Constraints from precision electroweak measurements reveal no evidence for new physics up to 5 -7 TeV, whereas naturalness requires new particles at around 1 TeV to address the stability of the electroweak scale. We show that this "little hierarchy problem" can be cured by introducing a symmetry for new particles at the TeV scale. As an example, we construct a little Higgs model with this new symmetry, dubbed T -parity, which naturally solves the little hierarchy problem and, at the same time, stabilize the electroweak scale up to 10 TeV. The model has many important phenomenological consequences, including consistency with the precision data without any finetuning, a stable weakly-interacting particle as the dark matter candidate, as well as collider signals completely different from existing little Higgs models, but rather similar to the supersymmetric theories with conserved R-parity.

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

A search for squarks and gluinos in final states containing high-p T jets, missing transverse momentum and no electrons or muons is presented. The data were recorded in 2012 by the ATLAS experiment in √ s = 8 TeV proton-proton collisions at the Large Hadron Collider, with a total integrated luminosity of 20.3 fb −1 . Results are interpreted in a variety of simplified and specific supersymmetry-breaking models assuming that R-parity is conserved and that the lightest neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 1330 GeV for a simplified model incorporating only a gluino and the lightest neutralino. For a simplified model involving the strong production of first-and second-generation squarks, squark masses below 850 GeV (440 GeV) are excluded for a massless lightest neutralino, assuming mass degenerate (single light-flavour) squarks. In mSUGRA/CMSSM models with tan β = 30, A 0 = −2m 0 and µ > 0, squarks and gluinos of equal mass are excluded for masses below 1700 GeV. Additional limits are set for non-universal Higgs mass models with gaugino mediation and for simplified models involving the pair production of gluinos, each decaying to a top squark and a top quark, with the top squark decaying to a charm quark and a neutralino. These limits extend the region of supersymmetric parameter space excluded by previous searches with the ATLAS detector.

A search for the electroweak production of charginos and sleptons decaying into final states with two electrons or muons is presented. The analysis is based on 139 fb −1 of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at √ s = 13 TeV. Three R-parity-conserving scenarios where the lightest neutralino is the lightest supersymmetric particle are considered: the production of chargino pairs with decays via either W bosons or sleptons, and the direct production of slepton pairs. The analysis is optimised for the first of these scenarios, but the results are also interpreted in the others. No significant deviations from the Standard Model expectations are observed and limits at 95% confidence level are set on the masses of relevant supersymmetric particles in each of the scenarios. For a massless lightest neutralino, masses up to 420 GeV are excluded for the production of the lightest-chargino pairs assuming W-boson-mediated decays and up to 1 TeV for slepton-mediated decays, whereas for slepton-pair production masses up to 700 GeV are excluded assuming three generations of mass-degenerate sleptons. Contents

A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb −1 of √ s = 8 TeV proton-proton collision data delivered by the Large HadronCollider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (χ ± 1 ) and next-to-lightest neutralino (χ 0 2 ) production with decays to lightest neutralino (χ 0 1 ) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons,χ ± 1 andχ 0 2 masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a masslessχ 0 1 . Keywords: Hadron-Hadron ScatteringArXiv ePrint: 1402.7029Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. The ATLAS collaboration 30 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] proposes the existence of supersymmetric particles, with spin differing by one-half unit with respect to that of their Standard Model (SM) partners. Charginos,χ ± 1,2 , and neutralinos,χ 0 1,2,3,4 , collectively referred to as electroweakinos, are the ordered mass eigenstates formed from the linear superposition of the SUSY partners of the Higgs and electroweak gauge bosons (higgsinos, winos and binos). Based on naturalness arguments [10,11], the lightest electroweakinos are expected to have mass of order 100 GeV and be accessible at the Large Hadron Collider (LHC). In the R-parity-conserving minimal supersymmetric extension of the SM (MSSM) [12][13][14][15][16], SUSY particles are pair-produced and the lightest SUSY particle (LSP), assumed in many models to be theχ 0 1 , is stable. Charginos and neutralinos can decay into leptonic final states via superpartners of neutrinos (ν, sneutrinos) or charged leptons (l, sleptons), or via W , Z or Higgs (h) bosons (χ ± i → ℓ ±ν , νl ± , W ±χ 0 j , Zχ JHEP04(2014)169µ or τ , referred to as leptons in the following) and missing transverse momentum originating from the two undetected LSPs and the neutrinos. The analysis is based on 20.3 fb −1 of proton-proton collision data recorded by ATLAS at a centre-of-mass energy of √ s = 8 TeV.Previous searches for charginos and neutralinos are documented in refs [17][18][19] by ATLAS, and in ref. [20] by CMS. Similar searches were conducted at the Tevatron [21, 22]. At LEP [23][24][25][26][27], searches for direct chargino production set a model-independent lower limit of 103.5 GeV at 95% confidence level (CL) on the mass of promptly decaying charginos. SUSY scenariosAmong the electroweakino pair-production processes leading to three leptons in the final state,χ ± 1χ 0 2 production has the largest cross-section in most of the MSSM parameter space. Several simplified supersymmetric models ("simplified models" [28])...