1. Dark Photons Stepan Stepanyan JLAB JLAB User GROUP Meeting May 29-31, 2013

2. 2 S. Stepanyan, UGM JLAB, May 29-31, 2013 HPS Collaboration Meeting, June 4-6 http://www.jlab.org/conferences/hps2013june/program.html Every one is welcome - note a general session with theory and experimental updates in the first half of Tuesday

3. Motivation from BSM physics 3 S. Stepanyan, UGM JLAB, May 29-31, 2013 The Heavy Photon (A ’ ) is:  a massive vector gauge boson, which kinetically mixes with the SM , inducing a weak coupling  e to electric charge,  ’ /    2  present in many extensions of the Standard Model below weak scale and is natural for string theories A ’ could explain the discrepancy between the measured and calculated value of the anomalous magnetic moment of the muon, a  = g – 2, (among the simplest new physics explanations is the existence of a new force mediator that couples to muons) Might be responsible for the discrepancy in PCR measured in muon and electron experiments R. Essig Existing constraints on heavy photons (A ’ ) - 90% confidence level limits from the beam dump experiments E141, E774, Orsay, and U70, the muon anomalous magnetic moment, KLOE, the test run results reported by APEX and MAMI, an updated estimate using a BaBar result, a constraint from supernova cooling, and an updated constraint from the electron anomalous magnetic moment. In the green band, the A ’ can explain the observed discrepancy between the calculated and measured muon anomalous magnetic moment.

4. Motivation from Dark Matter 4 S. Stepanyan, UGM JLAB, May 29-31, 2013 This new gauge boson, a heavy photon or dark photon, A ’, can be our way to reach to the dark sector Dark matter coupling to the A ’ can explain excess of positrons and electrons in cosmic rays via DM annihilation or decay into e + e – Inelastic scattering of DM on nucleus via A ’ can account for direct detection results Arkani-Hamed, Finkbeiner, Slatyer, Weiner, Pospelov & Ritz GeV - TeV MeV - GeV

5. Where and how to search for dark photons 5 S. Stepanyan, UGM JLAB, May 29-31, 2013 Both “naturalness” arguments and fits to astrophysical data suggest  ’ /    2 ~ 10 -4 – 10 -10 and m A ’ ~ MeV - GeV A ’ can be electroproduced the same way as radiatieve tridents in the fix target experiment ( J.D. Bjorken, R. Essig, P. Schuster, and N. Toro, Phys. Rev. D80, 2009, 075018 )

6. 6 S. Stepanyan, UGM JLAB, May 29-31, 2013 Fix target experiments: kinematics J.D. Bjorken, R. Essig, P. Schuster, and N. Toro, Phys. Rev. D80, 2009, 075018

7. 7 S. Stepanyan, UGM JLAB, May 29-31, 2013 Electromagnetic background Bethe-Heitler: dominant QED tridents: irreducible

8. 8 S. Stepanyan, UGM JLAB, May 29-31, 2013 Bump hunt and vertexing  QED tridents will be produced at Z=0 and will not produce a peak in M(e + e - )  A’ decays can extend to large Z and will produce a narrow peak in M(e + e - ) Excellent mass and vertex resolutions are needed

9. JLAB Experiments 9 S. Stepanyan, UGM JLAB, May 29-31, 2013 APEX

10. The A ʹ Experiment in Hall-A (APEX) 10 S. Stepanyan, UGM JLAB, May 29-31, 2013 Spectrometer-based search for 50- 500 MeV A’ decaying promptly to e+e- pairs

11. APEX test run 11 S. Stepanyan, UGM JLAB, May 29-31, 2013

12. APEX: run plan and expected reach 12 S. Stepanyan, UGM JLAB, May 29-31, 2013 Approved by JLab PAC 37 1 Month Beam Time – 6 days at 1,2,3 GeV – 12 days at 4.5 GeV) Active “sieve slit”: tagging by a Sci Fiber detector to improve spectrometer optics calibration A new septum optimized for full acceptance

13. 13 S. Stepanyan, UGM JLAB, May 29-31, 2013 Electron scattering of hydrogen gas target @ 1 MW Search for A‘ in “visible“ and “invisible“ decay modes

14. 14 S. Stepanyan, UGM JLAB, May 29-31, 2013 Windowless target, aggressively pumped Gas thickness ~10 19 cm -2 with 10mA beam yields~0.5 ab -1 /month Thin beryllium beampipe Si detector for proton recoil TPC + 0.5 T magnet o High track density o ~250 μm hit res. o Magnet confines low-p T backgrounds (e-p and Moller) Scintillators serves as veto for invisibles search

15. Beam test 15 S. Stepanyan, UGM JLAB, May 29-31, 2013 Demonstrate beam can pass through 2mm diameter aperture

16. 16 S. Stepanyan, UGM JLAB, May 29-31, 2013 Uses a large acceptance forward spectrometer in Hall-B to search for A ’ in the scattering of high energy (1.1 GeV, 2.2 GeV, and 6.6 GeV), high intensity (~500 nA) electron beams on tungsten target (0.125% r.l.) in the mass range of 20 MeV to 1000 MeV for couplings    > 10 -7 with bump hunt and    <5x10 -8 with displaced decay vertex search (unique to HPS) in the decay modes to e + e - and  +  - (unique to HPS) HPS is uniquely suited for detecting True Muonium, a (  +  - ) atom E XPERIMENT AT JLAB

17. HPS detector 17 S. Stepanyan, UGM JLAB, May 29-31, 2013 a 3-magnet chicane, with the second dipole as the analyzing magnet detection and identification of electrons and muons produced at angles  >15 mr detector package includes: 6-layer Silicon Vertex Tracker (SVT) installed inside the analyzing magnet vacuum chamber, Electromagnetic Calorimeter (ECal) and the muon system installed downstream of the analyzing magnet do avoid "wall of flame”, crated by Multiple Coulomb scattered beam particles and radiative secondaries, the detectors will be split into two identical parts, installed above and below the “dead zone” (beam plane)

18. 18 S. Stepanyan, UGM JLAB, May 29-31, 2013 Electron beam parameters The beamline optimizations performed for the 12 GeV CEBAF machine  X ≈280  m  Y ≈20  m The same optics optimization program was proven to work well for 6 GeV CEBAF Optimization parameters  X ≈300  m and  Y ≈10  m The close proximity of the silicon tracker to the beam line (Layer-1 SVT is at 500  m from the beam plane) puts tight constraints on the beam size and stability The vertex resolution will benefit from a small beam size (< 50  m) in the non- bend plane, Y direction Asymmetric beam profile is desirable (a small beam sizes in both dimensions will overheat the target foil)

19. x2 difference EGS5 vs. Geant4 HPS test run 19 S. Stepanyan, UGM JLAB, May 29-31, 2013 Large fraction of trigger rates and the tracker occupancies come from multiple Coulomb scattered electrons Two simulation tools, GEANT4 and EGS5 gave markedly different results in the rate estimates GEANT4 that was used for simulations of the background and trigger rates in the original proposal gives x2 higher rates than EGS5 Proposed run conditions, luminosity ~10 34 cm -2 s -1, were limited by trigger rates of ~35 kHz (DAQ limit is 50kHz, 100 Mb/s) The main goal - validate critical assumptions made in our simulations for rates and occupancies Other goal of the test run was to demonstrate the feasibility of the proposed apparatus and data acquisition systems S. Stepanyan, PEB MIT, March 14-16, 2012

20. Test Run Results: EGS5 is correct Multiple Coulomb scattering of beam electrons is the main contributor to the detector occupancies and determines the limits of sensitivity of the experiment In test run with photon beam, the angular distribution of the pair produced electrons and positions emerging from the converter has been studied to validate simulations 20 S. Stepanyan, UGM JLAB, May 29-31, 2013

21. True muonium with HPS 21 S. Stepanyan, UGM JLAB, May 29-31, 2013 HPS experiment has the potential to discover “true muonium", a bound state of a  +  - The (  +  - ) atom is hydrogen-like, and so has a set of excited states characterized by a principal quantum number n, with the binding energy of these states is E = -1407 eV/n 2 The (  +  - ) “atom" can be produced by an electron beam incident on a target, a similar way as the A ’ HPS will discover the 1S, 2S, and 2P true muonium bound states with its proposed run plan Search will require a vertex cut at about 1.5 cm to reject almost all QED background events, then look for a resonance at 2m  A. Banburski and P. Schuster, Phys. Rev. D 86, 093007 (2012) In two weeks of 6.6 GeV run HPS should see ~15 true muonium events

22. 22 S. Stepanyan, UGM JLAB, May 29-31, 2013 Summary Bump hunt region Displaced decay vertex search

23. 23 S. Stepanyan, UGM JLAB, May 29-31, 2013 HPS Collaboration Meeting, June 4-6 http://www.jlab.org/conferences/hps2013june/program.html Every one is welcome - note a general session with theory and experimental updates in the first half of Tuesday