Searcharxiv⌕ Search

arXiv subjects

Nikolai Uraltsev

Publications and source records attributed to Nikolai Uraltsev.

At least 19 recordsLinked to original sources
arXiv2012

B-> D* zero-recoil formfactor and the heavy quark expansion in QCD: a systematic study

We present a QCD analysis of heavy quark mesons focussing on the B -> D* formfactor at zero recoil, F_D*(1). An advanced treatment of the perturbative corrections in the Wilsonian approach is presented. We estimate the higher-order power corrections to the OPE sum rule and describe a refined analysis of the nonresonant continuum contribution. In the framework of a model-independent approach, we show that the inelastic contribution in the phenomenological part of the OPE is related to the mQ-dependence of the hyperfine splitting and conclude that the former is large, lowering the prediction for F_D*(1) down to about 0.86. This likewise implies an enhanced yield of radial and D-wave charm excitations in semileptonic B decays and alleviates the problem with the inclusive yield of the wide excited states. We also apply the approach to the expectation values of dimension 7 and 8 local operators and to a few other issues in the heavy quark expansion.

hep-ph↗
arXiv2012

Charm CP Violation and the Electric Dipole Moments from the Charm Scale

The reported CP asymmetry in D->K^+K^- / pi^+pi^- is argued to be too large to naturally fit the SM. If so, a new source of CP violation is implied in the Delta C=1 sector with a milliweak strength. CP-odd interactions in the flavor-diagonal sector are strongly constrained by the EDMs placing severe limitations on the underlying theory. While the largest effects usually come from the New Physics energy scale, they are strongly model-dependent. Yet the interference of the CP-odd forces manifested in D decays with the conventional CP-even Delta C=1 weak interaction generates at the charm scale a background level. It has been argued that the d_n in the SM is largely generated via such an interference, with mild KM-specific additional suppression. The reported CP asymmetry is expected to generate d_n of 30 to 100 times larger than in the SM, or even higher in certain model yet not quite natural examples. In the SM the charm-induced loop-less |d_n| is expected around 10^{-31}e*cm. On the technical side, we present a compact Ward-identity--based derivation of the induced scalar pion-nucleon coupling in the presence of the CP-odd interactions, which appears once the latter include the right-handed light quarks.

hep-ph↗
arXiv2010

Higher Order Power Corrections in Inclusive B Decays

We discuss order 1/m_b^4 and 1/m_b^5 corrections in inclusive semileptonic decay of a $B$ meson. We identify relevant hadronic matrix elements of dimension seven and eight and estimate them using the ground-state saturation approximation. Within this approach the effects on the integrated rate and on kinematic moments are estimated. The overall relative shift in V_{cb} turns out about +0.4% as applied to the existing fits. Similar estimates are presented for B -> X_s+γdecays.

hep-ph↗
arXiv2010

B -> D* at zero recoil revisited

We examine the B -> D* form factor at zero recoil using a continuum QCD approach rooted in the heavy quark sum rules framework. A refined evaluation of the radiative corrections as well as the most recent estimates of higher order power terms together with more careful continuum calculation are included. An upper bound on the form factor of F(1)< 0.93 is derived, based on just the positivity of inelastic contributions. A model-independent estimate of the inelastic contributions shows they are quite significant, lowering the form factor by about 6% or more. This results in an unbiased estimate F(1) \approx 0.86 with about three percent uncertainty in the central value.

hep-ph↗
arXiv2009

The Two Roads to "Intrinsic Charm" in B Decays

We describe two complementary ways to show the presence of higher order effects in the 1/m_Q expansion for inclusive B decays that have been dubbed "Intrinsic Charm". Apart from the lessons they can teach us about QCD's nonperturbative dynamics their consideration is relevant for precise extractions of |V_{cb}|: for they complement the estimate of the potential impact of 1/m_Q^4 contributions. We draw semiquantitative conclusions for the expected scale of Weak Annihilation in semileptonic B decays, both for its valence and non-valence components.

hep-ph↗
arXiv2004

Heavy quark expansion in beauty: recent successes and problems

The status of the QCD-based heavy quark expansion is briefly reviewed. A good agreement between properly applied theory and new precision data is observed. Critical remarks on certain recent claims from HQET are presented. Recent applications to the exclusive heavy flavor transitions are addressed. The `1/2 > 3/2' problem for the transitions into the charm P-wave states is discussed.

hep-ph↗
arXiv2004

On the strong-coupling effects in jet bremsstrahlung in the heavy flavor decays

We address the strong-coupling regime effects in the light-like quark jets where the radiated gluons are hard but highly collinear. These may lead to additional contributions in the invariant mass (or recoil energy) spectra, on top of the Fermi motion of the initial heavy quark in decays like b->s+γ. It is shown that the integer moments are nevertheless free from such effects; perturbation theory corrections for the moments are driven by α_s in the small coupling regime. They are modified nonperturbatively by the soft modes but not by the collinear modes.

hep-ph↗
arXiv2004

QCD in beauty decays: successes and puzzles

The status of the heavy quark expansion for inclusive B decays is briefly reviewed from the perspective of confronting theory with data and of extracting the heavy quark parameters. A good agreement between properly applied theory and new precision data is observed. Some recent applications to the exclusive heavy flavor transitions are addressed. I recall the `1/2 > 3/2' paradox for the transitions into the charm P-wave states.

hep-ph↗
arXiv2004

Perturbative corrections to the semileptonic b-decay moments: E^\ell_{cut} dependence and running-α_s effects in the OPE approach

We have calculated the perturbative corrections to all the structure functions in the semileptonic decays of a heavy quark. Assuming an arbitrary gluon mass as a technical tool allowed to obtain in parallel all the BLM corrections. We report the basic applications, viz. perturbative corrections to the hadronic mass and energy moments with full dependence on the charge lepton energy cut. In the adopted scheme with the OPE momentum scale separation around 1 GeV the perturbative corrections to are small and practically independent of E_{cut}; the BLM corrections are small, too. The corrections to the second mass squared moment show some decrease with E_{cut} consistent with the effect of the Darwin operator, within the previously estimated theoretical uncertainty. Perturbative corrections in the pole-type schemes appear significant and vary with E_\ell, decreasing the moments at higher cuts. The hardness of hadronic moments is quantitatively illustrated for different cuts on E_\ell.

hep-ph↗
arXiv2004

Moments of semileptonic B decay distributions in the 1/m_b expansion

We report the OPE-based predictions for a number of lepton energy and hadronic mass moments in the inclusive semileptonic B -> X_c \ellνdecays with a lower cut on the charged lepton energy. We rely on the direct OPE approach where no expansion in the charm mass is employed and the theoretical input is a limited set of underlying OPE parameters including m_b and m_c. A Wilsonian treatment with a `hard' cutoff is applied using running low-scale masses m_Q(μ) and kinetic expectation value μ_π^2(μ). This leaves for perturbative corrections only genuinely short-distance effects and makes them numerically small. Predictions are also given for the modified hadronic moments of the kinematic variable \cal N_X^2 which is a combination of M_X^2 and E_X. Measurement of such moments would allow a more reliable extraction of higher-order nonperturbative heavy quark parameters from experiment.

hep-ph↗
arXiv2003

A "BPS expansion" for B and D mesons

We analyze consequences of the approximation μ_π^2 -μ_G^2 << μ_π^2 (a `BPS' limit) for B and D mesons. It is shown that neglecting perturbative effects many power corrections would vanish to all orders in 1/m_Q, in particular those violating heavy flavor symmetry. Among them are corrections to B->D formfactors. A number of relations receive corrections only to the second order expanding around the BPS limit to any order in 1/m_Q, including both f_+ and f_- at zero recoil. This allows an accurate evaluation of {\cal F}_+ for B->D lν. Its perturbative renormalization is computed analytically in the required Wilsonian scheme, yielding the dominant 3% enhancement.

hep-ph↗
arXiv2003

Recent Advances in Semileptonic B Decays

Aspects of the OPE-based QCD theory of B decays are discussed. We have at least one nontrivial precision check of the OPE at the nonperturbative level in inclusive decays. The data suggest proximity to the `BPS' limit for B mesons. Its consequences are addressed and their accuracy qualified. It is suggested that theory-wise B->D \ellνnear zero recoil offers an accurate alternative way to measure |V_{cb}|. It is shown that the OPE is in a good standing when confronted by experiment. The alleged controversy between theory and data on certain decay channels is found to be an artefact of oversimplifying the OPE in presence of high experimental cuts severely degrading the effective hardness. The effects exponential in the latter are missed in the traditionally used expressions, yet they do not signify a breakdown of the 1/m_b expansion proper. They typically increase the apparent b quark mass in B->X_s+γby 70 MeV or more, together with an even more dramatic downward shift in the kinetic expectation value. The utility of the second moment of E_γis emphasized once the aforementioned effects have been included. Incorporating them brings different measurements into a good agreement, provided the OPE-based theory employs the robust approach.

hep-ph↗
arXiv2003

Theoretical Aspects of Semileptonic B decays

Strong interactions are addressed in connection to extracting |V_{cb}| and determining heavy quark parameters. A comprehensive approach allows a robust analysis not relying on a 1/m_c expansion; a percent defendable accuracy in |V_{cb}| becomes realistic. Some of the heavy quark parameters are already accurately known. We have at least one nontrivial precision check of the OPE at the nonperturbative level in inclusive decays. The alleged controversy between theory and BaBar data on is argued to be an artefact of oversimplifying the OPE for high cuts in lepton energy. Consequences of the proximity to the `BPS' limit are addressed and their accuracy qualified. It is suggested that theory-wise B->D lνnear zero recoil offers an accurate way for measuring |V_{cb}|.

hep-ph↗
arXiv2002

QCD Corrections in Gamma_sl(B)

Short-distance expansion of the total semileptonic B widths is reviewed for the OPE-conformable scheme employing low-scale running quark masses. The third- and fourth-order BLM corrections are given and the complete resummation of the BLM series presented. The effect of higher perturbative orders with running quark masses is found very small. Numerical consequences for |V_{cb}| are addressed.

hep-ph↗
arXiv2002

Strong interaction effects in semileptonic B decays

Strong interaction effects are addressed in connection to extracting |V_{cb}|. A comprehensive approach is described not relying on a 1/m_c expansion; it allows a percent accuracy without ad hoc assumptions about higher-order effects. An alternative to the M_X^2 variable is proposed improving convergence. Intrinsic hardness of integrated observables with a cut on E_\ell is discussed; it can be responsible for the behavior of reported by BaBar. Consequences of the proximity to the `BPS' limit are considered.

hep-ph↗
arXiv2002

On The Expected Photon Spectrum in B -> X_s + gamma and Its Uses

Measuring the photon energy spectrum in radiative B decays provides essential help for gaining theoretical control over semileptonic B transitions. The hadronic recoil mass distribution in B -> X_u \ellνpromises the best environment for determining |V_ub|. The theoretical uncertainties are largest in the domain of low values of the lepton pair mass q^2. Universality relations allow to describe this domain reliably in terms of the photon spectrum in B -> X_s + γ. A method is proposed to incorporate 1/m_b corrections into this relation. The low-E_γtail in radiative decays is important in the context of extracting |V_ub|. We argue that CLEO's recent fit to the spectrum underestimates the fraction of the photon spectrum below 2 GeV. Potentially significant uncertainties enter in the theoretical evaluation of the integrated end-point lepton spectrum or the B -> X_u \ellνwidth with a too high value of the lower cut on q^2 in alternative approaches to |V_ub|.

hep-ph↗
arXiv2001

On the chromomagnetic expectation value μ_G^2 and higher power corrections in heavy flavor mesons

The important parameter μ_G^2 of the heavy quark expansion is analyzed including perturbative and power corrections. It is found that μ_G^2(2GeV) is known with a few percent accuracy. The perturbative corrections are computed and found small. A nonperturbative relation is suggested which allows to control the power corrections. We conclude that μ_G^2(1GeV)=(0.35+0.03- 0.02)GeV^2. The two-loop expression for the effective `ME' radiation coupling alpha_s^(me)(ω) is given which improves reliability of the perturbative evolution of μ_G^2 towards the low momentum scale. On the nonperturbative side, we advocate the utility of combining the heavy quark expansion with expanding around the `BPS'-type approximation for the meson wavefunction, which implies relations μ_π^2 \approx μ_G^2 and -ρ_{LS}^3 \approx ρ_D^3 as well as similar ones for the nonlocal correlators.

hep-ph↗