16547801
Precise reconstruction of sparticle masses without ambiguities
We critically reexamine the standard applications of the method of kinematical endpoints for sparticle mass determination. We consider the typical decay chain in supersymmetry (SUSY) ? 02 ? ? 01, which yields a jet j, and two leptons ?n? and ?f. The conventional approaches use the upper kinematical endpoints of the individual distributions mj??, mj?(lo) = min {mj?n,mj?f} and mj?(hi) = max {mj?n,mj?f}, all three of which suffer from parameter space region ambiguities and may lead to multiple solutions for the SUSY mass spectrum. In contrast, we do not use mj??, mj?(lo) and mj?(hi), and instead propose a new set of (infinitely many) variables whose upper kinematic endpoints exhibit reduced sensitivity to the parameter space region. We then outline an alternative, much simplified procedure for obtaining the SUSY mass spectrum. In particular, we show that the four endpoints observed in the three distributions m2??, m2j?n?m2j?f and m2j?n+m2j?f are sufficient to completely pin down the squark mass m and the two neutralino masses m02 and m01, leaving only a discrete 2-fold ambiguity for the slepton mass m. This remaining ambiguity can be easily resolved in a number of different ways: for example, by a single additional measurement of the kinematic endpoint of any one out of the many remaining 1-dimensional distributions at our disposal, or by exploring the correlations in the 2-dimensional distribution of m2j?n?m2j?f versus m2??. We illustrate our method with two examples: the LM1 and LM6 CMS study points. An additional advantage of our method is the expected improvement in the accuracy of the SUSY mass determination, due to the multitude and variety of available measurements.
59038897
Dark matter particle spectroscopy at the LHC: generalizing MT2 to asymmetric event topologies
We consider SUSY-like missing energy events at hadron colliders and critically examine the common assumption that the missing energy is the result of two identical missing particles. In order to experimentally test this hypothesis, we generalize the subsystem MT2 variable to the case of asymmetric event topologies, where the two SUSY decay chains terminate in different “children” particles. In this more general approach, the endpoint MT2(max) of the MT2 distribution now gives the mass $$ {\tilde M_p}\left( {\tilde M_c^{(a)},\tilde M_c^{(b)}} \right) $$ of the parent particles as a function of two input children masses $$ \tilde M_c^{(a)} $$ and $$ \tilde M_c^{(b)} $$. We propose two methods for an independent determination of the individual children masses M(a)c and M(b)c. First, in the presence of upstream transverse momentum PUTM the corresponding function $$ {\tilde M_p}\left( {\tilde M_c^{(a)},\tilde M_c^{(b)},{P_{\text{UTM}}}} \right) $$ is independent of PUTM at precisely the right values of the children masses. Second, the previously discussed MT2 “kink” is now generalized to a “ridge” on the 2-dimensional surface $$ {\tilde M_p}\left( {\tilde M_c^{(a)},\tilde M_c^{(b)}} \right) $$. As we show in several examples, quite often there is a special point along that ridge which marks the true values of the children masses. Our results allow collider experiments to probe a multi-component dark matter sector directly and without any theoretical prejudice.
119191013
The role of interference in unraveling the ZZ-couplings of the newly discovered boson at the LHC
We present a general procedure for measuring the tensor structure of the coupling of the scalar Higgs-like boson recently discovered at the LHC to two Z bosons, including the effects of interference among different operators. To motivate our concern with this interference, we explore the parameter space of the couplings in the effective theory describing these interactions and illustrate the effects of interference on the differential dilepton mass distributions. Kinematic discriminants for performing coupling measurements that utilize the effects of interference are developed and described. We present projections for the sensitivity of coupling measurements that use these discriminants in future LHC operation in a variety of physics scenarios.
18459358
Interplay between Fermi gamma-ray lines and collider searches
A bstractWe explore the interplay between lines in the gamma-ray spectrum and LHC searches involving missing energy and photons. As an example, we consider a singlet Dirac fermion dark matter with the mediator for Fermi gamma-ray line at 130 GeV. A new chiral or local U(1) symmetry makes weak-scale dark matter natural and provides the axion or Z′ gauge boson as the mediator connecting between dark matter and electroweak gauge bosons. In these models, the mediator particle can be produced in association with a monophoton at colliders and it produces large missing energy through the decays into a DM pair or ZZ, Zγ with at least one Z decaying into a neutrino pair. We adopt the monophoton searches with large missing energy at the LHC and impose the bounds on the coupling and mass of the mediator field in the models. We show that the parameter space of the Z′ mediation model is already strongly constrained by the LHC 8TeV data, whereas a certain region of the parameter space away from the resonance in axion-like mediator models are bounded. We foresee the monophoton bounds on the Z′ and axion mediation models at the LHC 14 TeV.
119312637
From Lagrangians to Events: Computer Tutorial at the MC4BSM-2012 Workshop
This is a written account of the computer tutorial oered at the Sixth MC4BSM workshop at Cornell University, March 22-24, 2012. The tools covered during the tutorial include: FeynRules, LanHEP, MadGraph, CalcHEP, Pythia 8, Herwig++, and Sherpa. In the tutorial, we specify a simple extension of the Standard Model, at the level of a Lagrangian. The software tools are then used to automatically generate a set of Feynman rules, compute the invariant matrix element for a sample process, and generate both parton-level and fully hadronized/showered Monte Carlo event samples. The tutorial is designed to be self-paced, and detailed instructions for all steps are included in this write-up. Installation instructions for each tool on a variety of popular platforms are also provided.
118337925
Spin effects in the antler event topology at hadron colliders
A bstractWe investigate spin correlation effects in the “antler” event topology pp → A(A*) → B1B2 → (ℓ−C1)(ℓ+C2) at the LHC. We study the shapes of several kinematic variables, including the relative pseudorapidity, relative azimuthal angle and the energies of the two leptons, as well as several mass variables Mℓℓ, Meff, $ \sqrt {{{{s}_{{\min }}}}} $, MT2, MCT and MCTx. We focus on the two kinematic extremes of $\sqrt{S}$ — threshold and infinity — and derive analytical expressions for the differential distributions of several variables, most notably the cos $\theta_{{\ell -\ell +}}^{*}$ variable proposed by Barr in hep-ph/0511115. For all possible spin assignments of particles A, B and C, we derive the cos $\theta_{{\ell -\ell +}}^{*}$ differential distribution at threshold, including the effects of spin correlations. Our analytical results help identify the problematic cases for spin discrimination.
118380500
Simplified Models for LHC New Physics Searches
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 of 2010, the purpose of which was to develop a set of representative models that can be used to cover all relevant phase space in experimental searches. Particular emphasis is placed on searches relevant for the first similar to 50-500 pb(-1) of data and those motivated by supersymmetric models. This note largely summarizes material posted at http://lhcnewphysics.org/, which includes simplified model definitions, Monte Carlo material, and supporting contacts within the theory community. We also comment on future developments that may be useful as more data is gathered and analyzed by the experiments.
119444647
Self-accelerating Warped Braneworlds
Braneworld models with induced gravity have the potential to replace dark energy as the explanation for the current accelerating expansion of the Universe. The original model of Dvali, Gabadadze, and Porrati (DGP) demonstrated the existence of a 'self-accelerating' branch of background solutions, but suffered from the presence of ghosts. We present a new large class of braneworld models which generalize the DGP model. Our models have negative curvature in the bulk, allow a second brane, and have general brane tensions and localized curvature terms. We exhibit three different kinds of ghosts, associated to the graviton zero mode, the radion, and the longitudinal components of massive graviton modes. The latter two species occur in the DGP model, for negative and positive brane tension, respectively. In our models, we find that the two kinds of DGP ghosts are tightly correlated with each other, but are not always linked to the feature of self-acceleration. Our models are a promising laboratory for understanding the origins and physical meaning of braneworld ghosts, and perhaps for eliminating them altogether.
119239023
Multi-lepton signals of the Higgs boson
A bstractThe possibility of searching for the Higgs boson in channels with multiple non-resonant leptons is evaluated in light of recent advances in multi-lepton search techniques at the LHC. The total multi-lepton Higgs signal exceeds the four lepton gold-plated resonant mode, but is spread over many channels with same-sign di-lepton, tri-lepton, and four lepton final states. While any individual channel alone is not significant, the exclusive combination across multiple channels is shown to provide a sensitivity nearly comparable to other discovery level searches for the Higgs boson. We estimate that with 5 fb−1 of data, existing non-optimized multi-lepton searches at the LHC could exclude the Higgs boson to 95 % CL at a few times the predicted Standard Model cross section in the mass range 120 − 150 GeV. Refinements focused specifically on the Higgs boson signal are suggested that would further increase sensitivity. We illustrate the possibility of discerning patterns in production and decay modes using correlations across multiple channels by comparing sensitivities to Standard Model, Fermi-phobic, and b-phobic Higgs bosons.
118628340
Recovering Particle Masses from Missing Energy Signatures with Displaced Tracks
If physics beyond the Standard Model contains metastable new particle states, its signatures may manifest themselves at the LHC through the presence of displaced vertices or displaced tracks. Such signatures are common in low-scale supersymmetry breaking scenarios as well as a host of other well-studied new physics models. Here we analyze the kinematics of dual cascade decays with missing energy resulting from a stable non-interacting new particle state at the bottom of the decay chain. We find that if the final step of the decay chain involves a metastable particle, the resulting presence of displaced tracks provides strong constraints with which to solve for the unknown kinematic quantities lost through missing energy. In addition we develop techniques with which to recover all kinematic unknowns, even in situations where the unknown quantities outnumber the constraints. These techniques can all be performed using a very small number of events and can thus be applied to very early discovery level searches at the LHC.
54759462
Dark energy or modified gravity? An effective field theory approach
We take an Effective Field Theory (EFT) approach to unifying existing proposals for the origin of cosmic acceleration and its connection to cosmological observations. Building on earlier work where EFT methods were used with observations to constrain the background evolution, we extend this program to the level of the EFT of the cosmological perturbations — following the example from the EFT of Inflation. Within this framework, we construct the general theory around an assumed background which will typically be chosen to mimic ΛCDM, and identify the parameters of interest for constraining dark energy and modified gravity models with observations. We discuss the similarities to the EFT of Inflation, but we also identify a number of subtleties including the relationship between the scalar perturbations and the Goldstone boson of the spontaneously broken time translations. We present formulae that relate the parameters of the fundamental Lagrangian to the speed of sound, anisotropic shear stress, effective Newtonian constant, and Caldwell's parameter, emphasizing the connection to observations. It is anticipated that this framework will be of use in constraining individual models, as well as for placing model-independent constraints on dark energy and modified gravity model building.