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B. Grinstein

Publications and source records attributed to B. Grinstein.

12 recordsLinked to original sources
arXiv2025

The Compact X and Z and their Invisible Molecular Partners

We propose a model explaining the strong isospin violations observed in X(3872) decays. The X is assumed to be a compact isosinglet tetraquark and its charged partners to form an isotriplet, together with an additional neutral state mixing with the isosinglet. The isotriplet results from the loose binding of open charm mesons and, in isolation, has strongly suppressed production rates. However, its neutral component can still mix with the isosinglet and induce the large isospin violations observed. The compact-isosinglet/molecular-isotriplet pattern appears reversed when the Z(3900) resonances are considered. The Z particles observed correspond to a compact isotriplet and there is no evidence of a neutral isosinglet Z. We show that no isospin violation in the Z decays is expected.

hep-ph↗
arXiv2024

Radiative decays of X(3872) discriminate between the molecular and compact interpretations

Radiative decays X --> psi(1S) + gamma and X --> psi(2S) + gamma might be expected to have a ratio of branching fractions following the phase space volumes ratio. However data suggest the opposite, indicating a value for R=B(X --> psi^prime + gamma) / B(X --> psi +gamma) consistently larger than one. In this paper we present a calculation of R for both a compact Born-Oppenheimer cc-bar q-qbar state and a DD^* molecule. In the former case R~1 or larger is found, a value to be confronted with forthcoming high statistics data analyses. In the molecular picture, with D and D^* mesons described by the universal wave function used by Voloshin, Braaten and Kusunoki, we find that R would be of order 10^-2. A more precise experimental measure would be extremely helpful in clarifying the true nature of the X(3872).

hep-ph↗
arXiv2018

Lepton non-universality in $B$ decays and fermion mass structure

We consider the possibility that the neutral-current $B$ anomalies are due to radiative corrections generated by Yukawa interactions of quarks and leptons with new vector-like quark and lepton electroweak doublets and new Standard Model singlet scalars. We show that the restricted interactions needed can result from an underlying Abelian family symmetry and that the same symmetry can give rise to an acceptable pattern of quark and charged lepton masses and mixings, providing a bridge between the non-universality observed in the B-sector and that of the fermion mass matrices. We construct two simple models, one with a single singlet scalar in which the flavour changing comes from quark and lepton mixing and one with an additional scalar in which the flavour changing can come from both fermion and scalar mixing. We show that for the case the new quarks are much heavier than the new leptons and scalars the $B$ anomalies can be due to box diagrams with couplings in the perturbative regime consistent with the bounds coming from $B_s- \bar B_s$, $K- \bar K$ and $D- \bar D$ mixing as well as other lepton family number violating processes. The new states can be dark matter candidates and, in the two scalar model with a light scalar of O(60) GeV and vector-like lepton of O(100) GeV, there can be a simultaneous explanation of the B-anomalies, the muon anomalous magnetic moment and the dark matter abundance.

hep-ph↗
arXiv2016

Gauged Lepton Flavour

The gauging of the lepton flavour group is considered in the Standard Model context and in its extension with three right-handed neutrinos. The anomaly cancellation conditions lead to a Seesaw mechanism as underlying dynamics for all leptons; requiring in addition a phenomenologically viable setup leads to Majorana masses for the neutral sector: the type I Seesaw Lagrangian in the Standard Model case and the inverse Seesaw in the extended model. Within the minimal extension of the scalar sector, the Yukawa couplings are promoted to scalar fields in the bifundamental of the flavour group. The resulting low-energy Yukawa couplings are proportional to inverse powers of the vacuum expectation values of those scalars; the protection against flavour changing neutral currents differs from that of Minimal Flavor Violation. In all cases, the $\mu-\tau$ flavour sector exhibits rich and promising phenomenological signals.

hep-ph↗
arXiv2011

Ultraviolet Properties of the Higgs Sector in the Lee-Wick Standard Model

The Lee-Wick (LW) Standard Model (SM) offers a new solution to the hierarchy problem. We discuss, using effective potential techniques, its peculiar ultraviolet (UV) behaviour. We show how quadratic divergences in the Higgs mass Mh cancel as a result of the unusual dependence of LW fields on the Higgs background (in a manner reminiscent of Little Higgses). We then extract from the effective potential the renormalization group evolution of the Higgs quartic coupling lambda above the LW scale. After clarifying an apparent discrepancy with previous results for the LW Abelian Higgs model we focus on the LWSM. In contrast with the SM case, for any Mh, lambda grows monotonically and hits a Landau pole at a fixed trans-Planckian scale (never turning negative in the UV). Then, the perturbativity and stability bounds on Mh disappear. We identify a cutoff ~10^{16} GeV for the LWSM due to the hypercharge gauge coupling hitting a Landau pole. Finally, we also discuss briefly the possible impact of the UV properties of the LW models on their behaviour at finite temperature, in particular regarding symmetry nonrestoration.

hep-ph↗
arXiv2008

Decay b -> (c\bar{c}) s in the leading logarithm approximation

We consider an effective field theory for the nonleptonic decay in which a heavy quark decays into a pair of a heavy quark and antiquark having a small relative velocity and one relativistic (massless) quark. This effective theory is a combination of HQET, SCET, and a covariant modification of NRQCD. In the leading logarithm approximation the effective theory decay amplitude factorizes into the product of matrix elements of heavy-to-heavy and heavy-to-light currents. We discuss a possibility of factorization beyond the leading logarithm approximation and find it doubtful. The Wilson coefficients of the effective theory electro-weak (EWET) Lagrangian in the next-to-the leading logarithm approximation are calculated at the matching scale of the decay. The differential decay rate for the inclusive decay B -> J/ψ+h in the effective theory framework is evaluated.

hep-ph↗
arXiv2005

The Discovery Potential of a Super B Factory

The Proceedings of the 2003 SLAC Workshops on flavor physics with a high luminosity asymmetric e+e- collider. The sensitivity of flavor physics to physics beyond the Standard Model is addressed in detail, in the context of the improvement of experimental measurements and theoretical calculations.

hep-ph↗
arXiv2002

Testing factorization in B -> D(*)X decays

In QCD the amplitude for B0 -> D(*)+pi- factorizes in the large Nc limit or in the large energy limit Q >> Lambda_QCD where Q = {m_b, m_c, m_b-m_c}. Data also suggests factorization in exclusive processes B-> D* pi+ pi- pi- pi0 and B-> D* omega pi-, however by themselves neither large Nc nor large Q can account for this. Noting that the condition for large energy release in B0-> D+ pi- is enforced by the SV limit, m_b, m_c >> m_b-m_c >> Lambda, we propose that the combined large Nc and SV limits justify factorization in B -> D(*) X. This combined limit is tested with the inclusive decay spectrum measured by CLEO. We also give exact large Nc relations among isospin amplitudes for B -> D(*)X and B -> D(*) D-bar(*)X, which can be used to test factorization through exclusive or inclusive measurements. Predictions for the modes B-> D(*) pi pi, B-> D(*)K K-bar and B-> D(*) D-bar(*) K are discussed using available data.

hep-ph↗
arXiv1997

Chiral and Heavy Quark Symmetry Violation in B Decays

The most general Lagrangian consistent with chiral, heavy quark, and strong interaction symmetries to order $1/M$ and to linear order in the SU(3) vector and axial currents is presented. Two new dimensionful and five dimensionless couplings arise at this order. The heavy to light flavor changing current is derived to the same order, giving rise to two additional dimensionful constants and six dimensionless ones. The dimensionless parameters are shown to be irrelevant at \ord. The leading nonanalytic heavy quark and chiral symmetry violating corrections to heavy meson decay constants are computed, and implications to B decays are discussed; measurements of the experimentally accessible form factors for $D \to πl ν$ and $B \to πl ν$, along with knowledge of the $D^* D π$ coupling, determine the eight decay constants $f_D$, $f_{D_s}$, $f_{D^*}$, $f_{D_s^*}$, $f_B$, $ f_{B_s}$, $f_{B^*}$, and $f_{B_s^*}$, as well as one relation among the $B \to πl ν$ form factors, to \ord~. The ratio $R_1 = \frac{f_{B_s}}{f_B}/\frac{f_{D_s}}{f_D}$ is expressed in terms of two dimensionful couplings.

hep-ph↗
arXiv1995

Model-Independent Semileptonic Form Factors Using Dispersion Relations

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 nu form factors, and extract |V_cb|=0.0355^{+0.0029}_{-0.0025} from experimental data with a least squares fit. Our method eliminates model-dependent uncertainties inherent in choosing a parametrization for the extrapolation of the differential decay rate to threshold.

hep-ph↗
arXiv1992

Chiral Perturbation Theory for $f_{D_S}/f_D$ and $B_{B_S}/B_{B}$

The decay constants for the $D$ and $D_S$ mesons, denoted $f_D$ and $f_{D_S}$ respectively, are equal in the $SU(3)_V$ limit, as are the hadronic amplitudes for $B_S-\bar B_S$ and $B^0-\bar B^0$ mixing. The leading $SU(3)_V$ violating contribution to $\left( f_{D_S} / f_D \right)$ and to the ratio of hadronic matrix elements relevant for $B_S-\bar B_S$ and $B^0-\bar B^0$ mixing amplitudes are calculated in chiral perturbation theory. We discuss the formalism needed to include both meson and anti-meson fields in the heavy quark effective theory.

hep-ph↗