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Register a new userMono-top signature from stop decay at the HE-LHC
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Searching for the top squark (stop) is a key task to test the naturalness of SUSY. Different from stop pair production, single stop production relies on its electroweak properties and can provide some unique signatures. Following the single production process $pp to tilde t_1 tilde{chi}^-_1 to t tilde{chi}^0_1 tilde{chi}^-_1$, the top quark has two decay channels: leptonic channel and hadronic channel. In this paper, we probe the observability of these two channels in a simplified MSSM scenario. We find that, at the 27 TeV LHC with the integrated luminosity of ${cal L} = 15~text{ab}^{-1}$, $m_{tilde{t}_1}<1900$ GeV and $mu<750$ GeV can be excluded at $2sigma$ through the leptonic mono-top channel, while $m_{tilde{t}_1}<1200$ GeV and $mu<350$ GeV can be excluded at $2sigma$ through the hadronic channel.
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We survey the expected polarization of the top produced in the decay of a scalar top quark, $tilde t rightarrow {tilde t}chi_i^0, i =1-2$. The phenomenology is quite interesting, since the expected polarization depends both on the mixing in the stop and neutralino sectors and on the mass differences between the stop and the neutralino. We find that a mixed stop behaves almost like a right-handed stop due to the larger hypercharge that enters the stop/top/gaugino coupling and that these polarisation effects disappear, when $m_{tilde t_1} approx m_t+m_{tildechi^0_i}$. After a discussion on the expected top polarization from the decay of a scalar top quark, we focus on the interplay of polarization and kinematics at the LHC. We discuss different probes of the top polarization in terms of lab-frame observables. We find that these observables faithfully reflect the polarization of the parent top-quark, but also have a non-trivial dependence on the kinematics of the stop production and decay process. In addition, we illustrate the effect of top polarization on the energy and transverse momentum of the decay lepton in the laboratory frame. Our results show that both spectra are softened substantially in case of a negatively polarized top, particularly for a large mass difference between the stop and the neutralino. Thus, the search strategies, and the conclusions that can be drawn from them, depends not just on the mass difference $m_{tilde t} - m_{tildechi_{i}^{0}}$ due to the usual kinematic effects but also on the effects of top polarization on the decay kinematics the extent of which depends in turn on the said mass difference.
We study the possibility of discovering or excluding a light top squark (stop) based on top quark decays in the t-tbar events produced at the Fermilab Tevatron. In particular, we consider the Minimal Supersymmetric Standard Model with the sparticle spectrum m_{chi^+_1}+m_b, M_W+m_{chi^0_1}+m_b > m_{stop} > m_{chi^0_1}+m_c, where chi^0_1 is the lightest neutralino and chi^+_1 is the lightest chargino, so that t -> stop chi^0_1 and stop -> c chi^0_1. All other sparticle masses are assumed to be heavier than m_t. Such a spectrum seeks to explain the experimental values of alpha_s(M_Z^2), R_b and A_{LR} obtained from LEP/SLC data. We find that the prospect to observe a light stop via this channel at the Tevatron is very promising.
We investigate the possibility of observing the exotic decay mode of the top quark into the lightest stop ($tilde t_1$) and neutralino ($tildechi^0_1$) in the minimal supersymmetric standard model with R-parity at the upgraded Tevatron. First we determine the allowed range for the branching fraction $B(tto tilde t_1 tildechi^0_1)$ in the region of parameter space allowed by the $R_b$ data and the CDF $eegammagamma+{large ot} E_T$ event, and then consider all possible backgrounds and investigate the possibility of observing this final state at the Tevatron. We find that this final state is unobservable at Run 1. However, Run 2 can provide significant information on this new decay mode of the top quark: either discover it, or establish a strong constraint on the masses of $tilde t_1$ and $tildechi^0_1$ given approximately by $M_{tilde chi^0_1} > M_{tilde t_1} - 6$ GeV.
We study the effective field theory sensitivity of an LHC analysis for the $tau u$ final state with an associated b-jet. To illustrate the improvement due to the b-tagging, we first recast the recent CMS analysis in the $tau u$ channel, using an integrated luminosity of 35.9 fb$^{-1}$ at $sqrt{s}=13$ TeV, and provide limits on all the dimension-six effective operators which contribute to the process. The expected limits from the b-tagged analysis are then derived and compared. We find an improvement of approximately $sim 30%$ in the bounds for operators with a b quark. We also discuss in detail possible angular observables to be used as a discriminator between dimension-six operators with different Lorentz structure. Finally, we study the impact of these limits on some simplified scenarios aimed at addressing the observed deviations from the Standard Model in lepton flavor universality ratios of semileptonic B-meson decays. In particular, we compare the collider limits on those scenarios set by our analysis either with or without the b-tagging, assuming an integrated luminosity of 300 fb$^{-1}$, with relevant low-energy flavor measurements.
The production of supersymmetric stop-antistop pairs at the Large Hadron Collider (LHC) is studied including corrections from soft-gluon resummation up to next-to-next-to-leading logarithmic (NNLL) accuracy in the Mellin-space approach. Additionally, corrections to the hard-matching coefficient at one-loop and Coulomb contributions at two-loop order are considered. The NNLL corrections enhance the cross section for all stop masses at centre-of-mass energies of 8 and 13 TeV compared to the previously calculated predictions at next-to-leading logarithmic (NLL) accuracy. Furthermore, a slight increase in the dependence on the additional stop-mixing parameters is observed.
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