasma (QGP). In the last 25 years, experiments at CERN and Brookhaven have studied collisions of heavy ions looking for a suppression of charmonia/bottomonia, considered as a signature of the phase transition. After an introduction to the main concepts behind these studies and a short review of the SPS and RHIC results, I will describe the results obtained in Pb – Pb collisions by the ALICE experiment at the LHC. The ALICE findings will be critically compared to those of lower energy experiments, to CMS results, and to model calculations. The large cross-sections for heavy-quark production at LHC energies are expected to induce a novel production mechanism for charmonia in heavy-ion collisions, related to a recombination of [Formula: see text] pairs along the history of the collision and/or at hadronization. The occurrence of such a process at the LHC will be discussed. Finally, prospects for future measurements will be shortly addressed.

Constraints on the gluon PDF from top quark pair production at hadron colliders

Abstract Using the recently derived NNLO cross sections [1], we provide NNLO+NNLL theoretical predictions for top quark pair production based on all the available NNLO PDF sets, and compare them with the most precise LHC and Tevatron data. In this comparison we study in detail the PDF uncertainty and the scale, m t and α s dependence of the theoretical predictions for each PDF set. Next, we observe that top quark pair production provides a powerful direct constraint on the gluon PDF at large x, and include Tevatron and LHC top pair data consistently into a global NNLO PDF fit. We then explore the phenomenological consequences of the reduced gluon PDF uncertainties, by showing how they can improve predictions for Beyond the Standard Model processes at the LHC. Finally, we update to full NNLO+NNLL the theoretical predictions for the ratio of top quark cross sections between different LHC center of mass energies, as well as the cross sections for hypothetical heavy fourth-generation quark production at the LHC.

Massive non-planar two-loop four-point integrals with SecDec 2.1

We present numerical results for massive non-planar two-loop box integrals entering heavy quark pair production at NNLO, some of which are not known analytically yet. The results have been obtained with the program SecDec 2.1, based on sector decomposition and contour deformation, in combination with new types of transformations. Among the new features of version 2.1 is also the possibility to evaluate contracted tensor integrals, with no limitation on the rank. Program SummaryProgram title: SecDec 2.1Catalogue identifier: AEIR_v2_1Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEIR_v2_1.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 127558No. of bytes in distributed program, including test data, etc.: 2506220Distribution format: tar.gzProgramming language: Wolfram Mathematica, Perl, Fortran/C++.Computer: From a single PC to a cluster, depending on the problem.Operating system: Unix, Linux.RAM: Depends on the complexity of the problemClassification: 4.4, 5, 11.1.Catalogue identifier of previous version: AEIR_v2_0Journal reference of previous version: Comput. Phys. Comm. 184 (2013) 396External routines: BASES [1], CUBA [2]. The codes for both are included in the SecDec 2.1 distribution file.Does the new version supersede the previous version?: YesNature of problem: Extraction of ultraviolet and infrared singularities from parametric integrals appearing in higher order perturbative calculations in gauge theories. Numerical integration in the presence of integrable singularities (e.g. kinematic thresholds in massive multi-loop integrals). Flexibility to treat non-standard user defined functions.Solution method: Algebraic extraction of singularities in dimensional regularisation using iterated sector decomposition. This leads to a Laurent series in the dimensional regularisation parameter ε, where the coefficients are finite integrals over the unit-hypercube. Those integrals are evaluated numerically by Monte Carlo integration. The integrable singularities are handled by choosing a suitable integration contour in the complex plane, in an automated way.Reasons for new version: Several improvements and new features.Summary of revisions: Extended tensor integral option, flexibility to evaluate non-standard loop integral functions and to skip primary decomposition step, improvements in the user interface and the error treatment.Restrictions: Depending on the complexity of the problem, limited by CPU time or memory.Running time: Between a few minutes and several days, depending on the complexity of the problem.

Generalization of exotic quark searches

General limits on exotic heavy quarks $T$, $B$, and $X$ with masses above 300 GeV are presented for arbitrary branching fractions of $T\ensuremath{\rightarrow}{W}^{+}b$, $T\ensuremath{\rightarrow}Zt$, $T\ensuremath{\rightarrow}Ht$, $B\ensuremath{\rightarrow}{W}^{\ensuremath{-}}t$, $B\ensuremath{\rightarrow}Zb$, $B\ensuremath{\rightarrow}Hb$, and $X\ensuremath{\rightarrow}{W}^{+}t$. The results are based on a CMS search in final states with three isolated leptons ($e$ or $\ensuremath{\mu}$) or two isolated leptons with the same electric charge. Exotic heavy quark pair production through the strong interaction is considered. In the context of vectorlike quark models, $T$ quarks with a mass ${m}_{T}&lt;480\text{ }\text{ }\mathrm{GeV}$ and ${m}_{T}&lt;550\text{ }\text{ }\mathrm{GeV}$ are excluded for weak isospin singlets and doublets, respectively, and $B$ quarks with a mass ${m}_{B}&lt;480\text{ }\text{ }\mathrm{GeV}$ are excluded for singlets, all at a 95% confidence level. Mass limits at the 95% confidence level for $T$ and $B$ singlets, $(T,B)$ doublets, and $(X,T)$ doublets are presented as a function of the corresponding heavy quark masses. For equal mass ${m}_{T}={m}_{B}$ and ${m}_{X}={m}_{T}$, vectorlike quarks are excluded at the 95% confidence level with masses below 550 GeV for $T$ and $B$ singlets, 640 GeV for a $(T,B)$ doublet, and 640 GeV for a $(X,T)$ doublet.

Implications of LHCb measurements and future prospects

During 2011 the LHCb experiment at CERN collected 1.0 fb−1 of \(\sqrt{s} = 7\mbox{~TeV}\) pp collisions. Due to the large heavy quark production cross-sections, these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a dedicated experiment in the forward region at a hadron collider. This document discusses the implications of these first measurements on classes of extensions to the Standard Model, bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS. The physics potential of an upgrade to the LHCb detector, which would allow an order of magnitude more data to be collected, is emphasised.

Heavy Flavor Physics in PHENIX

Heavy quarks are good probes of the hot and dense medium created in relativistic heavy ion collisions since they are mainly generated early in the collision and interact with the medium in all collision stages. In addition, heavy flavor quarkonia production is thought to be uniquely sensitive to the deconfined medium of the Quark Gluon Plasma (QGP) through color screening. Heavy quark production has been studied by the PHENIX experiment at RHIC via measurements of single leptons from semi-leptonic decays, in both the electron channel at mid-rapidity and in the muon channel at forward rapidity. Large suppression and azimuthal anisotropy of single electrons have been observed in Au+Au collisions at GeV. These results suggest a large energy loss and strong flow of the heavy quarks in the hot, dense matter. The PHENIX experiment has also measured J/ψ production in p + p, d+Au, Cu+Cu, and Au+Au collisions at energies up to GeV. In central Au+Au at 200 GeV, more suppression is observed at forward rapidity than at central rapidity. This can be interpreted either as a sign of quark recombination, or as a hint of additional cold nuclear matter effects. Selected PHENIX results on open heavy flavor and heavy quarkonia production in p + p, d + Au, Cu+Cu, and Au+Au collisions are presented. The status of the recent PHENIX upgrade of the central Silicon Vertex Tracker (VTX) and its performance are elucidated.

Beyond the proton collinear factorization in heavy quark production in pA collisions at low x

We consider heavy quark production in high energy pA collisions and investigate the contribution of interactions of valence quarks of proton with the nucleus. The often made assumption that valence quarks of proton can be factored out is justified only if the nucleus saturation momentum is much smaller than the heavy quark mass. This is not the case in phenomenologically relevant situations. Breakdown of factorization manifests itself in substantial decrease of the cross section at large total and small relative transverse momenta of the heavy quark -antiquark pair.

Precision QCD at HERA

HERA was a high-energy electron/positron proton collider with a centre of mass energy of up to 320 GeV operated until summer 2007. The hadronic final state in the regime of deep-inelastic scattering as well as of photoproduction has been thoroughly investigated over a wide range of phase space with the two multi-purpose detectors H1 and ZEUS. The large available HERA data samples of about 0.5 fb−1 per experiment allowed stringent tests of quantum chromodynamics using a variety of different observables connected to the production of jets and heavy quarks. From the jet production measurements the strong coupling constant αs has been extracted with high precision. Some of the most recent results of the H1 and ZEUS collaborations are presented in this contribution.

Associated Production of Different Flavor Heavy Quarks Through &lt;span class="cmmi-10"&gt;W&lt;/span&gt;&lt;span class="cmsy-10"&gt;′ &lt;/span&gt;Bosons at the LHC

Associated production of different flavor heavy quarks can provide a probe for $W'$ bosons at the LHC. We study $pp\to t'\bar{b'}X$ process with the subsequent decays $t'\to W^+b$ and $\bar{b'}\to W^-W^+\bar b$, and compare the results with the $t\bar{b}$ final state. The effects of the $W'$ boson to the different flavor pair production cross section are shown to be important in some parameter region for the heavy quark masses of 600 and 700 GeV. We present accessible mass limits for the $W'$ boson at the LHC with $\sqrt s$=14 TeV.

Search for pair production of heavy top-like quarks decaying to a high-[formula omitted]W boson and a b quark in the lepton plus jets final state at [formula omitted] with the ATLAS detector

A search is presented for production of a heavy up-type quark (t′) together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb−1 of pp collisions at s=7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton+jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t′t¯′ signal when mt′≳400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t′→Wb)=1, a fourth-generation t′ quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass ∼126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t′→Wb) versus BR(t′→Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.

Heavy Flavour Production at HERA

Measurements of heavy-quark production at HERA constrain the structure of the proton, and have provided valuable indications of perturbative QCD. In this publication measurements of heavy-flavour jet cross sections in photoproduction and deep inelastic scattering (DIS), performed by the H1 and ZEUS collaborations, are presented using inclusive techniques for the signal extraction. The results are compared to next-to-leading order QCD calculations and found to be in reasonable agreement with theory predictions.

Polarized gluon studies with charmonium and bottomonium at LHCb and AFTER

Recently it has been put forward that linearly polarized gluons inside unpolarized protons affect the transverse momentum distribution of final state particles in hadronic collisions. They lead to a characteristic modulation of the differential cross section in Higgs production and to azimuthal asymmetries in, for instance, heavy quark pair production. Here we study the effect on charmonium and bottomonium production in hadronic collisions, such as at LHCb and at the proposed fixed target experiment AFTER at LHC. We focus mainly on the scalar and pseudoscalar quarkonia, eta_c, chi_{c0}, eta_b, chi_{b0}, which allow for an angular independent investigation. Within the framework of transverse momentum dependent factorization in combination with the nonrelativistic QCD based color-singlet quarkonium model, we show for small transverse momentum (q_T^2 \ll 4 M_Q^2) that the scalar and pseudoscalar quarkonium production cross sections are modified in different ways by linearly polarized gluons, while their effects on the production of higher angular momentum quarkonium states are strongly suppressed. Comparisons to chi_{c2}, chi_{b2} production can help to cancel out uncertainties. Together with the analogous study in Higgs production at LHC, quarkonium production can moreover be used to test the scale dependence of the linearly polarized gluon distribution over a large energy range.

Quantum chromodynamics results from HERA and JLAB

Recent QCD results from electron–proton interactions at HERA and JLAB are presented. Inclusive cross-section measurements as well as studies of the hadronic final state like jet production or the production of heavy quarks are discussed. The results are compared with perturbative QCD predictions and their impact on the determination of the parton density functions of the proton as well as of the strong coupling α s is discussed.