arXiv subjects

Marc Sher

Publications and source records attributed to Marc Sher.

At least 91 records · Page 5Linked to original sources

arXiv1998

The Anomalous Magnetic Moment of the Muon and Higgs-Mediated Flavor Changing Neutral Currents

In the two-Higgs doublet extension of the standard model, flavor-changing neutral couplings arise naturally. In the lepton sector, the largest such coupling is expected to be $μ-τ-ϕ#. We consider the effects of this coupling on the anomalous magnetic moment of the muon. The resulting bound on the coupling, unlike previous bounds, is independent of the value of other unknown couplings. It will be significantly improved by the upcoming E821 experiment at Brookhaven National Lab.

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arXiv1997

Supersymmetric model of quasi-degenerate neutrinos

We present a supersymmetric model of fermion masses, based on a non-Abelian family symmetry and the Froggatt-Nielsen mechanism, that can account for the solar and atmospheric neutrino problems via quasi-degenerate neutrinos. The model predicts that the ratio of neutrino mass squared splittings Δm^2_{12} / Δm^2_{23} is of order m_s^2/m_b^2, and the angles θ_{12} \sim m_d/m_s and θ_{23} \sim 1, which are of the desired orders of magnitude. We discuss the implications of the flavor structure of the neutrino sector on superparticle masses and mixing angles.

hep-ph↗

arXiv1997

$t \to cWW$ and $WW \to \bar{t}c + t\bar{c}$ in Extended Models

Jenkins has pointed out that the process $t\to cW^+W^-$ is GIM suppressed in the standard model. In this note, we calculate the branching ratio for a wide range of models, in which the decay occurs at tree level through exchange of a scalar, fermion or vector. In the case of scalar exchange, a scalar mass between $2m_W$ and 200 GeV leads to a resonant enhancement, giving a branching ratio as high as a few tenths of a percent. We then note that all of these models will also allow $W^+W^-\to \bar{t}c+t\bar{c}$, and we calculate the single-top/single-charm production rate at the LHC. The rates aren't negligibly small, but the background from single-top/single-bottom production will probably swamp the signal.

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arXiv1997

$K_L \to π^o ν\barν$ in the Aspon Model

An attractive alternative to the standard model of CP violation is the aspon model. In this model, CP is spontaneously broken, automatically solving the strong CP problem. Recently, it has been shown that CP violation in the B system is much smaller in the aspon model than in the standard model. Here we provide a complementary study by considering $K_L\to π^oν\barν$, which is almost entirely CP violating and free of hadronic uncertainties, and show that the rate in the aspon model is many orders of magnitude below the standard model rate. Observation of $K_L\to π^oν\barν$ would rule out the aspon model.

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arXiv1997

Millimeter-wave Signature of Strange Matter Stars

One of the most important questions in the study of compact objects is the nature of pulsars, including whether they consist of neutron matter or strange quark matter (SQM). However, few mechanisms for distinguishing between these two possibilities have been proposed. The purpose of this paper is to show that a strange star (one made of SQM) will have a vibratory mode with an oscillation frequency of approximately 250 GHz (millimeter wave). This mode corresponds to motion of the center of the expected crust of normal matter relative to the center of the strange quark core, without distortion of either. Radiation from currents generated in the crust at the mode frequency would be a SQM signature. We also consider effects of stellar rotation, estimate power emission and signal-to-noise ratio, and discuss briefly possible mechanisms for exciting the mode.

astro-ph↗

arXiv1996

The Coleman-Weinberg Phase Transition in Extended Higgs Models

In Coleman-Weinberg symmetry breaking, all dimensionful parameters vanish and the symmetry is broken by loop corrections. Before Coleman-Weinberg symmetry breaking in the Standard Model was experimentally ruled out, it had already been excluded on cosmological grounds. In this Brief Report, the cosmological analysis is carried out for Coleman-Weinberg models with extended Higgs sectors, which are not experimentally ruled out, and general constraints on such models are given.

hep-ph↗

arXiv1996

$K_L \to π^o ν\overlineν$ in Extended Higgs Models

The decay $K_L\rightarrow π^oν\overlineν$ is an excellent probe of the nature of CP violation. It is almost entirely CP-violating, and hadronic uncertainties are negligible. Experiments which hope to detect the decay are currently being planned. We calculate the decay rate in several extensions of the standard model Higgs sector, including the Liu-Wolfenstein two-doublet model of spontaneous CP-violation and the Weinberg three-doublet model. In a model with an extra doublet, with CP-violation arising from the CKM sector, the rate can increase by up to 50\%. However, in models in which the CP violation arises either entirely or predominantly from the Higgs sector, we find that the decay rate is much smaller than that of the standard model, unless parameters of the model are fine-tuned.

hep-ph↗

arXiv1995

Implications of a Higgs Discovery at LEP

If the Higgs boson has a mass below 130 GeV, then the standard model vacuum is unstable; if it has a mass below 90 GeV (i.e. within reach of LEP within the next two years), then the instability will occur at a scale between 800 GeV and 10 TeV. We show that precise determinations of the Higgs and top quark masses as well as more detailed effective potentialcalculations will enable one to pin down the location of the instability to an accuracy of about 25 percent. It is often said that ``the standard model must break down'' or ``new physics must enter'' by that scale. However, by considering a toy model for the new physics, we show that the lightest new particle (or resonance) could have a mass as much as an order of magnitude greater than the location of the instability, and still restabilize the vacuum.

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arXiv1995

New Bounds on R-Parity Violating Couplings

Bounds on R-parity violating couplings in the supersymmetric standard model are reviewed, and some new bounds arising from non-observation of certain rare B-decays (such as $B \rightarrow K^+K^-$) are presented. The focus is on baryon-number violating couplings, but it is also noted that many products of lepton- number violating and baryon-number violating couplings are not significantly constrained by proton decay bounds. The talk reviewed the work in a recent paper by Carlson, Roy and Sher (Phys. Letters, in press).

hep-ph↗

arXiv1995

Electroproduction and Hadroproduction of Light Gluinos

In a class of supergravity models, the gluino and photino are massless at tree level and receive small masses through radiative corrections. In such models, one expects a gluino-gluon bound state, the $R_0$, to have a mass of between 1.0 and 2.2 GeV and a lifetime between $10^{-10}$ and $10^{-6}$ seconds. Applying peturbative QCD methods (whose validity we discuss), we calculate the production cross sections of $R_0$'s in $e-p$, $π-p$, $K-p$, $\overline{p}-p$ and $p-p$ collisions. Signatures are also discussed.

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arXiv1995

New Bounds on R-parity Violating Couplings

We use information from rare nonleptonic decays of heavy-quark mesons to put new bounds on the magnitudes of certain product combinations of baryon nonconserving R-parity violating couplings in supersymmetric models. Product combinations of lepton and baryon nonconserving R-parity violating couplings are also considered in the light of existing bounds on nucleon decay. Contrary to popular impression, a few such combinations are shown to remain essentially unconstrained.

hep-ph↗

arXiv1995

Charged Leptons With Nanosecond Lifetimes

Some extensions of the standard model contain additional leptons which are vectorlike under weak isospin. A class of models is considered in which these leptons do not appreciably mix with the known leptons. In such models, the heavy charged lepton and the heavy neutrino are degenerate in mass, and the degeneracy is broken by radiative corrections. The mass splitting is calculated and found to be very weakly dependent on the lepton mass, varying from 250 to 330 MeV as the mass varies from 100 to 800 GeV. This result is {\it not} affected significantly by inclusion in a supersymmetric model in spite of the additional loops involving the superpartners. As a result, this fairly general class of models has a charged lepton whose lifetime varies in the narrow range from 0.5 to 2.0 nanoseconds, and which decays into neutrals plus a very low energy electron or muon.

hep-ph↗

arXiv1995

Bounds on $ΔB=1$ Couplings in the Supersymmetric Standard Model

The most general supersymmetric model contains baryon number violating terms of the form $\lm_{ijk}\ \ov{D}_i\ \ov{D}_j\ \ov{U}_k$ in the superpotential. We reconsider the bounds on these couplings, assuming that lepton number conservation ensures proton stability. These operators can mediate $n-\ov{n}$ oscillations and double nucleon decay. We show that neutron oscillations do not, as previously claimed, constrain the $\lm_{dsu}$ coupling; they do provide a bound on the $\lm_{dbu}$ coupling, which we calculate. We find that the best bound on $\lm_{dsu}$ arises from double nucleon decay into two kaons. There are no published limits on this process; experimenters are urged to examine this nuclear decay mode. Finally, the other couplings can be bounded by the requirement of perturbative unification.

hep-ph↗

arXiv1994

Bounds on $ΔB=1$ Couplings in the Supersymmetric Standard Model

The most general supersymmetric model contains baryon number violating terms of the form $λ_{ijk}\;\ov{D}_i\, \ov{D}_j\, \ov{U}_k$ in the superpotential. We reconsider the bounds on these couplings, assuming that lepton number conservation ensures proton stability. These operators can mediate $n - \ov{n}$ oscillations and double nucleon decay. We show that neutron oscillations do not, as previously claimed, constrain the $λ_{dsu}$ coupling; they do provide a bound on the $λ_{dbu}$ coupling, which we calculate. We find that the best bound on $λ_{dsu}$ arises from double nucleon decay into two kaons; the calculation is discussed in detail. There are no published limits on this process; experimenters are urged to examine this nuclear decay mode. Finally, the other couplings can be bounded by the requirement of perturbative unification.

hep-ph↗

arXiv1994

Solar Neutrino Oscillations in the Moon

The first three quarters of this paper consists of a pedagogical review of neutrino oscillations, including vacuum oscillations, constant density matter oscillations and variable density matter oscillations. We then address the question of whether MSW solar neutrino oscillations in the moon could be observed during a solar eclipse. For the small angle MSW solution, the effect is unmeasureable; for the large angle solution, we find an enhancement in the rate which can be as large as a factor of two. Since the center of the Sun, as seen from a typical solar neutrino detector (through a transparent Earth) is covered for roughly three hours per decade, a world-wide event rate of a few solar neutrino events per hour might be sufficient to observe this effect. This work constituted the undergraduate senior thesis of Brian Mason, and has been submitted to the American Journal of Physics.

hep-ph↗

arXiv1994

SUSY GUTs without light Higgs bosons

There are two different types of perturbative unification in SUSY GUTs. Perturbative gauge unification is necessary for the extremely successful prediction of $\sin^2θ_w$. ``Perturbative validity'' requires that {\it all} couplings (not just the gauge couplings) remain small; this assumption is essential to the proof that any SUSY GUT must have a light Higgs with a mass below 140 GeV. It is pointed out here that if the latter bound is violated (so that there need be no light Higgs bosons), then it is plausible that the successful prediction of $\sin^2θ_w$ will {\it not} be significantly altered.

hep-ph↗

arXiv1994

Precise Vacuum Stability Bound in the Standard Model (addendum)

This is an addendum to the paper of the above title published in Physics Letters B317, 159 (1993). In that paper, I found the lower bound to the Higgs mass as a function of the top quark mass one obtains by requiring that the standard model vacuum be stable. It included all higher order corrections to two-loops, precise definitions of the Higgs and top masses, etc. Unfortunately, the results were given for top quark masses between 120 and 160 GeV. In this addendum, I give the results for masses between 160 and 190 GeV. The main result, using a value of 0.117 for the strong coupling at the Z-mass, is M_Higgs > 132 + 2.2 (M_top-170) in GeV units. This increases (decreases) by 4.5 GeV if the strong coupling decreases (increases) by 0.007, and is accurate to 1 GeV in the top mass. This will be submitted to Phys. Lett. as an addendum.

hep-ph↗

arXiv1994

Photoproduction of Very Light Gluinos

Current experiments allow the possibility of gluino masses below about 600 MeV if the lifetime of the gluino is longer than 100 picoseconds. If the mass and lifetime are in this window, then photoproduction of pairs of gluino-gluon bound states can provide a means to observe them. The cross section is large enough that the window can be fully explored, up to lifetimes exceeding a microsecond, at high luminosity electron accelerators.

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