arXiv subjects

Marc Sher

Publications and source records attributed to Marc Sher.

At least 73 records · Page 4Linked to original sources

arXiv2004

Universal Extra Dimensions and Kaluza Klein Bound States

We study the bound states of the Kaluza-Klein (KK) excitations of quarks in certain models of Universal Extra Dimensions. Such bound states may be detected at future lepton colliders in the cross section for the pair production of KK-quarks near threshold. For typical values of model parameters, we find that "KK-quarkonia" have widths in the 10 - 100 MeV range, and production cross sections of order a few picobarns for the lightest resonances. Two body decays of the constituent KK-quarks lead to distinctive experimental signatures. We point out that such KK resonances may be discovered before any of the higher KK modes.

hep-ph↗

arXiv2003

Mu + N --> Tau + N at a Muon or Neutrino Factory

The experimental discovery of large nu_mu-nu_tau mixing indicates that analogous mixing in the charged lepton sector could be substantial. We consider the possibility that if a high intensity muon beam, perhaps at the early stages of a muon or neutrino factory, strikes a nuclear target, then conversion of some of the muons into tau leptons could occur (similar to the conversion of muons to electrons at MECO). Using current experimental limits on rare tau decays to bound the size of the relevant operators, we find that a 50 GeV muon beam, with 10^20 muons on target per year, could yield as many as 10^7 mu + N --> tau + N events per year. Backgrounds could be substantial, and we comment on the possibility of detection of this process.

hep-ph↗

arXiv2003

Tau-Mu Flavor Violation and the Scale of New Physics

Motivated by the strong experimental evidence of large nu_mu - nu_tau neutrino oscillations, we study existing constraints for related mu - tau flavor violation. Using a general bottom-up approach, we construct dimension-6 effective fermionic operators whose coefficients encode the scale of new physics associated with mu - tau flavor violation, which is a piece in the puzzle of the origin of neutrino oscillations. We survey existing experimental bounds on this scale, which arise mostly from tau and B decays. In many cases the new physics scale is constrained to be above a few TeV. We also discuss the operators which are either weakly constrained or, at present, subject to no experimental bounds.

hep-ph↗

arXiv2003

Probing the Light Pseudoscalar Window

Very light pseudoscalars can arise from the symmetry-breaking sector in many extensions of the Standard Model. If their mass is below 200 MeV, they can be long-lived and have interesting phenomenology. We discuss the experimental constraints on several models with light pseudoscalars, including one in which the pseudoscalar is naturally fermiophobic. Taking into account the stringent bounds from rare K and B decays, we find allowed parameter space in each model that may be accessible in direct production experiments. In particular, we study the photoproduction of light pseudoscalars at Jefferson Lab and conclude that a beam dump experiment could explore some of the allowed parameter space of these models.

hep-ph↗

arXiv2002

Tau Decays to Mu + Eta in Supersymmetric Models

The existence of large ν_μ-ν_τmixing suggests the likelihood of large smuon-stau mixing in supersymmetric models, leading to μand τnumber violation. In addition to interesting signatures in slepton and neutralino production and decay, this will lead to rare τdecays, such as τ--> mu γ. Recently, it has been pointed out that the τ--> 3μbranching ratio could be substantial in the large \tanβregion of parameter space, due to an induced μ-τ-Higgs vertex. In this paper, another signature, τ--> μηis considered. In the large \tanβregion, it is shown that the branching ratio of τ--> μηis 8.4 times the branching ratio of τ--> 3μ, independent of any unknown parameters, and it will thus give the most stringent bound on Higgs-mediated lepton flavor violation, and may provide its first signature. In the other regions of parameter space, where τ--> μγis the most prominent decay, the branching ratio for τ--> μηis always substantially lower.

hep-ph↗

arXiv2002

τ- μFlavor Violation as a Probe of the Scale of New Physics

Motivated by the recent strong experimental evidence of large ν_mu-ν_tau neutrino mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving tau-mu mixing with typical Lorentz structure (\bar{mu} Γtau)(\bar{q}^a Γ{q}^b), and discuss their relationship to the standard model gauge symmetry and the underlying flavor dynamics. We derive the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of tau, hadrons or heavy quarks. For operators involving at least one light quark (u,d,s), these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at the present, giving bounds of a few hundred GeV. A few scalar and pseudo-scalar operators are free from all current experimental constraints.

hep-ph↗

arXiv2002

Large Electric Dipole Moments of Heavy Neutrinos

In many models of CP violation, the electric dipole moment (EDM) of a heavy charged or neutral lepton could be very large. We present an explicit model in which a heavy neutrino EDM can be as large as $10^{-16}$ e-cm, or even a factor of ten larger if fine-tuning is allowed, and use an effective field theory argument to show that this result is fairly robust. We then look at the production cross section for these neutrinos, and by rederiving the Bethe-Block formula, show that they could leave an ionization track. It is then noted that the first signature of heavy neutrinos with a large EDM would come from $e^+e^-\to \bar{N}Nγ$, leading to a very large rate for single photon plus missing energy events, and the rate and angular distribution are found. Finally, we look at some astrophysical consequences, including whether these neutrinos could constitute the UHE cosmic rays and whether their decays in the early universe could generate a net lepton asymmetry.

hep-ph↗

arXiv2001

Large Electric Dipole Moments of Heavy Leptons

In many models of CP violation, the electric dipole moments (EDMs) of leptons scale as the cube of the lepton mass. In these models, the EDM of a 100 GeV heavy lepton would be a billion times greater than that of the muon, and could be as large as a 0.01 e-fermi. In other models, in which the heavy leptons have different properties from the lighter generations, a similarly large EDM can be obtained. A large EDM could dominate the electromagnetic properties of heavy leptons. The angular distribution and production cross-section of both charged and neutral heavy leptons with large dipole moments is calculated and discussed. The interesting possibility that a heavy neutrino with a large EDM could leave an ionization track in a drift chamber is investigated.

hep-ph↗

arXiv2001

Search for t->ch at e+e- Linear Colliders

We study the rare top-quark decay t->ch, where h is a generic Higgs boson, at a linear collider. If kinematically accessible, all models contain this decay at some level due to quark flavor mixing. Some models, such as Model III of the two-Higgs doublet model, have a tree-level top-charm-Higgs coupling, and the branching ratio is close to 0.5%. Others, such as the MSSM, have a coupling induced at one-loop, and can have a branching ratio in the range of 10^{-5} - 5x10^{-4}. We find that a linear collider of root(s) = 500 GeV and a luminosity of 500 fb^{-1} will begin to be sensitive to this range of the coupling.

hep-ph↗

arXiv2001

Extra neutral gauge bosons and Higgs bosons in an E_6-based model

Extra neutral gauge bosons and Higgs bosons in an effective low-energy $SU(2)_L \times SU(2)_I \times U(1)_Y \times U(1)_{Y^{\prime}}$ model, which is a subgroup of $E_6$, are studied. $SU(2)_I$ is a subgroup of $SU(3)_R$ and commutes with the electric charge operator, so the three corresponding gauge bosons are neutral. Electroweak precision experiments are used to put constraints on masses of the extra neutral gauge bosons and on the mixings between them and the ordinary Z boson, including constraints arising from a proposed measurment of the weak charge of the proton at Jefferson Lab. Bounds on and relationships of masses of Higgs bosons in the supersymmetric version of the model are also discussed.

hep-ph↗

arXiv2000

t-channel production of heavy charged leptons

We study the pair production of heavy charged exotic leptons at e+ e- colliders in the SU(2)_L x SU(2)_I x U(1)_Y model. This gauge group is a subgroup of the grand unification group E6; SU(2)_I commutes with the electric charge operator, and the three corresponding gauge bosons are electrically neutral. In addition to the standard photon and Z boson contributions, we also include the contributions from extra neutral gauge bosons. A t-channel contribution due to W_I-boson exchange, which is unsuppressed by mixing angles, is quite important. We calculate the left-right and forward-backward asymmetries, and discuss how to differentiate different models.

hep-ph↗

arXiv2000

Generations of Higgs Bosons in Supersymmetric Models

We examine extensions of the MSSM with more than one generation of Higgs bosons. If one assumes that a symmetry eliminates the tree-level FCNC, then the extra scalar bosons do not acquire VEVs, do not couple to fermions and do not mix with the ordinary Higgs bosons; the lightest is absolutely stable. The two lightest neutral scalars, $ϕ_S$, and $ϕ_P$, are degenerate in mass, and the mass difference between those and the lightest charged scalar, $ϕ_+$, is calculated. For most of the parameter space, the charged scalar is between a hundred MeV and a few GeV heavier than the neutral scalars. The $ϕ_+$ will decay at the vertex; the signature for this decay will be like that of a chargino with a nearly degenerate undetected neutralino. Next, the possibility that the symmetry that eliminates FCNC is a flavor symmetry is discussed. In an example, the U(2) model, tree-level FCNC processes can be calculated in terms of quark masses. The strongest constraint on this model is from $D-\bar{D}$ mixing, which should be within an order of magnitude of the current bound.

hep-ph↗

arXiv2000

Scalar Mediated FCNC at the First Muon Collider

In the most general two-Higgs doublet model (generally referred to as Model III), tree level flavor changing neutral couplings exist. It has been noted that the most natural value for such a coupling is of the order of the geometric mean of the Yukawa couplings of the two fermions. Bounds on those couplings that involve the second and third generations, from $τ,B,D$ and $μ$ physics, are very weak and are not sensitive to this "natural" value. In this paper, it is pointed out that the process μμ---> μτ, at a muon collider tuned to the scalar resonance, will easily reach this sensitivity if the scalar mass is below 140 GeV. Hundreds of events are expected for an integrated luminosity of an inverse femtobarn, and there appears to be no background. Failure to observe this process, if the scalar is below 140 GeV, would effectively rule out Model III.

hep-ph↗

arXiv1999

Quarks and Leptons Beyond the Third Generation

The possibility of additional quarks and leptons beyond the three generations already established is discussed. The make-up of this Report is (I) Introduction: the motivations for believing that the present litany of elementary fermions is not complete; (II) Quantum Numbers: possible assignments for additional fermions; (III) Masses and Mixing Angles: mass limits from precision electroweak data, vacuum stability and perturbative gauge unification; empirical constraints on mixing angles; (IV) Lifetimes and Decay Modes: their dependence on the mass spectrum and mixing angles of the additional quarks and leptons; the possibility of exceptionally long lifetimes; (V) Dynamical Symmetry Breaking: the significance of the top quark and other heavy fermions for alternatives to the elementary Higgs Boson; (VI) CP Violation: extensions to more generations and how strong CP may be solved by additional quarks; (VII) Experimental Searches: present status and future prospects; (VIII) Conclusions.

hep-ph↗

arXiv1999

Fourth Generation b-prime decays into b + Higgs

If a fourth generation quark exists whose mass is below 255 GeV, then the only two-body charged current decay, b'-->cW, is doubly-Cabibbo suppressed. For this reason, CDF has searched for the one-loop neutral current decay b' --> bZ, assuming that the branching ratio into bZ is 100%; an analysis giving the bounds on m_b' for smaller branching ratios is in preparation. In this Report, we examine the neutral current decay b' --> bH, which will occur if the Higgs mass is less than m_b'-m_b. Four different cases are examined: the sequential case, the non-chiral isosinglet case, the non-chiral isodoublet case, and a two-Higgs model with flavor-changing neutral currents. In the first three of these, the rates for b' --> bZ and b' --> bH are comparable, assuming comparable phase space factors; in the fourth, b' --> bH is completely dominant. Thus, we emphasize the importance of giving b' mass bounds as a function of the branching ratio into bZ, since the assumption of a 100% branching ratio for b' --> bZ may only be valid if the Higgs mass is near or above the b' mass.

hep-ph↗

arXiv1999

The Triple-Alpha Process and the Anthropically Allowed Values of the Weak Scale

In multiple-universe models, the constants of nature may have different values in different universes. Agrawal, Barr, Donoghue and Seckel have pointed out that the Higgs mass parameter, as the only dimensionful parameter of the standard model, is of particular interest. By considering a range of values of this parameter, they showed that the Higgs vacuum expectation value must have a magnitude less than 5.0 times its observed value, in order for complex elements, and thus life, to form. In this report, we look at the effects of the Higgs mass parameter on the triple-alpha process in stars. This process, which is greatly enhanced by a resonance in Carbon-12, is responsible for virtually all of the carbon production in the universe. We find that the Higgs vacuum expectation value must have a magnitude greater than 0.90 times its observed value in order for an appreciable amount of carbon to form, thus significantly narrowing the allowed region of Agrawal et al.

hep-ph↗

arXiv1998

Vacuum Stability Bounds in the Two-Higgs Doublet Model

In the standard model, the requirements of vacuum stability and the validity of perturbation theory up to the unification scale force the mass of the Higgs boson to be approximately between 130 GeV and 180 GeV. We re-examine these requirements in the (non-supersymmetric) two-Higgs doublet model, in the light of the large top quark mass, and constrain the masses of the Higgs bosons in this model. It is found that the mass of the charged Higgs boson must be lighter than 150 GeV. This bound is below the lower bound in the popular model-II two-Higgs doublet model, and thus we conclude that this model cannot be valid up to the unification scale. The bounds on the neutral Higgs scalars are also discussed.

hep-ph↗

arXiv1998

Scalar-Mediated Flavor-Changing Neutral Currents

The simplest extension of the standard model involves adding a scalar doublet--the so-called two-Higgs model. In general, the additional scalar will mediate tree-level flavor-changing neutral currents. Although one can arbitrarily impose a discrete symmetry to avoid these, it isn't necessary to do so; reasonable assumptions about the size of the flavor-changing couplings can make them sufficiently small as to avoid problems in the kaon sector. However, these same assumptions give much larger effects in the third-family case. We discuss the model, the "reasonable assumptions" on the size of the couplings, and examine phenomenological bounds on the couplings, showing that the most promising signatures are from $B_s --> μτ$, $μ--> eγ$, etc. We then include the newest result which shows potentially significant effects on the anomalous magnetic moment of the muon.

hep-ph↗