Search for supersymmetric particles in light gravitino scenarios and sleptons NLSP

A search for sleptons, neutralinos, charginos, sgoldstinos and heavy stable charged sleptons in the context of scenarios where the lightest supersymmetric particle is the gravitino, is presented. Data collected during 2000 with the DELPHI detector at centre-of-mass energies from 204 to 208 GeV were analysed and combined with all the data collected from 1995 to 1999 at lower energies. No evidence for the production of sleptons, neutralinos and charginos has been found, therefore new limits on the mass of these supersymmetric particles and on the model parameter space are set. The search for heavy stable charged sleptons also updates the stable sleptons mass limit. The absence of evidence for sgoldstino production allows limits to be set on its mass and on the scale of supersymmetry breaking. Received: 6 November 2002 / Published online: 14 February 2003     

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     

The SM as the quantum low-energy effective theory of the MSSM

In the framework of the minimal supersymmetric standard model we compute the one-loop effective action for the electroweak bosons obtained after integrating out the different sleptons, squarks, neutralinos and charginos, and present the result in terms of the physical sparticle masses. In addition we study the asymptotic behavior of the two-, three- and four-point Green's functions with external electroweak bosons in the limit where the physical sparticle masses are very large in comparison with the electroweak scale. We find that in this limit all the effects produced by the supersymmetric particles can either be absorbed in the standard model parameters and gauge bosons wave functions, or else they are suppressed by inverse powers of the supersymmetric particle masses. This work, therefore, completes the proof of decoupling of the heavy supersymmetric particles from the standard ones in the electroweak bosons effective action and in the sense of the Appelquist–Carazzone theorem; we started this proof in a previous work. From the point of view of effective field theories this work can be seen as a (partial) proof that the SM can indeed be obtained from the MSSM as the quantum low-energy effective theory of the latter when the SUSY spectra are much heavier than the electroweak scale. Received: 27 March 1999 / Revised version: 7 September 1999 / Published online: 27 January 2000     

Photonic events with missing energy in \mathrme^+\mathrme^- collisions at \sqrt{s} = 189 GeV

Photonic events with large missing energy have been observed in collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on and are derived for the case of stable and invisible . These limits apply to single and pair production of excited neutrinos (), to neutralino production () and to supersymmetric models in which and is a light gravitino. The case of macroscopic decay lengths of particle X is considered for , , when . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions. Received: 4 April 2000 / Published online: 27 November 2000     

Search for gauge mediated SUSY breaking topologies at \sqrt{{\mathrm s}} \sim 189 GeV

Searches for topologies characteristic of Gauge Mediated SUSY Breaking models (GMSB) are performed by analysing 173.6 of data collected at = 188.6 GeV with the ALEPH detector. These topologies include acoplanar photons, non-pointing single photon, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and four leptons plus missing energy final states. No evidence for these new phenomena is observed and limits on production cross sections and sparticle masses are derived. A scan of a minimal GMSB parameter space is performed and model dependent lower limits of about 45 GeV/ on the next-to-lightest supersymmetric particle (NLSP) mass and of about 9 TeV on the mass scale parameter are derived, independently of the NLSP lifetime. Received: 30 November 1999 / Published online: 14 April 2000     

Prospects for the measurement of B 0 s oscillations with the ATLAS detector at LHC

The feasibility for mass measurements of supersymmetric particles from minimal gauge mediated models in the ATLAS detector at the large hadron collider is studied using parameterized simulations. The covered models are characterized by having the supersymmetric tau as the only next-to-lightest supersymmetric particle (NLSP). Given independent measurements of the lightest Higgs mass and the NLSP lifetime, a sufficient number of supersymmetric particle mass measurements are performed such that all underlying model parameters are constrained.     

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     

Supersymmetric Higgs boson pair production: Discovery prospects at hadron colliders

We study the potential of hadron colliders in the search for the pair production of neutral Higgs bosons in the framework of the minimal supersymmetric standard model. We perform a detailed signal and background analysis, working out efficient kinematical cuts for the extraction of the signal. The important role of squark loop contributions to the signal is re-emphasized. If the signal is sufficiently enhanced by these contributions, it could even be observable at the next run of the upgraded Tevatron collider in the near future. At the LHC the pair production of light and heavy Higgs bosons might be detectable simultaneously. Received: 23 February 2000 / Revised version: 18 April 2000 / Published online: 31 August 2000     

Single superpartner production at Tevatron Run II

We study the single production of supersymmetric particles at Tevatron Run II which occur in the processes involving R-parity violating couplings of type . We focus on the single gaugino production which receive contributions from the resonant slepton production. We first calculate the amplitudes of the single gaugino production. Then we perform analyses of the single gaugino production based on the three charged leptons and like sign dilepton signatures. These analyses allow to probe supersymmetric particles masses beyond the present experimental limits, and many of the coupling constants down to values smaller than the low-energy bounds. Finally, we show that the studies of the single gaugino production offer the opportunity to reconstruct the and masses with a good accuracy in a model independent way. Received: 27 July 2000 / Revised version: 27 November 2000 / Published online: 23 February 2001     

Photon events with missing energy at \sqrt{s} =183 to 189 GeV

The production of single photons has been studied in the reaction at centre-of-mass energies of 183 GeV and 189 GeV. A previously published analysis of events with multi-photon final states accompanied by missing energy has been updated with 189 GeV data. The data were collected with the DELPHI detector and correspond to integrated luminosities of about 51 pb and 158 pb at the two energies. The number of light neutrino families is measured to be . The absence of an excess of events beyond that expected from Standard Model processes is used to set limits on new physics as described by supersymmetric and composite models. A limit on the gravitational scale is also determined. Received: 28 January 2000 / Revised version: 27 April 2000 / Published online: 26 July 2000     

Looking for physics beyond the Standard Model

Motivations for new physics beyond the Standard Model are presented. The most successful and best motivated option, supersymmetry, is described in some detail, and the associated searches performed at LEP are reviewed. These include searches for additional Higgs bosons and for supersymmetric partners of the standard particles. These searches constrain the mass of the lightest supersymmetric particle which could be responsible for the dark matter of the universe. To cite this article: P. Binétruy, J.-F. Grivaz, C. R. Physique 3 (2002) 1235–1243.     

Strong-interaction effects in stop pair production at e^+ e^- colliders

We discuss perturbative and non-perturbative strong-interaction effects in the pair production of stop squarks () at colliders. Events with an additional hard gluon allow to detect or exclude production even in scenarios with very small mass splitting between and an invisible lightest supersymmetric particle (LSP). Such events can also help to establish that transforms as a triplet under SU(3). We also carefully study non-perturbative fragmentation, which is currently not well understood: not only is the fragmentation function not known very well, but also there are ambiguities in the algorithm employed to model fragmentation. We present numerical results both for CERN LEP–183 and for a proposed future collider operating at center-of-mass energy GeV. Received: 19 February 1999 / Revised version: 12 March 1999 / Published online: 3 August 1999     

Pair production of charged MSSM Higgs bosons by gluon fusion

The production of pairs of charged Higgs bosons as predicted by the minimal supersymmetric standard model (MSSM) via the gluon fusion mechanism is investigated. The amplitudes at the leading one-loop order for the parton process are calculated with the complete set of MSSM particles. Numerical results are presented for the cross section of the inclusive hadron process at the LHC. Received: 20 September 1999 / Published online: 27 January 2000     

Searches for R-parity violating decays of Gauginos at 183 GeV at LEP

Searches for pair-produced charginos and neutralinos with R-parity violating decays have been performed using a data sample corresponding to an integrated luminosity of 56 pb collected with the OPAL detector at LEP at a centre-of-mass energy of 183 GeV. An important consequence of R-parity violation is that the lightest supersymmetric particle becomes unstable. The searches have been performed under the assumptions that the lightest supersymmetric particle promptly decays and that only one R-parity violating coupling is dominant for each of the decay modes considered. Such processes would yield multiple leptons, jets plus leptons, or multiple jets with or without significant missing energy in the final state. No excess of such events above Standard Model backgrounds has been observed. Limits are presented on the production cross-sections of gauginos in R-parity violating scenarios. Limits are also presented in the framework of the Minimal Supersymmetric Standard Model. Received: 14 December 1998 / Published online: 14 October 1999     

Investigation of phase-equivalent potentials by a halo transfer reaction

Using the supersymmetric quantum mechanics, we investigate the wave function-sensitive properties of the supersymmetric potentials which have received a lot of attention in the literature recently. We show that a superdeep potential and its phase-equivalent shallow-partner potential give very similar rms radius values for the weakly bound systems such as the deuteron and ¹¹Be nuclei. Although the corresponding eigenstates differ in the node-number, our investigation on the ¹¹Be(p, d )¹⁰Be single nucleon halo transfer reaction at 35 MeV show that also other physical quantities such as the cross-section angular distributions calculated using these wave functions reflect the nodal structure rather weakly. This lends support to two nearly equivalent treatments of the Pauli principle. Received: 15 May 2000 / Accepted: 14 September 2000     

Supersymmetric relics from the big bang

We consider the cosmological constraints on supersymmetric theories with a new, stable particle. Circumstantial evidence points to a neutral gauge/Higgs fermion as the best candidate for this particle, and we derive bounds on the parameters in the lagrangian which govern its mass and couplings. One favored possibility is that the lightest neutral supersymmetric particle is predominantly a photino $\text{～}\text{γ}$ with mass above $\text{1}\text{2}$ GeV, while another is that the lightest neutral supersymmetric particle is a Higgs fermion with mass above 5 GeV or less than O(100) eV. We also point out that a gravitino mass of 10 to 100 GeV implies that the temperature after completion of an inflationary phase cannot be above 10¹⁴ GeV, and probably not above 3 × 10¹² GeV. This imposes constraints on mechanisms for generating the baryon number of the universe.     

Implications of collider experiments for detecting cold dark matter

By investigating the space of parameters of the minimal supersymmetric extension of the Standard Model, it is shown that an observation of at least one supersymmetric particle in high-energy experiments would increase considerably the importance of highly sensitive low-energy experiments aimed at detecting cold-dark-matter particles assumed to be neutralinos, the lightest supersymmetric particles, which are stable. On the other hand, nonobservation of direct signals from dark matter in such experiments can have a pronounced effect on the strategy of high-energy searches for the light charged Higgs boson.     

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     

Chargino and neutralino decays revisited

We perform a comprehensive analysis of the decays of charginos and neutralinos in the minimal supersymmetric standard model where the neutralino is assumed to be the lightest supersymmetric particle. We focus, in particular, on the three-body decays of the next-to-lightest neutralino and the lightest chargino into the lightest neutralino and fermion–antifermion pairs and include vector boson, Higgs boson and sfermion exchange diagrams, where in the latter contribution the full mixing in the third generation is included. The radiative corrections to the heavy fermion and SUSY particle masses will also be taken into account. We present complete analytical formulae for the Dalitz densities and the integrated partial decay widths in the massless fermion case, as well as the expressions of the differential decay widths including the masses of the final fermions and the polarization of the decaying charginos and neutralinos. We then discuss these decay modes, in particular in scenarios where the parameter is large and in models without universal gaugino masses at the grand unification scale, where some new decay channels, such as decays into gluinos and pairs, open up. Received: 12 April 2001 / Published online: 8 June 2001     

Search for gauge mediated SUSY breaking topologies in ee collisions at centre-of-mass energies up to 209 GeV

A total of 628 bp^-1 of data collected with the ALEPH detector at centre-of-mass energies from 189 to 209 GeV is analysed in the search for gauge mediated SUSY breaking (GMSB) topologies. These topologies include two acoplanar photons, non-pointing single photons, acoplanar leptons, large impact parameter leptons, detached slepton decay vertices, heavy stable charged sleptons and multi-leptons plus missing energy final states. No evidence is found for new phenomena, and lower limits on masses of supersymmetric particles are derived. A scan of a minimal GMSB parameter space is performed and lower limits are set for the next-to-lightest supersymmetric particle (NLSP) mass at 54 GeV/c² and for the mass scale parameter at 10 TeV/c², independently of the NLSP lifetime. Including the results from the neutral Higgs boson searches, a NLSP mass limit of 77 GeV/c² is obtained and values of up to 16 TeV/c² are excluded. Received: 14 March 2002 / Published online: 20 September 2002