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Benjamin Grinstein

Publications and source records attributed to Benjamin Grinstein.

122 records · Page 7Linked to original sources
arXiv1995

SU(3) Corrections to B -> D l nu Form Factors at O(1/M)

We compute the O(1/M,m_s) heavy quark and SU(3) corrections to B_s -> D_s e nu form factors. In the limit of vanishing light quark mass, B_s -> D_s e nu form factors are given in terms of the the B -> D e nu form factors, the leading order chiral parameter g, and two O(1/M) chiral parameters $g_1$ and $g_2$. All the chiral parameters can be extracted, in principle, from other heavy meson decays. Analytic counterterms proportional to the strange quark mass are presented for completeness, but no predictive power remains when they are included. Anomalously large loop corrections warn of poor convergence of the heavy quark chiral symmetry expansion for these processes. This suggests that naive extrapolations of B \to D form factors relying on heavy quark and chiral symmetries, as often used in monte carlo simulations of lattice QCD, may incur large errors.

hep-ph↗
arXiv1994

Form Factors in the Heavy Quark and Chiral Limit: Pole dominance in $\bar B\to πe \barν_e$

It is shown that the form factors in the semileptonic decay of ground state heavy mesons to light pseudoscalar mesons are dominated by the vector meson pole at all momentum transfers. First, the general approaches to modeling form factors in terms of quark model, pole dominance, or hybrid shapes are reviewed. It is then shown that in the combined limits of heavy quarks, chiral symmetry, and large~$\Nc$ the form factor is precisely pole-dominated. It is also shown that in a two-dimensional QCD model pole-dominance occurs for any heavy quark mass. Corrections to this limit are discussed and, to illustrate the nature of the approximations, an explicit exact calculation of the approach to the limit in a two-dimensional model is given.

hep-ph↗
arXiv1993

Exact Heavy to Light Meson Form Factors in the Combined Heavy Quark, Large $N_c$ and Chiral Limits

We demonstrate that the form factors of local operators between a heavy meson state (like the~$B$) and a light pseudoscalar state (like the pion) are given exactly by a single pole form in the combined heavy quark, large $N_c$ (number of colors) and chiral limits. We discuss the deviations from this exact result from finite heavy quark masses, non-zero light quark masses and finite $N_c$. We comment on some implications of this result.

hep-ph↗
arXiv1993

Light Threshold Effects in Supersymmetric Grand Unified Theories

Supersymmetric Grand Unified Theories have a rich spectrum of particles barely heavier than the intermediate vector bosons. As their non-supersymmetric counterparts, they lead to many relations among low energy observables. But the precise form of the predictions is modified by the extended spectrum. If the masses of these new particles are comparable to $M_Z$, the standard computation of their effect becomes inaccurate. We present a detail discussion of the correct procedure, and carry out the relevant computations to one loop order. The procedure we propose has the advantage over other existing methods that two-loop running of gauge couplings can be incorporated readily and consistently. Attention is paid to the special treatment that the top and Higgs particles must receive. The size of the effect is explored for a range of parameters in the minimal supersymmetric $SU(5)$ grand-unified theory with radiative breaking. It is found that the naive (leading-log) computation can be fairly inaccurate.

hep-ph↗
arXiv1993

Light-Quark, Heavy-Quark Systems: An Update

We review many of the recently developed applications of Heavy Quark Effective Theory techniques. After a brief update on Luke's theorem, we describe striking relations between heavy baryon form factors, and how to use them to estimate the accuracy of the extraction of $|V_{cb}|$. We discuss factorization and compare with experiment. An elementary presentation, with sample applications, of reparametrization invariance comes next. The final and most extensive chapter in this review deals with phenomenological lagrangians that incorporate heavy-quark spin-flavor as well as light quark chiral symmetries. We compile many interesting results and discuss the validity of the calculations.

hep-ph↗
arXiv1993

On a precise calculation of (f_{B_s}/f_B) / (f_{D_s}/f_D) and its implications on the interpretation of B - \bar B mixing

We observe that quantities like (f_{B_s}/f_B) / (f_{D_s} /f_D) are predicted to be unity both by heavy quark and by light quark flavor symmetries. Hence, the deviation from the symmetry prediction must be simultaneously small in both symmetry breaking parameters, i.e., order of the ratio of light to heavy quarks masses. We estimate the size of the correction. We observe that the ratio of (ΔM/Γ) for B_s - \bar B_s to B - \bar B mixing can be expressed in terms of the measurable ratio f_{D_s} / f_D with good precision. We comment on applications of these ideas to other processes.

hep-ph↗
arXiv1993

Constraints on Extended Technicolor Models from B -> mu+ mu- X

A recent study by Randall and Sundrum shows that models of Extended Technicolor (ETC) have interesting implications on rare B decays. We extend their study to the decay B -> mu+ mu- X. ETC models with a GIM mechanism predict a decay rate that is a factor of order 30 above the Standard Model, violating the experimental upper bound by a factor of 2-4. ``Traditional" ETC models predict a decay rate that is a factor of order 4 above the Standard Model, and will be probed when an improvement in the sensitivity of experiments by a factor of order 2-4 is achieved.

hep-ph↗
arXiv1993

$\bar B\to\bar K\e^+\e^-$ in Chiral Perturbation Theory

The combined use of chiral $SU(3)$ and heavy quark symmetries allows one to relate the hadronic form factors for the decay $\bar B\to \bar K\e^+\e^-$ to those for $\bar B\toπ\e^-\barν$. We investigate departures from the symmetry limit which arise from chiral symmetry breaking. The analysis uses chiral perturbation theory and the heavy quark limit to compute the relevant hadronic matrix elements. We estimate the size of $SU(3)$ corrections by computing, at one loop order, the leading nonanalytic dependence on the light quark masses. The calculation is trustworthy only in the portion of the Dalitz plot in which the momentum of the kaon or pion is small. We find the corrections to be~$\sim40\%$.

hep-ph↗
arXiv1992

On Constraints for Heavy-Meson Form Factors

We examine the recent work of de Rafael and Taron where model-independent bounds on the Isgur-Wise function are presented. We first argue that the bounds cannot hold in as much generality as implied. We show that the effects of resonances omitted in their discussion (such as heavy-heavy ``onium'' states below threshold) modify the bound. The resulting bound is much weaker but may be useful where the size of the additional contribution may be computed or estimated.

hep-ph↗
arXiv1992

Comment On ``Grand Unification and Supersymmetric Threshold"

Barbieri and Hall have argued that threshold effects at the scale of grand-unification wipe out predictions on the SUSY scale, M_S. Using triviality arguments we give upper bounds on ultraheavy particles, while proton stability gives lower bounds on the mass of the higgs color-triplet. We find no useful lower bound on the $Σ$ supermultiplet, but if the strong coupling constant is as large as recent experiments suggest, unification in the minimal SUSY SU(5) model requires that the $Sigma$ masses be $\sim 10^{-7}M_V$ and that the color octet and weak triplet be split in mass by a factor of $\sim$100.

hep-ph↗
arXiv1992

Heavy Hadron Form Factor Relations for $m_c\ne\infty$ and $\bas(m_c)\ne0$

First order power corrections to current matrix elements between heavy meson or $Λ_\Q$ baryon states are shown to vanish at the zero recoil point to all orders in QCD. Five relations among the six form factors that parametrize the semileptonic decay $Λ_b \to Λ_c e \overlineν$ are also demonstrated to exist to all orders in the strong coupling at order $1/\mQ$ . The $O(\bas(m_c)/m_c)$ form factor relations are displayed.

hep-ph↗
arXiv1992

Heavy Mesons in Two Dimensions

The large mass limit of QCD uncovers symmetries that are not present in the QCD lagrangian. These symmetries have been applied to physical (finite mass) systems, such as B and D mesons. We explore the validity of this approximation in the 't Hooft model (two-dimensional QCD in the large-N approximation). We find that the large mass approximation is good, even at the charm mass, for form factors, but it breaks down for the pseudoscalar decay constant.

hep-ph↗