We discuss a general theoretical framework for the study of bound states made of two heavy quarks. For states below threshold, the framework has been developed to the level of providing precision studies for several charmonium and bottomonium observables. For states above threshold, like some of the newly discovered charmonium-like resonances at the B factories, a systematic description exists so far only for specific systems.
In this work, we revise the conventional description of J/Psi(1S), Y(1S), Psi'(2S) and Y'(2S) elastic photo- and electroproduction off a nucleon target within the color dipole picture and carefully study various sources of theoretical uncertainties in calculations of the corresponding electroproduction cross sections. For this purpose, we test the corresponding predictions using a bulk of available dipole cross section parametrisations obtained from deep inelastic scattering data at HERA. Specifically, we provide the detailed analysis of the energy and hard-scale dependencies of quarkonia yiel…
Properties of quarkonia - like states in the charm and bottom sector have been studied in the frame work of relativistic Dirac formalism with a linear confinement potential. We have computed the mass spectroscopy and decay properties (vector decay constant and leptonic decay width) of several quarkonia - like states. Present study is also intended to identify some of the unexplained states as mixed P-wave and mixed S-D wave states of Charmonia and Bottomonia. The results indicate that the X(4140) state can be an admixture of two P states of charmonium. And the charmonium like states X(4630) an…
In our quest to win a deeper understanding of how QCD actually works, the study of the binding of heavy quarkonia and heavy-flavor hadrons to atomic nuclei offers enormous promise. Modern experimental facilities such as FAIR, Jefferson Lab at 12 GeV and J-PARC offer exciting new experimental opportunities to study such systems. These experimental advances are complemented by new theoretical approaches and predictions, which will both guide these experimental efforts and be informed and improved by them. This review will outline the main theoretical approaches, beginning with QCD itself, summar…
We discuss the exclusive $J/ψ$ photoproduction process, as measured recently at LHCb, as a means of constraining and ultimately determining the low $x$ and low $Q$ gluon PDF. The scale dependence of the theoretical prediction for this process is shown to be systematically improved via a taming of the known $\bar{\text{MS}}$ result, this amounts to resumming logarithmically enhanced small-$x$ terms and implementing a small-$Q$ power correction. The cross section level predictions allow the behaviour of the gluon PDF in the low $(x, Q)$ domain to be determined.
The objective of this paper is to assess the current theoretical understanding of the extensive set of quarkonium observables (for both charmonia and bottomonia) that have been attained in ultrarelativistic heavy-ion collisions over two orders of magnitude in center-of-mass energy. We briefly lay out and compare the currently employed theoretical frameworks and their underlying transport coefficients, and then analyze excitation functions of quarkonium yields to characterize the nature of the varying production mechanisms. We argue that an overall coherent picture of suppression and regenerati…
In this work we study the radiative decay widths of vector quarkonia for the process of $J/ψ(nS)\rightarrowη_{c}(nS)γ$ and $Υ(nS)\rightarrowη_{b}(nS)γ$ (for principal quantum numbers $n=1, 2, 3$) in the framework of Bethe-Salpeter equation under the covariant instantaneous ansatz using a $4\times 4$ form of BSE. The parameters of the framework were determined by a fit to the mass spectrum of ground states of pseudoscalar and vector quarkonia, such as; $η_{c}$, $η_{b}$, $J/ψ$ and $Υ$. These input parameters so fixed were found to give good agreements with data on mass spectra of ground and exci…
Production cross sections of heavy quarkonia are considered as useful tools to study various aspects of QCD. We have entered a new era of quarkonium production phenomenology, with the help of new measurements from the LHC giving access to more varieties of observables, and recent theoretical developments that provided us a better understanding of the short-distance process in which quarkonia are produced. With a more concrete understanding of perturbative corrections and a prolific collection of data, a satisfactory description of the quarkonium production mechanism might now be within reach. …
Within the nonrelativistic quantum chromodynamics framework, we make a comprehensive study on the exclusive production of excited charmonium and bottomonium in $e^+e^-\to γ^*/Z^0 \to|(Q\bar{Q})[n]\rangle +γ$ ($Q=c$ or $b$ quarks) at future $Z$ factory, where the $[n]$ represents the color-singlet $n^1S_0,~n^3S_1,~n^1P_0$ and $n^3P_J$ ($n=1,2,3,4; J=0,1,2$) Fock states. The "improved trace technology" is adopted to derive the analytic expressions at the amplitude level, which is useful for calculating the complicated $nP$-wave channels. Total cross sections, differential distributions, and unce…
In this work we calculate the mass spectrum of charmonium for $1P,...,3P$ states of $0^{++}$ , $1^{++}$, as well as for $1S,...,4S$ states of $0^{-+}$, and $1S, ...,5D$ states of $1^{--}$ along with the two-photon decay widths of ground and first excited state of $0^{++}$ quarkonia for the process, $O^{++}\rightarrow γγ$ in the framework of a QCD motivated Bethe-Salpeter Equation. In this $4\times 4$ BSE framework, the coupled Salpeter equations are first shown to decouple for the confining part of interaction, under heavy-quark approximation, and analyically solved, and later the one-gluon-ex…
While our visible Universe could be a 3-brane, some cosmological scenarios consider that other 3-branes could be hidden in the extra-dimensional bulk. Matter disappearance toward a hidden brane is mainly discussed for neutron - both theoretically and experimentally - but other particles are poorly studied. Recent experimental results offer new constraints on positronium or quarkonium invisible decays. In the present work, we show how a two-brane Universe allows for such invisible decays. We put this result in the context of the recent experimental data to constrain the brane energy scale $M_B$…
We analyze the validity of a commonly used identification between structures of the virtual photon $γ^*\to Q\bar Q$ and vector meson $V\to Q\bar Q$ transitions. In the existing studies of $S$-wave vector-meson photoproduction in the literature, such an identification is typically performed in the light-front (LF) frame while the radial component of the meson wave function is rather postulated than computed from the first principles. The massive photon-like $V\to Q\bar Q$ vertex, besides the $S$-wave component, also contains an extra $D$-wave admixture in the $Q\bar Q$ rest frame. However, the …
An overview is given of the theoretical developments on heavy quarks and quarkonia in heavy-ion collisions as reported at the "Hard and EM Probes 2018" conference. Specifically, we address progress in the understanding of heavy-flavor diffusion and its hadronization, quarkonium transport and the extraction of quarkonium melting temperatures, energy loss of heavy quarks at high momentum, and their rescattering in small colliding systems.
