arXiv subjects

Ludovica Pirelli

Publications and source records attributed to Ludovica Pirelli.

10 recordsLinked to original sources

arXiv2026

Finite density lattice QCD without extrapolation: Bulk thermodynamics with physical quark masses from the canonical ensemble

Quantum Chromodynamics (QCD) at finite density is most often formulated on the lattice as a grand canonical ensemble. Since lattice QCD has a complex action problem at finite baryo-chemical potential ($μ_B$), its results at finite density are indirect: e.g. in the form of a set of expansion coefficients. In contrast, the canonical formulation offers direct results for integer-valued net-baryon number. In this work we present for the first time results in the canonical formulation with physical quark masses. To this end we use a high statistics finite-volume lattice ($16^3\times8$) data set that we generated at $μ_B=0$ with our 4HEX staggered action. We extend the canonical ensemble to non-integer net-baryon number and connect the results back to the grand canonical ensemble. Unlike reweighing to real $μ_B$, this method can also be used with rooted staggered quarks. For densities where the sign problem can be overcome by brute force computing power, this scheme provides lattice QCD results (e.g. for pressure, baryon density) directly, without relying on any extrapolation in the baryo-chemical potential. In this work we chart the phase diagram by studying bulk thermodynamic observables, which we show to be feasible up to $μ_B\approx500$~MeV.

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arXiv2025

Finite density lattice QCD without extrapolations

Finite density lattice QCD usually relies on extrapolations in baryon chemical potential ($μ_B$), be it Taylor expansion, T' expansion (\cite{Borsanyi:2021sxv}) or analytical continuation. However, their range of validity is difficult to control. In the canonical formulation, the baryon density is the parameter of the system, not $μ_B$. Here we demonstrate that we can access finite density QCD in the canonical formulation with physical quark masses. We present first results with both the strangeness ($n_S$) and baryon ($n_B$) densities as parameters. Specifically, we compute the QCD pressure and chemical potentials as functions of $n_B$ and $n_S$. Our computations rely on high-statistics simulations with 2+1 4HEX-staggered fermions.

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arXiv2025

Search for a Lee-Yang edge singularity in high-statistics Wuppertal-Budapest data

Near a critical endpoint the Lee-Yang edge singularity approaches the real axis in the complex chemical potential plane. In the vicinity of the critical point the functional form of this approach depends on the universality class. Assuming a three dimensional Ising critical point in the QCD phase diagram the location of the critical endpoint can be extrapolated provided that the position of the Lee-Yang edge singularity is known at multiple temperatures. A popular method to estimate the position of a singularity is to model the free energy as a rational function of the baryon chemical potential $μ_\text{B}$. The parameters of this model can be constrained by the cumulants of the net baryon density taken at $μ_\text{B}^2\leq0$ . Using high-statistics simulations on a lattice $16^3\times8$ by the Wuppertal-Budapest Collaboration we estimate the location of the closest singularity in the QCD phase diagram. We also compare various models for the functional form of the free energy and discuss the predictive power of this approach.

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arXiv2024

QCD deconfinement transition line up to $μ_B=400$ MeV from finite volume lattice simulations

The QCD cross-over line in the temperature ($T$) -- baryo-chemical potential ($μ_B$) plane has been computed by several lattice groups by calculating the chiral order parameter and its susceptibility at finite values of $μ_B$. In this work we focus on the deconfinement aspect of the transition between hadronic and Quark Gluon Plasma (QGP) phases. We define the deconfinement temperature as the peak position of the static quark entropy ($S_Q(T,μ_B)$) in $T$, which is based on the renormalized Polyakov loop. We extrapolate $S_Q(T,μ_B)$ based on high statistics finite temperature ensembles on a $16^3\times 8$ lattice to finite density by means of a Taylor expansion to eighth order in $μ_B$ (NNNLO) along the strangeness neutral line. For the simulations the 4HEX staggered action was used with 2+1 flavors at physical quark masses. In this setup the phase diagram can be drawn up to unprecedentedly high chemical potentials. Our results for the deconfinement temperature are in rough agreement with phenomenological estimates of the freeze-out curve in relativistic heavy ion collisions. In addition, we study the width of the deconfinement crossover. We show that up to $μ_B \approx 400$ MeV, the deconfinement transition gets broader at higher densities, disfavoring the existence of a deconfinement critical endpoint in this range. Finally, we examine the transition line without the strangeness neutrality condition and observe a hint for the narrowing of the crossover towards large $μ_B$.

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arXiv2024

Chiral and deconfinement properties of the QCD crossover have a different volume and baryochemical potential dependence

The crossover from hadronic to quark matter is understood to be both a deconfinement as well as a chiral symmetry restoring transition. Here, we study observables related to both aspects using lattice simulations: the Polyakov loop and its derivatives and the chiral condensate and its derivatives. At zero baryochemical potential, and infinite volume, the chiral and deconfinement crossover temperatures almost agree. However, chiral and deconfinement related observables have a qualitatively different chemical potential and volume dependence. In general, deconfinement related observables have a milder volume dependence. Furthermore, while the deconfinement transition appears to get broader with increasing $μ_B$, the width as well as the strength of the chiral transition is approximately constant. Our results are based on simulations at zero and imaginary chemical potentials using 4stout-improved staggered fermions with $N_τ=12$ time-slices and physical quark masses.

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arXiv2023

Nonperturbative running of the tensor operator for $N_\rm{f}=3$ QCD from the chirally rotated Schrödinger Functional

We study the Renormalisation Group (RG) running of the non-singlet tensor operator, for $N_\mathrm{\scriptstyle f}=3$ QCD with Wilson fermions in a mixed action setup, with standard Schrödinger Functional (SF) boundary conditions for sea quarks and chirally rotated Schrödinger Functional ($χ$SF) boundary conditions for valence quarks. Based on a recursive finite-size scaling technique we compute non-perturbatively the tensor step-scaling function for an energy range between a hadronic scale and an electroweak scale, above which perturbation theory may be safely applied. Our result is expressed as the RG-running factor $T^{\mathrm{RGI}}/[ T(μ_{\mathrm{had}})]_{\scriptstyle \rm R}$, where the numerator is the scale independent (Renormalisation Group Invariant - RGI) tensor operator and the denominator is its renormalised counterpart at a hadronic scale $μ_{\mathrm{had}} = 233(8)$~MeV in a given scheme. We determine the step-scaling function in four distinct renormalisation schemes. We also compute the renormalisation parameters of these schemes at $μ_{\mathrm{had}}$ which, combined with the RG-running factor, gives the scheme-independent quantity $Z^{\mathrm{RGI}}_{\mathrm T}(g_0^2)$ in four schemes and for a range of bare gauge couplings in which large volume hadronic matrix element simulations are performed by the CLS consortium in $N_\mathrm{\scriptstyle f}=2+1$ QCD. All four results are compatible and also agree with a recent determination based on a unitary setup for Wilson quarks with Schrödinger Functional boundary conditions~arXiv:2309.04314 . This provides a strong universality test.

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arXiv2022

RG-running of the tensor currents for $N_f$ =3 QCD in a $χSF$ setup

We present the preliminary results of the non-perturbative running of the flavour non-singlet tensor operator in the high-energy range $2~\rm{GeV}\lesssim μ\lesssim 128~\rm{GeV}$ in $N_f=3$ massless QCD, comparing four different definitions of the renormalisation constant. We use the configuration ensembles of arXiv:1802.05243 and arXiv:1607.06423, subject to Schrödinger functional (SF) boundary conditions, and valence quarks with chirally rotated Schrödinger functional ($χ$SF) boundary conditions. Provided that boundary counterterms have been appropriately tuned, this results in O($a$) improvement of the tensor operator, without the need of a dimension-4 Symanzik counterterm (proportional to $c_T$).

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arXiv2021

Nonperturbative running of the quark mass for $N_f=3$ QCD from the chirally rotated Schrödinger Functional

We study the Renormalisation Group (RG) running of the quark mass, for $N_f=3$ QCD with Wilson fermions in a mixed action setup, with standard Schrödinger Functional (SF) boundary conditions for sea quarks and chirally rotated Schrödinger Functional ($χ$SF) boundary conditions for valence quarks. This necessitates the tuning of the boundary factor $z_f(g_0^2)$ of the $χ$SF valence action, in order to ensure that QCD symmetries are fully recovered in the continuum. The properties of this novel setup are monitored through the ratios $Z_S/Z_P$ and $Σ_S/Σ_P$ of the renormalisation parameters and step scaling functions of the scalar and pseudoscalar densities. Where comparison is possible, our $Z_S/Z_P$ results are found to agree with previous determinations, based on a mass ratio method arXiv:1906.03445 and Ward identities arXiv:2005.01352, arXiv:2101.10969, with Schrödinger Functional boundary conditions. The behaviour of $Σ_S/Σ_P$ confirms the theoretical expectations of $χ$SF QCD, related to the restoration of the theory's symmetries in the continuum limit. From the step scaling function of the pseudoscalar density we obtain the quark mass RG-running function from hadronic to perturbative energy scales. This is fully compatible with the earlier result obtained in a similar setup for Wilson quarks with Schrödinger Functional boundary conditions arXiv:1802.05243 and provides a strong universality test for the two lattice setups.

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arXiv2021

Quark mass RG-running for $N_f$ =3 QCD in a $χSF$ setup

We compute the nonperturbative quark mass RG-running in the range $Λ_{QCD}\lessapproxμ\lessapprox M_W$ for $N_f=3$ massless QCD with a mixed action approach: sea quarks are regularised using nonperturbatively $O(a)$-improved Wilson fermions with Schrödinger functional (SF) boundary conditions, employing the configurations of 1802.05243, while valence quarks are regularised using nonperturbatively $O(a)$-improved Wilson fermions with chirally rotated Schrödinger functional boundary conditions ($χ$SF). Our result is compatible with its SF counterpart of ref.1802.05243, confirming the universality of $χ$SF and SF in the continuum limit. We also establish the optimal tuning strategy for the critical hopping parameter $κ_c$ and the $χ$SF boundary counterterm coefficient $z_{\rm f}$. We work in two energy regimes with two different definitions of the coupling: SF-coupling for 2 GeV $\lessapproxμ\lessapprox M_W$ and GF-coupling for $Λ_{QCD} \lessapproxμ\lessapprox 2 GeV$.

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arXiv2021

Renormalization $\&$ improvement of the tensor operator for $N_f=3$ QCD in a $χ$SF setup

We present preliminary results of the non-perturbative renormalization group (RG) running of the flavor non-singlet tensor operator. We employ the $χ$SF scheme for $N_f=3$ QCD using ensembles generated by the ALPHA collaboration for the computation of the quark mass running. The $χ$SF property of automatic $O(a)$ improvement prevents the $O(a)$ mixing of the correlation functions.

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