StandardSU(5) supergravity grand unification model and predictions

A review is given of the StandardSU(5) supergravity model. This model has passed an important check regarding unification of the electro-weak and the strong couplings using high precision LEP data. It is shown that for a significant domain of the parameter space the model also satisfies constraints on the SUSY spectrum from CDF and LEP, as well as proton stability and cosmological relic density constraints.     

Should we still believe in constrained supersymmetry?

We calculate partial Bayes factors to quantify how the feasibility of the constrained minimal supersymmetric standard model (CMSSM) has changed in the light of a series of observations. This is done in the Bayesian spirit where probability reflects a degree of belief in a proposition and Bayes’ theorem tells us how to update it after acquiring new information. Our experimental baseline is the approximate knowledge that was available before LEP, and our comparison model is the Standard Model with a simple dark matter candidate. To quantify the amount by which experiments have altered our relative belief in the CMSSM since the baseline data we compute the partial Bayes factors that arise from learning in sequence the LEP Higgs constraints, the XENON100 dark matter constraints, the 2011 LHC supersymmetry search results, and the early 2012 LHC Higgs search results. We find that LEP and the LHC strongly shatter our trust in the CMSSM (with M ₀ and M _1/2 below 2 TeV), reducing its posterior odds by approximately two orders of magnitude. This reduction is largely due to substantial Occam factors induced by the LEP and LHC Higgs searches.     

Signals of spontaneous R parity breaking at LEP and at a linear collider

We study the production of neutralinos and charginos at LEP and at a linear collider in the case of spontaneously broken R-parity. We first investigate the constraints on the single neutralino and chargino production from the LEP 1 experiments, and then we consider the production at LEP 2 and at a linear collider. We concentrate on the supersymmetric model, where the spontaneous R-parity breaking is inevitable and is associated with the breaking of the LR-symmetry. Received: 26 February 1998 / Published online: 9 April 1998     

Brane vector phenomenology

Local oscillations of the brane world are manifested as massive vector fields. Their coupling to the Standard Model can be obtained using the method of nonlinear realizations of the spontaneously broken higher-dimensional space–time symmetries, and to an extent, are model independent. Phenomenological limits on these vector field parameters are obtained using LEP collider data and dark matter constraints.     

The contribution of Bc mesons to the search for B → τ ντ decays at LEP

We study the contribution of B_c mesons to the search for B → τν_τ decays. We find that at LEP the contributions from B_u and B_c mesons can be comparable. This observation can have a relevant impact on the extraction of constraints on new physics (such as charged-Higgs contributions) from current LEP limits on B → τν final states. Inclusion of the B_c contribution can reduce the current L3 limit on tan β/M_H from 0.38 GeV⁻¹ (90% CL) down to 0.27 GeV⁻¹ (90% CL).     

Unitarity constraints on the electroweak three vector boson vertices

Partial wave unitarity applied to fermion-pair annihilation into WZ, Wγ or W⁺W⁻ severely limits any deviation of the ZWW and γWW three vector boson vertices from the standard model prediction. These constraints are quantified and experimental consequences, in particular for W-pair production at LEP, are discussed.     

Leptoquark signal from global analysis

Data from HERA, LEP and the Tevatron as well as from low energy experiments are used to constrain the Yukawa couplings for scalar and vector leptoquarks in the Buchmüller–Rückl–Wyler effective model. In the limit of very high leptoquark masses constraints on the coupling to the mass ratio are derived using the contact-interaction approximation. For finite masses the coupling limits are studied as a function of the leptoquark mass. Some leptoquark models are found to describe the existing experimental data much better than the standard model. The increase in the global probability observed for models including or leptoquark production/exchange corresponds to an effect of more than . Assuming that a real leptoquark signal is observed, the allowed region in the –M plane is calculated. The leptoquark signal mostly results from the new data on the atomic parity violation in cesium, but is also supported by recent LEP2 measurements, unitarity violation in the CKM matrix and HERA high- results. Received: 21 June 2000 / Revised version: 31 August 2000 / Published online: 23 October 2000     

Process Z{\rightarrow}h(A)+\gamma in the 2HDM and the experimental constraints from LEP

The one-loop branching ratios for the process are calculated in the general two Higgs doublet model (Model II), taking into account existing constraints on the model parameters. For Higgs boson masses below 50 GeV and of the fraction of such Z decays are at the level of , but can be significantly stronger for very low or high , where the dependence of these results on other model parameters like and the mass of the charged Higgs boson is found to be of little importance. The results are compared to the LEP measurements, which are sensitive to branching ratios of of the order for masses GeV, but approach for low masses. By relating the expectation to the experimental limits, constraints on the parameter space of the 2HDM are derived. Received: 28 November 1998 / Published online: 27 April 1999     

Can the minimal supersymmetric standard model with light bottom squark and light gluino survive Z-peak constraints?

In the framework of the minimal supersymmetric model we examine the Z-peak constraints on the scenario of one light bottom squark (sbottom) ( approximately 2-5.5 GeV) and light gluino (approximately 12-16 GeV), which has been successfully used to explain the excess of bottom quark production in hadron collisions. Such a scenario is found to be severely constrained by the CERN LEP Z-peak observables, especially by R(b), due to the large effect of gluino-sbottom loops. To account for the R(b) data in this scenario, the other mass eigenstate of sbottom, i.e., the heavier one, must be lighter than 125 (195) GeV at 2sigma(3sigma) level, which, however, is disfavored by CERN LEP II experiments.     

The Process e+e-→tc in Technicolor Model with a Massless Scalar Doublet

At LEPⅡ (the energy: √s = 200 GeV), we carry out precise calculations on the process e⁺e^-→tc in the framework of technicolor model with a massless scalar doublet. The results show that the process is unmeasurably small in this TC model.     

All possible lightest supersymmetric particles in R-parity violating minimal supergravity models

We investigate, which lightest supersymmetric particles can be obtained via a non-vanishing lepton- or baryon-number violating operator at the grand unification scale within the R-parity violating minimal supergravity model. We employ the full one-loop renormalization group equations. We take into account restrictions from the anomalous magnetic moment of the muon and b→sγ$b\to s\gamma$, as well as collider constraints from LEP and the Tevatron. We also consider simple deformations of minimal supergravity models.     

Constraints on SUSY models from LEP

We consider a particular supersymmetric extension of the standard model involving a light singlet and explainingM _Weak?M _Planck naturally, without detailed assumptions about a GUT or supergravity sector. Imposingm _cl>45 GeV for the lightest chargino andm _H1>20 GeV for the lightest Higgs scalar, the model survives all other constraints due to recent LEP results; it predicts, however, supersymmetric and Higgs particles to be seen in the near future.     

Tests of the Electroweak Theory at LEP

LEP offers a rich choice of tests of the electroweak theory such as the measurement of hadronic and leptonic cross sections, leptonic forward-backward asymmetries, τ polarization asymmetries, partial widths and forward-backward asymmetries of heavy quark flavours, of the inclusive qq charge asymmetry and of final state radiation in hadronic events. We discuss experimental aspects of these measurements and their theoretical parametrization and summarize the results available so far. We present several analyses which reveal specific aspects of the results, such as their constraints on Standard Model parameters and on new particles, the sensitivity to deviations from the Standard Model multiplet structure and an analysis in a framework which provides a model independent search for new physics.     

The b \to X_s \gamma rate and Higgs boson limits in the constrained minimal supersymmetric model

New NLO calculations have become available using resummed radiative corrections. Using these calculations we perform a global fit of the supergravity inspired constrained minimal supersymmetric model. We find that the resummed calculations show similar constraints as the LO calculations, namely that only with a relatively heavy supersymmetric mass spectrum of (1 TeV) the b– Yukawa unification and the rate can coexist in the large scenario. The resummed calculations are found to reduce the renormalization scale uncertainty considerably. The low scenario is excluded by the present Higgs limits from LEP II. The constraint from the Higgs limit in the plane is severe, if the trilinear coupling at the GUT scale is fixed to zero, but is considerably reduced for . The relatively heavy SUSY spectrum required by corresponds to a Higgs mass of GeV in the CMSSM. Received: 14 February 2001 / Revised version: 22 March 2001 / Published online: 29 June 2001     

Search for SUSY in the AMSB scenario with the DELPHI detector

The DELPHI experiment at the LEP e ⁺ e^- collider collected almost 700 pb^-1 at centre-of-mass energies above the Z⁰ mass pole and up to 208 GeV. Those data were used to search for SUSY in the Anomaly Mediated SUSY Breaking (AMSB) scenario with a flavour independent common sfermion mass parameter. The searches covered several possible signatures experimentally accessible at LEP, with either the neutralino, the sneutrino or the stau being the Lightest Supersymmetric Particle (LSP). They included: the search for nearly mass-degenerate chargino and neutralino, which is a typical feature of AMSB; the search for Standard-Model-like or invisibly decaying Higgs boson; the search for stable staus; the search for cascade decays of SUSY particles resulting in the LSP and a low multiplicity final state containing neutrinos. No evidence of a signal was found, and thus constraints were set in the space of the parameters of the model.Received: 3 September 2003, Revised: 3 February 2004, Published online: 9 March 2004     

Dark light-Higgs bosons

We study a limit of the nearly Peccei-Quinn-symmetric next-to-minimal supersymmetric standard model possessing novel Higgs and dark matter (DM) properties. In this scenario, there naturally coexist three light singletlike particles: a scalar, a pseudoscalar, and a singlinolike DM candidate, all with masses of order 0.1-10 GeV. The decay of a standard model-like Higgs boson to pairs of the light scalars or pseudoscalars is generically suppressed, avoiding constraints from collider searches for these channels. For a certain parameter window annihilation into the light pseudoscalar and exchange of the light scalar with nucleons allow the singlino to achieve the correct relic density and a large direct-detection cross section consistent with the DM direct-detection experiments, CoGeNT and DAMA/LIBRA, preferred region simultaneously. This parameter space is consistent with experimental constraints from LEP, the Tevatron, Υ, and flavor physics.     

Explaining DAMPE results by dark matter with hierarchical lepton-specific Yukawa interactions

We propose to interpret the DAMPE electron excess at 1.5 Te V through scalar or Dirac fermion dark matter(DM) annihilation with doubly charged scalar mediators that have lepton-specific Yukawa couplings. The hierarchy of such lepton-specific Yukawa couplings is generated through the Froggatt-Nielsen mechanism, so that the dark matter annihilation products can be dominantly electrons. Stringent constraints from LEP2 on intermediate vector boson production can be evaded in our scenarios. In the case of scalar DM, we discuss one scenario with DM annihilating directly to leptons and another scenario with DM annihilating to scalar mediators followed by their decays. We also discuss the Breit-Wigner resonant enhancement and the Sommerfeld enhancement in the case where the s-wave annihilation process is small or helicity-suppressed. With both types of enhancement, constraints on the parameters can be relaxed and new ways for model building can be opened in explaining the DAMPE results.     

Studying the D ∗ K molecular structure of D^{+}_{s1}(2460) in the Bethe–Salpeter approach

To explain both the possible superluminal neutrino propagation and all the known phenomenological constraints/observations on Lorentz violation, the Background-Dependent Lorentz Violation (BDLV) has been proposed. We study the BDLV in a model-independent way. Assuming that the Lorentz violation on the Earth is much larger than those on the interstellar scale, we automatically escape all the astrophysical constraints on Lorentz violation. For the BDLV from the effective field theory, we present a simple model and discuss the possible solutions to the theoretical challenges of the superluminal neutrino propagation such as the Bremsstrahlung effects for muon neutrinos and the pion decays. Also, we address the Lorentz violation constraints from the LEP and KamLAMD experiments. For the BDLV from the Type IIB string theory with D3-branes and D7-branes, we point out that the D3-branes are flavour blind, and all the SM particles are the conventional particles as in the traditional SM when they do not interact with the D3-branes. Thus, we not only can naturally avoid all the known phenomenological constraints on Lorentz violation, but also can naturally explain all the theoretical challenges. Interestingly, the energy-dependent photon velocities may be tested at the experiments.