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BABAR searches for a dark photon at low mass.

BaBar searches for dark photon at low mass

A recent search for a dark photon in e+e- collisions performed by BaBar, and published on Phys. Rev. Lett. 113, 201801 (2014) (PDF), has been featured in a Physics Viewpoint article: New Light Shed on Dark Photons.

From astrophysical and cosmological observations dark matter is believed to account for about 25% of the energy of the Universe, to be compared with the only 5% accounted by the standard matter (i.e., the matter described by the Standard Model of Particle Physics). The nature of the dark matter is still unknown and many theoretical models have been developed.

In analogy with the electromagnetism, some of the proposed solutions hypothesize the existence of a new "dark" force, whose mediator, the so-called dark photon A', connects the dark sector with the Standard Model (SM), coupling to both dark-matter particles and standard-matter particles.

The interactions of the dark photon with the ordinary matter could also explain the difference between the value of the anomalous magnetic moment of the muon measured experimentally and that predicted by the SM.

BaBar searched for decays of the dark-photon to a pair of leptons in the reaction e+e- → γ A', with the subsequent decay A' → e+e- or A' → μ+μ-.

We observed no statistically significant deviations from the standard model prediction in the mass spectrum of the reconstructed lepton pairs, and we set stringent limits on the existence of a dark photon in the mass range from 20 MeV/c2 to about 10 GeV/c2. In particular, our data ruled out almost completely the dark photon as an explanation for the measured value of the muon anomalous magnetic moment.

BaBar is an international collaboration of approximately 400 physicists from Canada, France, Germany, India, Italy, Israel, Netherlands, Norway, Russia, Saudi Arabia, Spain, the U.K., and the U.S., which is based at the SLAC National Accelerator Laboratory, which is operated by Stanford University for the U.S. Department of Energy Office of Science. The Collaboration operated the BaBar experiment, which recorded the collisions of more than 9 billion pairs of electrons and positrons between 1999 and 2008, The BaBar experiment was originally constructed to answer completely different questions driven by the desire to understand why the universe contains matter with no antimatter. BaBar data helped confirm the matter-antimatter theory for which two researchers won the 2008 Nobel Prize in Physics. The BaBar data continue to be analyzed for various studies—including this one.

This work is supported by DOE and NSF (USA), NSERC (Canada), CEA and CNRS-IN2P3 (France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands), NFR (Norway), MES (Russia), MICIIN (Spain) and STFC (United Kingdom). Individuals have received support from the Marie Curie EIF (European Union) and the A. P. Sloan Foundation (USA).

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