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Thomas Mehen

Publications and source records attributed to Thomas Mehen.

At least 19 recordsLinked to original sources
arXiv2025

Relativistic corrections to exclusive photoproduction of Quarkonia near-threshold

Non-relativistic QCD (NRQCD) is used to calculate the relativistic correction to the amplitude for exclusive photoproduction of vector Quarkonia in the near-threshold region within the generalized parton distribution (GPD) framework. The relativistic corrections are found to be large for $J/\psi$, and lead to a breakdown of the GPD moment expansion near threshold. Cross-sections for both $J/\psi$ and $\Upsilon$ are calculated with the former being compared to the data. We also demonstrate the presence of endpoint divergences for the relativistic correction away from the near-threshold regime.

hep-ph↗
arXiv2024

Probing nonperturbative transverse momentum dependent PDFs with chiral perturbation theory: the $\bar{d}-\bar{u}$ asymmetry

We use chiral perturbation theory to study the long distance regime of transverse momentum dependent parton distribution functions (TMD PDFs). Chiral corrections to the TMD PDFs are computed from proton to pion/baryon splittings. For consistent power counting, we find that the fraction of the proton's momentum that a pion may carry must be kept small. We make predictions for a $\bar{d}-\bar{u}$ asymmetry in the proton's TMD PDFs and find that the effective theory gives a natural exponential suppression of the TMD PDF at long distances. We then explore the effects that additional nonperturbative physics may have on the TMD $\bar{d}-\bar{u}$ asymmetry.

hep-ph↗
arXiv2024

Transverse Momentum Dependent PDFs in Chiral Effective Theory

We develop a theoretical framework to match transverse momentum dependent parton distribution functions (TMD PDFs) onto chiral effective theory operators. In this framework the TMD PDF is expressed as a convolution of TMD hadronic distribution functions, which describe fluctuations of initial states into intermediate hadrons in chiral perturbation theory, and short distance matching coefficients, which are the TMD PDFs of intermediate hadrons in the chiral limit. The various limits of the matching condition are explored and an operator product expansion is applied to the high energy TMD matching coefficients, allowing them to be written in terms of the collinear valence PDFs of intermediate hadrons. As an example, we calculate the isovector TMD hadronic distribution functions for the proton at leading order in the chiral expansion.

hep-ph↗
arXiv2023

Polarized $J/\psi$ production in semi-inclusive DIS at large $Q^2$: Comparing quark fragmentation and photon-gluon fusion

We compare the relative importance of different mechanisms for polarized $J/\psi$ production in semi-inclusive deep inelastic scattering processes at large $Q^2$. The transverse momentum dependent (TMD) factorization framework and nonrelativistic quantum chromodynamics are used to study the leading contributions from light quark fragmentation to polarized $J/\psi$, and compared to direct production via photon-gluon fusion, which can proceed through color-singlet as well as color-octet mechanisms. We identify kinematic regimes where light quark fragmentation dominates, allowing for the extraction of the $^3S_1^{[8]}$ matrix element, as well as regimes where photon gluon fusion dominates, suggesting that the gluon TMD parton distribution function can be probed.

hep-ph↗
arXiv2023

Polarized TMD fragmentation functions for $J/\psi$ production

We calculate the matching, at leading order, of the transverse momentum-dependent fragmentation functions (TMDFFs) for light quarks and gluons fragmenting to a $J/\psi$ onto polarized nonrelativistic QCD (NRQCD) TMDFFs. The NRQCD TMDFFs have an operator-product-expansion in terms of nonperturbative NRQCD production matrix elements. Using the results we obtain, we make predictions for the light quark fragmentation contribution to the production of polarized $J/\psi$ in semi-inclusive deep inelastic scattering (SIDIS) both for unpolarized and longitudinally polarized beams. These results are an important contribution to polarized $J/\psi$ production in SIDIS, and thus are needed for comparison with experiments at the future Electron-Ion Collider.

hep-ph↗
arXiv2023

TMD Handbook

This handbook provides a comprehensive review of transverse-momentum-dependent parton distribution functions and fragmentation functions, commonly referred to as transverse momentum distributions (TMDs). TMDs describe the distribution of partons inside the proton and other hadrons with respect to both their longitudinal and transverse momenta. They provide unique insight into the internal momentum and spin structure of hadrons, and are a key ingredient in the description of many collider physics cross sections. Understanding TMDs requires a combination of theoretical techniques from quantum field theory, nonperturbative calculations using lattice QCD, and phenomenological analysis of experimental data. The handbook covers a wide range of topics, from theoretical foundations to experimental analyses, as well as recent developments and future directions. It is intended to provide an essential reference for researchers and graduate students interested in understanding the structure of hadrons and the dynamics of partons in high energy collisions.

hep-ph↗
arXiv2023

Quantum Information Science and Technology for Nuclear Physics. Input into U.S. Long-Range Planning, 2023

In preparation for the 2023 NSAC Long Range Plan (LRP), members of the Nuclear Science community gathered to discuss the current state of, and plans for further leveraging opportunities in, QIST in NP research at the Quantum Information Science for U.S. Nuclear Physics Long Range Planning workshop, held in Santa Fe, New Mexico on January 31 - February 1, 2023. The workshop included 45 in-person participants and 53 remote attendees. The outcome of the workshop identified strategic plans and requirements for the next 5-10 years to advance quantum sensing and quantum simulations within NP, and to develop a diverse quantum-ready workforce. The plans include resolutions endorsed by the participants to address the compelling scientific opportunities at the intersections of NP and QIST. These endorsements are aligned with similar affirmations by the LRP Computational Nuclear Physics and AI/ML Workshop, the Nuclear Structure, Reactions, and Astrophysics LRP Town Hall, and the Fundamental Symmetries, Neutrons, and Neutrinos LRP Town Hall communities.

nucl-ex↗
arXiv2023

Strong decays of $T_{cc}^+$ at NLO in an effective field theory

The $T_{cc}^+$ exotic meson, discovered by the LHCb Collaboration in 2021, can be interpreted as a molecular state of $D^{(*)0}$ and $D^{(*)+}$ mesons. We compute next-to-leading-order (NLO) contributions to the strong decay of $T_{cc}^+$ in an effective field theory for $D$ mesons and pions, considering contributions from one-pion exchange and final-state rescattering. Corrections to the total width, as well as the differential distribution in the invariant mass of the final-state $D$ meson pair are computed. The results remain in good agreement with LHCb experimental results when the NLO contributions are added. The leading uncertainties in the calculation come from terms which depend on the scattering length and effective range in $D$ meson scattering.

hep-ph↗
arXiv2022

Minimal Entanglement and Emergent Symmetries in Low-energy QCD

We study low-energy scattering of spin-1/2 baryons from the perspective of quantum information science, focusing on the correlation between entanglement minimization and the appearance of accidental symmetries. The baryon transforms as an octet under the SU(3) flavor symmetry and its interactions below the pion threshold are described by contact operators in an effective field theory (EFT) of QCD. Despite there being 64 channels in the 2-to-2 scattering, only six independent operators in the EFT are predicted by SU(3). We show that successive entanglement minimization in SU(3)-symmetric channels are correlated with increasingly large emergent symmetries in the EFT. In particular, we identify scattering channels whose entanglement suppression are indicative of emergent SU(6), SO(8), SU(8), and SU(16) symmetries. We also observe the appearance of non-relativistic conformal invariance in channels with unnaturally large scattering lengths. Improved precision from lattice simulations could help determine the degree of entanglement suppression, and consequently the amount of accidental symmetry, in low-energy QCD.

quant-ph↗
arXiv2021

$T_{cc}^+$ decays: Differential spectra and two-body final states

The recently discovered tetraquark, $T_{cc}^+$, has quark content $cc\bar{u}\bar{d}$ and a mass that lies just below open charm thresholds. Hence it is reasonable to expect the state to have a significant molecular component. We calculate the decay of the $T_{cc}^+$ in a molecular interpretation using effective field theory. In addition we calculate differential spectra as a function of the invariant mass of the final state charm meson pair. These are in good agreement with spectra measured by LHCb. We also point out that if shallow bound states of two pseudoscalar charm mesons exist, then two-body decays to those bound states and a single pion or photon can significantly enhance the width of the $T_{cc}^+$.

hep-ph↗
arXiv2021

Symmetry from Entanglement Suppression

Symmetry is among the most fundamental and powerful concepts in nature, whose existence is usually taken as given, without explanation. We explore whether symmetry can be derived from more fundamental principles from the perspective of quantum information. Starting with a two-qubit system, we show there are only two minimally entangling logic gates: the Identity and the SWAP, where SWAP interchanges the two states of the qubits. We further demonstrate that, when viewed as an entanglement operator in the spin-space, the $S$-matrix in the two-body scattering of fermions in the $s$-wave channel is uniquely determined by unitarity and rotational invariance to be a linear combination of the Identity and the SWAP. Realizing a minimally entangling $S$-matrix would give rise to global symmetries, as exemplified in Wigner's spin-flavor symmetry and Schr\"odinger's conformal invariance in low energy Quantum Chromodynamics. For $N_q$ species of qubit, the Identity gate is associated with an $[SU(2)]^{N_q}$ symmetry, which is enlarged to $SU(2N_q)$ when there is a species-universal coupling constant.

hep-th↗
arXiv2020

Prospects for quarkonium studies at the high-luminosity LHC

Prospects for quarkonium-production studies accessible during the upcoming high-luminosity phases of the CERN Large Hadron Collider operation after 2021 are reviewed. Current experimental and theoretical open issues in the field are assessed together with the potential for future studies in quarkonium-related physics. This will be possible through the exploitation of the huge data samples to be collected in proton-proton, proton-nucleus and nucleus-nucleus collisions, both in the collider and fixed-target modes. Such investigations include, among others, those of: (i) J/psi and Upsilon produced in association with other hard particles; (ii) chi(c,b) and eta(c,b) down to small transverse momenta; (iii) the constraints brought in by quarkonia on gluon PDFs, nuclear PDFs, TMDs, GPDs and GTMDs, as well as on the low-x parton dynamics; (iv) the gluon Sivers effect in polarised-nucleon collisions; (v) the properties of the quark-gluon plasma produced in ultra-relativistic heavy-ion collisions and of collective partonic effects in general; and (vi) double and triple parton scatterings.

hep-ph↗
arXiv2020

Quarkonium Semiclassical Transport in Quark-Gluon Plasma: Factorization and Quantum Correction

We study quarkonium transport in the quark-gluon plasma by using the potential nonrelativistic QCD (pNRQCD) effective field theory and the framework of open quantum systems. We argue that the coupling between quarkonium and the thermal bath is weak using separation of scales, so the initial density matrix of the total system factorizes and the time evolution of the subsystem is Markovian. We derive the semiclassical Boltzmann equation for quarkonium by applying a Wigner transform to the Lindblad equation and carrying out a semiclassical expansion. We resum relevant interactions to all orders in the coupling constant at leading power of the nonrelativistic and multipole expansions. The derivation is valid for both weakly coupled and strongly coupled quark-gluon plasmas. We find reaction rates in the transport equation factorize into a quarkonium dipole transition function and a chromoelectric gluon distribution function. For the differential reaction rate, the definition of the momentum dependent chromoelectric gluon distribution function involves staple-shaped Wilson lines. For the inclusive reaction rate, the Wilson lines collapse into a straight line along the real time axis and the distribution becomes momentum independent. The relation between the two Wilson lines is analogous to the relation between the Wilson lines appearing in the gluon parton distribution function (PDF) and the gluon transverse momentum dependent parton distribution function (TMDPDF). The centrality dependence of the quarkonium nuclear modification factor measured by experiments probes the momentum independent distribution while the transverse momentum dependence and measurements of the azimuthal angular anisotropy may be able to probe the momentum dependent one. We discuss one way to indirectly constrain the quarkonium in-medium real potential by using the factorization formula and lattice calculations. The leading quantum correction to the semiclassical transport equation of quarkonium is also worked out. The study can be easily generalized to quarkonium transport in cold nuclear matter, which is relevant for quarkonium production in eA collisions in the future Electron-Ion Collider.

hep-ph↗
arXiv2020

Operator Counting and Soft Blocks in Chiral Perturbation Theory

Chiral perturbation theory (ChPT) is a low-energy effective field theory of QCD and also a nonlinear sigma model based on the symmetry breaking pattern ${\rm SU}(N_f)\times {\rm SU}(N_f)\to {\rm SU}(N_f)$. In the limit of massless $N_f$ quarks, we enumerate the independent operators without external sources in ChPT using an on-shell method, by counting and presenting the soft blocks at each order in the derivative expansion, up to ${\cal O}(p^{10})$. Given the massless on-shell condition and total momentum conservation, soft blocks are homogeneous polynomials of kinematic invariants exhibiting the Adler's zero when any external momentum becomes soft and vanishing. In addition, soft blocks are seeds for recursively generating all tree amplitudes of Nambu-Goldstone bosons without recourse to ChPT, and in one-to-one correspondence with the "low energy constants" which are the Wilson coefficients. Relations among operators, such as those arising from equations of motion, integration-by-parts, hermiticity, and symmetry structure, manifest themselves in the soft blocks in simple ways. We find agreements with the existing results up to NNNLO, and make a prediction at N$^4$LO.

hep-ph↗
arXiv2020

An effective field theory approach to quarkonium at small transverse momentum

In this work we apply effective field theory (EFT) to observables in quarkonium production and decay that are sensitive to soft gluon radiation, in particular measurements that are sensitive to small transverse momentum. Within the EFT framework we study $χ_Q$ decay to light quarks followed by the fragmentation of those quarks to light hadrons. We derive a factorization theorem that involves transverse momentum distribution (TMD) fragmentation functions and new quarkonium TMD shape functions. We derive renormalization group equations, both in rapidity and virtuality, which are used to evolve the different terms in the factorization theorem to resum large logarithms. This theoretical framework will provide a systematic treatment of quarkonium production and decay processes in TMD sensitive measurements.

hep-ph↗
arXiv2019

Revisiting $X(3872)\to D^0 \bar{D}^0 π^0$ in XEFT

The calculation of the decay $X(3872)\to D^0 \bar{D}^0 π^0$ in effective field theory is revisited to include final state $π^0 D^0$, $π^0 \bar{D}^0$and $D^0\bar{D}^0$ rescattering diagrams. These introduce significant uncertainty into the prediction for the partial width as a function of the binding energy. The differential distribution in the pion energy is also studied for the first time. The normalization of the distribution is again quite uncertain due to higher order effects but the shape of the distribution is unaffected by higher order corrections. Furthermore the shape of the distribution and the location of the peak are sensitive to the binding energy of $X(3872)$. The shape is strongly impacted by the presence of virtual $D^{*0}$ graphs which highlights the molecular nature of the $X(3872)$. Measurement of the pion energy distribution in the decay $X(3872)\to D^0 \bar{D}^0 π^0$ can reveal interesting information about the binding nature of the $X(3872)$.

hep-ph↗
arXiv2019

Fate of Heavy Quark Bound States inside Quark-Gluon Plasma

Transport equations have been applied successfully to describe the quarkonium evolution inside the quark-gluon plasma, which include both plasma screening effects and recombination. We demonstrate how the quarkonium transport equation is derived from QCD by using the open quantum system framework and effective field theory. Weak coupling and Markovian approximations used in the derivation are justified from a separation of scales. By solving the equations numerically, we study the Upsilon production in heavy ion collisions.

hep-ph↗