Sunday, 15 June 2014

Weekend Plot: BaBar vs Dark Force

BaBar was an experiment studying 10 GeV electron-positron collisions. The collider is long gone, but interesting results keep appearing from time to time.  Obviously, this is not a place to discover new heavy particles. However, due to the large luminosity and clean experimental environment,  BaBar is well equipped to look for light and very weakly coupled particles that can easily escape detection in bigger but dirtier machines like the LHC. Today's weekend plot is the new BaBar limits on dark photons:

Dark photon is a hypothetical spin-1 boson that couples to other particles with the strength proportional to their electric charges. Compared to the ordinary photon, the dark one is assumed to have a non-zero mass mA' and the coupling strength suppressed by the factor ε. If ε is small enough the dark photon can escape detection even if mA' is very small, in the MeV or GeV range. The model was conceived long ago, but in the previous decade it has gained wider popularity as the leading explanation of the PAMELA anomaly.  Now, as PAMELA is getting older, she is no longer considered a convincing evidence of new physics. But the dark photon model remains an important benchmark - a sort of spherical cow model for light hidden sectors. Indeed, in the simplest realization, the model is fully described by just two parameters: mA' and ε, which makes it easy to present and compare results of different searches.

In electron-positron collisions one can produce a dark photon in association with an ordinary photon, in analogy to the familiar process of e+e- annihilation into 2 photons. The dark photon then decays to a pair of electrons or muons (or heavier charged particles, if they are kinematically available). Thus, the signature is a spike in the e+e- or μ+μ- invariant mass spectrum of γl+l- events. BaBar performed this search to obtain world's best limits on dark photons in the mass range 30 MeV - 10 GeV, with the upper limit on ε in the 0.001 ballpark. This does not have direct consequences for the explanation of the  PAMELA anomaly, as the model works with a smaller ε too. On the other hand, the new results close in on the parameter space where the minimal dark photon model  can explain the muon magnetic moment anomaly (although one should be aware that one can reduce the tension with a trivial modification of the model, by allowing the dark photon to decay into the hidden sector).

So, no luck so far, we need to search further. What one should retain is that finding new heavy particles and finding new light weakly interacting particles seems equally probable at this point :)

15 comments:

Robert L. Oldershaw said...


No doubt the paisley rainbow and the pot of whatever particles is just over the next hill.

If one is neck-deep in the Big Muddy, what is there to do but to press on.

Alex said...

As long as we're talking about new results, any comment on the EXO paper in Nature?

Jester said...

The paper is on arxiv since February. But that's true: I should write one day about the progress in searches for the neutrinoless beta decay.

Anonymous said...

I think the first link is wrong.

Jester said...

Ooops, sorry, fixed.

Anonymous said...

Jester -- Any comments on LHCb's latest talks on 2.5 sigma results showing 25% discrepancy in universality of b decays predicted by std model? https://cds.cern.ch/record/1706212/files/LHCb-TALK-2014-108.pdf
and http://www.symmetrymagazine.org/article/june-2014/lhcb-glimpses-possible-sign-of-new-physics

Jester said...

Yeah, I'm aware of this, but i need to do some more research to decide whether it's serious.

Cédric Bardot said...

No exciting news either from the "God" space accelerator and its American cosmic rays spectrometer AMS02 (the only living part in the sky of the late SSC so to speak, thanks to Samuel Ting ;-)
http://ams.nasa.gov/Documents/AMS_Publications/NASA%20JUNE-2014C.pdf

Robert L. Oldershaw said...


LHC(CMS): No "excited quarks"

askaboutphysics.dk said...

Along the lines of Dark Photons it would be interesting on your take on the new keV line seen by chandra at ~7keV that 'could' originate from sterile neutrinos. http://iopscience.iop.org/0004-637X/789/1/13/

If I understand correctly (im a particle physicist and not an astronomer) the line is seen in 3 seperate channels above 3 sigma, but at the limit of instrumentation, so I guess it could well be a dud still...

ask said...

Hi Jester,
Along the lines of Dark Photons, do you have any comments to the claim of 3+ sigma sterile neutrino spectral line at 7keV from Chandra?
e.g. : http://arxiv.org/abs/1402.2301

I guess it might be hard to say if it is anything but instrumentation for anybody but the people involved in the experiment...

Alex said...

So, you ever gonna blog again or not?

Anonymous said...

There is no dark matter in Bob Holdom's model (He mixed 2 U(1)s Kinetically - which in fact one can get most naturally in supergravity with a nontrivial gauge kinetic function). Dark Matter came into the picture later on... Dark Photon is a poor label anyway, the extra spin 1 particle mass discussed is much closer to a Z prime mass than a photon (which has zero mass)...nevertheless....let the jargon parade on.

David Brown said...

"Dark photon is a hypothetical spin-1 one boson that couples to other particles with a strength proportional to their electric charges." My impression is that the majority of physicists strongly doubt the existence of the dark photon and that the graviton, the inflaton, the axion, the matter magnetic monopole, and the antimatter magnetic monopole are the only undiscovered particles that they would vote in favor of. Is my impression correct?

Jester said...

Guys, sorry for late replies, I was closed for summer.

askaboutphysics.dk: i blogged about it here http://resonaances.blogspot.fr/2014/02/signal-of-neutrino-dark-matter.html
and there will soon be a follow-up.

alex: Yes :)

Anon: It's true but "dark photon" is a cool name, better than "hidden photon". Personally i call dark photon rather than Z' as long as it is light enough such that its couplings to matter are proportional to the electric charge.

David: It's true that currently there's neither theoretical nor phenomenological motivations for dark photons. It's just a toy model for new physics that is light but hidden from the usual collider searches.