Sunday, 26 April 2015

Weekend plot: dark photon update

Here is a late weekend plot with new limits on the dark photon parameter space:

The dark photon is a hypothetical massive spin-1 boson mixing with the ordinary photon. The minimal model is fully characterized by just 2 parameters: the mass mA' and the mixing angle ε. This scenario is probed by several different experiments using completely different techniques.  It is interesting to observe how quickly the experimental constraints have been improving in the recent years. The latest update appeared a month ago thanks to the NA48 collaboration. NA48/2 was an experiment a decade ago at CERN devoted to studying CP violation in kaons. Kaons can decay to neutral pions, and the latter can be recycled into a nice probe of dark photons.  Most often,  π0 decays to two photons. If the dark photon is lighter than 135 MeV, one of the photons can mix into an on-shell dark photon, which in turn can decay into an electron and a positron. Therefore,  NA48 analyzed the π0 → γ e+ e-  decays in their dataset. Such pion decays occur also in the Standard Model, with an off-shell photon instead of a dark photon in the intermediate state.  However, the presence of the dark photon would produce a peak in the invariant mass spectrum of the e+ e- pair on top of the smooth Standard Model background. Failure to see a significant peak allows one to set limits on the dark photon parameter space, see the dripping blood region in the plot.

So, another cute experiment bites into the dark photon parameter space.  After this update, one can robustly conclude that the mixing angle in the minimal model has to be less than 0.001 as long as the dark photon is lighter than 10 GeV. This is by itself not very revealing, because there is no  theoretically preferred value of  ε or mA'.  However, one interesting consequence the NA48 result is that it closes the window where the minimal model can explain the 3σ excess in the muon anomalous magnetic moment.

Friday, 17 April 2015

Antiprotons from AMS

This week the AMS collaboration released the long expected measurement of the cosmic ray antiproton spectrum.  Antiprotons are produced in our galaxy in collisions of high-energy cosmic rays with interstellar matter, the so-called secondary production.  Annihilation of dark matter could add more antiprotons on top of that background, which would modify the shape of the spectrum with respect to the prediction from the secondary production. Unlike for cosmic ray positrons, in this case there should be no significant primary production in astrophysical sources such as pulsars or supernovae. Thanks to this, antiprotons could in principle be a smoking gun of dark matter annihilation, or at least a powerful tool to constrain models of WIMP dark matter.

The new data from the AMS-02 detector extend the previous measurements from PAMELA up to 450 GeV and significantly reduce experimental errors at high energies. Now, if you look at the  promotional material, you may get an impression that a clear signal of dark matter has been observed.  However,  experts unanimously agree that the brown smudge in the plot above is just shit, rather than a range of predictions from the secondary production. At this point, there is certainly no serious hints for dark matter contribution to the antiproton flux. A quantitative analysis of this issue appeared in a paper today.  Predicting  the antiproton spectrum is subject to large experimental uncertainties about the flux of cosmic ray proton and about the nuclear cross sections, as well as theoretical uncertainties inherent in models of cosmic ray propagation. The  data and the predictions are compared in this Jamaican band plot. Apparently, the new AMS-02 data are situated near the upper end of the predicted range.

Thus, there is no currently no hint of dark matter detection. However, the new data are extremely useful to constrain models of dark matter. New constraints on the annihilation cross section of dark matter  are shown in the plot to the right. The most stringent limits apply to annihilation into b-quarks or into W bosons, which yield many antiprotons after decay and hadronization. The thermal production cross section - theoretically preferred in a large class of WIMP dark matter models - is in the  case of b-quarks excluded for the mass of the dark matter particle below 150 GeV. These results provide further constraints on models addressing the hooperon excess in the gamma ray emission from the galactic center.

More experimental input will allow us to tune the models of cosmic ray propagation to better predict the background. That, in turn, should lead to  more stringent limits on dark matter. Who knows... maybe a hint for dark matter annihilation will emerge one day from this data; although, given the uncertainties,  it's unlikely to ever be a smoking gun.

Thanks to Marco for comments and plots. 

Wednesday, 1 April 2015

What If, Part 1

This is the do-or-die year, so Résonaances will be dead serious. This year, no stupid jokes on April Fools' day: no Higgs in jail, no loose cables, no discovery of supersymmetry, or such. Instead, I'm starting with a new series "What If" inspired  by XKCD.  In this series I will answer questions that everyone is dying to know the answer to. The first of these questions is

If HEP bloggers were Muppets,
which Muppet would they be? 

Here is  the answer.

  • Gonzo the Great: Lubos@Reference Frame (on odd-numbered days)
    The one true uncompromising artist. Not treated seriously by other Muppets, but adored by chicken.
  • Animal: Lubos@Reference Frame (on even-numbered days)
    My favorite Muppet. Pure mayhem and destruction. Has only two modes: beat it, or eat it.
  • Swedish Chef: Tommaso@Quantum Diaries Survivor
    The Muppet with a penchant for experiment. No one understands what he says but it's always amusing nonetheless.
  • Kermit the Frog: Matt@Of Particular Significance
    Born Muppet leader, though not clear if he really wants the job.
  • Miss Piggy: Sabine@Backreaction
    Not the only female Muppet, but certainly the best known. Admired for her stage talents but most of all for her punch.
  • Rowlf: Sean@Preposterous Universe
    The real star and one-Muppet orchestra. Impressive as an artist or and as a comedian, though some complain he's gone to the dogs.

  • Statler and Waldorf: Peter@Not Even Wrong
    Constantly heckling other Muppets from the balcony, yet every week back for more.
  • Fozzie Bear:  Jester@Résonaances
    Failed stand-up comedian. Always stressed that he may not be funny after all.
If you have a match for  Bunsen, Beaker, or Dr Strangepork, let me know in the comments.

In preparation:
-If theoretical physicists were smurfs... 

-If LHC experimentalists were Game of Thrones characters...
-If particle physicists lived in Middle-earth... 

-If physicists were cast for Hobbit's dwarves... 
and more.