Kinematic Edges Research Articles

Many leptons at the LHC from the next-to-minimal supersymmetric standard model

We present a benchmark in the parameter space of the next-to-minimal supersymmetric standard model (NMSSM) that provides for a dramatic multilepton signal and no jets containing 5 or more leptons resulting from the cascade decays of the third lightest neutralino, ${\ensuremath{\chi}}_{3}^{0}$, and the lightest chargino, ${\ensuremath{\chi}}_{1}^{\ifmmode\pm\else\textpm\fi{}}$, via light charged sleptons. This is a very clean signal with almost no standard model (SM) background. In some cases, a total signal of $\ensuremath{\ge}3\text{ }\mathrm{\text{leptons}}+0\text{ }\mathrm{\text{jets}}$ can be detected at the $5\ensuremath{\sigma}$ level at the LHC running at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$ with approximately $3\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data and with less than $1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ when running at $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$. In addition, kinematic edges in the invariant mass distributions of 2, 3, and 4 leptons are easily detectable with large integrated luminosities ($\ensuremath{\sim}600\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$) which can lead to simple measurements of the mass differences of heavy particles in the decay chains, including all combinations of the three lightest neutralinos.

Distinguishing dark matter stabilization symmetries using multiple kinematic edges and cusps

We emphasize that the stabilizing symmetry for dark matter (DM) particles does not have to be the commonly used parity (${Z}_{2}$) symmetry. We therefore examine the potential of the colliders to distinguish models with parity stabilized DM from models in which the DM is stabilized by other symmetries. We often take the latter to be a ${Z}_{3}$ symmetry for illustration. We focus on signatures where a single particle, charged under the DM stabilization symmetry decays into the DM and standard model (SM) particles. Such a ${Z}_{3}$-charged mother particle can decay into one or two DM particles along with the same SM particles. This can be contrasted with the decay of a ${Z}_{2}$-charged mother particle, where only one DM particle appears. Thus, if the intermediate particles in these decay chains are off-shell, then the reconstructed invariant mass of the SM particles exhibits two kinematic edges for the ${Z}_{3}$ case but only one for the ${Z}_{2}$ case. For the case of on-shell intermediate particles, distinguishing the two symmetries requires more than the kinematic edges. In this case, we note that certain decay chain topologies of the mother particle which are present for the ${Z}_{3}$ case (but absent for the ${Z}_{2}$ case) generate a cusp in the invariant mass distribution of the SM particles. We demonstrate that this cusp is generally invariant of the various spin configurations. We further apply these techniques within the context of explicit models.

Using subsystemMT2for complete mass determinations in decay chains with missing energy at hadron colliders

We propose to use the MT2 concept to measure the masses of all particles in SUSY-like events with two unobservable, identical particles. To this end we generalize the usual notion of MT2 and define a new MT2(n,p,c) variable, which can be applied to various subsystem topologies, as well as the full event topology. We derive analytic formulas for its endpoint MT2{max}(n,p,c) as a function of the unknown test mass Mc of the final particle in the subchain and the transverse momentum pT due to radiation from the initial state. We show that the endpoint functions MT2{max}(n,p,c)(Mc,pT) may exhibit three different types of kinks and discuss the origin of each type. We prove that the subsystem MT2(n,p,c) variables by themselves already yield a sufficient number of measurements for a complete determination of the mass spectrum (including the overall mass scale). As an illustration, we consider the simple case of a decay chain with up to three heavy particles, X2 -> X1 -> X0, which is rather problematic for all other mass measurement methods. We propose three different MT2-based methods, each of which allows a complete determination of the masses of particles X0, X1 and X2. The first method only uses MT2(n,p,c) endpoint measurements at a single fixed value of the test mass Mc. In the second method the unknown mass spectrum is fitted to one or more endpoint functions MT2{max}(n,p,c)(Mc,pT) exhibiting a kink. The third method is hybrid, combining MT2 endpoints with measurements of kinematic edges in invariant mass distributions. As a practical application of our methods, we show that the dilepton W+W- and tt-bar samples at the Tevatron can be used for an independent determination of the masses of the top quark, the W boson and the neutrino, without any prior assumptions.

Mixed sneutrinos, dark matter, and the CERN LHC

We study the phenomenology of supersymmetric models in which gauge-singlet scalars mix with the minimal supersymmetric standard model (MSSM) sneutrinos through weak-scale $A$ terms. After reviewing the constraints on mixed-sneutrino dark matter from measurements of ${\ensuremath{\Omega}}_{\mathrm{CDM}}$ and from direct-detection experiments, we explore mixed-sneutrino signatures relevant to the LHC. For a mixed-sneutrino lightest supersymmetric particle (LSP) and a right-handed slepton next-to-lightest supersymmetric particle (NLSP), decays of the lightest neturalino can produce opposite-sign, same-flavor (OSSF) dileptons with an invariant-mass distribution shifted away from the kinematic end point. This signature is possible for parameters that lead to a cosmologically viable mixed-sneutrino LSP. We also consider signatures that require larger mixing angles than preferred for mixed-sneutrino dark matter, but which are possible regardless of whether a mixed-sneutrino is the LSP. In some parameter regions, the charginos and neutralinos produced in cascades all decay dominantly to the lighter sneutrinos, leading to a kinematic edge in the jet-lepton invariant-mass distribution from the decay chain $\stackrel{\texttildelow{}}{q}\ensuremath{\rightarrow}{\ensuremath{\chi}}^{\ensuremath{-}}q\ensuremath{\rightarrow}{\stackrel{\texttildelow{}}{\ensuremath{\nu}}}^{*}lq$, without an OSSF dilepton signature. We explore the possibility of using mass-estimation methods to distinguish this mixed-sneutrino jet-lepton signature from an MSSM one. Finally, we consider signatures associated with Higgs-lepton or $Z$-lepton production in cascades involving the heavier sneutrinos.

Determining SUSY model parameters and masses at the LHC using cross-sections, kinematic edges and other observables.

We address the problem of mass measurements of supersymmetric particles at the Large Hadron Collider, using the ATLAS detector as an example. By using Markov Chain sampling techniques to combine standard measurements of kinematic edges in the invariant mass distributions of decay products with a measurement of a missing $p_T$ cross-section, we show that the precision of mass measurements at the LHC can be dramatically improved, even when we do not assume that we have measured the kinematic endpoints precisely, or that we have identified exactly which particles are involved in the decay chain causing the endpoints. The generality of the technique is demonstrated in a preliminary investigation of a non-universal SUGRA model, in which we relax the requirements of mSUGRA by breaking the degeneracy of the GUT scale gaugino masses. The model studied is compatible with the WMAP limits on dark matter relic density.

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.