arXiv subjects

Marc Sher

Publications and source records attributed to Marc Sher.

At least 55 records · Page 3Linked to original sources

arXiv2009

Higgs Masses in the Four Generation MSSM

In the Minimal Supersymmetric Standard Model (MSSM) with three generations of fermions, there is a stringent upper bound on the mass of the lightest neutral Higgs h, and the mass of the charged Higgs H^+, must be close (within tens of GeV) to the heavier neutral Higgs, H, and the pseudoscalar Higgs A. In this Brief Report, we show that in the four generation MSSM, the upper bound on the h mass is much higher, as high as 400 GeV, and H^+ is generally much heavier than the A, allowing the H^+ -> A W^+ decay, potentially changing search strategies for both the charged Higgs and the pseudoscalar. The H mass, on the other hand, remains within tens of GeV of the charged Higgs mass.

hep-ph↗

arXiv2009

The Fourth Generation t-prime in Extensions of the Standard Model

We study the effects of a fourth generation t' quark in various extensions of the standard model. In the Randall-Sundrum model, the decay t'--> t Z has a large branching ratio that could be detected at the Large Hadron Collider (LHC). We also look at the two-Higgs doublet models I, II and III, and note that, in the latter, the branching ratio of t' --> t H, where H is a Higgs scalar or pseudoscalar, is huge and we discuss detection at the LHC. A few comments about the minimal supersymmetric standard model are also included.

hep-ph↗

arXiv2008

Production of Single Heavy Charged Leptons at a Linear Collider

A sequential fourth generation of quarks and leptons is allowed by precision electroweak constraints if the mass splitting between the heavy quarks is between 50 and 80 GeV. Although heavy quarks can be easily detected at the LHC, it is very difficult to detect a sequential heavy charged lepton, L, due to large backgrounds. Should the L mass be above 250 GeV, it can not be pair-produced at a 500 GeV ILC. We calculate the cross section for the one-loop process e+e- -> L tau. Although the cross section is small, it may be detectable. We also consider contributions from the two Higgs doublet model and the Randall-Sundrum model, in which case the cross section can be substantially higher.

hep-ph↗

arXiv2008

Single Production of Heavy Charged Leptons at the ILC

This paper has been withdrawn by the authors due to a coding error and a missed diagram. The calculation has been completely redone, with very different results, and with an additional author, and will be submitted to the arxiv shortly.

hep-ph↗

arXiv2008

Extra Gauge Invariance from an Extra Dimension

We describe higher-dimensional theories whose low-energy 4D descriptions contain larger gauge or global symmetry groups. As an example, we construct a Higgsless SU(2) x U(1) model of electroweak symmetry breaking. The 5D SU(2) gauge invariance contains both the weak SU(2) gauge group and a custodial symmetry that protects the rho parameter. Fermions obtain isospin-violating masses while maintaining universal gauge couplings among all three generations. As a further example, we construct a model of chiral color based on a single SU(3).

hep-ph↗

arXiv2008

Experimental constraints on fourth generation quark masses

The existing bounds from CDF on the masses of the fourth generation quarks, t' and b', are reexamined. The bound of 256 GeV on the t' mass assumes that the primary decay of the t' is into q+W, which is not the case for a substantial region of parameter space. The bound of 268 GeV on the b' mass assumes that the branching ratio for b' --> b+Z is very large, which is not only not true for much of parameter space, but is {\em never} true for b' masses above 255 GeV. In addition, it is assumed that the heavy quarks decay within the silicon vertex detector, and for small mixing angles this will not be the case. The experimental bounds, including all of these effects, are found as a function of the other heavy quark mass and the mixing angle.

hep-ph↗

arXiv2007

Holographic Electroweak Symmetry Breaking from D-branes

We observe several interesting phenomena in a technicolor-like model of electroweak symmetry breaking based on the D4-D8-D8bar system of Sakai and Sugimoto. The benefit of holographic models based on D-brane configurations is that both sides of the holographic duality are well understood. We find that the lightest technicolor resonances contribute negatively to the Peskin-Takeuchi S-parameter, but heavy resonances do not decouple and lead generically to large, positive values of S, consistent with standard estimates in QCD-like theories. We study how the S parameter and the masses and decay constants of the vector and axial-vector techni-resonances vary over a one-parameter family of D8-brane configurations. We discuss possibilities for the consistent truncation of the theory to the first few resonances and suggest some generic predictions of stringy holographic technicolor models.

hep-th↗

arXiv2006

New Bounds on Isotropic Lorentz Violation

Violations of Lorentz invariance that appear via operators of dimension four or less are completely parameterized in the Standard Model Extension (SME). In the pure photonic sector of the SME, there are nineteen dimensionless, Lorentz-violating parameters. Eighteen of these have experimental upper bounds ranging between 10^{-11} and 10^{-32}; the remaining parameter, k_tr, is isotropic and has a much weaker bound of order 10^{-4}. In this Brief Report, we point out that k_tr gives a significant contribution to the anomalous magnetic moment of the electron and find a new upper bound of order 10^{-8}. With reasonable assumptions, we further show that this bound may be improved to 10^{-14} by considering the renormalization of other Lorentz-violating parameters that are more tightly constrained. Using similar renormalization arguments, we also estimate bounds on Lorentz violating parameters in the pure gluonic sector of QCD.

hep-ph↗

arXiv2006

Top Pair Production in Randall-Sundrum Models

If the lowest lying Kaluza-Klein states in Randall-Sundrum (RS1) models have masses in the 10-100 TeV range, direct production of these states at the LHC or ILC is impossible, and electroweak precision measurements may not be sufficiently sensitive. We address the possibility that high-precision measurements of top pair production at the ILC may provide the first evidence of these states. We consider RS1 models with fermions on and off the brane, with bulk left and right handed mass terms, discuss brane kinetic terms and calculate corrections to top pair production in these models.

hep-ph↗

arXiv2006

Implementation of the Multiple Point Principle in the Two-Higgs Doublet Model of type II

The multiple point principle (MPP) is applied to the non--supersymmetric two-Higgs doublet extension of the Standard Model (SM). The existence of a large set of degenerate vacua at some high energy scale caused by the MPP results in a few relations between Higgs self-coupling constants which can be examined at future colliders. The numerical analysis reveals that these MPP conditions constrain the mass of the SM--like Higgs boson to lie below 180 GeV for a wide set of MPP scales $Λ$ and $\tanβ$.

hep-ph↗

arXiv2005

3-3-1 Models with Unique Lepton Generations

We study previously unconsidered 3-3-1 models which are characterized by each lepton generation having a different representation under the gauge group. Flavor-changing neutral currents in the lepton sector occur in these models. To satisfy constraints on mu to 3e decays, the Z' must be heavier than 2 to 40 TeV, depending on the model and assignments of the leptons. These models can result in very unusual Higgs decay modes. In most cases the mu-tau decay state is large (in one case, it is the dominant mode), and in one case, the Higgs to s-sbar rate dominates.

hep-ph↗

arXiv2005

Experimentally Probing the Shape of Extra Dimensions

In brane world scenarios in which only gravity can propagate in the extra dimensions, effects on the gravitational force may be experimentally testable if there are two or three large extra dimensions. The strength of the force at distances smaller than the compactification radius will be sensitive to the volume of the extra dimensions, but the determination of the shape requires knowing the gravitational potential at intermediate scales. We determine the dependence of the potential vs. distance as a function of both the relative size of the extra dimensions and the possible angle between the extra dimensional unit vectors, and show that high precision measurements of the gravitational force will allow the determination of the shape of the extra dimensions.

hep-ph↗

arXiv2005

Kaluza-Klein Mesons in Universal Extra Dimensions

In models with universal extra dimensions, the isosinglet Kaluza-Klein (KK) quarks, q^1, have very narrow widths, of O(5-10) MeV, and will thus hadronize. Studies of KK-quarkonia, \bar{q}^1 q^1, show very sharp resonances and dramatic signatures at the Linear Collider. In this Brief Report, we consider the possibility of detecting KK-mesons, \bar{q}^1 q, and show that detection at a Linear Collider is unlikely.

hep-ph↗

arXiv2005

Neutrino Masses in the Effective Rank-5 Subgroups of E_6 I: Non-supersymmetric Case

The neutral fermion sectors of E_6-inspired low energy models, in particular the Alternative Left-Right and Inert models, are considered in detail within the non-supersymmetric scenario. We show that in their simplest form, these models always predict, for each generation, the lightest neutrino to be an SU(2)_L singlet, as well as two extra neutrinos with masses of the order of the up-quark mass. In order to recover Standard Model phenomenology, additional assumptions in the form of discrete symmetries and/or new interactions are needed. These are classified as the Discrete Symmetry (DS), Higher Dimensional Operators (HDO), and Additional Neutral Fermion (ANF) methods. The DS method can solve the problem, but requires additional Higgs doublets that do not get vacuum expectation values. The HDO method predicts no sterile neutrino, and that the active neutrinos mix with a heavy isodoublet neutrino, thus slightly suppressing the couplings of active neutrinos, with interesting phenomenological implications. The ANF method also predicts this suppression, and also naturally includes one or more "sterile" neutrinos. This scenario allows the existence of sterile neutrino(s) in either a 3+1 or 2+2 structure at low energies, which are favored by the LSND result.

hep-ph↗

arXiv2005

Neutrino Masses in the Effective Rank-5 Subgroups of E_6 II: Supersymmetric Case

We present a complete analysis of the neutral fermion sector of supersymmetric E_6-inspired low energy models containing an extra SU(2), concentrating on the Alternate Left-Right and Inert models. We show that the R-parity conserving scenario always exhibits a large Dirac mass for ν_L with maximal mixing with an isosinglet neutrino, and that R-parity violating scenarios do not change the picture other than allowing further mixing with another isosinglet. In order to recover Standard Model phenomenology, additional assumptions in the form of discrete symmetries and/or new interactions are needed. We introduce and investigate Discrete Symmetry method and Higher Dimensional Operators as mechanisms for solving the neutrino mass and mixing problems in these models.

hep-ph↗

arXiv2004

Universal Extra Dimensions and Kaluza-Klein Bound States

We study the bounds states of the Kaluza-Klein (KK) excitations of quarks in 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 the ``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.

hep-ph↗

arXiv2004

FCNC and Rare B Decays in 3-3-1 Models

An interesting extension of the Standard Model is based on the electroweak gauge group SU(3)_L x U(1). It requires three generations to cancel anomalies, treats the third generation differently than the first two, and has a rich phenomenology. There are several models, distinguished by the embedding of the charge operator into the SU(3)_L group and by the choice of fermion representations. In this Brief Report, we consider flavor-changing neutral currents in these models, concentrating on the P-Pbar mass difference, where P=(K, D, B, B_s), as well as B --> K l^+ l^-, B --> mu^+ mu^- and B_s --> mu^+ mu^- decays. Although the P-Pbar mass difference has been considered previously in some models, the rare B decays are new. We find that the strongest bounds come from the B-Bbar and Bs-Bs-bar mass differences.

hep-ph↗

arXiv2004

Lorentz and CPT Violation in the Higgs Sector

Colladay and Kostelecky have proposed a framework for studying Lorentz and CPT violation in a natural extension of the Standard Model. Although numerous bounds exist on the Lorentz and CPT violating parameters in the gauge boson and fermion sectors, there are no published bounds on the parameters in the Higgs sector. We determine these bounds. The bounds on the CPT-even asymmetric coefficients arise from the one-loop contributions to the photon propagator, those from the CPT-even symmetric coefficients arise from the equivalent c_{μν} coefficients in the fermion sector, and those from the CPT-odd coefficient arise from bounds on the vacuum expectation value of the Z-boson.

hep-ph↗