Search for R-parity violating decays of scalar fermions at LEP

A search for pair produced scalar fermions with couplings that violate R-parity has been performed using a data sample corresponding to an integrated luminosity of 56 pb at a centre-of-mass energy of 183 GeV collected with the OPAL detector at LEP. An important consequence of R-parity breaking interactions is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and stop quarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield multi-leptons, jets plus leptons or multi-jets, with or without missing energy, in the final state. No significant excess of such events has been observed. Limits on the production cross-sections of scalar fermions in R-parity violating scenarios are obtained. Mass exclusion regions are also presented in the framework of the Constrained Minimal Supersymmetric Standard Model. Received: 16 March 1999 / Published online: 14 October 1999     

Search for R-parity violating decays of sfermions at LEP

A search for pair-produced sfermions, the scalar supersymmetric partners of the Standard Model fermions, under the assumption that R-parity is not conserved has been performed using data collected with the OPAL detector at LEP. The data samples analysed correspond to an integrated luminosity of about 610 pb^-1 collected at centre-of-mass energies of GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle is expected to be unstable. Searches for R-parity violating decays of charged sleptons, sneutrinos and squarks have been performed under the assumptions that the lightest supersymmetric particle decays promptly and that only one of the R-parity violating couplings is dominant for each of the decay modes considered. Such processes would yield final states consisting of leptons, jets, or both, with or without missing energy. No significant signal-like excess of events has been observed with respect to the Standard Model expectations. Limits on the production cross-sections of sfermions in R-parity violating scenarios are obtained. Constraints on the supersymmetric particle masses are also presented in an R-parity violating framework analogous to the Constrained Minimal Supersymmetric Standard Model.Received: 25 June 2003, Published online: 13 February 2004     

Searches at HERA for squarks in R-parity violating supersymmetry

A search for squarks in R-parity violating supersymmetry is performed in collisions at HERA at a centre of mass energy of 300 GeV, using H1 data corresponding to an integrated luminosity of 37 pb. The direct production of single squarks of any generation in positron-quark fusion via a Yukawa coupling is considered, taking into account R-parity violating and conserving decays of the squarks. No significant deviation from the Standard Model expectation is found. The results are interpreted in terms of constraints within the Minimal Supersymmetric Standard Model (MSSM), the constrained MSSM and the minimal Supergravity model, and their sensitivity to the model parameters is studied in detail. For a Yukawa coupling of electromagnetic strength, squark masses below 260 GeV are excluded at 95% confidence level in a large part of the parameter space. For a 100 times smaller coupling strength masses up to 182 GeV are excluded. Received: 28 February 2001 / Published online: 29 June 2001     

Exploring the SUSY Higgs sector ate +e− linear colliders: a synopsis

We discuss the Higgs scenario in the minimal supersymmetric extension of the Standard Model ate ⁺e^? linear colliders operating in the c.m. energy range between 300 and 500 GeV. Besides decays of the Higgs particles into ordinary fermions and cascade decays, we analyze also decays into gaugino/Higgsinos and in particular, neutral Higgs decays into the lightest supersymmetric particles which are invisible ifR-parity is conserved. The cross sections for the various production channels of SUSY Higgs particles ine ⁺e^? collisions are discussed in detail. The lightest Higgs boson cannot escape detection, and in major parts of the MSSM parameter space all five Higgs particles can be observed.     

A supersymmetric extension of the standard model with bilinear R-parity violation

The minimum supersymmetric standard model with bilinear R-parity violation is studied systematically. As we consider low-energy supersymmetry, we examine the structure of the bilinear R-parity violating model carefully. We analyze the mixing of, e.g., Higgs bosons with sleptons, neutralinos with neutrinos and charginos with charged leptons in the model. Possible and some important physics results, e.g. that the lightest Higgs may be heavier than the weak Z-boson at tree level, are obtained. The Feynman rules for the model are derived in the 't Hooft–Feynman gauge, which is convenient if perturbative calculations are needed beyond the tree level. Received: 26 April 1999 / Revised version: 28 June 1999 / Published online: 16 November 1999     

Testing the mechanism for the LSP stability at the LHC

The lightest supersymmetric particle (LSP) is a natural candidate for the cold dark matter of the universe. In this Letter we discuss how to test the mechanism responsible for the LSP stability at the LHC. We note that if R-parity is conserved dynamically one should expect a Higgs boson which decays mainly into two right-handed neutrinos (a “leptonic” Higgs) or into two sfermions. The first case could exhibit spectacular lepton number violating signals with four secondary vertices due to the long-lived nature of right-handed neutrinos. These signals, together with the standard channels for the discovery of SUSY, could help to establish the underlying theory at the TeV scale.     

New physics effects from B meson decays

In this talk, we point out some of the present and future possible signatures of physics beyond the Standard Model from B-meson decays, taking R-parity conserving and violating supersymmetry as illustrative examples. An expanded version is available on hep-ph archive.     

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189 GeV at LEP

Cross-sections and angular distributions for hadronic and lepton pair final states in collisions at a centre-of-mass energy near 189 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a sneutrino in supersymmetric theories with R-parity violation. A search for the indirect effects of the gravitational interaction in extra dimensions on the and final states is also presented. Received: 9 July 1999 / Published online: 17 February 2000     

Search for an LSP gluino at LEP with the DELPHI detector

In some supersymmetric models, the gluino () is predicted to be light and stable. In that case, it would hadronize to form R-hadrons. In these models, the missing energy signature of the lightest supersymmetric particle is no longer valid, even if R-parity is conserved. Therefore, such a gluino is not constrained by hadron collider results, which looked for the decay . Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c² is excluded at the 95% confidence level in these models. Then, R-hadrons produced in the squark decays were searched for in the data collected by DELPHI at the centre-of-mass energies of 189 to 208 GeV, corresponding to an overall integrated luminosity of 609 Pb^-1. The observed number of events is in agreement with the Standard Model predictions. Limits at 95% confidence level are derived on the squark masses from the excluded regions in the plane () GeV/c², and GeV/c² for purely left squarks. GeV/c², and GeV/c² independent of the mixing angle. Received: 6 November 2002 / Published online: 15 January 2003     

One-prong \tau decays with kaons

Cross-sections for hadronic, and lepton pair final states in collisions at =183 GeV, measured with the OPAL detector at LEP, are presented and compared with the predictions of the Standard Model. Forward-backward asymmetries for the leptonic final states have also been measured. Cross-sections and asymmetries are also presented for data recorded in 1997 at =130 and 136 GeV. The results are used to measure the energy dependence of the electromagnetic coupling constant , and to place limits on new physics as described by four-fermion contact interactions or by the exchange of a new heavy particle such as a leptoquark, or of a squark or sneutrino in supersymmetric theories with R-parity violation. Received: 3 July 1998 / Published online: 15 October 1998     

Top squark and neutralino decays in a <Emphasis Type="Italic">R</Emphasis>-parity violating model constrained by neutrino oscillation data

In a R-parity violating (RPV) model of neutrino mass with three bilinear couplings μ _i and three trilinear couplings λ′ _i33, where i is the lepton index, we find six generic scenarios each with a distinctive pattern of the trilinear couplings consistent with the neutrino oscillation data. These patterns may be reflected in direct RPV decays of the lighter top squark or in the RPV decays of the lightest superparticle, assumed to be the lightest neutralino. Typical signal sizes at the Tevatron RUN II and the LHC have been estimated and the results turn out to be encouraging.      

Mass bounds on a very light neutralino

Within the Minimal Supersymmetric Standard Model (MSSM) we systematically investigate the bounds on the mass of the lightest neutralino. We allow for non-universal gaugino masses and thus even consider massless neutralinos, while assuming in general that R-parity is conserved. Our main focus is on laboratory constraints. We consider collider data, precision observables, and also rare meson decays to very light neutralinos. We then discuss the astrophysical and cosmological implications. We find that a massless neutralino is allowed by all existing experimental data and astrophysical and cosmological observations.     

Neutrino mass patterns, R-parity violating supersymmetry and associated phenomenology

Motivated by the recent super-Kamiokande results on atmospheric neutrinos, we incorporate massive neutrinos, with large angle oscillation between the second and third generations, in a theory with R-parity violating supersymmetry. The general features of such a theory are briefly reviewed. We emphasize its testability through the observation of comparable numbers of muons and taus, produced together with the W-boson, in decays of the lightest neutralino. A distinctly measurable decay gap is another remarkable feature of such a scenario.     

R_b and R_{\ell} in MSSM without R-parity

We examined and couplings in the minimal supersymmetric model with explicit trilinear R-parity violating interactions. We found that the top quark couplings and can give sizable contributions through top quark loops. When deriving the bounds from and data, we also take into account the loop contributions of R-parity conserving interactions. The bounds from are found to be stronger than those from and serve as the hitherto strongest bounds for some couplings. Received: 14 October 1999 / Revised version: 2 May 2001 / Published online: 8 June 2001     

The neutrinoless double-beta decay: A test for new physics

The neutrinoless double-beta decay is not allowed in the Standard Model (SM) but it is allowed in most Grand Unified Theories (GUTs). The neutrino must be a Majorana particle (identical with its antiparticle) and must have a mass to allow the neutrinoless double-beta decay. Apart of one claim that the neutrinoless double-beta decay in ⁷⁶Ge is measured, one has only upper limits for this transition probability. But even the upper limits allow to give upper limits for the electron Majorana neutrino mass and upper limits for parameters of GUTs and the minimal R-parity violating supersymmetric model. One further can give lower limits for the vector boson mediating mainly the right-handed weak interaction and the heavy mainly right-handed Majorana neutrino in left-right symmetric GUTs. For that, one has to assume that the specific mechanism is the leading one for the neutrinoless double-beta decay and one has to be able to calculate reliably the corresponding nuclear matrix elements. In the present contribution, one discusses the accuracy of the present status of calculating the nuclear matrix elements and the corresponding limits of GUTs and supersymmetric parameters.     

LHC (CMS) discovery potential for models with effective supersymmetry and nonuniversal gaugino masses

We investigate squark and gluino pair production at LHC (CMS) with subsequent decays into quarks, leptons, and the lightest supersymmetric particles (LSP) in models with effective supersymmetry, where the third generation of squarks is relatively light, whilst the first two generations of squarks are heavy. We consider the general case of nonuniversal gaugino masses. The visibility of a signal through an excess over Standard Model background in (n≥2) jets+(m≥0) leptons+E _T ^miss events depends rather strongly on the relation between the LSP, second-neutralino, gluino, and squark masses and decreases with increasing LSP mass. We find that, for a relatively heavy gluino, it is very difficult to detect a SUSY signal even for light third-generation squarks $(m_{\tilde q_3 } \leqslant 1TeV)$ if the LSP mass is close to the third-generation squark mass.     

Particle mass limits in minimal and nonminimal supersymmetric models

A review of the Higgs and neutralino sector of supersymmetric models is presented. This includes the upper limit on the mass of the lightest Higgs boson in the minimal supersymmetric standard model, as well as models based on the standard model gauge groupSU(2) _L xU(l) _Y with extended Higgs sectors. We then discuss the Higgs sector of left-right supersymmetric models, which conserveR-parity as a consequence of gauge invariance, and present a calculable upper bound on the mass of the lightest Higgs boson in these models. We also discuss the neutralino sector of general supersymmetric models based on the SM gauge group. We show that, as a consequence of gauge coupling unification, an upper bound on the mass of the lightest neutralino as a function of the gluino mass can be obtained.     

On the production of the chargino and W boson at the interaction of neutralino with high-energy photons

The process of the photoproduction of the chargino and W boson on nonrelativistic neutralinos has been considered within the supersymmetric Standard Model. It has been shown that the energy of the chargino in the limit of high energies of the initial photon approaches a constant limit and its decays are accompanied by the production of leptons with energies in a certain range whose width depends only on the masses of the chargino and neutralino. The importance of this process for the detection of dark matter in the observations of cosmic rays from nearest active galactic nuclei and in experiments at modern colliders has been discussed.     

Photon signatures for low energy supersymmetry breaking and broken R-parity

The possible phenomenological consequences of R-parity violating interactions in the framework of low energy supersymmetry breaking are studied. It is pointed out that even very weak R-parity violation would completely overshadow one of the basic signatures of low energy supersymmetry breaking models, that is the decay of the next to lightest supersymmetric particle into a photon (lepton) and missing energy. Thus, the observation of these decays would put very strong limits on R-parity violating couplings. Vice-versa, if R-parity violation is established experimentally, before a detailed knowledge of the spectrum is obtained, it will be very difficult to distinguish gravity mediated from low energy gauge mediated supersymmetry breaking scenarios. Those conclusions are very model independent. We also comment on the possibility of mixing between charged and neutral leptons with charginos and neutralinos, respectively, and its phenomenological consequences for the photon (lepton) signatures, in scenarios where this mixing is generated by the presence of bilinear or trilinear R-parity violating terms in the superpotential.     

Prospects of squark and slepton searches at a gamma–gamma collider

We examine the prospects of detecting sfermions at a gamma–gamma collider. Once produced, a slepton can decay into a pair of quarks (jets) through R-parity violating interactions. Similarly, a squark may decay into a lepton–quark pair. Analyzing the corresponding Standard Model backgrounds, namely 4-jet and dilepton plus dijet final states respectively, we show that the sfermion can be detected almost right upto the kinematic limit and its mass determined to a fair degree of accuracy. Similar statements also hold for nonsupersymmetric leptoquarks and diquarks.