RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Search for a dark photon in π 0 Dalitz decays by PHENIX
experiment at RHIC
Y. Akiba∗1 and Y. Yamaguchi∗2 for PHENIX Collaboration

Dark photon limit

Several models of dark matter suggest the existenceof a “dark photon,” an U (1) gauge boson in the dark
matter sector that mixes with an ordinary photon1) .
In these models, a dark photon is responsible for annihilation of a pair of dark matter particles into an e+ e−pair. Such annihilation can explain the positron excess
observed by PAMELLA, FERMI, and AMS-2 satellite
experiments. A dark photon can also explain the 3
σ level deviation of the anomalous magenetic momentof the muon (muon g − 2) from the Standard Modelcalculation.
In the simplest version of these models, a dark photon U mixes with a QED photon γ with a very small Expected reach with 1.3M events & 3MeV of σFig. 1. Limit on the dark photon mixing parameter ϵ2 as amixing term of the Lagrangianfunction of dark photon mass. See the text for details.

ϵ QED µν
Lmix = − FµνFdark ,2
where ϵ is the mixing parameter.
The dark photon can have a small mass MU . If
MU is greater than twice the electron mass me , it candecay into an e+ e− pair. In the natural version of themodel, this is the only decay mode. This means thefollowing:(1) A dark photon can be produced in any processthat can produce virtual photon with a small
fraction ϵ2 .
(2) Once produced, it decays exclusively into ane+ e− pair.
(3) The decay width of dark photon is very narrowand practically zero due to the small coupling ϵ2Therefore, if the mass of the dark photon is less thanthat of π 0 , a clear signal of dark photon should show
up as a narrow peak in e+ e− pair mass spectrum of π 0
Dalitz decays π 0 → e+ e− γ.
We searched for the signal of dark photon in π 0Dalitz decay data measured by PHENIX experiment
at RHIC. PHENIX is very well suited for this search
since it has a very good electron identification capability, a high mass resoluiton of low mass e+ e− pairs,
and a high statistics data sample of π 0 Dalitz decay.
We used the data set of p + p collision in 2006 run√
and d+Au collisions in 2008 run, both at sN N =200 GeV. We analyzed approximately 1.3 million e+ e−pairs for the search.
We did not find any significant peak in the Dalitz
pair mass spectrum. Thus, we set the upper limit on
the mixing parameter ϵ2 as a function of dark photon
mass MU from our null search result. In setting the∗1∗2upper limit, we employed the CLs method, a statisticalmethod now widely used by the LHC experiments for
new particle searches.
The preliminary results of the search is shown in
Fig.1. The magenta band shows the observed 90 %CL upper limit on ϵ2 . The width of the band represents the systematic uncertainty of the limit. The
fluctuation of the limit is due to statistical fluctation of
the background continuum, i.e., Dalitz pair mass spectrum. If the background e+ e− mass spectrum fluctuate up (down), the upper limit on the dark photon becomes higher (lower). The dashed curve represents the“expected level” of the upper limit if there is no suchstatistical flucutation. The green and yellow bands are
expected statistical fluctuation of the upper limit at 1σand 2σ level, respectively. The fluctuation of the observed limit is within approximately 2σ level.
The gray band in the figure shows the 90 % CL region that can explain the muon g − 2 deviation. The
dashed curve shows the upper limit on ϵ2 from electrong − 2 at 2σ level. Together with the limit from electron
g −2, our results have excluded almost all region of themuon g − 2 band.
We are now working to finalize the data. We will
publish the final results soon.
References1) Nima Arkani-Hamed et al., Phys. Rev. D79: 015014(2009).

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