Friday, 30 May 2014

Planck 2014 Liveblog: Day Five Session 2

We come at last to the end of the conference.  Last night was the banquet, so I regrettably missed the first session this morning.  The second morning session looks to be related to leptonic physics.

11:00 am: Same-Sign Tetra-Leptons from Type II Seesaw, Eung Jin Chun

Type II Seesaw is the one where neutrino masses come from a scalar SU(2) triplet with smal VEV.  Decay patterns of doubly-charged scalar to two-lepton final states directly gives information on the associated Yukawa couplings; as with SM Higgs, neutrino masses generated by a single Yukawa.  This is obvious collider probe.

A lot of discussion on various constraints on model (low energy flavour, collider searches, Higgs widths, EWPO, vacuum stability).  Main phenomenological feature: small mass splittings among (non-SM-like-Higgs) scalars (less than W mass).

SS4L signal of title comes from oscillations among the neutral triplet scalars, mediated by the (small) doublet-triplet mixing.  The charge offset is carried by Ws that decay to jets.  Cross sections are small but (irreducible) backgrounds are essentially zero.

11:30 am: What does gravity do with axions?, Sacha Davidson

The title of this talk has changed a lot.  Indeed, the topic has changed.

The core of the question is: can we distinguish WIMP CDM and axion CDM (using LSS)?  Answer is ... maybe.  Stress energy tensor is different.  But hard to be quantitative.

Assume inflation before PQ phase transition.  Follows that have U(1) topological defects.  Leads to one PQ string per horizon at PQPT.  Persists to QCDPT at which point mixing with axions triggers axion oscillations, strings become cold particles.

12:00 pm: Reading low energy neutrino data with leptogenesis, Pasquale Di Bari

In Copenhagen, Pasquale was too loud for me to actually follow.  Here, he is less painful than many speakers have been this week.

Can we probe LG with neutrino physics?  Answer (for high-scale LG) traditionally thought to be no.  However, with no evidence for TeV LG and the measurement of θ13, should we reconsider?  In particular, the moderately sized reactor angle makes it easier to measure CPV in the lepton sector.

Planck upper limit on neutrino masses means we are approaching the regime where we can actually distinguish NH and IH (quasi-degenerate spectra becoming disfavoured).

Even the simplest seesaw models have too many parameters to say things easily.  How to deal with this?  Old idea: assume flavour structure unimportant, hierarchical spectra (so N2 to N1 decay dominates).  This idea still feasible.

May be worried about pre-existing asymmetry.  However, based on solar mass splitting RH neutrinos will generically wash out "any" such asymmetry.

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