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M. Beneke

Publications and source records attributed to M. Beneke.

At least 127 records · Page 7Linked to original sources
arXiv1998

QCD analysis of inclusive B decay into charmonium

We compute the decay rates and $H$-energy distributions of $B$ mesons into the final state $H+X$, where $H$ can be any one of the $S$-wave or $P$-wave charmonia, at next-to-leading order in the strong coupling. We find that a significant fraction of the observed $J/ψ$, $ψ'$ and $χ_c$ must be produced through $c\bar{c}$ pairs in a colour octet state and should therefore be accompanied by more than one light hadron. At the same time we obtain stringent constraints on some of the long-distance parameters for colour octet production.

hep-ph↗
arXiv1998

New results on heavy quarks near threshold

We review in brief the threshold expansion, a method to perform the expansion of Feynman integrals near the heavy quark-antiquark threshold, and its relation to the construction of two effective theories, non-relativistic QCD (NRQCD) and potential-NRQCD. We then summarize recent next-to-next-to-leading order results on the decay $J/Ψ\to l^+ l^-$, the bottom quark mass from $Υ$ sum rules and the top-anti-top production cross section near threshold in $e^+ e^-$ collisions.

hep-ph↗
arXiv1998

A quark mass definition adequate for threshold problems

Recent calculations of heavy quark cross sections near threshold at next-to-next-to-leading order have found second-order corrections as large as first-order ones. We analyse long-distance contributions to the heavy quark potential in momentum and coordinate space and demonstrate that long-distance contributions in momentum space are suppressed as $Λ_{QCD}^2/q^2$. We then show that the long-distance sensitivity of order $Λ_{QCD} r$ introduced by the Fourier transform to coordinate space cancels to all orders in perturbation theory with long-distance contributions to the heavy quark pole mass. This leads us to define a subtraction scheme -- the `potential subtraction scheme' -- in which large corrections to the heavy quark potential and the `potential-subtracted' quark mass are absent. We compute the two-loop relation of the potential-subtracted quark mass to the $\bar{\rm MS}$ quark mass. We anticipate that threshold calculations expressed in terms of the scheme introduced here exhibit improved convergence properties.

hep-ph↗
arXiv1997

Two-loop Correction to the Leptonic Decay of Quarkonium

Applying asymptotic expansions at threshold, we compute the two-loop QCD correction to the short-distance coefficient that governs the leptonic decay $ψ\to l^+ l^-$ of a S-wave quarkonium state and discuss its impact on the relation between the quarkonium non-relativistic wave function at the origin and the quarkonium decay constant in full QCD.

hep-ph↗
arXiv1997

Asymptotic expansion of Feynman integrals near threshold

We present general prescriptions for the asymptotic expansion of massive multi-loop Feynman integrals near threshold. As in the case of previously known prescriptions for various limits of momenta and masses, the terms of the threshold expansion are associated with subgraphs of a given graph and are explicitly written through Taylor expansions of the corresponding integrands in certain sets of parameters. They are manifestly homogeneous in the threshold expansion parameter, so that the calculation of the given Feynman integral near the threshold reduces to the calculation of integrals of a much simpler type. The general method is illustrated by two-loop two-point and three-point diagrams. We discuss the use of the threshold expansion for problems of physical interest, such as the next-to-next-to-leading order heavy quark production cross sections close to threshold and matching calculations and power counting in non-relativistic effective theories.

hep-ph↗
arXiv1997

Inelastic photoproduction of polarised $J/ψ$

The comparison of $J/ψ$ photoproduction data in the inelastic region with theoretical predictions based on the NRQCD approach has remained somewhat ambiguous and controversial, in particular at large values of the inelasticity variable $z$. We study the polar and azimuthal decay angular distribution of $J/ψ$ mesons as functions of $z$ and transverse momentum $p_t$. Future measurements of decay angular distributions at the HERA $ep$ collider will provide a new test of theoretical approaches to factorisation between perturbation theory and quarkonium bound-state dynamics and shed light on the colour-octet production fraction in various regions of $z$ and $p_t$.

hep-ph↗
arXiv1997

Kinematic Enhancement of Non-Perturbative Corrections to Quarkonium Production

In this letter we address issues involved in quarkonium production near the boundaries of phase space. It is shown that higher-order non-perturbative contributions are enhanced in this kinematic region and lead to a breakdown of the non-relativistic (NRQCD) expansion. This breakdown is a consequence of sensitivity to the kinematics of soft gluon radiation and to the difference between partonic and hadronic phase space. We show how these large corrections can be resummed giving the dominant contribution to the cross section. The resummation leads to the introduction of non-perturbative, universal distribution functions. We discuss the importance of these shape functions for several observables, in particular the energy distribution of photo-produced $J/ψ$ close to the endpoint.

hep-ph↗
arXiv1997

Non-relativistic effective theory for Quarkonium Production in Hadron Collisions

I review recent progress in understanding inclusive quarkonium production in hadron collisions. The first part focuses on non-relativistic QCD as an effective theory. I discuss its differences from and similarities with effective theories describing bound states of a single heavy quark, as far as matching calculations beyond tree-level and power counting are concerned. The second part summarizes predictions for charmonium and bottomonium production at collider and fixed target experiments and their comparison with data. The emphasis here is on novel signatures due to color octet production, polarization of quarkonia and the $χ_1/χ_2$ ratio in fixed target collisions.

hep-ph↗
arXiv1997

Large-order Behaviour due to Ultraviolet Renormalons in QCD

Ultraviolet renormalons, contrary to their infrared counterparts, lead to a universal contribution to the large-order behaviour of perturbative expansions in QCD. In this letter, we determine nature of the leading ultraviolet renormalon singularity for the inclusive hadroproduction cross section in $e^+ e^-$ annihilation, for hadronic $τ$ decays and the moments of deep-inelastic scattering structure functions. We comment on the relevance of ultraviolet renormalons to estimates in low orders of perturbation theory.

hep-ph↗
arXiv1997

Two-Loop Anomalous Dimension of the Chromo-Magnetic Moment of a Heavy Quark

We find that the anomalous dimension (in the $\bar{MS}$ scheme) of the chromo-magnetic operator $g_s\bar h_vσ_{μν} G^{μν} h_v$ in the heavy-quark effective theory is γ_{mag} = (C_Aα_s / 2π) [1 + ( 17/18 C_A - 13/18 T_F n_f)α_s/π+ O(α_s^2)] generalizing the one-loop expression known previously. To derive the two-loop result, we use the reparametrization invariance and the virial theorem. After performing infrared subtractions, all two-loop integrals are of propagator type and are evaluated by a recurrence relation for tensor integrals.

hep-ph↗
arXiv1997

Phenomenology of Power Corrections in Fragmentation Processes in e^+ e^- Annihilation

We analyse power corrections to longitudinal and transverse fragmentation processes in e^+ e^- annihilation, based on the assumption of ultraviolet dominance of power corrections. Under this assumption, we determine the dependence of power corrections on the scaling variable x from the infrared renormalon asymptotics of leading power coefficient functions. Our results suggest that the longitudinal and transverse gluon fragmentation coefficient functions receive corrections of order 1/(x Q)^2. The power expansion breaks down at x<Λ/Q and has to be resummed. This resummation leads to 1/Q corrections to the longitudinal and transverse cross section, which cancel for the total cross section. We provide a simple parametrization of the x dependence of 1/Q^2 corrections to fragmentation processes and investigate perturbative corrections to the longitudinal cross section in higher orders, in view of a determination of the strong coupling.

hep-ph↗
arXiv1996

Direct J/psi and psi' Polarization and Cross Sections at the Tevatron

Transverse polarization of ${}^3 S_1$ charmonium states, produced directly in $p\bar{p}$ collisions at asymptotically large transverse momentum p_t, has emerged as the most prominent test of color octet contributions and spin symmetry in quarkonium production. We present predictions for the polar angle distribution at moderate values of p_t \sim 4 - 20 GeV, covered by the Tevatron Run I data. We update the fits of NRQCD matrix elements and discuss their theoretical uncertainties. With our best fit values, no transverse polarization is expected at p_t \sim 5 GeV, but the angular distribution is predicted to change dramatically as p_t increases to 20 GeV.

hep-ph↗
arXiv1996

On the infrared sensitivity of the longitudinal cross section in $e^+ e^-$ annihilation

We have calculated the contributions proportional to $β_0^nα_s^{n+1}$ to the longitudinal fragmentation function in $e^+e^-$ annihilation to all orders of perturbation theory. We use this result to estimate higher-order perturbative corrections and nonperturbative corrections to the longitudinal cross section $σ_L$ and discuss the prospects of determining $α_s$ from $σ_L$. The structure of infrared renormalons in the perturbative expansion suggests that the longitudinal cross section for hadron production with fixed momentum fraction $x$ receives nonperturbative contributions of order $1/(x^2Q^2)$, whereas the total cross section has a larger, $1/Q$ correction. This correction arises from very large longitudinal distances and is related to the behaviour of the Borel integral for the cross section with fixed $x$ at large values of the Borel parameter.

hep-ph↗
arXiv1996

Lifetime Difference of Neutral $B_s$ Mesons and its Implications

We discuss the calculation of the width difference $ΔΓ_{B_s}$ between the $B_s$ mass eigenstates to next-to-leading order in the heavy quark expansion. $1/m_b$-corrections are estimated to reduce the leading order result by typically $30\%$. The error of the present estimate $(ΔΓ/Γ)_{B_s} = 0.16^{+0.11}_{-0.09}$ could be substantially improved by pinning down the value of $\langle \bar{B}_s |(\bar b_is_i)_{S-P}(\bar b_js_j)_{S-P}| B_s\rangle$ and an accuracy of $10\%$ in $(ΔΓ/Γ)_{B_s}$ should eventually be reached. We briefly mention strategies to measure $(ΔΓ/Γ)_{B_s}$, and its implications for constraints on $ΔM_{B_s}$, CKM parameters and the observation of CP violation in untagged $B_s$ samples.

hep-ph↗
arXiv1996

Power Corrections, Renormalons and Resummation

I briefly review three topics of recent interest concerning power corrections, renormalons and Sudakov resummation: (a) $1/Q$ corrections to event shape observables in $e^+ e^-$ annihilation, (b) power corrections in Drell-Yan production and (c) factorial divergences that arise in resummation of large infrared (Sudakov) logarithms in moment or `real' space.

hep-ph↗