Thursday, 15 October 2015

CosPA 2015 Liveblog: Day Four Session Four

The final parallel session of CosPA.

3:45 pm: The e+e- collider experiments to search for dark matter, Kihyeon Cho

Looks like we are talking about low-energy experiments and light DM: BaBar & Belle, not ILC.  In particular, hidden photons.  Also searches for long-lived hidden Higgs; did he mention results there?

BaBar limits for invisible upsilon decays.

4:10 pm: Exploring top quark FCNC at hadron colliders in association with flavour physics, Yeo Woong Yoon

 Connection with B to Dτν anomaly.  More than 3σ from SM expectations in ratios R; expect most hadronic uncertainties to have factored out.  BaBar, Belle & LHCb average is 3.9σ from SM.

Could be from charged Higgs, which obviously couples more strongly to heavy quarks & leptons.  However, MSSM and Type II 2HDM can not explain the observations; as excluded as the SM.

Effective operator explanations: four possible solutions.  Need sizable NP contributions, comparable to SM, in at least one of two operator channels.  In MSSM, one operator proportional to tan β and one to cotan β.  This is related to problem.

Type III 2HDM (where both Higgs doublets couple to all fields) give additional terms, could explain anomaly.  But in this model, must worry about FCNC.  More constraining for down-type FCNC.  Work in alignment limit to get SM-like Higgs couplings to work.  Adopt Cheng-Sher ansatz, where flavour violating terms proportional to masses; suppresses violation for light fermions.

Fit to B to Dτν data.  Prefers large FCNC couplings.  These generate single-top production at loop level; turns out not to give meaningful constraints.  Also generates tree-level same sign top production; much more constraining.  Indeed, is actually in some tension with B to Dτν preferred region.

Another process to consider is b to sγ.  Possible to cancel different unrelated components, but this is a fine tuning.  Tight constraints.

Putting it all together, is still possible to satisfy all constraints.  Lower bound on coupling of charged Higgs to taus, typically quite large.  Likely to affect SM-like Higgs decay to taus.

Question: what of perturbativity?  okay... would like to see some numerical values here.  Back of the envelope, he's working with couplings > 1 so some reason to worry.

4:35 pm: New resonance shapes of heavy Higgs bosons due to interference with the continuum background, Jeonghyeon Song

2HDM.  Alignment limit.

I just realised I recently saw a seminar from Jeonghyeon, and there's a lot of overlap with that talk.  There was something about this that worried me, and I've now tracked down the paper that concerned me.  However, what went wrong in (the initial version of) that paper is different to what is being done here and here  The idea is simple: for some values of the phase between a resonance signal and the continuum takes the desired value, the interference on the resonance can lead to a suppressed signal, instead of the usual resonant peak.  And this is not that esoteric, if it can show up in a Type-II 2HDM.

That said, you can't get large rates and large interference suppression in the simplest model.  The ZZ final state has small interference, while the γγ final state suffers from small cross sections.  This can be modified by adding new decay modes to e.g. dark matter.  It's not quite clear to me why this enhances this signal.

5:00 pm: Beyond the Dark Matter effective field theory and a simplified model approach at Colliders, Myeonghun Park

 Model with fermion DM and scalar mediator that mixes with Higgs.  Modification to mono-jet searches: localisation of DM invariant mass around mh, even for very heavy mediator.  Efficiences different to simplified model.

Mixing with Higgs implies scalar couples to tops.  ttbar plus MET becomes potentially interesting channel.  Efficiencies different again.

5:25 pm: Gravitational Compton scatering from the worldline formalism, Naser Ahmadiniaz

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