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V. M. Braun

Publications and source records attributed to V. M. Braun.

At least 109 records · Page 6Linked to original sources
arXiv2000

Evolution of twist-three parton distributions in QCD beyond the large N_c limit

We formulate a consistent 1/N_c^2 expansion of the QCD evolution equations for the twist-three quark distributions g_2(x,Q^2), h_L(x,Q^2) and e(x,Q^2) based on the interpretation of the evolution as a three-particle quantum-mechanical problem with hermitian Hamiltonian. Each distribution amplitude can be decomposed in contributions of partonic components with DGLAP-type scale dependence. We calculate the 1/N_c^2 corrections to the evolution of the dominant component with the lowest anomalous dimension - the only one that survives in the large-N_c limit - and observe a good agreement with the exact numerical results for N_c=3. The 1/N_c^2 admixture of operators with higher anomalous dimensions is shown to be concentrated at a few lowest partonic components and in general is rather weak.

hep-ph↗
arXiv1999

Pion Form Factor in QCD at Intermediate Momentum Transfers

We present a quantitative analysis of the electromagnetic pion form factor in the light-cone sum rule approach, including radiative corrections and higher-twist effects. The comparison to the existing data favors the asymptotic profile of the pion distribution amplitude and allows to estimate the deviation: $(\int du/u ϕ_π(u))/ (\int du/u ϕ^{\rm as}_π(u))=$ 1.1$\pm$ 0.1 at the scale 1 GeV. Special attention is payed to the precise definition and interplay of soft and hard contributions at intermediate momentum transfer, and to matching of the sum rule to the perturbative QCD prediction. We observe a strong numerical cancellation between the soft (end point) contribution and power suppressed hard contributions of higher twist, so that the total nonperturbative correction to the usual pQCD result turns out to be of order 30% for $Q^2\sim 1$ GeV$^2$.

hep-ph↗
arXiv1999

QCD Sum Rules for Heavy Flavors

I give a short summary of QCD sum rule results for hadrons involving a heavy quark, with emphasis on recent developments.

hep-ph↗
arXiv1999

Baryon Distribution Amplitudes in QCD

We develop a new theoretical framework for the description of leading twist light-cone baryon distribution amplitudes which is based on integrability of the helicity $λ=3/2$ evolution equation to leading logarithmic accuracy. A physical interpretation is that one can identify a new `hidden' quantum number which distinguishes components in the $λ=3/2$ distribution amplitudes with different scale dependence. The solution of the corresponding evolution equation is reduced to a simple three-term recurrence relation. The exact analytic solution is found for the component with the lowest anomalous dimension for all moments $N$, and the WKB-type expansion is constructed for other levels, which becomes asymptotically exact at large $N$. Evolution equations for the $λ=1/2$ distribution amplitudes (e.g. for the nucleon) are studied as well. We find that the two lowest anomalous dimensions for the $λ=1/2$ operators (one for each parity) are separated from the rest of the spectrum by a finite `mass gap'. These special states can be interpreted as scalar diquarks.

hep-ph↗
arXiv1998

Higher Twist Distribution Amplitudes of Vector Mesons in QCD: Twist-4 Distributions and Meson Mass Corrections

We present a systematic study of twist-4 light-cone distribution amplitudes of vector mesons in QCD, which is based on conformal expansion. The structure of meson mass corrections is studied in detail. A complete set of distribution amplitudes is constructed, which satisfies all (exact) equations of motion and constraints from conformal expansion. Nonperturbative input parameters are estimated from QCD sum rules. Our study suggests that meson mass corrections may present a dominant source of higher twist effects in exclusive processes.

hep-ph↗
arXiv1998

The Operator Product Expansion, Non-perturbative Couplings and the Landau Pole: Lessons from the O(N) $σ$-Model

We obtain the operator product expansion of the self-energy in the O(N) non-linear $σ$-model to all orders in the coupling and the large momentum, and to next-to-leading order in 1/N. In the light of this result we discuss recent suggestions that there may be additional power corrections from short distances, associated with defining the coupling constant non-perturbatively. The non-linear $σ$-model provides no evidence for such `non-standard' power corrections. We also find that the OPE converges for sufficiently large external momentum, presumably because there are no multi-particle thresholds at arbitrarily high energies in the 1/N expansion.

hep-ph↗
arXiv1998

Handbook of Higher Twist Distribution Amplitudes of Vector Mesons in QCD

We give a summary of existing results on higher twist distribution amplitudes of vector mesons in QCD. Special attention is payed to meson mass corrections which turn out to be large. A ``shopping list'' is presented of most important nonperturbative parameters which enter distribution amplitudes.

hep-ph↗
arXiv1998

Exclusive Semileptonic and Rare B-Meson Decays in QCD

We present the first complete results for the semileptonic and rare radiative form factors of B-mesons weak decay into a light vector-meson (rho, omega, K*, phi) in the light-cone sum-rule approach. The calculation includes radiative corrections, higher-twist corrections and SU(3)-breaking effects. The theoretical uncertainty is investigated in detail. A simple parametrization of the form factors is given in terms of three parameters each. We find that the form factors observe several relations inspired by heavy-quark symmetry.

hep-ph↗
arXiv1998

Higher Twist Distribution Amplitudes of Vector Mesons in QCD: Formalism and Twist Three Distributions

We present a systematic study of twist three light-cone distribution amplitudes of vector mesons in QCD, which is based on conformal expansion and takes into account meson and quark mass corrections. A complete set of distribution amplitudes is constructed for rho, omega, K* and phi mesons, which satisfies all (exact) equations of motion and constraints from conformal expansion. Nonperturbative input parameters are calculated from QCD sum rules, including an update of SU(3) breaking corrections in the leading twist distributions.

hep-ph↗
arXiv1998

Integrability of three-particle evolution equations in QCD

We show that Brodsky-Lepage evolution equation for the leading twist spin 3/2 baryon distribution amplitude is completely integrable and reduces to the three-particle XXX_{s=-1} Heisenberg spin chain. Trajectories of the anomalous dimensions are identified and calculated using the 1/N expansion. Extending this result, we prove integrability of the evolution equations for twist 3 quark-gluon operators in the large N_c limit and derive explicit expressions for the corresponding integrals of motion.

hep-ph↗
arXiv1998

Light-Cone Sum Rules

A short introduction is given to the method of light-cone sum rules, their theoretical background and main modifications. The discussion is concentrated but not restricted to the applications to heavy quark decays.

hep-ph↗
arXiv1997

Radiative Corrections to the Decay $B\to πeν$ and the Heavy Quark Limit

We calculate radiative corrections to the light-cone sum rule for the semileptonic form factor in $B\toπeν$ decays and thus remove the major uncertainty in determining the CKM mixing angle $|V_{ub}|$ by this method. We discuss the remaining uncertainties and perspectives for further studies. The structure of the radiative corrections suggests factorization of soft (end-point) and hard rescattering contributions in heavy-to-light decays in the heavy quark limit.

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

Use and Misuse of QCD Sum Rules in Heavy-to-light Transitions: the Decay $B\toρe ν$ Reexamined

The existing calculations of the form factors describing the decay $B\toρe ν$ from QCD sum rules have yielded conflicting results at small values of the invariant mass squared of the lepton pair. We demonstrate that the disagreement originates from the failure of the short-distance expansion to describe the $ρ$ meson distribution amplitude in the region where almost the whole momentum is carried by one of the constituents. This limits the applicability of QCD sum rules based on the short-distance expansion of a three-point correlation function to heavy-to-light transitions and calls for an expansion around the light-cone, as realized in the light-cone sum rule approach. We derive and update light-cone sum rules for all the semileptonic form factors, using recent results on the $ρ$ meson distribution amplitudes. The results are presented in detail together with a careful analysis of the uncertainties, including estimates of higher-twist effects, and compared to lattice calculations and recent CLEO measurements. We also derive a set of ``improved'' three-point sum rules, in which some of the problems of the short-distance expansion are avoided and whose results agree to good accuracy with those from light-cone sum rules.

hep-ph↗
arXiv1997

Q^2 Evolution of Chiral-Odd Twist-3 Distributions h_L(x, Q^2) and e(x, Q^2) in Large-N_c QCD

We prove that the twist-3 chiral-odd parton distributions obey simple Gribov-Lipatov-Altarelli-Parisi evolution equations in the limit $N_c\to\infty$ and give analytic results for the corresponding anomalous dimensions. To this end we introduce an evolution equation for the corresponding three-particle twist-3 parton correlation functions and find an exact analytic solution. For large values of n (operator dimension) we are further able to collect all corrections subleading in N_c, so our final results are valid to $O(1/N_c^2\cdot \ln(n)/n)$ accuracy.

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.

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