Friday, 24 May 2013

Planck 2013 Day Five Live Blog 2

Okay, some much-needed coffee and it's time for the second session of the morning.

11:00am: Joseph Conlon, "Origin and Phenomenology of Dark Radiation"

You have a microphone, no need to shout.

If dark matter, why not dark radiation?  Beyond the Planck constraints, of course.

An interesting point, raised by Joseph in an earlier Q&A session, is directly about those constraints.  The Planck measurements alone don't look too promising, but in terms of using the CMB alone the one-sigma deviation is consistent with pre-Planck data; while combining with direct measurements of the Hubble rate, the Planck result is still two sigma high compared to the SM expectation.

The theoretical perspective is that the inflaton decays may produce dark radiation in the form of e.g. axions or hidden photons.  Irrespective of the initial conditions, the Universe passes through a phase dominated by matter with Planck-suppressed couplings (due to radiation redshifting away and Planck-suppressed couplings giving the longest lifetimes).

Specific context (large volume string theory) to get actual predictions for branching rates to dark radiation.  Advantage: one modulus much lighter than the others (relates to overall volume).  So can ignore all others.  Additionally, always exist a light/massless axion.

Decay to axion uniquely determined by Kahler potential.  Matter is a bit trickier but comes down to Higgs decays, again from Kahler potential.  One "free" parameter; minimal scenario, where coupling equals 1, excluded.

Cosmic axion background: generically, axions produced with roughly predictable temperature from inflaton decay; and due to weak scattering, temperature only changes by expansion.  Much higher temperature than photons (typically a factor of a million).  The axion energy in the cosmic background is then huge.  Try to observe from conversion to photons in the presence of large magnetic fields.

Potential luminosity huge.  Typical axion coupling limits would lead to light brighter than whole cluster.  Additionally, long-standing excess from Coma cluster.  Is this an explanation?  Some possible tests, e.g. compare morphology to magnetic field (if it can be measured).

11:30am: Takeo Moroi, "Probing BSM Physics with Inflationary Gravitational Waves"

Inflation seems strongly supported by modern astrophysical observations.  What happened after inflation?  Gravitational waves can probe the pre-CMB era due to their weak interactions.  Indeed, they can even work back to before the regime probed by neutrinos.

Of course, we haven't even measured the cosmic neutrino background yet.  Nor any gravitational waves.  But let's be hopeful, right?

The first basic result is that inflationary gravitational waves have amplitude proportional to the expansion rate at that time.  Super-horizon waves have power spectrum that goes as a-2; sub-horizon as a-4.  (a is the scale factor.)  In the simplest case, with nothing special post-inflation, this leads to a spectrum today constant for long wavelengths that entered the horizon after reheating; then at short wavelengths the spectrum decreases with decreasing wavelength.  The turn-over point is related to the reheating temperature.

A good, a plot on the possible future sensitivities.  The accessible tensor-to-scalar ratio is not great; proposed experiments can get a factor of 100 better than Planck limits for optimal frequencies.  The theoretical limit is another three orders of magnitude better.

Some consideration of possible deviations from e.g. cosmic phase transitions (potential energy dominates), temporary matter domination or production of dark radiation.  Generic behaviour: drop in spectrum at key frequency.  Transition point related to temperature of deviation, drop in power spectrum related to amount of energy injected.  Slope of transition related to timescale.

This is all very interesting, though it does feel a bit speculative right now.

12:00pm: Marek Olechowski, "New Regions in the NMSSM with a 125 GeV Higgs"

This differs from Michael's talk on Wednesday in that we are taking the Higgs-singlet coupling to be small, less than about 0.1.  Also the pure singlet sector is left general.

Even if the NMSSM only contributes 5 to 10 GeV to the Higgs mass, it lets the stops be a factor of two lighter, so less fine tuning.  Of course, there's still the direct search limits to consider.

The NMSSM contributes to the Higgs mass in two ways; the usual direct contribution from the tree-level singlet coupling, and an additional contribution from the mixing.  The former requires large singlet-Higgs coupling, hence focus on the latter.  In order for mixing with the singlet to increase the Higgs mass, the singlet must be lighter than the Higgs.  Unfortunately, in the limit where you ignore the heavy Higgs the singlet would need to have a coupling to the Z that would have let it be seen at LEP.

Mixing with the heavy Higgs can help by introducing an additional suppression to the singlet-bottom coupling.  There are still non-trivial constraints from the LEP searches for Higgs to two jets.  However, these constraints are weaker allowing the desired 5 or so GeV contribution to the Higgs mass.

Suppressing the singlet decay to bottom quarks will enhance the SM-like Higgs branching ratio.  That is not yet constrained.

A bigger problem might be the non-observation of the singlet as yet, since it has an enhanced diphoton rate.  Fit to the claimed mass difference?  (i.e. degenerate with SM-like Higgs.)  Probably not due to insufficient shift to mass.

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