Color-singlet Clusters Research Articles

Heavy-flavor meson and baryon production in high-energy nucleus-nucleus collisions

We present a new model for the description of heavy-flavor hadronization in relativistic heavy-ion collisions. We explore its effect on the charmed hadron yields and kinematic distributions once the latter is applied at the end of transport calculations used to simulate the propagation of heavy quarks in the deconfined fireball produced in the collision. The model is based on the formation of color-singlet clusters through the recombination of charm quarks with light antiquarks or diquarks from the same fluid cell. This local mechanism of color neutralization leads to a strong space-momentum correlation, which provides a substantial enhancement of charmed baryon production and of the collective flow of all charmed hadrons.

In-medium hadronization of heavy quarks and its effect on charmed meson and baryon distributions in heavy-ion collisions

We present a new model for the description of heavy-quark hadronization in relativistic heavy-ion collisions in the presence of a reservoir of lighter thermal particles with which recombination can occur leading to the formation of color-singlet clusters. Color neutralization is assumed to occur locally, within the same fluid cell occupied by the heavy quark and it proceeds via the recombination of the latter with light antiquarks (quarks) or diquarks (anti-diquarks), which are assumed to be present in the medium with thermal abundance as effective degrees of freedom around the QCD crossover temperature T_c. Typically the resulting color-singlet clusters have quite low invariant mass, in most of the cases below 4 GeV, and in this case they are assumed to undergo an isotropic 2-body decay in their local rest-frame. Heavier clusters are instead fragmented as Lund strings. The possibility of recombination with light diquarks enhances the yields of charmed baryons, in qualitative agreement with recent measurements. The assumption of local color neutralization leads to a strong space-momentum correlation, which provides a substantial enhancement of the collective flow of the final-state charmed hadrons, affecting both their momentum and their angular distributions.

Possible light neutral boson and particle mass quantization * *Supported by the National Natural Science Foundation of China (10175091, 11305007)

Qualities of nucleons, such as the fundamental parameter mass, might be modified in extreme conditions relative to those of isolated nucleons. We show the ratio of the EMC-effect tagged nucleon mass to that of the free one ( ); these values are derived from the nuclear structure function ratio between heavy nuclei and deuterium measured in the electron Deep Inelastic Scattering (DIS) reaction in 0.3 0.7. The increase in with is phenomenologically interpreted via the release of a color-singlet cluster formed by sea quarks and gluons in bound nucleons holding high momentum in the nucleus, from which the mass and fraction of non-nucleonic components in nuclei can be deduced. The mass of color-singlet clusters released per short range correlated (SRC) proton in the high momentum region ( 2 fm ) is extracted to be 16.890 0.016 MeV/c , which evidences the possibility of a light neutral boson and quantized mass of matter.

Medium-induced color flow softens hadronization

Medium-induced parton energy loss, resulting from gluon exchanges between the QCD matter and partonic projectiles, is expected to underly the strong suppression of jets and high-$p_T$ hadron spectra observed in ultra-relativistic heavy ion collisions. Here, we present the first color-differential calculation of parton energy loss. We find that color exchange between medium and projectile enhances the invariant mass of energetic color singlet clusters in the parton shower by a parametrically large factor proportional to the square root of the projectile energy. This effect is seen in more than half of the most energetic color-singlet fragments of medium-modified parton branchings. Applying a standard cluster hadronization model, we find that it leads to a characteristic additional softening of hadronic spectra. A fair description of the nuclear modification factor measured at the LHC may then be obtained for relatively low momentum transfers from the medium.

The role of in-medium color flow for high-pT particle production in AA collisions

The suppression of high-pT single-hadron spectra in heavy-ion collisions is usually interpreted as due to parton energy loss of high-momentum quarks and gluons propagating in the plasma. Here, we discuss to what extent this partonic picture must be complemented by a picture of medium-modified hadronization. In particular, we show how color exchange with the medium modifies the properties of color-singlet clusters arising from the parton branchings, producing a softening of the hadron spectra.

Color-singlet clustering of partons and recombination model for hadronization of quark-gluon plasma

SU$({N}_{c})$ color-singlet restriction, along with flavor and spin symmetry, on a thermal partonic ensemble is shown to recombine the partons with internal color structure into color-singlet multiquark clusters, which can be identified with various hadronic modes at a given temperature. This can provide a possible basis for a recombination model for hadronization of quark-gluon plasma. This also leads to a natural explanation for the ratio of (anti)protons to pions and the quark number scaling of the elliptic flow coefficient in relativistic heavy-ion collisions.

Dihyperons in chiral color dielectric model

The mass of the dibaryon having spin, parityJπ = 0+, isospinI = 0 and strangeness—2 is computed using chiral color dielectric model. The bare wave function is constructed as a product of two color-singlet three-quark clusters and then it is properly antisymmetrized by considering appropriate exchange operators for spin, flavor and color. Color magnetic energy due to gluon exchange, meson self energy and energy correction due to center of mass motion are computed. The calculation shows that the mass of the particle is 80 to 160 MeV less than twice λ mass.

Strange quark production in a statistical effective model

An effective model with constituent quarks as fundamental degrees of freedom is used to predict the relative strangeness production pattern in both high energy elementary and heavy ion collisions. The basic picture is that of the statistical hadronization model, with hadronizing color-singlet clusters assumed to be at full chemical equilibrium at constituent quark level. Thus, by assuming that at least the ratio between strange and non-strange constituent quarks survives in the final hadrons, the apparent undersaturation of strange particle phase space observed in the data can be accounted for. In this framework, the enhancement of relative strangeness production in heavy ion collisions in comparison with elementary collisions is mainly owing to the excess of initial non-strange matter over antimatter and the so-called canonical suppression, namely the constraint of exact color and flavor conservation over small volumes.

Baryon density correlations in high temperature hadronic matter

As part of an ongoing effort to characterize the high temperature phase of QCD, in a numerical simulation using the staggered fermion scheme, we measure the quark baryon density in the vicinity of a fixed test quark at high temperature and compare it with similar measurements at low temperature and at the crossover temperature. We find an extremely weak correlation at high temperature, suggesting that small color singlet clusters are unimportant in the thermal ensemble. We also find that at T=0.75 ${\mathit{T}}_{\mathit{c}}$ the total induced quark number shows a surprisingly large component attributable to baryonic screening. A companion simulation of a simple flux tube model produces similar results and also suggests a plausible phenomenological scenario: As the crossover temperature is approached from below, baryonic states proliferate. Above the crossover temperature the mean size of color singlet clusters grows explosively, resulting in an effective electrostatic deconfinement.

Baryon density correlations in the quark plasma

As part of an ongoing effort to characterize the high temperature phase of QCD, we measure the quark baryon density in the vicinity of a fixed test quark and compare it with similar measurements at low temperature and at the crossover temperature. Such an observable has also been studied by the Vienna group. We find an extremely weak correlation at high temperature, suggesting that small color singlet clusters are unimportant in the thermal ensemble. We also find that at T = 0.75 T c the induced quark number shows a surprisingly large component attributable to baryonic screening. A simulation of a simple flux tube model produces results that suggest a plausible scenario: As the crossover temperature is approached from below, baryonic states proliferate. Above the crossover temperature the mean size of color singlet clusters grows explosively, resulting in an effective electrostatic deconfinement.

A QCD model for jet fragmentation including soft gluon interference

A new model for hadronic jet fragmentation in hard processes is presented. It is based on a QCD parton branching mechanism with correct treatment of leading collinear and infra-red singularities (i.e. including soft gluon interference). Hadronization occurs via preconfinement of colour singlet clusters, which decay according to a simple phase-space scheme. Although tightly constrained, the model gives a good account of existing e + e − annihilation data. It predicts significant differences between quark and gluon jets. Comparisons with datadata on the CERN p̄p collider jets suggest that a large fraction of them are gluon jets. The model predicts soft gluon interference effects in hadron distributions which are probably not yet observable for pions but should be clear for kaons and baryons.

Baryon production in e +e − annihilations

A dynamical mechanism for the production of baryons in e +e − annihilations both on and off the upsilon resonance is presented. Parton showers are produced using an approximation to QCD perturbation theory. Color singlet clusters are formed and “decay” into hadrons through a simple phase-space process. Baryon-antibaryon pairs are produced in the decay of high mass clusters and are more abundant at the upsilon since the initial state of three gluons results in more high mass clusters than does the quark-antiquark initial state of the continuum.

A QCD model for e +e − annihilation

A QCD model for e +e − annihilation is presented, and its consequences are compared with experimental data. The model involves production of a shower of partons described by a simple approximation to QCD perturbation theory, and decay of colour singlet clusters of produced partons into hadrons through a simple phase space process. The model reproduces most known theoretical features of QCD, and, with certain choices of parameters, appears to correspond well with experimental results.

Size of colour singlets in QCD jets

Abstract By using the jet calculus technique, the size of the colour singlet clusters of emitted partons of mass Q 0 is estimated. For α(Q 2 0 ) π ⪅1 , the average mass of these clusters turns out to be a few GeV, thus suggesting phenomenological applications of preconfinement in pertubbative QCD. This result is very sensitive to the correct treatment of infrared singularities.

Quantitative properties of colour singlet clusters in QCD jets

Using a Monte Carlo technique we have made detailed calculations of the properties of colour singlet clusters in QCD jets, first discussed by Amati and Veneziano. Although the results confirm a mass damping for colour singlets, we find that, quantitatively, it is not enough to justify a phenomenological use of colour singlets as an intermediate step in the materialization of QCD quanta into hadrons.

The jet mass in leading log QCD

The distribution of the jet mass is computed in the leading log approximation. A Sudakov-like damping factor broadens the distribution compared to the first-order result. The leading quark-gluon pair in a jet is seen to have a damped mass spectrum similar to that for color singlet clusters.

Space-time description of colour screening via jet calculus techniques

The branching process characterizing the perturbative development of a chromodynamic jet up to the formation of colour singlet clusters is studied in real space-time with jet calculus techniques. Whereas the space-time separation of generic partons increases indefinitely with energy (or momentum transfer), such a separation remains finite within each colour singlet system in its own rest frame. These results add further appeal and overall consistency to the preconfinement picture.

Towards an algebraic quantum chromodynamics

We outline a quantum theory of quarks and gluons based on fields with values taken from a noncommutative Jordan algebra. These fields automatically satisfy a triality rule: Quark-antiquark and three-quark states are color singlets. If the elements of the algebra are position dependent, the theory leads to a minimal gauge-invariant coupling between quarks and gluons. The quantization of such a theory is outlined; we find that only color-singlet clusters of quarks and antiquarks have particle properties. The color-nonsinglet fields do not support a representation of the Lorentz group; in fact, their phases are undefined. We conjecture that this can be remedied by a coupling between space-time and flavor symmetries as suggested by Hawking and Pope. Such a coupling naturally leads to one-third-integer values of the quark charges.

Color singlet distributions and mass damping in perturbative QCD

x-Distributions and mass spectrum of color-singlet clusters are given in the leading log approximation. The mass spectrum has an inverse power damping, thus supporting the recently proposed preconfinement idea. The multiplicity of such singlets is also given in terms of the parton multiplicity.