1. Jin Huang Brookhaven National Lab For the PHENIX Collaboration A MERICAN P HYSICAL S OCIETY A PRIL M EETING, A PRIL 13, 2015, B ALTIMORE, M ARYLAND Outline: ● Motivation ● Detector Setup ● Analysis ● Results ● Future

2. APS April Meeting 2015 Jin Huang 2 Cosmic Positron fraction VS energy  Among many experimental hints for dark matter, muon (g-2) anomaly and cosmic ray positron excess strongly motivates models of a U(1) dark photon, that weekly coupled to EM photon  Experimentally, this dark photon can be searched through peak finding over a large flux of virtual photons, including Dalitz decay of π 0 /η → γ A’ → γ + (e + e - )  PHENIX@RHIC accumulated large statistics of Dalitz decay data for a dark photon search  A similar approach was/is being also used by COSY-WASA, HADES, NA48/2 ? ? Muon (g-2) Theory VS exp. A’ * *

3. The most versatile hadron collider in the world, and world’s first and only spin-polarized proton collider Two major experiments as of today Pioneering High Energy Nuclear Interaction eXperiment (PHENIX) Solenoidal Tracker At RHIC (STAR) APS April Meeting 2015 Jin Huang 3

4. APS April Meeting 2015 Jin Huang 4 :Used in this search

5. 2006 Run: p + p 2008: d + Au 2009 Run: p + p w/ hadron blind detector (X 0 ~2.4%) APS April Meeting 2015 Jin Huang 5 Detection of e + e - pairs in PHENIX Central arms spectrometers (-0.35< η <0.35, 2×90⁰ in φ) Vertex determination: beam-beam counter Tracking: drift chamber (DC) + pad chamber (PC), 0.39% X 0 before tracker in 2006/2008 B dL~1.2 T m, δp/p ~ 1% (+) 1.1% × (p / (GeV/c)) → Δm ee ≈ 3.1 MeV Electron trigger & ID: ring imaging Cherenkov detector (hadron rejection >1000) + electromagnetic calorimeter (hadron rejection ~ 10) e+e+ e-e- √s NN = 200 GeV

6.  In the search region (30 < m ee < 90 MeV/c 2 ), observed e + e - pairs contain Dalitz decay of π 0 /η and background contributions  Category of background: ◦ Combinatorial of e + e - that are born through uncorrelated process ◦ e + e - possibly correlated through non- Dalitz origin of the same pion, or through jet productions  Background estimates: ◦ 2009: same-sign pairs of e ± e ± → combinatorial + correlated background ◦ 2006/08: mix event + simulation  Selection region where background < 10% of pair yield for this search APS April Meeting 2015 Jin Huang 6

7. APS April Meeting 2015 Jin Huang 7 Search window No peak observed 

8.  Under assumption of a narrow dark photon mass width, the expected peak width is detector resolution dominated  Width quantified through simulation  We estimated possible events with dark photon candidates under 90% confidence limit using CLs approach  No dark photon signal was observed  Upper limit of event count is given APS April Meeting 2015 Jin Huang 8 Simulated dark photon mass peak and width Color coding in p T Color coding in m ee

9.  Upper limit of of dark photon-QED photon coupling is given in mass range of 30-90 GeV  Preliminary release Oct. 2013, final result published in PRC rapid communication (2015)  Together with published data, almost rule out muon g-2 preferred band  Exciting to see new search coming to this region too: ◦ NA48/2 (as in Cenci’s talk, arXiv:1504.00607) ◦ BELL (II) (as in Jaegle’s talk) ◦ ALICE, APEX, HPS, DarkLight, MESA, VEPP3, Mu3e,... APS April Meeting 2015 Jin Huang 9 PHENIX Collaboration, arXiv:1409.0851, PRC 91, 031901 (2015)

10.  The PHENIX collaboration carried out a search for dark photon through Dalitz decay of π 0 /η  No dark photon signal was observed in 2006-2009 data  Upper limit of dark photon coupling to QED photon over the mass range 30 < m ee < 90 MeV/c 2  With published world data, dark photon as explanation for observed (g-2) anomaly almost ruled out APS April Meeting 2015 Jin Huang 10 Future directions @ PHENIX : use of recent high luminosity data and silicon detector to perform displaced vertex search. PHENIX Collaboration, arXiv:1409.0851, PRC 91, 031901 (2015)

11. APS April Meeting 2015 Jin Huang 11 ~20002017→2020~2025Time Current PHENIX sPHENIX (+fsPHENIX) An EIC detector  14y+ operation 100+M$ investment  Broad spectrum of physics (QGP, Hadron Physics, DM)  147 published papers to date  Last run in this form 2016  Comprehensive central upgrade based on BaBar magnet  Rich jet and beauty quarkonia physics program → nature of QGP  fsPHENIX : forward tracking, Hcal and muon ID → Spin, CNM  PHENIX upgrade path leads to a capable EIC detector  Large coverage of tracking, calorimetry, and PID  Welcome new collaborators and new ideas Welcome new collaborators and new ideas! Documented: http://www.phenix.bnl.gov/plans.html RHIC: A+A, spin-polarized p+p, spin-polarized p+A EIC: e+p, e+A

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17. APS April Meeting 2015 Jin Huang 17 JHEP 1502 (2015) 157

18. 1312.4992 C. Hearty Taku Gunji talk @ BNL workshop APS April Meeting 2015 Jin Huang 18 Excising and future data – courtesy: Bertrand Echenard

19.  PHENIX: Pioneering High Energy Nuclear Interaction eXperiment One of the largest experiment @ RHIC  14y+ operation 100+M$ investment  Broad spectrum of physics ◦ QGP, Spin puzzle, New Physics ◦ 147 published papers to date APS April Meeting 2015 Jin Huang 19 18.5 m