Richard F. Lebed

Bounding noncommutative QCD

Abstract Jurco, Moller, Schraml, Schupp, and Wess have shown how to construct noncommutative SU(N) gauge theories from a consistency relation. Within this framework, we present the Feynman rules for noncommutative QCD and compute explicitly the most dangerous Lorentz-violating operator generated through radiative corrections. We find that interesting effects appear at the one-loop level, in contrast to conventional noncommutative U(N) gauge theories, leading to a stringent bound. Our results are consistent with others appearing recently in the literature that suggest collider limits are not competitive with low-energy tests of Lorentz violation for bounding the scale of spacetime noncommutativity.

Constraints on Form Factors for Exclusive Semileptonic Heavy to Light Meson Decays

We use rigorous QCD dispersion relations to derive model-independent bounds on the {ital {bar B}}{r_arrow}{pi}{ital l}{bar {nu}}, {ital D}{r_arrow}{pi}{ital {bar l}}{nu}, and {ital D}{r_arrow}{ital {bar K}{bar l}}{nu} form factors. These bounds are particularly restrictive when the value of the observable form factor at one or more kinematic points is assumed. With reasonable assumptions we find {ital f}{sub {ital B}}{le}195 MeV and that the shape of the form factor becomes severely constrained. These constraints are useful both for model discrimination and for model-insensitive extraction of Cabibbo-Kobayashi-Maskawa mixing parameters.

Maximal neutrino mixing from a minimal flavor symmetry

The authors study a number of models, based on a non-Abelian discrete group, that successfully reproduce the simple and predictive Yukawa textures usually associated with U(2) theories of flavor. These models allow for solutions to the solar and atmospheric neutrino problems that do not require altering successful predictions for the charged fermions or introducing sterile neutrinos. Although Yukawa matrices are hierarchical in the models they consider, the mixing between second- and third-generation neutrinos is naturally large. They first present a quantitative analysis of a minimal model proposed in earlier work, consisting of a global fit to fermion masses and mixing angles, including the most important renormalization group effects. They then propose two new variant models: The first reproduces all important features of the SU(5) x U(2) unified theory with neither SU(5) nor U(2). The second demonstrates that discrete subgroups of SU(2) can be used in constructing viable supersymmetric theories of flavor without scalar universality even though SU(2) by itself cannot.

The Pentaquark Candidates in the Dynamical Diquark Picture

Abstract Starting with the dynamical picture of the exotic c c ¯ -containing states XYZ as the confinement-induced hadronization of a rapidly separating pair of a compact diquark and antidiquark, we describe the pentaquark candidates P c + ( 4380 ) and P c + ( 4450 ) in terms of a confined but rapidly separating color-antitriplet diquark cu and color-triplet “triquark” c ¯ ( u d ) . This separation explains the relatively small P c + widths, despite these 5-quark systems lying far above both the J / ψ p and Λ c D ¯ ( ⁎ ) 0 thresholds. The P c + states are predicted to form isospin doublets with neutral partners P c 0 .

U(2) flavor physics without U(2) symmetry

Grupo de Fisica Teorica Universidade Catolica de Petropolis (UCP), Rua Barao do Amazonas 124, 25.685-000 Petropolis-RJThe authors present a model of fermion masses based on a minimal, non-Abelian discrete symmetry that reproduces the Yukawa matrices usually associated with U(2) theories of flavor. Mass and mixing angle relations that follow from the simple form of the quark and charged lepton Yukawa textures are therefore common to both theories. They show that the differing representation structure of the horizontal symmetry allows for new solutions to the solar and atmospheric neutrino problems that do not involve modification of the original charged fermion Yukawa textures, or the introduction of sterile neutrinos.

Model-independent extraction of |Vcb| using dispersion relations

Abstract We present a method for parametrizing heavy meson semileptonic form factors using dispersion relations, and from it produce a two-parameter description of the B → B elastic form factor. We use heavy quark symmetry to relate this function to B → D ∗ l ν form factors, and extract | V cb | = 0.037 −0.002 +0.003 from experimental data w least squares fit. Our method eliminates model-dependent uncertainties inherent in choosing a parametrization for the extrapolation of the differential decay rate to threshold. The method also allows a description of B → Dl ν form factors accurate to 1% in terms of two parameters.

Supersymmetric noncommutative QED and Lorentz violation

We consider Lorentz-violating operators induced at the loop level in softly-broken supersymmetric noncommutative QED. Dangerous operators forbidden in the supersymmetric limit are generated via finite corrections, with the scale of supersymmetry breaking serving as a gauge-invariant regulator. We compare the most dangerous loop effects to those obtained in noncommutative theories truncated by a momentum-space cutoff, and find significantly improved bounds.

Phenomenology of Large Nc QCD

These lectures are designed to introduce the methods and results of large Nc QCD in a presentation intended for nuclear and particle physicists alike. Beginning with definitions and motivations of the approach, we demonstrate that all quark and gluon Feynman diagrams are organized into classes based on powers of 1/Nc. We then show that this result can be translated into definite statements about mesons and baryons containing arbitrary numbers of constituents. In the mesons, numerous well-known phenomenological properties follow as immediate consequences of simply counting powers of Nc, while for the baryons, quantitative large Nc analyses of masses and other properties are seen to agree with experiment, even when “large” Nc is set equal to its observed value of 3. Large Nc reasoning is also used to explain some simple features of nuclear interactions.

Minimal Lee-Wick extension of the Standard Model

Abstract We consider a minimal Lee–Wick (LW) extension to the Standard Model in which the fields providing the most important contributions to the cancellation of quadratic divergences are the lightest. Partners to the SU(2) gauge bosons, Higgs, top quark, and left-handed bottom quark are retained in the low-energy effective theory, which is valid up to approximately 10 TeV; the remaining LW partners appear above this cutoff and complete the theory in the ultraviolet. We determine the constraints on the low-energy spectrum from the electroweak parameters S and T , and find LW states within the kinematic reach of the LHC at the 95% confidence level.

Baryon mass splittings in the 1/Nc expansion

The {ital I}=0,1,2,3 mass splittings of the spin-1/2 octet and spin-3/2 decuplet baryons are analyzed in the 1/{ital N}{sub {ital c}} expansion combined with perturbative flavor breaking. We show there is considerable experimental evidence that the baryon masses satisfy the hierarchy predicted by this expansion. Since flavor symmetry-breaking suppression factors alone are not sufficient to describe the observed hierarchy, we conclude that there is firm evidence for the 1/{ital N}{sub {ital c}} expansion in the baryon masses. Our analysis differs from nonrelativistic SU(6).