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Lisa Randall

Publications and source records attributed to Lisa Randall.

140 records · Page 8Linked to original sources
arXiv1995

Could the Supersymmetric Higgs Particles Naturally be Pseudo-Goldstone Bosons?

The doublet-triplet splitting problem is perhaps the most problematic aspect of supersymmetric grand unified theories. It can be argued that the most natural reason for the Higgs doublets to be light is that they are pseudo-Goldstone bosons associated with the spontaneous breakdown of an accidental global symmetry. In this paper we discuss the possibility of implementing this idea in the SU(6) model of refs. \cite{Zur,Bar2,Bar3,Bar4}. We show that although it is simple to generate an accidental symmetry of the renormalizable terms of the potential, it is quite difficult to construct a model which allows for the preservation of the accidental symmetry in the nonrenormalizable terms. We summarize the constraints on such models and then give three different ways to construct a superpotential where the dangerous mixing terms are sufficiently suppressed even in the presence of nonrenormalizable operators. With these examples we demonstrate the existence of consistent models implementing the Higgs as pseudo-Goldstone boson scheme. We extend one of the three examples to include fermion masses. We also show that when restricted to regular group embeddings the only possible models without light triplets are trivial generalizations of the SU(6) model we consider.

hep-ph↗
arXiv1994

The R Axion From Dynamical Supersymmetry Breaking

All generic, calculable models of dynamical supersymmetry breaking have a spontaneously broken $R$ symmetry and therefore contain an $R$ axion. We show that the axion is massive in any model in which the cosmological constant is fine-tuned to zero through an explicit $R$-symmetry-breaking constant. In visible-sector models, the axion mass is in the 100 MeV range and thus evades astrophysical bounds. In nonrenormalizable hidden-sector models, the mass is of order of the weak scale and can have dangerous cosmological consequences similar to those already present from other fields. In renormalizable hidden- sector models, the axion mass is generally quite large, of order $10^7$ GeV. Typically, these axions are cosmologically safe. However, if the dominant decay mode is to gravitinos, the potentially large gravitino abundance that arises from axion decay after inflation might affect the successful predictions of big-bang nucleosynthesis. We show that the upper bound on the reheat temperature after standard inflation can be competitive with or stronger than bounds from thermal gravitino production, depending on the model and the gravitino mass.

hep-ph↗
arXiv1994

Low-Energy Kahler Potentials in Supersymmetric Gauge Theories with (ALMOST) Flat Directions

We derive the supersymmetric low-energy effective theory of the D-flat directions of a supersymmetric gauge theory. The Kahler potential of Affleck, Dine and Seiberg is derived by applying holomorphic constraints which manifestly maintain supersymmetry. We also present a simple procedure for calculating all derivatives of the Kahler potential at points on the flat direction manifold. Together with knowledge of the superpotential, these are sufficient for a complete determination of the spectrum and the interactions of the light degrees of freedom. We illustrate the method on the example of a chiral abelian model, and comment on its application to more complicated calculable models with dynamical supersymmetry breaking.

hep-th↗
arXiv1994

Solving the Cosmological Moduli Problem with Weak Scale Inflation

Many models of supersymmetry breaking involve particles with weak scale mass and Planck mass suppressed couplings. Coherent production of such particles in the early universe destroys the successful predictions of nucleosynthesis. We show that this problem may be solved by a brief period of weak scale inflation. Furthermore the inflaton potential for such an inflation naturally arises from the same assumptions which lead to the cosmological problem. Successful baryogenesis and preservation of density fluctuations for large scale structure formation are also possible in this scenario.

hep-ph↗
arXiv1993

The ACCMM Model and the Heavy Quark Expansion

The ACCMM model predicts the lepton spectrum from $B$ meson decay by assuming the meson disintegrates into a spectator quark of definite mass and momentum distribution and an off shell $b$ quark whose decay leptons (boosted into the rest frame of the meson) determine the lepton spectrum. In this letter, we show that one can define a model dependent $b$ quark mass so that the spectrum derived from the ACCMM model agrees very well with the free quark decay spectrum far from the endpoint. Near the endpoint, there is some disagreement, indicating the result is more model dependent. The integrated spectra are however very nearly identical. These results are in accordance with expectations based on the heavy quark effective theory. The detailed analytic study of this model helps elucidate the HQET result, and in particular the meaning of the $b$ quark mass. We conclude that for LEP experiments, free quark decay might be as general and more simple than the ACCMM ansatz for modeling the inclusive charged lepton spectrum from $B$ meson decay.

hep-ph↗
arXiv1993

Naturally Large Tan $β$

We show that if there are only two Higgs doublets in the supersymmetric standard model, large $\tanβ$ requires a fine tuning in the parameters of the Lagrangian of order ($1/\tanβ$), which cannot be explained by any approximate symmetry. With an extended Higgs sector, large $\tanβ$ can be natural. We give an explicit example with four doublets in which it is possible to achieve large $\tanβ$ as a result of an approximate symmetry, without any light superpartners. The approximate symmetry can be extended to explain all the hierarchies in the quark mass matrix.

hep-ph↗
arXiv1993

The Rate for $B\bbar$ Production Accompanied by a Single Pion

We study the rate for the production of ${B B^\pm π^\mp}$, where the sign of the charged pion tags the flavor content of the neutral $B$ meson. We estimate this branching ratio, employing the heavy meson chiral effective theory. We find that at center of mass energy of approximately 12 GeV, a $B$ meson pair should be produced as often with and without an accompanying charged pion. We also calculate two pion production at this center of mass energy, and find that it is negligible, as is the rate for rho production. We consider the implications for CP violation studies. (Invited Talk, 1993 Electroweak Rencontres de Moriond)

hep-ph↗
arXiv1993

Constraints on a Massive Dirac Neutrino Model

We examine constraints on a simple neutrino model in which there are three massless and three massive Dirac neutrinos and in which the left handed neutrinos are linear combinations of doublet and singlet neutrinos. We examine constraints from direct decays into heavy neutrinos, indirect effects on electroweak parameters, and flavor changing processes. We combine these constraints to examine the allowed mass range for the heavy neutrinos of each of the three generations.

hep-ph↗
arXiv1993

$b \rightarrow s γ$ and $B_s \rightarrow μ^+ μ^-$ in Extended Technicolor Models

The rates of the rare flavor-changing processes, $b \rightarrow s γ$ and $B_s \rightarrow μ^+ μ^-$ are estimated in extended technicolor models with and without a GIM mechanism. We find the $b \rightarrow s γ$ rate in ETC models with a GIM mechanism to be at most slightly larger than the standard model rate, whereas there is no significant extra model-independent contribution in other ETC scenarios. In the case of $B_s \rightarrow μ^+ μ^-$, ETC models with a GIM mechanism can yield a rate up to two orders of magnitude bigger than that of the standard model, whereas generic ETC scenarios are likely to give a rate which is about an order of magnitude bigger than that of the standard model.

hep-ph↗
arXiv1993

Why a Scalar Explanation of the L3 Events is Implausible

We investigate the question of whether an additional light neutral scalar can explain the $l^+ l^- γγ$ events with high invariant mass photon pairs recently observed by the L3 collaboration. We parameterize the low energy effects of the unknown dynamics in terms of higher dimensional effective operators. We show that operators which allow for the scalar to be produced and decay into photon pairs will allow other observable processes that should have been seen in current experiments.

hep-ph↗
arXiv1993

The Rate for $e^+e^-\to B B^\pm π^\mp$ and its Implications for the Study of CP Violation, $B_s$ Identification, and the Study of $B$ Meson Chiral Perturbation Theory

H.~Yamamoto has proposed employing $B$ mesons produced in conjunction with a single charged pion at an $Υ$ resonance for studies of CP violation in the neutral $B$ meson system at a symmetric $e^+$-$e^-$ collider. The sign of the charged pion would tag the neutral $B$ meson. We estimate this branching ratio, employing the heavy meson chiral effective field theory. We find a negligible branching ratio to $B B^{\pm} π^{\mp}$ at the $Υ$(5S) and a branching ratio of only a few percent at the $Υ$(6S). However, if nonresonant studies of neutral $B$ mesons should prove feasible, Yamamoto's proposal could be a good method for tagging neutral $B$'s for the study of CP violation at a symmetric collider. We also explore the possibility of studying $B_s$ at the $Υ$(5S). The rate is low but depends sensitively on the precise value of the mass of the $B_s$. The background we compute is comparable to the rate at the largest allowed value of the $B_s$ mass. Finally, we discuss the extraction of the axial pion coupling to $B$ mesons from measurement of the $B\bbarπ$ branching fraction in a restricted region of phase space, where chiral perturbation theory should work well.

hep-ph↗
arXiv1992

Heavy Meson Hyperfine Splittings: A Puzzle for Heavy Quark Chiral Perturbation Theory

We show that there is a large discrepancy between the expected light flavor dependence of the heavy pseudoscalar--vector mass splittings and the measured values. We demonstrate that the one--loop calculation is unreliable. Moreover, agreement with experiment requires the leading dependence on SU(3) symmetry breaking to be nearly cancelled, so that the heavy quark mass dependence is unknown and the expected dependence on the light quark mass is not realized.

hep-ph↗
arXiv1992

The QCD Scale in the Heavy Quark Expansion

We argue that consistency of the combined heavy quark and chiral effective lagrangian requires the QCD scale which multiplies $1/M$ in the heavy quark expansion to be the chiral symmetry breaking scale, $Λ_{CSB}$, rather than the QCD scale, $Λ_{QCD}$. This means that either there is large uncertainty in the accuracy with which the heavy quark effective theory can be applied to $c$ quarks or the cutoff scale of the heavy quark chiral effective theory is lower than has been assumed.

hep-ph↗