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Register a new userDetecting a Light Stop from Top Decays at the Tevatron
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S. Mrenna
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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.
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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.
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.
Top quark decays are of particular interest as a mean to test the standard model (SM) predictions, both for the dominant ($tto b+W$) and rare decays ($tto q+W, cV, cVV,cphi^0,bWZ$). As the latter are highly suppressed, they become an excellent window to probe the predictions of theories beyond the SM. In particular, in this paper, we evaluate the corrections from new physics to the CKM-suppressed SM top quark decay $tto q+W$ ($q=d,s$), both within the effective lagrangian approach and the MSSM and we discuss the perspectives to probe those predictions at the ILC.
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 explore the possibility that the right-handed top quark is composite. We examine the consequences that compositeness would have on $t bar{t}$ production at the Tevatron, and derive a weak constraint on the scale of compositeness of order a few hundred GeV from the $t bar{t}$ inclusive cross section. More detailed studies of differential properties of $t bar{t}$ production could potentially improve this limit. We find that a composite top can result in an enhancement of the $t bar{t} t bar{t}$ production rate at the LHC (of as much as $10^3$ compared to the Standatd Model four top rate). We explore observables which allow us to extract the four top rate from the backgrounds, and show that the LHC can either discover or constrain top compositeness for wide ranges of parameter space.
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