Charm Quark Pair Research Articles

Measurements of J/ψ Production vs. Event Multiplicity in Forward Rapidity in p + p Collisions in the PHENIX Experiment

J/ψ, a charmonium bound state made of a charm and an anti-charm quark, was discovered in the 1970s and confirmed the quark model. Because the mass of charm quarks is significantly above the quantum chromodynamics (QCD) scale ΛQCD, charmonia are considered excellent probes to test perturbative quantum chromodynamics (pQCD) calculations. In recent decades, they have been studied extensively at different high-energy colliders. However, their production mechanisms, which involve multiple scales, are still not very well understood. Recently, in high-multiplicity p+p collisions at RHIC and at the LHC, a significant enhancement of J/ψ production yield has been observed, which suggests a strong contribution of multi-parton interaction (MPI). This is different from the traditional pQCD picture, where charm quark pairs are produced from a single hard scattering between partons in p+p collisions. In this work, we will report the J/ψ normalized production yield as a function of normalized charged particle multiplicity over a board range of rapidity and event multiplicity in the J/ψ→μ+μ− channel with PHENIX Run 15 p+p data at s=200 GeV. The results are compared with PYTHIA 8 simulations with the MPI option turned on and off. Finally, the outlooks of J/ψ in p+Au and Au+p collisions, along with color glass condensate (CGC) predictions and the multiplicity-dependent ψ(2S)/J/ψ ratio in p+p data, will be briefly discussed.

Overview of quarkonium production in ALICE

ALICE is an experiment mainly dedicated to the study of the quark-gluon plasma (QGP), a state of nuclear matter where quarks and gluons are deconfined, due to high temperature and density. The production of quarkonia, bound states of either a charm and anti-charm quark pair for charmonia or a bottom and anti-bottom quark pair for bottomonia, is a very useful probe of this state of matter. Indeed, the presence of the QGP modifies their production yields, due to a competition between medium-induced suppression and, at least for charmonia, a recombination mechanism in medium or at hadronization. The measurement of quarkonium production in proton-proton (pp) collisions is also essential to investigate its production mechanism as well as to test quantum chromodynamics (QCD) models. It also provides a reference for the investigation of the properties of the QGP in nucleus–nucleus collisions. Measurements in proton–nucleus collisions allow one to study the cold nuclear matter effects. In addition, high multiplicity pp collisions are useful to investigate multiparton interactions and to search for collectivity in small systems. In this contribution, we will report on the latest quarkonium production results, obtained with the ALICE detector in pp collisions, for different center-of-mass energies, at midrapidity and forward rapidity. We will also present the measurements of the quarkonium nuclear modification factor in p–Pb collisions at sNN=8.16TeV. Finally, in Pb–Pb collisions at sNN=5.02TeV, we will report on the new ψ(2S) measurement, the nuclear modification factor of the prompt and non-prompt J/ψ, as well as the first measurement of quarkonium polarization, the latter carried out also as a function of the event plane. All the measurements will be compared to theoretical predictions.

Kurt Gottfried

Kurt Gottfried

Precise evaluation of and axion-like particle production * *Supported in part by National Science Foundation of China (11875179, 11775130, 11635009)

We study the decay of the SM Higgs boson to a massive charm quark pair at the next-to-next-to-leading order QCD and next-to-leading order electroweak. At the second order of QCD coupling, we consider the exact calculation of flavour-singlet contributions where the Higgs boson couples to the internal top and bottom quark. Helpful information on the running mass effects related to Yukawa coupling may be obtained by analyzing this process. High precision production for within the SM makes it possible to search for new physics that may induce relatively large interactions related to the charm quark. As an example, we evaluate the axion-like particle associate production with a charm quark pair in the Higgs decay and obtain some constraints for the corresponding parameters under some assumptions.

Measurement of quarkonium production in ALICE

ALICE is designated to study the quark–gluon plasma (QGP), a state of matter where, due to high temperature and density, quarks and gluons are deconfined. One of the probes used to investigate this state of matter is quarkonium states, bound states of either a charm and anti-charm quark pair (charmonia) or a bottom and anti-bottom quark pair (bottomonia). The presence of the QGP is expected to modify the quarkonium production yields in a very specific way due to a balance between medium-induced suppression, and a recombination mechanism or a hadronization mechanism. To understand the the properties of the QGP in nucleus-nucleus collisions, it is essential to measure the quarkonium differential yields in proton–proton collisions, as it provides a reference and allows the investigation of quarkonium production mechanisms, as well as in proton–nucleus collisions to understand the cold nuclear matter effects that appear. In this contribution, the latest results for quarkonium production measured with the ALICE detector in pp collisions at different collision energies are reported. The measurements of the nuclear modification factor and anisotropic flow in Pb–Pb collisions at sNN=5.02TeV and in p–Pb at sNN=8.16TeV at mid- and forward rapidity are also reported. All measurements are compared to various theoretical predictions.

J/ψ meson production in association with an open charm hadron at the LHC: A reappraisal

We critically (re)examine the associated production process of a $J/\psi$ meson plus an open charm hadron at the LHC in the proton-proton ($pp$) and proton-lead ($p{\rm Pb}$) collisions. Such a process is very intriguing in the sense of tailoring to explore the double parton structure of nucleons and to determine the geometry of partons in nuclei. In order to interpret the existing $pp$ data with the LHCb detector at the center-of-mass energy $\sqrt{s}=7$ TeV, we introduce two overlooked mechanisms for the double parton scattering (DPS) and single parton scattering (SPS) processes. Besides the conventional DPS mode, where the two mesons are produced almost independently in the two separate scattering subprocesses, we propose a novel DPS mechanism that the two constituent (heavy) quarks stemming from two hard scatterings can form into a composite particle, like the $J/\psi$ meson, during the hadronization phase. However, it turns out the corresponding contribution is small in $J/\psi+c\bar{c}$ hadroproduction. On the contrary, we point out that the resummation of the initial state logarithms due to gluon splitting into a charm quark pair is crucial to understand the LHCb measurement, which was overlooked in the literature. We perform a proper matching between the perturbative calculations in different initial-quark flavor number schemes, generically referring to the variable flavor number scheme. The new variable flavor number scheme calculation for the process strongly enhance the SPS cross sections, almost closing the discrepancies between theory and experiment. Finally, we present our predictions for the forthcoming LHCb measurement in $p{\rm Pb}$ collisions at $\sqrt{s_{NN}}=8.16$ TeV. Some interesting observables are exploited to set up the control regions of the DPS signal and to probe the impact-parameter dependent parton densities in lead.

Modification of the spectra of correlated charm-anticharm quark pairs in the quark-gluon plasma * * This work was supported by Chinese government scholarship (CSC) for foreigner graduate students and by NSFC (11675079)

Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. We study the interactions of single heavy (charm) quarks and correlated charm and anticharm ( ) quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions. Besides studying the traditional observables, the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates, we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes. We quantified the smearing of pair azimuthal correlations with an increasing thermal relaxation rate: while the (nearly) back-to-back correlations among pairs are almost completely washed out at low transverse momentum (pT), these correlations at high pT largely survive the pair diffusion. This provides a novel observable for diagnosing the properties of QGP.

A search for the standard model Higgs boson decaying to charm quarks

A direct search for the standard model Higgs boson, H, produced in association with a vector boson, V (W or Z), and decaying to a charm quark pair is presented. The search uses a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb−1, collected by the CMS experiment at the LHC in 2016, at a centre-of-mass energy of 13 TeV. The search is carried out in mutually exclusive channels targeting specific decays of the vector bosons: W → ℓν, Z → ℓℓ, and Z → νν, where ℓ is an electron or a muon. To fully exploit the topology of the H boson decay, two strategies are followed. In the first one, targeting lower vector boson transverse momentum, the H boson candidate is reconstructed via two resolved jets arising from the two charm quarks from the H boson decay. A second strategy identifies the case where the two charm quark jets from the H boson decay merge to form a single jet, which generally only occurs when the vector boson has higher transverse momentum. Both strategies make use of novel methods for charm jet identification, while jet substructure techniques are also exploited to suppress the background in the merged-jet topology. The two analyses are combined to yield a 95% confidence level observed (expected) upper limit on the cross section sigma left(mathrm{VH}right)mathrm{mathcal{B}}left(mathrm{H}to mathrm{c}overline{mathrm{c}}right) of 4.5 left({2.4}_{-0.7}^{+1.0}right) pb, corresponding to 70 (37) times the standard model prediction.

Class imbalance techniques for high energy physics

A common problem in a high energy physics experiment is extracting a signal from a much larger background. Posed as a classification task, there is said to be an imbalance in the number of samples belonging to the signal class versus the number of samples from the background class. In this work we provide a brief overview of class imbalance techniques in a high energy physics setting. Two case studies are presented: (1) the measurement of the longitudinal polarization fraction in same-sign WW scattering, and (2) the decay of the Higgs boson to charm-quark pairs.

Exclusive heavy quark-pair production in ultraperipheral collisions

In this article, we study the fully differential observables of exclusive production of heavy (charm and bottom) quark pairs in high-energy ultraperipheral $pA$ and $AA$ collisions. In these processes, the nucleus $A$ serves as an efficient source of the photon flux, while the QCD interaction of the produced heavy-quark pair with the target ($p$ or $A$) proceeds via an exchange of gluons in a color singlet state, described by the gluon Wigner distribution. The corresponding predictions for differential cross sections were obtained by using the dipole $S$-matrix in the McLerran--Venugopalan saturation model with impact parameter dependence for the nucleus target, and its recent generalization, for the proton target. Prospects of experimental constraints on the gluon Wigner distribution in this class of reactions are discussed.

Higgs physics at CMS

The most recent results of a Higgs boson production and properties from the CMS Collaboration using the LHC Run 1 and Run 2 data are reported. These include analyses of a Higgs boson decaying to a pair of photons, four leptons via Z boson pair decays and the associated production of a Higgs boson with top quark pair, predicted by the Standard Model (SM). The studies of a Higgs boson decays to a pair of bottom quarks, a pair of tau leptons and a charm quark pair are also presented. The Higgs boson production via vector boson fusion (VBF) and decaying to invisible particles is reported here as well. The analysis of a Higgs boson decay to a pair of muons is also presented. The study of a Higgs boson pair production at 13 TeV is performed as well and projections of a Higgs boson self couplings together with the couplings to other particles at the HL-LHC are made.

Quarkonium production in pp and p-A collisions with ALICE at the LHC

Quarkonia are mesons formed of either a charm and anti-charm quark pair (J/ψ, ψ(2S)) or a beauty and anti-beauty quark pair (ϒ(1S), (2S) and (3S)). We report on forward rapidity (2:5 < y < 4) J/ψ and ψ(2S) production measured in pp collisions at √s = 13 TeV, using data collected at the LHC in 2015. The results will be compared with similar measurements performed at √s = 2.76, 5.02, 7 and 8 TeV. They will be further compared to NRQCD and FONLL calculations, which describe prompt and non-prompt charmonium production respectively. Results of the J/ψ nuclear modification factor as a function of collision centrality in p–Pb collisions at [see formula in PDF] = 8.16 TeV, at forward and backward rapidities, will also be presented. These measurements will be compared with Run-1 (2009 – 2013) results as well as theoretical calculations and will be interpreted in terms of cold nuclear matter effects.

Production of primordial J/ψ in relativistic p+p and heavy-ion collisions

$J/\ensuremath{\psi}$ observables of $p+p$ collisions at $\sqrt{s}=200\phantom{\rule{0.16em}{0ex}}\mathrm{GeV}$ and 2.76 TeV are calculated using the pythia event generator for the production of $c$ and $\overline{c}$ and the Wigner density formalism. The charm quark momentum from pythia is tuned in such a way that the transverse momentum and rapidity distribution correspond to the fixed-order next-to-leading logarithm calculations and hence to the experimental data for open charm mesons. Using the Wigner density of the charmonium wave function we calculate for each charm quark pair the probability to form a charmonium. As a result, the experimental data on the total $J/\ensuremath{\psi}$ yield, its rapidity and transverse momentum distribution as well as the fractions of feed-down from excited states of charmonium are reproduced. Applying the same approach to relativistic heavy-ion collisions, we find, if quark gluon plasma (QGP) effects are ignored, a considerable enhancement of the primordial $J/\ensuremath{\psi}$'s.

Radiative hc/b decays to η or η′

Motivated by recent measurements of the radiative decay rates of the $P$-wave spin singlet charmonium ${h}_{c}$ to the light meson $\ensuremath{\eta}$ or ${\ensuremath{\eta}}^{\ensuremath{'}}$ by the BESIII Collaboration, we investigate the decay rates of these channels at order $\ensuremath{\alpha}{\ensuremath{\alpha}}_{s}^{4}$. The photon is radiated mainly from charm quark pairs in the lowest-order Feynman diagrams, because the diagrams where a photon radiated from light quarks are suppressed by ${\ensuremath{\alpha}}_{s}$ or the relative charm quark velocity $v$, due to charge-parity conservation. The form factors of two gluons to $\ensuremath{\eta}$ or ${\ensuremath{\eta}}^{\ensuremath{'}}$, which are the major mechanism for $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ production, are employed. $\ensuremath{\eta}({\ensuremath{\eta}}^{\ensuremath{'}})$ is treated as a light-cone object when we consider that the parent charmonium mass is much heavier than that of the final light meson. We obtain the branching ratio $\mathcal{B}({h}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}{\ensuremath{\eta}}^{\ensuremath{'}})=(1.9{4}_{\ensuremath{-}0.51}^{+0.70})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in the nonrelativistic QCD approach, which is in agreement with the BESIII measurement. The prediction of the branching ratio of ${h}_{c}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\eta}$ is also within the range of experimental error after including the larger uncertainty of the total decay width ${\mathrm{\ensuremath{\Gamma}}}_{{h}_{c}}$. The applications of these formulas to the radiative decays to $\ensuremath{\eta}({\ensuremath{\eta}}^{\ensuremath{'}})$ of the $P$-wave spin singlet bottomonium ${h}_{b}(nP)$ are presented. These studies will shed some light on the $\ensuremath{\eta}\ensuremath{-}{\ensuremath{\eta}}^{\ensuremath{'}}$ mixing effects, the flavor SU(3) symmetry breaking, and the nonperturbative dynamics of charmonium and bottomonium.

Free energy versus internal energy potential for heavy-quark systems at finite temperature

Using the QCD sum rule with its operator product expansion reliably determined from lattice calculations for the pressure and energy density of hot QCD matter, we calculate the strength of the $J/\ensuremath{\psi}$ wave function at the origin and find that it decreases with temperature when the temperature is in the vicinity of the transition temperature. This result is shown to follow exactly that obtained from the solution of the Schr\"odinger equation for a charm and anticharm quark pair with a temperature independent quark mass using the free energy from lattice calculations as the potential and is in sharp contrast to that using the deeper potential associated with the internal energy, which shows an enhanced strength of the $J/\ensuremath{\psi}$ wave function at the origin. Our result thus suggests that the free energy potential from lattice calculations is the appropriate heavy-quark potential for analyzing the charmonium spectrum at a finite temperature.

Measurements of J/ψ → e+e− with ALICE at the LHC

The ALICE detector provides excellent capabilities to study quarkonium production at the Large Hadron Collider (LHC). Heavy quarkonia, bound states of charm or beauty quark anti-quark pairs such as the J/ψ, are expected to be produced by initial hard processes. Thus they will provide insight into the earliest and hottest stages of AA collisions where the formation of a Quark-Gluon Plasma (QGP) is expected. Furthermore, high-precision data from pp collisions represent an essential baseline for the measurement of nuclear modifications in heavy-ion collisions and serve also as a crucial test for several models of quarkonium hadroproduction. In addition, the study of pA collisions allows to investigate nuclear modifications due to Cold Nuclear Matter (CNM) effects. In ALICE, J/ψ were measured in pp and Pb-Pb collisions down to pT = 0 via their di-electron decay channel in the central barrel (|y| < 0.8). Results on the nuclear modification factor (RAA) at central rapidities in Pb-Pb collisions at = 2.76 TeV will be shown and their implications discussed. A separation of the prompt and non-prompt components is also possible down to pT of the J/ψ of 2 GeV/c.

Charmonia formation in quark-gluon plasma

Using the color evaporation model, the cross section for charmonium production in $\text{p}+\text{p}$ collision is calculated in quark-gluon plasma. The threshold energy for open charms is given by the free-energy potential from lattice calculations, the initial charm quark pairs by the pythia simulations, and their time evolution by solving the Langevin equation. It is found that the threshold energy, which decreases with temperature, reduces the cross section while the invariant mass of charm pairs which decreases by collisions enhances it. As a result, charmonium production is suppressed by $20%\ensuremath{\sim}50%$ depending on temperature and charm diffusion constant while $J/\ensuremath{\psi}$ production is similar or enhanced compared with in vacuum.

Azimuthal correlation of charm quark pair produced in heavy ion collision

It is known that heavy quarks are produced in pairs. There always exists a correlation in azimuthal angle in transverse momentum plane between the members in a pair. The quark and anti-quark in a pair may lose different amount of energy while traveling through QGP and it is expected that the correlation of QQ̄ will alter considerably. Also in a complete different picture, collective flow of the medium can change the correlation of heavy quark pair. Here we present the effect of charm quark energy loss based on Wang-Huang-Sarcevic model on its correlation. The flow feature is shown using a naive model based on Cautle-Páic model.

SHARE with CHARM

SHARE with CHARM program (SHAREv3) implements the statistical hadronization model description of particle production in relativistic heavy-ion collisions. Given a set of statistical parameters, SHAREv3 program evaluates yields and therefore also ratios, and furthermore, statistical particle abundance fluctuations. The physical bulk properties of the particle source is evaluated based on all hadrons produced, including the fitted yields. The bulk properties can be prescribed as a fit input complementing and/or replacing the statistical parameters. The modifications and improvements in the SHARE suite of programs are oriented towards recent and forthcoming LHC hadron production results including charm hadrons. This SHAREv3 release incorporates all features seen previously in SHAREv1.x and v2.x and, beyond, we include a complete treatment of charm hadrons and their decays, which further cascade and feed lighter hadron yields. This article is a complete and self-contained manual explaining and introducing both the conventional and the extended capabilities of SHARE with CHARM. We complement the particle list derived from the Particle Data Group tabulation composed of up, down, strange $u,d,s$ quarks (including resonances) with hadrons containing charm $c,\bar c$ quarks. We provide a table of the charm hadron decays including partial widths. The branching ratios of each charm hadron decays add to unity, which is achieved by including some charm hadron decay channels based on theoretical consideration in the absence of direct experimental information. A very successful interpretation of all available LHC results has been already obtained using this program.

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.