Monday, 9 November 2015

KIAS-CFHEP Workshop Liveblog: Day One Session Four

The final session of the day is also the most intense plenary session, with four talks.
4:00 pm: Lepton Flavour Violating Processes in Mirror Fermion Models (with Non-Sterile Electroweak Scale νR), Tzu-Chiang Yuan

Mirror matter but SM gauge group.  Restores parity at high scales, contributes to LFV observables.  Connection to PMNS vs CKM flavour structure.

Scalar triplet gives Majorana mass to right-handed neutrino, will singlet gives Dirac mass.  Consider case of EW-scale singlet VEV and electron-Yukawa size couplings.

Add an A4 symmetry, a load more Higgs scalars and slide after slide of formulae.  Four PMNS matrices; I'm sorry, in what way have you gained anything?

4:30 pm: CP Violation in Baryonic b-hadron Decays, Chao-Qiang Geng

Looks like we have a connection to pentaquarks.

SM has only a single CP-violating phase.  However, we know that we need BSM physics for baryogenesis.  CP violation has not yet been observed in the charged B system, so that is a candidate place to search.  Charmless 3-body decay modes, despite small branching ratios (10-6), are higher than the corresponding 2-body decays.  This is due to threshold enhancements.

Use a CP-violating ratio of channels, to cancel hadronic uncertainties. I think this is about precision SM computations, though I kind of fell asleep in the middle there.

CP asymmetries in pentaquark decays problematic to explain.  NP, or intrinsic charm so can access Vub.

5:00 pm: Interference effects on the heavy Higgs resonance shape, Jeonghyeon Song

It's another talk I've heard before!  Well, the slides look new.  Plus having seen it before I can follow the talk much more easily.  Results relevant when background cross section (amplitude) dominant.  Cross section contains three pieces: BW resonant peak, imaginary interference term and real interference term.

Real interference piece is odd across resonant peak.  If resolution is poor, expect this might integrate to zero (but under study).  Have a BW distribution with a correction factor.  Interference term can suppress peaks, exactly cancelling them or leading to a dip rather than a peak.

Example is gluon fusion followed by decay to tt.  Imaginary part non-zero from tops on-shell, and quite sizeable too.  However, as discussed in previous talks, experimental smearing makes it unlikely to be seen.  γγ gives better resolution but unobservable signals.  ZZ suffers from both signal and backgrounds dominated by loops of tops, which have similar phases.  Top-phobic line where heavy Higgs does not couple to t offers some hope, but no regions where large effects are seen (30% at most).

Finally, open up decays to other Higgs states, e.g. H+W-..  This enhances the heavy Higgs width and thus the possible signals to order 1 effects, at the cost of suppressed cross section.

What about using jets to suppress backgrounds? Tricky because of interference
What of uncertainties? Not included here.
What of ττ?  Interference is problem.  Gluon fusion happens at 2 loops, calculation not done.
What of higher order corrections? Good question.

5:30 pm: Beyond the DM effective theory & a Simplified Model, Myeonghun Park

It's déjà vu all over again.

Combine simplified DM models with simplified topologies for LHC searches.  One important point seems to be that effective operators can be problematic at LHC as might not preserve gauge symmetry, or might not have simple UV completions.  Example, scalar operators UV completed by additional scalar that mixes with Higgs.  This leads to two diagrams which destructively interfere.  What's more, the different masses of the two propagators result in different efficiencies at LHC, which is problematic for interpretations using simplified topologies.

For example, Higgs portal models will give complications for e.g. mono-X searches.  Kinematic distributions always localised near Higgs mass, which is not the case for effective operators. This problem persists in other channels, e.g. tt + MET.

Focus on fermionic DM, scalar mediator.  What of others?  Should be same.

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