arXiv subjects

Pok Man Lo

Publications and source records attributed to Pok Man Lo.

At least 37 records · Page 2Linked to original sources

arXiv2020

Multiplicity dependence of (multi-)strange baryons in the canonical ensemble with phase shift corrections

The increase in strangeness production with charged particle multiplicity, as seen by the ALICE collaboration at CERN in p-p, p-Pb and Pb-Pb collisions, is investigated in the hadron resonance gas model taking into account interactions among hadrons using S-matrix corrections based on known phase shift analyses. Strangeness conservation is taken into account in the framework of the canonical strangeness ensemble. A very good description is obtained for the variation of the strangeness content in the final state as a function of the number of charged hadrons in the mid-rapidity region using the same fixed temperature value as obtained in the most central Pb-Pb collisions. It is shown that the number of charged hadrons is linearly proportional to the volume of the system. For small multiplicities the canonical ensemble with local strangeness conservation restricted to mid-rapidity leads to a stronger suppression of (multi-)strange baryons than seen in the data. This is compensated by introducing a global conservation of strangeness in the whole phase-space which is parameterized by the canonical correlation volume larger than the fireball volume at the mid-rapidity. The results on comparing the hadron resonance gas model with and without S-matrix corrections, are presented in detail. It is shown that the interactions introduced by the phase shift analysis via the S-matrix formalism are essential for a better description of the yields data.

hep-ph↗

arXiv2020

Deconfinement in the presence of a strong magnetic field

We study the impact of a finite magnetic field on the deconfinement phase transition for heavy quarks by computing the fluctuations of the Polyakov loops. It is demonstrated that the explicit Z(3) breaking field increases with the magnetic field, leading to a decrease in the (pseudo) critical temperatures and a shrinking first-order region in the phase diagram. Phenomenological equations that capture the behaviors of the Z(3) breaking field at strong and weak magnetic fields for massive and massless quarks are given. Lastly, we explore the case of dynamical light quarks and demonstrate how an improved constituent quark mass function can enforce the correct magnetic field dependence of the deconfinement temperature in an effective model, as observed in Lattice QCD calculations.

hep-ph↗

arXiv2020

Density of States of a Coupled-Channel System

We demonstrate how an effective density of states can be derived from the S-matrix describing a coupled-channel system. Besides the locations of poles, the phase of the determinant of the S-matrix encodes essential details in characterizing the dynamics of resonant and non-resonant interactions. The density of states is computed for the two channel scattering problem ($ππ, K \bar{K}$, S-wave), and the influences from the various dynamical structures: poles, roots, branch cuts, and Riemann sheets, are examined.

hep-ph↗

arXiv2019

Fourier coefficients of the net-baryon number density and chiral criticality

We investigate the Fourier coefficients $b_k(T)$ of the net--baryon number density in strongly interacting matter at nonzero temperature and density. The asymptotic behavior of the coefficients at large $k$ is determined by the singularities of the partition function in the complex chemical potential plane. Within a QCD-like effective chiral model, we show that the chiral and deconfinement properties at nonzero baryon chemical potential are reflected in characteristic $k$-- and $T$-- dependences of the Fourier coefficients. We also discuss the influence of the Roberge-Weiss transition on these coefficients. Our results indicate that the Fourier expansion approach can provide interesting insights into the criticality of QCD matter.

hep-ph↗

arXiv2019

Thermal contribution of unstable states

Within the framework of the Lee model, we analyze in detail the difference between the energy derivative of the phase shift and the standard spectral function of the unstable state. The fact that the model is exactly solvable allows us to demonstrate the construction of these observables from various exact Green functions. The connection to a formula due to Krein, Friedal, and Lloyd is also examined. We also directly demonstrate how the derivative of the phase shift correctly identifies the relevant interaction contributions for consistently including an unstable state in describing the thermodynamics.

hep-ph↗

arXiv2019

The thermal proton yield anomaly in Pb-Pb collisions at the LHC and its resolution

We propose a resolution of the discrepancy between the proton yield predicted by the statistical hadronization approach and data on hadron production in ultra-relativistic nuclear collisions at the LHC. Applying the S-matrix formulation of statistical mechanics to include pion-nucleon interactions, we reexamine their contribution to the proton yield, taking resonance widths and the presence of nonresonant correlations into account. The effect of multi-pion-nucleon interactions is estimated using lattice QCD results on the baryon-charge susceptibility. We show that a consistent implementation of these features in the statistical hadronization model, leads to a reduction of the predicted proton yield, which then quantitatively matches data of the ALICE collaboration for Pb-Pb collisions at the LHC.

hep-ph↗

arXiv2018

Thermodynamics of strange baryon system from coupled-channel analysis and missing states

We study the thermodynamics of the strange baryon system using an S-matrix formulation of statistical mechanics. For this purpose, we employ an existing coupled-channel study involving $\bar{K} N$, $πΛ$, and $πΣ$ interactions in the $S=-1$ sector. A novel method is proposed to extract an effective phase shift due to the interaction, which can subsequently be used to compute various thermal observables via a relativistic virial expansion. As an application of the calculation scheme, we compute the correlation of the net baryon number with strangeness ($χ_{BS}$) for an interacting hadron gas. We show that the S-matrix approach, which entails a consistent treatment of resonances and naturally incorporates the additional hyperon states which are not listed by the Particle Data Group, leads to an improved description of the lattice data over the Hadron Resonance Gas model.

hep-ph↗

arXiv2018

Polyakov loop fluctuations in the presence of external fields

We study the implications of the spontaneous and explicit Z(3) center symmetry breaking for the Polyakov loop susceptibilities. To this end, ratios of the susceptibilities of the real and imaginary parts, as well as of the modulus of the Polyakov loop are computed within an effective model using a color group integration scheme. We show that the essential features of the lattice QCD results of these ratios can be successfully captured by the effective approach. Furthermore we discuss a novel scaling relation in one of these ratios involving the explicit breaking field, volume, and temperature.

hep-ph↗

arXiv2017

Overlap between Lattice QCD and HRG with in-medium effects and parity doubling

We investigate the fluctuations and correlations involving baryon number in hot hadronic matter with modified masses of negative-parity baryons, in the context of the hadron resonance gas. Temperature-dependent masses are adopted from the recent lattice QCD results and from a chiral effective model which implements the parity doubling structure with respect to the chiral symmetry. Confronting the baryon number susceptibility, baryon-charge correlation, and baryon-strangeness correlation and their ratios with the lattice QCD data, we show that the strong downward mass shift in hyperons can accidentally reproduce some correlation ratios, however it also tends to overshoot the individual fluctuations and correlations. This indicates, that in order to correctly account for the influence of the chiral symmetry restoration on the fluctuation observables, a consistent framework of in-medium effects beyond hadron mass shifts is required.

hep-ph↗

arXiv2017

S-matrix analysis of the baryon electric charge correlation

We compute the correlation of the net baryon number with the electric charge ($χ_{BQ}$) for an interacting hadron gas using the S-matrix formulation of statistical mechanics. The observable $χ_{BQ}$ is particularly sensitive to the details of the pion-nucleon interaction, which are consistently incorporated in the current scheme via the empirical scattering phase shifts. Comparing to the recent lattice QCD studies in the $(2+1)$-flavor system, we find that the natural implementation of interactions and the proper treatment of resonances in the S-matrix approach lead to an improved description of the lattice data over that obtained in the hadron resonance gas model.

hep-ph↗

arXiv2017

S-matrix formulation of thermodynamics with N-body scatterings

We apply a phase space expansion scheme to incorporate the N-body scattering processes in the S-matrix formulation of statistical mechanics. A generalized phase shift function suitable for studying the thermal contribution of $N \rightarrow N$ processes is motivated and examined in various models. Using the expansion scheme, we revisit how the hadron resonance gas model emerges from the S-matrix framework, and consider an example of structureless scattering in which the phase shift function can be exactly worked out. Finally we analyze the influence of dynamics on the phase shift function in a simple example of 3- and 4-body scattering.

hep-ph↗

arXiv2017

Resonance decay dynamics and their effects on $p_T$-spectra of pions in heavy-ion collisions

We examine the influence of resonance decay dynamics on momentum spectra of pions in heavy-ion collisions. Taking the decay $ω\rightarrow 3 π$ as an example, we demonstrate how details of the decay matrix element can modify the physical observables. Such a dynamical effect is commonly neglected in statistical models. To remedy the situation, we formulate a theoretical framework for incorporating hadron dynamics into the analysis, which can be straightforwardly extended to describe general N-body decays.

hep-ph↗

arXiv2017

Effects of $ρ$-meson width on pion distributions in heavy-ion collisions

The influence of the finite width of $ρ$ meson on the pion momentum distribution is studied quantitatively in the framework of the S-matrix approach combined with a blast-wave model to describe particle emissions from an expanding fireball. We find that the proper treatment of resonances which accounts for their production dynamics encoded in data for partial wave scattering amplitudes can substantially modify spectra of daughter particles originating in their two body decays. In particular, it results in an enhancement of the low-$p_T$ pions from the decays of $ρ$ mesons which improves the quantitative description of the pion spectra in heavy ion collisions obtained by the ALICE collaboration at the LHC energy.

hep-ph↗

arXiv2017

Repulsive interactions and their effects on the thermodynamics of hadron gas

We compare two approaches in modeling repulsive interactions among hadrons: the excluded volume approximation and the S-matrix formalism. These are applied to study the thermodynamics of the $πN Δ$ system. It is shown that the introduction of an extraneous repulsion between pions and nucleons via the excluded volume approach, in addition to the interaction that generates the $Δ$-resonance, is incompatible with the analysis based on the physical phase shift of pion-nucleon scattering in the $P_{33}$ channel. This finding suggests that the repulsive force between hadrons is interaction-channel dependent and is hence unlikely to be captured by a single phenomenological parameter. The S-matrix approach employed here can be used, however, to provide useful estimates of the magnitude of the effective eigenvolume.

nucl-th↗

arXiv2015

Matching the Hagedorn mass spectrum with Lattice QCD results

Based on recent Lattice QCD (LQCD) results obtained at finite temperature, we discuss modeling of the hadronic phase of QCD in the framework of Hadron Resonance Gas (HRG) with discrete and continuous mass spectra. We focus on fluctuations of conserved charges, and show how a common limiting temperature can be used to constrain the Hagedorn exponential mass spectrum in different sectors of quantum number, through a matching of HRG and LQCD. For strange baryons, the extracted spectra are found to be consistent with all known and expected states listed by the Particle Data Group (PDG). The strange-mesonic sector, however, requires additional states in the intermediate mass range beyond that embodied in the database.

nucl-th↗

arXiv2015

Strangeness fluctuations from $K-π$ interactions

Motivated by recent lattice QCD studies, we explore the effects of interactions on strangeness fluctuations in strongly interacting matter at finite temperature. We focus on S-wave $Kπ$ scattering and discuss the role of the $K_0^*(800)$ and $K^*(1430)$ resonances. The interaction contribution is obtained within the S-matrix formulation of thermodynamics, using the empirical $Kπ$ phase shifts as input. We find that the simplified treatment of the interactions in this channel, employed in the hadron resonance gas approach, leads to an overestimate of the strangeness fluctuations. Thus, our calculation indicates that broad resonances must be treated with caution, when modeling QCD thermodynamics in the hadronic phase.

hep-ph↗

arXiv2014

Polyakov loop fluctuations and deconfinement in the limit of heavy quarks

We explore the influence of heavy quarks on the deconfinement phase transition in an effective model for gluons interacting with dynamical quarks in color SU(3). With decreasing quark mass, the strength of the explicit breaking of the Z(3) symmetry grows and the first-order transition ends in a critical endpoint. The nature of the critical endpoint is examined by studying the longitudinal and transverse fluctuations of the Polyakov loop, quantified by the corresponding susceptibilities. The longitudinal susceptibility is enhanced in the critical region, while the transverse susceptibility shows a monotonic behavior across the transition point. We investigate the dependence of the critical endpoint on the number of quark flavors at vanishing and finite quark density. Finally we confront the model results with lattice calculations and discuss a possible link between the hopping parameter and the quark mass.

hep-ph↗

arXiv2013

Polyakov loop fluctuations in SU(3) lattice gauge theory and an effective gluon potential

We calculate the Polyakov loop susceptibilities in the SU(3) lattice gauge theory using the Symanzik improved gauge action on different-sized lattices. The longitudinal and transverse fluctu- ations of the Polyakov loop, as well as, that of its absolute value are considered. We analyze their properties in relation to the confinement-deconfinement phase transition. We also present results based on simulations of (2+1)-flavor QCD on 32^3 x 8 lattice using Highly Improved Staggered Quark (HISQ) action by the HotQCD collaboration. The influences of fermions on the Polyakov loop fluctuations are discussed. We show, that ratios of different susceptibilities of the Polyakov loop are sensitive probes for critical behavior. We formulate an effective model for the Polyakov loop potential and constrain its parameters from existing quenched lattice data including fluctuations. We emphasize the role of fluctuations to fully explore the thermodynamics of pure gauge theory within an effective approach.

hep-lat↗