Friday, 29 May 2015

Planck 2015 Liveblog: Day Five, Session Three

The final session (not counting the summary talk) of the conference.

3:00 pm: "Searches Strategies for Heavy Quark Partners at LHC run-II", Thomas Flacke

Number of quark partners, depending of course on the fermion embedding.  Production can be divided into QCD pair production and various EW single production channels.  Their decay signals vary; some types have a single dominant decay, but the possibility of decays to bW, tZ and th in differnt regions of parameter space make a top-like top partner more interesting.

Classic exclusions from run-I based on pair production (less model-dependent).  Some single-production bounds from ATLAS (only).

At run II, single production will be relatively more important simply on kinematic grounds.  Additionally, boosted decay products (and hence the use of jet substructure) become relevant.

Some projected exclusion contours that look both reasonable and good. Limits could go up to 1 or 2 TeV, depending no the type of top partner.

3:50 pm: "How to Avoid Unnatural Hierarchical Thermal Leptogenesis", Jackson Clarke

Hierarchical Leptogenesis in Type I See-Saw is unnatural.  This is because the CP asymmetry, needed for leptogenesis, is proportional to the right-handed neutrino Yukawas.  Demanding that the loop corrections to the Higgs be below 1 TeV gives an upper bound on the NR mass of 3 x 107 GeV.  This is to be compared to the standard lower bound for leptogenesis, 108 GeV.

Original calculation was done in 1-flavour model.  Do things vary in 3-flavour model?  Get some benefits from mixing matrix, but it turns out that the constraint remains.

To solve this, move to a 2HDM where second Higgs couples to right-handed neutrinos.  This has a small VEV, which turns out to avoid the naturalness and leptogenesis bounds.  Some model-building to see if things can work.  Example here involves softly breaking Z2 together with a large, positive mass parameter for the second Higgs.

Various constraints (flavour, avoiding reintroducing fine tuning).  Notable one is the vacuum stability; need theory to remain under control at least to the see-saw scale.

4:00 pm: "Stabilizing the Higgs potential with a Z'", Venus Keus

Restrict ourselves to a heavy Z', 3 TeV, above even the current SSM constraints.  Running is not trivial now due to mixings of states, both vector kinetic mixing and also mixing involving dark Higgses or fermions.  Work is done in a highly general basis, allowing range of charges for SM fields.

Get extra, positive combination to running of Higgs quartic.  Also get extra negative contribution to top Yukawa.

Find regions in parameter space where Higgs potential is absolutely stable.  Reasonable chunks of parameter space.  Interestingly, if Z' can be light (leptophobic, 200 GeV) can get stable potential even if Higgs is not directly charged under the new U(1).

What about Yukawas with dark scalars?  Assumed small.
What of future constraints?  Will make things worse.

4:15 pm: "One loop corrections to the Higgs EW chiral Lagrangian", J.J. Sanz-Cillero

Non-linear Higgs EFT: computed UV divergences at NLO.  Consider the "κ framework" of coupling deviations from the SM.  This is fine for low precision, but need to use EFTs to handle precision data.

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