This document proposes a collection of simplified models relevant to the design of new-physics searches at the Large Hadron Collider (LHC) and the characterization of their results. Both ATLAS and CMS have already presented some results in terms of simplified models, and we encourage them to continue and expand this effort, which supplements both signature-based results and benchmark model interpretations. A simplified model is defined by an effective Lagrangian describing the interactions of a small number of new particles. Simplified models can equally well be described by a small number of masses and cross-sections. These parameters are directly related to collider physics observables, making simplified models a particularly effective framework for evaluating searches and a useful starting point for characterizing positive signals of new physics. This document serves as an official summary of the results from the 'Topologies for Early LHC Searches' workshop, held at SLAC in September
Dark matter could have an electroweak origin, yet communicate with the visible sector exclusively through gravitational interactions. In a set-up addressing the hierarchy problem, we propose a new dark matter scenario where gravitational mediators, arising from the compactification of extra-dimensions, are responsible for dark matter interactions and its relic abundance in the Universe. We write an explicit example of this mechanism in warped extradimensions and work out its constraints. We also develop a dual picture of the model, based on a four-dimensional scenario with partial compositeness. We show that Gravity-mediated Dark Matter is equivalent to a mechanism of generating viable dark matter scenarios in a strongly-coupled, near-conformal theory, such as in composite Higgs models. *
We propose to use the M T 2 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 M T 2 and define a new M (n,p,c) T 2 variable, which can be applied to various subsystem topologies, as well as the full event topology. We derive analytic formulas for its endpoint M (n,p,c) T 2,max as a function of the unknown test massM c of the final particle in the subchain and the transverse momentum p T due to radiation from the initial state. We show that the endpoint functions M (n,p,c) T 2,max (M c , p T ) may exhibit three different types of kinks and discuss the origin of each type. We prove that the subsystem M (n,p,c) T 2 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, X 2 → X 1 → X 0 , which is rather problematic for all other mass measurement methods. We propose three different M T 2 -based methods, each of which allows a complete determination of the masses of particles X 0 , X 1 and X 2 . The first method only uses M (n,p,c) T 2 endpoint measurements at a single fixed value of the test massM c . In the second method the unknown mass spectrum is fitted to one or more endpoint functions M (n,p,c) T 2,max (M c , p T ) exhibiting a kink. The third method is hybrid, combining M T 2 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 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. 42 -2 - [18,19,27,38,39].• III. M T 2 methods. These methods explore the transverse invariant mass variable M T 2 originally proposed in [13] and later used and developed in [17,24,26,28,37,40,41,45].Recently it was shown that under certain circumstances, the endpoint of the M T 2 distribution, when considered as a function of the unknown test massM 0 of the lightest new particle X 0 , exhibits a kink and the true mass M 0 of X 0 , i.e. atM 0 = M 0 [29-32,36].One could also combine two or more of these techniques into a hybrid method, e.g. a mixed polynomial and endpoint method [34], a mixed M T 2 and endpoint method [33,43], or a mixed M T 2 and polynomial method [47,48]. In Section 2 we shall describe in detail each of these three basic approaches I -III. We shall then contrast them to each other and discuss their pros and cons. In particular, we shall concentrate on their applicability as a function of the length of the decay chain, i.e. the number n of intermediate resonances in Fig. 1. We shall find that for sufficiently long decay chains, namely n ≥ 3, each method I -III by itself is able to completely determine the unknown particle spectrum, at least as a matter of principle. Therefore, if Nature is so ki...
A search for narrow resonances with a mass of at least 1 TeV in the dijet mass spectrum is performed using pp collisions at root s = 7 TeV corresponding to an integrated luminosity of 1 fb(-1), collected by the CMS experiment at the LHC. No resonances are observed. Upper limits at the 95 confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, and gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95 confidence level: string resonances with mass less than 4.00 TeV, E-6 diquarks with mass less than 3.52 TeV, excited quarks with mass less than 2.49 TeV, axigluons and colorons with mass less than 2.47 TeV, and W' bosons with mass less than 1.51 TeV. These results extend previous exclusions from the dijet mass search technique. (C) 2011 CERN. Published by Elsevier B.V. All rights reserved
Abstract:We consider a class of on-shell constrained mass variables that are 3+1 dimensional generalizations of the Cambridge M T 2 variable and that automatically incorporate various assumptions about the underlying event topology. The presence of additional onshell constraints causes their kinematic distributions to exhibit sharper endpoints than the usual M T 2 distribution. We study the mathematical properties of these new variables, e.g., the uniqueness of the solution selected by the minimization over the invisible particle 4-momenta. We then use this solution to reconstruct the masses of various particles along the decay chain. We propose several tests for validating the assumed event topology in missing energy events from new physics. The tests are able to determine: 1) whether the decays in the event are two-body or three-body, 2) if the decay is two-body, whether the intermediate resonances in the two decay chains are the same, and 3) the exact sequence in which the visible particles are emitted from each decay chain.
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