More Like this

1. Gluon Parton Distribution of the Pion from Lattice QCD

   Zhouyou Fan, Huey-Wen Lin ( January 2021 , Physics letters)

   We present the first determination of the x-dependent pion gluon distribution from lattice QCD using the pseudo-PDF approach. We use lattice ensembles with 2+1+1 flavors of highly improved staggered quarks (HISQ), generated by MILC Collaboration, at two lattice spacings a≈0.12 and 0.15~fm and three pion masses Mπ≈220, 310 and 690 MeV. We use clover fermions for the valence action and momentum smearing to achieve pion boost momentum up to 2.29 GeV. We find that the dependence of the pion gluon parton distribution on lattice spacing and pion mass is mild. We compare our results from the lightest pion mass ensemble with the determination by JAM and xFitter global fits.
2. Production of pions, kaons, (anti-)protons and $$\phi $$ mesons in Xe–Xe collisions at $$\sqrt{s_{\mathrm{NN}}}$$ = 5.44 TeV

   https://doi.org/10.1140/epjc/s10052-021-09304-4  

   Acharya, S. ; Adamová, D. ; Adler, A. ; Adolfsson, J. ; Aglieri Rinella, G. ; Agnello, M. ; Agrawal, N. ; Ahammed, Z. ; Ahmad, S. ; Ahn, S. U. ; et al ( July 2021 , The European Physical Journal C)

   Abstract The first measurement of the production of pions, kaons, (anti-)protons and $$\phi $$ ϕ mesons at midrapidity in Xe–Xe collisions at $$\sqrt{s_{\mathrm{NN}}} = 5.44~\text {TeV}$$ s NN = 5.44 TeV is presented. Transverse momentum ( $$p_{\mathrm{T}}$$ p T ) spectra and $$p_{\mathrm{T}}$$ p T -integrated yields are extracted in several centrality intervals bridging from p–Pb to mid-central Pb–Pb collisions in terms of final-state multiplicity. The study of Xe–Xe and Pb–Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe–Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the $$\phi $$ ϕmore »-to-pion ratio with increasing final-state multiplicity.« less
3. Production of light-flavor hadrons in pp collisions at $$\sqrt{s}~=~7\text { and }\sqrt{s} = 13 \, \text { TeV} $$

   https://doi.org/10.1140/epjc/s10052-020-08690-5  

   Acharya, S. ; Adamová, D. ; Adler, A. ; Adolfsson, J. ; Aggarwal, M. M. ; Rinella, G. Aglieri ; Agnello, M. ; Agrawal, N. ; Ahammed, Z. ; Ahmad, S. ; et al ( March 2021 , The European Physical Journal C)

   Abstract The production of $$\pi ^{\pm }$$ π ± , $$\mathrm{K}^{\pm }$$ K ± , $$\mathrm{K}^{0}_{S}$$ K S 0 , $$\mathrm{K}^{*}(892)^{0}$$ K ∗ ( 892 ) 0 , $$\mathrm{p}$$ p , $$\phi (1020)$$ ϕ ( 1020 ) , $$\Lambda $$ Λ , $$\Xi ^{-}$$ Ξ - , $$\Omega ^{-}$$ Ω - , and their antiparticles was measured in inelastic proton–proton (pp) collisions at a center-of-mass energy of $$\sqrt{s}$$ s = 13 TeV at midrapidity ( $$|y|<0.5$$ | y | < 0.5 ) as a function of transverse momentum ( $$p_{\mathrm{T}}$$ p T ) using the ALICE detector at the CERN LHC. Furthermore, the single-particle $$p_{\mathrm{T}}$$ p T distributions of $$\mathrm{K}^{0}_{S}$$ K S 0 , $$\Lambda $$ Λ , and $$\overline{\Lambda }$$ Λ ¯ in inelastic pp collisions at $$\sqrt{s} = 7$$ s = 7  TeV are reported here for the first time. The $$p_{\mathrm{T}}$$ p T distributions are studied at midrapidity within the transverse momentum range $$0\le p_{\mathrm{T}}\le 20$$ 0 ≤ p T ≤ 20 GeV/ c , depending on the particle species. The $$p_{\mathrm{T}}$$ p T spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower $$\sqrt{s}$$ smore »and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high $$p_{\mathrm{T}}$$ p T with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and $$x_{\mathrm{T}}\equiv 2p_{\mathrm{T}}/\sqrt{s}$$ x T ≡ 2 p T / s scaling properties of hadron production are also studied. As the collision energy increases from $$\sqrt{s}$$ s = 7–13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of $$\sqrt{s}$$ s , while ratios for multi-strange hadrons indicate enhancements. The $$p_\mathrm{{T}}$$ p T -differential cross sections of $$\pi ^{\pm }$$ π ± , $$\mathrm {K}^{\pm }$$ K ± and $$\mathrm {p}$$ p ( $$\overline{\mathrm{p}}$$ p ¯ ) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for $$\pi ^{\pm }$$ π ± and $$\mathrm{p}$$ p ( $$\overline{\mathrm{p}}$$ p ¯ ) at high $$p_\mathrm{{T}}$$ p T .« less
4. Identifying the nature of the QCD transition in relativistic collision of heavy nuclei with deep learning

   https://doi.org/10.1140/epjc/s10052-020-8030-7  

   Du, Yi-Lun ; Zhou, Kai ; Steinheimer, Jan ; Pang, Long-Gang ; Motornenko, Anton ; Zong, Hong-Shi ; Wang, Xin-Nian ; Stöcker, Horst ( June 2020 , The European Physical Journal C)

   Abstract Using deep convolutional neural network (CNN), the nature of the QCD transition can be identified from the final-state pion spectra from hybrid model simulations of heavy-ion collisions that combines a viscous hydrodynamic model with a hadronic cascade “after-burner”. Two different types of equations of state (EoS) of the medium are used in the hydrodynamic evolution. The resulting spectra in transverse momentum and azimuthal angle are used as the input data to train the neural network to distinguish different EoS. Different scenarios for the input data are studied and compared in a systematic way. A clear hierarchy is observed in the prediction accuracy when using the event-by-event, cascade-coarse-grained and event-fine-averaged spectra as input for the network, which are about 80%, 90% and 99%, respectively. A comparison with the prediction performance by deep neural network (DNN) with only the normalized pion transverse momentum spectra is also made. High-level features of pion spectra captured by a carefully-trained neural network were found to be able to distinguish the nature of the QCD transition even in a simulation scenario which is close to the experiments.
5. The Drell-Yan process with pions and polarized nucleons

   https://doi.org/10.1007/JHEP02(2021)166  

   Bastami, S. ; Gamberg, L. ; Parsamyan, B. ; Pasquini, B. ; Prokudin, A. ; Schweitzer, P. ( February 2021 , Journal of High Energy Physics)

   A bstract The Drell-Yan process provides important information on the internal struc- ture of hadrons including transverse momentum dependent parton distribution functions (TMDs). In this work we present calculations for all leading twist structure functions de- scribing the pion induced Drell-Yan process. The non-perturbative input for the TMDs is taken from the light-front constituent quark model, the spectator model, and available parametrizations of TMDs extracted from the experimental data. TMD evolution is im- plemented at Next-to-Leading Logarithmic precision for the first time for all asymmetries. Our results are compatible with the first experimental information, help to interpret the data from ongoing experiments, and will allow one to quantitatively assess the models in future when more precise data will become available.