Les Houches

This is an old revision of the document!

Responsible: Ken Mimasu

Members: Jose Mi No, Chris Pollard, Sanjoy Biswas, Adam Falkowski

Idea

Can we see heavy Higgs bosons with masses above the ttbar threshold decaying into top pairs about the large continuum QCD background?

Clearly this represents a relatively difficult search compared to your typical resonant analysis. Thats said, it seems important to be sure whether or not anything can be done at the LHC in this particular final state. It corresponds to a large part of i.e. the 2HDM parameter space where you are close to alignment but have relatively heavy new states without large mass splittings. In this case, there are few other ways in which they could be seen at the LHC.

Things to take into account:

• The overwhelming size of said QCD background
• Large interference effects between signal and (t-channel) background, producing a peak-dip structure
• Potentially large widths of Higgs bosons, especially in scenarios with enhanced top couplings i.e. aligned 2HDM with low tangent beta
• Experimentalists have begun to look into this
• Considering associated production with a Z(gamma?) via t/b loops would change the background to ttZ(gamma), which may have more manageable rates - but what would be our signal cross section in this case?

Literature

http://arxiv.org/abs/hep-ph/9404359

One of the earlier papers discussing this channel. They quantify the peak-dip structure of the signal and emphasise the fact that, depending on the heavy Higgs mass, the total cross section can be reduced, enhanced or stay roughly the same as the SM value. They conclude that the effect may potentially be observed at the LHC if the ttbar mass resolution is smaller than the width of the top.

http://arxiv.org/abs/1503.01656

Maltoni et al. implementation of HZ production in Madgraph5_aMC@NLO. The ZH production cross section looks to be O(1) fb in their benchmark scenarios what take masses of 350, 440 and 500 GeV. This is to be compared with a recent calculation by Frixione et al. (http://arxiv.org/abs/1504.03446) of the background ttZ process quoting O(500) fb cross sections at LO with additional, non-negligible k-factors.