Publications

2023

• Gauge fixing and physical symmetries
  
  • van Egmond Duifje Maria
  • Reinosa Urko
  Physical Review D, American Physical Society, 2023, 108 (5), pp.054029. We analyze how gauge fixing, which is required by any practical continuum approach to gauge systems, can interfere with the physical symmetries of such systems. In principle, the gauge fixing procedure, which deals with the (unphysical) gauge symmetry, should not interfere with the other (physical) symmetries. In practice, however, there can be an interference which takes two different forms. First, depending on the considered gauge, it might not always be simple or possible to devise approximation schemes that preserve the physical symmetry constraints on (gauge-independent) observables. Second, even at an exact level of discussion, the (gauge-dependent) effective action for the gauge field, and thus the related vertex functions, may not reflect the physical symmetries of the problem. We illustrate these difficulties using a very general class of gauge fixings that contains the usual gauge fixings as particular cases. Using background field techniques, we then propose specific gauge choices that allow one to keep the physical symmetries explicit, both at the level of the observables and at the level of the effective action for the gauge field. Our analysis is based on the notion of invariance modulo gauge transformations. This is not only a convenient framework to discuss symmetries in the presence of unphysical degrees of freedom, but it also allows one to reinterpret certain well known phenomena in gauge theories without the need to invoke the conceptually annoying ``breaking of a gauge symmetry''. (10.1103/PhysRevD.108.054029)
  DOI : 10.1103/PhysRevD.108.054029
• The tensor Harish-Chandra-Itzykson-Zuber integral I: Weingarten calculus and a generalization of monotone Hurwitz numbers
  
  • Collins Benoît
  • Gurau Razvan
  • Lionni Luca
  J.Eur.Math.Soc., 2023. We study a generalization of the Harish-Chandra - Itzykson - Zuber integral to tensors and its expansion over trace-invariants of the two external tensors. This gives rise to natural generalizations of monotone double Hurwitz numbers, which count certain families of constellations. We find an expression of these numbers in terms of monotone simple Hurwitz numbers, thereby also providing expressions for monotone double Hurwitz numbers of arbitrary genus in terms of the single ones. We give an interpretation of the different combinatorial quantities at play in terms of enumeration of nodal surfaces. In particular, our generalization of Hurwitz numbers is shown to enumerate certain isomorphism classes of branched coverings of a bouquet of $D$ 2-spheres that touch at one common non-branch node. (10.4171/JEMS/1315)
  DOI : 10.4171/JEMS/1315
• A unimodular-like string effective description
  
  • Kehagias Alex
  • Partouche Hervé
  • Toumbas Nicolaos
  Nuclear Physics B, North-Holland ; Elsevier [1967-....], 2023, 991, pp.116196. We show that vanishing of the sigma-model beta-functions for the metric $g_{\mu\nu}$, antisymmetric tensor $B_{\mu\nu}$ and dilaton $\Phi$ of any string-theory model in $D$ dimensions at tree level can be obtained from a universal effective action, provided that the field combination $\sqrt{-g}e^{-2\Phi}$ is non-dynamical. The central-charge deficit term is omitted from the action and appears as an integration constant at the level of the field equations. The analysis is done at leading order in $\alpha'$, i.e. at the two-derivative level in the effective description. It can be extended to incorporate higher order string-loop corrections, Ramond-Ramond forms and moduli fields. In all cases, the non-dynamical field combination turns out to be $\sqrt{-g}\, U$, where $U$ is the effective potential. When certain fluxes are fixed and the moduli fields are stabilized, no cosmological term will arise in the action. Instead, the cosmological constant will appear as an integration constant as in unimodular modifications of general relativity. (10.1016/j.nuclphysb.2023.116196)
  DOI : 10.1016/j.nuclphysb.2023.116196
• Medium-induced radiation with vacuum propagation in the pre-hydrodynamics phase
  
  • Andres Carlota
  • Apolinário Liliana
  • Dominguez Fabio
  • Martinez Marcos Gonzalez
  • Salgado Carlos A.
  Journal of High Energy Physics, Springer, 2023, 03, pp.189. The recent discovery of the potential of jet quenching observables to constrain the initial stages after a heavy-ion collision makes imperative to have a better understanding of the process of medium-induced radiation before the formation of the quark-gluon plasma (QGP) and its impact on observables at high-$p_T$. In this work, we generalize the BDMPS-Z framework for medium-induced radiation to account for additional emissions occurring before the creation of the QGP. For simplicity, we assume that during the pre-hydrodynamics phase the hard parton propagates as in vacuum. This set-up, allows us to isolate the contribution from the additional initial radiation by comparing with the usual scenarios in which the emitter is created inside the medium but with different starting points. Using both a numerical implementation of the fully resummed emission spectrum and the usual analytical approximations, we find that replacing an initial slab of the medium by vacuum yields to a significant reduction of the emission spectrum for low radiated gluon energies, while the high-energy tails remain largely unmodified. Finally, we assess the effect of replacing the initial medium by vacuum propagation on the single-inclusive particle suppression $R_{AA}$ and high-$p_T$ azimuthal asymmetry $v_2$. Our findings indicate that considering vacuum propagation prior to hydrodynamization leads to an increase in the $v_2$, thus corroborating the importance of the treatment of jet quenching in the initial stages for the correct description of both observables. (10.1007/JHEP03(2023)189)
  DOI : 10.1007/JHEP03(2023)189
• Thermodynamic Phase Diagram of Two-Dimensional Bosons in a Quasicrystal Potential
  
  • Zhu Zhaoxuan
  • Yao Hepeng
  • Sanchez-Palencia Laurent
  Physical Review Letters, American Physical Society, 2023, 130 (22), pp.220402. Quantum simulation of quasicrystals in synthetic bosonic matter now paves the way to the exploration of these intriguing systems in wide parameter ranges. Yet thermal fluctuations in such systems compete with quantum coherence, and significantly affect the zero-temperature quantum phases. Here we determine the thermodynamic phase diagram of interacting bosons in a two-dimensional, homogeneous quasicrystal potential. Our results are found using quantum Monte Carlo simulations. Finite-size scaling is carefully considered and the quantum phases are systematically distinguished from thermal phases. In particular, we demonstrate stabilization of a genuine Bose glass phase against the normal fluid in sizable parameter ranges. Our results for strong interactions are interpreted using a fermionization picture and experimental relevance is discussed. (10.1103/PhysRevLett.130.220402)
  DOI : 10.1103/PhysRevLett.130.220402