Gliding Down the QCD Transition Line, from Nf = 2 till the Onset of Conformality

Abstract

We review the hot QCD transition with varying number of flavours, from two till the onset of the conformal window. We discuss the universality class for Nf=2, along the critical line for two massless light flavours, and a third flavour whose mass serves as an interpolator between Nf=2 and Nf=3. We identify a possible scaling window for the 3D O(4) universality class transition, and its crossover to a mean field behaviour. We follow the transition from Nf=3 to larger Nf, when it remains of first order, with an increasing coupling strength; we summarise its known properties, including possible cosmological applications as a model for a strong electroweak transition. The first order transition, and its accompanying second order endpoint, finally morphs into the essential singularity at the onset of the conformal window, following the singular behaviour predicted by the functional renormalisation group.

Scaling region of the 3D Ising universality class in finite temperature QCD

“…In particular, we detailed the application of our results in finite temperature QCD, where an increasing amount of results coming from recent collision experiments needs to be compared with theoretical predictions, in order to get evidence on the conjectured location of the critical ending point in the (𝑇, 𝜇) phase diagram. The scaling region charting performed here, as well as other new complementary results [11][12][13][14][15], moves in this direction. The goal of constructing a "reference frame" in the neighbourhood of the Ising critical point was efficiently reached using a parametric representation of the model in three dimensions.…”

Phase Transitions in Particle Physics

“…It is also suggested that the phase transition might stay second-order up to the onset of the conformal window at 9 ≲ N * f ≲ 12. These studies connect naturally to the conformal window of strong interactions [44,45,46], to be further discussed in Section 5.…”

Baryogenesis and Dark Matter in Multiple Hidden Sectors

“…However, its precise value for a larger number of flavours is fairly ambiguous. In the literature, the value varies significantly so there is a large uncertainty associated with the six flavours T c [67][68][69][70][71].…”