arXiv subjects

Thomas Mehen

Publications and source records attributed to Thomas Mehen.

At least 55 records · Page 3Linked to original sources

arXiv2013

Anomalous dimensions of the double parton fragmentation functions

Double parton fragmentation is a process in which a pair of partons produced in the short-distance process hadronize into the final state hadron. This process is important for quarkonium production when the transverse momentum is much greater than the quark mass. Resummation of logarithms of the ratio of these two scales requires the evolution equations for double parton fragmentation functions (DPFF). In this paper we complete the one-loop evaluation of the anomalous dimensions for the DPFF. We also consider possible mixing between the DPFF and single parton power suppressed gluon fragmentation and show that such effects are sub-leading.

hep-ph↗

arXiv2012

The Systematics of Quarkonium Production at the LHC and Double Parton Fragmentation

In this paper we discuss the systematics of quarkonium production at the LHC. In particular, we focus on the necessity to sum logs of the form log(Q/p_perp) and log(p_perp/m_Q). We show that the former contributions are power suppressed, while the latter, whose contribution in fragmentation is well known, also arise in the short distance (i.e., non-fragmentation) production mechanisms. Though these contributions are suppressed by powers of m_Q/p_perp, they can be enhanced by inverse powers of v, the relative velocity between heavy quarks in the quarkonium. In the limit p_perp >> m_Q short distance production can be thought of as the fragmentation of a pair of partons (i.e., the heavy quark and anti-quark) into the final state quarkonium. We derive an all order factorization theorem for this process in terms of double parton fragmentation functions (DPFF) and calculate the one-loop anomalous dimension matrix for the DPFF.

hep-ph↗

arXiv2011

On the Role of Charmed Meson Loops in Charmonium Decays

We investigate the effect of intermediate charmed meson loops on the M1 radiative decays $J/ψ\to η_c γ$ and $ψ'\rightarrowη^{(\prime)}_cγ$ as well as the isospin violating hadronic decays $ψ'\rightarrow J/ψ\,π^0(η)$ using heavy hadron chiral perturbation theory (HH$χ$PT). The calculations include tree level as well as one loop diagrams and are compared to the latest data from CLEO and BES-III. Our fit constrains the couplings of 1S and 2S charmonium multiplets to charmed mesons, denoted $g_2$ and $g_2^\prime$, respectively. We find that there are two sets of solutions for $g_2$ and $g_2^\prime$. One set, which agrees with previous values of the product $g_2 g_2^\prime$ extracted from analyses that consider only loop contributions to $ψ'\rightarrow J/ψ\,π^0(η)$, can only fit data on radiative decays with fine-tuned cancellations between tree level diagrams and loops in that process. The other solution for $g_2$ and $g_2^\prime$ leads to couplings that are smaller by a factor of 2.3. In this case tree level and loop contributions are of comparable size and the numerical values of the tree level contributions to radiative decays are consistent with estimates based on the quark model as well as non-relativistic QCD (NRQCD). This result shows that tree level HH$χ$PT couplings are as important as the one loop graphs with charmed mesons in these charmonium decays. The couplings $g_2$ and $g_2^\prime$ are also important for the calculations of the decays of charmed meson bound states, such as the X(3872), to conventional charmonia.

hep-ph↗

arXiv2011

The decay of the X(3872) into χ_{cJ} and the Operator Product Expansion in XEFT

XEFT is a low energy effective theory for the X(3872) that can be used to systematically analyze the decay and production of the X(3872) meson, assuming that it is a weakly bound state of charmed mesons. In a previous paper, we calculated the decays of X(3872) into χ_{cJ} plus pions using a two-step procedure in which Heavy Hadron Chiral Perturbation Theory (HH\chiPT) amplitudes are matched onto XEFT operators and then X(3872) decay rates are then calculated using these operators. The procedure leads to IR divergences in the three-body decay X(3872) \to χ_{cJ} ππwhen virtual D mesons can go on-shell in tree level HH\chiPT diagrams. In previous work, we regulated these IR divergences with the $D^{*0}$ width. In this work, we carefully analyze X(3872) \to χ_{cJ} π^0 and X(3872) \to χ_{cJ} ππusing the operator product expansion (OPE) in XEFT. Forward scattering amplitudes in HH\chiPT are matched onto local operators in XEFT, the imaginary parts of which are responsible for the decay of the X(3872). Here we show that the IR divergences are regulated by the binding momentum of the X(3872) rather than the width of the D^{*0} meson. In the OPE, these IR divergences cancel in the calculation of the matching coefficients so the correct predictions for the X(3872) \to χ_{c1} ππdo not receive enhancements due to the width of the D^{*0}. We give updated predictions for the decay X(3872) \to χ_{c1} ππat leading order in XEFT.

hep-ph↗

arXiv2011

Heavy Quark Symmetry Predictions for Weakly Bound B-Meson Molecules

Recently the Belle collaboration discovered two resonances, Zb(10610) and Zb(10650), that lie very close to the B\bar{B}^* and B^*\bar{B}^* thresholds, respectively. It is natural to suppose that these are molecular states of bottom and anti-bottom mesons. Under this assumption, we introduce an effective field theory for the Zb(10610) and Zb(10650), as well as similar unobserved states that are expected on the basis of heavy quark spin symmetry. The molecules are assumed to arise from short-range interactions that respect heavy quark spin symmetry. We use the theory to calculate line shapes in the vicinity of B^{(*)}\bar{B}^{(*)} thresholds as well as two-body decay rates of the new bottom meson bound states. We derive new heavy quark spin symmetry predictions for the parameters appearing in the line shapes as well as the total and partial widths of the states.

hep-ph↗

arXiv2011

Radiative Decays X(3872) -> psi(2S)+gamma and psi(4040) -> X(3872)+gamma in Effective Field Theory

Heavy hadron chiral perturbation theory (HHchiPT) and XEFT are applied to the decays X(3872) -> psi(2S) + gamma and psi(4040) -> X(3872) + gamma under the assumption that the X(3872) is a molecular bound state of neutral charm mesons. In these decays the emitted photon energies are 181 MeV and 165 MeV, respectively, so HHchiPT can be used to calculate the underlying D^0 bar{D}^{0*}+ bar{D}^0 D^{0*} -> psi(2S) + gamma or psi(4040) -> (D^0 bar{D}^{0*}+ bar{D}^0 D^{0*}) + gamma transition. These amplitudes are matched onto XEFT to obtain decay rates. The decays receive contributions from both long distance and short distance processes. We study the polarization of the psi(2S) in the decay X(3872) -> psi(2S) + gamma and the angular distribution of X(3872) in the decay psi(4040) -> X(3872) + gamma and find they can be used to differentiate between different decay mechanisms as well as discriminate between 2^{-+} and 1^{++} quantum number assignments of the X(3872).

hep-ph↗

arXiv2010

Pair Production of Color-Octet Scalars at the LHC

Heavy colored scalar particles, which exist in many models of new physics, can be pair produced at the Large Hadron Collider (LHC) via gluon-gluon fusion and possibly form quarkonium-like bound states. If the scalars are also charged under the electroweak gauge group, these bound states can then decay into electroweak bosons. This yields a resonant cross section for final states such as gamma gamma that can exceed Standard Model backgrounds. This paper studies this process in the Manohar-Wise model of color-octet scalars (COS). Important threshold logarithms and final state Coulomb-like QCD interactions are resummed using effective field theory. We compute the resummed cross section for gluon-gluon fusion to COS pairs at the LHC as well as the resonant cross section for octetonium decaying to gamma gamma. The latter cross section exceeds the Standard Model di-photon cross section when the COS mass is less than 500 (350) GeV for sqrt{s} = 14 (7) TeV. Nonobservation of resonances below these energies can significantly improve existing bounds on COS masses.

hep-ph↗

arXiv2010

Scattering of an Ultrasoft Pion and the X(3872)

The identification of the X(3872) as a loosely-bound charm-meson molecule allows it to be described by an effective field theory, called XEFT, for the D^* Dbar, D Dbar^* and D Dbar pi sector of QCD at energies small compared to the pion mass. We point out that this effective field theory can be extended to the sector that includes an additional pion and used to calculate cross sections for the scattering of a pion and the X(3872). If the collision energy is much smaller than the pion mass, the cross sections are completely calculable at leading order in terms of the masses and widths of the charm mesons, pion masses, and the binding energy of the X(3872). We carry out an explicit calculation of the cross section for the breakup of the X(3872) into D^{*+} Dbar^{*0} by the scattering of a very low energy pi^+.

hep-ph↗

arXiv2009

Octetonium at the LHC

Several models of new physics, such as grand unified theories, Pati-Salam models, chiral color models, etc., predict the existence of an $SU(2)_L$ doublet of color-octet scalars (COS). In the Manohar-Wise model, the Yukawa couplings of the COS are assumed to be consistent with Minimal Flavor Violation ensuring constraints from flavor physics are satisfied even for relatively light scalars. In this simple model we consider the production of color singlet bound states of COS that we call octetonium. Octetonium are mainly produced via gluon-gluon fusion and have significant production cross sections at the LHC. They can decay to pairs of gluons or electroweak gauge bosons. If the masses of the octetonia are 1 TeV or less, these states will be visible as resonances in $γγ, W^+W^-, ZZ$, and $ γZ$.

hep-ph↗

arXiv2009

Factorization and resummation for single color-octet scalar production at the LHC

Heavy colored scalar particles appear in a variety of new physics (NP) models and could be produced at the Large Hadron Collider (LHC). Knowing the total production cross section is important for searching for these states and establishing bounds on their masses and couplings. Using soft-collinear effective theory, we derive a factorization theorem for the process $pp\to SX$, where $S$ is a color-octet scalar, that is applicable to any NP model provided the dominant production mechanism is gluon-gluon fusion. The factorized result for the inclusive cross section is similar to that for the Standard Model Higgs production, however, differences arise due to color exchange between initial and final states. We provide formulae for the total cross section with large (partonic) threshold logarithms resummed to next-to-leading logarithm (NLL) accuracy. The resulting $K$-factors are similar to those found in Higgs production. We apply our formalism to the Manohar-Wise model and find that the NLL cross section is roughly 2 times (3 times) as large as the leading order cross section for a color-octet scalar of mass of 500 GeV (3 TeV). A similar enhancement should appear in any NP model with color-octet scalars.

hep-ph↗

arXiv2009

Color Octet Scalar Bound States at the LHC

One possible extension of the Standard Model scalar sector includes SU(2)_L doublet scalars that are color octets rather than singlets. We focus on models in which the couplings to fermions are consistent with the principle of minimal flavor violation (MFV), in which case these color octet scalars couple most strongly to the third generation of quarks. When the Yukawa coupling of color octet scalars to Standard Model fermions is less than unity, these states can live long enough to bind into color-singlet spin-0 hadrons, which we call octetonia. In this paper, we consider the phenomenology of octetonia at the Large Hadron Collider (LHC). Predictions for their production via gluon-gluon fusion and their two-body decays into Standard Model gauge bosons, Higgs bosons, and \bar{t}t are presented.

hep-ph↗

arXiv2008

Hadronic Decays of the X(3872) to chi_{cJ} in Effective Field Theory

The decays of the X(3872) to P-wave quarkonia are calculated under the assumption that it is a shallow bound state of neutral charmed mesons. The X(3872) is described using an effective theory of nonrelativistic D mesons and pions (X-EFT). We calculate X(3872) decays by first matching heavy hadron chiral perturbation theory (HHchiPT) amplitudes for D^0 bar{D}^{*0} -> chi_{cJ} + (pi^0,pi+pi) onto local operators in X-EFT, and then using these operators to calculate the X(3872) decays. This procedure reproduces the factorization theorems for X(3872) decays to conventional quarkonia previously derived using the operator product expansion. For single pion decays, we find nontrivial dependence on the pion energy from HHchiPT diagrams with virtual D mesons. This nontrivial energy dependence can potentially modify heavy quark symmetry predictions for the relative sizes of decay rates. At leading order, decays to final states with two pions are dominated by the final state chi_{c1} + pi^0 + pi^0, with a branching fraction just below that for the decay to chi_{c1} + pi^0. Decays to all other final states with two pions are highly suppressed.

hep-ph↗

arXiv2008

Nonperturbative Charming Penguin Contributions to Isospin Asymmetries in Radiative B decays

Recent experimental data on the radiative decays B -> V gamma, where V is a light vector meson, find small isospin violation in B -> K^* gamma while isospin asymmetries in B -> rho gamma are of order 20%, with large uncertainties. Using Soft-Collinear Effective Theory, we calculate isospin asymmetries in these radiative B decays up to O(1/m_b), also including O(v alpha_s) contributions from nonperturbative charming penguins (NPCP). In the absence of NPCP contributions, the theoretical predictions for the asymmetries are a few percent or less. Including the NPCP can significantly increase the isospin asymmetries for both B -> V gamma modes. We also consider the effect of the NPCP on the branching ratio and CP asymmetries in B^\pm -> V^\pm gamma.

hep-ph↗

arXiv2007

On Non-Relativistic Conformal Field Theory and Trapped Atoms: Virial Theorems and the State-Operator Correspondence in Three Dimensions

The field theory of nonrelativistic fermions interacting via contact interactions can be used to calculate the properties of few-body systems of cold atoms confined in harmonic traps. The state-operator correspondence of Non-Relativistic Conformal Field Theory (NRCFT) shows that the energy eigenvalues (in oscillator units) of N harmonically trapped fermions can be calculated from the scaling dimensions of N-fermion operators in the NRCFT. They are also in one-to-one correspondence with zero-energy, scale-invariant solutions to the N-body problem in free space. We show that these two mappings of the trapped fermion problem to free space problems are related by an automorphism of the SL(2,R) algebra of the conformal symmetry of fermions at the unitary limit. This automorphism exchanges the internal Hamiltonian of the gas with the trapping potential and hence provides a novel method for deriving virial theorems for trapped Fermi gases at the unitary limit. We also show that the state-operator correspondence can be applied directly in three spatial dimensions by calculating the scaling dimensions of two- and three-fermion operators and finding agreement with known exact results for energy levels of two and three trapped fermions at the unitary limit.

cond-mat.other↗

arXiv2007

Demonstration of the Equivalence of Soft and Zero-Bin Subtractions

Calculations of collinear correlation functions in perturbative QCD and Soft-Collinear Effective Theory (SCET) require a prescription for subtracting soft or zero-bin contributions in order to avoid double counting the contributions from soft modes. At leading order in $λ$, where $λ$ is the SCET expansion parameter, the zero-bin subtractions have been argued to be equivalent to convolution with soft Wilson lines. We give a proof of the factorization of naive collinear Wilson lines that is crucial for the derivation of the equivalence. We then check the equivalence by computing the non-Abelian two-loop mixed collinear-soft contribution to the jet function in the quark form factor. These results provide strong support for the equivalence, which can be used to give a nonperturbative definition of the zero-bin subtraction at lowest order in $λ$.

hep-ph↗

arXiv2007

On The Equivalence of Soft and Zero-Bin Subtractions

Zero-bin subtractions are required to avoid double counting soft contributions in collinear loop integrals in Soft-Collinear Effective Theory (SCET). In traditional approaches to factorization, double counting is avoided by dividing jet functions by matrix elements of soft Wilson lines. In this paper, we compare the two approaches to double counting, studying the quark form factor and deep inelastic scattering (DIS) as x_B \to 1 as examples. We explain how the zero-bin subtractions in SCET are required to reproduce the well-established factorization theorem for DIS as x_B \to 1. We study one-loop virtual contributions to the quark form factor and real gluon emission diagrams in DIS. The two approaches to double counting are equivalent if dimensional regularization (DR) is used to regulate infrared (IR) divergences. We discuss in detail ambiguities in the calculation of one-loop scaleless integrals in DR in SCET and perturbative QCD. We also demonstrate a nontrivial check of the equivalence of the zero-bin subtraction and the soft Wilson line subtraction in the virtual two-loop Abelian contributions to the quark form factor.

hep-ph↗

arXiv2006

Implications of SU(2)_L x U(1) Symmetry for SIM(2) Invariant Neutrino Masses

We consider SU(2)_L x U(1) gauge invariant generalizations of a nonlocal, Lorentz violating mass term for neutrinos that preserves a SIM(2) subgroup. This induces Lorentz violating effects in QED as well as tree-level lepton family number violating interactions. Measurements of g_e - 2 with trapped electrons severely constrain possible SIM(2) mass terms for electrons which violate C invariance. We study Lorentz violating effects in a C invariant and SIM(2) invariant extension of QED. We examine the Lorentz violating interactions of nonrelativistic electrons with electromagnetic fields to determine their impact on the spectroscopy of hydrogen-like atoms and g_e - 2 measurements with trapped electrons. Generically, Lorentz violating corrections are suppressed by m_ν^2/m_e^2 and are within experimental limits. We study one-loop corrections to electron and photon self-energies and point out the need for a prescription to handle IR divergences induced by the nonlocality of the theory. We also calculate the tree level contribution to μto e + γfrom SIM(2) invariant mass terms.

hep-ph↗

arXiv2006

Doubly Heavy Baryons and Quark-Diquark Symmetry in Quenched and Partially Quenched Chiral Perturbation Theory

We extend the chiral Lagrangian with heavy quark-diquark symmetry to quenched and partially quenched theories. These theories are used to derive formulae for the chiral extrapolation of masses and hyperfine splittings of doubly heavy baryons in lattice QCD simulations. A quark-diquark symmetry prediction for the hyperfine splittings of heavy mesons and doubly heavy baryons is rather insensitive to chiral corrections in both quenched and partially quenched QCD. Extrapolation formulae for the doubly heavy baryon electromagnetic transition moments are also determined for the partially quenched theory.

hep-lat↗