QCD transition at the physical point, and its scaling window from twisted mass Wilson fermions

The nature of the QCD chiral phase transition in the limit of vanishing quark masses has remained elusive for a long time, since it cannot be simulated directly on the lattice and is strongly cutoff-dependent. We report on a comprehensive ongoing study using unimproved staggered fermions with Nf ∈ [2, 8] mass-degenerate flavours on Nτ ∈ {4, 6, 8} lattices, in which we locate the chiral critical surface separating regions with first-order transitions from crossover regions in the bare parameter space of the lattice theory. Employing the fact that it terminates in a tricritical line, this surface can be extrapolated to the chiral limit using tricritical scaling with known exponents. Knowing the order of the transitions in the lattice parameter space, conclusions for approaching the continuum chiral limit in the proper order can be drawn. While a narrow first-order region cannot be ruled out, we find initial evidence consistent with a second-order chiral transition in all massless theories with Nf ≤ 6, and possibly up to the onset of the conformal window at 9 ≲ $$ {N}_{\mathrm{f}}^{\ast } $$ N f ∗ ≲ 12. A reanalysis of already published $$ \mathcal{O} $$ O (a)-improved Nf = 3 Wilson data on Nτ ∈ [4, 12] is also consistent with tricritical scaling, and the associated change from first to second-order on the way to the continuum chiral limit. We discuss a modified Columbia plot and a phase diagram for many-flavour QCD that reflect these possible features.

show abstract

“…A similar investigation with twisted mass Wilson fermions based on slightly larger pion masses confirms this behaviour [26].…”

Section: Jhep11(2021)141supporting

confidence: 58%

On the order of the QCD chiral phase transition for different numbers of quark flavours

Cuteri

Philipsen

Sciarra

2021

J. High Energ. Phys.

View full text Add to dashboard Cite

show abstract

“…The critical temperature in the chiral limit, eq. ( 21) agrees quantitatively with the fRG-estimate in [12], while it is consistent with the lattice estimates in [11] with T c0 = 132 +3 −6 MeV, and [80] with T c0 = 134 +6 −4 MeV. We emphasise that the coincidence of the chiral transition temperature in the chiral limit as well as for all light current quark masses investigated with the fRG results is rather non-trivial in view of the different approximations involved: while the present work includes, in contradistinction to [12], the full quarkgluon vertex for all temperatures, the fRG-computation in [12] includes multi-scattering vertices of the scalarpseudoscalar four-quark channel (σ and pion).…”

Section: Chiral Limit Ofsupporting

confidence: 84%

“…This situation calls for a combined functional-lattice study: the chiral limit (and the continuum limit) are difficult to access within lattice computations, while functional computations are well-controlled in the chiral limit. M , for various pion masses from the present computation (fRG-DSE) in comparison with results from [12] (fRG), and recent lattice QCD studies: [11] (hotQCD) with Nτ = 8, 12 and [80] (KLT) with a fixed scaled approach. These results are also depicted in Figure 7.…”

Section: B Combined Error Estimate For the Chiral Transition Temperaturementioning

confidence: 80%

“…A detailed discussion in view of the universality class of QCD in the chiral limit of the functional QCD results in the present work and in [12], as well as the lattice results of [11,80] is given in Section V. For the time being, we just remark that the approximation to functional QCD with the fRG in [12] incorporates criticality in terms of the dynamics of the order parameter and the order parameter potential. This has been tested both in generic low energy effective theories and in QCD, see [75].…”

Section: A Transition Temperature and Transition Linementioning

confidence: 87%

“…A first main result is given by the chiral transition temperature T c (m l ) in dependence of the ratio H = m l /m s of current quark masses, that can be derived from the magnetic equation of state: the transition temperature is obtained from the peak of the susceptibility of the renormalised light chiral condensate defined in Section III A. The results are discussed in Section IV A, including a comparison with the respective lattice results, [11,[77][78][79][80], as well as the functional QCD study (fRG) in [12]. For the comparison with the latter results we also present T c (m π ).…”

Section: Magnetic Equation Of Statementioning

confidence: 99%

See 2 more Smart Citations

Phase structure of 2+1-flavour QCD and the magnetic equation of state

Gao,

Pawlowski

2021

Preprint

View full text Add to dashboard Cite

We determine the chiral phase structure of 2 + 1-flavour QCD in dependence of temperature and the light flavour quark mass with Dyson-Schwinger equations. Specifically, we compute the renormalised chiral condensate and its susceptibility. The latter is used to determine the (pseudo)critical temperature for general light current quark masses. In the chiral limit we obtain a critical temperature of about 141 MeV. This result is in quantitative agreement with recent functional renormalisation group results in QCD, and is compatible with the respective lattice results. We also compute the order parameter potential of the light chiral condensate and map out the regime in the phase diagram which exhibits quasi-massless modes, and discuss the respective chiral dynamics.

show abstract