Heavy Partons Research Articles

Equation of State and Energy Loss of Hot and Dense Quark-Gluon matter from Holographic Black Holes

By using gravity/gauge correspondence, we construct a holographic model, constrained to mimic the lattice QCD equation of state at zero density, to investigate the temperature and baryon chemical potential dependence of the equation of state. We also obtained the energy loss of light and heavy partons within the hot and dense plasma represented by the heavy quark drag force, Langevin diffusion coefficients and jet quenching parameter at the critical point and across the first-order transition line predicted by the model.

Heavy-flavour jet properties and correlations from small to large systems measured by ALICE

The early production of heavy-flavour partons makes them an excellent probes for investigating the evolution of QCD systems. Jets tagged by the presence of a heavy-flavour hadron give access to the kinematics of the heavy partons and allow for comparisons of their production, propagation and fragmentation across different systems. Whilst traversing the quark-gluon plasma (QGP), charm and bottom quarks lose energy through interactions with the medium, at a different rate relative to light quarks and gluons. To constrain the energy loss in the QGP. the nuclear modification factor of D0-tagged jets is measured in the 0-10% most central Pb-Pb collisions at √SNN = 5.02 GeV/c. The properties of charm fragmentation are also investigated in pp collisions at √S = 5.02 GeV/c through measurements of the production and the momentum fraction of the D0 with respect to its jet.

Nonperturbative effects on radiative energy loss of heavy quarks

The radiative energy loss of fast partons traveling through the quark-gluon plasma (QGP) is commonly studied within perturbative QCD (pQCD). Nonperturbative (NP) effects, which are expected to become important near the critical temperature, have been much less investigated. Here, we utilize a recently developed T -matrix approach to incorporate NP effects for gluon emission off heavy quarks propagating through the QGP. We set up four cases that contain, starting from a Born diagram calculation with color- Coulomb interaction, an increasing level of NP components, by subsequently including (remnants of ) confining interactions, resummation in the heavy-light scattering amplitude, and off-shell spectral functions for both heavy and light partons. For each case we compute the power spectra of the emitted gluons, heavy-quark transport coefficients (drag and transverse-momentum broadening, hat{q} ), and the path-length dependent energy loss within a “QGP brick” at fixed temperature. Investigating the differences in these quantities between the four cases illustrates how NP mechanisms affect gluon radiation processes. While the baseline perturbative processes experience a strong suppression of soft radiation due to thermal masses of the emitted gluons, confining interactions, ladder resummations and broad spectral functions (re-)generate a large enhancement toward low momenta and low temperatures. For example, for a 10 GeV charm quark at 200 MeV temperature, they enhance the transport coefficients by up to a factor of 10, while the results smoothly converge to perturbative results at sufficiently hard scales.

Energy loss of heavy quarks in the isotropic collisional hot QCD medium

The collisional energy loss of heavy partons (charm and bottom quarks) has been determined within the framework of semi-classical transport theory implying the Bhatnagar–Gross–Krook (BGK) collisional kernel. Hot QCD medium effects have been incorporated while employing a quasi-particle description of the medium in terms of effective gluons, quarks and antiquarks with respective temperature dependent effective fugacities. The momentum dependence of the energy loss for the charm and the bottom quark has been investigated. It is observed that with the increase in momentum of the heavy quarks, the loss increases sharply for the smaller values and reaches saturation later. Furthermore, as compared to the charm quark, the bottom quark loses less energy at a particular momentum and collisional frequency. The energy loss is seen to increase with increasing collisional frequency. We also provide a comparative study of the results obtained using the BGK kernel rather than those using the relaxation time approximation (RTA) kernel and found them to be consistent with each other. The medium effects in all the situations are seen to play a quite significant role.

Linearized Boltzmann transport model for jet propagation in the quark-gluon plasma: Heavy quark evolution

A Linearized Boltzmann Transport (LBT) model coupled with hydrodynamical background is established to describe the evolution of jet shower partons and medium excitations in high energy heavy-ion collisions. We extend the LBT model to include both elastic and inelastic processes for light and heavy partons in the quark-gluon plasma. A hybrid model of fragmentation and coalescence is developed for the hadronization of heavy quarks. Within this framework, we investigate how heavy flavor observables depend on various ingredients, such as different energy loss and hadronization mechanisms, the momentum and temperature dependences of the transport coefficients, and the radial flow of the expanding fireball. Our model calculations show good descriptions of the $D$ meson suppression and elliptic flow observed at the LHC and RHIC. The prediction for the Pb-Pb collisions at $\sqrt{s_\mathrm{NN}}$=5.02~TeV is provided.

New parton distribution functions from a global analysis of quantum chromodynamics

We present new parton distribution functions (PDFs) up to next-to-next-to-leading order (NNLO) from the CTEQ-TEA global analysis of quantum chromodynamics. These differ from previous CT PDFs in several respects, including the use of data from LHC experiments and the new D0 charged lepton rapidity asymmetry data, as well as the use of more flexible parametrization of PDFs that, in particular, allows a better fit to different combinations of quark flavors. Predictions for important LHC processes, especially Higgs boson production at 13 TeV, are presented. These CT14 PDFs include a central set and error sets in the Hessian representation. For completeness, we also present the CT14 PDFs determined at the leading order (LO) and the next-to-leading order (NLO) in QCD. Besides these general-purpose PDF sets, we provide a series of (N)NLO sets with various $\alpha_s$ values and additional sets in general-mass variable flavor number (GM-VFN) schemes, to deal with heavy partons, with up to 3, 4, and 6 active flavors.

Heavy-flavour production in ALICE at the LHC

ALICE at the LHC is the experiment dedicated to study the physics of nucleus-nucleus collisions. The apparatus is well suited for the measurement of heavy-quark hadron production, making use of the high spatial resolution provided by the tracking detectors and the excellent particle identification, which are distinctive of the ALICE apparatus. Results from proton–proton collisions at s=2.76 and 7 TeV, and from Pb–Pb collisions at sNN=2.76 TeV are presented. The measurements in pp collisions provide an important test of perturbative QCD predictions. The precise vertex reconstruction together with the electron identification, allows the separation of the charm and the beauty components. Furthermore, the pp results are essential as a reference for the measurements in heavy-ion collisions. Nuclear modification factors were measured for D mesons, for electrons and for muons from heavy-flavour hadron decays. The elliptic flow of D mesons is also discussed. These measurements are important because they will provide information on the Quark–Gluon Plasma produced in heavy-ion collisions, via the energy loss of the heavy partons in the strongly interacting medium, and hints on the medium thermalization.

Hadron Multiplicity in Pb+Pb Collisions at √s = 2.76 TeV from Color Glass Condensate

The pseudo-rapidity distribution of charged hadron multiplicity in Pb+Pb collisions at √s = 2.76 TeV is studied by using Color Glass Condensate dynamics in the fixed coupling case. We fit the HERA experimental data with heavy parton mass effect to obtain saturation exponent λ = 0.22. It is found that the charged hadron multiplicity can only describe after including heavy parton mass effect due to a large amount of heavy partons produced at large hadron collider (LHC) energy. The pomeron loop effect contribution to hadron production is also investigated. It shows that charged hadron multiplicity is underestimated by including the pomeron loop effect, which indicates that the pomeron loop effect may not exist at LHC energy.

General mass scheme for jet production in DIS

We propose a method for calculating DIS jet production cross sections in QCD at NLO accuracy with consistent treatment of heavy quarks. The scheme relies on the dipole subtraction method for jets, which we extend to all possible initial state splittings with heavy partons, so that the Aivazis-Collins-Olness-Tung massive collinear factorization scheme can be applied. As a first check of the formalism we recover the Aivazis-Collins-Olness-Tung result for the heavy quark structure function using a dedicated Monte Carlo program.

Open Heavy Flavor Production in Heavy Ion Collisions

The interaction of heavy partons, charm and beauty, with the matter created in heavy ion collisions has been of great interest in recent years. Heavy partons were predicted to interact less strongly with the matter than light partons. In apparent contrast to these predictions, unexpectedly strong suppression of non-photonic electrons from heavy flavor decays has been seen. However, significant experimental uncertainties remain, both in the measurements themselves and in the separation of the contribution from charm and beauty, which have complicated the interpretation of these results. The current experimental situation is critically reviewed and prospects for making these measurements more easily interpretable discussed.

Heavy Flavor Production and Energy Loss with Two-Particle Correlations at PHENIX

Heavy quarks are a valuable probe of the hot, dense medium created in a heavy ion collision, and are an important test of proposed mechanisms of energy loss. It was discovered that single non-photonic electrons are suppressed at a similar level to light hadrons, implying a comparable level of energy loss between light and heavy partons. Because theory has had a difficult time explaining the level of heavy quark energy loss, it is crucial to better understand charm and bottom suppression. Electron-hadron correlations have been used at PHENIX to study heavy flavor in both p + p and Au+Au collisions. In p + p the ratio of charm to bottom production has been measured using mass correlations through a partial reconstruction of the D meson. Electron-hadron angular correlations have also been used to study medium modification of heavy flavor, and we see hints of energy loss effects. A complementary study of correlated electron-muon pairs provides a clean measurement of heavy flavor production in a rapidity range not yet studied.

The relic abundance of long-lived heavy colored particles

Long-lived colored particles with masses m200 GeV are allowed by current accelerator searches, and are predicted by a number of scenarios for physics beyond the standard model. We argue that such ``heavy partons'' effectively have a geometrical cross section (of order 10 mb) for annihilation at temperatures below the QCD deconfinement transition. The annihilation process involves the formation of an intermediate bound state of two heavy partons with large orbital angular momentum. The bound state subsequently decays by losing energy and angular momentum to photon or pion emission, followed by annihilation of the heavy partons. This decay occurs before nucleosynthesis for m1011 GeV for electrically charged partons and mTeV for electrically neutral partons. This implies that heavy parton lifetimes as long as 1014 sec are allowed even for heavy partons with m ~ TeV decaying to photons or hadrons with significant branching fraction.

Charm production in the STAR experiment at RHIC

The single electron spectrum over a sufficiently broad \(p_{\rm T}\) range provides an indirect measurement of charm and beauty production at RHIC energies. Heavy-quarks are produced in parton-parton scattering in the initial phase of the collision and thus provide important information about the initial configuration of the colliding nuclei. Even more important, the final state spectra reflect the interactions of these heavy partons with the medium and thus allow us to probe the properties of the hot and dense system created in heavy-ion collisions. We present preliminary measurements of electron and positron \(p_{\rm T}\) spectra in pp and d + Au as well as preliminary elliptic flow measurements in Au + Au collisions at \(\sqrt{s_{NN}} = 200\) GeV performed by the STAR experiment. We describe the measurement techniques used to discriminate electrons from hadrons and compare the results with theoretical calculations.

When is a heavy quark not a parton? Charged Higgs production and heavy quark mass effects in the QCD-based parton model

Several issues pertaining to the application of the QCD-based parton model to new physics processes involving heavy partons are described and quantitatively studied using charged Higgs boson production as a prime example. The naive parton model predictions are found to over-estimate the actual cross section by a factor of 2 to 5, depending on the top-quark and Higgs masses. The role of the top quark as a “parton” is examined by a detailed study of the cancellation between the straight parton model contribution and a subtraction term required by QCD corrections. The accuracy of the zero-mass method for evaluating the first-order QCD correction is assessed (in light of the potentially large mass of the top quark) by a quantitative analysis of the cancellation of mass singularities between the correction terms. A pragmatic procedure for calculation based on a renormalization scheme without the heavy quark-parton is formulated and compared with the usual perturbative QCD formalism. The energy ranges over which heavy quarks (or other particles) should or should not be naturally treated as “partons” are delineated. Properly evolved parton distribution functions relevant to the specific renormalization schemes considered are employed for all the numerical studies in order to ensure consistency in the QCD framework.

Ultra-heavy particle production from heavy partons at hadron colliders

The existence of heavy partons in the proton can have important consequences for hard scattering processes at high-energy hadron colliders. Nevertheless, by naively using heavy-parton distribution functions, one may be double-counting, since the origin of the heavy partons is due to pair production from gluons inside the proton. Using perturbative QCD, we develop a technique by which heavy-parton distribution functions may be used in a consistent manner. We illustrate the technique with two examples of the production of ultra-heavy particles: heavy squark production due to gluino-quark fusion and heavy charged Higgs production via t b fusion.

Fragmentation of hadrons from heavy quark partons

A model is presented to describe hadron fragmentation off light and heavy partons. Fragmentation functions are parametrized by one variable. When a heavy parton of a new flavour fragments, a heavy hadron tends to carry away most of the parton momentum, leaving light hadron ( π and K) spectra softer than those from light partons.

Parton fragmentation model for inclusive pion spectra in electron-positron annihilation

The inclusive pion spectra in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation are analyzed in the parton fragmentation model with nonvanishing transverse momenta and with heavy partons. The nonscaling phenomena in the small-$x$ region are explained quite satisfactorily in this model.