Xumeu Planells

Dilepton excess from local parity breaking in baryon matter

Abstract We propose that local parity breaking induced by a large-scale fluctuation of topological charge at large temperatures and/or condensation of pseudoscalar mesons in the isotriplet channel for large baryon densities may be responsible for the substantial dilepton excess that is found for low invariant masses and moderate values of p T in central heavy ions collisions. This insofar unexplained enhancement could be understood by a combination of two effects leading to an excess of both e + e − and μ + μ − pairs with respect to theoretical predictions based on conventional hadronic processes: (a) a modification of the dispersion relation of photons and vector mesons propagating in such a nuclear medium due to local parity breaking; (b) the appearance of new decay channels, forbidden by parity conservation in QCD in the usual vacuum. Possible signatures of this effect and perspectives for its detection are discussed.

An effective QCD Lagrangian in the presence of an axial chemical potential

We consider the low energy realization of QCD in terms of mesons when an axial chemical potential is present; a situation that may be relevant in heavy ion collisions. We shall demonstrate that the presence of an axial charge has profound consequences on meson physics. The most notorious effect is the appearance of an explicit source of parity breaking. The eigenstates of strong interactions do not have a definite parity and interactions that would otherwise be forbidden compete with the familiar ones. In this work we focus on scalars and pseudoscalars that are described by a generalized linear sigma model. We comment briefly on the screening role of axial vectors in formation of effective axial charge and on the possible experimental relevance of our results, whose consequences may have been already seen at RHIC.

Chemical potentials and parity breaking: the Nambu–Jona-Lasinio model

We consider the ‘two flavour’ Nambu–Jona-Lasinio model in the presence of a vector and an axial external chemical potential and study the phase structure of the model at zero temperature. The Nambu–Jona-Lasinio model is often used as a toy replica of QCD and it is therefore interesting to explore the consequences of adding external vector and axial chemical potentials in this model, mostly motivated by claims that such external drivers could trigger a phase where parity could be broken in QCD. We are also motivated by some lattice analysis that attempt to understand the nature of the so-called Aoki phase using this simplified model. Analogies and differences with the expected behaviour in QCD are discussed and the limitations of the model are pointed out.

Analysis of dilepton angular distributions in a parity breaking medium

We investigate how local parity breaking due to large topological fluctuations may affect hadron physics. A modified dispersion relation is derived for the lightest vector mesons

Abnormal dilepton yield from parity breaking in dense nuclear matter

\ensuremath{\rho}

Searching for P- and CP-odd effects in heavy ion collisions

and

Abnormal dilepton yield from local parity breaking in heavy-ion collisions

\ensuremath{\omega}

Abnormal enhancement of dilepton yield in central heavy-ion collisions from local parity breaking

. They exhibit a mass splitting depending on their polarization. We present a detailed analysis of the angular distribution associated to the lepton pairs created from these mesons searching for polarization dependencies. We propose two angular variables that carry information related to the parity breaking effect. Possible signatures for experimental detection of local parity breaking that could potentially be seen by the PHENIX and STAR collaborations are discussed.

An effective theory for QCD with an axial chemical potential

At finite density parity can be spontaneously broken in strong interactions with far reaching implications. In particular, a time‐dependent pseudoscalar background would modify QED by adding a Chern‐Simons term to the lagrangian. As a striking consequence we propose a novel explanation for the dilepton excess observed in heavy ion collisions at low invariant masses. The presence of local parity breaking due to a time‐dependent pseudoscalar condensate substantially modifies the dispersion relation of photons and vector mesons propagating in such a medium, changing the ρ spectral function and resulting in a potentially large excess of dileptons with respect to the predictions based in a ‘cocktail’ of known processes.

Аномальное усиление выхода дилептонов в центральных столкновениях тяжелых ионов вследствие локального нарушения четности@@@Abnormal enhancement of dilepton yield in central heavy-ion collisions from local parity breaking

In this talk we will summarize the main results from our recent work concerning the possibility that a new metastable phase occurs in some heavy ion collisions (HIC). This phase would be characterized by the breaking of two characteristic symmetries of strong interactions; namely P and CP. We investigate the experimental consequences of parity breaking in such a situation and propose suitable observables to elucidate the presence this phenomenon.