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M. Shifman

Publications and source records attributed to M. Shifman.

At least 199 records · Page 11Linked to original sources
arXiv1995

A Closer Look at Perturbative Corrections in the b->c Semileptonic Transitions

We comment on two recent calculations of the second order perturbative corrections in the heavy flavor semileptonic transitions within the Brodsky-Lepage-Mackenzie approach. It is pointed out that the results do not show significant enhancement either in the inclusive $b\rightarrow c$ decays or in the exclusive amplitudes at zero recoil provided that the expansion parameter is chosen in a way appropriate to the kinematics at hand. The values of the second-order coefficients inferred from the BLM-type calculations appear to be of order unity in the both cases. Thus, in both cases no significant uncertainty in extracting $V_{cb}$ can be attributed to perturbative effects. The theoretical accuracy is mostly determined by the existing uncertainty in $1/m_c^2$ nonperturbative corrections in the exclusive $B\rightarrow D^*$ amplitude, and, to a lesser extent, by the uncertainty in the estimated value of the kinetic energy matrix element $μ_π^2$ in the case of $Γ_{\rm sl}(b\rightarrow c)$. The theoretical accuracy of the inclusive method of determination of $V_{cb}$ seemingly competes with and even exceeds the experimental accuracy.

hep-ph↗
arXiv1994

Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions

A study of two-dimensional QCD on a spatial circle with Majorana fermions in the adjoint representation of the gauge groups SU(2) and SU(3) has been performed. The main emphasis is put on the symmetry properties related to the homotopically non-trivial gauge transformations and the discrete axial symmetry of this model. Within a gauge fixed canonical framework, the delicate interplay of topology on the one hand and Jacobians and boundary conditions arising in the course of resolving Gauss's law on the other hand is exhibited. As a result, a consistent description of the residual $Z_N$ gauge symmetry (for SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum of the model is determined analytically in the limit of a small circle. There, the Born-Oppenheimer approximation is justified and reduces the vacuum problem to simple quantum mechanics. The issue of fermion condensates is addressed and residual discrepancies with other approaches are pointed out.

hep-th↗
arXiv1994

Sum Rules for Heavy Flavor Transitions in the SV Limit

We show how sum rules for the weak decays of heavy flavor hadrons can be derived as the moments of spectral distributions in the small velocity (SV) limit. This systematic approach allows us to determine corrections to these sum rules, to obtain new sum rules and it provides us with a transparent physical interpretation; it also opens a new perspective on the notion of the heavy quark mass. Applying these sum rules we derive a lower bound on the deviation of the exclusive form factor $F_{B->D^*}$ from unity at zero recoil; likewise we give a field-theoretical derivation of a previously formulated inequality between the expectation value for the kinetic energy operator of the heavy quark and for the chromomagnetic operator. We analyze how the known results on nonperturbative corrections must be understood when one takes into account the normalization point dependence of the low scale parameters. Relation between the field-theoretic derivation of the sum rules and the quantum-mechanical approach is elucidated.

hep-ph↗
arXiv1994

How to Measure Kinetic Energy of the Heavy Quark Inside B Mesons?

We discuss how one can determine the average kinetic energy of the heavy quark inside heavy mesons from differential distributions in $B$ decays. A new, so-called third, sum rule for the $b\rightarrow c$ transition is derived in the small velocity (SV) limit. Using this sum rule and the existing data on the momentum dependence in the $B\rightarrow D^*$ transition (the slope of the Isgur-Wise function) we obtain a new lower bound on the parameter $μ_π^2 = (2M_B)^{-1}\langle B |\bar b (i\vec{D})^2 b |B\rangle $ proportional to the average kinetic energy of $b$ quark inside $B$ meson. The existing data suggest $μ_π^2 > 0.4$~GeV$^2$ and (from the ``optical'' sum rule) $\overlineΛ > 500$ MeV, albeit with some numerical uncertainties.

hep-ph↗
arXiv1994

Fractons in Twisted Multiflavor Schwinger Model

We consider two-dimensional QED with several fermion flavors on a finite spatial circle. A modified version of the model with {\em flavor-dependent} boundary conditions $ψ_p(L) = e^{2πip/ N} ψ_p(0)$, $p = 1, \ldots , N$ is discussed ($N $ is the number of flavors). In this case a non-contactable contour in the space of the gauge fields is {\em not} determined by large gauge transformations. The Euclidean path integral acquires the contribution from the gauge field configurations with fractional topological charge. The configuration with $ν= 1/N$ is responsible for the formation of the fermion condensate $\langle\barψ_p ψ_p\rangle_0$. The condensate dies out as a power of $L^{-1}$ when the length $L$ of the spatial box is sent to infinity. Implications of this result for non-abelian gauge field theories are discussed in brief.

hep-th↗
arXiv1994

$|V_{cb}|$ from OPE Sum Rules for Heavy Flavor Transitions

We derive a model-independent upper bound for the axial-vector form factor of the $B\ra D^*$ transition at zero recoil, $F_{B\ra D^*}$. The form factor turns out to be noticeably less than unity. The deviation of $F_{B\ra D^*}$ from unity is larger than previously anticipated. Using our estimate we extract $|V_{cb}|$ from the measured exclusive rate of $B\ra D^* lν$ extrapolated to the point of zero recoil. The central `` exclusive" value of $|V_{cb}|$ is in agreement with the value obtained from the inclusive semileptonic width $Γ(B\ra X_c lν)$. We argue that the theoretical uncertainty in determining $|V_{cb}|$ from the total inclusive width is significantly reduced if a constraint on the quark mass difference $m_b - m_c$ stemming from the heavy quark expansion is taken into account.

hep-ph↗
arXiv1994

Heavy Quark Distribution Function in QCD and the AC$^2$M$^2$ Model

We show that the phenomenological \ACM ansatz is consistent with QCD through order $1/m_b$ in the description of $B\ra l\bar ν_l+X_u$ and $B\ra γ+X_s$ transitions, including their energy spectra and differential distributions. This suggests a concrete realization for the QCD distribution function, which we call the ``Roman'' function. On the other hand the \ACM model description of the end-point domain in $B\ra l\bar ν_l + X_c$ is incompatible with QCD: a different distribution function enters the description of $b\ra c$ decays as compared to the transitions to the massless quarks. Both observations -- the validity of the {\ACM}-like description for heavy-to-light transitions and the emergence of the new distribution function in the $b\ra c$ case -- are in contradiction to a recent claim in the literature. The intrinsic limitation of the \ACM model could reveal itself in different values of the effective $b$ quark mass from fits of the $Λ_b$ and $B$ decays.

hep-ph↗
arXiv1994

Non-Leptonic Decays of Beauty Hadrons -- from Phenomenology to Theory

`Anyone who keeps the ability to see beauty never grows old' Franz Kafka. In the last few years considerable progress has been achieved in our understanding of the decays of heavy flavour hadrons. One can now calculate inclusive transition rates in QCD proper through an expansion in inverse powers of the heavy flavour quark mass without recourse to phenomenological assumptions. The non-perturbative contributions are treated systematically in this way; they are found to produce corrections of order a few percent in beauty decays, i.e. typically somewhat smaller than the perturbative corrections. One finds, among other things: (a) The lifetime of $B^-$ mesons is predicted to be longer than that of $B^0$ mesons by several percent. (b) The QCD prediction for the semileptonic branching ratio of $B$ mesons appears to exceed present experimental values. We discuss the implications of this discrepancy. The phenomenological engineering that has been developed for the description of {\em exclusive} two-body modes of $B$ mesons has reached a mature stage and awaits more precise and detailed experimental tests. First steps towards a genuine QCD treatment of these modes are being made.

hep-ph↗
arXiv1993

The Baffling Semileptonic Branching Ratio of $B$ Mesons

The apparent gap between the measured and the expected value for the semileptonic branching ratio of $B$ mesons has become more serious over the last year. This is due to the improved quality of the data and to the increasing maturity of the theoretical treatment of non-perturbative corrections. We discuss various theoretical options to reduce the semileptonic $B$ branching ratio; among the more spectacular resolutions of the apparent puzzle is the possibility of an unorthodox enhancement in non-perturbative corrections or even of an intervention by `New Physics'. Phenomenological implications of such scenarios are pointed out.

hep-ph↗
arXiv1993

QCD Predictions for Lepton Spectra in Inclusive Heavy Flavour Decays

We derive the lepton spectrum in semileptonic beauty decays from a nonperturbative treatment of QCD; it is based on an expansion in $1/m_Q$ with $m_Q$ being the heavy flavour quark mass. The leading corrections arising on the $1/m_Q$ level are completely expressed in terms of the difference in the mass of the heavy hadron and the quark. Nontrivial effects appear in $1/m_Q^2$ terms affecting mainly the endpoint region; they are different for meson and baryon decays as well as for beauty and charm decays.

hep-ph↗
arXiv1992

A QCD ``Manifesto'' on Inclusive Decays of Beauty and Charm

A selfconsistent treatment of nonperturbative effects in inclusive nonleptonic and semileptonic decays of beauty and charm hadrons is presented. It is illustrated by calculating semileptonic branching ratios, lifetime ratios, radiative decay rates and the lepton spectra in semileptonic decays.

hep-ph↗
arXiv1992

Nonperturbative QCD efects in weak hadronic decays

QCD-based analysis of nonfactorizable parts of weak nonleptonic amplitudes is reported. Nonperturbative effects due to soft gluon exchange play a key role leading to the emergence of a dynamical rule of discarding $1/N_c$ corrections.

hep-ph↗
arXiv1992

The Rule of Discarding $1/N_c$ in Inclusive Weak Decays. II

We calculate the leading preasymptotic correction to the inclusive width $b\rightarrow \bar c c s$ (two massive quarks in the final state) due to the operator $\vec σ\vec H$. It is found that this correction tends to cancel the $1/N_c$ part of the inclusive width calculated using naive factorization. The absolute value of the effect is of order 0.25. We also discuss how quark-hadron duality is realized in the inclusive decays.

hep-ph↗
arXiv1992

The Rule of Discarding $1/N_c$ in Inclusive Weak Decays

We continue to investigate the effects of the (dominant) subleading operator of the $\vecσ\vec H$ type in the $1/N_c$ parts of the weak nonleptonic amplitudes. If the previous work concentrated on exclusive decays now we analyse inclusive widths in a certain kinematic limit. Deviations from the parton model predictions are found due to soft-gluon color exchange. In the interference (i.e. $1/N_c$) terms they are stronger than in the non-interference terms. The sign is negative so that the $1/N_c$ parts tend to cancel. The absolute value of the effect is of order 1 in D-mesons and of order 0.50 in B mesons.

hep-ph↗
arXiv1992

The Isgur-Wise Function in Small Velocity Limit

We discuss the Isgur-Wise function $ξ(y)$ in the small velocity (SV) limit within the QCD sum rule method. The behavior of $ξ(y)$ in the SV limit is sensitive to the particular form of the duality relations used to decontaminate the sum rule predictions from the continuum contribution. Peculiarities of the duality relations in the problem at hand are revealed. It is shown that the proper requirements of duality and angular isotropy for S wave states lead to an unambiguous form of the sum rules for the Isgur-Wise function. We illustrate the constraints due to these requirements using a toy model of the harmonic oscillator. The slope parameter and the shape of $ξ(y)$ are determined.

hep-ph↗