arXiv subjects

B. C. Allanach

Publications and source records attributed to B. C. Allanach.

At least 55 records · Page 3Linked to original sources

arXiv2013

The dark side of the $μ$: on multiple solutions to renormalisation group equations, and why the CMSSM is not necessarily being ruled out

When solving renormalisation group equations in a quantum field theory, one often specifies the boundary conditions at multiple renormalisation scales, such as the weak and grand-unified scales in a theory beyond the standard model. A point in the parameter space of such a model is usually specified by the values of couplings at these boundaries of the renormalisation group flow, but there is no theorem guaranteeing that such a point has a unique solution to the associated differential equations, and so there may exist multiple, phenomenologically distinct solutions, all corresponding to the same point in parameter space. We show that this is indeed the case in the constrained minimal supersymmetric standard model (CMSSM), and we exhibit such solutions, which cannot be obtained using out-of-the-box computer programs in the public domain. Some of the multiple solutions we exhibit have CP-even lightest Higgs mass predictions between 124 and 126 GeV. Without an exhaustive 11-dimensional MSSM parameter scan per CMSSM parameter point to capture all of the multiple solutions, CMSSM phenomenological analyses are incomplete.

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arXiv2013

Uncertainty in Electroweak Symmetry Breaking in Models With High Scale Supersymmetry Breaking and its Impact on Interpretations of Searches For Supersymmetric Particles

Some regions of parameter space of the minimal supersymmetric standard model (MSSM) with high scale supersymmetry breaking have extreme sensitivity of electroweak symmetry breaking (EWSB) to the top quark mass through renormalisation group evolution effects. This leads to uncertainties in the predictions which need to be taken into account in the interpretation of searches for supersymmetric particles in these regions. As an example, we provide estimates of the current uncertainties on the position in parameter space of the region which does not break electroweak symmetry in the constrained MSSM (CMSSM). The position of the boundary of EWSB can vary by up to 2 TeV in m_0 due to the uncertainties coming from the current measurement errors on the top quark mass and from higher order corrections. In this dangerous region, for fixed CMSSM parameters the neutralino lightest supersymmetric particle mass has an associated large uncertainty of order 100%. These uncertainties therefore have a profound effect on the interpretation of LHC supersymmetric particle searches in terms of the CMSSM. We also show how to ameliorate poor convergence of the iterative numerical algorithm that calculates the MSSM spectrum near the boundary of EWSB.

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arXiv2012

R-Parity Violating Supersymmetry Explanation for Large t tbar Forward-Backward Asymmetry

We propose a supersymmetric explanation for the anomalously high forward backward asymmetry in top pair production measured by CDF and D0. We suppose that it is due to the t-channel exchange of a right-handed sbottom which couples to d_R and t_R, as is present in the R-parity violating minimal supersymmetric standard model. We show that all Tevatron and LHC experiments' t tbar constraints may be respected for a sbottom mass between 300 and 1200 GeV, and a large Yukawa coupling >2.2, yielding A_{FB} up to 0.18. The non Standard Model contribution to the LHC charge asymmetry parameter is Delta A_C^y=0.017-0.045, small enough to be consistent with current measurements but non-zero and positive, allowing for LHC confirmation in the future within 20 fb^-1. A small additional contribution to the LHC t tbar production cross-section is also predicted, allowing a further test. We estimate that 10 fb^-1 of LHC luminosity would be sufficient to rule out the proposal to 95% confidence level, if the measurements of the t tbar cross-section turn out to be centred on the Standard Model prediction.

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arXiv2012

Including R-parity violation in the numerical computation of the spectrum of the minimal supersymmetric standard model: SOFTSUSY

Current publicly available computer programs calculate the spectrum and couplings of the minimal supersymmetric standard model under the assumption of R-parity conservation. Here, we describe an extension to the SOFTSUSY program which includes R-parity violating effects. The user provides a theoretical boundary condition upon the high-scale supersymmetry breaking R-parity violating couplings. Successful radiative electroweak symmetry breaking, electroweak and CKM matrix data are used as weak-scale boundary conditions. The renormalisation group equations are solved numerically between the weak scale and a high energy scale using a nested iterative algorithm. This paper serves as a manual to the R-parity violating mode of the program, detailing the approximations and conventions used.

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arXiv2012

SOFTSUSY: a program for calculating supersymmetric spectra

SOFTSUSY is a program which accurately calculates the spectrum of superparticles in the CP-conserving Minimal Supersymmetric Standard Model (MSSM), with a full flavour mixing structure. The program solves the renormalisation group equations with theoretical constraints on soft supersymmetry breaking terms provided by the user. Weak-scale gauge coupling and fermion mass data (including one-loop finite MSSM corrections) are used as a boundary condition, as well as successful radiative electroweak symmetry breaking. The program can also calculate a measure of fine-tuning. The program structure has been designed to easily generalise to extensions of the MSSM. This article serves as a self-contained guide to prospective users, and indicates the conventions and approximations used.

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arXiv2012

Les Houches 2011: Physics at TeV Colliders New Physics Working Group Report

We present the activities of the "New Physics" working group for the "Physics at TeV Colliders" workshop (Les Houches, France, 30 May-17 June, 2011). Our report includes new agreements on formats for interfaces between computational tools, new tool developments, important signatures for searches at the LHC, recommendations for presentation of LHC search results, as well as additional phenomenological studies.

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arXiv2012

Computation of Neutrino Masses in R-parity Violating Supersymmetry: SOFTSUSY3.2

The program SOFTSUSY can calculate tree-level neutrino masses in the R-parity violating minimal supersymmetric standard model (MSSM) with real couplings. At tree-level, only one neutrino acquires a mass, in contradiction with neutrino oscillation data. Here, we describe an extension to the SOFTSUSY program which includes one-loop R-parity violating effects' contributions to neutrino masses and mixing. Including the one-loop effects refines the radiative electroweak symmetry breaking calculation, and may result in up to three massive, mixed neutrinos. This paper serves as a manual to the neutrino mass prediction mode of the program, detailing the approximations and conventions used.

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arXiv2012

Searches for New Physics: Les Houches Recommendations for the Presentation of LHC Results

We present a set of recommendations for the presentation of LHC results on searches for new physics, which are aimed at providing a more efficient flow of scientific information between the experimental collaborations and the rest of the high energy physics community, and at facilitating the interpretation of the results in a wide class of models. Implementing these recommendations would aid the full exploitation of the physics potential of the LHC.

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arXiv2012

Hide and Seek With Natural Supersymmetry at the LHC

Gluinos that result in classic large missing transverse momentum signatures at the LHC have been excluded by 2011 searches if they are lighter than around 800 GeV. This adds to the tension between experiment and supersymmetric solutions of the naturalness problem, since the gluino is required to be light if the electroweak scale is to be natural. Here, we examine natural scenarios where supersymmetry is present, but was hidden from 2011 searches due to violation of R-parity and the absence of a large missing transverse momentum signature. Naturalness suggests that third generation states should dominate gluino decays and we argue that this leads to a generic signature in the form of same-sign, flavour-ambivalent leptons, without large missing transverse momentum. As a result, searches in this channel are able to cover a broad range of scenarios with some generality and one should seek gluinos that decay in this way with masses below a TeV. We encourage the LHC experiments to tailor a search for supersymmetry in this form. We consider a specific case that is good at hiding: baryon number violation, and estimate that the most constraining existing search from 2011 data implies a lower bound on the gluino mass of 550 GeV.

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arXiv2012

Benchmark Models, Planes, Lines and Points for Future SUSY Searches at the LHC

We define benchmark models for SUSY searches at the LHC, including the CMSSM, NUHM, mGMSB, mAMSB, MM-AMSB and p19MSSM, as well as models with R-parity violation and the NMSSM. Within the parameter spaces of these models, we propose benchmark subspaces, including planes, lines and points along them. The planes may be useful for presenting results of the experimental searches in different SUSY scenarios, while the specific benchmark points may serve for more detailed detector performance tests and comparisons. We also describe algorithms for defining suitable benchmark points along the proposed lines in the parameter spaces, and we define a few benchmark points motivated by recent fits to existing experimental data.

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arXiv2011

Interpreting a 1 fb^-1 ATLAS Search in the Minimal Anomaly Mediated Supersymmetry Breaking Model

Recent LHC data significantly extend the exclusion limits for supersymmetric particles, particularly in the jets plus missing transverse momentum channels. The most recent such data have so far been interpreted by the experiment in only two different supersymmetry breaking models: the constrained minimal supersymmetric standard model (CMSSM) and a simplified model with only squarks and gluinos and massless neutralinos. We compare kinematical distributions of supersymmetric signal events predicted by the CMSSM and anomaly mediated supersymmetry breaking (mAMSB) before calculating exclusion limits in mAMSB. We obtain a lower limit of 900 GeV on squark and gluino masses at the 95% confidence level for the equal mass limit, tan(beta)=10 and mu>0.

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arXiv2011

Supersymmetry With Prejudice: Fitting the Wrong Model to LHC Data

We critically examine interpretations of hypothetical supersymmetric LHC signals, fitting to alternative wrong models of supersymmetry breaking. The signals we consider are some of the most constraining on the sparticle spectrum: invariant mass distributions with edges and end-points from the golden cascade decay chain \tilde{q}_L -> q χ_2^0 (-> \tilde{l}^{\pm} l^{\mp} q) -> χ_1^0 l^+ l^- q. We assume a CMSSM point to be the `correct' one, and fit the signals instead to minimal gauge mediated supersymmetry breaking models (mGMSB) with a neutralino quasi-stable lightest supersymmetric particle, minimal anomaly mediation (mAMSB) and large volume string compactification models (LVS). mAMSB and LVS can be unambiguously discriminated against the CMSSM for the parameter point assumed and 1 inverse femtobarn of LHC data at 14 TeV. However, mGMSB would not be discriminated on the basis of the kinematic end-points alone, and would require further, more detailed investigation. The best-fit points of mGMSB and CMSSM look remarkably similar, making experimental discrimination at the LHC appear unlikely by any means.

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arXiv2011

Discovery reach for generic supersymmetry at the LHC: MT2 versus missing transverse momentum selections for pMSSM searches

Different search strategies for supersymmetry have been employed by the LHC general-purpose experiments using early data. As proven by their early results, these strategies are promising, but raise the question of how well they will generalize for the future. We address this question by studying two thousand phenomenological minimal supersymmetric standard model parameter space points that come from a fit to indirect and cosmological data. We examine the 5 sigma discoverability of the points employing a typical ATLAS-type search based on missing transverse momentum (MET), a search based on an optimised MT2 cut and a combination of the two, taking into account standard model backgrounds. The discovery reach of the strategies can depend strongly on the systematic uncertainty in the background, subject to the stringency of the cuts and the details of the background simulation. By combining the MET and MT2 based strategies, with an integrated luminosity of 1 fb^-1 (10 fb^-1) at 7 TeV, 4-8% (42%) of the points are discoverable, depending on the systematic uncertainty on the background. At 14 TeV and with 10 fb^-1, 96% of the points are discoverable. While the majority of points can be discovered by both strategies at sqrt{s} = 14 TeV and with 1 fb^-1, there are some that are left undiscovered by a MET search strategy, but which are discovered by the MT2 strategy, and vice versa, therefore it is essential that one performs both in parallel. We discuss some of the factors that can make points more difficult to observe.

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arXiv2011

Impact of CMS Multi-jets and Missing Energy Search on CMSSM Fits

Recent CMS data significantly extend the direct search exclusion for supersymmetry. We examine the impact of such data on global fits of the constrained minimal supersymmetric standard model (CMSSM) to indirect and cosmological data. By simulating supersymmetric signal events at the LHC, we construct a likelihood map for the recent CMS data, validating it against the exclusion region calculated by the experiment itself. A previous CMSSM global fit is then re-weighted by our likelihood map. The CMS results nibble away at the high fit probability density region, transforming probability distributions for the scalar and gluino masses. The CMS search has a non-trivial effect on tan beta due to correlations between the parameters implied by the fits to indirect data.

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arXiv2011

The impact of the ATLAS zero-lepton, jets and missing momentum search on a CMSSM fit

Recent ATLAS data significantly extend the exclusion limits for supersymmetric particles. We examine the impact of such data on global fits of the constrained minimal supersymmetric standard model (CMSSM) to indirect and cosmological data. We calculate the likelihood map of the ATLAS search, taking into account systematic errors on the signal and on the background. We validate our calculation against the ATLAS determinaton of 95% confidence level exclusion contours. A previous CMSSM global fit is then re-weighted by the likelihood map, which takes a bite at the high probability density region of the global fit, pushing scalar and gaugino masses up.

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arXiv2010

Selecting a Model of Supersymmetry Breaking Mediation

We study the problem of selecting between different mechanisms of supersymmetry breaking in the Minimal Supersymmetric Standard Model using current data. We evaluate the Bayesian evidence of four supersymmetry breaking scenarios: mSUGRA, mGMSB, mAMSB and moduli mediation. The results show a strong dependence on the dark matter assumption. Using the inferred cosmological relic density as an upper bound, minimal anomaly mediation is at least moderately favoured over the CMSSM. Our fits also indicate that evidence for a positive sign of the mu parameter is moderate at best. We present constraints on the anomaly and gauge mediated parameter spaces and some previously unexplored aspects of the dark matter phenomenology of the moduli mediation scenario. We use sparticle searches, indirect observables and dark matter observables in the global fit and quantify robustness with respect to prior choice. We quantify how much information is contained within each constraint.

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arXiv2010

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 $μ$ 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.

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arXiv2009

SUSY Les Houches Accord 2

The Supersymmetry Les Houches Accord (SLHA) provides a universal set of conventions for conveying spectral and decay information for supersymmetry analysis problems in high energy physics. Here, we propose extensions of the conventions of the first SLHA to include various generalisations: the minimal supersymmetric standard model with violation of CP, R-parity, and flavour, as well as the simplest next-to-minimal model.

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