Accurate Mass Determinations in Decay Chains with Missing Energy

Abstract

Many beyond the standard model theories include a stable dark matter candidate that yields missing or invisible energy in collider detectors. If observed at the CERN Large Hadron Collider, we must determine if its mass and other properties (and those of its partners) predict the correct dark matter relic density. We give a new procedure for determining its mass with small error. DOI: 10.1103/PhysRevLett.100.252001 PACS numbers: 13.85.Qk, 14.80.Ly One of the most dramatic possibilities for the Large Hadron Collider (LHC) is the observation of events with large missing energy compatible with the production of a stable, weakly interacting particle that could explain the Universe's relic dark matter content. Many beyond the standard model (SM) theories contain such a particle, denoted N. In particular, in the minimal supersymmetric standard model (MSSM) the lightest neutralino~0 1 is stable if R parity is conserved. Each LHC event must contain two N's that each emerge at the end of a chain decay. For example, in the MSSM, a large production rate is associated with the squark pair,qq , production, and eachq can have substantial probability to decay viaq ! q~0 2 ! q'' ! q' '~0 1 (' e; ; ), where~0 2 andl are the second lightest neutralino and slepton, respectively. More generally, we will use the notation Z ! 7 Y ! 7 5 X ! 7 5 3 1 N , where particles 7, 5, and 3 are standard model jets or leptons and Z, Y, and X are intermediate on-shell resonances. This event structure is illustrated in Fig. 1. This Letter gives a procedure for accurately determining M Z , M Y , M X , and M N for this topology.Many mass determination procedures examine only one decay chain at a time [1][2][3][4]. This often does not allow one to solve for the event's missing momenta. An exception is a very long decay chain starting from the gluino [2]. Considering both decay chains simultaneously gives us more information and allows a better determination of the masses [5][6][7]. For the decay chains of Fig. 1, if all particles can be correctly located on the decay chains and there are no experimental effects, by considering two events we can solve for all the 4-momenta in both events and determine all the masses up to a discrete ambiguity. After examining a small number of event pairings, a unique solution will emerge.Assuming we can isolate LHC events with the topology in Fig. 1 and using m N m N

Determining the dark matter particle mass through antler topology processes at lepton colliders

“…(7), (10), and (12). The ellipse shape of the contour will become manifest when extending to larger regions.…”

“…energy and frame. There exist many attempts to determine the missing particle mass at the LHC, such as endpoint methods [11], polynomial methods [12], M T 2 methods [13], and the matrix element method [14]. Recently, we studied the "antler decay" diagram [15], as illustrated in Fig.…”

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

“…Given such a large multi-lepton signal, it is then possible to estimate several mass differences from the kinematic edges that will be observable in the invariant mass distributions of the leptons when enough integrated luminosity is accumulated [35][36][37][38][39][40][41][42]. Analytic expressions for the kinematic edges in the invariant mass distributions of 2, 3, and 4 leptons resulting from a sequence of two-body decays are given in Appendix A.…”

Extracting gluino endpoints with event topology patterns

“…Another approach is to use the family of M T 2 -based kinematic variables , which often serves the event-by-event best lower bound on the unknown particle mass of interest. The third option is the polynomial method [33][34][35][36][37][38][39], which attempts to determine all the missing momenta in the event by solving the kinematic constraints inherent to the process. This allows to measure all intermediate particle masses simultaneously.…”