Heavy-quark Recombination Research Articles

Observation of a {varLambda}_b^0 \u2212 {overline{varLambda}}_b^0 production asymmetry in proton-proton collisions at sqrt{s} = 7 and 8 TeV

This article presents differential measurements of the asymmetry between {varLambda}_b^0 and {overline{varLambda}}_b^0 baryon production rates in proton-proton collisions at centre-of-mass energies of sqrt{s} = 7 and 8 TeV collected with the LHCb experiment, corresponding to an integrated luminosity of 3 fb−1. The {varLambda}_b^0 baryons are reconstructed through the inclusive semileptonic decay {varLambda}_b^0 → {varLambda}_c^{+} μ− overline{nu} μX. The production asymmetry is measured both in intervals of rapidity in the range 2.15 < y < 4.10 and transverse momentum in 2 < pT< 27 GeV/c. The results are found to be incompatible with symmetric production with a significance of 5.8 standard deviations for both sqrt{s} = 7 and 8 TeV data, assuming no CP violation in the decay. There is evidence for a trend as a function of rapidity with a significance of 4 standard deviations. Comparisons to predictions from hadronisation models in Pythia and heavy-quark recombination are provided. This result constitutes the first observation of a particle-antiparticle asymmetry in b-hadron production at LHC energies.

Λc+/Λc−andΛb0/Λ¯b0production asymmetry at the LHC from heavy quark recombination

The asymmetries in the forward region production cross section of Lambda_c^+/Lambda_c^- and Lambda_b^0/\bar{Lambda}_b^0 are predicted using the heavy quark recombination mechanism for pp collisions at 7 TeV and 14 TeV. Using non-perturbative parameters determined from various previous experiments, we find that A_p(Lambda_c^+/Lambda_c^-) ~ 1-2% and A_p(Lambda_b^0/\bar{Lambda}_b^0) ~ 1-3% in the forward region covered by the LHCb experiment.

Quarkonium measurements in the STAR experiment

Abstract Calculations of Quantum Chromodynamics on lattice showed that under conditions of high energy density or high temperature nuclear matter undergoes a phase transition from a state of quarks and gluons confined in hadrons to a deconfined state, the Quark–Gluon Plasma (QGP). Such conditions were present in first moments after the Big Bang and can be created in laboratory by colliding of heavy ions with sufficient energy. One of the most prominent signatures of QGP formation is the quarkonium suppression in central heavy-ion collisions arising from the Debye screening of the quark–antiquark potential in hot and dense nuclear matter. However, cold nuclear matter effects and heavy quark recombination could influence the measured quarkonium yields. Measurements of J / ψ at different collision energies can shed new light on understanding the interplay of these mechanisms for J / ψ production. The production mechanism of J / ψ is not well understood yet and the simultaneous measurement of J / ψ transverse momentum spectra and polarization in p + p collisions can significantly constrain possible production scenarios. In this proceedings, we report the recent STAR measurements of J / ψ production at mid-rapidity in p + p , d + Au , Au + Au at s NN = 200 GeV and in Au + Au collisions at s NN = 39 GeV and s NN = 62 GeV .

D±production asymmetry at the LHC from heavy quark recombination

The asymmetry in the forward region production cross section of D^\pm is calculated using the heavy-quark recombination mechanism for pp collisions at 7 TeV. By suitable choices of four non-perturbative parameters, our calculated results can reproduce those obtained at LHCb. We find A_p ~ -1% when integrated over 2.0 GeV < p_T < 18 GeV and 2.2< eta <4.75, which agrees with A_p=-0.96\pm0.26\pm0.18% as measured by LHCb. Furthermore, the calculated distributions in eta and p_T agree reasonably well with those obtained at LHCb.

Dynamical heavy-quark recombination and the nonphotonic single-electron puzzle at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

We show that the single, nonphotonic electron nuclear modification factor ${R}_{\mathit{AA}}^{e}$ is affected by the thermal enhancement of the heavy-baryon-to-heavy-meson ratio in relativistic heavy-ion collisions with respect to proton-proton collisions. We make use of the dynamical quark recombination model to compute such a ratio and show that this produces a sizable suppression factor for ${R}_{\mathit{AA}}^{e}$ at intermediate transverse momenta. We argue that this suppression factor needs to be considered, in addition to the energy loss contribution, in calculations of ${R}_{\mathit{AA}}^{e}$.

The influence of direct D-meson production on the determination of the nucleon strangeness asymmetry via dimuon events in neutrino experiments

Experimentally, the production of oppositely charged dimuon events by neutrino and antineutrino deep inelastic scattering (DIS) is used to determine the strangeness asymmetry inside a nucleon. Here we point out that the direct production of a D-meson in DIS may have a substantial influence on the measurement of nucleon strangeness distributions. Direct D-meson production is via heavy quark recombination (HQR) and via light-quark fragmentation from perturbative QCD (LQF-P). To see the influence precisely, we compute the direct D-meson productions via HQR and LQF-P quantitatively and estimate their corrections to the analysis of the strangeness asymmetry. The results show that HQR has a stronger effect than LQF-P does, and the former may influence the experimental determination of the nucleon strangeness asymmetry.

Ds Asymmetry in Photoproduction

By adopting two models of strange and antistrange quark distributions inside nucleon, the light-cone meson-baryon fluctuation model and the effective chiral quark model, we calculate the [Formula: see text] asymmetry in photoproduction in the framework of heavy-quark recombination mechanism. We find that the effect of asymmetry of strange sea to the Ds asymmetry is considerable and depending on the different models. Therefore, we expect that with the further study in electroproduction, e.g. at HERA and CEBAF, the experimental measurements on the [Formula: see text] asymmetry may impose a strong restriction on the strange-antistrange distribution asymmetry models.

Formula omitted] asymmetry in photoproduction

Considering of the possible difference in strange and antistrange quark distributions inside nucleon, we investigate the Ds+–Ds− asymmetry in photoproduction in the framework of heavy-quark recombination mechanism. We adopt two distribution models of strange sea, those are the light-cone meson–baryon fluctuation model and the effective chiral quark model. Our results show that the asymmetry induced by the strange quark distributions is distinct, which is measurable in experiments. And, there are evident differences between the predictions of our calculation and previous estimation. Therefore, the experimental measurements on the Ds+–Ds− asymmetry may impose a unique restriction on the strange–antistrange distribution asymmetry models.

Λc+/Λc−asymmetry in hadroproduction from heavy-quark recombination

Asymmetries in the hadroproduction of charm particles directly probe power corrections to the QCD factorization theorems. In this paper, the heavy-quark recombination mechanism, a power correction that explains charm meson asymmetries, is extended to charm baryons. In this mechanism, a light quark participates in the hard scattering that creates a charm quark and they hadronize together into a charm baryon. This provides a natural and economical explanation for the \Lambda_c^+/\Lambda_c^- asymmetries measured in \pi N and p N collisions.

Charm production asymmetries from heavy-quark recombination

Charm asymmetries in fixed-target hadroproduction experiments are sensitive to power corrections to the QCD factorization theorem for heavy quark production. A power correction called heavy-quark recombination has recently been proposed to explain these asymmetries. In heavy-quark recombination, a light quark or antiquark participates in a hard scattering which produces a charm–anticharm quark pair. The light quark or antiquark emerges from the scattering with small momentum in the rest frame of the charm quark, and together they hadronize into a charm particle. The cross section for this process can be calculated within perturbative QCD up to an overall normalization. Heavy-quark recombination explains the observed D meson and Λc asymmetries with a minimal set of universal nonperturbative parameters.

Leading-particle effect from heavy-quark recombination.

The leading particle effect in charm hadroproduction is an enhancement of the cross section for a charmed hadron D in the forward direction of the beam when the beam hadron has a valence parton in common with the D. The large D(+)/D(-) asymmetry observed by the E791 experiment is an example of this phenomenon. We show that the heavy-quark recombination mechanism provides an economical expla-nation for this effect. In particular, the D(+)/D(-) asymmetry can be fit reasonably well using a single parameter whose value is consistent with a recent determination from charm photoproduction.

Bproduction asymmetries in perturbativeQCD

This paper explores a new mechanism for B production in which a b quark combines with a light parton from the hard-scattering process before hadronizing into the B hadron. This recombination mechanism can be calculated within perturbative QCD up to a few nonperturbative constants. Though suppressed at large transverse momentum by a factor Lambda_QCD m_b/p_t^2 relative to b quark fragmentation production, it can be important at large rapidities. A signature for this heavy-quark recombination mechanism in proton-antiproton colliders is the presence of rapidity asymmetries in B cross sections. Given reasonable assumptions about the size of nonperturbative parameters entering the calculation, we find that the asymmetries are only significant for rapidities larger than those currently probed by collider experiments.

Charm-anticharm asymmetries in photoproduction from heavy-quark recombination

The asymmetries between charm and anticharm mesons observed in fixed-target photoproduction experiments are an order of magnitude larger than the asymmetries predicted by conventional perturbative QCD. We show that these charm meson asymmetries can be explained by a heavy-quark recombination mechanism for heavy meson production. In this process, a charm quark combines with a light antiquark from the hard-scattering process and they subsequently hadronize into a state including the charm meson. This recombination mechanism can be calculated within perturbative QCD up to some nonperturbative constants. After using symmetries of QCD to reduce the number of free parameters to two, we obtain a good fit to all the data on the asymmetries for charmed mesons from the E687 and E691 experiments.

Photonic production of Bc-mesons

The cross section of the γγ → B c(B c ∗) bc process is calculated. It is shown that near threshold the pseudoscalar state production is much suppressed with respect to the vector one. At large energies their ratio becomes σ(B c ∗) σ(B c) ∼ 4 . The process of heavy quark recombination dominates in the production of B c(B c ∗) states. The fragmentation process b → B c comes into play at high p t values only, while its contribution remains nondominant.