arXiv subjects

Seyong Kim

Publications and source records attributed to Seyong Kim.

At least 55 records · Page 3Linked to original sources

arXiv2021

Reconstruction of bottomonium spectral functions in thermal QCD using Kernel Ridge Regression

We discuss results for bottomonium at nonzero temperature obtained using NRQCD on Fastsum Generation 2L ensembles, as part of the Fastsum collaboration's programme to determine the spectrum of the bottomonium system as a function of temperature using a variety of approaches. Here we give an update on results for spectral functions obtained using Kernel Ridge Regression. We pay in particular attention to the generation of training data and introduce the notion of using lattice QCD ensembles to learn how to improve the generation of training data. A practical implementation is given.

hep-lat↗

arXiv2019

News from bottomonium spectral functions in thermal QCD

New results on bottomonium at nonzero temperature are presented, using the FASTSUM Generation 2L ensembles. Preliminary results for spectral function reconstruction using Kernel Ridge Regression, a machine learning technique, are shown as well and compared to results from the Maximum Entropy Method.

hep-lat↗

arXiv2019

Spectral quantities in thermal QCD: a progress report from the FASTSUM collaboration

In order to study spectral quantities in thermal QCD, the FASTSUM collaboration employs anisotropic lattice simulations with N_f=2+1 flavours of Wilson fermions. Here we discuss our Generation 2 and Generation 2L ensembles, which differ in the pion mass. The focus is on observables related to the light quarks and chiral symmetry restoration.

hep-lat↗

arXiv2019

Non-relativistic susceptibility and a dark matter application

When thermal rate equations are derived for the evolution of slow variables, it is often practical to parametrize the right-hand side with chemical potentials. To close the system, the chemical potentials are subsequently re-expressed in terms of the slow variables, which involves the consideration of a "susceptibility". Here we study a non-relativistic situation in which chemical potentials are large compared with the temperature, as is relevant for late-time pair annihilations in dark matter freeze-out. An order-of-magnitude estimate and a lattice simulation are presented for a susceptibility dominated by bound states of stop-like mediators. After this "calibration", the formalism is applied to a model with Majorana singlet dark matter, confirming that masses up to the multi-TeV domain are viable in the presence of sufficient (though not beyond a limit) mass degeneracy in the dark sector.

hep-ph↗

arXiv2019

Studies of a thermally averaged p-wave Sommerfeld factor

Thermal pair annihilation of heavy particles, such as dark matter or its co-annihilation partners, can be strongly influenced by attractive interactions. We investigate the case that pair annihilation proceeds through a velocity-suppressed $p$-wave operator, in the presence of an SU(3) gauge force. Making use of a non-relativistic effective theory, the thermal average of the pair-annihilation rate is estimated both through a resummed perturbative computation and through lattice simulation, in the range $M/T \sim 10 ... 30$. Bound states contribute to the annihilation process and enhancement factors of up to $\sim 100$ can be found.

hep-ph↗

arXiv2019

In-medium heavy quarkonium from lattice NRQCD

We present the final results from a multi-year study of the in-medium spectral properties of heavy quarkonium bound states on the lattice. In this work we combine high statistics $N_f=2+1$ ensembles from the HotQCD collaboration with the effective theory NRQCD and improved Bayesian spectral reconstruction methods. We corroborate with high precision the hierarchical in-medium modification of quarkonium states with respect to their vacuum binding energy and provide updated values on melting temperatures. In particular we are able to understand previous disagreements between different Bayesian methods as resulting from underestimated systematic uncertainties. The main quantitative result is a robust determination of the in-medium mass shifts of quarkonium ground states, which we find are negative, consistent with the behavior observed in strongly coupled pNRQCD potential based computations.

hep-lat↗

arXiv2018

Quarkonium in-medium properties from realistic lattice NRQCD

We present the final results of our high statistics study on the properties of bottomonium and charmonium at finite temperature. We focus on the temperature range around the crossover transition $150\leq T\leq 410$MeV, relevant for current heavy ion collision experiments. The QCD medium degrees of freedom which consist of dynamical u,d, and s quarks and gluons are captured by realistic state-of-the art ($m_π\approx 161$MeV) lattice QCD simulations of the HotQCD collaboration. For the heavy quarks we deploy the non-relativistic effective field theory of QCD, NRQCD. The in-medium properties of quarkonium are deduced from their spectral functions, which are reconstructed using improved and novel Bayesian approaches. Through a systematic analysis we shed light on the origin of the discrepancies in melting temperatures previously reported in the literature, showing that they are owed to underestimated methods uncertainties of the deployed spectral reconstructions. Our simulations corroborate a picture of sequential in-medium modification, ordered according to the vacuum binding energy of the states. As a central quantitative result, our study reveals how the mass of the heavy quarkonium ground state reduces as temperature increases. The observed spectral modifications are interpreted in the light of, and compared to previous studies based on the complex lattice potential for heavy quarkonium. Thus for the first time we provide a robust picture of in-medium heavy quarkonium modification in the quark-gluon plasma consistent among different non-relativistic methods. We also critically discuss the perspectives for improving on these results.

hep-lat↗

arXiv2017

High statistics study of in-medium S- and P-wave quarkonium states in lattice Non-relativistic QCD

Many measurements of quarkonium suppression at the LHC, e.g. the nuclear modification factor $R_{AA}$ of $J/Ψ$, are well described by a multitude of different models. Thus pinpointing the underlying physics aspects is difficult and guidance based on first principles is needed. Here we present the current status of our ongoing high precision study of in-medium spectral properties of both bottomonium and charmonium based on NRQCD on the lattice. This effective field theory allows us to capture the physics of quarkonium without modeling assumptions in a thermal QCD medium. In our study a first principles and realistic description of the QCD medium is provided by state-of-the-art lattices of the HotQCD collaboration at almost physical pion mass. Our updated results corroborate a picture of sequential modification of states with respect to their vacuum binding energy. Using a novel low-gain variant of the Bayesian BR method for reconstructing spectral functions we find that remnant features of the Upsilon may survive up to $T\sim400$MeV, while the $χ_b$ signal disappears around $T\sim270$MeV. The $c\bar{c}$ analysis hints at melting of $χ_c$ below $T\sim190$MeV while some $J/Ψ$ remnant feature might survive up to $T\sim245$MeV. An improved understanding of the numerical artifacts in the Bayesian approach and the availability of increased statistics have made possible a first quantitative study of the in-medium ground state masses, which tend to lower values as $T$ increases, consistent with lattice potential based studies.

hep-lat↗

arXiv2017

Heavy Flavours in Quark-Gluon Plasma

Recent progresses in lattice studies of heavy quark and quarkonium at non-zero temperature are discussed. Formulating a tail of spectral functions as a transport coefficient allows lattice determination of momentum diffusion coefficient ($κ$) for charm quark in the heavy quark mass limit and lattice determination of heavy quark/heavy anti-quark chemical equilibration rate in NRQCD. Quenched lattice study on a large volume gives $κ/T^3 = 1.8 \cdots 3.4$ in the continuum limit. A recent study with $N_f = 2+1$ configurations estimates the charmonium chemical equilibration rate $Γ_{\rm chem}$. At $T = 400$ MeV with $M \sim 1.5$ GeV, $Γ_{\rm chem}^{-1} \sim 150$ fm/c. Earlier results from the two studies (with different lattice setups and with different Bayesian priors) which calculate bottomonium correlators using NRQCD and employ Bayesian method to calculate spectral functions are summarized: $Υ(1S)$ survives upto $T \sim 1.9 T_c$ and excited states of $Υ$ are sequentially suppressed. The spectral functions of $χ_{b1}$ channel shows a Bayesian prior dependence of its thermal behavior: the $χ_{b1}$ spectral function with MEM prior shows melting above $T_c$ but that with a new Bayesian prior hints survival of $χ_{b1}$ upto $\sim 1.6 T_c$. Preliminary results from the efforts to understand the difference in the behavior of $χ_{b1}$ spectral function is given.

hep-lat↗

arXiv2017

On thermal corrections to near-threshold annihilation

We consider non-relativistic "dark" particles interacting through gauge boson exchange. At finite temperature, gauge exchange is modified in many ways: virtual corrections lead to Debye screening; real corrections amount to frequent scatterings of the heavy particles on light plasma constituents; mixing angles change. In a certain temperature and energy range, these effects are of order unity. Taking them into account in a resummed form, we estimate the near-threshold spectrum of kinetically equilibrated annihilating TeV scale particles. Weakly bound states are shown to "melt" below freeze-out, whereas with attractive strong interactions, relevant e.g. for gluinos, bound states boost the annihilation rate by a factor 4...80 with respect to the Sommerfeld estimate, thereby perhaps helping to avoid overclosure of the universe. Modestly non-degenerate dark sector masses and a way to combine the contributions of channels with different gauge and spin structures are also discussed.

hep-ph↗

arXiv2016

Rapid thermal co-annihilation through bound states in QCD

The co-annihilation rate of heavy particles close to thermal equilibrium, which plays a role in many classic dark matter scenarios, can be "simulated" in QCD by considering the pair annihilation rate of a heavy quark and antiquark at a temperature of a few hundred MeV. We show that the so-called Sommerfeld factors, parameterizing the rate, can be defined and measured non-perturbatively within the NRQCD framework. Lattice measurements indicate a modest suppression in the octet channel, in reasonable agreement with perturbation theory, and a large enhancement in the singlet channel, much above the perturbative prediction. The additional enhancement is suggested to originate from bound state formation and subsequent decay. Making use of a Green's function based method to incorporate thermal corrections in perturbative co-annihilation rate computations, we show that qualitative agreement with lattice data can be found once thermally broadened bound states are accounted for. We suggest that our formalism may also be applicable to specific dark matter models which have complicated bound state structures.

hep-ph↗

arXiv2015

In-medium quarkonium properties from a lattice QCD based effective field theory

In order to understand the experimental data on heavy quarkonium production in heavy ion collisions at RHIC and LHC it is necessary (though not sufficient) to pinpoint the properties of heavy $Q\bar{Q}$ bound states in the deconfined quark-gluon plasma, including their dissolution. Here we present recent results on the temperature dependence of bottomonium and charmonium correlators, as well as their spectral functions in a lattice QCD based effective field theory called NRQCD, surveying temperatures close to the crossover transition $140 {\rm MeV} < T< 249 {\rm MeV}$. The spectra are reconstructed based on a novel Bayesian prescription, whose systematic uncertainties are assessed. We present indications for sequential melting of different quarkonium species with respect to their vacuum binding energies and give estimates on the survival of S-wave and P-wave ground states.

hep-lat↗

arXiv2015

Lattice NRQCD study of quarkonium at non-zero temperature

To study the in-medium modification of quarkonium properties, charmonium correlators at $140.4 (β=6.664) \le T \le 221 (β= 7.280)$ (MeV) are calculated using the NRQCD formalism on $48^3 \times 12$ gauge configurations with dynamical $N_f = 2 + 1$ flavors of Highly Improved Staggered Quarks (HISQ). To determine the "zero energy shift" for these lattices, we perform a fine zero temperature scan ($β=6.664, 6.740, 6.800, 6.880, 6.950, 7.030, 7.150$ and $7.280$). We find that the temperature dependence of charmonium correlators is stronger than the temperature dependence of bottomonium correlators in a given channel. This fits into the expected pattern of sequential quarkonium melting.

hep-lat↗

arXiv2015

Nonrelativistic lattice study of stoponium

We calculate the bound state properties of stoponium using lattice formulation of nonrelativistic effective field theory for stop which is moving nonrelativistically in the rest frame of stoponium. Our calculation method is similar to that employed in lattice nonrelativistic quantum chromodynamics (NRQCD) studies for charmonium and bottominum. Using $16^3 \times 256$ quenched lattice gauge field configurations at $a^{-1} = 50(1) {\rm GeV}$, we obtain the stopoinium mass and the lattice matrix element which is related to the wavefunction at the origin for the $1S$ state and find that the lattice $|R_{1S} (0)|^2/M_{1S}^3$ is $3.5 \sim 4$ larger than that from a potential model calculation for $200 {\rm GeV} \le M_{1S} \le 800 {\rm GeV}$.

hep-lat↗

arXiv2015

Lattice NRQCD study of S- and P-wave bottomonium states in a thermal medium with $N_f=2+1$ light flavors

We investigate the properties of S-and P-wave bottomonium states in the vicinity of the deconfinement transition temperature. The light degrees of freedom are represented by dynamical lattice QCD configurations of the HotQCD collaboration with $N_f = 2 + 1$ flavors. Bottomonium correlators are obtained from bottom quark propagators, computed in nonrelativistic quantum chromodynamics (NRQCD) under the background of these gauge field configurations. The spectral functions for the $^3S_1$ ($Υ$) and $^3P_1$ ($χ_{b1}$) channel are extracted from the Euclidean time correlators using a novel Bayesian approach in the temperature region $140 {\rm MeV} \le T \le 249 {\rm MeV}$ and the results are contrasted to those from the standard maximum entropy method. We find that the new Bayesian approach is far superior to the maximum entropy method. It enables us to study reliably the presence or absence of the lowest state signal in the spectral function of a certain channel, even under the limitations present in the finite temperature setup. We find that $χ_{b1}$ survives up to $T=249 {\rm MeV}$, the highest temperature considered in our study and put stringent constraints on the size of the medium modification of $Υ$ and $χ_{b1}$ states.

hep-lat↗

arXiv2014

Quark-gluon plasma phenomenology from anisotropic lattice QCD

The FASTSUM collaboration has been carrying out simulations of N_f=2+1 QCD at nonzero temperature in the fixed-scale approach using anisotropic lattices. Here we present the status of these studies, including recent results for electrical conductivity and charge diffusion, and heavy quarkonium (charm and beauty) physics.

nucl-th↗

arXiv2014

Lattice NRQCD study on in-medium bottomonium spectra using a novel Bayesian reconstruction approach

We present recent results on the in-medium modification of S- and P-wave bottomonium states around the deconfinement transition. Our study uses lattice QCD with $N_f=2+1$ light quark flavors to describe the non-perturbative thermal QCD medium between $140$MeV$<T<249$MeV and deploys lattice regularized non-relativistic QCD (NRQCD) effective field theory to capture the physics of heavy quark bound states immersed therein. The spectral functions of the $^3S_1$ $(Υ)$ and $^3P_1$ $(χ_{b1})$ bottomonium states are extracted from Euclidean time Monte Carlo simulations using a novel Bayesian prescription, which provides higher accuracy than the Maximum Entropy Method. Based on a systematic comparison of interacting and free spectral functions we conclude that the ground states of both the S-wave $(Υ)$ and P-wave $(χ_{b1})$ channel survive up to $T=249$MeV. Stringent upper limits on the size of the in-medium modification of bottomonium masses and widths are provided.

hep-lat↗

arXiv2014

NRQCD based S- and P-wave bottomonium spectra at finite temperature from $48^3 \times 12$ lattices with $N_f=2+1$ light HISQ flavors

We study S-wave and P-wave bottomonium spectral functions at non-zero temperature in 2+1 flavor QCD using the NRQCD formulation for bottom quarks. We use a novel Bayesian approach to reconstruct the spectral functions and find that $χ_{b1}$ survives up to $T=249 {\rm MeV}$. We also study the effect of different temporal discretizations of the NRQCD formalism on the bottomonium correlation functions

hep-lat↗