arXiv subjects

Rudolph C. Hwa

Publications and source records attributed to Rudolph C. Hwa.

At least 19 recordsLinked to original sources

arXiv2023

Universal Energy Dependence of Measured Temperatures for Baryons Produced in Heavy-ion Collisions

From the data on baryon production in heavy-ion collisions it is shown that a set of measurable functions exist, one for each baryon type, that depend exponentially on transverse momenta up to the maximum values detected for all collision energies ranging from the low end of RHIC BES to the high end of CERN LHC. The implied temperatures satisfy a scaling law in collision energy with a universal exponent for all baryon types which is the novel discovery. A self-similar thermal source is implied. Furthermore, it is shown how the scaling behavior depends on centralities. Those features in the data are related to simple properties of light and strange quarks by use of the recombination model. Prediction of $ϕ$ meson production is made and then verified by existing data.

hep-ph↗

arXiv2021

Centrality and transverse momentum dependence of hadrons in Pb+Pb collisions at LHC

The transverse momentum spectra of seven identified hadrons produced in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ and 5.02 TeV at the CERN Large Hadron Collider (LHC) have been investigated in the framework of the recombination model (RM). Soft, semihard and hard partons all play important roles in our model and are uniformly treated for all hadrons produced.The investigation has been extended to non-central collisions and the parameters controlling the momentum degradation of semihard partons have been tuned. Our study shows good agreement between theoretical results and experimental data. To be able to provide a coherent explanation for the production of all identified hadrons ($π$, p, K, $Λ$, $ϕ$, $Ξ$, $Ω$) for transverse momenta as high as 14 GeV/c (for $π$, p and K), and for all centralities, is an unprecedented achievement that supports the RM as a sensible model to combine various mechanisms of parton production with a universal scheme of hadronization.

hep-ph↗

arXiv2019

A unified study of the production of all identified hadrons over wide ranges of transverse momenta at LHC

The production of all identified hadrons at the CERN Large Hadron Collider (LHC) is studied with emphasis on the $p_T$ distributions up to 20 GeV/c in central collisions at $\sqrt{s_{NN}}=2.76$ TeV. The parton recombination model is used to determine the hadronic \ppt\ spectra from the quark \dis s. From the heavy hyperon spectra it is known from earlier studies that the $u, d, s$ thermal \dis s in \ppt\ are exponential with large inverse slopes that cannot be identified with any temperature in conventional fluid models. They are used as inputs in our model together with shower partons determined from our treatment of momentum degradation that uses high-\ppt\ pion data as input. Those thermal and shower partons are used to calculate the $p_T$ \dis s of all observed hadrons ($π, K, p, Λ$, $Ξ$, $Ω$ and $ϕ$) over wide ranges of \ppt, so the system is highly constrained. We show how well the LHC data can be reproduced with only a few parameters to adjust. Centrality dependence has not been studied. What is learned is that minijets are important, not only in giving rise to abundant shower partons, but also in the conversion of semihard partons in the medium to soft partons that enhance the thermal partons. Since the conversion process can occur throughout the expansion phase of the high-density medium, this work provides the basis for questioning the validity of the assumption of rapid equilibration.

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arXiv2018

Universal Formula for Baryon Spectra in Heavy-ion Collisions and its Implications

In an unconventional presentation of the data on the transverse momentum spectra of baryons produced in heavy-ion collisions, regularities are found that make possible the discovery of a universal formula valid for $p, Λ, Ξ$ and $Ω$. The formula describes the baryon distributions over wide ranges of $p_T (0.5 \ ^<_\sim\ p_T \ ^<_\sim\ 5$ GeV/c) for $0.06 \ ^<_\sim\ \sqrt {s_{NN}} \ ^<_\sim\ 3$ TeV, except for very peripheral collisions. Some aspects of their empirical properties are derived in the recombination model, resulting in a revelation of some features of the light and strange quark distributions before hadronization. Interpretation of the inverse slopes of their exponential behavior leads to an implication that cannot accommodate the conventional description of fluid flow. This is mainly a study of phenomenology without detailed model input.

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arXiv2016

Phenomenological Implications of the $p_T$ Spectra of $ϕ$ and $Ω$ produced at LHC and RHIC

The data on the $p_T$ spectra of $ϕ$ and $Ω$ at LHC can be presented in a format that shows exponential behavior up to $p_T\approx 6$ GeV/c with the same slope for both particles and for nearly all centralities. They are empirical properties that are shared at lower energies with the inverse slope showing a power-law dependence on $\sqrt{s_{NN}}$. The shared properties of the spectra are shown to emerge naturally from the recombination model. No flow is needed. We find experimental hints for the possibility that $ϕ$ and $Ω$ are mostly produced in the ridge that are generated by minijets. Appropriate experimental test is suggested.

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arXiv2016

Observable Properties of Quark-Hadron Phase Transition at the Large Hadron Collider

Quark-hadron phase transition is simulated by an event generator that incorporates the dynamical properties of contraction due to QCD confinement forces and randomization due to the thermal behavior of a large quark system on the edge of hadronization. Fluctuations of emitted pions in the $(η,ϕ)$ space are analyzed using normalized factorial moments in a wide range of bin sizes. The scaling index $ν$ is found to be very close to the predicted value in the Ginzburg-Landau formalism. The erraticity indices $μ_q$ are determined in a number of ways that lead to the same consistent values. They are compared to the values from the Ising model, showing significant difference in a transparent plot. Experimental determination of $ν$ and $μ_q$ at the LHC are now needed to check the reality of the theoretical study and to provide guidance for improving the model description of quark-hadron phase transition.

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arXiv2014

Recognizing Critical Behavior amidst Minijets at the Large Hadron Collider

The transition from quarks to hadrons in a heavy-ion collision at high energy is usually studied in two different contexts that involve very different transverse scales: local and non-local. Models that are concerned with the $p_T$ spectra and azimuthal anisotropy belong to the former, i.e., hadronization at a local point in $(η,ϕ)$ space, such as the recombination model. The non-local problem has to do with quark-hadron phase transition where collective behavior through near-neighbor interaction can generate patterns of varying sizes in the $(η,ϕ)$ space. The two types of problems are put together in this paper both as brief reviews separately and to discuss how they are related to each other. In particular, we ask how minijets produced at LHC can affect the investigation of multiplicity fluctuations as signals of critical behavior. It is suggested that the existing data from LHC have sufficient multiplicities in small $p_T$ intervals to make feasible the observation of distinctive features of clustering of soft particles, as well as voids, that characterize the critical behavior at phase transition from quarks to hadrons, without any ambiguity posed by the clustering of jet particles.

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arXiv2013

Centrality and Transverse Momentum Dependencies of Minijets and Hadrons in Au-Au Collisions

In the study of hadron production in Au-Au collisions at RHIC minijets play an important role in generating shower partons in the intermediate \ppt\ region. Momentum degradation of the hard and semihard partons as they traverse the inhomogeneous medium at various azimuthal angles results in a complicated convolution of geometrical, nuclear and dynamical factors that cannot usually be described in a transparent way. In this work a compact formula is found that represents the inclusive distributions of minijets of any parton type at the surface of the medium for any collision centrality. They take into account the contributions from all initiating partons created at any point in the medium. By comparing with the case of no energy loss, a ratio has been determined that is analogous to the nuclear modification factor for minijets. Phenomenological reality of such distributions is examined by calculating the hadronization of the minijets in the recombination model. Good fits of the data on pion, kaon and proton production throughout the intermediate \ppt\ region have been obtained by adjusting the parameters controlling the magnitude of the thermal partons and the degradation rates of the semihard partons. The result gives support to the minijet spectra at any centrality on the one hand, and the hadronization procedure used on the other. An important property made manifest in this study is that quarks and gluons must not lose energy in the same way because the partons form mesons and baryons differently by recombination and the momenta of quarks and gluons must be degraded at different rates in order to reproduce the experimental pion and proton spectra. This is a feature that renders invalid the notion of parton-hadron duality or other hadronization schemes based on similar ideas.

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arXiv2013

Effects of Minijets on Common Observables in Heavy-Ion Collisions with Uncommon Implications

In this brief review of the observable effects of minijets in heavy-ion collisions the main points emphasized are that the quadruple moment $v_2(p_T,b)$ and the hadronic ($π$ and $p$) spectra at low $p_T$ can both be reproduced by minijet contributions to the recombination of thermal and shower partons. Without using hydrodynamics the minijet approach does not trace the evolution of the expanding system. The thermal distribution of the medium partons at the time of hadronization is assumed, but rapid thermalization initially is not required so as to allow minijets to leave their footprints on the system in the final state. Azimuthal anisotropy due to minijets is directly calculated in the momentum space without any fluid assumption relating the spatial eccentricity to $v_2$. There are no more parameters used, compared to the hydro approach in fitting the data on $v_2$ and $p_T$ spectra. Thus both approaches satisfy the sufficiency condition for a viable description of the dynamical process involved.

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arXiv2012

Effects of Minijets on Hadronic Spectra and Azimuthal Harmonics in Au-Au Collisions at 200 GeV

The production of hadrons in heavy-ion collisions at RHIC in the low transverse-momentum ($p_T$) region is investigated in the recombination model with emphasis on the effects of minijets on the azimuthal anisotropy. Since the study is mainly on the hadronization of partons at late time, the fluid picture is not used to trace the evolution of the system. The inclusive distributions at low $p_T$ are determined as the recombination products of thermal partons. The $p_T$ dependencies of both pion and proton have a common exponential factor apart from other dissimilar kinematic and resonance factors, because they are inherited from the same pool of thermal partons. Instead of the usual description based on hydrodynamics, the azimuthal anisotropy of the produced hadrons is explained as the consequence of the effects of minijets, either indirectly through the recombination of enhanced thermal partons in the vicinity of the trajectories of the semihard partons, or directly through thermal-shower recombination. Although our investigation is focussed on the single-particle distribution at midrapidity, we give reasons why a component in that distribution can be identified with the ridge, which together with the second harmonic $v_2$ is due to the semihard partons created near the medium surface that lead to calculable anisotropy in $ϕ$. It is shown that the higher azimuthal harmonics, $v_n$, can also be well reproduced without reference to flow. The $p_T$ and centrality dependencies of the higher harmonics are prescribed by the interplay between TT and TS recombination components. The implication of the success of this drastic departure from the conventional approach is discussed.

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arXiv2012

Ridge and Transverse Correlation at Separated Rapidities

A simple phenomenological relationship between the ridge distribution in $Δη$ and the single-particle distribution in $η$ can be established from the PHOBOS data on both distributions. The implication points to the possibility that there is no long-range longitudinal correlation. An interpretation of the relationship is then developed, based on the recognition that longitudinal uncertainty of the initial configuration allows for non-Hubble-like expansion at early time. It is shown that the main features of the ridge structure can be explained in a model where transverse correlation stimulated by semihard partons is the principal mechanism.

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arXiv2012

Multi-minijet Contribution to Hadronic Spectra and Correlations in Pb-Pb Collisions at 2.76 TeV and beyond

In heavy-ion collisions at very high energy the density of produced jets can be so high that the possibility of hadrons produced by recombination of shower partons in overlapping minijets may become important. We study such multi-minijet contribution to the hadron spectra and to dihadron correlation in Pb-Pb collisions at 2.76 TeV at LHC. We adjust the parameter controlling the momentum degradation of semihard partons by fitting the charged-particle distribution up to $p_T\sim 16$ GeV/c. The relative magnitudes of different identified hadrons and of various partonic components are fixed by the recombination formalism. We find that the coalescence of shower patons from adjacent miniijets can be as much as from single jets for meson production, and even more so for proton, but never dominant over other components. In 3-shower-parton recombination the ratio of 2-jet to 1-jet contributions increases with collision energy; its maximum can exceed 2 at 5.5 TeV. Two-hadron correlation exhibits a broad peak on transverse rapidities, confirming that minijets play a central role at low $p_T$.

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arXiv2011

Spectra of Identified Hadrons in Pb-Pb Collisions at LHC

The transverse momentum distributions of identified hadrons produced in Pb-Pb collisions at the Large Hadron Collider (LHC) are studied in the low and intermediate range for $p_T<5$ GeV/c. All four spectra ($π, K, p, Λ$) can be well reproduced in the recombination model based on a common thermal parton distribution of light and strange quarks and on shower partons emitted in hard and semihard jets. Two essential parameters are adjusted to fit the data, one being the inverse slope of the thermal distribution and the other revealing the degree of momentum degradation in the medium. Various combinations of thermal and shower parton components are calculated, showing the dominance of minijets. The effect of minijets is to produce harder baryons than mesons resulting in their ratio to peak at around $p_T \sim 3$ GeV/c. Substantial portion of the jet energy is found to be lost to the dense medium before the partons emerge at the surface to undergo hadronization by recombination.

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arXiv2011

Local Multiplicity Fluctuations as a Signature of Critical Hadronization at LHC

In central Pb-Pb collisions at LHC the multiplicity of particles produced is so high that it should become feasible to examine the nature of transition from the deconfined quark-gluon state to the confined hadron state by methods that rely on the availability of high multiplicity events. We consider four classes of the transition process ranging from critical behavior to totally random behavior, depending on whether or not there is clustering of quarks and on whether or not there is contraction of dense clusters due to confinement. Fluctuations of bin multiplicities in each event are quantified, and then the event-by-event fluctuations of spatial patterns are analyzed. A sequence of measures are proposed and are shown to be effective in capturing the essence of the differences among the classes of simulated events. It is demonstrated that a specific index has a low value for critical transition but a larger value if the hadronization process is random.

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arXiv2011

Ridge Formation Induced by Jets in $pp$ Collisions at 7 TeV

An interpretation of the ridge phenomenon found in pp collisions at 7 TeV is given in terms of enhancement of soft partons due to energy loss of semihard jets. A description of ridge formation in nuclear collisions can directly be extended to pp collisions, since hydrodynamics is not used, and azimuthal anisotropy is generated by semihard scattering. Both the p_T and multiplicity dependencies are well reproduced. Some suggestions are made about other observables.

hep-ph↗

arXiv2011

Inclusive Ridge Distributions in Heavy-Ion Collisions

The formation of ridges induced by semihard scattering in nuclear collisions is included in the description of single-particle distributions for both pion and proton at low transverse momenta. The ridge component is characterized by an azimuthal dependent factor that is derived in the study of the ridge structure in two-particle correlation distributions involving triggers. It is shown that the inclusive ridge can reproduce the observed data on $v_2(p_T)$ if the base component underlying the ridge has no azimuthal dependence. A common description of pion and proton spectra is given in the recombination model that can smoothly join the low- and intermediate-$p_T$ regions. All the important properties of single-particle distributions in those regions can be satisfactorily described in this approach.

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arXiv2010

Phenomenological Relationship between the Ridge and Inclusive Distributions

A relationship between the ridge distribution in $Δη$ and the single-particle distribution in $η$ is proposed. It is then verified by use of the data from PHOBOS on both distributions. The implication seems to point to the possibility that there is no long-range longitudinal correlation. An interpretation of the phenomenological observation along that possibility is developed.

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arXiv2010

Effects of Near-side Two-jet Recombination on Away-side Correlation at LHC

The creation of partons with transverse momenta around 10 GeV/c in Pb-Pb collisions at LHC is expected to be so copious that the parton density may be high enough for neighboring jet cones to overlap. If the shower partons in adjacent jets can recombine to form trigger particles, the distribution of hadrons on the away side should be distinguishable from that due to the recoil of a single harder parton that can produce a similar trigger particle by fragmentation. The two-particle azimuthal correlation on the away side is generated by simulation of the two trigger types. Moments of the correlation distributions are calculated to exhibit their differences. A measure that can be determined by suitable analysis of the LHC data is suggested. It may reveal the footprint of two-jet recombination.

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