Benjamin Grinstein

Professor

Ph.D. Harvard, 1984

You may also search for Grinstein in the Los Alamos Particle Physics eprint archives or for in SLAC's Spires (with number of citations).

For general information on particle physics visit The Particle Adventure List of Sites.

Some recent papers
Grand Unification and the Principle of Minimal Flavor Violation
Falsifying Models of New Physics Via WW Scattering
Shape and soft functions of HQET and SCET in the 't Hooft Model
Phenomenology of minimal lepton flavor violation
The CP asymmetry in B0(t) ---> K(S) pi0 gamma in the standard model
Minimal flavor violation in the lepton sector
Factorization in B -> K pi l+ l- decays
A Precision model independent determination of |V(ub)| from B ---> pi e nu
Chiral symmetry and exclusive B decays in the SCET.

and some recent talks:
Three lectures on QCD: 1, 2, 3, at the CERN Latin American School of Prticles and Fields
Long Distance Effects in Exclusive Radiative B Decays At SLAC 10^36 Workshop, May 2003

Introduction and general work interests

My main interest has been in the study of weak interactions, and, in particular, decays of "strange," "charm" and "bottom" quarks. The weak force is responsible for these decays; analogous decays of the "down" quark underlie the radioactive decays of elements. Four out of seventeen fundamental parameters in the standard model of strong and electroweak interactions (SM) are called "Mixing Angles", and determine the rate of decays of quarks. The decay of the bottom quark sensitively depends on two of these. There is great interest in understanding the decays of bottom quarks because the determination of two fundamental parameters is at stake.

Quarks are never found in isolation. They experience a "strong force" which binds them into "mesons" (containing a quark and an antiquark) or "baryons" (containing three quarks). When a quark in a meson (or a baryon) decays into other quarks we don't quite see a quark decay in the lab. Instead what is seen is that the meson carrying the decaying quark decays into other mesons (and possibly baryons). So we measure decay rates of mesons and baryons, not of the more basic quarks.

The hard theoretical question is: given a decay rate of a meson, what is the underlying decay rate of the quark?

A meson with one beauty antiquark and one up or down quark is called a B-meson. The b-quark is much heavier than the up or down quarks and than the mass equivalent of the binding enery of the quarks in the meson. Therefore b-quarks in B-mesons pretty much sit in the middle of the meson without moving (much like the sun in our solar system, or like a nucleus in an atom). This apparently inncuous observation has led to a method for computing the decay rate of B-mesons in term of the decay rate of underlying b-quarks. The method is called Heavy Quark Effective Theory (HQET). With it we can make predictions of a few specific decay rates at very particular kinematic points, that is, when the outgoing particles have very specific momenta.

Specifically, HQET gives, with little uncertainty, the semileptonic decay rate B --> D* + e + nu (that is, of a B-meson into a D-vector-meson, an electron and an antineutrino) when the D* is at rest relative to the rest frame of the originating B-meson (the point of "zero recoil").

I have been very involved in the development of HQET and of its applications to physical processes. For a review see, eg, An Introduction to Heavy Mesons.

Not so Recent Work (1995-97)

Non-leptonic decays of B-mesons are hardest to analyze. Recent theoretical proposals suggest that the inlcusive non-leptonic decay rate of B mesons (ie, the decay rate into anything excluding leptons in the final state) is identical to the decay rate of the b quark into lighter quarks. One can try to verify or disprove these proposals by considering a simpler version of the world in which the strong interactions are exactly soluble. In "Explicit Quark-hadron Duality in Heavy-light Meson Weak Decays in the 't Hooft Model" we compute the nonleptonic weak decay width of a heavy-light meson in 1+1 spacetime dimensions with a large number of QCD colors (the 't Hooft model) as a function of the heavy quark mass. In this limit, QCD is exactly soluble, and decay modes are dominated by two-particle final states. We compare the results to the tree-level partonic decay width of the heavy quark in order to test quark-hadron duality in this universe. We find that this duality is surprisingly well satisfied in the heavy quark limit, in that the difference between the sum of exclusive partial widths and the tree-level partonic width approaches a constant as M --> infinity, and the deviation is well-fit by a small 1/M correction.

While HQET gives the decay rate B --> D* + e + nu at the fixed kinematic point of zero recoil, experimentally the rate is measured at other kinematic points. An extrapolation to sero recoil is necesary. There is an inherent systematic uncertainty in the extrapolation if one does not have a priori a functional form for the extrapolating function. In a series of papers (see Model-Independent Determinations of B -> D l nu , D* lnu Form Factors, Model-Independent Extraction of |V_{cb}| Using Dispersion Relations and Precision Corrections to Dispersive Bounds on Form Factors ) we have developped a form for this extrapolating function directly from the theory of strong interactions. In the last of this series we have demonstrated that all sorts of corrections ignored before are negligible.

I will add to this. In the mean time, for other recent work, click you may search for "Grinstein" in in the Los Alamos Particle Physics eprint archive.

Next to Not So Recent Work (1993-94)

I have investigated the accuracy of approximate heavy quark symmetries in planar two-dimensional QCD. Surprisingly the form factors for semileptonic decay, B -> pi e nu, into a massless pion, are given by a single simple pole.[e-Print Archive:hep-ph/9401303]

I have also been interested in the symmetry violations to predictions based on chiral and heavy quark symmetries. The relations between the endpoint form factors for B -> Ke+e- and B -> pi e nu are modified at the 40% level,[e-Print Archive:hep-ph/9306310] while doubly protected quantities, like (fBs/fB)/(fDs/fD) are predicted with better than 10% accuracy.[e-Print Archive:hep-ph/9308266 and hep-ph/9402340] Chiral and heavy quark symetries can be used to find relations between form factors for the semileptonic decays of mesons with a b-quark into mesons with a c-quark, and one can systematically study the deviation from the symmetry limit.[e-Print Archive:hep-ph/9502311]

We have also been interested in the low energy predictions of Grand Unified Supergravity theories. We have developed a method of computation of light particle threshold effects, and made postdictions of the measured strengths of the electroweak and strong interactions.[e-Print Archive:hep-ph/9308329]

Selected Publications:

Junegone Chay, Howard Georgi, Benjamin Grinstein, LEPTON ENERGY DISTRIBUTIONS IN HEAVY MESON DECAYS FROM QCD. Phys.Lett.B247:399-405,1990.

Adam F. Falk, Benjamin Grinstein, Michael E. Luke, LEADING MASS CORRECTIONS TO THE HEAVY QUARK EFFECTIVE THEORY. Nucl.Phys.B357:185-207,1991.

Adam F. Falk, Howard Georgi, Benjamin Grinstein, Mark B. Wise, HEAVY MESON FORM-FACTORS FROM QCD. Nucl.Phys.B343:1-13,1990.

Benjamin Grinstein, THE STATIC QUARK EFFECTIVE THEORY. Nucl.Phys.B339:253-268,1990.

Benjamin Grinstein, Roxanne Springer, Mark B. Wise, STRONG INTERACTION EFFECTS IN WEAK RADIATIVE ANTI-B MESON DECAY. Nucl.Phys.B339:269-309,1990.

Nathan Isgur, Daryl Scora, Benjamin Grinstein, Mark B. Wise, SEMILEPTONIC B AND D DECAYS IN THE QUARK MODEL. Phys.Rev.D39:799,1989.

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Last Updated 8/27/97
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