Partial U(1)Arestoration and η enhancement in high-energy heavy-ion collisions

Abstract

We calculate the thermally averaged rates for the η-π conversion and η scattering using the Di Vecchia-Veneziano model and t' Hooft model, which incorporate explicitly the U (1) A anomaly. Assuming an exponential suppression of the U (1) A anomaly, we also take into account the partial restoration of U (1) A symmetry at high temperatures. We find that the chemical equilibrium between η and π breaks up considerably earlier than the thermal equilibrium.Two distinct scenarios for the η freeze-out are discussed and the corresponding chemical potentials are calculated. We predict an enhancement of the thermal η-production as a possible signal of the partial U (1) A restoration in high-energy heavy-ion collisions.

In-medium reduction of theη'mass √sNN= 200 GeV Au+Au collisions

“…Physically, B −1 is limited by T FO , so the trapped η ′ -s may gain only moderate transverse momenta. Hence, the enhancement mostly appears at low p T [6][7][8] just as in the first simulations. However, now the slope of "hole" of the λ * (m T ) curve is determined by B −1 , and, for certain values of the model parameters, the data can be reproduced quantitatively.…”

“…will not be able to leave the hot and dense region through thermal fluctuation since they cannot compensate for the missing mass [7,8], and thus will be trapped in the hot and dense region until it disappears. As the energy density of the medium is dissolved, the effect of QCD instantons increases and the trapped η ′ mesons regain their free mass and appear at low p T .…”

“…After this perfect fluid of quarks rehadronizes, a hot and dense hadronic matter may be created, where the U A (1) symmetry of the strong interactions may temporarily be restored [6][7][8]. Recent lattice QCD calculations indicate that such chirally symmetric but hadronic matter may exist below the critical temperature for quark deconfinement [9].…”

Functional renormalization for chiral andUA(1)symmetries at finite temperature

“…The mass shift due to the chiral restoration enhances or reduces the hadron thermal production, for instance for the kaon yields and ratios [7][8][9]. The partial restoration of the U A (1) symmetry is closely related to the production of η meson and spin-excited hadrons in hot medium [10,11].…”

Relations among correlation functions in the high temperature phase of QCD with broken SU(3)

“…There has also been recent interest in the behavior of the SU(2) singlet pseudoscalars (η and η ′ ) at high temperatures [13,14,7]. However the correlators in both of these channels require quark-line disconnected pieces.…”