Wednesday, 6 May 2015

Pheno 2015 Liveblog: Day 2 Session 2

The second session of the day is the dark matter session.  I'm looking forward to it!

11:00 am: EFT: Theory and Practice, Aneesh Manohar

This is a general overview, with an eye on Higgs physics.  However, it has definite applicability to modern DM searches.  Remember, EFTs need the full infinite series of operators for quantum consistency.  Our calculations are a power series in the cut-off scale.  There is a many-to-one relationship between the high and low energy theories.  Finally, you cannot resum this infinite series.

Classic examples include quantum mechanics (e.g. NRQED) or chiral pertubation theory.  The low energy theories are easier to use.

Focus on SMEFT.  Differs from some EFTs in assume that there is a fundamental scalar that breaks the gauge symmetry; this fixes the Higgs couplings.  Some theories use the non-linear sigma model and let the scalar couplings float.  Also, impose B and L conservation.  At dimension 6, 59 types of operators; once flavour structure is added, 2499 independent new parameters.

Important, only recently noted point is that dim-6 operators give corrections to SM coupling running.  This is same order as leading contribution from thos operators, must be included.

One odd result at the theoretical level.  Anomalous dimension matrix shows peculiar "holomorphic" structure.  Accident at one-loop or clue pointing to something more fundamental?

11:35 am: Dark Matter Direct Detection, Xiangdong Ji

An interesting feature in DD searches is how the rate of improvement keeps getting better!  (At moderate to high masses, but on a logarithmic scale of cross section.)  In recent years and in the near future, Xenon-based experiments are offering the best limits and the best rate of improvement.  This means we are living in an exciting experimental time.

The advantages of Xenon are several.  It's "cheap"; about $1 million for 1 tonne.  It offers a double signal that can discriminate nuclear and electron recoils, while simultaneously locating the event within the detector.  Xenon is self-shielding so this cuts backgrounds down substantially.

The most recent limits are from LUX, which reported results based on a 200 kg detector.  Long-term plans for an upgrade to 10 tonne.  This talk focuses on PandaX, now running with a 500 kg detector.  In the medium term we also have Xenon-1T, with a 1-tonne detector.

Amusingly, PandaX is not located in an old mine but an active Hydroelectric plant on the Yalong river.  This diverts the river 18km through a mountain, with 2.5km overhead.  The experimental hall is off the access road that runs parallel to the water tunnels.  The cosmic background is better (smaller) than in any other lab currently in use, plus the radioactive background is very low.

First run, based on 120kg detector, complete.  Sensitivity generally worse than Xenon/LUX, but better at low mass (by construction).  The paper came out on the arXiv today!

PandaX-II will start taking data soon, based on 500 kg detector.  This will (unsurprisingly) beat the current LUX limits.  The main question is if they can get results out before Xenon-1T publishes; their must larger detector will give much stronger limits.

What are the "accidental" background events? When random independent background events overlap to fake a signal by coincidence.  Background light in detector, plus surface scattering where two normal signals overlap.

12:10 pm: Dark Matter Theory and Searches, Tracy Slatyer

Focus is phenomenological: 3.5 keV line, GCE excess, and impact of results from Fermi, AMS-02 and Planck.

The 3.5 keV line
Claimed by two groups a year ago.  Follow-up studies: some found it, others didn't.  Simplest explanaion would be sterile neutrino; this strongly ruled out by non-detection in dwarfs and stacked galaxy samples.  Alternatives that seem to work are excited DM and conversion of axion-like particles in a magnetic field.

There is debate over whether this could be a K or Cl spectral line.  The Astro-H experiment is planned to launch next year that could resolve this.

6 years old now.  Extends out to 10 degrees from galactic plane; quite a long way.  Roughly spherically symmetric.  Spectrum peaks at around 2 GeV.  Fermi has stated in symposium talk that excess exists, but have not yet published anything.

DM interpretation prefers light masses annihilating into quarks.  Heavy states seem also able to fit, albeit less well except for the Higgs.

New Results
Dwarf limits are seriously challenging the DM interpretation.  There remains room to explain everything for now, but the near future should become clearer.  The dwarf galaxy experiments have sufficient sensitivity to observe a signal, if it exists.

In last couple of months, discovered nine new dwarfs.  Controversy over signal seen at Reticulum II with varying significance and the question of what the J-factor of that dwarf actually is.

AMS-02 measurements recently updated.  The most interesting are the antiproton results; may be DM, but the astrophysics are complicated.  However, they are good enough to put some limits on GCE models.

DM a good fit to GCE?  Not exactly, but spectrum is distinct from any other excess seen.  Excess also robust to choice of spectrum, and improvement over data explanation without DM-like signal is dramatic.  Not statistics, could be systematics?

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