Invariant mass distributions in cascade decays

We derive analytical expressions for the shape of the invariant mass distributions of massless Standard Model endproducts in cascade decays involving massive New Physics (NP) particles, D -> Cc -> Bbc -> Aabc, where the final NP particle A in the cascade is unobserved and where two of the particles a, b, c may be indistinguishable. Knowledge of these expressions can improve the determination of NP parameters at the LHC. The shape formulas are composite, but contain nothing more complicated than logarithms of simple expressions. We study the effects of cuts, final state radiation and detector effects on the distributions through Monte Carlo simulations, using a supersymmetric model as an example. We also consider how one can deal with the width of NP particles and with combinatorics from the misidentification of final state particles. The possible mismeasurements of NP masses through `feet' in the distributions are discussed. Finally, we demonstrate how the effects of different spin configurations can be included in the distributions.Comment: 39 pages, 14 figures (colour), JHEP clas

A hybrid method for determining particle masses at the Large Hadron Collider with fully identified cascade decays

A new technique for improving the precision of measurements of SUSY particle masses at the LHC is introduced. The technique involves kinematic fitting of events with two fully identified decay chains. We incorporate both event ETmiss constraints and independent constraints provided by kinematic end-points in experiment invariant mass distributions of SUSY decay products. Incorporation of the event specific information maximises the information used in the fit and is shown to reduce the mass measurement uncertainites by ~30% compared to conventional fitting of experiment end-point constraints for the SPS1a benchmark model.Comment: 10 pages, 2 .eps figures, JHEP3 styl

Measuring R-parity-violating couplings in dilepton production at the LHC

We revisit the issue of probing R-violating couplings of supersymmetric theories at hadronic colliders, particularly at the LHC. Concentrating on dimuon production, an evaluation of the optimal sensitivity to the R-violating coupling is performed through a maximum likelihood analysis. The measurement uncertainties are evaluated through a study of fully generated events processed through a fast simulation of the ATLAS detector. It is found that a host of R-violating couplings can be measured to a statistical accuracy of better than 10%, over a significant part of the m_{tilde f} -- lambda parameter space still allowed by low energy measurements. Since the bounds thus obtained do not simply scale as the squark mass, one can do significantly better at the LHC than at the Tevatron. The same analysis can also be extended to assess the reach of the LHC to effects due to any non-SM structure of the four-fermion amplitude, caused by exchanges of new particles with different spins such as leptoquarks and gravitons that are suggested by various theoretical ideas.Comment: 28 pages, 14 figures (uses JHEP3.cls

Dark Matter and LHC: What is the Connection?

We review what can (and cannot) be learned if dark matter is detected in one or more experiments, emphasizing the importance of combining LHC data with direct, astrophysical and cosmological probes of dark matter. We briefly review the conventional picture of a thermally produced WIMP relic density and its connection with theories of electroweak symmetry breaking. We then discuss both experimental and theoretical reasons why one might generically expect this picture to fail. If this is the case, we argue that a combined effort bringing together all types of data -- combined with explicitly constructed theoretical models -- will be the only way to achieve a complete understanding of the dark matter in our universe and become confident that any candidate actually provides the relic density.Comment: 25 pages, 2 figures, Invited review for Modern Physics Letters

A Clean Slepton Mixing Signal at the LHC

In supersymmetric scenarios where the scalar tau lepton is stable or long-lived, a search for a decay mode chi0 --> stau + mu at the LHC has a good sensitivity to the flavor mixing in the scalar lepton sector. We demonstrate that the sensitivities to the mixing angle at the level of sin(theta)=0.15 are possible with an integrated luminosity of 100fb^{-1} if the total production cross section of supersymmetric particles is of the order of 1pb. The sensitivity to the mixing parameter can be better than the experimental bound from the tau --> mu + gamma decay depending on model parameters.Comment: 7 pages, 2 figures, 1 table, the stau resolution corrected. version to appear in JHE

Complementarity of Gamma-ray and LHC Searches for Neutralino Dark Matter in the Focus Point Region

We study the complementarity between the indirect detection of dark matter with gamma-rays in H.E.S.S. and the supersymmetry searches with ATLAS at the Large Hadron Collider in the Focus Point region within the mSUGRA framework. The sensitivity of the central telescope of the H.E.S.S. II experiment with an energy threshold of ~ 20 GeV is investigated. We show that the detection of gamma-ray fluxes of O(10^-12) cm-2s-1 with H.E.S.S. II covers a substantial part of the Focus Point region which may be more difficult for LHC experiments. Despite the presence of multi-TeV scalars, we show that LHC will be sensitive to a complementary part of this region through three body NLSP leptonic decays. This interesting complementarity between H.E.S.S. II and LHC searches is further highlighted in terms of the gluino mass and the two lightest neutralino mass difference.Comment: 11 pages, 6 figures, accepted for publication in Phys. Rev. D on February 7, 2008 (slight modifications: references updated and text improved

Measurement of the Gluino Mass via Cascade Decays for SPS 1a

If R-parity conserving supersymmetry is realised with masses below the TeV scale, sparticles will be produced and decay in cascades at the LHC. In the case of a neutral LSP, which will not be detected, decay chains cannot be fully reconstructed, complicating the mass determination of the new particles. In this paper we extend the method of obtaining masses from kinematical endpoints to include a gluino at the head of a five-sparticle decay chain. This represents a non-trivial extension of the corresponding method for the squark decay chain. We calculate the endpoints of the new distributions and assess their applicability by examining the theoretical distributions for a variety of mass scenarios. The precision with which the gluino mass can be determined by this method is investigated for the mSUGRA point SPS 1a. Finally we estimate the improvement obtained from adding a Linear Collider measurement of the LSP mass.Comment: 40 pages; extended discussion of error

Gravitino Dark Matter Scenarios with Massive Metastable Charged Sparticles at the LHC

We investigate the measurement of supersymmetric particle masses at the LHC in gravitino dark matter (GDM) scenarios where the next-to-lightest supersymmetric partner (NLSP) is the lighter scalar tau, or stau, and is stable on the scale of a detector. Such a massive metastable charged sparticle would have distinctive Time-of-Flight (ToF) and energy-loss ($dE/dx$) signatures. We summarise the documented accuracies expected to be achievable with the ATLAS detector in measurements of the stau mass and its momentum at the LHC. We then use a fast simulation of an LHC detector to demonstrate techniques for reconstructing the cascade decays of supersymmetric particles in GDM scenarios, using a parameterisation of the detector response to staus, taus and jets based on full simulation results. Supersymmetric pair-production events are selected with high redundancy and efficiency, and many valuable measurements can be made starting from stau tracks in the detector. We recalibrate the momenta of taus using transverse-momentum balance, and use kinematic cuts to select combinations of staus, taus, jets and leptons that exhibit peaks in invariant masses that correspond to various heavier sparticle species, with errors often comparable with the jet energy scale uncertainty.Comment: 23 pages, 10 figures, updated to version published in JHE