118403976
Guide to transverse projections and mass-constraining variables
This paper seeks to demonstrate that many of the existing mass-measurement variables proposed for hadron colliders (m{sub T}, m{sub eff}, m{sub T2}, missing p-vector{sub T}, h{sub T}, {radical}(s-circumflex){sub min}, etc.) are far more closely related to each other than is widely appreciated, and indeed can all be viewed as a common mass-bound specialized for a variety of purposes. A consequence of this is that one may understand better the strengths and weaknesses of each variable, and the circumstances in which each can be used to best effect. In order to achieve this, we find it necessary first to revisit the seemingly empty and infertile wilderness populated by the subscript 'T' (as in 'pe{sub T}') in order to remind ourselves what this process of transversification actually means. We note that, far from being simple, transversification can mean quite different things to different people. Those readers who manage to battle through the barrage of transverse notation distinguishing 'T' from 'v' or or from 'o', and 'early projection' from 'late projection', will find their efforts rewarded towards the end of the paper with (i) a better understanding of how collider mass variables fit together, (ii) an appreciation of how these variables could bemore » generalized to search for things more complicated than supersymmetry, (iii) will depart with an aversion to thoughtless or naieve use of the so-called 'transverse methods' of any of the popular computer Lorentz-vector libraries, and (iv) will take care in their subsequent papers to be explicit about which of the 61 identified variants of the 'transverse mass' they are employing.« less
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.
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.
206259922
Superpartner mass measurement technique using 1D orthogonal decompositions of the Cambridge transverse mass variable M(T2).
We propose a new model-independent technique for mass measurements in missing energy events at hadron colliders. We illustrate our method with the most challenging case of a single-step decay chain. We consider inclusive same-sign chargino pair production in supersymmetry, followed by leptonic decays to sneutrinos χ+ χ+ → ℓ+ ℓ'+ ν(ℓ)ν(ℓ') and invisible decays ν(ℓ) → ν(ℓ) χ(1)(0). We introduce two one-dimensional decompositions of the Cambridge MT2 variable: M(T2∥) and M(T2⊥), on the direction of the upstream transverse momentum P→T and the direction orthogonal to it, respectively. We show that the sneutrino mass Mc can be measured directly by minimizing the number of events N(Mc) in which MT2 exceeds a certain threshold, conveniently measured from the end point M(T2⊥)(max) (Mc).
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.
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.
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.
119195510
Quantum aspects of massive gravity II: non-Pauli-Fierz theory
We investigate the non-Pauli-Fierz (nPF) theory, a linearized massive gravity with a generic graviton mass term, which has been ignored due to a ghost in its spectrum and the resultant loss of unitarity. We first show that it is possible to use the Lee-Wick mechanism, a unitarization through the decay of a ghost, in order to handle the sixth mode ghost of nPF, and then check for the quantum consistency. Once proven to be consistent, nPF could become a viable candidate for a large distance modification of gravity, because it naturally solves the intrinsic problems that most darkenergy/modified gravity models suffer from: It smoothly converges to general relativity at short distances, and the small graviton mass necessary to modify gravity at large scales can be stable under the radiative corrections from the minimal gravity-to-matter coupling.
26327559
Quantum aspects of massive gravity
We consider the effect of quantum interactions on Pauli–Fierz massive gravity. With generic graviton cubic interactions, we observe that the 1-loop counterterms do not conform to the tree level structure of Pauli–Fierz action, resulting in the reappearance of the sixth mode ghost. Then to explore the quantum effects to the full extent, we calculate the resummed graviton propagator with an arbitrary interaction and analyze its complete structure, from which a minimal condition for the absence of the ghost is obtained.
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.
118013034
Searching for $t \to c h$ with Multi-Leptons
The results of a multi-lepton search conducted by the CMS collaboration with 5 fb$^{-1}$ of data collected from 7 TeV $pp$ collisions are used to place the first bound on the rare flavor-changing decay of the top quark to a Higgs boson and charm quark. Combining results from a number of exclusive three- and four-lepton search channels yields an estimated upper limit of ${\rm Br}(t \to ch) < 2.7%$ for a Higgs boson mass of 125 GeV. The sensitivity of future dedicated searches for $t \to c h$ could be improved by adding exclusive same sign di-lepton channels, as well as by sub-dividing channels based on $b$-quark tagging and partial kinematic top quark and Higgs boson tagging. This bound may be interpreted more widely within a range of new physics processes that yield final states with a $W$-boson in association with a Higgs boson. For such processes with kinematics that are similar to top--anti-top production and decay, the estimated limit on cross section times branching ratio corresponds to roughly $\sigma \cdot {\rm Br}(pp \to WhX) < 9$ pb.