arXiv subjects

Thomas Mehen

Publications and source records attributed to Thomas Mehen.

At least 37 records · Page 2Linked to original sources

arXiv2019

Perturbative Corrections to Heavy Quark-Diquark Symmetry Predictions for Doubly Heavy Baryon Hyperfine Splittings

Doubly heavy baryons $\left(QQq\right)$ and singly heavy antimesons $\left(\bar{Q}q\right)$ are related by the heavy quark-diquark (HQDQ) symmetry because in the $m_Q \to \infty$ limit, the light degrees of freedom in both the hadrons are expected to be in identical configurations. Hyperfine splittings of the ground states in both systems are nonvanishing at $O(1/m_Q)$ in the heavy quark mass expansion and HQDQ symmetry relates the hyperfine splittings in the two sectors. In this paper, working within the framework of Non-Relativistic QCD (NRQCD), we point out the existence of an operator that couples four heavy quark fields to the chromomagnetic field with a coefficient that is enhanced by a factor from Coulomb exchange. This operator gives a correction to doubly heavy baryon hyperfine splittings that scales as $1/m_Q^2 \times α_S/r$, where $r$ is the separation between the heavy quarks in the diquark. This correction can be calculated analytically in the extreme heavy quark limit in which the potential between the quarks in the diquark is Coulombic. In this limit, the correction is $O(α_s^2/m_Q)$ and comes with a small coefficient. For values of $α_s$ relevant to doubly charm and doubly bottom systems, the correction to the hyperfine splittings in doubly heavy baryons is only a few percent or smaller. We also argue that nonperturbative corrections to the prediction for the hyperfine splittings are suppressed by $Λ^2_{\rm QCD}/m_Q^2$ rather than $Λ_{\rm QCD}/m_Q$. Corrections should be $\approx 10\%$ in the charm sector and smaller in heavier systems.

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arXiv2019

Double Heavy baryons and Corrections to Heavy Quark-Diquark Symmetry Prediction for Hyperfine Splitting

In the $m_Q\rightarrow\infty$ limit, the hyperfine splittings in the ground state doubly heavy baryons $\left(QQq\right)$ and single heavy antimesons $(\bar{Q}q)$ are related by heavy quark-diquark symmetry (HQDQ) as the light degrees of freedom in both the hadrons are expected to have identical configurations. In this article, working within the framework of nonrelativistic QCD (NRQCD), we study the perturbative and nonperturbative corrections to the HQDQ symmetry hyperfine splitting relation that scale as ${\cal O}\left(α_s^2\right)$ and $Λ_{\rm QCD}^2/m_Q^2$ respectively. In the extreme heavy quark limit, the perturbative corrections to hyperfine splitting of doubly charm or bottom baryons are a few percent or smaller. The nonperturbative corrections to hyperfine splitting are of order $10\%$ in the case of doubly charm baryons and $1\%$ or smaller in doubly bottom baryons.

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arXiv2019

Quarkonium In-Medium Transport Equation Derived from First Principles

We use the open quantum system formalism to study the dynamical in-medium evolution of quarkonium. The system of quarkonium is described by potential non-relativistic QCD while the environment is a weakly coupled quark-gluon plasma in local thermal equilibrium below the melting temperature of the quarkonium. Under the Markovian approximation, it is shown that the Lindblad equation leads to a Boltzmann transport equation if a Wigner transform is applied to the system density matrix. Our derivation illuminates how the microscopic time-reversibility of QCD is consistent with the time-irreversible in-medium evolution of quarkonium states. Static screening, dissociation and recombination of quarkonium are treated in the same theoretical framework. In addition, quarkonium annihilation is included in a similar way, although the effect is negligible for the phenomenology of the current heavy ion collision experiments. The methods used here can be extended to study quarkonium dynamical evolution inside a strongly coupled QGP, a hot medium out of equilibrium or cold nuclear matter, which is important to studying quarkonium production in heavy ion, proton-ion, and electron-ion collisions.

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arXiv2018

From Underlying Event Sensitive To Insensitive: Factorization and Resummation

In this paper we study the transverse energy spectrum for the Drell-Yan process. The transverse energy is measured within the central region defined by a (pseudo-) rapidity cutoff. Soft-collinear effective theory (SCET) is used to factorize the cross section and resum large logarithms of the rapidity cutoff and ratios of widely separated scales that appear in the fixed order result. We develop a framework which can smoothly interpolate between various regions of the spectrum and eventually match onto the fixed order result. This way a reliable calculation is obtained for the contribution of the initial state radiation to the measurement. By comparing our result for Drell-Yan against Pythia we obtain a simple model that describes the contribution from multiparton interactions (MPI). A model with little or no dependence on the primary process gives results in agreement with the simulation. Based on this observation we propose MPI insensitive measurements. These observables are insensitive to the MPI contributions as implemented in Pythia and we compare against the purely perturbative result obtained with the standard collinear factorization.

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arXiv2017

Transverse Vetoes with Rapidity Cutoff in SCET

We consider di-jet production in hadron collisions where a transverse veto is imposed on radiation for (pseudo-)rapidities in the central region only, where this central region is defined with rapidity cutoff. For the case where the transverse measurement (e.g., transverse energy or min $p_T$ for jet veto) is parametrically larger relative to the typical transverse momentum beyond the cutoff, the cross section is insensitive to the cutoff parameter and is factorized in terms of collinear and soft degrees of freedom. The virtuality for these degrees of freedom is set by the transverse measurement, as in typical transverse-momentum dependent observables such as Drell-Yan, Higgs production, and the event shape broadening. This paper focuses on the other region, where the typical transverse momentum below and beyond the cutoff is of similar size. In this region the rapidity cutoff further resolves soft radiation into (u)soft and soft-collinear radiation with different rapidities but identical virtuality. This gives rise to rapidity logarithms of the rapidity cutoff parameter which we resum using renormalization group methods. We factorize the cross section in this region in terms of soft and collinear functions in the framework of soft-collinear effective theory, then further refactorize the soft function as a convolution of the (u)soft and soft-collinear functions. All these functions are calculated at one-loop order. As an example, we calculate a differential cross section for a specific partonic channel, $q q' \to q q'$, for the jet shape angularities and show that the refactorization allows us to resum the rapidity logarithms and significantly reduce theoretical uncertainties in the jet shape spectrum.

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arXiv2017

Implications of Heavy Quark-Diquark Symmetry for Excited Doubly Heavy Baryons and Tetraquarks

We give heavy quark-diquark symmetry predictions for doubly heavy baryons and tetraquarks in light of the recent discovery of the $Ξ_{cc}^{++}$ by LHCb. For five excited doubly charm baryons that are predicted to lie below the $Λ_c D$ threshold, we give predictions for their electromagnetic and strong decays using a previously developed chiral Lagrangian with heavy quark-diquark symmetry. Based on the mass of the $Ξ_{cc}^{++}$, the existence of a doubly heavy bottom $I=0$ tetraquark that is stable to the strong and electromagnetic decays has been predicted. If the mass of this state is below 10405 MeV, as predicted in some models, we argue using heavy quark-diquark symmetry that the $J^P=1^+$ $I=1$ doubly bottom tetraquark state will lie just below the open bottom threshold and likely be a narrow state as well. In this scenario, we compute strong decay width for this state using a new Lagrangian for tetraquarks which is related to the singly heavy baryon Lagrangian by heavy quark-diquark symmetry.

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arXiv2017

Dynamical Screening of $α$-$α$ Resonant Scattering and Thermal Nuclear Scattering Rate in a Plasma

We use effective field theory and thermal field theory to study the dynamical screening effect in the QED plasma on the $α$-$α$ scattering at the $^8$Be resonance. Dynamical screening leads to an imaginary part of the potential which results in a thermal width for the resonance and dominates over the previously considered static screening effect. As a result, both the resonance energy and width increase with the plasma temperature. Furthermore, dynamical screening can have a huge impact on the $α$-$α$ thermal nuclear scattering rate. For example, when the temperature is around $10$ keV, the rate is suppressed by a factor of about $900$. We expect similar thermal suppressions of nuclear reaction rates to occur in those reactions dominated by an above threshold resonance with a thermal energy. Dynamical screening effects on nuclear reactions can be relevant to cosmology and astrophysics.

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arXiv2017

Transverse Momentum Dependent Fragmenting Jet Functions with Applications to Quarkonium Production

We introduce the transverse momentum dependent fragmenting jet function (TMDFJF), which appears in factorization theorems for cross sections for jets with an identified hadron. These are functions of $z$, the hadron's longitudinal momentum fraction, and transverse momentum, $\boldsymbol{\mathrm{p}}_{\perp}$, relative to the jet axis. In the framework of Soft-Collinear Effective Theory (SCET) we derive the TMDFJF from both a factorized SCET cross section and the TMD fragmentation function defined in the literature. The TMDFJFs are factorized into distinct collinear and soft-collinear modes by matching onto SCET$_+$. As TMD calculations contain rapidity divergences, both the renormalization group (RG) and rapidity renormalization group (RRG) must be used to provide resummed calculations with next-to-leading-logarithm prime (NLL') accuracy. We apply our formalism to the production of $J/ψ$ within jets initiated by gluons. In this case the TMDFJF can be calculated in terms of NRQCD (Non-relativistic quantum chromodynamics) fragmentation functions. We find that when the $J/ψ$ carries a significant fraction of the jet energy, the $p_T$ and $z$ distributions differ for different NRQCD production mechanisms. Another observable with discriminating power is the average angle that the $J/ψ$ makes with the jet axis.

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arXiv2017

NRQCD Confronts LHCb Data on Quarkonium Production within Jets

We analyze the recent LHCb measurement of the distribution of the fraction of the transverse momentum, $z(J/ψ)$, carried by the $J/ψ$ within a jet. LHCb data is compared to analytic calculations using the fragmenting jet function (FJF) formalism for studying $J/ψ$ in jets. Logarithms in the FJFs are resummed using DGLAP evolution. We also convolve hard QCD partonic cross sections, showered with PYTHIA, with leading order Non Relativistic Quantum Chromodynamics (NRQCD) fragmentation functions and obtain consistent results. Both approaches use Madgraph to calculate the hard process that creates the jet initiating parton. These calculations give reasonable agreement with the $z(J/ψ)$ distribution that was shown to be poorly described by default PYTHIA simulations in the LHCb paper. We compare our predictions for the $J/ψ$ distribution using various extractions of nonperturbative NRQCD long-distance matrix elements (LDMEs) in the literature. NRQCD calculations agree with LHCb data better than default PYTHIA regardless of which fit to the LDMEs is used. LDMEs from fits that focus exclusively on high transverse momentum data from colliders are in good agreement with the LHCb measurement.

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arXiv2016

Analytic and Monte Carlo Studies of Jets with Heavy Mesons and Quarkonia

We study jets with identified hadrons in which a family of jet-shape variables called angularities are measured, extending the concept of fragmenting jet functions (FJFs) to these observables. FJFs determine the fraction of energy, z, carried by an identified hadron in a jet with angularity, τ_a. The FJFs are convolutions of fragmentation functions (FFs), evolved to the jet energy scale, with perturbatively calculable matching coefficients. Renormalization group equations are used to provide resummed calculations with next-to-leading logarithm prime (NLL') accuracy. We apply this formalism to two-jet events in e^+ e^- collisions with B mesons in the jets, and three-jet events in which a J/ψis produced in the gluon jet. In the case of B mesons, we use a phenomenological FF extracted from e^+ e^- collisions at the Z^0 pole evaluated at the scale μ= m_b. For events with J/ψ, the FF can be evaluated in terms of Non-Relativistic QCD (NRQCD) matrix elements at the scale μ=2 m_c. The z and τ_a distributions from our NLL' calculations are compared with predictions from monte carlo event generators. While we find consistency between the predictions for B mesons and the J/ψdistributions in τ_a, we find the z distributions for J/ψdiffer significantly. We describe an attempt to merge PYTHIA showers with NRQCD FFs that gives good agreement with NLL' calculations of the z distributions.

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arXiv2016

An Effective Field Theory Approach to the Stabilization of $^8$Be in a QED Plasma

We use effective field theory to study the $\mathrmα$-$\mathrmα$ resonant scattering in a finite-temperature QED plasma. The static plasma screening effect causes the resonance state $^8$Be to live longer and eventually leads to the formation of a bound state when $m_D\gtrsim 0.3$ MeV. We speculate that this effect may have implications on the rates of cosmologically and astrophysically relevant nuclear reactions involving $\mathrmα$ particles.

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arXiv2016

Jet Shapes in Dijet Events at the LHC in SCET

We consider the class of jet shapes known as angularities in dijet production at hadron colliders. These angularities are modified from the original definitions in e+e- collisions to be boost invariant along the beam axis. These shapes apply to the constituents of jets defined with respect to either k_T-type (anti-k_T, C/A, and k_T) algorithms and cone-type algorithms. We present an SCET factorization formula and calculate the ingredients needed to achieve next-to-leading-log (NLL) accuracy in kinematic regions where non-global logarithms are not large. The factorization formula involves previously unstudied "unmeasured beam functions," which are present for finite rapidity cuts around the beams. We derive relations between the jet functions and the shape-dependent part of the soft function that appear in the factorized cross section and those previously calculated for e+e- collisions, and present the calculation of the non-trivial, color-connected part of the soft-function to O(α_s). This latter part of the soft function is universal in the sense that it applies to any experimental setup with an out-of-jet p_T veto and rapidity cuts together with two tagged jets and it is independent of the choice of jet (sub-)structure measurement. In addition, we implement the recently introduced soft-collinear refactorization to resum logarithms of the jet size, valid in the region of non-enhanced non-global logarithm effects. While our results are valid for all 2 \to 2 channels, we compute explicitly for the qq' \to qq' channel the color-flow matrices and plot the NLL resummed differential dijet cross section as an explicit example, which shows that the normalization and scale uncertainty is reduced when the soft function is refactorized. For this channel, we also plot the jet size R dependence, the p_T^{\rm cut} dependence, and the dependence on the angularity parameter a.

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arXiv2016

New Tests of NRQCD from Quarkonia Within Jets

I review the current status of quarkonium production theory based on the non-relativistic QCD factorization formalism (NRQCD). While this theory describes much of the world's data on J/ψand Υproduction, there are still outstanding problems, most notably the polarization of quarkonia at large p_T in hadron colliders. In this talk we will present new tests of NRQCD involving the distribution of quarkonia within jets. The distribution of hadrons within jets is determined by nonperturbative functions called fragmenting jet functions (FJFs). FJFs are convolutions of fragmentation functions, evolved to the jet energy scale, with perturbatively calculable matching coefficients. I show how the FJFs for quarkonia can be calculated in NRQCD in terms of a few NRQCD long-distance matrix elements (LDME), so the dependence of the cross section on the energy fraction of the heavy quarkonium, z, is sensitive to the underlying production mechanism, and therefore provides a new test of NRQCD. The jet energy and z dependence of the cross section can be used to perform an independent extraction of NRQCD LDME. Finally, I describe ongoing work building on this result. This includes comparison of analytic resummed calculations with Monte Carlo simulations for two-jet events in e^+e^- collisions with B mesons, and three-jet events with J/ψ, as well as the definition of boost invariant jet substructure variables and calculation of a boost invariant soft function that are necessary for analytic calculations of jet cross sections at the Large Hadron Collider.

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arXiv2015

Hadronic Loops versus Factorization in EFT calculations of $X(3872) \to χ_{cJ} π^0$

We compare two existing approaches to calculating the decay of molecular quarkonium states to conventional quarkonia in effective field theory, using $X(3872) \to χ_{cJ} π^0$ as an example. In one approach the decay of the molecular quarkonium proceeds through a triangle diagram with charmed mesons in the loop. We argue this approach predicts excessively large rates for $Γ[X(3872) \to χ_{cJ}π^0]$ unless both charged and neutral mesons are included and a cancellation between these contributions is arranged to suppress the decay rates. This cancellation occurs naturally if the $X(3872)$ is primarily in the $I=0$ $D \bar{D}^{*} +c.c.$ scattering channel. The factorization approach to molecular decays calculates the rates in terms of tree-level transitions for the $D$ mesons in the $X(3872)$ to the final state, multiplied by unknown matrix elements. We show that this approach is equivalent to hadronic loops approach if the cutoff on the loop integrations is taken to be a few hundred MeV or smaller, as is appropriate when the charged $D$ mesons have been integrated out of XEFT.

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arXiv2014

Probing Quarkonium Production Mechanisms with Jet Substructure

We use fragmenting jet functions (FJFs) in the context of quarkonia to study the production channels predicted by NRQCD (3S_1^(1), 3S_1^(8), 1S_0^(8), 3P_J^(8)). We choose a set of FJFs that give the probability to find a quarkonium with a given momentum fraction inside a cone-algorithm jet with fixed cone size and energy. This observable gives several lever arms that allow one to distinguish different production channels. In particular, we show that at fixed momentum fraction the individual production mechanisms have distinct behaviors as a function of the the jet energy. As a consequence of this fact, we arrive at the robust prediction that if the depolarizing 1S_0^(8) matrix element dominates, then the gluon FJF will diminish with increasing energy for fixed momentum fraction, z, and z > 0.5.

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arXiv2014

Production of Stoponium at the LHC

Although the Large Hadron Collider (LHC) has not observed supersymmetric (SUSY) partners of the Standard Model particles, their existence is not ruled out yet. One recently explored scenario in which there are light SUSY partners that have evaded current bounds from the LHC is that of a light long-lived stop quark. In this paper we consider light stop pair production at the LHC when the stop mass is between 200 and 400 GeV. If the stops are long-lived they can form a bound state, stoponium, which then undergoes two-body decays to Standard Model particles. By considering the near-threshold production of such a pair through the gluon-gluon fusion process and taking into account the strong Coulombic interactions responsible for the formation of this bound state, we obtain factorization theorems for the stop pair inclusive and differential production cross sections. We also perform a resummation of large threshold logarithms up to next-to-next-to-leading logarithmic accuracy using well-established renormalization group equations in an effective field theory methodology. These results are used to calculate the invariant mass distributions of two photons or two Z bosons coming from the decay of the stoponium at the LHC. For our choices of SUSY model parameters, the stoponium is not detectable above Standard Model backgrounds in γγor ZZ at 8 TeV, but will be visible with 400 fb^(-1) of accumulated data if its mass is below 500 GeV when the LHC runs at 14 TeV.

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arXiv2014

Exotic Quarkonium Spectroscopy: X(3872), Z_b(10610), and Z_b(10650) in Non-Relativistic Effective Theory

This talk summarizes recent developments in quarkonium spectroscopy. I comment on the relation between the Z_b(10610) and Z_b(10650) and recently observed Z_c(3900) and Z_c(4025) states. Then I discuss a number of calculations using non-relativistic effective field theory for the X(3872), Z_b(10610), and Z_b(10650), under the assumption that these are shallow molecular bound states of charm or bottom mesons.

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arXiv2013

Line shapes in Υ(5S) \to B^{()} \bar{B}^{()}πwith Z(10610) and Z(10650) using effective field theory

The Belle collaboration recently discovered two resonances, $Z_b(10610)$ and $Z_b(10650)$ --- denoted $Z_b$ and $Z_b^\prime$ --- in the decays $Υ(5S) \to Υ(nS) π^+ π^-$ for $n$ = 1, 2, or 3, and $Υ(5S) \to h_b(mP) π^+ π^-$ for $m = 1$ or 2. These resonances lie very close to the $B^* \bar{B}$ and $B^* \bar{B}^*$ thresholds, respectively. A recent Belle analysis of the three-body decays $Υ(5S) \to [B^{(*)} \bar{B}^{(*)}]^{\mp} π^\pm$ gives further evidence for the existence of these states. In this paper we analyze this decay using an effective theory of $B$ mesons interacting via strong short-range interactions. Some parameters in this theory are constrained using existing data on $Υ(5 S) \to B^{(*)}\bar{B}^{(*)}$ decays, which requires the inclusion of heavy quark spin symmetry (HQSS) violating operators. We then calculate the differential distribution for $Υ(5S) \to B^{(*)} \bar{B}^{(*)} π$ as a function of the invariant mass of the $B^{(*)} \bar{B}^{(*)}$ pair, obtaining qualitative agreement with experimental data. We also calculate angular distributions in the decay $Υ(5S) \to Z_b^{(\prime)} π$ which are sensitive to the molecular character of the $Z_b^{(\prime)}$.

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