arXiv subjects

N. G. Uraltsev

Publications and source records attributed to N. G. Uraltsev.

At least 19 recordsLinked to original sources

arXiv1997

Key Distributions for Charmless Semileptonic B Decay

We present theoretical predictions for a few phenomenologically interesting distributions in the semileptonic b->u decays which are affected by Fermi motion. The perturbative effects are incorporated at the one-loop level and appear to be very moderate. Our treatment of Fermi motion is based directly on QCD, being encoded in the universal distribution function F(x). The decay distributions in the charged lepton energy, invariant mass of hadrons, hadron energy, and q^2 are given. We note that typically about 90% of all decay events are expected to have M_X < M_D; this feature can be exploited in determination of |V_ub|.

hep-ph↗

arXiv1996

The Heavy Quark Expansion

I review the status of the modern theoretical approach to weak decays of heavy flavor hadrons based on the 1/m_Q expansion in QCD. The qualitative features are explained and the subtleties in simultaneously incorporating perturbative and power-suppressed effects are addressed. A few topical phenomenological applications are discussed in quantitative detail.

hep-ph↗

arXiv1996

Operator Product Expansion, Heavy Quarks, QCD Duality and its Violations

The quark (gluon)-hadron duality constitutes a basis for the theoretical treatment of a wide range of inclusive processes -- from hadronic τdecays and R_{e^+e^-}, to semileptonic and nonleptonic decay rates of heavy flavor hadrons. A theoretical analysis of these processes is carried out by using the operator product expansion in the Euclidean domain, with subsequent analytic continuation to the Minkowski domain. We formulate the notion of the quark (gluon)-hadron duality in quantitative terms, then classify various contributions leading to violations of duality. A prominent role in the violations of duality seems to belong to the so called exponential terms which, conceptually, may represent the (truncated) tail of the power series. A qualitative model, relying on an instanton background field, is developed, allowing one to get an estimate of the exponential terms. We then discuss a number of applications, mostly from heavy quark physics.

hep-ph↗

arXiv1996

On the Problem of Boosting Nonleptonic b Baryon Decays

The constituent picture of hadrons implies certain quantum mechanical inequalities which must hold in the potential models. Basing on this qualitative consideration I argue that it is not easy to increase significantly the scale of the flavor-dependent 1/m_b^3 effects within the heavy quark expansion preserving the conventional constituent picture of heavy flavor hadrons. I briefly address the physical consequences one might expect if the effects of weak scattering and interference are attempted to be pushed above the 10% level within 1/m_b expansion not invoking qualitatively different mechanisms including violations of duality.

hep-ph↗

arXiv1995

Are IR Renormalons a Good Probe for the Strong Interaction Domain?

We study the origin of non-analyticity in α_s of a short-distance QCD observable to demonstrate that the infrared renormalons, the same-sign factorial growth of the perturbative expansion, is a universal phenomenon that originates entirely from the small coupling domain. In particular, both the position and the nature of the singularity of the Borel transform of the perturbative series prove to be independent of whether the running coupling α(k^2) becomes singular at some finite scale ("Landau pole"), or stays finite down to k^2=0. We argue that getting hold of the infrared renormalons per se can help next to nothing in quantifying non-perturbative effects.

hep-ph↗

arXiv1995

Do Higher Order Perturbative Corrections Upset |V_{cb}| and |V_{ub}| Determined from Semileptonic Widths?

It is shown that large perturbative corrections found previously for semileptonic beauty and charm decays are associated with using inappropriate pole masses. The latter, in the perturbative expansion, suffer from the 1/m_Q infrared renormalon which is absent in the widths, which leads to similar large corrections in m_Q. Pole masses are neither measured directly in experiment. If the widths are related to parameters determined in experiment, the overall impact of the calculated second order corrections becomes strongly suppressed and leads to less than 1\% change in |V_{cb}| and |V_{ub}|. Even in charm decays the perturbative corrections appear to be very moderate in the consistent OPE-compliant treatment. The updated estimate of |V_{cb}| is given, based on recent accurate determination of m_b and α_s(1 GeV). The theoretical accuracy of determination of |V_{ub}| from Γ_{sl}(b->u) appears to be good as well.

hep-ph↗

arXiv1995

b -> s +γ: A QCD Consistent Analysis of the Photon Energy Distribution

The photon energy distribution in the inclusive b -> s+γtransitions is a combination of two components: the first component, soft physics, is determined by the so called primordial distribution function, while the second component, perturbative physics, is governed by the hard gluon emission. A simple ansatz is suggested for the primordial distribution function which obeys the QCD constraints known so far. We then discuss in detail how the hard gluon emission affects the energy distribution. An extension of the Sudakov approximation is worked out incorporating the Brodsky-Lepage-Mackenzie prescription and its generalizations. We explicitly calculate the marriage of nonperturbative with perturbative effects in the way required by OPE, introducing separation scale μ. A few parameters, such as m_b and μ_π^2 affect the shape of the distribution and, thus, can be determined by matching to the experimental data. The data, still scarce, while not giving precise values for these parameters, yield consistency with theory: the current values of the above parameters lie within experimental uncertainty. On the theoretical side we outline a method allowing one to go beyond the practical version of OPE.

hep-ph↗

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

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

Comment on the Renormalization Group Improvement in Exclusive $b\ra c $ Transitions

Using rather general consideration I argue that the numerical impact of the existing next-to-leading logarithmic summations of terms $\log{m_b/m_c}$ for perturbative factors $η_A$, $η_V$ in the exclusive zero recoil semileptonic transitions is irrelevant, and adopting corresponding corrections beyond the exact one loop result for both estimating the values of these formfactors and their theoretical uncertainty, is misleading. The central theoretical value if taken literally is then $η_A\simeq 0.97$.

hep-ph↗

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

The Pole Mass of The Heavy Quark. Perturbation Theory and Beyond

The key quantity of the heavy quark theory is the quark mass $m_Q$. Since quarks are unobservable one can suggest different definitions of $m_Q$. One of the most popular choices is the pole quark mass routinely used in perturbative calculations and in some analyses based on heavy quark expansions. We show that no precise definition of the pole mass can be given in the full theory once non-perturbative effects are included. Any definition of this quantity suffers from an intrinsic uncertainty of order $\Lam /m_Q$. This fact is succinctly described by the existence of an infrared renormalon generating a factorial divergence in the high-order coefficients of the $α_s$ series; the corresponding singularity in the Borel plane is situated at $2π/b$. A peculiar feature is that this renormalon is not associated with the matrix element of a local operator. The difference $\La \equiv M_{H_Q}-m_Q^{pole}$ can still be defined in Heavy Quark Effective Theory, but only at the price of introducing an explicit dependence on a normalization point $μ$: $\La (μ)$. Fortunately the pole mass $m_Q(0)$ {\em per se} does not appear in calculable observable quantities.

hep-ph↗

arXiv1994

On the Motion of Heavy Quarks inside Hadrons: Universal Distributions and Inclusive Decays

In previous papers we have pointed out that there exists a QCD analog of the phenomenological concept of the so called Fermi motion for the heavy quark inside a hadron. Here we show in a more detailed way how this comes about and we analyze the limitations of this concept. Non-perturbative as well as perturbative aspects are included. We emphasize both the similarities and the differences to the well-known treatment of deep inelastic lepton-nucleon scattering. We derive a model-independent {\em lower} bound on the kinetic energy of the heavy quark inside the hadron.

hep-ph↗

arXiv1993

$D_s$ Lifetime, $m_b$, $m_c$ and $|V_{cb}|$ in the Heavy Quark Expansion

We present some straightforward applications of the QCD heavy quark expansion, stated in previous papers [1-3], to the inclusive widths of heavy flavour hadrons. We address the question of the $D_s$ lifetime and argue that -- barring Weak Annihilation (WA) -- $τ(D_s)$ is expected to exceed $τ(D^0)$ by several percent; on the other hand WA could provide a difference of up to $10\div20\%$ of {\it any} sign. We extract $m_c$, $m_b$ and $|V_{cb}|$ from $Γ\ind{SL}(D^+)$ and $Γ\ind{SL}(B)$. The values of the quark masses are somewhat higher, but compatible with estimates from QCD sum rules; we obtain $|V_{cb}|\simeq 0.043$ for $τ(B)=1.4$ psec and $BR_{SL}(B)=10.5$\% . We discuss the associated uncertainties in the $1/m_Q$ expansion as well as some consequences for other electroweak decays.

hep-ph↗

arXiv1993

Anathematizing the Guralnik and Manohar Inequality for \barΛ

There is a recent claim by Guralnik and Manohar \cite{GM} to have established a rigorous lower bound on $\bar Λ$, the asymptotic difference between the mass of a heavy flavour {\em hadron} and that of the heavy flavour {\em quark}. We point out the flaw in their reasoning and discuss the underlying physical problem. An explicit counterexample to the GM bound is given; one can therefore not count on a refined proof to re-establish this bound. *********** Uses LaTeX No figures No macros file used.

hep-ph↗

arXiv1993

Weak Annihilation and the Endpoint Spectrum in Semileptonic B Decays

A general relationship is formulated between the contributions of `Weak Annihilation' (WA) to inclusive semileptonic decays of heavy flavour hadrons and the matrix elements of four fermion operators. We argue that nonperturbative contributions from low energy hadronic final states provide the dominant impact of WA on the semileptonic $b\ra u$ width of $B^-$ mesons. In that case WA affects the lepton spectrum mainly in its endpoint region -- i.e. beyond the kinematical boundary for $b\ra c$ transitions -- and we expect thelater to look quite different in $B^-$ than in $B_d$ decays. At present we are unable to make a truly quantitative prediction; yet a detailled experimental comparison of the $B^-$ and the $B_d$ lepton spectra will enable one to determine the size of the relevant matrix elements and to check directly to what extent the factorization approximation works here. Using universal analyticity properties of the decay amplitude we analyze WA in the perturbative regime and rederive our earlier results about the absence of a power enhancement due to gluon emission in a more general way. We address also the problem of the invariant mass of hadrons in the final state in semileptonic $B$ decays and identify nontrivial cancellations in $\aver{M^2\ind{hadr}}$.

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↗