arXiv subjects
Publications and source records attributed to Rudolph C. Hwa.
An overview of the correlation among high-p_T hadrons produced in heavy-ion collisions is presented. Emphasis is placed on the physical processes that can quantitatively account for the data on correlations in p_T, ηand ϕon the near and far sides. Predictions are made on processes that have no observable correlations for hadrons produced at intermediate and higher p_T at RHIC and LHC.
A model of parton multiple scattering in a dense and expanding medium is described. The simulated results reproduce the general features of the data. In particular, in the intermediate trigger momentum region there is a dip-bump structure, while at higher trigger momentum the double bumps merge into a central peak. Also, a new measure is proposed to quantify the azimuthal distribution with the virtue that it suppresses the statistical fluctuations event-by-event, while enhancing the even-structure of the signal.
The production of pions and protons in the $p_T$ range between 10 and 20 GeV/c for Pb+Pb collisions at LHC is studied in the recombination model. It is shown that the dominant mechanism for hadronization is the recombination of shower partons from neighboring jets when the jet density is high. Protons are more copiously produced than pions in that $p_T$ range because the coalescing partons can have lower momentum fractions, but no thermal partons are involved. The proton-to-pion ratio can rise beyond 20. When such a high $p_T$ hadron is used as a trigger particle, there will not be any associated particles that are not in the background.
An approach is proposed to analyze the azimuthal distribution of particles produced on the away side in heavy-ion collisions without background subtraction. Measures in terms of factorial moments are suggested that can suppress the statistical background, while giving clear distinction between one-jet and two-jet event structures on the away side. It is also possible to map the position and strength of the recoil jet to suitably chosen asymmetry moments.
We relate the fragmentation functions of partons into mesons and baryons in the framework of recombination of shower partons. The results are in reasonable agreement with the data. The implication is that the meson and baryon fragmentation functions are not independent when hadronization of the shower partons are taken into account. The conclusion therefore closes a conceptual gap in the system of fragmentation functions whose $Q^2$ evolution has been more extensively studied than their interrelationship.
We study the production of hadrons in Au+Au collisions in the region $0.6 0$, where $η'=η-y_{\rm beam}$, depending on $p_T$. We show how hadrons can be produced in that region when the hadronization process is parton recombination. The inclusive $x$-distributions for proton and pion production are calculated with momentum degradation taken into account. The results show that the proton yield is significantly higher than that of the pions in the TFR. Without particle identification the existing data cannot be used for comparison with our result on the $p/π$ ratio. Without $p_T$ determination it is not feasible to relate the $x$ distribution to the experimental $η'$ distribution. Nevertheless, on theoretical grounds we have shown why the production of hadrons in the TFR is not forbidden by momentum conservation.
Autocorrelation of two pions produced in heavy-ion collisions at intermediate $p_T$ is calculated in the framework of the recombination model. The differences of the pseudo-rapidities and azimuthal angles of the two pions are related to the angle between two shower partons in a jet. It is shown how the autocorrelation distribution reveals the properties of jet cone of the shower partons created by high-$p_T$ partons in hard collisions.
Some of the puzzles on hadron production at intermediate $p_T$ found at RHIC are explained as natural consequences of parton recombination. In that framework for hadronization the correlation among hadrons produced in jets can be calculated. Some new results on both near-side and away-side jet structures are presented.
We study the pion distributions in jets produced in heavy-ion collisions in association with trigger particles restricted to definite momentum ranges. We determine the centrality dependence of the associated particle distributions for both d+Au and Au+Au collisions in the recombination model. The central-to-peripheral ratios show significant dependence on centrality for Au+Au collisions, but negligible dependence for d+Au collisions. The results are in agreement with the qualitative features of the data.
Correlations among hadrons in jets produced in heavy-ion collisions are discussed in the framework of the recombination model. The basic correlation at the parton level is among the shower partons arising from kinematical constraint. The resultant correlation between hadrons at intermediate $p_T$ is amazingly rich in characteristics.
Correlation between shower partons is first studied in high $p_T$ jets. Then in the framework of parton recombination the correlation between pions in heavy-ion collisions is investigated. Since thermal partons play very different roles in central and peripheral collisions, it is found that the correlation functions of the produced hadrons behave very differently at different centralities, especially at intermediate $p_T$. The correlation function that can best exhibit the distinctive features is suggested. There is not a great deal of overlap between what we can calculate and what has been measured. Nevertheless, some aspects of our results compare favorably with experimental data.
We study the characteristics of correlation between particles in jets produced in heavy-ion collisions. In the framework of parton recombination we calculate the $η$ and $ϕ$ distributions of a pion associated with a trigger particle. The origin of the pedestal in $Δη$ is related to the longitudinal expansion of the thermal partons that are enhanced by the energy loss of hard partons traversing the bulk medium. The peaks in $Δη$ and $Δϕ$ are related to the same angular spread of the shower partons in a jet cone. No artificial short- or long-range correlations are put in by hand. A large part of the correlation between hadrons in jets is due to the correlation among the shower partons arising from momentum conservation. Recombination between thermal and shower partons dominates the correlation characterisitics in the intermediate $p_T$ region.
Hadron production at all $p_T$ in heavy-ion collisions in the framework of parton recombination is reviewed. It is shown that the recombination of thermal and shower partons dominates the hadron spectra in the intermediate $p_T$ region. In $d+Au$ collisions, the physics of particle production at any $η$ is basically the same as at $η=0$. The Cronin effect is described as a result of the final-state instead of the initial-state interaction. The suppression of $R_{CP}$ at high $η$ is due to the reduction of the soft parton density on the deuteron side, thus resulting in less pions produced by recombination, an explanation that requires no new physics. In $Au+Au$ collisions large $p/π$ ratio is obtained because the thermal partons can contribute to the formation of proton more than they do to the pion.
Hadron production at forward rapidities in d+Au collisions is studied in the framework of parton recombination in the final state. Multiple scattering and gluon saturation in the initial state are not explicitly considered. The recombination of soft and shower partons is found to be important. The soft parton densities are determined by extrapolation from the parametrization fixed at η=0 with no unconstrained adjustments. The suppression of R_{CP} observed at high ηis understood as the simple consequence of the reduction of the soft parton density on the deuteron side compared to that on the gold side. The effect of momentum degradation responsible for baryon stopping is also considered. The asymmetry of backward-to-forward spectra can be reproduced within the same framework without any change in the basic physics.
Several puzzles about the data at high $p_T$ in heavy-ion collisions are listed. The resolution of them all is given in the framework of parton recombination. More specifically, it is the recombination of the soft and semi-hard shower partons that enhances the region $3 0$ without the introduction of any new physics.
The difference between the structures of jets produced in heavy-ion and hadronic collisions can best be exhibited in the correlations between particles within those jets. We study the dihadron correlations in jets in the framework of parton recombination. Two types of triggers, $π^+$ and proton, are considered. It is shown that the recombination of thermal and shower partons makes the most important contribution to the spectra of the associated particles at intermediate $p_T$. In $pp$ collisions the only significant contribution arises from shower-shower recombination, which is negligible in heavy-ion collisions. Moments of the associated-particle distributions are calculated to provide simple summary of the jet structures for easy comparison with experiments.
We develop the notion of shower partons and determine their distributions in jets in the framework of the recombination model. The shower parton distributions obtained render a good fit of the fragmentation functions. We then illustrate the usefulness of the distributions in a problem where a jet is produced in the environment of thermal partons as in heavy-ion collisions. The recombination of shower and thermal partons is shown to be more important than fragmentation. Application to the study of two-particle correlation in a jet is also carried out.
The recombination model is applied to the production of phi and Omega at all pt in central Au+Au collisions. Since no light quarks are involved in the hadronization, those hidden-strange particles present a clean slate for the study of the role of strange quarks in large-pt physics. We find that shower s quarks have negligible effect for pt<6 GeV/c, in which range the thermal s quarks make the dominant contributions to the formation of phi and Omega. We show that the same effective temperature of the s quarks is responsible for the shape of the spectra of both phi and Omega. We predict that the ratio of Omega to phi will show a peak at pt \approx 6 GeV/c due to the effect of the hard partons. We also give reasons on the basis of the pt dependence that phi cannot be formed by means of K+K- coalescence.