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Benjamin Grinstein

Publications and source records attributed to Benjamin Grinstein.

At least 91 records · Page 5Linked to original sources
arXiv2004

The photon polarization in B -> X gamma in the standard model

The standard model prediction for the $B\to X_{s,d}γ$ decay amplitude with a right-handed photon is believed to be tiny, suppressed by $m_{s,d}/m_b$, compared to the amplitude with a left-handed photon. We show that this suppression is fictitious: in inclusive decays, the ratio of these two amplitudes is only suppressed by $g_s/(4π)$, and in exclusive decays by $Λ_{QCD}/m_b$. The suppression is not stronger in $B\to X_dγ$ decays than it is in $B\to X_sγ$. We estimate that the time dependent CP asymmetries in $B\to K^*γ$, $ργ$, $K_Sπ^0γ$, and $π^+π^-γ$ are of order 0.1 and that they have significant uncertainties.

hep-ph↗
arXiv2004

Exclusive rare B -> K*e+e- decays at low recoil: controlling the long-distance effects

We present a model-independent description of the exclusive rare decays B-> K* e+e- in the low recoil region (large lepton invariant mass q^2 ~ m_b^2). In this region the long-distance effects from quark loops can be computed with the help of an operator product expansion in 1/Q, with Q={m_b, \sqrt{q^2}}. Nonperturbative effects up to and including terms suppressed by Lambda/Q and mc^2/mb^2 relative to the short-distance amplitude can be included in a model-independent way. Based on these results, we propose an improved method for determining the CKM matrix element |V{ub}| from a combination of rare and semileptonic B and D decays near the zero recoil point. The residual theoretical uncertainty from long distance effects in this |V{ub}| determination comes from terms in the OPE of order alpha_s(Q)Λ/mb, alpha_s^2(Q), mc^4/mb^4$ and duality violations and is estimated to be below 10%.

hep-ph↗
arXiv2004

Theory of the endpoint region of exclusive rare B decays

In the ratio of ratios of rates for radiative and semileptonic decays of B and D to K* and rho mesons the non-computable form factors mostly cancel out. The resulting method for the determination of Vub is largely free of hadronic uncertainties, raising the prospect of a precise determination of Vub.

hep-ph↗
arXiv2004

SU(3) Decay Amplitudes of Pentaquarks into Decuplet Baryons

Evidence for the existence of exotic, pentaquark baryons was first found in the invariant mass spectrum of a baryon-meson pair, with the baryon in the octet of flavor SU(3). Exotic pentaquarks may also decay into members of the spin-3/2 baryon decuplet. We calculate relations between decay rates of the exotic pentaquark into a baryon decuplet and an octet of pseudoscalar mesons based on SU(3) flavor symmetry, including the leading flavor symmetry breaking term. We consider all possible representations for an exotic pentaquark, namely, that it belongs to either a \bar{10}, 27, or 35 representation of flavor SU(3). In addition, we use the approximate SU(6) spin-flavor symmetry of baryons to derive relations between the reduced matrix elements of our calculation and those of an existing analogous calculation for decays into the baryon octet. By comparing several decay rates of exotics into both octet and decuplet baryons, our results could be used to elucidate the SU(3) nature of pentaquarks.

hep-ph↗
arXiv2004

Heavy quark symmetry in B -> D(*) ell nu spectra

We calculate heavy quark symmetry breaking in the slopes and curvatures of the $B\to D^{(*)}\ell\barν$ spectra at zero recoil, including the order $α_s^2β_0$ corrections. We point out that the theoretical uncertainties in the differences between $B\to D$ and $B\to D^*$ slopes and curvatures are smaller than in the deviations of the slopes and curvatures themselves from their infinite mass limits. We find that the central values of the current experimental results for the difference of the slopes differ from our calculations when QCD sum rules are used to estimate subleading Isgur-Wise functions. A better understanding of the shapes of the $B\to D^{(*)}\ell\barν$ spectra may also help to reduce the error of $|V_{cb}|$ extracted from the zero recoil limit of $B\to D^*\ell\barν$. We argue that heavy quark symmetry requires that the same fitting procedure be used in the experimental determinations of the shape parameters and $|V_{cb}|$ from the $B\to D\ell\barν$ and $B\to D^{*}\ell\barν$ spectra.

hep-ph↗
arXiv2003

Detector resolution effects on hadronic mass moments in B -> X l nu

We show how to relate moments of the reconstructed hadronic invariant mass distribution to the moments of the physical spectrum in a model independent way. The only information needed on detector resolution functions are their first few moments. Theoretical predictions for the first three moments as functions of non-perturbative parameters are given for various lepton energy cuts.

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arXiv2002

Subleading corrections to the V_ub determination from exclusive B decays

It has been proposed to determine the CKM matrix element |V_ub| in a model-independent way from a combination of rare and semileptonic B and D decays near the zero recoil point. An essential ingredient in such a determination is a heavy quark symmetry relation connecting the form-factors appearing in B -> K* e+e- to semileptonic form factors relevant for B -> rho eν. We estimate the leading corrections to this symmetry relation, of order α_s(m_b) and Λ/m_b, pointing out that they can be as large as 20%, depending on the value of the matrix element of a dimension-4 operator. Dimensional analysis estimates of this matrix element give a corresponding uncertainty in |V_ub| of the order of a few percent.

hep-ph↗
arXiv2002

Symmetry-breaking corrections to heavy meson form factor relations

In the heavy quark limit, the form factors for semileptonic and rare radiative B decays into light mesons are related by heavy quark spin symmetry. Here we compute the leading corrections of order Λ/m_Q to these symmetry relations, showing also how to include hard gluon effects to any order in α_s(m_Q). The subleading correction to the form factor relation for B\to πcan be computed exactly in the soft pion limit. For decays into vector mesons, the matrix elements of two local dimension-4 operators are needed, one of which vanishes in the constituent quark model. A few applications are briefly discussed.

hep-ph↗
arXiv2001

Global duality in heavy flavor decays in the 't Hooft model

We average the decay width of a heavy meson in the t' Hooft model over the heavy quark mass M with a smooth weight function. The averaging has support over a few resonance spacings. We use the previously determined heavy meson decay width which differs from the free quark width by a 1/M correction. In contrast, we find that the averaged meson and quark widths differ by a 1/M^2 correction. We speculate on the relevance of our results to the phenomenologically relevant case of 3+1 dimensional QCD.

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arXiv2001

Bulk Observers in Non-Factorizable Geometries

We consider five dimensional non-factorizable geometries where the transverse dimension is bounded and the remaining (parallel) dimensions are not. We study the construction of effective theories at distances much longer than the transverse size. An observer unable to resolve the transverse direction can only measure distances along the parallel dimensions, but the non-factorizable geometry makes the length of a curve along the parallel dimension sensitive to where on the transverse direction the curve lies. We show that long geodesics that differ in their endpoints only by shifts along the transverse direction all have the same length to within the observer's resolution. We argue that this is the correct notion of distance in the effective theory for a bulk observer. This allows us to present a consistent interpretation of what is measured by observers that live either on a brane or in the bulk.

hep-th↗
arXiv2001

Radion Stabilization by Brane Matter

We find a static solution to Einstein's field equations on a five-dimensional orbifold with a compact S_1/Z_2 fifth direction and Poincare invariant 3+1 sections. The solution describes a theory with bulk cosmological constant and 3-branes at the orbifold fixed points which carry matter density and pressure in addition to tension. The radius of the fifth dimension is determined by the matter content of the branes. The ratio of the space and time components of the metric depends on the fifth coordinate. Thus, the speed of propagation of massless fields is path dependent. For example, bulk and brane fields propagate with different speeds.

hep-th↗
arXiv2000

On a Covariant Determination of Mass Scales in Warped Backgrounds

We propose a method of determining masses in brane scenarios which is independent of coordinate transformations. We apply our method to the scenario of Randall and Sundrum (RS) with two branes, which provides a solution to the hierarchy problem. The core of our proposal is the use of covariant equations and expressing all coordinate quantities in terms of invariant distances. In the RS model we find that massive brane fields propagate proper distances inversely proportional to masses that are not exponentially suppressed. The hierarchy between the gravitational and weak interactions is nevertheless preserved on the visible brane due to suppression of gravitational interactions on that brane. The towers of Kaluza-Klein states for bulk fields are observed to have different spacings on different branes when all masses are measured in units of the fundamental scale. Ratios of masses on each brane are the same in our covariant and the standard interpretations. Since masses of brane fields are not exponentiated, the fundamental scale of higher-dimensional gravity must be of the order of the weak scale.

hep-th↗
arXiv2000

Long-distance effects in $B\to Vγ$ radiative weak decays

A systematic approach to long-distance effects in exclusive radiative weak $B$ decays is presented, based on a combination of the heavy quark limit with perturbative QCD. The dominant long-distance effects, connected with weak annihilation and $W$-exchange topologies, can be computed in a model-independent way using experimental data on $B$ radiative leptonic decays. Nonfactorizable corrections vanish in the chiral limit and to leading twist. The left-handed photon amplitudes are shown to be enhanced relative to the right-handed ones, both in the long- and short-distance parts of the $\bar B$ decay amplitudes. We discuss the implications of our results for the extraction of $|V_{td}|$.

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arXiv2000

Short Distance Analysis of $\bar B -> D^{(*)0} e+e-$ and $\bar B -> J/psi e+e-$

Over a large fraction of phase space a combination of an operator product and heavy quark expansions effectively turn the decay $\bar B -> D^{(*)0} e+e-$ into a ``short distance'' process, i.e., one in which the weak and electromagnetic interactions occur through single local operators. These processes have an underlying W-exchange quark diagram topology and are therefore Cabibbo allowed but suppressed by combinatoric factors and short distance QCD corrections. Our technique allows a clearer exploration of these effects. For the decay $\bar B_{d,s} -> J/psi(η_c) e+e-$ one must use a non-relativistic (NRQCD) expansion, in addition to an operator product expansion and a heavy quark effective theory expansion. We estimate the decay rates for $\bar B_{d,s} -> J/psi e+e-$, $\bar B_{d,s} -> eta_c e+e-$, $\bar B_{d,s} -> D^{*0} e+e-$ and $\bar B_{d,s} -> D^{0} e+e-$.

hep-ph↗
arXiv2000

Adding Matter to Poincare Invariant Branes

A solution to the cosmological constant problem has been proposed in which our universe is a 3-brane in a 5-dimensional spacetime. With a bulk scalar, the field equations admit a Poincare invariant brane solution regardless of the value of the cosmological constant (tension) on the brane. However, the solution does not include matter in the brane. We find new exact static solutions with matter density and pressure in the brane. We study small perturbations about these solutions. None seem consistent with observational cosmology. As a byproduct we find a class of new matterless static solutions and a non-static solution which, curiously, requires the string value for the dilaton coupling.

hep-th↗
arXiv2000

Operator Product Expansion for Exclusive Decays: B^+ ->Ds^+ e+e- and B^+ -> Ds^{*+} e+e-

The decays $B^+\to D_{s,d}^+e^+e^-$ and $B^+\to D_{s,d}^{*+}e^+e^-$ proceed through a weak and an electromagnetic interaction. This is a typical ``long distance'' process, usually difficult to compute systematically. We propose that over a large fraction of phase space a combination of an operator product and heavy quark expansions effectively turns this process into one in which the weak and electromagnetic interactions occur through a local operator. Moreover, we use heavy quark spin symmetry to relate all the local operators that appear in leading order of the operator expansion to two basic ones. We use this operator expansion to estimate the decay rates for $B^+\to D_{s,d}^{(*)+}e^+e^-$.

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