RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-4. Hadron Physics

Search for dark photons from neutral meson decays in p + p and√d+Au collisions at sN N =200 GeV†
Y. Akiba∗1 and Y. Yamaguchi∗2,∗3 for PHENIX Collaboration

ε QED µν
Lmix = − Fµν
Fdark ,
2
where ε parameterizes the mixing strength. Dark photons can then mix with the QED photon through allprocesses that involves QED photons.
The PHENIX experiment searched for possible decays of π 0 , η → γU , U → e+ e− by examining theinvariant mass mee of e+ e− pairs in a large sampleof Dalitz decays, π 0 , η → γe+ e− for 30 < mU < 90
MeV/c2 in the dark photon parameter space. Theweak coupling of the dark photon with the QED photon implies that the natural width of the dark photon
is very narrow. Thus if the dark photon mass is in thisrange, a clear dark photon signal should appear as anarrow peak in the e+ e− mass spectrum.
We used the data set of p + p collisions in the 2006
and 2009 runs and d+Au collisions in the 2008 run,√at sN N = 200 GeV, but did not find any significant
signal. Thus, we set the upper limit on the mixing
parameter ε2 as a function of the dark photon massmU .
Fig.1 shows the limits determined by the PHENIX
along with the 90% confidence level (CL) limits fromother experiments and the 2σ upper limit theoretically calculated from (g − 2)e . The band indicates therange of parameters that would allow the dark photon to explain the (g − 2)µ anomalies with 90% CL.
The PHENIX experiment limits exclude the values of†∗1∗2∗3Condensed from the article in Phys. Rev. C91:031901(R)(2015)
RIKEN Nishina Center
CNS, University of Tokyo
Stony Brook University

10-4A1 (MAMI)

electron g-2 2σKLOE
ε 90% CLThe standard model (SM) of particle physics provides unprecedented numerical accuracy for quantities
such as the anomalous magnetic moment of the electron (g − 2)e . Hence, measurements that lie outside
SM predictions warrant a special investigation. One
such result is the measured value of (g − 2)µ , which
deviates from SM calculations by 3.6σ. An intriguing
explanation for this discrepancy has been proposed byadding a “dark photon”1) . This possibility has recently
gained more relevance because it provides a simultaneous explanation of various beyond-the-standard-model
phenomena in addition to (g − 2)µ . These includes thepositron excess observed by PAMELLA, FERMI, andAMS-2 satellite experiments.
A simple formulation of the dark sector postulatesa “dark photon” U of mass mU that mixes with theQED photon via the “kinetic coupling” term in theLagrangian-510WASAmuon g-2 90% CLHADESPHENIXBaBar-610102030405060708090 100mU [MeV/c 2]

Fig. 1. A compilation of the limits of the U γ mixing parameter, showing the PHENIX results, together withother experiments and the band indicating the range ofmass and coupling parameters favored by the (g − 2)µanomaly at 90% CL. Also shown is the 2σ upper limit
obtained from (g − 2)e .

the coupling favored by the (g − 2)µ anomaly above
mU > 36 MeV/c2 . Recently, BaBar reported stricter
limits from a search of the reaction e+ e− → γU, U →l+ l− , excluding values of the preferred (g − 2)µ region
for mU > 32 MeV/c22) . As a result, nearly all theavailable parameter space that would allow the dark
photon to explain the (g − 2)µ results are ruled out
at 90% CL by independent experiments. The entire
parameter space to explain the (g − 2)µ anomaly by
the dark photon can be excluded at 85% CL by thePHENIX data alone. The level of compatibility between our data and the coupling strength favored forthe (g − 2)µ anomaly is 10% with a statistical test.
In summary, the PHENIX results set limits for thecoupling of a dark photon to the QED photon over themass range 30 < mU < 90 MeV/c2 . Combining withthe BaBar results, the dark photon is ruled out at
90% CL as an explanation for the (g − 2)µ anomaly
for mU > 32 MeV/c2 , leaving only a small remaining
part of the parameter space in the region 29 < mU <32 MeV/c2 .
References
1) P. Fayet, Phys. Rev. D75:115017(2007).
2) J. Lees et al. (BaBar Collaboration), Phys. Rev.
Lett.113:201801 (2014).

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