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.     

SUSY background to neutral MSSM Higgs boson searches

Within the Minimal Supersymmetric Standard Model (MSSM) the production and decay of superpartners can give rise to backgrounds for Higgs boson searches. Here MSSM background processes to the vector boson fusion channel with the Higgs boson decaying into two tau leptons or two W-bosons are investigated, giving rise to dilepton plus missing transverse momentum signals of the Higgs boson. Starting from a scenario with relatively small masses of the supersymmetric (SUSY) particles, with concomitant large cross section of the background processes, one obtains a first conservative estimate of the background. Light chargino pair production plus two jets, lightest and next-to-lightest neutralino production plus two jets as well as slepton pair production plus two jets are identified as important contributions to the irreducible SUSY background. Light chargino and next-to-lightest neutralino production plus two jets and next-to-lightest neutralino pair production plus two jets give rise to reducible backgrounds, which can be larger than the irreducible ones in some scenarios. The relevant distributions are shown and additional cuts for MSSM background reduction are discussed. Extrapolation to larger squark masses is performed and shows that MSSM backgrounds are quite small for squark masses at the current exclusion limits.     

Neutrinoless double beta decay in the LHC era

Once neutrinoless double beta decay is discovered, the question which mechanism triggers the decay becomes crucial for drawing any conclusion about the concrete physics underlying the process, like the neutrino Majorana mass. For example, in the minimal supersymmetric extension with R-parity violation both neutrino Majorana masses and superpartners can trigger the decay. We show that in this case, if the decay is triggered by superpartners, there exist good prospects to observe single slepton production at the LHC. Resonant single slepton production at the LHC can therefore discriminate between the neutrinoless double beta decay mechanism and others.     

Mass restricting variables in semi-invisible production at the LHC

We outline some of the popular mass restricting variables for the semi-invisible productions at the Large Hadron Collider. In this context, heavy resonating mass, if produced through antler decay topology may already be detectable. New mass variables constructed by applying this mass constraint proved to have an array of interesting properties, including a new kink solution at the true masses of the produced particles. This enables one to measure the mass of the invisible particle and the parent particle simultaneously. This variable in turn can also be applied in reconstructing such events with the momenta of invisible particles. This feature is further demonstrated with the Higgs boson decaying into a pair of third-generation tau-lepton (\(\tau \)) and thus exploring direct Higgs coupling with the leptonic sector. Dominant discovery signatures rely upon the hadronic decay of tau which is associated with a pair of invisible neutrinos. Exploiting the already measured Higgs mass bound, present technique is capable of providing unique event reconstruction. Moreover, a significant efficiency enhancement is demonstrated in comparison with the existing methods.     

Anomalous muon production in e+e− annihilations as evidence for heavy leptons

We have measured inclusive muon production in e⁺e^? annihilation for CMS energies between 3.6 and 5.0 GeV. Above 4 GeV the cross section cannot be explained by conventional sources like higher order QED processes or inclusive production of the $\text{J}\text{?}$ (3.1). It is, however, compatible with the pair production of heavy particles of a mass of about 1.9 GeV/c². Spin assignment and decay parameters are investigated.     

Neutrino telescopes as a direct probe of supersymmetry breaking

We consider models where the scale of supersymmetry breaking lies between 5 x 10(6) and 5 x 10(8) GeV. In this class of theories, which includes models of mediated supersymmetry breaking, the lightest supersymmetric particle is the gravitino, and the next to lightest is typically a long-lived charged slepton with a lifetime between a microsecond and a second, depending on its mass. We investigate the production of these particles by the diffuse flux of high energy neutrinos colliding with nucleons in the Earth, and the potential for their observation in large ice or water Cerenkov detectors. The small production cross section is partially compensated by the very long range of sleptons. The signal, two well-separated parallel tracks, has very little background. Using the Waxman-Bahcall limit for the neutrino flux results in up to four events a year in km3 experiments.     

Mass determination of new states at hadron colliders

We propose an improved method for hadron-collider mass determination of new states that decay to a massive, long-lived state like the LSP in the MSSM. We focus on pair-produced new states which undergo three-body decay to a pair of visible particles and the new invisible long-lived state. Our approach is to construct a kinematic quantity which enforces all known physical constraints on the system. The distribution of this quantity calculated for the observed events has an endpoint that determines the mass of the new states. However we find it much more efficient to determine the masses by fitting to the entire distribution and not just the end point. We consider the application of the method at the LHC for various models and demonstrate that the method can determine the masses within about 6 GeV using only 250 events. This implies the method is viable even for relatively rare processes at the LHC such as neutralino pair production.     

Muon anomalous magnetic moment in the supersymmetric economical 3-3-1 model

We investigate the muon anomalous magnetic moment in the context of a supersymmetric version of the economical 3-3-1 model. We compute the 1-loop contribution of superpartner particles. We show that the contribution of superparticle loops become significant when tanγ is large. We investigate the cases of both small and large values of tanγ. We find the region of the parameter space where the slepton masses of a few hundred GeV are favored by the muon g–2 for small tanγ (tanγ ≈ 5). Numerical estimation gives the mass of supersymmetric particles, the mass of gauginos m _G ≈ 700 GeV, and the light slepton mass \(m_{\tilde L} \) of the order of O (100) GeV. When tanγ is large (tanγ ≈ 60), the charged slepton mass \(m_{\tilde L} \) and the gaugino mass m _G are O(1) TeV, while the sneutrino mass ≈450 GeV is in the reach of the LHC.     

Exclusive decay of 1 heavy quarkonium into photon and two pions

We study the exclusive decay of 1⁻⁻ heavy quarkonium into one photon and two pions in the kinematic region, where the two-pion system has a invariant mass which is much smaller than the mass of heavy quarkonium. Neglecting effects suppressed by the inverse of the heavy quark mass, the decay amplitude can be factorized, in which the nonperturbative effect related to heavy quarkonium is represented by a non-relativistic QCD matrix element, and that related to the two pions is represented by a distribution amplitude of two gluons in the isoscalar pion pair. By taking the asymptotic form for the distribution amplitude and by using chiral perturbative theory we are able to obtain numerical predictions for the decay. Numerical results show that the decay of J/ψ can be observed at BEPC and at CESR. Experiment observation of this process in this kinematic region at BEPC and CESR can provide information about how gluons are converted into the two pions and may supply a unique approach to study I=0 s-wave ππ scattering.     

Central exclusive meson pair production in the perturbative regime at hadron colliders

The central exclusive production (CEP) of heavy resonance states that subsequently decay into meson pairs, \(M\overline{M}\), is an important signature for such processes at hadron colliders. However, there is a potentially important background from the direct QCD production of meson pairs, as mediated for example by the exclusive \(gg\to M\overline{M}\) hard scattering subprocess. This is in fact an interesting process in its own right, testing novel aspects of perturbative QCD technology. We explicitly calculate the \(gg \to M\overline{M}\) helicity amplitudes for different meson states within the hard exclusive formalism, and comment on the application of MHV techniques to the calculation. Using these results, we describe how meson pair CEP can be calculated in the perturbative regime, and present some sample numerical predictions for a variety of final states. We also briefly consider the dominant non-perturbative contributions, which are expected to be important when the meson transverse momentum is small.     

Mass shell technique for measuring masses of a pair of semi-invisibly decaying particles

Motivated by evidence for the existence of dark matter, many new physics models predict the pair production of new particles, followed by the decays into two invisible particles, leading to a momentum imbalance in the visible system. For the cases where all four components of the vector sum of the two "missing" momenta are measured from the momentum imbalance, we present analytic solutions of the final state system in terms of measurable momenta, with the mass shell constraints taken into account. We then introduce new variables which allow the masses involved in the new physics process, including that of the dark matter particles, to be extracted efficiently. These are compared with a selection of variables in the literature, and possible applications at lepton and hadron colliders are discussed.     

Search for events with leptonic jets and missing transverse energy in pp collisions at √s=1.96 TeV

We present the first search for pair production of isolated jets of charged leptons in association with a large imbalance in transverse energy in pp collisions using 5.8 fb?1 of integrated luminosity collected by the D0 detector at the Fermilab Tevatron Collider. No excess is observed above the standard model background, and the result is used to set upper limits on the production cross section of pairs of supersymmetric chargino and neutralino particles as a function of "dark-photon" mass, where the dark photon is produced in the decay of the lightest supersymmetric particle.     

Flavor and LHC searches for new physics

Uncovering the physics of electroweak symmetry breaking (EWSB) is the raison-d’etre of the LHC. Flavor questions, it would seem, are of minor relevance for this quest, apart from their role in constraining the possible structure of EWSB physics. In this short review article, we outline, using flavor-dependent slepton physics as an example, how flavor can affect both searches for supersymmetry, and future measurements aimed at understanding the nature of any new discoveries. If the production cross-sections for supersymmetry are relatively low, as indicated by the fact that it has not revealed itself yet in standard searches, the usual assumptions about the superpartner spectra need rethinking. Furthermore, one must consider more intricate searches, such as lepton-based searches, which could be susceptible to flavor effects. We start by reviewing the flavor structure of existing frameworks for mediating supersymmetry breaking, emphasizing flavor-dependent models proposed recently. We use the kinematic endpoints of invariant mass distributions to demonstrate how flavor dependence can impact both searches for supersymmetry and the Inverse Problem. We also discuss methods for measuring small-mass splittings and mixings at the LHC, both in models with a neutralino LSP and in models with a charged slepton (N)LSP.     

Test of lepton flavour violation at the LHC

We study lepton flavour violating decays of neutralinos and sleptons within the minimal supersymmetric standard model, assuming two and three generation mixings in the slepton sector. We take into account the most recent bounds on flavour violating rare lepton decays. Taking the SPS1a’ scenario as an example, we show that some of the lepton flavour violating branching ratios of neutralinos and sleptons can be sizable (∼5–10%). We study the impact of the lepton flavour violating neutralino and slepton decays on the di-lepton mass distributions measured at the LHC. We find that they can result in novel and characteristic edge structures in the distributions. In particular, double-edge structures can appear in the eτ and μτ mass spectra if is the lightest slepton. The appearance of these remarkable structures provides a powerful test of supersymmetric lepton flavour violation at the LHC.     

Pair production in the gravitational field of a cosmic string

We show that many elementary particle physics processes, such as pair production by a high energy photon, that take place in Minkowski space only if a non-uniform external field provides for momentum non-conservation, do occur in the space-time around a straight cosmic string, even though the space is locally flat and there is no local gravitational potential. We exemplify this mechanism through the evaluation of the cross section per unit length of string for the decay of a massless scalar particle into a pair of massive particles. The cross sections for this kind of processes are typically small. Nevertheless, it is interesting to realize how these reactions occur due to topological properties of space, rather than to the action of a local field.     

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.     

Signatures of MSSM charged Higgs bosons via chargino–neutralino decay channels at the LHC

We assess the potential of detecting a charged Higgs boson of the MSSM at the LHC via its decays into a chargino and a neutralino. We focus our attention on the region of parameter space with and , where identification of the via other decay modes has proven to be ineffective. Searching for means to plug this hole, we simulate the decays and – the former can yield a single hard lepton (from the chargino decay) while the latter can yield three leptons (from the chargino and neutralino decays). Coupled with the dominant top quark + charged Higgs boson production mode, the resulting signature is one or three hard, isolated leptons, substantial missing transverse momentum and a reconstructed (via a 3-jet invariant mass) top quark. The single lepton channel is swamped by background processes; however, with suitable cuts, a trilepton signal emerges. While such a signal suffers from a low number of surviving events (after cuts) and is dependent on several MSSM input parameters (notably , and slepton masses), it does fill at least some of the void left by previous investigations. Received: 21 October 2000 / Published online: 23 February 2001     

Simulation of sextet diquark production

We present a method for simulating the production and decay of particles in the sextet representation of SU(3) _C including the simulation of QCD radiation. Results from the Monte Carlo simulation of sextet diquark production at the LHC including both resonant and pair production are presented. We include limits on resonant diquark production from recent ATLAS results and perform the first simulation studies of the less model dependent pair production mechanism.     

\mu\rightarrow e\gamma decay in the left-right supersymmetric model

We calculate the dipole amplitude for the decay and related processes in the left-right supersymmetric model when parity breaking occurs at a considerably large scale. The low-energy flavor violation in the model originates either from the nonvanishing remnants of the left-right symmetry in the slepton mass matrix or from the direct flavor changing lepton-slepton-neutralino interaction. The result is found to be large and already accessible with current experimental accuracy for supersymmetric masses not far above the electroweak scale. It also provides nontrivial constraints on the lepton mixing in the model. Received: 8 June 1998 / Published online: 15 October 1998