1. Richard MilnerHERA symposium June 30, 20071 HERMES physics, a historical * perspective Why was HERMES proposed? How did the concept of HERMES evolve? What were the physics goals? * and personal Outline The physics which motivated HERMES My personal path to HERMES

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5. Richard MilnerHERA symposium June 30, 20075 This worthless equation…

6. Richard MilnerHERA symposium June 30, 20076 HERMES originated in 1987 from a number of sources EMC spin results => quarks carry only a fraction of the proton’s spin Significant development in polarized internal gas target technology HERA had the potential for longitudinally polarized electron/positron beams Common interests of scientists in Europe and North America Seek to understand proton structure in terms of the quarks (and gluons) of QCD Desire to be technically innovative

7. Richard MilnerHERA symposium June 30, 20077 The structure of the proton ~ 1985 Structure and dynamics understood in terms of quarks: gluons assumed to carry half of the proton’s momentum but essentially `invisible’ Theoretical QCD inspired models based on quarks and confinement, e.g. the MIT bag model EMC Effect => quark momentum modified in the nucleus Violation of Gottfried Sum Rule => flavor asymmetry of the sea established No lattice QCD

8. Richard MilnerHERA symposium June 30, 20078 The EMC Effect Geesaman, et al., Ann. Rev. Nucl. Part. Sc. 45, 337 (1995) J.J. Aubert et al. Phys. Lett. B 123, 275 (1983)

9. Richard MilnerHERA symposium June 30, 20079 The MIT Bag Model Quarks are treated as massless particles inside a bag of finite dimension The quarks are infinitely massive outside the bag Confinement results from the balance of pressure on the bag walls from the outside and the pressure resulting from the kinetic energy of the quarks inside The bag pressure constant, B, is related to the equilibrium radius of the bag R as B ~ R -4 Inside the bag perturbative QCD applies The total color charge of the matter inside the bag must be colorless => valid hadronic bags only contain qqq and q A.Chodos et al., Phys. Rev. D 9, 3471 (1974) Models of the Nucleon Bhaduri

10. Richard MilnerHERA symposium June 30, 200710 Constituent quark model of proton All pairs of colored quarks are antisymmetric under interchange Pairs must be symmetric under interchange of other quantum numbers Suppose symmetric in momentum space => identical favors couple symmetrically, i.e. to S=1 Coupling S=1 with the S=½ of the remaining valence quark to form J=½ yields

11. Richard MilnerHERA symposium June 30, 200711 Proton spin in constituent quark model

12. Richard MilnerHERA symposium June 30, 200712 Quark fraction of proton spin Experimentally g A /g V ~ 1.25

13. Richard MilnerHERA symposium June 30, 200713 Asymmetries

14. Richard MilnerHERA symposium June 30, 200714 Nucleon magnetic moments

15. Richard MilnerHERA symposium June 30, 200715 Integrals (2·Z in Bj’s paper) isovector octet singlet Isovector, SU(3) flavor octet and singlet axial charges

16. Richard MilnerHERA symposium June 30, 200716 Sum Rules Bjorken Sum Rule (1966) Ellis Jaffe Sum Rules (1974) when α s -> 0, and Δs = 0.

17. Richard MilnerHERA symposium June 30, 200717 Ellis-Jaffe Sum Rules Ellis-Jaffe (1974) predicts I p = 0.175 ± 0.018

18. Richard MilnerHERA symposium June 30, 200718 SLAC E80, E130 Data taken in late 1970’s 22.7 GeV polarized electron beam p B ~ 80%, I ~ 10 nA Polarized beam generated from 6 Li atomic beam 8 GeV spectrometer and custom larger acceptance spectrometer used DNP butanol polarized proton target p T ~ 60% Asymmetry measured in region 0.1 < x < 0.7, 1 < Q 2 < 10 Asymmetries independent of Q 2 Proton integral = 0.165 ± 0.05 Consistent with Ellis-Jaffe Sum Rule Assuming the BSR, this implied that the neutron integral should be small

19. Richard MilnerHERA symposium June 30, 200719 E130 SLAC Experiment

20. Richard MilnerHERA symposium June 30, 200720 E80 and E130 Data 5/9 x-dependence of A 1 in the valence region confirms the quark model predictions

21. Richard MilnerHERA symposium June 30, 200721 Expected neutron asymmetry after E130 Hughes and Kuti Ann. Rev. Nucl. and Part. Sc. 33, 611 (1983) 0

22. Richard MilnerHERA symposium June 30, 200722 EMC Experiment Data taken in mid 1980’s 100-200 GeV muon beam, flux ~ 2 x 10 7 s -1 Muon beam naturally polarized p B ~ 80% DNP 1 meter long ammonia polarized target p T ~ 80% Asymmetries measured in the range 0.01 < x < 0.7 and 3 < Q 2 < 30 (GeV/c) 2 I first became aware of the results of the EMC experiment from Terry Sloan at the SLAC Workshop on Electronuclear Physics with Internal Targets on January 5-8, 1987

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24. Richard MilnerHERA symposium June 30, 200724 EMC CERN Experiment

25. Richard MilnerHERA symposium June 30, 200725 EMC Polarized Target

26. Richard MilnerHERA symposium June 30, 200726 Dilution Factor

27. Richard MilnerHERA symposium June 30, 200727 EMC Data

28. Richard MilnerHERA symposium June 30, 200728 Violation of Ellis-Jaffe Sum Rule! Surprise results from effects of sea quarks/gluons at low x Sizable neutron integral Sizable, negative polarization of strange quarks!

29. Richard MilnerHERA symposium June 30, 200729 Polarized lepton beams In ~ 1987 available beams included - low intensity, highly polarized muon beams - high intensity, low polarization electron beams Interest in studying the spin structure of the nucleon drove the polarized electron beam technology in a major way At SLAC, ~ 80% polarized electron beams were developed At DESY, HERMES drove the development of ~65% polarized electron/positron beams using the Sokolov- Ternov effect Precision beam polarimetry was developed

30. Richard MilnerHERA symposium June 30, 200730 SLAC Polarized Electron Source

31. Richard MilnerHERA symposium June 30, 200731 See talk by Erhard Steffens

32. Richard MilnerHERA symposium June 30, 200732 Spinoff I: Strangeness in the Proton From EMC data and assuming SU(3) flavor symmetry, it was concluded that the polarization of the strange quarks was negative Δs ~ -0.10±0.04 When taken together with other data from pion and neutrino scattering, this led to a major effort to observe sizable effects of strange quarks For example, a worldwide program of parity violating electron scattering from the nucleon was carried out at Bates, Mainz, and Jefferson Lab No clear evidence for the effects of strange quarks has been observed

33. Richard MilnerHERA symposium June 30, 200733 G. Cates

34. Richard MilnerHERA symposium June 30, 200734 Spinoff II: GDH Sum Rule Measurements Causality, unitarity, Lorentz and gauge invariance => low Q 2 sum rule This has been a major focus of effort at ELSA, Mainz, JLab with electron beams and at GRAAL, Spring-8 and LEGS/BNL with photon beams

35. Richard MilnerHERA symposium June 30, 200735 My personal path to HERMES

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37. Richard MilnerHERA symposium June 30, 200737 OF PEGASYS

38. Richard MilnerHERA symposium June 30, 200738 PEGASYS Experimental Layout

39. Richard MilnerHERA symposium June 30, 200739 OF PEGASYS

40. Richard MilnerHERA symposium June 30, 200740 PEGASYS projection for A n 1

41. Richard MilnerHERA symposium June 30, 200741 - Terry Sloan gave a talk on the semi-inclusive unpolarized measurements with the EMC experiment - Privately, he told us of the unexpected results on the proton spin asymmetries - Terry encouraged the idea of an internal target experiment to measure A 1 n using polarized 3 He - Bob Jaffe went off to the SLAC library to write a paper

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44. Richard MilnerHERA symposium June 30, 200744 Meeting at DESY, September 30 th 1987

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48. Richard MilnerHERA symposium June 30, 200748 HERMES became the official name of the collaboration about June 1989 Costas Papanicolas, then at the University of Illinois, came up with the name

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50. Richard MilnerHERA symposium June 30, 200750 Semi-inclusive deep inelastic scattering F. Close and R.M., Phys. Rev. D44, 3691 (1991)

51. Richard MilnerHERA symposium June 30, 200751 Rates for SIDIS

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53. Richard MilnerHERA symposium June 30, 200753 August 1992

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57. Richard MilnerHERA symposium June 30, 200757 HERMES Physics Goals Measure the valence quark spin distribution in the nucleon Test the Bjorken Sum Rule Provide a stringent test for models of the nucleon Measure semi-inclusive deep inelastic scattering to probe the polarization of the sea Transverse target polarization was to constrain the magnitude of g 2 !

58. Richard MilnerHERA symposium June 30, 200758 HERMES Was a technically innovative experiment Owes its existence to the DESY laboratory being open to new ideas Has had a physics scope and impact far beyond that which motivated its beginnings Has been (together with ZEUS and H1) the major source of new insight into the fundamental structure of matter over the last decade Has educated and trained new generations of hadron physicists Was conceived, realized and carried out in a truly international spirit of cooperation by physicists largely from institutions outside of DESY