Wednesday, 7 May 2014

Pheno 2014 Liveblog: Day Two Session 2

This session is about dark matter and other particle astrophysics.  Again, the post-coffee break session is more interesting to me.  I'm not sure if I should be happy or sad about this; it lets me sleep in if I need to without missing too much, but it also leaves me with less motivation to get up.  Plus, if the interesting talks came first I could skip the second session and get some work done.

11:00 am: Direct Searches for WIMP Dark Matter, Karen Gibson

We are starting with the Bullet Cluster, that favourite evidence, but slightly more detail than usually expected.  Also Planck, rotation curves etc. Generally the sound quality has been quite good at this conference, but for some reason I'm finding it hard to hear Karen clearly.

Focusing on WIMPs, hardly a surprise at a conference like this or given the title.

Looking ahead, she's used a lovely slide from the Snowmass study last year:
I like this, because not only does it show the current limits and near-future prospects, it also shows the neutrino background.  Anything in the yellow region is essentially invisible at terrestial experiments, because the signal is smaller than the rate from astrophysical neutrinos.  So it gives us a good idea of the future of this field, both in the short and long terms.  We also see that even simple SUSY with neutralino DM, the iconic WIMP model, contains regions of parameter space where it will never be seen directly.  (It is the red circle in the lower-right of the plot.)

Another nice slide, actually before this, shows the three essential signals from DM scattering in a detector: Ionisation, phonons and light.  Interestingly, some experiments use only a single signal and some use two.  None use three, which is interesting but might just be a material limitation.

KG is a member of the LUX collaboration, so it's not surprising that she's mostly ignoring the anomalies at DAMA/CoGeNT/CDMS etc, since LUX seems to rule them out.  We're getting an overview of the various current/future noble liquid experiments, and a few comments on other searches.  Interesting to note the relative "low" cost of these experiments; LUX, current state of the art, cost $4M.

Also interesting to look ahead to LUX-ZEPLIN, next generation experiment.  Looking at the plot above, see that this will scrape the edge of the neutrino background and probe a number of asymmetric DM models.

In general, this was a perfectly fine review talk.  Unfortunately I didn't see too much I haven't seen before, but then, we were told yesterday that half the attendees are grad students which makes me feel old means the plenary speakers should be covering this background material.

11:35 am: Dark Matter Theory and Searches, Pearl Sandick

The last five years have been an age of unceasing DM anomalies in many different types of experiments.  This has made things rather fun from the model-building side.  We have an abundance of experimental data and theoretical approaches.

SUSY DM.  General parameter space issue.  Start with highly constrained models (Polonyi mSUGRA, two new parameters) and see if we can explain observations (Higgs, DM relic density).  Surprise surprise, cannot; so gradually extend model to see if we can fix things.

Interestingly, three-parameter model seems to work (not ruled out).  We have m0, m1/2 and mUni, with the last a unification scale.  This being free requires some non-trivial unification scale physics.  DM is Higgsino-like with Higgs funnel annihilation.

pMSSM, hard to understand why points are/are not allowed.

Simplified models with some sparticles light, others heavy.  Light sleptons for aμ, light gauginos for DM.  Resonance models to ask question: can DD probe thermal WIMP?  If annihilating through Higgs resonance, answer is will take ~ 10 years.

12:10 pm: Recent Excitement in Astro-Particle Physics, Alexander Kusenko

Intro slide promises PeV neutrinos and keV gamma rays.  That's new, and different to usual talks, so I guess I won't be leaving early.  First we have blazars, AGN jets.  Surprise here is  that we expect these to be sharply cut-off at high energies due to γγ to ee, with one photon from the blazar and one from the extragalactic background.

This has two problems.  First, the source spectra need to be unphysically hard, and second the appearances to us are universal but (because of different distances) the spectra would not be.  The problem seems to be with very distant AGNs.  Proposed NP solutions include photon-ALP mixing, or Lorentz violation.

Possible SM explanation: regeneration of high-energy photons from high-energy proton ICS.  Same target (low-energy photons) in both cases.  This has interesting neutrino predictions at high energies.  Claim is that this is entirely consistent with the IceCube discovery.

"More explanations for the IceCube PeV neutrinos than actual events".

Now on to 3.5 keV X-Ray line.  Interesting, but not yet discovery; will have an answer within two years from new experiment with much better energy resolution.  Consider sterile neutrinos, one of the  more plausible particles in this mass range.  A number of  possible models and ways to generate them in the early universe.  Oscillations off resonance minimal possible production channel, but limited at this mass to only produce a fraction (~10 %) of DM.

Alternative model based on EW-scale Higgs singlet that gives neutrinos mass.  Requires tiny Yukawa.

Alternative explanation: light moduli.  These fields will generically be set to non-minimal values during inflation, then oscillate at late times.  This corresponds to a DM density that is far too large (15 orders of magnitude).  This is the moduli problem of SUSY, string theories etc.  Can this be solved by anthropic arguments?  Initial value is set randomly, so there will exist (small) patches where the DM density is correct.  Claimed that DM/B must be less than 10, else only black holes exist.  Compare observation, DM/B ~ 5.  Note this is within standard inflation, not a multiverse/landscape scenario.

The final question: can we distinguish them?  Moduli are very cold, while sterile neutrinos have measurable free-streaming length.  This seems to point to looking at structure formation, which does point towards DM which is slightly warm.


  1. Thank you for taking the time to publish this information very useful! איך בוחרים יין אדום

  2. No problem, I mostly do this as it's the only way to get me to take notes :)