We propose a novel method for probing sleptons in compressed spectra at hadron colliders. The process under study is slepton pair production in R-parity conserving supersymmetry, where the slepton decays to a neutralino LSP of mass close to the slept
on mass. In order to pass the trigger and obtain large missing energy, an energetic mono-jet is required. Both leptons need to be detected in order to suppress large standard model backgrounds with one charged lepton. We study variables that can be used to distinguish the signal from the remaining major backgrounds, which include tt, WW+jet, Z+jet, and single top production. We find that the dilepton MT2, bound by the mass difference, can be used as an upper bound to efficiently reduce the backgrounds. It is estimated that sleptons with masses up to about 150 GeV can be discovered at the 14 TeV LHC with 100/fb integrated luminosity.
Jet radiation patterns are indispensable for the purpose of discriminating partons with different quantum numbers. However, they are also vulnerable to various contaminations from the underlying event, pileup, and radiation of adjacent jets. In order
to maximize the discrimination power, it is essential to optimize the jet radius used when analyzing the radiation patterns. We introduce the concept of jet radiation radius which quantifies how the jet radiation is distributed around the jet axes. We study the color and momentum dependence of the jet radiation radius, and discuss two applications: quark-gluon discrimination and $W$ jet tagging. In both cases, smaller (sub)jet radii are preferred for jets with higher PTs, albeit due to different mechanisms: the running of the QCD coupling constant and the boost to a color singlet system. A shrinking cone W jet tagging algorithm is proposed to achieve better discrimination than previous methods.
We present a new strategy to uncover light, quasi-degenerate Higgsinos, a likely ingredient in a natural supersymmetric model. Our strategy focuses on Higgsinos with inter-state splittings of O(5-50) GeV that are produced in association with a hard,
initial state jet and decay via off-shell gauge bosons to two or more leptons and missing energy, $pp to j + text{MET}, + 2^+, ell$. The additional jet is used for triggering, allowing us to significantly loosen the lepton requirements and gain sensitivity to small inter-Higgsino splittings. Focusing on the two-lepton signal, we find the seemingly large backgrounds from diboson plus jet, $bar tt$ and $Z/gamma^* + j$ can be reduced with careful cuts, and that fake backgrounds appear minor. For Higgsino masses $m_{chi}$ just above the current LEP II bound ($mu simeq 110,$) GeV we find the significance can be as high as 3 sigma at the LHC using the existing 20 fb$^{-1}$ of 8 TeV data. Extrapolating to LHC at 14 TeV with 100 fb$^{-1}$ data, and as one example $M_1 = M_2 = 500$ GeV, we find 5 sigma evidence for $m_{chi} lesssim, 140,$ GeV and 2 sigma evidence for $m_{chi} lesssim, 200,$ GeV . We also present a reinterpretation of ATLAS/CMS monojet bounds in terms of degenerate Higgsino ($delta m_{chi} ll 5,$) GeV plus jet production. We find the current monojet bounds on $m_{chi}$ are no better than the chargino bounds from LEP II.
Stops with the mass nearly degenerate with the top mass, decaying into tops and soft neutralinos, are usually dubbed stealth stops. Their kinematics looks very similar to that of the standard tops events, which leads to events with little or no exces
s of missing transverse energy. This complicates the probing of this region of the stop parameter space by hadron colliders, rendering the application of standard searching techniques challenging. In this Snowmass white paper we reanalyze the spin correlation approach to the search of stealth stops, focusing on the feasibility of this search at the 14 TeV LHC. We find, while the statistical limitations significantly shrink compared to the low-luminosity 8 TeV run, the systematic PDF uncertainties pose the main obstacle. We show that the current understanding of PDFs probably does not allow us to talk about top and stop discrimination via spin correlation in the inclusive sample. On the other hand the systematic uncertainties significantly shrink if only events with low center of mass energy are considered, rendering the search in this region feasible.
We show that the tracking system in a collider detector can be used to efficiently identify boosted massive particles from their QCD backgrounds. We examine variables defined with tracking information which are sensitive to jet radiation patterns, in
cluding charged particle multiplicity and N-subjettiness. These variables are barely correlated with variables sensitive to the hard splitting scale in the jet, such as the filtered jet mass. Therefore these two kinds of variables should be combined to optimize the discriminating power. We illustrate the method with $W$ jet tagging. It is shown that for jet PT=500 GeV, one can gain a factor of 1.6 in statistical significance by combining filtered jet mass and charged multiplicity, over filtered mass alone. Adding N-subjettiness increases the factor to 1.8.
At the LHC, top quark pairs are dominantly produced from gluons, making it difficult to measure the top quark forward-backward asymmetry. To improve the asymmetry measurement, we study variables that can distinguish between top quarks produced from q
uarks and those from gluons: the invariant mass of the top pair, the rapidity of the top-antitop system in the lab frame, the rapidity of the top quark in the top-antitop rest frame, the top quark polarization and the top-antitop spin correlation. We combine all the variables in a likelihood discriminant method to separate quark-initiated events from gluon-initiated events. We apply our method on models including G-primes and W-primes motivated by the recent observation of a large top quark forward-backward asymmetry at the Tevatron. We have found that the significance of the asymmetry measurement can be improved by 10% to 30%. At the same time, the central values of the asymmetry increase by 40% to 100%. We have also analytically derived the best spin quantization axes for studying top quark polarization as well as spin-correlation for the new physics models.
Models of top condensation can provide both a compelling solution to the hierarchy problem as well as an explanation of why the top-quark mass is large. The spectrum of such models, in particular topcolor-assisted technicolor, includes top-pions, top
-rhos and the top-Higgs, all of which can easily have large top-charm or top-up couplings. Large top-up couplings in particular would lead to a top forward-backward asymmetry through $t$-channel exchange, easily consistent with the Tevatron measurements. Intriguingly, there is destructive interference between the top-mesons and the standard model which conspire to make the overall top pair production rate consistent with the standard model. The rate for same-sign top production is also small due to destructive interference between the neutral top-pion and the top-Higgs. Flavor physics is under control because new physics is mostly confined to the top quark. In this way, top condensation can explain the asymmetry and be consistent with all experimental bounds. There are many additional signatures of topcolor with large tu mixing, such as top(s)+jet(s) events, in which a top and a jet reconstruct a resonance mass, which make these models easily testable at the LHC.
A method is proposed for distinguishing highly boosted hadronically decaying Ws (W-jets) from QCD-jets using jet substructure. Previous methods, such as the filtering/mass-drop method, can give a factor of ~2 improvement in S/sqrt(B) for jet pT > 200
GeV. In contrast, a multivariate approach including new discriminants such as R-cores, which characterize the shape of the W-jet, subjet planar flow, and grooming-sensitivities is shown to provide a much larger factor of ~5 improvement in S/sqrt(B). For longitudinally polarized Ws, such as those coming from many new physics models, the discrimination is even better. Comparing different Monte Carlo simulations, we observe a sensitivity of some variables to the underlying event; however, even with a conservative estimates, the multivariate approach is very powerful. Applications to semileptonic WW resonance searches and all-hadronic W+jet searches at the LHC are also discussed. Code implementing our W-jet tagging algorithm is publicly available at http://jets.physics.harvard.edu/wtag
We study methods for reconstructing the momenta of invisible particles in cascade decay chains at hadron colliders. We focus on scenarios, such as SUSY and UED, in which new physics particles are pair produced. Their subsequent decays lead to two dec
ay chains ending with neutral stable particles escaping detection. Assuming that the masses of the decaying particles are already measured, we obtain the momenta by imposing the mass-shell constraints. Using this information, we develop techniques of determining spins of particles in theories beyond the standard model. Unlike the methods relying on Lorentz invariant variables, this method can be used to determine the spin of the particle which initiates the decay chain. We present two complementary ways of applying our method by using more inclusive variables relying on kinematic information from one decay chain, as well as constructing correlation variables based on the kinematics of both decay chains in the same event.
We propose a dark matter model with standard model singlet extension of the universal extra dimension model (sUED) to explain the recent observations of ATIC, PPB-BETS, PAMELA and DAMA. Other than the standard model fields propagating in the bulk of
a 5-dimensional space, one fermion field and one scalar field are introduced and both are standard model singlets. The zero mode of the new fermion is identified as the right-handed neutrino, while its first KK mode is the lightest KK-odd particle and the dark matter candidate. The cosmic ray spectra from ATIC and PPB-BETS determine the dark matter particle mass and hence the fifth dimension compactification scale to be 1.0-1.6 TeV. The zero mode of the singlet scalar field with a mass below 1 GeV provides an attractive force between dark matter particles, which allows a Sommerfeld enhancement to boost the annihilation cross section in the Galactic halo to explain the PAMELA data. The DAMA annual modulation results are explained by coupling the same scalar field to the electron via a higher-dimensional operator. We analyze the model parameter space that can satisfy the dark matter relic abundance and accommodate all the dark matter detection experiments. We also consider constraints from the diffuse extragalactic gamma-ray background, which can be satisfied if the dark matter particle and the first KK-mode of the scalar field have highly degenerate masses.
