Jet wake from linearized hydrodynamics

We explore how to improve the hybrid model description of the particles originating from the wake that a jet produced in a heavy ion collision leaves in the droplet of quark-gluon plasma (QGP) through which it propagates, using linearized hydrodynamics on a background Bjorken flow. Jet energy and momentum loss described by the hybrid model become currents sourcing linearized hydrodynamics. By solving the linearized hydrodynamic equations numerically, we investigate the development of the wake in the dynamically evolving droplet of QGP, study the effect of viscosity, scrutinize energy-momentum conservation, and check the validity of the linear approximation. We find that linearized hydrodynamics works better in the viscous case because diffusive modes damp the energy-momentum perturbation produced by the jet. We calculate the distribution of particles produced from the jet wake by using the Cooper-Frye prescription and find that both the transverse momentum spectrum and the distribution of particles in azimuthal angle are similar in shape in linearized hydrodynamics and in the hybrid model. Their normalizations are different because the momentum-rapidity distribution in the linearized hydrodynamics analysis is more spread out, due to sound modes. Since the Bjorken flow has no transverse expansion, we explore the effect of transverse flow by using local boosts to add it into the Cooper-Frye formula. After including the effects of transverse flow in this way, the transverse momentum spectrum becomes harder: more particles with transverse momenta bigger than 2 GeV are produced than in the hybrid model. Although we defer implementing this analysis in a jet Monte Carlo, as would be needed to make quantitative comparisons to data, we gain a qualitative sense of how the jet wake may modify jet observables by computing proxies for two example observables: the lost energy recovered in a cone of varying open angle, and the fragmentation function. We find that linearized hydrodynamics with transverse flow effects added improves the description of the jet wake in the hybrid model in just the way that comparison to data indicates is needed. Our study illuminates a path to improving the description of the wake in the hybrid model, highlighting the need to take into account the effects of both transverse flow and the broadening of the energy-momentum perturbation in spacetime rapidity on particle production.

A New Description of Transverse Momentum Spectra of Identified Particles Produced in Proton-Proton Collisions at High Energies

The transverse momentum spectra of identified particles produced in high energy proton-proton p + p collisions are empirically described by a new method with the framework of the participant quark model or the multisource model at the quark level, in which the source itself is exactly the participant quark. Each participant (constituent) quark contributes to the transverse momentum spectrum, which is described by the TP-like function, a revised Tsallis–Pareto-type function. The transverse momentum spectrum of the hadron is the convolution of two or more TP-like functions. For a lepton, the transverse momentum spectrum is the convolution of two TP-like functions due to two participant quarks, e.g., projectile and target quarks, taking part in the collisions. A discussed theoretical approach seems to describe the p + p collisions data at center-of-mass energy s = 200 GeV , 2.76 TeV, and 13 TeV very well.

Effect of in-medium mass-shift on transverse-momentum spectrum and elliptic anisotropy of ϕ meson

We study the effect of in-medium mass-shift on transverse-momentum spectrum and elliptic anisotropy of [Formula: see text] meson. It is found that the mass-shift enhances the [Formula: see text] yields and suppresses the elliptic flow [Formula: see text] in large momentum region, and the effects increase with the increasing mass-shift. The effects are various for different sources and decrease with the increasing collision energy. We further study the effects for parts of all [Formula: see text] meson with mass-shift, and the effects decrease with the decreasing probability of [Formula: see text] meson with mass-shift. A possible method to detect the mass-shift of [Formula: see text] meson via transverse-momentum spectrum and [Formula: see text] is proposed.

Studies of gluon TMDs and their evolution using quarkonium-pair production at the LHC

$$J/\psi $$J/ψ- or $$\Upsilon $$Υ-pair production at the LHC are promising processes to study the gluon transverse momentum distributions (TMDs) which remain very poorly known. In this article, we improve on previous results by including the TMD evolution in the computation of the observables such as the pair-transverse-momentum spectrum and asymmetries arising from the linear polarization of gluons inside unpolarized protons. We show that the azimuthal asymmetries generated by the gluon polarization are reduced compared to the tree level case but are still of measurable size (in the 5–10% range). Such asymmetries should be measurable in the available data sets of $$J/\psi $$J/ψ pairs and in the future data sets of the high-luminosity LHC for $$\Upsilon $$Υ pairs.

The transverse momentum spectrum of weak gauge bosons at N$${}^3$$LL + NNLO

We present accurate QCD predictions for the transverse momentum ($$p_\perp $$p⊥) spectrum of electroweak gauge bosons at the LHC for $$13~\mathrm {TeV} $$13TeV collisions, based on a consistent combination of a NNLO calculation at large $$p_\perp $$p⊥ and N$$^3$$3LL resummation in the small $$p_\perp $$p⊥ limit. The inclusion of higher order corrections leads to substantial changes in the shape of the differential distributions, and the residual perturbative uncertainties are reduced to the few percent level across the whole transverse momentum spectrum. We examine the ratio of $$p_\perp $$p⊥ distributions in charged- and neutral-current Drell–Yan production, and study different prescriptions for the estimate of perturbative uncertainties that rely on different degrees of correlation between these processes. We observe an excellent stability of the ratios with respect to the perturbative order, indicating a strong correlation between the corresponding QCD corrections.