Benjamin C. Allanach

SUSY Les Houches accord: interfacing SUSY spectrum calculators, decay packages, and event generators

An accord specifying a unique set of conventions for supersymmetric extensions of the Standard Model together with generic file structures for 1) supersymmetric model specifications and input parameters, 2) electroweak scale supersymmetric mass and coupling spectra, and 3) decay tables is presented, to provide a universal interface between spectrum calculation programs, decay packages, and high energy physics event generators.

Precise determination of the neutral Higgs boson masses in the MSSM

We present the implementation of the radiative corrections of the Higgs sector in three public computer codes for the evaluation of the particle spectrum in the Minimal Supersymmetric Standard Model, SoftSusy, SPheno and SuSpect. We incorporate the full one–loop corrections to the Higgs boson masses and the electroweak symmetry breaking conditions, as well as the two–loop corrections controlled by the strong gauge coupling and the Yukawa couplings of the third generation fermions. We include also �� � � � �

Measuring sparticle masses in non-universal string inspired models at the LHC

We demonstrate that some of the suggested five supergravity points for study at the LHC could be approximately derived from perturbative string theories or M-theory, but that charge and colour breaking minima would result. As a pilot study, we then analyse a perturbative string model with non-universal soft masses that are optimised in order to avoid global charge and colour breaking minima. By combining measurements of up to six kinematic edges from squark decay chains with data from a new kinematic variable, designed to improve slepton mass measurements, we demonstrate that a typical LHC experiment will be able to determine squark, slepton and neutralino masses with an accuracy sufficient to permit an optimised model to be distinguished from a similar standard SUGRA point. The technique thus generalizes SUSY searches at the LHC.

Fitting the phenomenological MSSM

We perform a global Bayesian fit of the phenomenological minimal supersymmetric standard model (pMSSM) to current indirect collider and dark matter data. The pMSSM contains the most relevant 25 weak-scale MSSM parameters, which are simultaneously fit using nested sampling Monte Carlo techniques in more than 15 years of CPU time. We calculate the Bayesian evidence for the pMSSM and constrain its parameters and observables in the context of two widely different, but reasonable, priors to determine which inferences are robust. We make inferences about sparticle masses, the sign of the {mu} parameter, the amount of fine-tuning, dark matter properties, and the prospects for direct dark matter detection without assuming a restrictive high-scale supersymmetry breaking model. We find the inferred lightest CP-even Higgs boson mass as an example of an approximately prior-independent observable. This analysis constitutes the first statistically convergent pMSSM global fit to all current data.

Multidimensional mSUGRA likelihood maps

We calculate the likelihood map in the full 7-dimensional parameter space of the minimal supersymmetric standard model assuming universal boundary conditions on the supersymmetry breaking terms. Simultaneous variations of m{sub 0}, A{sub 0}, M{sub 1/2}, tan{beta}, m{sub t}, m{sub b} and {alpha}{sub s}(M{sub Z}) are applied using a Markov chain Monte Carlo algorithm. We use measurements of b{yields}s{gamma}, (g-2){sub {mu}} and {omega}{sub DM}h{sup 2} in order to constrain the model. We present likelihood distributions for some of the sparticle masses, for the branching ratio of B{sub s}{sup 0}{yields}{mu}{sup +}{mu}{sup -} and for m{sub {tau}}{sub -tilde}-m{sub {chi}{sub 1}{sup 0}}. An upper limit of 2x10{sup -8} on this branching ratio might be achieved at the Tevatron, and would rule out 29% of the currently allowed likelihood. If one allows for non-thermal-neutralino components of dark matter, this fraction becomes 35%. The mass ordering allows the important cascade decay q-tilde{sub L}{yields}{chi}{sub 2}{sup 0}{yields}l-tilde{sub R}{yields}{chi}{sub 1}{sup 0} with a likelihood of 24{+-}4%. The stop-coannihilation region is highly disfavored, whereas the light Higgs region is marginally disfavored.

Theoretical uncertainties in sparticle mass predictions from computational tools

We estimate the current theoretical uncertainty in sparticle mass pre- dictions by comparing several state-of-the-art computations within the minimal su- persymmetric standard model (MSSM). We nd that the theoretical uncertainty is comparable to the expected statistical errors from the Large Hadron Collider (LHC), and signicantly larger than those expected from a future e + e Linear Collider (LC). We quantify the theoretical uncertainty on relevant sparticle observables for both LHC and LC, and show that the value of the error is signicantly dependent upon the supersymmetry (SUSY) breaking parameters. We also present the theoretical uncertainty induced in fundamental scale SUSY breaking parameters when they are tted from LHC measurements. Two regions of the SUSY parameter space where accurate predictions are particularly dicult are examined in detail: the large tan and focus point regimes.

Natural priors, CMSSM fits and LHC weather forecasts

Previous LHC forecasts for the constrained minimal supersymmetric standard model (CMSSM), based on current astrophysical and laboratory measurements, have used priors that are flat in the parameter tanxa0β, while being constrained to postdict the central experimental value of MZ. We construct a different, new and more natural prior with a measure in μ and B (the more fundamental MSSM parameters from which tanxa0β and MZ are actually derived). We find that as a consequence this choice leads to a well defined fine-tuning measure in the parameter space. We investigate the effect of such on global CMSSM fits to indirect constraints, providing posterior probability distributions for Large Hadron Collider (LHC) sparticle production cross sections. The change in priors has a significant effect, strongly suppressing the pseudoscalar Higgs boson dark matter annihilation region, and diminishing the probable values of sparticle masses. We also show how to interpret fit information from a Markov Chain Monte Carlo in a frequentist fashion; namely by using the profile likelihood. Bayesian and frequentist interpretations of CMSSM fits are compared and contrasted.

Bayesian selection of sign μ within mSUGRA in global fits including WMAP5 results

We study the properties of the constrained minimal supersymmetric standard model (mSUGRA) by performing fits to updated indirect data, including the relic density of dark matter inferred from WMAP5. In order to find the extent to which ?? ?0, we compare the Bayesian evidence values for these models, which we obtain straightforwardly and with good precision from the recently developed multi-modal nested sampling (`MULTINEST) technique. We find weak to moderate evidence for the ??>?0 branch of mSUGRA over ?? ?0)/P(?? ?0 in mSUGRA. The MULTINEST technique also delivers probability distributions of parameters and other relevant quantities such as superpartner masses. We explore the dependence of our results on the choice of the prior measure used. We also use the Bayesian evidence to quantify the consistency between the mSUGRA parameter inferences coming from the constraints that have the largest effects: (g?2)?, BR(b?s?) and cold dark matter (DM) relic density ?DMh2.

750 GeV diphoton excess explained by a resonant sneutrino in R-parity violating supersymmetry

We explain the recent excess seen by ATLAS and CMS experiments at around 750 GeV in the di-photon invariant mass as a narrow width sneutrino decaying to di-photons via a stau loop in R-parity violating Supersymmetry. The stau mass is predicted to be somewhere between half the resonant sneutrino mass and half the sneutrino mass plus 14 GeV. The scenario also predicts further signal channels at an invariant mass of 750 GeV, the most promising being into di-jets and

Exploring small extra dimensions at the Large Hadron Collider

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