Tuesday, 10 November 2015

KIAS-CFHEP Workshop Liveblog: Day Two Session One

We had a very nice reception last night, together with cheese—good cheese—which is not common here in Korea.  Today we have morning plenary talks on Cosmology, followed by the first parallel sessions after lunch.

9:30 am: Recent results on keV-MeV mass particles in cosmology, astrophysics, and experiment, Josef Pradler

Two basic subjects: keV dark photons, and long-lived MeV particles in cosmology.  Dividing line is the threshold for e+e decay.

Light dark photons cannot be thermal WIMP, but can be generated by quantum fluctuations during inflation (or some kind of misalignment mechanism).  The first point of concern for very light states is astrophysical constraints.  The Sun core temperature of 1 keV means anything lighter than that will be abundantly produced.  As a general rule they beat everything due to a combination of size and time.  Different objects (sun/red giants/other) give bounds at different masses.

How can these limits be improved?  If dark photon is DM, use direct search experiments.  Different signal: dark photon absorption. Xenon 10 superior for masses around 10 eV.  Xenon 100 not so good (different mass range) but Xenon 1T can be impressive.

Moving to MeV-scale dark photons, focus is on very small kinetic mixing.  This makes dark photon long-lived, so constraintes from CMB/BBN.  BBN of interest; a moderately complex interaction chain, so limits might not be trivial, and also the Lithium anomaly.

How to modify standard BBN picture?  Change in timing (change Hubble rate, extra light degrees of freedom); non-equilibrium process (decays during BBN); catalysis (long-lived charged particles).  Second case is relevant here.  Interestingly, excluded regions are only islands that (to me) are surprisingly small.  (Still cover orders of magnitude in mass and mixing.)

The Lithium-7 problem: a deficit in the measured 7Li abundance compared to expectations, by a factor of a few.  Difficult to explain using standard alterations to BBN mentioned above due to precise deuterium abundance measurements.  So instead use anti-catalytic state that directly disassociates 7Be (the precursor to 7Li).  This is feasible as 7Be is the least bound nucleus relevant for BBN.

Particle physics requirements: for 5 MeV particle, required cross sections are small compared to photo-nuclear but large compared to weak interactions, with lifetimes comparable to β decay.  Consider an ALP with freeze-in.  Redshift of light states leads to enhanced probability distribution at low masses, which improves efficiency.

New light state that couples to quarks: look for it at neutrino experiments with hadronic drivers.  LSND (huge luminosity) sets strongest limits.

CMB/BBN constraints depend on initial conditions? Yes, took most conservative approach.  Could be stronger with additional sources.
Supernovae cooling/neutron star constraints?  Yes, on plots.

10:10 am: Cosmic ray "anomalies" & indirect dark matter searches, Pasquale Serpico

A long successful history of discoveries using astrophysical observations.  Why, then, so many false alarms of indirect DM detection?  Essentially, a contrast between the simple systems used in particle physics vs complex systems in astrophysics.  A common source of astrophysical anomalies is using a too simple model.  These can lead to statistically sound deviations in models that are under control, but do not correspond to genuinely new physics.

Example 1: positron cosmic ray anomaly.  Even ten years ago, believe only secondary positrons needed to explain data.  Measurement of rise in flux misinterpreted as source of WIMP instead of previously-proposed astrophysical source of primary positrons.  Not previously included as not required before PAMELA and AMS-02 gave new data.  But combining new data with lack of signals in other channels, e.g. Planck, confirmed that no consistent DM explanation for data: astrophysics alone is sufficient.

Next case: GCE.  Important point is significance has improved but no independent confirmation from other channels.  But new types of signals possible.  e.g. before Fermi, no ms Pulsars known in gamma rays; now the dominant source!  Objection from DM camp is that no members of the required population have been seen.  But this depends on how far away the pulsars found so far are, which determines their absolute luminosity.  But clear prediction: even if found none so far,pass-8 Fermi data should find 10 to 40 pulsars.  Most recent data finds some evidence that these exist.

Finally, consider 2015 antiproton anomaly from AMS.  Clear discrepancy between data and old theoretical prediction based on secondary production.  However, already in 2011 pointed out that old theory inconsistent with newer data.  PAMELA/AMS agree (eventually) on a broken power law for proton/Helium fluxes, with the break at 300 GeV.  Using this new data together with old propagation models, already have much better agreement with data (marginal but ok).

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