Light stop decays: implications for LHC searches

Abstract

We investigate the flavour-changing neutral current decay of the lightest stop into a charm quark and the lightest neutralino and its four-body decay into the lightest neutralino, a down-type quark and a fermion pair. These are the relevant stop search channels in the low-mass region. The SUSY-QCD corrections to the two-body decay have been calculated for the first time and turn out to be sizeable. In the four-body decay both the contributions from diagrams with flavour-changing neutral current couplings and the mass effects of final state bottom quarks and τ leptons have been taken into account, which are not available in the literature so far. The resulting branching ratios are investigated in detail. We find that in either of the decay channels the branching ratios can deviate significantly from 1 in large parts of the allowed parameter range. Taking this into account, the experimental exclusion limits on the stop, which are based on the assumption of branching ratios equal to 1, are considerably weakened. This should be taken into account in future searches for light stops at the next run of the LHC, where the probed low stop mass region will be extended.

Two-loop contributions of the order O α t α s $$ \mathcal{O}\left({\alpha}_t{\alpha}_s\right) $$ to the masses of the Higgs bosons in the CP-violating NMSSM

“…The difference between the two schemes is more pronounced for large absolute values of A t . As expected the difference in the masses obtained using the two schemes, 9) becomes much smaller when going from one to two loops. The lower panels of figure 7 show that it drops from some 15 − 25% difference to a value below 1.5%.…”

“…Up to now, however, no SUSY particles have been discovered, and the LHC has put lower limits of around 1.5 TeV on the gluino mass and the squark masses of the first two generations. On the other hand, from analysis strategies based on monojet-like and charm-tagged event selections it can be concluded that the mass of the lightest stop can still be rather light [3][4][5][6][7][8][9], down to about 240 GeV for arbitrary neutralino masses [5]. The stops provide the dominant contribution to the Higgs mass corrections and play a crucial…”

Two light stops in the NMSSM and the LHC

“…As pointed out already,t 1 could undergo the two-body FV decay to cχ 0 1 and the 4B decay to bff χ 0 1 . Recently, the implications of the competition between these decay modes have been much appreciated [107][108][109][110][111], in particular, in the context of compressed scenarios in the MSSM with a lightt 1 . 11 These are subsequently followed up by the LHC collaborations [89,91].…”

Beyond Standard Model Collider Phenomenology of Higgs Physics and Supersymmetry

“…Recent work to study this question [153,154] has showed that the answer is, as would be expected, heavily dependent on the pattern and degree of flavour violation. However in general, this 2-body decay becomes more likely as ∆m gets smaller, and both flavour violating scenarios they considered had a near 100% BR to 4-body by ∆m < ∼ 20 GeV.…”

“…The question for which regions of parameter space is this likely to be the only decay channel, is answered by looking at the 2-body decay branching ratios as discussed above and in [153,154]. When ∆m < ∼ 5 GeV this decay is not allowed, and for a ∆m of at least 110 GeV it is likely to have a BR ∼ 100%.…”