arXiv subjects

Johan Rathsman

Publications and source records attributed to Johan Rathsman.

44 records · Page 3Linked to original sources

arXiv2001

Disentangling running coupling and conformal effects in QCD

We investigate the relation between a postulated skeleton expansion and the conformal limit of QCD. We begin by developing some consequences of an Abelian-like skeleton expansion, which allows one to disentangle running-coupling effects from the remaining skeleton coefficients. The latter are by construction renormalon-free, and hence hopefully better behaved. We consider a simple ansatz for the expansion, where an observable is written as a sum of integrals over the running-coupling. We show that in this framework one can set a unique Brodsky-Lepage-Mackenzie (BLM) scale-setting procedure as an approximation to the running-coupling integrals, where the BLM coefficients coincide with the skeleton ones. Alternatively, the running-coupling integrals can be approximated using the effective charge method. We discuss the limitations in disentangling running coupling effects in the absence of a diagrammatic construction of the skeleton expansion. Independently of the assumed skeleton structure we show that BLM coefficients coincide with the conformal coefficients defined in the small $β_0$ (Banks-Zaks) limit where a perturbative infrared fixed-point is present. This interpretation of the BLM coefficients should explain their previously observed simplicity and smallness. Numerical examples are critically discussed.

hep-ph↗

arXiv2001

Conformal Expansions: A Template for QCD Predictions

The use of conformal expansions for predictions in quantum chromodynamics is discussed as a way to avoid renormalization scheme and scale ambiguities, as well as factorial growth of perturbative coefficients due to renormalons. Special emphasis is given to the properties of an assumed skeleton expansion and its relation to the Banks-Zaks expansion. The relation of BLM scale-setting to the skeleton expansion is also discussed and new criteria for the applicability of BLM scale-setting are presented.

hep-ph↗

arXiv1999

Odderon-Pomeron Interference

We show that the asymmetry in the fractional energy of charm versus anticharm jets produced in high energy diffractive photoproduction is sensitive to the interference of the Odderon $(C = -)$ and Pomeron $(C = +)$ exchange amplitudes in QCD. We predict the dynamical shape of the asymmetry in a simple model and estimate its magnitude to be of the order 15% using an Odderon coupling to the proton which saturates constraints from proton-proton vs. proton-antiproton elastic scattering. Measurements of this asymmetry at HERA could provide firm experimental evidence for the presence of Odderon exchange in the high energy limit of strong interactions.

hep-ph↗

arXiv1999

Conformal Symmetry as a Template: Commensurate Scale Relations and Physics Renormalization Schemes

Commensurate scale relations are perturbative QCD predictions which relate observable to observable at fixed relative scale, such as the "generalized Crewther relation", which connects the Bjorken and Gross-Llewellyn Smith deep inelastic scattering sum rules to measurements of the e+ e- annihilation cross section. We show how conformal symmetry provides a template for such QCD predictions, providing relations between observables which are present even in theories which are not scale invariant. All non-conformal effects are absorbed by fixing the ratio of the respective momentum transfer and energy scales. In the case of fixed-point theories, commensurate scale relations relate both the ratio of couplings and the ratio of scales as the fixed point is approached. In the case of the $α_V$ scheme defined from heavy quark interactions, virtual corrections due to fermion pairs are analytically incorporated into the Gell-Mann Low function, thus avoiding the problem of explicitly computing and resumming quark mass corrections related to the running of the coupling. Applications to the decay width of the Z boson, the BFKL pomeron, and virtual photon scattering are discussed.

hep-ph↗

arXiv1999

The Two-Loop Scale Dependence of the Static QCD Potential including Quark Masses

The interaction potential V(Q^2) between static test charges can be used to define an effective charge $α_V(Q^2)$ and a physically-based renormalization scheme for quantum chromodynamics and other gauge theories. In this paper we use recent results for the finite-mass fermionic corrections to the heavy-quark potential at two-loops to derive the next-to-leading order term for the Gell Mann-Low function of the V-scheme. The resulting effective number of flavors $N_F(Q^2/m^2)$ in the $α_V$ scheme is determined as a gauge-independent and analytic function of the ratio of the momentum transfer to the quark pole mass. The results give automatic decoupling of heavy quarks and are independent of the renormalization procedure. Commensurate scale relations then provide the next-to-leading order connection between all perturbatively calculable observables to the analytic and gauge-invariant $α_V$ scheme without any scale ambiguity and a well defined number of active flavors. The inclusion of the finite quark mass effects in the running of the coupling is compared with the standard treatment of finite quark mass effects in the $\bar{MS}$ scheme.

hep-ph↗

arXiv1999

A Generalised Area Law for Hadronic String Reinteractions

A new model for hadronic string reinteractions based on a generalised area law is presented. The model describes both the hadronic final states in $e^+e^-$ annihilation and the diffractive structure function in deep inelastic scattering. The model also predicts a shift in the W-mass reconstructed from hadronic decays of W-pairs of the order 65 MeV.

hep-ph↗

arXiv1998

An Analytic Extension of the $\bar{MS}$ Renormalizaton Scheme

The conventional definition of the running coupling $α_{\bar{MS}}(μ)$ in quantum chromodynamics is based on a solution to the renormalization group equations which treats quarks as either completely massless at a renormalization scale $μ$ above their thresholds or infinitely massive at a scale below them. The coupling is thus nonanalytic at these thresholds. In this paper we present an analytic extension of $α_{\bar{MS}}(μ)$ which incorporates the finite-mass quark threshold effects into the running of the coupling. This is achieved by using a commensurate scale relation to connect $α_{\bar{MS}}(μ)$ to the physical $α_V$ scheme at specific scales, thus naturally including finite quark masses. The analytic-extension inherits the exact analyticity of the $α_V$ scheme and matches the conventional $\bar {MS}$ scheme far above and below mass thresholds. Furthermore just as in $α_V$ scheme, there is no renormalization scale ambiguity, since the position of the physical mass thresholds is unambiguous.

hep-ph↗

arXiv1997

Hints for Enhanced b -> sg from Charm and Kaon Counting

Previously, motivation for enhanced b -> sg from new flavor physics has centered on discrepancies between theory and experiment. Here two experimental hints are considered: (1) updated measurements of the charm multiplicity and BR(Bbar -> X_{ccbars}) at the Upsilon (4S) imply BR(B -> Xnocharm}) \approx 12.4 \pm 5.6 %, (2) the Bbar} -> K- X and Bbar -> K+/K- X branching fractions are in excess of conventional Bbar -> Xc -> KX yields by about 16.9 \pm 5.6 % and 18 \pm 5.3 %, respectively. JETSET 7.4 was used to estimate kaon yields from ssbar popping in Bbar -> Xcubard decays. JETSET 7.4 Monte Carlos for BR(Bbar -> Xsg) \sim 15 % imply that the additional kaon production would lead to 1 σagreement with observed charged and neutral kaon yields. The Ks momentum spectrum would be consistent with recent CLEO bounds in the end point region. Search strategies for enhanced b -> sg are discussed in light of large theoretical uncertainty in the standard model fast kaon background from b -> s penguin operators.

hep-ph↗