Neutralino phenomenology at LEP2 in supersymmetry with bilinear breaking of R-parity

We discuss the phenomenology of the lightest neutralino in models where an effective bilinear term in the superpotential parametrizes the explicit breaking of R-parity. We consider supergravity scenarios where the lightest supersymmetric particle (LSP) is the lightest neutralino and which can be explored at LEP2. We present a detailed study of the LSP decay properties and general features of the corresponding signals expected at LEP2. We also contrast our model with gauge mediated supersymmetry breaking.     

Search for resonant second generation slepton production at the Fermilab Tevatron

We present a search for supersymmetry in the R-parity violating resonant production and decay of smuons and muon sneutrinos in the channels mu-->chi(1)(0)mu, mu-->chi(2,3,4)(0)mu, and nu(mu)-->chi(1,2)(+/-)mu. We analyzed 0.38 fb(-1) of integrated luminosity collected between April 2002 and August 2004 with the D0 detector at the Fermilab Tevatron Collider. The observed number of events is in agreement with the standard model expectation, and we calculate 95% C.L. limits on the slepton production cross section times branching fraction to gaugino plus muon, as a function of slepton and gaugino masses. In the framework of minimal supergravity, we set limits on the coupling parameter lambda(211)('), extending significantly previous results obtained in Run I of the Tevatron and at the CERN LEP collider.     

Bosonic quartic couplings at LEP2

We list the set of and conserving anomalous quartic vector bosons self-couplings which can be tested at LEP2 through triple vector boson production. We show how this set can be embedded in manifestly gauge invariant operators exhibiting an global symmetry. We derive bounds on these various couplings and show the most relevant distributions that can enhance their contribution. We also find that an collider running at 500 GeV can improve the LEP2 limits by as much as three-orders of magnitude. Received: 9 September 1999 / Revised version: 19 October 1999 / Published online: 17 February 2000     

Effects of R-Parity Violating Supersymmetry in Top Pair Production at Linear Colliders with Polarized Beams

In the minimal supersymmetric standard model with R-parity violation, the lepton number violating top quark interactions can contribute to the top pair production at a linear collider via tree-level u-channel squark exchange diagrams. We calculate such contributions and find that in the allowed range of these R-violating couplings, the top pair production rate as well as the top quark polarization and the forward-backward asymmetry can be significantly altered. By comparing the unpolarized beams with the polarized beams, we find that the polarized beams are more powerful in probing such new physics.     

Signals of universal extra dimension at the international linear collider

In the minimal universal extra dimension model, single production of n = 2 gauge bosons provides a unique discriminating feature from supersymmetry. We discuss how the proposed international linear collider can act as a n = 2 factory, much in the same vein as LEP.      

Associated stop-Higgs production at a linear collider and extraction of stop parameters

We calculate stop-stop-Higgs production at a linear collider. Combining the measurements from the pair production of the lightest stop and that of the mass of the Higgs, we show how, in a scenario where only the lightest stop and the lightest Higgs are accessible, one could extract the mass of the heavier stop and infer some useful information on the supersymmetric parameters. Received: 19 November 1998 / Published online: 18 June 1999     

Single Chargino Production with R-Parity Lepton Number Violation in Photon-Photon Collisions

We examine the process γγ→ x+ γ^- at photon-photon collider in the minimal supersymmetric standard model with R-parity violation,where all the one-loop diagrams are considered.We mainly discuss the effects of bilinear breaking terms,and conclude that their contributions may be important compared with trilinear terms.Our results show that the events of this process could be detectable at photon-photon colliders,if the values of the parameters are favorable.     

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.     

Space Anisotropy Search at Colliders

In the framework of model with Lorentz violation (LV) we discuss a physical observables for q\(\bar q\) pair production at lepton–lepton colliders and describe the experimental signal to be detected. We obtain a conservative limits on Lorentz-violating dimensionless coupling for quark sector from LEP data. We also make a phenomenological prediction for LV model at the future lepton collider.     

Searching for supersymmetry at LEP/LHC

For theep collider at LEP/LHC, we study the production and the signal coming from sleptons and squarks for several processes. We show how the supersymmetric signal can be extracted from the Standard Model background processes. In particular the large backgrounds due to singleW andZ production are considered. All the results are based on exact calculations of the matrix elements at tree level. Total cross sections and various kinematical distributions are presented.     

Dark and visible matter with broken R-parity and the axion multiplet

A small breaking of R-parity reconciles thermal leptogenesis, gravitino dark matter and primordial nucleosynthesis. We find that the same breaking relaxes cosmological bounds on the axion multiplet. Naturally expected spectra become allowed and bounds from late particle decays become so weak that they are superseded by bounds from non-thermal axion production. In this sense, the strong CP problem serves as an additional motivation for broken R-parity.     

Production of exotic E6 leptons in ep collisions

We consider the possibility of producing single, charged E₆ exotic leptons in ep collisions at high energies via flavor-changing couplings to neutral gauge bosons. We find that at HERA energies the production rate for these particles is quite small due to small mixing effects. At higher energies, as may be possible at a LEP ep collider, the production rates become reasonably large: ⋍100–1000 events per year.     

Probing R-parity Violating Interactions via pp^-→eμ＋X Channel on Tevatron

We investigate the lepton flavor violation processes pp→eμX induced by R-parity violating interactions at the Tevatron hadron collider. The theoretical calculation and Monte Carlo simulation demonstrate that with a set of suitable cuts on experimental observables, one might be capable to reduce the standard model physical background to a controllable level so that the signals of R-parity violating interactions could be detected distinctively. Furthermore, clear sneutrino information can be abstracted from the purified event sample where other SUSY scalar quark ``pollution' is heavily suppressed. We conclude that with a reasonable assumption of 10 fb^-1 integrated luminosity, the experiments at the Tevatron machine would have potential to discover sneutrino in the region of m_{\tilde{ν}}≤400 GeV via lepton flavor violation eμ production channels, or extend the mass scale constraint up to m_{\tilde{ν}}≥ 550 GeV at 95% CL.     

Testing locality at colliders via Bell''s inequality?

We consider a measurement of correlated spins at LEP and show that it does not constitute a general test of local-realistic theories via Bell's inequality. The central point of the argument is that such tests, where the spins of two particles are inferred from a scattering distribution, can be described by a local hidden variable theory. We conclude that with present experimental techniques it is not possible to test locality via Bell's inequality at a collider experiment. Finally we suggest an improved fixed-target it is not possible to test locality via Bell's inequality at a collider experiment. Finally we suggest an improved fixed-target experiment as a viable test of Bell's inequality.     

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.     

Search for single leptoquark and squark production in electron-photon scattering at $\sqrt{s_{\rm ee}}=189$ GeV at LEP

A search for first generation scalar and vector leptoquarks (LQ) as well as for squarks () in R-parity violating SUSY models with the direct decay of the into Standard Model particles has been performed using ee collisions collected with the OPAL detector at LEP at an ee centre-of-mass energy of 189 GeV. The data correspond to an integrated luminosity of about 160 pb. The dominant process for this search is , where a photon, which has been radiated by one of the beam electrons, serves as a source of quarks. The numbers of selected events found in the two decay channels are in agreement with the expectations from Standard Model processes. This result allows to set lower limits at the 95% confidence level on the mass of first generation scalar and vector leptoquarks, and of squarks in R-parity violating SUSY models. For Yukawa couplings to fermions larger than , the mass limits range from 121 GeV to GeV ( GeV to GeV) depending on the branching ratio of the scalar (vector) leptoquark state. Furthermore, limits are set on the Yukawa couplings for leptoquarks and for squarks, and on as a function of the scalar leptoquark/squark mass. Received: 22 May 2001 / Published online: 25 January 2002     

Technihadron production and decay at LEP2

The simple “straw-man” model of low-scale technicolor contains light color-singlet technihadrons, which mix with the electroweak gauge bosons. We present lepton collider production rates at the parton level, and show that experiments at LEP2 may be sensitive to the presence of technirho and techniomega states with masses 10–20 GeV beyond the center-of-mass energy because of the mixing. The exact sensitivity depends on several parameters, such as the technipion mass, the technipion mixing angle, and the charge of the technifermions. In an appendix, we describe the implementation of the model into the event generator PYTHIA for particle-level studies at lepton and hadron colliders.     

Search for scalar fermions and long-lived scalar leptons at centre-of-mass energies of 130 GeV to 172 GeV

Data taken by DELPHI during the 1995 and 1996 LEP runs have been used to search for the supersymmetric partners of electron, muon and tau leptons and of top and bottom quarks. The observations are in agreement with standard model predictions. Limits are set on sfermion masses. Searches for long lived scalar leptons from low scale supersymmetry breaking models exclude stau masses below 55 GeV/c at the 95% confidence level, irrespective of the gravitino mass. Received: 13 July 1998 / Published online: 19 November 1998     

Investigation of vacuum polarization in t-channel radiative Bhabha scattering

We discuss the possibility of a precision measurement of vacuum polarization in t-channel radiative Bhabha scattering at a high luminosity collider. For illustration, the achievable precision is estimated for the BaBar experiment at PEP-II and for the OPAL experiment at LEP. Received: 6 June 2001 / Published online: 21 September 2001     

Investigation of the dynamics of gluon distributions in the production of heavy quarks and quarkonia at the LEP2 collider

The inclusive production of heavy quarks and quarkonia in photon-photon collisions at the LEP2 collider is considered within the semihard (k _T-factorization) QCD approach. The dependence of the total and differential cross sections for the production of heavy (c and b) quarks and D* and J/ψ mesons on the choice of unintegrated gluon distribution is studied. The transition of a $c\bar c$ charmed pair to observed J/ψ mesons is described on the basis of the color-singlet model. The results of the calculations are compared with currently available experimental data obtained by the L3, OPAL, ALEPH, and DELPHI Collaborations. It is shown that the polarization properties of J/ψ mesons at the LEP2 collider are sensitive to the behavior of unintegrated gluon distributions. This means that experimental investigations of the polarization properties of quarkonia in photon-photon collisions may provide a direct test of the dynamics of gluon distributions in the photon.