Hampton University and Jefferson Lab Virginia, USA

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JPOS09: Int. Workshop on Positrons at Jefferson Lab, March 25-27, 2009. OLYMPUS @ DESY A proposal to definitively determine the contribution of multiple photon exchange in elastic lepton-nucleon scattering. Michael Kohl. Hampton University and Jefferson Lab Virginia, USA. Overview. - PowerPoint PPT Presentation

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  • OLYMPUS @ DESYA proposal to definitively determine the contribution of multiple photon exchange in elastic lepton-nucleon scattering

    Hampton University and Jefferson Lab Virginia, USA

    Michael KohlJPOS09: Int. Workshop on Positrons at Jefferson Lab, March 25-27, 2009

  • *IntroductionMotivation from previous dataDescription of the proposed experimentSummary

  • *Nucleon Elastic Form Factors Fundamental quantitiesDefined in context of single-photon exchange

    Describe internal structure of the nucleonsRelated to spatial distribution of charge and magnetism

    Rigorous tests of nucleon modelsDetermined by quark structure of the nucleonUltimately calculable by Lattice-QCDInput to nuclear structure and parity violation experiments

    50 years of ever increasing activity

    Tremendous progress in experiment and theory over last decadeNew techniques / polarization experimentsUnexpected results

  • *General definition of the nucleon form factor

    Sachs Form Factors In One-photon exchange approximation above form factors are observables of elastic electron-nucleon scattering

  • *tGM2GE2=180o=0o

  • *

  • *

  • *Double polarization in elastic/quasielastic ep or en scattering: Recoil polarization or (vector) polarized target Polarized cross section

    Double spin asymmetry = spin correlationAsymmetry ratio (Super ratio) independent of polarization or analyzing power1,2H(e,ep), 1,2H(e,ep),

    Nucleon Form Factors and Polarization2H(e,en), 2H(e,en) , 3He(e,en),

  • *All Rosenbluth data from SLAC and Jlab in agreement Dramatic discrepancy between Rosenbluth and recoil polarization techniqueMulti-photon exchange considered best candidateJefferson LabProton Form Factor RatioDramatic discrepancy!

  • *Q2/(GeV/c)2mpGpE/GpM00 2 4 6 8 101Proton Form Factor RatioIachello 1973:

    Drop of the ratio already suggested by VMDF. Iachello et al., PLB43 (1973) 191 F. Iachello, nucl-th/0312074A.V. Belitsky et al., PRL91 (2003) 092003G. Miller and M. Frank, PRC65 (2002) 065205S. Brodsky et al., PRD69 (2004) 076001Quark angular momentumHelicity non-conservation

  • *Two-Photon Exchange: A Lot of TheoryP.A.M. Guichon and M. Vanderhaeghen, PRL91 (2003) 142303: Formalism TPE effect could be largeP.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) 034612, PRL91 (2003) 142304: Nucl. Theory elastic half, Delta oppositeM.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331: Importance of higher-order radiative effectsY.C. Chen et al., PRL93 (2004) 122301: Partonic calculation, TPE large at high Q2A.V. Afanasev and N.P. Merenkov, PRD70 (2004) 073002: Large logarithms in normal beam asymmetryA.V. Afanasev, S.J. Brodsky, C.E. Carlson, Y.C. Chen, M. Vanderhaeghen, PRD72 (2005) 013008: high Q2, small effect on asym., larger on x-sec., TPE on R smallY.C. Chen, C.W. Kao, S.N. Yang, PLB652 (2007) 269: Model-independent TPE large D. Borisyuk, A. Kobushkin, PRC74 (2006)065203; 78 (2008) 025208: TPE effect rather small!M. Kuhn, H. Weigel, EPJA38 (2008) 295: TPE in Skyrme Model D.Y. Chen et al., PRC78 (2008) 045208: TPE for timelike form factorsM. Gorchtein, C.J. Horowitz, arXiv:0811.0614 [hep-ph]: gamma-Z box

    Two-photon exchange theoretically suggestedInterference of one- and two-photon amplitudes

  • *TPE Predictions for e+/e- RatioY.C. Chen, C.W. Kao, S.N. Yang PLB652 (2007) 269Q2 = 1.75-5.0 (GeV/c)2Q2 = 3.0 (GeV/c)2D. Borisyuk, A. Kobushkin, PRC74 (2006)065203P.G. Blunden, W. Melnitchouk, J.A. Tjon PRC72 (2005) 034612, PRL91 (2003) 142304 TPE effect largeTPE effect smallTPE effect sizeableTPE effect smallA.V. Afanasev, S.J. Brodsky,C.E. Carlson, Y.C. Chen, M. Vanderhaeghen, PRD72 (2005) 013008

  • *Precision comparison of positron-proton and electron-proton elastic scattering over a sizable range at Q2 ~ 2-3 (GeV/c)2J. Arrington, PRC 69 (2004) 032201(R)SLAC data=180o=0oAt low : ~ 0.01 to 0.8 (GeV/c)2At high : ~ 1-5 (GeV/c)2Experiments to Verify 2g Exchange

  • *Elastic electron-proton topositron-proton ratio (P. Blunden)

  • *BLAST @ 2.0 GeV Q2 = 0.62.2 (GeV/c)2Elastic electron-proton topositron-proton ratio (P. Blunden)

  • *

  • *pOsitron-proton andeLectron-proton elastic scattering to test thehYpothesis of Multi- Photon exchange UsingDoriS2008 Full proposal2009/10 Transfer of BLAST2011/12 OLYMPUS Running

  • *Electrons/positrons (100mA) in multi-GeV storage ring DORIS at DESY, Hamburg, GermanyUnpolarized internal hydrogen target (buffer system) 3x1015 at/cm2 @ 100 mA L = 2x1033 / (cm2s)Redundant monitoring of luminosity pressure, temperature, flow, current measurements small-angle elastic scattering at high epsilon / low Q2Large acceptance detector for e-p in coincidence BLAST detector from MIT-Bates available

    Measure ratio of positron-proton to electron-proton unpolarized elastic scattering to 1% stat.+sys.

  • *Left-right symmetricLarge acceptance: 0.1 < Q2/(GeV/c)2 < 0.8 20o < q < 80o, -15o < < 15oCOILS Bmax = 3.8 kGDRIFT CHAMBERS Tracking, PID (charge) dp/p=3%, dq = 0.5oCERENKOV COUNTERS e/p separationSCINTILLATORS Trigger, ToF, PID (p/p)NEUTRON COUNTERS Neutron tracking (ToF)

  • *BatesUNHMITASU

  • *Charge +/-CoplanarityKinematicsTiming

    Advantages of magnetic field: suppression of background 2-3% momentum resolution = 0.5o and = 0.5o

  • ***Ph.D. work of C. Crawford (MIT) and A. Sindile (UNH)Impact of BLAST data combined with cross sections on separation of GpE and GpMErrors factor ~2 smallerReduced correlationDeviation from dipole at low Q2!C.B. Crawford et al., PRL 98 (2007) 052301

  • **Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU)*E. Geis, M.K., V. Ziskin et al., PRL 101 (2008) 042501

  • Electrons/positrons (100mA) in multi-GeV storage ring DORIS at DESY, Hamburg, GermanyUnpolarized internal hydrogen target (buffer system) 3x1015 at/cm2 @ 100 mA L = 2x1033 / (cm2s)Large acceptance detector for e-p in coincidence BLAST detector from MIT-Bates availableMeasure ratio of positron-proton to electron-proton unpolarized elastic scattering to 1% stat.+sys. Redundant monitoring of luminosity (pressure, temperature, flow, current measurements) small-angle elastic scattering at high epsilon / low Q2

  • Forward telescopes2 tGEM telescopes, 3 tracking planes 3.9 msr, 10o, R=160 cm, dR=10 cm10o

  • Forward angle electron/positron telescopes or trackers with good angular and vertex resolutionCoincidence with proton in BLASTHigh rate capability

    GEM technology

    MIT protoype:

    Telescope of 3 Triple GEM prototypes (10 x 10 cm2) using TechEtch foils

    F. Simon et al., Nucl. Instr. and Meth. A 598 (2009) 432

  • i = e+ or e-j= pos/neg polarityGeometric proton efficiency:Ratio in singlepolarity jGeometric lepton efficiency:

  • Change between electrons and positrons every other dayChange BLAST polarity every other dayLeft-right symmetry Super ratio:Cycle of four states ijRepeat cycle many times

  • 1000 hours eachfor e+ and e-Lumi=2x1033 cm-2s-1500 hours eachfor e+ and e-Lumi=2x1033 cm-2s-1

  • *Experiments to Verify 2g ExchangeExperiment proposals to verify hypothesis:

    e+/e- ratio: CLAS/PR04-116 secondary e+/e- beam Novosibirsk/VEPP-3storage ring / internal target BLAST@DORIS/DESYstorage ring / internal target SSA:PR05-15 (Hall A)e-dependence: PR04-119 (polarized), PR05-017 (unpolarized)VEPP3CLAS

  • *Significant effect theoretically predicted, size uncertainConvinced from feasibility of proposed experiment (2006)Contacted DESY, idea presented to PRC in May 2007Submitted letter of intent in June 2007Presented to DOE at MIT review in July 2007Intern. collaboration (~50 scientists, 11 inst.), April 2008Submitted full proposal on invitation by DESY (Sep 2008)Next steps: Fund raising in the US and in Europe (2009) Transfer of BLAST detector (2009-2010) Construction of new components (2009-2011) Running in 2011 and 2012, before DORIS to shut down

  • *Nucleon Elastic Form Factors All that we know and most of what we believe [in nuclear physics][] is based on electron scattering and one-photon exchange (W. Turchinetz)

    The electromagnetic probe is well understood, hence (a common phrase in many articles)

    The elastic form factors characterize the simplest processin nuclear physics, namely elastic scattering (straightforward, one should think)

    If we dont understand the form factors, we will not have understood anything (my take on the importance of OLYMPUS)Science = Belief replaced by knowledge

    ****Let me now turn to the physics.From Rosenbluth it is hard to determine GM at very low Q2 and GE at very high Q2, fueling the idea to measure the ratio GE/GM through double polarization oberservables****Such double polarization observables can be measured in either recoil polarization or polarized target experiments. The polarized cross section section contains an asymmetry term that is related to the form factors through the dependence on the target or recoil polarization components. The polarization observable is an interference effect.****PR04-116: Engineering run in 2006, full proposal approved in January 2007, no schedule yet*PR04-116: Engineering run in 2006, full proposal approved in January 2007, no schedule yet**If two-photon exchange is responsible, then the most direct way to test this is a comparison of electron-proton to positron-proton cross section ratio at small epsilon.We have realized that such a measurement could be done with the BLAST detector at storage ring experiment with electrons and positrons, covering kinematics with a signature in excess of 4%. *If two-photon exchange is responsible, then the most direct way to test this is a comparison of electron-proton to positron-proton cross section ratio at small epsilon.We have realized that such a measurement could be done with the BLAST detector at storage ring experiment with electrons and positrons, covering kinematics with a signature in excess of 4%. *One of the rare places in this world providing e+ and e- beams of this energy could be the DORIS storage ring at DESY. We have recently documented this idea and established contact with DESY on this.***The neutron counter array is asymmetric to enhance the sensitivity to Gen as will be clear in a minute.*The neutron counter array is asymmetric to enhance the sensitivity to Gen as will be clear in a minute.*Let me show you some overview on selection of elastic events.Elastic scattering involves the coincidence of a negative and a positve track. Additional conditions such as coplanarity or kinematical constraints that relate angles and momenta allow for an extremely clean selection of elastic events, shown here is the angle correlation of elastic events from hydrogen. In the deuterium case, there is a significant contribution by the quasielastic e,ep channel, however the time of flight clearly separates protons from deuterons. Note that these spectra are essentially background-free.

    *No data from double polarization below 0.4First double pol data with polarized targetExperimental erros improved in overlapping region

    *World data from double pol, consistent with Galster except at low and high Q2FW fit slightly better. Confirmed by BLAST, only half of the dataset so far.Various models, e.g. dispersion theory: does not quite fit at low Q2 which has a consequence:*******PR04-116: Engineering run in 2006, full proposal approved in January 2007, no schedule yet**

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