arXiv subjects

J. T. Lenaghan

Publications and source records attributed to J. T. Lenaghan.

13 recordsLinked to original sources

arXiv2004

Subprocess Size in Hard Exclusive Scattering

The interaction region of hard exclusive hadron scattering can have a large transverse size due to endpoint contributions, where one parton carries most of the hadron momentum. The endpoint region is enhanced and can dominate in processes involving multiple scattering and quark helicity flip. The endpoint Fock states have perturbatively short lifetimes and scatter softly in the target. We give plausible arguments that endpoint contributions can explain the apparent absence of color transparency in fixed angle exclusive scattering and the dimensional scaling of transverse rho photoproduction at high momentum transfer, which requires quark helicity flip. We also present a quantitative estimate of Sudakov effects.

hep-ph↗

arXiv2003

Fluctuation Induced Critical Behavior at Non-Zero Temperature and Chemical Potential

We discuss phase transitions in relativistic systems as a function of both chemical potential and temperature. The presence of a chemical potential explicitly breaks Lorentz invariance and may additionally break other internal symmetries. This introduces new subtleties in the determination of the critical properties. We discuss separately three characteristic effects of a nonzero chemical potential. Firstly, we consider only the explicit breaking of Lorentz invariance using a scalar field theory with a global U(1) symmetry. Secondly, we study the explicit breaking of an internal symmetry in addition to Lorentz invariance using two--color QCD at nonzero baryonic chemical potential. Finally, we consider the spontaneous breaking of a symmetry using three-color QCD at nonzero baryonic and isospin chemical potential. For each case, we derive the appropriate three-dimensional effective theory at criticality and study the effect of the chemical potential on the fixed point structure of the $β$-functions. We find that the order of the phase transition is not affected by the explicit breaking of Lorentz invariance but is sensitive to the breaking of additional symmetries by the chemical potential.

hep-ph↗

arXiv2003

Melting the Diquark Condensate in Two-Color QCD: A Renormalization Group Analysis

We use a Landau theory and the epsilon expansion to study the superfluid phase transition of two-color QCD at nonzero temperature, T, and baryonic chemical potential, mu. At low T, and for N_f flavors of massless quarks, the global SU(N_f) \times SU(N_f) \times U(1) symmetry is spontaneously broken by a diquark condensate down to Sp(N_f) \times Sp(N_f) for any mu > 0. As the temperature increases, the diquark condensate melts, and at sufficiently large T the symmetry is restored. Using renormalization group arguments, we find that in the presence of the chiral anomaly term there can be a second order phase transition when N_f=2 or N_f >= 6, while the transition is first order for N_f=4. We discuss the relevance of these results for the emergence of a tricritical point recently observed in lattice simulations.

hep-ph↗

arXiv2002

Saturation and Pion Production in Proton-Nucleus Collisions

We study the effects of gluon saturation on pion production in high energy proton-nucleus collisions using the color glass condensate model. At high p_perp, we show that the p_perp-distribution of gluons behaves as ~ 1/(p_perp^7) in accordance with both conventional perturbative QCD calculations and experiment. Fragmentation of gluons into pions leads to a rapidity dependent depletion of pions relative to the conventional perturbative QCD predictions. We argue that these clear and systematic differences provide a signal for the onset of gluon saturation which is accessible in upcoming experiments.

hep-ph↗

arXiv2002

The K/pi ratio from condensed Polyakov loops

We perform a field-theoretical computation of hadron production in large systems at the QCD confinement phase transition associated with restoration of the Z(3) global symmetry. This occurs from the decay of a condensate for the Polyakov loop. From the effective potential for the Polyakov loop, its mass just below the confinement temperature T_c is in between the vacuum masses of the pion and that of the kaon. Therefore, due to phase-space restrictions the number of produced kaons is roughly an order of magnitude smaller than that of produced pions, in agreement with recent results from collisions of gold ions at the BNL-RHIC. From its mass, we estimate that the Polyakov loop condensate is characterized by a (spatial) correlation scale of 1/m_\ell ~ 1/2 fm. For systems of deconfined matter of about that size, the free energy may not be dominated by a condensate for the Polyakov loop, and so the process of hadronization may be qualitatively different as compared to large systems. In that vein, experimental data on hadron abundance ratios, for example K/pi, in high-multiplicity pp events at high energies should be very interesting.

hep-ph↗

arXiv2002

Dashen's Phenomenon in Gauge Theories with Spontaneously Broken Chiral Symmetries

We examine Dashen's phenomenon in the Leutwyler--Smilga regime of QCD with any number of colors and quarks in either the fundamental or adjoint representations of the gauge group. In this limit, the theories only depend on simple combinations of quark masses, volume, chiral condensate and vacuum angle. Based upon this observation, we derive simple expressions for the chiral condensate and the topological density and show that they are in fact related. By examining the zeros of the various partition functions, we elucidate the mechanism leading to Dashen's phenomena in QCD.

hep-th↗

arXiv2001

The $θ$-Vacua and the Leutwyler--Smilga Scaling Regime

The partition function of QCD is studied in the Leutwyler--Smilga scaling regime for an arbitrary number of quark flavors and masses including the contributions from all winding numbers. For $N_f=2$ and degenerate quark masses, the partition function becomes independent of the quark masses at $θ=π$ and subsequently the scalar chiral condensate vanishes. There is a discontinuity at $θ=π$ in the first derivative of the energy density with respect to $θ$ corresponding to the first--order phase transition in which CP is spontaneously broken, known as Dashen's phenomena. These properties are found to be insensitive to both the pattern of chiral symmetry breaking and the representation of the quark fields.

hep-ph↗

arXiv2001

The Superfluid and Conformal Phase Transitions of Two-Color QCD

The phase structure of two-color QCD is examined as a function of the chemical potential and the number of light quark flavors. We consider effective Lagrangians for two-color QCD containing the Goldstone excitations, spin-one particles and negative intrinsic parity terms. We discuss the possibility of a conformal phase transition and the enhancement of the global symmetries as the number of flavors is increased. The effects of a quark chemical potential on the spin-one particles and on the negative intrinsic parity terms are analyzed. It is shown that the phase diagram that is predicted by the linearly realized effective Lagrangian at tree-level matches exactly that predicted by chiral perturbation theory.

hep-ph↗

arXiv2001

First-Order Chiral Phase Transition in High-Energy Collisions: Can Nucleation Prevent Spinodal Decomposition?

We discuss homogeneous nucleation in a first-order chiral phase transition within an effective field theory approach to low-energy QCD. Exact decay rates and bubble profiles are obtained numerically and compared to analytic results obtained with the thin-wall approximation. The thin-wall approximation overestimates the nucleation rate for any degree of supercooling. The time scale for critical thermal fluctuations is calculated and compared to typical expansion times for high-energy hadronic or heavy-ion collisions. We find that significant supercooling is possible, and the relevant mechanism for phase conversion might be that of spinodal decomposition. Some potential experimental signatures of supercooling, such as an increase in the correlation length of the scalar condensate, are also discussed.

hep-ph↗

arXiv2000

Influence of the U(1)_A Anomaly on the QCD Phase Transition

The SU(3)_{r} \times SU(3)_{\ell} linear sigma model is used to study the chiral symmetry restoring phase transition of QCD at nonzero temperature. The line of second order phase transitions separating the first order and smooth crossover regions is located in the plane of the strange and nonstrange quark masses. It is found that if the U(1)_{A} symmetry is explicitly broken by the U(1)_{A} anomaly then there is a smooth crossover to the chirally symmetric phase for physical values of the quark masses. If the U(1)_{A} anomaly is absent, then there is a phase transition provided that the σmeson mass is at least 600 MeV. In both cases, the region of first order phase transitions in the quark mass plane is enlarged as the mass of the σmeson is increased.

hep-ph↗

arXiv2000

Hydrodynamic Models for Heavy-Ion Collisions, and beyond

A generic property of a first-order phase transition in equilibrium, and in the limit of large entropy per unit of conserved charge, is the smallness of the isentropic speed of sound in the ``mixed phase''. A specific prediction is that this should lead to a non-isotropic momentum distribution of nucleons in the reaction plane (for energies around 40 AGeV in our model calculation). On the other hand, we show that from present effective theories for low-energy QCD one does not expect the thermal transition rate between various states of the effective potential to be much larger than the expansion rate, questioning the applicability of the idealized Maxwell/Gibbs construction. Experimental data could soon provide essential information on the dynamics of the phase transition.

nucl-th↗

arXiv2000

Chiral Symmetry Restoration at Nonzero Temperature in the SU(3) \times SU(3) Linear Sigma Model

We study patterns of chiral symmetry breaking at zero temperature and its subsequent restoration at nonzero temperature within the $SU(3)_{r} \times SU(3)_{\ell}$ linear sigma model. Gap equations for the masses of the scalar and pseudoscalar mesons and the non-strange and strange quark condensates are systematically derived in the Hartree approximation via the Cornwall-Jackiw-Tomboulis formalism. In the chiral limit, the chiral symmetry restoring transition is found to be first order, as predicted by universality arguments. Taking the experimental values for the meson masses, however, the transition is crossover. The absence of the $U(1)_A$ anomaly is found to drive this transition closer to being first order. At large temperatures, the mixing angles between octet and singlet states approach ideal flavor mixing.

nucl-th↗

arXiv2000

The O(N) Model at Finite Temperature: Renormalization of the Gap Equations in Hartree and Large-N Approximation

The temperature dependence of the sigma meson and pion masses is studied in the framework of the O(N) model. The Cornwall-Jackiw-Tomboulis formalism is applied to derive gap equations for the masses in the Hartree and large-N approximations. Renormalization of the gap equations is carried out within the cut-off and counter-term renormalization schemes. A consistent renormalization of the gap equations within the cut-off scheme is found to be possible only in the large-N approximation and for a finite value of the cut-off. On the other hand, the counter-term scheme allows for a consistent renormalization of both the large-N and Hartree approximations. In these approximations, the meson masses at a given nonzero temperature depend in general on the choice of the cut-off or renormalization scale. As an application, we also discuss the in-medium on-shell decay widths for sigma mesons and pions at rest.

nucl-th↗