Removing gaps in the exclusion of top squark parameter space

High-Precision Differential Predictions for Top-Quark Pairs at the LHC

We present the first complete next-to-next-to-leading order (NNLO) QCD predictions for differential distributions in the top-quark pair production process at the LHC. Our results are derived from a fully differential partonic Monte Carlo calculation with stable top quarks which involves no approximations beyond the fixed-order truncation of the perturbation series. The NNLO corrections improve the agreement between existing LHC measurements [V. Khachatryan et al. (CMS Collaboration), Eur. Phys. J. C 75, 542 (2015)] and standard model predictions for the top-quark transverse momentum distribution, thus helping alleviate one long-standing discrepancy. The shape of the top-quark pair invariant mass distribution turns out to be stable with respect to radiative corrections beyond NLO which increases the value of this observable as a place to search for physics beyond the standard model. The results presented here provide essential input for parton distribution function fits, implementation of higher-order effects in Monte Carlo generators, as well as top-quark mass and strong coupling determination.

Opening Up the Compressed Region of Top Squark Searches at 13 TeV LHC

Light top superpartners play a key role in stabilizing the electroweak scale in supersymmetric theories. For R-parity conserved supersymmetric models, traditional searches are not sensitive to the compressed regions. In this Letter, we propose a new method targeting this region, with top squark and neutralino mass splitting ranging from m_{t[over ˜]}-m_{χ}≳m_{t} to about 20 GeV. In particular, we focus on the signal process in which a pair of top squarks are produced in association with a hard jet, and we define a new observable R_{M} whose distribution has a peak in this compressed region. The position of the peak is closely correlated with m_{t[over ˜]}. We show that for the 13 TeV LHC with a luminosity of 3000  fb^{-1}, this analysis can extend the reach of the top squark in the compressed region to m_{t[over ˜]} around 800 GeV.

Monojetlike Searches for Top Squarks with a b Tag

The LHC searches for light compressed top squarks have resulted in considerable bounds in the case where the top squark decays to a neutralino and a charm quark. However, in the case where the top squark decays to a neutralino, a bottom quark, and two fermions via an off-shell W boson, there is currently a significant unconstrained region in the top-squark-neutralino mass plane, still allowing for top squark masses in the range 90-140 GeV. In this Letter we propose a new monojetlike search for light top squarks, optimized for the four-body decay mode, in which at least one b-tagged jet is required. We show that, by using the existing 8 TeV LHC data set, such a search would cover the entire unconstrained region. Moreover, in the process of validating our tools against an ATLAS monojet search, we show that the existing limit can be extended to exclude also top squark masses below 100 GeV.