1. Richard MilnerDESY April 6, 20091 OLYMPUS Overview Motivation for the experiment Progress to date on the experiment The path forward

2. Richard MilnerDESY April 6, 20092 Elastic Electron Scattering from Proton ++… Dirac, Pauli FF Sachs FF

3. Richard MilnerDESY April 6, 20093 Nucleon elastic form factors Fundamental observables describing the distribution of charge and magnetism in the proton and neutron Defined in the context of single photon exchange Experimentally, data well described (to first order) by an exponential spatial fall off of nucleon’s charge and magnetism ~ e -µr => dipole form factor G D (Q 2 ) ~ (1 + Q 2 /0.71) -2 At Q 2 >> 1, σ ~ σ Mott G 2 D ~ Q -12 FF determined by quark structure of proton Will be calculable in lattice QCD

4. Richard MilnerDESY April 6, 20094 Unpolarized Elastic e-N Scattering For ~ 50 years unpolarized cross section measurements have determined the elastic FF G p E and G p M using the Rosenbluth separation σ red = dσ/dΩ [ε(1+  )/σ Mott ] =  G M 2 + εG E 2  = Q 2 /4M 2 ε = [ 1 + 2(1+  )tan 2 θ/2 ] -1

5. Richard MilnerDESY April 6, 20095 Rosenbluth Separation GM2GM2 GE2GE2 θ=180 o θ=0 o

6. Richard MilnerDESY April 6, 20096 All Rosenbluth data from SLAC and Jlab in agreement. Dramatic discrepancy between Rosenbluth and recoil polarization technique Jefferson Lab Proton Form Factor Ratio Dramatic discrepancy!

7. Richard MilnerDESY April 6, 20097  P.A.M. Guichon and M. Vanderhaeghen, PRL91, 142303 (2003)  P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72, 034612 (2005), PRL91, 142304 (2003)  M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22, 331 (2004)  Y.C. Chen et al.,PRL93, 122301 (2004)  A.V. Afanasev and N.P. Merenkov, PRD70, 073002 (2004) + +.... Calculation of Two Photon Exchange Effects Effects

8. Richard MilnerDESY April 6, 20098 Estimation of TPE Contribution P.G. Blunden et al., Phys. Rev. C 72, 034612 (2005)

9. Richard MilnerDESY April 6, 20099 How do we verify that the TPE contribution interpretation is correct? Precision comparison of positron-proton and electron-proton elastic scattering over a sizable ε range at Q 2 ~ 2-3 (GeV/c) 2 J. Arrington PRC 69, 032201(R) (2004) SLAC data Θ=180 o Θ=0 o At low ε : ~ 0.01 to 0.8 (GeV/c) 2 At high ε : ~ 1-5 (GeV/c) 2

10. Richard MilnerDESY April 6, 200910 TPE calculation comparison with e + p/e - p scattering data P.G. Blunden et al., Phys. Rev. C 72, 034612 (2005)

11. Richard MilnerDESY April 6, 200911 e + p/e - p cross section ratio P.G. Blunden et al., Phys. Rev. C 72, 034612 (2005)

12. Richard MilnerDESY April 6, 200912 Proton form factor ratio Blunden et al.

13. Richard MilnerDESY April 6, 200913 The OLYMPUS Experiment The OLYMPUS Experiment Electrons/positrons (100mA) in multi-GeV storage ring DORIS at DESY, Hamburg, Germany Unpolarized internal hydrogen target (like HERMES) 3 x 10 15 at/cm 2 @ 100 mA → L = 2x10 33 / (cm 2 s) Measure elastic e + /e - proton scattering to 1% precision at 2 GeV energy with ε range from 0.4 to 1 at high Q 2 ~ 2-3 (GeV/c) 2 using the existing Bates Large Acceptance Spectrometer Toroid Experiment requires switching from e + beam to e - beam on timescale of ≤ 1 day. Redundant monitoring of luminosity, pressure, temperature, flow, current measurements - small-angle elastic scattering at high ε and low Q 2

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16. Richard MilnerDESY April 6, 200916 OLYMPUS kinematics

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21. Richard MilnerDESY April 6, 200921 Projected OLYMPUS uncertainties

22. Richard MilnerDESY April 6, 200922 Control of systematics Luminosity monitors BLAST @ DORIS 10 o Change BLAST polarity once a day Change between electrons and positrons regularly Left-right symmetry θ→0 o, ε→1 TPE effects → 0

23. Richard MilnerDESY April 6, 200923 Control of systematics i = e+ or e- j= pos/neg polarity of BLAST field Geometric proton efficiency: Ratio in single polarity j Geometric lepton efficiency:

24. Richard MilnerDESY April 6, 200924 Control of systematics Change between electrons and positrons regularly Change BLAST polarity every day Left-right symmetry provides additional redundancy – two identical experiments simultaneously taking data Super ratio: Cycle of four states ij Repeat cycle many times

25. Richard MilnerDESY April 6, 200925 December 17 th 2008

26. Richard MilnerDESY April 6, 200926 Timeline OLYMPUS proposal conditionally approved by DESY in December 2008 Requests submitted to funding agencies: December 2008 (Germany) and January 2009 (US) Remove ARGUS experiment in Summer 2009 Ship BLAST/OLYMPUS detector and OLYMPUS target in summer/fall 2010 Modify DORIS beamline and install OLYMPUS target in DORIS in winter 2010/11 shutdown. Install complete OLYMPUS experiment in summer 2011 Commission in fall 2011 Take data in 2012

27. Richard MilnerDESY April 6, 200927 Summary The OLYMPUS experiment has been approved by DESY conditional on the funds being available. A schedule for the installation, commissioning and data taking has been established. Detailed discussion of the technical issues are getting underway. The critical path issue now is to secure the necessary funding required to carry out OLYMPUS.