1. Lepton Hadron Scattering, An Overview Abhay Deshpande RIKEN BNL Research Center Snowmass 2001 Working Group M5 July 3, 2001

2. Lepton Hadron Colliders, Snowmass01 July 3, 20012 Experiments and The Standard Model Standard Model (SM) of High Energy Physics: 1. Electroweak Physics: Electro-Magnetic-Weak Interactoins mediated by gauge bosons:  W(+/-) and Z. Interactions between charged particles: quarks & leptons (3 families) 2. QCD Physics: Strong Interactions mediated by gauge boson: “g”, the gluon. Carry color charge! Distinct from EW carriers of forces in the fact that they can interact with other gluons “g-g” interaction 3. Gravitational Physics  no direct connection found yet Experimental Tests of the SM 1. Lepton-Lepton Scattering 2. Lepton-Hadron Scattering: Probing the structure of matter and strong interactions:  Topic of this working group 3. Hadron-Hadron Scattering All three methods are complementary. Investigate the universality of physical laws leading towards a better understanding of structure of matter and nature of above interactions.

3. Lepton Hadron Colliders, Snowmass01 July 3, 20013 Lepton Hadron Scattering Deep Inelastic Scattering A point like probe as in a lepton probes the inside of the hadron at high enough energies… exchange of  or Z for neutral currents and W’s (not shown) for charged currents

4. Lepton Hadron Colliders, Snowmass01 July 3, 20014 Lepton Hadron Scattering Deep Inelastic Scattering! Fixed target experiments: e-p, e-A scattering  Unpolarized & Polarized DIS (SLAC/DESY)  -p,  -A scattering  Unpolarized and polarized DIS (CERN/FNAL) -p, -A scattering  Semi-un-polarized (FNAL) Collider Experiments e-p scattering  Semi-un-polarized ONLY (DESY) Unpolarized ScatteringSemi-un-polarized ScatteringPolarized Scattering

5. Lepton Hadron Colliders, Snowmass01 July 3, 20015 HERA at DESY Proton Electron

6. Lepton Hadron Colliders, Snowmass01 July 3, 20016 H1 Detector SchematicZEUS Detector Schematic HERA Collider Detectors Very close to 4  acceptance Good calorimetry, tracking & vertex resolution

7. Lepton Hadron Colliders, Snowmass01 July 3, 20017 HERA Kinematic Range with Collider Detectors Significant increase in kinematic range beyond the fixed target experimental era. Many new aspects of QCD were discovered and understood because of the extended range Unresolved problems…..

8. Lepton Hadron Colliders, Snowmass01 July 3, 20018 Experimental Excitement from HERA Future Experimental Measurements! Highlights from HERA I Proton structure & QCD  Unexpected rise of F2, Role of gluon Jet physics, Precise  S determination Electroweak physics Low x phyisics  High parton density matter, Diffractive physics Photon structure  Jet physics, Prompt photons Physics beyond SM  Lepto-quarks, Contact interactions, excited leptons etc…. HERA II Luminosity Upgrade Spin rotators for electron beams for ZEUS and H1  Polarization of electron beams H1 and ZEUS Detector upgrades Apology: Will not cover all that HERA has done!

9. Lepton Hadron Colliders, Snowmass01 July 3, 20019 Proton Structure function F 2 (x,Q2) Scaling violation explicitly seen…  Beyond the fixed target regime H1 and ZEUS data in agreement. Further, pQCD predictions at NLO describe data impressively over many decades in x and Q2. Studies have resulted in the determination of gluon distribution, precise determination of  S Rise in F 2 at low x

10. Lepton Hadron Colliders, Snowmass01 July 3, 200110 Gluon Determination from F 2 QCD Analysis Perturbative QCD analysis of F2 allows us to access the gluon Distribution in the proton. Recent results from DIS2001, Bologna

11. Lepton Hadron Colliders, Snowmass01 July 3, 200111 F L Longitudinal Structure Function Presnt extractions of FL based on extrapolations of pQCD in to a region of high y Ideally, to get FL free of assumptions, one needs data at different CM energies It has been suggested to run HERA at lower proton beam energy… 400 GeV E. Elsen, H1, DIS2001 RUNNING ACCELERATORS AT LOWER ENERGY CONSEQUENCES TO THE LUMINOSITY? HOW CAN THE EFFETCS BE MINIMIZED?

12. Lepton Hadron Colliders, Snowmass01 July 3, 200112 Transition region: Low Q2 Photoproduction  pQCD U. Fricke, ZEUS, DIS2001

13. Lepton Hadron Colliders, Snowmass01 July 3, 200113 Observations…. Is it really a hint of something new happening at low x? If so we need to go past the HERA kinematics in low x and at the same time keep a “respectable” Q2 so that some of these perturbative calculations/fitting procedures are still valid  THERA/eVLHC Other ways: There are models that predict that at low x the partonic densities might be very large, and as such the DGLAP formalism breaks down…. Predominantly gluonic matter at very high density. Remember gluons are bosons, and at high enough densities they might form “Bose-Einstein Condensates” which form the high density matter.  Some have called this: Color Glass Condensate: A yet undiscovered state of matter…. When this (Glass) shatters…Quark Gluon Plasma is formed… Bottom line is: to create the high gluon density objects we might as well work with HEAVY IONS for DIS. This would be effectively going to the high parton densities I.e. LOW x!  Electon Ion Collider (BNL)

14. Lepton Hadron Colliders, Snowmass01 July 3, 200114 Diffractive Events at HERA Early observation at HERA The diffractive cross sections is unexpectedly large at HERA: ~typically 5-6%  Explanations based on pomeron exchange exist.. But..

15. Lepton Hadron Colliders, Snowmass01 July 3, 200115 Diffraction at HERA… Larger than expected  Gluonic in origin Consequences to interpretation at low x, low-intermediate Q2 Saturation models (Color Glass Condensate) do indeed account for the higher than expected diffractive events  and predict that if e-A collisions are performed, due to even higher density of gluons in the nuclei, the expected diffractive cross sections might be as large as 40% A very clear and robust diffractive signal could be seen. Since these SAME high parton density models also address the low x structure function behavior in details… they provide a compelling alternative to the pomeron ideas…. How are they connected? THERA/eVLHC and e-A physics at high enough energies such as EIC would allow us to directly look at this.

16. Lepton Hadron Colliders, Snowmass01 July 3, 200116  S  Determination from jet measurements… ZEUS H1

17. Lepton Hadron Colliders, Snowmass01 July 3, 200117  S  Determination from inclusive jet measurements…

18. Lepton Hadron Colliders, Snowmass01 July 3, 200118 Universality! Determination of the strong coupling constant from different methods. Lead to astonishingly consistent values We must be doing this right.. Beauty and complimentarity of measurement techniques! S. Betke The smallest and the third smallest Uncertainty comes from DIS!

19. Lepton Hadron Colliders, Snowmass01 July 3, 200119 Electroweak Physics at HERA At large Q2 the electromagnetic and weak currents assume equal strength. At high Q2 situation limited by statistics Over 8 orders of magnitude these measurements have been compared with the predictions from SM H1 and ZEUS data in excellent agreement Further, data are in excellent agreement with the Standard Model. These and such tests will improve further in HERA II. AND HERA will access Physics beyond the SM… if it exists…

20. Lepton Hadron Colliders, Snowmass01 July 3, 200120 xF 3 Measurement at HERA First measurement of xF3 at HERA Agrees well with the PDFs evolved from much lower Q2 Statistics limited HERA II will consolidate this measurement The polarized partner of xF3 is Structure function g 5. Can be measured If we have polarized ep colliders… Capable of having e+/e- beams! Can we assure that future e-p facilities would Always have e+ and e- beams?

21. Lepton Hadron Colliders, Snowmass01 July 3, 200121 Rutherford would have been proud of this measurement….. W. Marciano’s talk: Critical dimensions below which physics beyond the SM might appear are 10 e –17 cm

22. Lepton Hadron Colliders, Snowmass01 July 3, 200122 Excess of high pT lepton events vs. SM SM H1 Detector

23. Lepton Hadron Colliders, Snowmass01 July 3, 200123 SM Processes Considered as Possible sources Of events

24. Lepton Hadron Colliders, Snowmass01 July 3, 200124 High pT leptons

25. Lepton Hadron Colliders, Snowmass01 July 3, 200125 Other searches of Beyond SM Contact Interaction Various SUSY searches R-parity violating lepto-quarks Compositeness Searches of extra-dimensions …..  All of these would be possible with the large luminosity at HERA II and with future machines like THERA/eVLHC

26. Lepton Hadron Colliders, Snowmass01 July 3, 200126 Sensitivity to  s Color Dynamics Sensitivity to Photon Structure function Sensitivity to Proton structure function Why high ET jets? Soft effects minimized Fragmentation/Hadronization effects small Hard Scale  reliable NLO A Great Test of QCD! Photon Structure from High E T Photoprodction

27. Lepton Hadron Colliders, Snowmass01 July 3, 200127 Photon Structure… (Cont) NLO QCD predictions using available PDFs describe the measured cross sections. ZEUS sees deviations Further interest and studies needed and expected…. HERAII Ideal measurement for future colliders (polarized and un-polarized!)

28. Lepton Hadron Colliders, Snowmass01 July 3, 200128 Summary Slide… for future unpolarized DIS Great anticipation based on present data towards HERA II This however will be only a luminosity upgrade Interesting physics may lie beyond the kinematic reach of HERA Higher energy machines need to be designed…  Plans for THERA, DIS with VLHC? High density partonic matter could be found in high energy e-A scattering… Diffractive signals from eA physics at high energies might tell us something about the very low x region  Plan for EIC at RHIC?  Discovery of a new state of matter… Color Glass Condensate…?  Detailed studies of diffraction, F L, in general gluons in heavy ions Upcoming talks will tell us what these future machines could do….

29. Lepton Hadron Colliders, Snowmass01 July 3, 200129 Polarized DIS Electron beams up to 50 GeV/c on fixed (solid and gaseous) targets were predominantly used at SLAC (EXXX serise) and DESY (HERMES) Muon beams 100-200 GeV/c on fixed (solid) targets were used at CERN(EMC,SMC) Compared to un-polarized DIS, the kinematic range is extremely limited!

30. Lepton Hadron Colliders, Snowmass01 July 3, 200130 How far does polarized DIS have to go!

31. Lepton Hadron Colliders, Snowmass01 July 3, 200131 Cross Section Polarized Structure Functions: Transversity Distributions: Polarized and Unpolarized DIS

32. Lepton Hadron Colliders, Snowmass01 July 3, 200132 Physics Goals of Past Experiments

33. Lepton Hadron Colliders, Snowmass01 July 3, 200133 Polarized DIS … Towards a Collider. These goals needed only inclusive DIS measurements  Have lead to results which can only be improved by making new measurements at low x…. Higher sqrt(s) … higher energy beams and in collider mode: Polarized Collider necessary As experiments got more ambitious they started looking at the target fragmentation… and tried to identify the final states using sophisticated detectors: tracking, particle ID, vertexing etc. on hadronic final states.  Lea to the results on semi-inclusive DIS mostly relevant in intermediate to high x Next step could be to extend these to low x and a detailed study of target fragmentation region which would benefit a lot in Colllider mode DIS.

34. Lepton Hadron Colliders, Snowmass01 July 3, 200134 pQCD analysis results using world data set QCD Analysis of available Data Sets: (SMC Phys, Rev. D58:112002, 1998)

35. Lepton Hadron Colliders, Snowmass01 July 3, 200135 Polarized Parton Distributions Polarized Singlet quark distributions and the nonsinglet quark distributions determined rather accurately Polarized gluon distribution has largest uncertainty Principle uncertainties come from: a) assumed initial parton distribution functional form b) uncertainty in  S c) factorization and renormalization scale etc… All related to “lack of data in the low x” region.

36. Lepton Hadron Colliders, Snowmass01 July 3, 200136 Lack of low x data… consequences Q2 = 10 GeV2 Regge/QCD SMC Results

37. Lepton Hadron Colliders, Snowmass01 July 3, 200137 Neutron structure function… E154/SLAC Consequence: Unertainties in low x for Bjorken sum rule… Acceptable? No… measure low x!

38. Lepton Hadron Colliders, Snowmass01 July 3, 200138 Ellis Jaffe Sum Rule Violated!

39. Lepton Hadron Colliders, Snowmass01 July 3, 200139 Ellis Jaffe Sum Rule…. Violated!

40. Lepton Hadron Colliders, Snowmass01 July 3, 200140 Bjorken Sum Rule Confirmed!

41. Lepton Hadron Colliders, Snowmass01 July 3, 200141 How to access gluons…? We know the quarks carry small amount of spin… and that gluons may have a significant role in this…. We saw what we need to determine gluon distributions… what other ways can we access gluon distributions? ….. Next Generation Experiments

42. Lepton Hadron Colliders, Snowmass01 July 3, 200142  G from Photon-Gluon-Fusion (PGF) Photon Gluon Fusion in DIS:  Di-Jet events  2-High-pT hadron events HERMES Collaboration At high Sqrt(s) the theoretical interpretation is without ambiguities or uncertainties! At low Sqrt(s) however things may not be as simple…..

43. Lepton Hadron Colliders, Snowmass01 July 3, 200143 High pT Hadron:PGF HERMES Results No estimate of theoretical Uncertainty

44. Lepton Hadron Colliders, Snowmass01 July 3, 200144  G from high-pT hadron…. PYTHIA used at very low scales… caution! VMD component’s reliability questionable, and no reliable uncertainty estimate possible. High pT, possible in future lepton-hadron colliders will overcome this problems. H1 has already published such a result on G using high-pt hadrons.

45. Lepton Hadron Colliders, Snowmass01 July 3, 200145 Scale dependencies in this business… M. Stratmann & W. Vogelsang Proceedings of SPIN2000, Osaka, Japan. At low energies (such as HERMES/COMPASS) result strongly dependent on the value of the scale At higher energies the scale dependence of the result significantly reduced… look at curves for eRHIC, HERA, RHIC, Tevatron… Need a polarized high energy collider!

46. Lepton Hadron Colliders, Snowmass01 July 3, 200146 Semi-Inclusive Physics: HERMES/SMC SMC: Tags only charge of h HERMES: Has particle ID Flavor separated valence quark distributions in nucleon accessed

47. Lepton Hadron Colliders, Snowmass01 July 3, 200147 Semi-Inclusive & Inclusive Asymmetries Newly installed RICH would do excellent particle ID  expect to see Asymmetries for K/  instead of charged hadrons soon.

48. Lepton Hadron Colliders, Snowmass01 July 3, 200148 Transversity Probability of finding a quark oriented transverse to the proton momentum One of the last unmeasured spin structure function of the proton Unmeasured: because it’s a chiral odd structure function and does not couple directly to observables in experiments Suggested by Jaffe et al.: Why not measure a PRODUCT Transversity and something else the product of the two is a chiral even structure function  Then if you can unfold that other function, to get at transversity! Approach will be tried by HERMES, COMPASS and RHIC Spin The “other function” which needs to be measured is the Collin’s function. Hermes collaboration recently measured this for the first time for pions.

49. Lepton Hadron Colliders, Snowmass01 July 3, 200149 Recent excitement in polarized DIS: Single spin azimuthal asymmetries…HERMES First attempt at measuring Collin’s function Measured for positive and negative pions Found that positive pion asymmetries are positive, while the negative pions are consistent with zero. This now in future can be used for getting to the transversity measurements.

50. Lepton Hadron Colliders, Snowmass01 July 3, 200150 Crossing the x-Q2 Barrier in DIS: Low-x surprises! Elastic e-p scattering (SLAC, 1950s) Q2 ~ 1 GeV2 Finite Size of proton Inelastic e-p scattering (SLAC, 1960s) Q2 > 1 GeV2 Parton structure of the proton Inelastic  -p scattering off p/d at CERN (1980s) Q2 > 1 GeV2 Unpolarized EMC effect Inelastic e-p scattering at HERA/DESY (1990s) Q2 > 1 GeV2 Unexpected rise of F2, Study of pQCD and QCD through various physics processes, diffraction… What NEXT?  A higher energy collider? Or Denser Target “A”?

51. Lepton Hadron Colliders, Snowmass01 July 3, 200151 Stern & Gehrlach (1921): Space quantization associated with direction Goudschmidt & Ulhenbeck (1926): Atomic fine structure and electron spin magnetic moment Stern (1933): Proton anomalous magnetic moment Kusch (1947): Electron anomalous magnetic moment Prescott & Yale-SLAC Collaboration (1978): EW interference in polarized e-d DIS, parity non-conservation EMC (1989): Proton spin crisis/puzzle Low x! HERMES (1999): ???? Transverse spin asymmetries ???? Physics with “Spin” full of Surprises! Future spin experiments may be best done In collider mode at high energies

52. Lepton Hadron Colliders, Snowmass01 July 3, 200152 How can we build upon all this? The accelerator physics will pave the way… -- High energy, high luminosity lepton hadron colliders, high polarizations of beam  Access new kinematic regions for polarized and unpolarized DIS -- Variable beam energies… without much effect on luminosity… -- Ability in Linac as well as rings to have high -polarized e+/e- beams… -- Highly dense interaction regions We should know this months from all the specialists who have gathered here: -- How we can achieve this… and -- What journey we can embark upon within and outside of the Standard Model with such exotic tools… Challenges for Accelerator Physicists