Browsing by Subject "Quarkonium"

The J/ψ particle has been the source of much research since its

discovery in 1974. It provides an important probe of quantum

chromodynamics and has lead to many important insights into the

interactions of quarks and gluons in bound states. The rate of J/ψ

production was found to be much higher than expected at hadron

colliding experiments. Non-relativistic quantum chromodynamics was

developed in order to address these issues. This theory predicts a

strong spin-alignment not observed in data. All previous measurements

of J/ψ production have overlooked the hadronic environment the

J/ψ is produced in. This work is the first exploration of the

radiation surrounding J/ψ events measured at ATLAS at

√s=8 TeV. This is the first measurement of the

separation between the J/ψ and a matched jet and the second

measurement of the momentum fraction shared between the jet and the

J/ψ. These variables probe the radiation environment around the

J/ψ, and provide a new ways to understand quarkonia production.

The production of heavy quarkonium in heavy ion collisions has been used as an important probe of the quark-gluon plasma (QGP). Due to the plasma screening effect, the color attraction between the heavy quark antiquark pair inside a quarkonium is significantly suppressed at high temperature and thus no bound states can exist, i.e., they ``melt". In addition, a bound heavy quark antiquark pair can dissociate if enough energy is transferred to it in a dynamical process inside the plasma. So one would expect the production of quarkonium to be considerably suppressed in heavy ion collisions. However, experimental measurements have shown that a large amount of quarkonia survive the evolution inside the high temperature plasma. It is realized that the in-medium recombination of unbound heavy quark pairs into quarkonium is as crucial as the melting and dissociation. Thus, phenomenological studies have to account for static screening, dissociation and recombination in a consistent way. But recombination is less understood theoretically than the melting and dissociation. Many studies using semi-classical transport equations model the recombination effect from the consideration of detailed balance at thermal equilibrium. However, these studies cannot explain how the system of quarkonium reaches equilibrium and estimate the time scale of the thermalization. Recently, another approach based on the open quantum system formalism started being used. In this framework, one solves a quantum evolution for in-medium quarkonium. Dissociation and recombination are accounted for consistently. However, the connection between the semi-classical transport equation and the quantum evolution is not clear.

In this dissertation, I will try to address the issues raised above. As a warm-up project, I will first study a similar problem: $\alpha$-$\alpha$ scattering at the $^8$Be resonance inside an $e^-e^+\gamma$ plasma. By applying pionless effective field theory and thermal field theory, I will show how the plasma screening effect modifies the $^8$Be resonance energy and width. I will discuss the need to use the open quantum system formalism when studying the time evolution of a system embedded inside a plasma. Then I will use effective field theory of QCD and the open quantum system formalism to derive a Lindblad equation for bound and unbound heavy quark antiquark pairs inside a weakly-coupled QGP. Under the Markovian approximation and the assumption of weak coupling between the system and the environment, the Lindblad equation will be shown to turn to a Boltzmann transport equation if a Wigner transform is applied to the open system density matrix. These assumptions will be justified by using the separation of scales, which is assumed in the construction of effective field theory. I will show the scattering amplitudes that contribute to the collision terms in the Boltzmann equation are gauge invariant and infrared safe. By coupling the transport equation of quarkonium with those of open heavy flavors and solving them using Monte Carlo simulations, I will demonstrate how the system of bound and unbound heavy quark antiquark pairs reaches detailed balance and equilibrium inside the QGP. Phenomenologically, my calculations can describe the experimental data on bottomonium production. Finally I will extend the framework to study the in-medium evolution of heavy diquarks and estimate the production rate of the doubly charmed baryon $\Xi_{cc}^{++}$ in heavy ion collisions.

Quantum chromodynamics (QCD), the theory of quarks and gluons, is known to be

the correct description of strong nuclear interactions. At high energy and momenta,

one can use QCD directly to compute quantities of physical interest related to the

strong force. At low energies and momenta, one should use a different description in

terms of the degrees of freedom relevant at that scale. Two approaches to achieve

this end are effective field theories and gauge/gravity dualities. The former involves

a field theory more or less like QCD itself, but with states which are composites

of quarks and gluons. Then a perturbative expansion is made not in terms of the

gauge coupling but instead in terms of the momentum of the fields. This approach

dates back to the 1970s and is on firm theoretical footing. Gauge/gravity dualities

are a newer and less understood technique, which relates the physics of the strong

interactions to a different but likely equivalent theory in a higher dimensional space-

time, where the quantity of interest can be computed more readily. We employ

both effective field theories and gauge/gravity dualities to study the physics of ex-

otic quarkonium states, that is bound states containing a heavy quark-antiquark pair

which nevertheless cannot be be understood working only with the standard quark

model of hadrons. Candidates for such states, long speculated to exist, have recently

been observed at particle colliders, so that the theory of exotic quarkonium is now

of great experimental importance.

While bound states of heavy quarks $Q\bar{Q}$ called quarkonia have been studied for nearly a half-century, their production is still not well understood. We examine how techniques from Effective Field Theories (EFTs) of the strong force can be used to probe the production of the $J/\psi(c\bar{c})$. The focus will be to study how quarkonia are produced in jets, highly-collimated sprays of hadrons ubiquitous at particle colliders. We review the study of quarkonium production using Non-relativistic QCD (NRQCD) and the study of jet substructure observables using the Soft-Collinear Effective Theory (SCET). The concept of Fragmenting Jet Functions (FJF), which describe a hadron's energy distribution inside a jet of measured substructure, is extended to jets where the angularity is measured or where the transverse momenta of a hadron relative to the jet axis is measured. Predictions of the energy distribution of $J/\psi$ in jets at the LHC using FJFs are compared with the latest LHCb data using various extractions of the non-perturbative NRQCD long-distance-matrix-elements (LDMEs) in the literature. These distributions are also calculated using a modification of the Pythia Monte Carlo, which is shown to have an unphysical model of quarkonium production and gives results consistent with our FJF calculations. Our predictions of the energy of $J/\psi$ produced in jets fit the data much better than default Monte Carlo results. We also demonstrate that LDMEs extracted from high transverse momentum data do a better job at predicting the LHCb measurements.