arXiv Journal 2021-01-11

想来想去，觉得还是把每次在arXiv上扫过的文章简单记录下来。

2021-01-11

• hep-ph: 2 papers
• hep-th: 2 papers
• hep-lat: 1 paper

hep-ph: 2 papers

Title: QCD pressure: renormalization group optimized perturbation theory confronts lattice [arXiv:2101.02124]
Abstract: The quark contribution to the QCD pressure, Pq, is evaluated up to next-to-leading order (NLO) within the renormalization group optimized perturbation theory (RGOPT) resummation approach. To evaluate the complete QCD pressure we simply add the perturbative NLO contribution from massless gluons to the resummed Pq. Despite of this unsophisticated approximation our results for P = Pq + Pg at the central scale M∼2πT show a remarkable agreement with ab initio lattice predictions for T >∼ 0.25 GeV. We also show that by being imbued with RG properties, the RGOPT produces a drastic reduction of the embarrassing remnant scale dependence that plagues both standard thermal perturbative QCD and hard thermal loop perturbation theory (HTLpt) applications.
Comments: I wonder how to calculate the pressure inside hadrons from their partons. Look at Eq. (1) and references before, maybe we can learn something from it.

Title: Parametrization of Quark and Gluon Generalized Parton Distributions in a Dynamical Framework [arXiv:2101.01826]
Abstract: We present a parametrization of the chiral even generalized parton distributions,H,E, ̃H, ̃E, for the quark, antiquark and gluon, in the perturbative QCD-parton framework. Parametric analytic forms are given as a function of two equivalent sets of variables x,ξ,t(symmetric frame) and X,ζ,t(asymmetric frame), at an initial scale,Qo². In the X > ζregion a convenient and flexible form is obtained as the product of a Regge term ∝ X^−α+α′t, describing the low X behavior, times a spectator model-based functional form depending on various mass parameters; the behavior at X < ζ, is determined using the generalized parton distributions symmetry and polynomiality properties. The parameters are constrained using data on the flavor separated nucleon electromagnetic elastic form factors, the axial and pseudoscalar nucleon form factors, and the parton distribution functions from both the deep inelastic unpolarized and polarized nucleon structure functions. For the gluon distributions we use, in particular, constraints provided by recent lattice QCD moments calculations.The parametrization’s kinematical range of validity is: 0.0001≤X≤0.85, 0.01≤ζ≤0.85, 0≤−t≤1 GeV2, 2≤Q2≤100 GeV². With the simultaneous description of the quark, anti-quark and gluon sectors, this parametrization represents a first tool enabling a global QCD analysis of deeply virtual exclusive experiments.
Comments:

1. I am still not familiar with GPD. Maybe we can learn some of it from this paper.
2. Why they are considering these two parametrisations? Are they smart ways to parametrise? Why they are only valid in those kinematical regions, and how they break apart as kinematics going out of those regions?
3. We can also learn some of fittings of GPD from this paper.

hep-th: 2 papers

Title: Spacetime as a quantum circuit [arXiv:2101.01185]
Abstract: We propose that finite cutoff regions of holographic spacetimes represent quantum circuits that map between boundary states at different times and Wilsonian cutoffs,and that the complexity of those quantum circuits is given by the gravitational action. The optimal circuit minimizes the gravitational action. This is a generalization of both the “complexity equals volume” conjecture to unoptimized circuits, and path integral optimization to finite cutoffs. Using tools from holographic TT, we find that surfaces of constant scalar curvature play a special role in optimizing quantum circuits. We also find an interesting connection of our proposal to kinematic space, and discuss possible circuit representations and gate counting interpretations of the gravitational action.
Comments: Interesting topic as always. However I am not familiar with holography. Maybe we should focus on:

1. What’s a quantum circuit? In the abstract, the paper says that The optimal circuit minimizes the gravitational action. I wonder how.

Anytime, feel free to quit this topic.

Title: Learning scattering amplitudes by heart [arXiv:2101.02884]
Abstract: The canonical forms associated to scattering amplitudes of planar Feynman diagrams are interpreted in terms of masses of projectives, defined as the modulus of their central charges, in the hearts of certain t-structures of derived categories of quiver representations and, equivalently, in terms of cluster tilting objects of the corresponding cluster categories.
Comments: I only care two points on this paper:

1. Can I learn some ABCs of scattering amplitudes, as this paper advertises?
2. What is the heart?

hep-lat: 1 paper

Title: Quark distribution inside a pion in many-flavor 2+1 dimensional QCD using lattice: UV listens to IR [arXiv:2101.02224]
Abstract: We study the changes in the short-distance quark structure of the Nambu-Goldstone boson when the long-distance symmetry-breaking scales are depleted controllably. We achieve this by studying the valence Parton Distribution Function (PDF) of pion in 2+1 dimensional two-color QCD, with the number N of massless quarks as the tunable parameter that slides the theory from being strongly broken for N= 0 to the conformal window for N >4, where the theory is gapped by the fixed finite volume. We perform our study non-perturbatively using lattice simulations with N= 0,2,4,8 flavors of nearly massless two-component Wilson-Dirac sea quarks and employ the leading-twist formalism (LaMET/SDF) to compute the PDF of pion at a fixed valence mass. We find that the relative variations in the first few PDF moments are only mild compared to the changes in decay constant, but the shape of the reconstructed x-dependent PDF itself dramatically changes from being broad in the scale-broken sector to being sharply peaked in the near-conformal region, best reflected in PDF shape observables such as the cumulants.
Comments:

1. quark distribution inside a pion which is a Goldstone boson is an interesting topic.
2. How the 2+1 dimensional 2-color QCD look like? Learn how to tune these parameters.
3. Check on how to reconstruct x-dependent PDFs from lattice results.