SUSY Particles Research Articles

GRACE/SUSY: Automatic generation of tree amplitudes in the minimal supersymmetric standard model

GRACE/SUSY is a program package for generating the tree-level amplitude and evaluating the corresponding cross section of processes of the minimal supersymmetric extension of the standard model (MSSM). The Higgs potential adopted in the system, however, is assumed to have a more general form indicated by the two-Higgs-doublet model. This system is an extension of GRACE for the standard model (SM) of the electroweak and strong interactions. For a given MSSM process the Feynman graphs and amplitudes at tree-level are automatically created. The Monte Carlo phase space integration by means of BASES gives the total and differential cross sections. When combined with SPRING, an event generator, the program package provides us with the simulation of the SUSY particle productions.

Searches for SUSY particles at LEP

Searches for production of sleptons, squarks, charginos, neutralinos and neutral Higgs bosons are performed with the data collected by the ALEPH, DELPHI, L3 and OPAL detectors at LEP at centre-of-mass energies up to 209.2 GeV. No evidence for any such signal is observed. Preliminary LEP mass lower limits are reported in the framework of the minimal supersymmetric extension of the standard model.

The CALET, CALorimetric Electron Telescope, mission for the International Space Station

The CALorimetric Electron Telescope, CALET, mission is proposed for the Japanese Experiment Module Exposure Facility of the International Space Station. Major goals of the mission are precise measurements of the electrons in a few GeV - 10 TeV and the gamma-rays in 100 MeV - several TeV, keeping an energy resolution of a few % over 100 GeV. From the measurements, a systematic investigation of high-energy electromagnetic process in universe will be performed. A detection of SUSY particle which is a candidate of the dark matter would also be expected. The detector is composed of an imaging calorimeter of scintillating fibers and a total absorption calorimeter. Total thickness of the absorber is 45 r.1 for electro-magnetic particles and 2.1 m.f.p for protons. Total weight of the payload is nearly 2,200 kg, and the effective geometrical factor should be ∼ 1.0 m 2sr. The hadron rejection power can be 10 6 for electrons.

Supersymmetry at LHC

This document describes prospects for discovering Supersymmetry and studying its properties at Large Hadron Collider (LHC). ATLAS and CMS collaborations have enormous potential to discover Supersymmetry, if it exists at mass scale less than about 2 TeV. Methods and potentials to determine the properties of SUSY particles using various kinematic distributions are also summarised.

Status of the ATLAS project

The construction status of the ATLAS detector is presented. Within a tight schedule the project is on track to record the first physics events from the Large Hadron Collider in spring 2006 with an initial detector configuration that is capable of searching successfully for bench-mark processes like Higgs and SUSY particle productions.

Investigation of no-scale supersymmetry breaking models with a gaugedU(1)B−Lsymmetry

Noscale supersymmetry (SUSY) breaking model is investigated in the minimal extension of the minimal supersymmetric standard model (MSSM) with a gauged U(1)_{B-L} symmetry. We specifically consider a unification-inspired model with the gauge groups SU(3)_{C} \times SU(2)_{L} \times U(1)_Y \times U(1)_{B-L} \subset SU(5) \times U(1)_{5} for illustration. While the noscale boundary condition at the grand unification scale (M_{G}\simeq 2\times 10^{16} GeV) in the MSSM is not consistent with phenomenological constraints, we show that it is if the gaugino of the U(1)_{5} multiplet is several times heavier than the gauginos of the MSSM. However, if SU(5) \times U(1)_{5} is further embedded in a larger simple group, e.g. SO(10), the noscale boundary condition at M_{G} is inconsistent with phenomenological constraints. If we relax the noscale boundary condition and allow non-zero soft scalar masses for the Higgs fields which spontaneously break the U(1)_{5} symmetry, the resultant spectrum of SUSY particles becomes consistent with all the phenomenological constraints, even if we impose the GUT relation on the gauge coupling and the gaugino mass of the U(1)_{5}. In this case, the SUSY CP problem is also solved, since the condition B\mu =0 at the boundary can be imposed consistently with the electroweak symmetry breaking.

Associated production ofH±andW∓in high-energye+e−collisions in the minimal supersymmetric standard model

We study the associated production of the charged Higgs boson and W^{\pm} gauge boson in high energy e+e- collisions in the Minimal Supersymmetric Standard Model (MSSM). This associated production, which first arises at the one loop level, offers the possibility of producing the charged Higgs boson at the e+e- collider with mass more than half the center-of-mass energy, when the charged Higgs pair production is kinematically forbidden. We present analytic and numerical results for the cross section for e+e- --> W+ H- in the full MSSM, taking into account the previously uncalculated contributions from supersymmetric (SUSY) particles. We find that the contributions of the SUSY particles enhance the cross section over most of SUSY parameter space, especially when the SUSY particles are light, ~200 GeV. With favorable SUSY parameters, at small tan beta, this process can yield more than ten W^{\pm}H^{\mp} events for m_{H^{\pm}} <~ 350 GeV in 500 fb-1 at a 500 GeV e+e- collider, or m_{H^{\pm}} <~ 600 GeV in 1000 fb-1 at a 1000 GeV collider. 80% left-handed polarization of the e- beam improves these reaches to m_{H^{\pm}} <~ 375 GeV and m_{H^{\pm}} <~ 670 GeV, respectively.

Updated Constraints on the Minimal Supergravity Model

Recently, refinements have been made on both the theoretical and experimental determinations of the i.) mass of the lightest Higgs scalar (m_h), ii.) relic density of cold dark matter in the universe (Omega_CDM h^2), iii.) branching fraction for radiative B decay BF(b \to s \gamma), iv.) muon anomalous magnetic moment (a_\mu), and v.) flavor violating decay B_s \to \mu^+\mu^-. Each of these quantities can be predicted in the MSSM, and each depends in a non-trivial way on the spectra of SUSY particles. In this paper, we present updated constraints from each of these quantities on the minimal supergravity (mSUGRA) model as embedded in the computer program ISAJET. The combination of constraints points to certain favored regions of model parameter space where collider and non-accelerator SUSY searches may be more focussed.

The Tevatron Collider Run 2 prospects for discoveries in particle physics

The chances of discovering the Standard Model Higgs boson in Run 2 at the Fermilab Tevatron Collider are discussed. The reach of a search for MSSM Higgs boson and for other Susy particles is also mentioned. The large integrated luminosity potentially offered by the upgraded Tevatron in the years before the start of LHC will make an exciting physics program possible in the next several years. Let aside the luminosity, we can rely now on two much more powerful detectors than in run 1. This is a very real point of strength of the run 2 Tevatron program. The progress of the Tevatron from spring this year has been slow but steady. From this, there is no reason for being pessimistic, but admittedly no particular reason for being optimistic as well. CDF will be able to produce physics quality data early in 2002. After that, data will flow for years and years. We expect to be able to publish the first papers based on the new data in fall 2002.

Detection of Bs→ μ+μ− at the tevatron Run II and constraints on the SUSY parameter space

A measurement of the branching ratio for the rare decay mode B s → μ + μ − at the Tevatron is an opportunity to test various supersymmetric scenarios. We investigate the prospects for studying this mode in Run II and estimate that CDF would be sensitive to this decay for a branching ratio >1.2×10 −8 with 15 fb −1 (or, if a similar analysis holds for D0, >6.5×10 −9 for the combined data). We calculate the branching ratio in minimal supergravity (mSUGRA) parameter space, and find that tan β>30 can be probed. (This mSUGRA parameter space cannot be probed by direct production of SUSY particles at Run II.) Including other experimental constraints on the mSUGRA parameter space, one finds that CDF B s → μ + μ − measurements would be able to cover the full mSUGRA parameter space for tan β=50 if the muon g μ −2 anomaly exceeds ∼11×10 −10, and about half the allowed parameter space for tan β=40. A large branching ratio >7(14)×10 −8 (feasible with only 2 fb −1) would be sufficient to exclude the mSUGRA model for tan β⩽50(55). Dark matter neutralino–proton detection cross sections are examined in the allowed region, and should be large enough to be accessible to future planned experiments. Combined measurements of B s → μ + μ −, the Higgs mass m h and the muon g μ −2 anomaly would be sufficient to determine the μ>0 mSUGRA parameters (or show the model is inconsistent with the data). We also briefly discuss the B s → μ + μ − decay in R-parity violating models. There, for some models, the branching ratio can be large enough to be detected even for small tan β and large m 1/2.

Supersymmetric scalar production in the electroweak Sudakov regime of lepton colliders

We consider the production of supersymmetric (SUSY) scalar pairs at lepton colliders, for c.m. energies much larger than the mass of the heaviest SUSY (real or virtual) particle involved in the process. In that energy regime, we derive the leading and subleading terms of the electroweak Sudakov logarithms in the MSSM, first working at one loop with physical states and then resumming to all orders with asymptotic expansions. We show that the first order of the resummed expression reproduces the physical one loop approximation, and compute systematically the possible effects on various observables both at one loop and to all orders. We discuss the regimes and the observables where the one loop approximation can or cannot be trusted, working in an energy range between 1 TeV and 4 TeV under a ``light'' SUSY mass assumption. As a by-product of our analysis, we propose a determination of the MSSM parameter $\mathrm{tan}\ensuremath{\beta}$ showing how a relative accuracy $\ensuremath{\simeq}25%$ can be easily achieved in the region $\mathrm{tan}\ensuremath{\beta}\ensuremath{\gtrsim}14,$ under reasonable experimental assumptions.

Reach of the Fermilab Tevatron and CERN LHC for gaugino mediated SUSY breaking models

In supersymmetric models with gaugino mediated SUSY breaking (inoMSB), it is assumed that SUSY breaking on a hidden brane is communicated to the visible brane via gauge superfields which propagate in the bulk. This leads to GUT models where the common gaugino mass $m_{1/2}$ is the only soft SUSY breaking term to receive contributions at tree level. To obtain a viable phenomenology, it is assumed that the gaugino mass is induced at some scale $M_c$ beyond the GUT scale, and that additional renormalization group running takes place between $M_c$ and $M_{GUT}$ as in a SUSY GUT. We assume an SU(5) SUSY GUT above the GUT scale, and compute the SUSY particle spectrum expected in models with inoMSB. We use the Monte Carlo program ISAJET to simulate signals within the inoMSB model, and compute the SUSY reach including cuts and triggers approriate to Fermilab Tevatron and CERN LHC experiments. We find no reach for SUSY by the Tevatron collider in the trilepton channel. %either with or without %identified tau leptons. At the CERN LHC, values of $m_{1/2}=1000$ (1160) GeV can be probed with 10 (100) fb$^{-1}$ of integrated luminosity, corresponding to a reach in terms of $m_{\tg}$ of 2150 (2500) GeV. The inoMSB model and mSUGRA can likely only be differentiated at a linear $e^+e^-$ collider with sufficient energy to produce sleptons and charginos.

Prospects for new physics observations in diffractive processes at the LHC and Tevatron

We study the double-diffractive production of various heavy systems (e.g. Higgs, dijet, \(t\bar{t}\) and SUSY particles) at LHC and Tevatron collider energies. In each case we compute the probability that the rapidity gaps, which occur on either side of the produced system, survive the effects of soft rescattering and QCD bremsstrahlung effects. We calculate both the luminosity for different production mechanisms, and a wide variety of subprocess cross sections. The results allow numerical predictions to be readily made for the cross sections of all these processes at the LHC and the Tevatron collider. For example, we predict that the cross section for the exclusive double-diffractive production of a 120 GeV Higgs boson at the LHC is about 3 fb, and that the QCD background in the \(b\bar{b}\) decay mode is about 4 times smaller than the Higgs signal if the experimental missing-mass resolution is 1 GeV. For completeness we also discuss production via \(\gamma\gamma\) or WW fusion.

The exotic double-charge exchange μ−→e + conversion in nuclei

The formalism for the neutrinoless ( μ −,e +) conversion is investigated in detail and the relevant nuclear matrix elements for light intermediate neutrinos in the case of 27Al( μ −,e +) 27Na are calculated. The nucleus 27Al is going to be used as a stopping target in the MECO experiment at Brookhaven, one of the most sensitive probes expected to reach a sensitivity in the branching ratio of the order 10 −16 within the next few years. The relevant transition operators are constructed utilizing a variety of mechanisms present in current gauge theories, with emphasis on the intermediate neutrinos, both light and heavy, and heavy SUSY particles. The nuclear wave functions, both for the initial state and all excited final states are obtained in the framework of 1s–0d shell model employing the well-known and tested Wildenthal realistic interaction. In the case of the light intermediate neutrinos the transition rates to all excited final states up to 25 MeV in energy are calculated. We find that the imaginary part of the amplitude is dominant. The total rate is calculated by summing over all these partial transition strengths. We also find that the rate due to the real part of the amplitude is much smaller than the corresponding quantity found previously by the closure approximation.

Searching for L-violating supersymmetry at the LHC

The possibility to simulate lepton number violating supersymmetric models has been introduced into the recently updated PYTHIA event generator, now containing 1278 decay channels of SUSY particles into SM particles via lepton number violating interactions. This generator has been used in combination with the ATLFAST detector simulation to study the impact of lepton number violation (LV) on event topologies in the ATLAS detector, and trigger menus designed for LV-SUSY are proposed based on very general considerations. In addition, a rather preliminary analysis is presented on the possibility for ATLAS to observe a signal above the background in several mSUGRA scenarios, using a combination of primitive cuts and neural networks to optimize the discriminating power between signal and background events over regions of parameter space rather than at individual points. It is found that a 5 sigma discovery is possible roughly for m_{1/2} < 1TeV and m_0 < 2TeV with an integrated luminosity of 30fb^{-1}, corresponding to one year of data taking with the LHC running at ``mid-luminosity'', L = 3*10^{33}s^{-1}cm^{-2}.

The spectrum of the MSSM with non-standard supersymmetry breaking

The Minimal Supersymmetric Standard Model (MSSM) can include two soft breaking terms which are often neglected: a non-analytic scalar trilinear coupling and a Higgsino bilinear term. A set of high-scale boundary conditions consistent with the reparameterisation invariance which the model possesses is obtained. The three-family renormalisation group equations for the MSSM with these terms are presented. The ranges of the universal high-scale values of these couplings which lead to an acceptable TeV-scale theory are obtained, as is the supersymmetric particle spectrum at this scale. The effect of the new terms on fine-tuning is presented. SOFTSUSY, an existing program for calculating SUSY particle spectra, has been used, with as few modifications as possible.

Higgs, SUSY and the standard model at γγ colliders

In this report, I surveyed physics potential of the γγ option of a linear e +e − collider with the following questions in mind: What new discovery can be expected at a γγ collider in addition to what will be learned at its ‘ parent’ e +e − linear collider? By taking account of the hard energy spectrum and polarization of colliding photons, produced by Compton back-scattering of laser light off incoming e − beams, we find that a γγ collider is most powerful when new physics appears in the neutral spin-zero channel at an invariant mass below about 80% of the c.m. energy of the colliding e −e − system. If a light Higgs boson exists, its properties can be studied in detail, and if its heavier partners or a heavy Higgs boson exists in the above mass range, they may be discovered at a γγ collider. CP property of the scalar sector can be explored in detail by making use of linear polarization of the colliding photons, decay angular correlations of final state particles, and the pattern of interference with the Standard Model amplitudes. A few comments are given for SUSY particle studies at a γγ collider, where a pair of charged spinless particles is produced in the s-wave near the threshold. Squark-onium may be discovered. An e ±γ collision mode may measure the Higgs- Z-γ coupling accurately and probe flavor oscillations in the slepton sector. As a general remark, all the Standard Model background simulation tools should be prepared in the helicity amplitude level, so that simulation can be performed for an arbitrary set of Stokes parameters of the incoming photon beams.

Constraints on the minimal supergravity model and prospects for SUSY particle production at future linear e+e− colliders

We perform a complete analysis of the supersymmetric particle spectrum in the Minimal Supergravity (mSUGRA) model where the soft SUSY breaking scalar masses, gaugino masses and trilinear couplings are unified at the GUT scale, so that the electroweak symmetry is broken radiatively. We show that the present constraints on the Higgs boson and superparticle masses from collider searches and precision measurements still allow for large regions of the mSUGRA parameter space where charginos, neutralinos, sleptons and top squarks as well as the heavier Higgs particles, are light enough to be produced at the next generation of $\ee$ linear colliders with center of mass energy around $\sqrt{s} \sim 800$ GeV, with sizeable cross sections. An important part of this parameter space remains even when we require that the density of the lightest neutralinos left over from the Big Bang, which we calculate using standard assumptions, falls in the range favored by current determinations of the Dark Matter density in the Universe. Already at a c.m. energy of 500 GeV, SUSY particles can be accessible in some parameter range, and if the energy is increased to $\sqrt{s} \simeq 1.2$ TeV, the $e^+e^-$ collider will have a reach for high precision studies of SUSY particles in a range that is comparable to the discovery range of the LHC.

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 $\chi_1^0$ 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 be also 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 $\tan \beta$ 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 $q\bar{q}$ pairs, open up.

Testing neutrino mixing at future collider experiments

Low energy supersymmetry with bilinear breaking of R parity leads to a weak-scale seesaw mechanism for the atmospheric neutrino scale and a radiative mechanism for the solar neutrino scale. The model has striking implications for collider searches of supersymmetric particles. Assuming that the lightest SUSY particle is the lightest neutralino we demonstrate that (i) the neutralino decays inside the detector even for tiny neutrino masses and (ii) measurements of the neutrino mixing angles lead to predictions for the ratios of various neutralino decay branching ratios implying an independent test of neutrino physics at future colliders, such as the CERN Large Hadron Collider or a Linear Collider. We study the lightest neutralino decay branching ratio predictions taking into account present supersymmetric particle mass limits as well as restrictions coming from neutrino physics, with emphasis on the solar and atmospheric neutrino anomalies.