Monday, 25 May 2015

Planck 2015 Liveblog: Day One, Session Three

The first session after lunch is very neutrino-focused.

3:00 pm: "Neutrino Lanscape and the Road to New Physics", Jose Valle

Observational motivation for new physics.  Claim that neutrino masses need new physics.  I was at a talk in Minneapolis a few weeks ago where that claim got a lot of discussion; essentially, it comes down to a matter of how the SM is defined, but it is certainly possible to extend the SM in a sensible manner to get neutrino masses from the Higgs VEV.

Of course, if you do that, we have not discovered all the SM, as we have not found thr right-handed neutrinos needed.  So we either need NP or to complete the SM.  However, the claim that Jose has made that we must have another Higgs to break lepton number is not true.

If we do have a second Higgs, which is comparably massive to the SM Higgs, then the system must be simultaneously minimised.  In particular we will get contributions to the Higgs invidisible width.

Neutrino mixings (PMNS matrix entries) unexplained.  Though that's also true for the CKM matrix.

Dynamics in generation of neutrino masses (expanding the see-saw operator).  Radiative neutrino masses.  GUTs.  This feels like a list of unrelated topics.

Particle needed for GUT affect stability of Higgs potential?  Yes, interesting work to be done here.
Recent Daya Bay results on the low side of previous results; will that have an effect on searches for CP violation?  Hard to say at this time.

3:30 pm: "Novel Scenarios for Neutrino Majorana Mass generation and Leptogenesis from Kalb-Ramond Torsion", Nick Mavromatos

Another talk from the title = abstract school.  This is another string theory talk.

Baryon asymmetry is motivation for BSM physics.  Right-handed neutrinos can provide extra CP violation.  Talk about νMSM as simple solution to baryogenesis and dark matter problems.

The sound quality for this talk is suddenly quite bad.  And I can't read the slides.  Fuck it.

4:00 pm: "Probing Dark Matter Interactions with Cosmic Colliders", S Sarkar

I hope we can end this session with something I am interested in.

This talk is based on recent results suggesting DM might have large self-interactions.  Can we measure DM self-interactions by looking for off-sets between DM reconstructed through lensing, and visible matter?

DM in a galaxy such as ours should have many substructures orbiting one another, stable on the relevant cosmological timescales.  But this is problematic, with observations not agreeing with numerical simulations; the cusp-vs-core, too-big-to-fail and missing-satellite problems.  Only now do we have the technology to probe the affects of DM self-interactions on this problem, despite it having been proposed back in 1999.

(There might be astrophysical explanations; baryons only now being added to simulations for similar reasons.  But it's still interesting to consider the self-interacting problem.)

Self-interactions need to be large, order a hadronic cross section, to have an effect.  (Contrast standard WIMPs which are fifteen orders of magnitude smaller.)  Ideas exist, including strongly-interacting DM, mirror DM and atomic DM.  These can be probed using astrophysics, even if the interactions with the SM are small.  Hidden valley-type models a (LHC-testable) example of such models.

Now on to the impact of cosmological collisions.  Bullet-cluster (and similar) give upper bounds on self-interactions, but these are of the order a barn, comparable to hadronic scattering.  Additional constraints from cluster cores, halo ellipticity and subhalo evaporation; but σ ~ 1 cm2/g still fine, even without including velocity-dependence.

DM interactions decelerate halos, so can be modeled as an effective drag force.  For simplicity, consider two limits; long-range force (massless mediator) and a hard, velocity-independent force.

Key recent observation is Abell 3827, cluster with four bright elliptical galaxies within 10 kpc.  Notable separation between DM and visible matter.  Bounds where claimed much stronger than other systems, but based on an erroneous understanding of gravitational effects.  In particular, need self-interaction forces to overcome gravitational forces which leads to usual size as above.

Final claim is that this separation, if explained by self-interactions, needs large cross sections of the scale 1 cm2/g.  This can be tested with about 30-40 other systems, which exist, we just need to find them.

What about tidal forces?  Possible, but we have no evidence for it.
What interactions can be probed?  Right not, only 1 cm2/g.
Right-handed neutrinos?  Only care about interaction strength, not its particle origin, in these kinds of observations.

No comments:

Post a Comment