Heavy Quark Pair Research Articles

How theZc(3900)Reveals the Spectra of Charmonium Hybrids and Tetraquarks

Flavor-exotic tetraquark mesons have recently been observed in the heavy-quark pair sectors of QCD, including two isospin multiplets in the b b-bar sector, Z_b(10610) and Z_b(10650), and one isospin multiplet in the c c-bar sector, Z_c(3900). We identify Z_b and Z_c as tetraquark mesons that are analogs of quarkonium hybrids with the gluon field replaced by an isospin-1 excitation of the light-quark fields. Given the identification of Y(4260) and Z_c(3900) as a ground-state charmonium hybrid and tetraquark, respectively, lattice QCD calculations of the charmonium spectrum can be used to estimate the masses of the lowest four spin-symmetry multiplets of charmonium hybrids and tetraquarks. The Z_b(10610) and Z_b(10650) can be assigned to excited-state multiplets of bottomonium tetraquarks, resulting in estimates of the masses of the ground-state multiplets of bottomonium hybrids and tetraquarks.

Linear polarization of gluons and photons in unpolarized collider experiments

Abstract We study azimuthal asymmetries in heavy quark pair production in unpolarized electron-proton and proton-proton collisions, where the asymmetries originate from the linear polarization of gluons inside unpolarized hadrons. We provide cross section expressions and study the maximal asymmetries allowed by positivity, for both charm and bottom quark pair production. The upper bounds on the asymmetries are shown to be very large depending on the transverse momentum of the heavy quarks, which is promising especially for their measurements at a possible future Electron-Ion Collider or a Large Hadron electron Collider. We also study the analogous processes and asymmetries in muon pair production as a means to probe linearly polarized photons inside unpolarized protons. For increasing invariant mass of the muon pair the asymmetries become very similar to the heavy quark pair ones. Finally, we discuss the process dependence of the results that arises due to differences in color flow and address the problem with factorization in case of proton-proton collisions.

The real radiation antenna functions for $ S\to Q\overline{Q}gg $ at NNLO QCD

We analyze, in the antenna subtraction framework, the real radiation antenna functions for processes involving the production of a pair of heavy quarks and two gluons by an uncolored initial state at NNLO QCD. We provide explicit expressions for these functions and discuss their infrared singular behaviour. Our main results are the corresponding integrated antenna functions which are computed analytically. They are expressed in terms of harmonic polylogarithms.

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.

Azimuthal correlation among jets produced in association with a bottom or top quark pair at the LHC

Angular correlation of jets produced in association with a massive scalar, vector or tensor boson is crucial in the determination of their spin and CP properties. We study jet angular correlations in events with a high mass bottom quark pair or a top quark pair and two jets at the LHC, whose cross-section is dominated by the virtual gluon fusion sub-processes when appropriate kinematic selection cuts (vector-boson fusion cuts) are applied. We evaluate helicity amplitudes for sub-processes initiated by qq, qg and gg collisions in the limit where the intermediate gluons are collinear to the initial partons. We first obtain a general expression for the azimuthal angle correlations among the dijets and t t-bar or b b-bar, in terms of the gg to t t-bar or b b-bar helicity amplitudes in the real gluon approximation of the full matrix elements, and find simple analytic expressions in the two kinematic limits, the production of a heavy quark pair near the threshold, and in the relativistic limit where the invariant mass of the heavy quark pair is much larger than the quark mass. For b b-bar + 2 jets we find strong azimuthal angle correlations which are distinct from those expected for events with a CP-even or odd scalar boson which may decay into a b b-bar pair. For t t-bar + 2 jets we find that the angular correlations are similar to that of a CP-odd scalar+2 jets near the threshold M_(t t-bar) ~ 2 m_t, while in the relativistic limit they resemble the distribution for b b-bar + 2 jets. These correlations in the standard QCD processes will help establish the experimental technique to measure the spin and CP properties of new particles produced via gluon fusion at the LHC.

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 ${m}_{T}<480\text{ }\text{ }\mathrm{GeV}$ and ${m}_{T}<550\text{ }\text{ }\mathrm{GeV}$ are excluded for weak isospin singlets and doublets, respectively, and $B$ quarks with a mass ${m}_{B}<480\text{ }\text{ }\mathrm{GeV}$ 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.

Quarkonium formation time in quark-gluon plasma

The quarkonium formation time in a quark-gluon plasma (QGP) is determined from the space-time correlator of heavy quark vector currents using the quarkonium in-medium mass and wave function obtained from heavy quark potentials extracted from the lattice QCD. It is found that the formation time of a quarkonium increases with the temperature of the QGP and diverges near its dissociation temperature. Also, the quarkonium formation time is longer if the heavy quark potential is taken to be the free energy from lattice calculations for a heavy quark pair, compared to that based on the more negative internal energy.

Nuclear modification of vector boson production in proton–lead collisions at the LHC

In anticipating the upcoming proton–lead run at the LHC in the near future, we present predictions for the nuclear modification factor of transverse momentum spectrum of Z0 production and transverse momentum broadening of vector boson (J/ψ, ϒ, W/Z0) production in proton–lead collisions at s=5 TeV, respectively. We find that the measurement of nuclear modification factor of Z0 production provides a clean and unambiguous test of the nuclear anti-shadowing proposed in the recent EPS09. In addition, the dramatic difference in transverse momentum broadening between the heavy quarkonium and W/Z0 production could be a signature prediction of QCD parton multiple scattering, and it provides valuable information on color neutralization of a produced heavy quark pair when it transmutes into a physical quarkonium.

GENXICC2.1: An improved version of GENXICC for hadronic production of doubly heavy baryons

We present an improved version of GENXICC, which is a generator for hadronic production of the doubly heavy baryons $\Xi_{cc}$, $\Xi_{bc}$ and $\Xi_{bb}$ and has been raised by C.H. Chang, J.X. Wang and X.G. Wu [Comput. Phys. Commun. 177 (2007) 467; Comput. Phys. Commun. 181 (2010) 1144]. In comparison with the previous GENXICC versions, we update the program in order to generate the unweighted baryon events more effectively under various simulation environments, whose distributions are now generated according to the probability proportional to the integrand. One Les Houches Event (LHE) common block has been added to produce a standard LHE data file that contains useful information of the doubly heavy baryon and its accompanying partons. Such LHE data can be conveniently imported into PYTHIA to do further hadronization and decay simulation, especially, the color-flow problem can be solved with PYTHIA8.0.

J/ψ Measurements with the ALICE Experiment at the LHC

The ALICE detector is the dedicated heavy-ion experiment at the Large Hadron Collider (LHC) at CERN. It provides excellent capabilities to study quarkonium production in proton-proton (pp) and heavy-ion (A-A) collisions in the kinematic range of |y| < 0.9 and 2.5 < y < 4.0 down to a transverse momentum of zero. Quarkonia, bound states of heavy (charm or bottom) quark anti-quark pairs such as the J/ψ are expected to be produced by hard processes. Thus they will provide insights into the earliest and hottest stages of A-A collisions where the formation of a Quark-Gluon Plasma is expected. Results of the inclusive and prompt J/psi production cross section in pp collisions at =2.76 and 7 TeV will be presented. A polarization measurement in the forward region and the dependence of the J/ψ yield on the charged particle multiplicity in pp collisions will be discussed. The nuclear modification factor (RAA) at forward rapidity in Pb-Pb collisions at =2.76 TeV will be shown and its implications discussed.

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.

Relativistic correction to charmonium dissociation temperature

By solving the covariant relativistic Schrödinger equations for a pair of heavy quarks, we obtained the wave functions for the ground and excited quarkonium states at finite temperature. In comparison with the non-relativistic calculation, the J/ψ dissociation temperature determined by the infinity size or zero binding energy of the system increases 7%–13%, when the central potential varies between the free energy and internal energy.

A Fuller Picture of the Higgs Boson

Two collaborations at the Tevatron have combined data from their searches for the Higgs boson and report evidence of a new particle decaying into heavy quark pairs. This could be the first experimental evidence that the same mechanism that gives mass to the carriers of the weak force also underlies the mass of quarks.

Searching for new heavy chiral quark pairs via their annihilation to multiple vector bosons

Drawing analogy of replacing the nucleon by heavy chiral quark $Q$, the pion by Goldstone boson $G$, and $\pi NN$ coupling by $GQQ$ coupling, we construct a statistical model for $Q\bar Q \to nG$ annihilation, i.e. into $n$ longitudinal weak bosons. This analogy is becoming prescient since the LHC direct bound $m_Q > 611$ GeV implies strong Yukawa coupling. Taking $m_Q \in (1, 2)$ TeV, the mean number $<n_G>$ ranges from 6 to over 10, with negligible two or three boson production. With individual $t'$ or $b'$ decays suppressed either by phase space or quark mixing, and given the strong Yukawa coupling, $Q\bar Q\to nV_L$ is the likely outcome for very heavy $Q\bar Q$ production at the LHC.

Search for pair-produced heavy quarks decaying toWqin the two-lepton channel at(s)=7 TeVwith the ATLAS detector

A search is presented for heavy-quark pair production (QQbar) under the decay hypothesis QQbar to WqWq with q = u,d,c,s,b. The search is performed with 1.04 fb^-1 of integrated luminosity from pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the CERN LHC. Dilepton final states are selected, requiring large missing transverse momentum and at least two jets. Mass reconstruction of heavy quark candidates is performed by assuming that the W boson decay products are nearly collinear. The data are in agreement with Standard Model expectations; a heavy quark with mass less than 350 GeV is excluded at 95% confidence level.

Debye mass and heavy quark potential in a PNJL quark plasma

We calculate the Debye mass for the screening of the heavy quark potential in a plasma of massless quarks coupled to the temporal gluon background governed by the Polyakov loop potential within the PNJL model in RPA approximation. We give a physical motivation for a recent phenomenological fit of lattice data by applying the calculated Debye mass with its suppression in the confined phase due to the Polyakov-loop to a description of the temperature dependence of the singlet free energy for QCD with a heavy quark pair at infinite separation. We compare the result to lattice data.

The Magnetic Mass of Transverse Gluon, the B-Meson Weak Decay Vertex and the Triality Symmetry of Octonion

With an assumption that in the Yang-Mills Lagrangian, a left-handed fermion and a right-handed fermion both expressed as the quaternion make an octonion which possesses the triality symmetry, I calculate the magnetic mass of the transverse self-dual gluon from three loop diagram, in which a heavy quark pair is created and two self-dual gluons are interchanged. The magnetic mass of the transverse gluon depends on the mass of the pair created quarks, and in the case of charmed quark pair creation, the magnetic mass m mag becomes approximately equal to T c at $${T=T_c\sim 1.14\Lambda_{\overline{MS}} \sim 260}$$ MeV. A possible time-like magnetic gluon mass from two self-dual gluon exchange is derived, and corrections in the B-meson weak decay vertices from the two self-dual gluon exchange are also evaluated.

Double heavy meson production through double parton scattering in hadronic collisions

It is shown that the contribution from double parton scattering to the inclusive double heavy meson yield is quite comparable with the usually considered mechanism of their production at the LHC energy. For some pairs of heavy flavored quarks in the final state the double parton scattering will be a dominant mode of their production.

Charm and bottom photoproduction at HERA with MC@NLO

We apply the MC@NLO formalism to the production of heavy-quark pairs in pointlike photon–hadron collisions. By combining this result with its analogue relevant to hadron–hadron collisions, we obtain NLO predictions matched to parton showers for the photoproduction of QQ¯ pairs. We compare MC@NLO results to the measurements of c- and b-flavoured hadron observables performed by the H1 and ZEUS Collaborations at HERA.

Ultrasoft NLL running of the nonrelativistic O(v) QCD quark potential

Using the nonrelativistic effective field theory vNRQCD, we determine the contribution to the next-to-leading logarithmic (NLL) running of the effective quark-antiquark potential at order v (1/mk) from diagrams with one potential and two ultrasoft loops, v being the velocity of the quarks in the c.m. frame. The results are numerically important and complete the description of ultrasoft next-to-next-to-leading logarithmic (NNLL) order effects in heavy quark pair production and annihilation close to threshold.