Lessons from HERA A. Caldwell, Max-Planck-Institut f. Physik

1. Lessons from HERAA. Caldwell, Max-Planck-Institut f. Physik Overview of main results A detector for small/large-x A. Caldwell, MPI f. Physik

2. ZEUS detector • Main features: • Solenoid • Focus on central rapidities • Excellent hadronic resolution • general purpose detector A. Caldwell, MPI f. Physik

3. p e±L,R H1 detector: better EM calorimetry, hadronic calorimeter less precise. A. Caldwell, MPI f. Physik

4. HERA Kinematics Ee=27.5 GeV EP=920 GeV r * s=(k+P)2 = (320 GeV)2 CM energy squared Q2=-(k-k`)2 virtualiy W2=(q+P)2*P CM energy squared Transverse distance scale probed: r  hc/Q McAllister, Hofstadter Ee=188 MeV rmin=0.4 fm Bloom et al. 10 GeV 0.05 fm CERN, FNAL fixed target 500 GeV 0.007 fm HERA 50 TeV 0.0007 fm / A. Caldwell, MPI f. Physik

5. What about a longitudinal distance scale ? Proton  momentum frame Proton rest frame • = E/Q2 Lifetime of hadronic = W2/2MPQ2 fluctuations of photon = 1/2Mpx x=10-4  1000 fm, x>0.1  <1 fm x=Q2/2P q fraction of P momentum carried by struck quark Proton collection of ‘frozen’ partons on time scale of interaction. Variations of cross section with x, Q2 attributed to details of proton structure. Sensible at small-x ? small x corresponds to scattering of dipole with proton. At large Q2, dipole small, reduced radiation, color transparency. At small Q2, highly evolved dipole, hadron-hadron scattering. A. Caldwell, MPI f. Physik

6. Proton  momentum frame Proton rest frame • d2/dW dQ2 =  (T +  L) • is flux of photons T,L are cross sections for transversely, longitudinally polarized photons to scatter from proton  is the relative flux Rutherford d2/dxdQ2=22/xQ4[(1+(1-y)2)F2 - y2FL] F2 = f e2f x {q(x,Q2) + q(x,Q2) } ef is quark charge q(x,Q2) is quark density FL = 0 in LO (QPM), non-zero after gluon radiation. Key test of our understanding F2 = Q2/42 (T +  L) In dipole model, photon qq wfn extracted from T,L A. Caldwell, MPI f. Physik

7. Hadron-hadron scattering cross section versus CM energy *P scattering cross section versus CM energy (Q20). Same energy dependence observed s0.08 vs W2 0.08 Higher energy – more time for evolution of wfns - short lived fluctuations become visible. A. Caldwell, MPI f. Physik

8. Cross sections as a function of Q2 The rise of F2 with decreasing x observed at HERA is strongly dependent on Q2 Equivalently, strongly rising *P cross section with W at high Q2 A. Caldwell, MPI f. Physik

9. The behavior of the rise with Q2 Below Q20.5 GeV2, see same energy dependence as observed in hadron-hadron interactions. Observe transition from partons to hadrons (constituent quarks) in data. Distance scale  0.3 fm ?? What physics causes this transition ? Hadron-hadron scattering energy dependence (Donnachie-Landshoff) A. Caldwell, MPI f. Physik

10. In general, NLO pQCD (DGLAP) fits do a good job of reproducing the data over the full measurement range. A. Caldwell, MPI f. Physik

11. Analysis of F2 in terms of parton densities (quarks and gluons) • NLO DGLAP fits can follow the data accurately, yield parton densities. BUT: • many free parameters (18-30) • form of parametrization fixed (not given by theory) • Constraints, e.g., dsea=usea put in by hand. Is this correct ? Need more constraints to untangle parton densities. A. Caldwell, MPI f. Physik

12. See breakdown of NLO DGLAP approach ... Gluon density known with good precision at larger Q2. For Q2=1, gluons go negative. NLO, so not impossible, BUT – cross sections such as L also negative ! A. Caldwell, MPI f. Physik

13. FL shows tremendous variations when attempt to calculate at different orders. But FL is an observable – unique result. FL is hard to measure but a very sensitive test. Should be measured. A. Caldwell, MPI f. Physik

14. There are also significant uncertainties at large x Fractional error A. Caldwell, MPI f. Physik

15. Statistics at large-x from HERA II From 1 fb-1 with Ep=920 GeV. 15% measurement at x>0.7 . Will be a first measurement of this quantity. A. Caldwell, MPI f. Physik

16. Detector acceptance, performance:Key issue for detector builders Main detector acceptance Shifted vertex run The Q2=1 GeV2 region not covered continuously. Very interesting region  focus of new experiment. Large-x also has limited coverage in DIS regime. Beamline Cal A. Caldwell, MPI f. Physik

17. Detector Acceptance and Kinematics e P Small-x: scattered particles in electron direction. Scattered electron energy+angle determines kinematics e P Large-x: scattered jet in proton direction. Electron angle depends on Q2. Jet energy gives x. Jet acceptance critical. A. Caldwell, MPI f. Physik

18. Diffractive Surprises ‘Standard DIS event’ Detector activity in proton direction Diffractive event No activity in proton direction A. Caldwell, MPI f. Physik

19. Diffraction • There is a large diffractive cross section, even in DIS (ca. 20 %) • The diffractive and total cross sections have similar energy dependences. Data suggests simple physics – what is it ? • Key detector issues: • Need to guarantee proton intact. • Cover full W range • Good MX resolution • Experience: measuring scattered proton gives cleanest measurements, but acceptance limited in PT,xL. Tag/reject proton breakup gives full acceptance, but need to work on PT resolution. A. Caldwell, MPI f. Physik

20. Exclusive Processes (VM and DVCS) VM  • Clean process, but • was it really elastic ? • limited W coverage – depends on rapidity coverage of tracking system. A. Caldwell, MPI f. Physik

21. epeVp (V=,,,J/) epep (as QCD process) Energy dependence of exclusive processes Rise similar again to that seen in total cross section. Summary of different Vector mesons Need bigger lever arm in W to see energy dependence more precisely. Need to distinguish elastic from proton dissociation events for small impact parameter scans of proton. A. Caldwell, MPI f. Physik

22. Select energetic forward pions, with kTp ~ Q2, in low x DIS events. Simple DGLAP description fails. BFKL reasonable agreement. CCFM fails. Jet results similar. Parton radiation at low x not well understood! p0 Forward jet and particle production A. Caldwell, MPI f. Physik

23. In the end, we need to understand QCD radiation and to solve QCD at large distance scales – non-perturbative physics ! track CAL A. Caldwell, MPI f. Physik

24. Open Questions – Next Steps • Measure the behavior of inclusive, diffractive and exclusive reactions in the region near Q2=1 GeV2 to understand parton to hadron transition. Best is full acceptance detector for scattered electrons at small angles. • Measure FL over widest possible kinematical range, as this is a crucial observable for testing our understanding of radiation processes in QCD. Need large y  small electron energies. Start with maximum possible electron beam energy, good e/ separation, electron resolution. • Measure exclusive processes (VM production, DVCS) over wide W range to precisely pin down energy dependence of cross section. Need t-dependence of cross sections to get 3-D map of proton. Need to verify scattering elastic. Maximum rapidity coverage of tracking, good PT resolution. A. Caldwell, MPI f. Physik

25. Open Questions – Next Steps • Measure forward jet cross sections over widest possible rapidity range, to study radiation processes over the full rapidity range from the proton to the scattered quark. Need calorimetry and tracking at high rapidity. • Measure structure functions at high-x. Need large luminosities, calorimetry at high rapidity. • And, do it all with nuclei ! A. Caldwell, MPI f. Physik

26. HERA II 2003-2007 Spin Rotators successfully implemented New physics possibilities due to big increase in data sets, polarization of the beam A. Caldwell, MPI f. Physik

27. helicity suppression EW Physics Measure CC cross sections as a function of the lepton charge and polarization. Sensitive to right-handed weak currents (up to WR=400 GeV). Classic test of EW interaction. A. Caldwell, MPI f. Physik

28. Deeply Inelastic eP scattering: Charged Current e- W- , W- scatters on u,c,d, s e+ W+ , W+ scatters on d,s,u,c Neutral Current Measure distribution of quarks and gluons in the proton (structure functions) as a function of a transverse resolution scale (Q – 4 momentum transferred) and a longitudinal momentum scale (x – fraction of proton momentum). A. Caldwell, MPI f. Physik

29. Large-x Charged Current interactions allow to constrain separately u and d valence quark density at high x and Q2 _ _ --- s (e-p) ~ x (u+c) + (1-y2) x (d+s) _ _ --- s (e+p) ~ x (u+c) + (1-y2) x(d+s) Put emphasis on eR+ to get max sensitivity to d(x),s(x) [u(x) primarily from F2NC]. Require >500 pb-1 for large-x measurements. A. Caldwell, MPI f. Physik

30. HERA-III Initiative • Two letters of intent were submitted to the DESY PRC (May 7,8 2002) • H1 Collaboration • Focus initially on eD: isospin symmetry of sea at small-x • uv/dv at large-x • diffraction on n, D • Discuss also interest in: eA, F2,FL at small Q2, forward jets, • spin structure • New Collaboration • Optimized detector for precision structure function measurements, forward jets and particle production over a • large rapidity interval (eP, eD, eA) A. Caldwell, MPI f. Physik

31. Possible upgrades of H1 detector Upgrade in electron direction for low Q2 F2 and FL measurements + very forward proton, neutron detectors Upgrade in proton direction for forward particle, jets measurements A. Caldwell, MPI f. Physik

32. Precision eD measurements • Universality of PDF‘s at small-x, isospin symmetry Can measure F2p-F2n w/o nuclear corrections if can tag scattered nucleon A. Caldwell, MPI f. Physik

33. Simulation with H1 based on 50 pb-1 • Flavor dependence at high-x Behavior of F2p/F2n as x  1 SU(6) symmetry F2p/F2n = 2/3 Dominant scalar diquark F2p/F2n = 1/4 Can measure this ratio w/o need to correct for nuclear effects if can tag scattered nucleon. A. Caldwell, MPI f. Physik

34. A new detector to study strong interaction physics p Si tracking stations EM Calorimeter Hadronic Calorimeter Compact – fits in dipole magnet with inner radius of 80 cm. Long - |z|5 m e A. Caldwell, MPI f. Physik

35. The focus of the detector is on providing complete acceptance in the low Q2 region where we want to probe the transition between partons and more complicated objects. Tracking acceptance Q2=100 Q2=10 Q2=1 Q2=0.1 W=0 W=315 GeV A. Caldwell, MPI f. Physik

36. Tracking acceptance in proton direction Huge increase in tracking acceptance compared to H1 And ZEUS. Very important for forward jet, particle production, particle correlation studies. ZEUS,H1 This region covered by calorimetry Accepted  4 Si stations crossed. A. Caldwell, MPI f. Physik

37. FL Measurement Range d2/dxdQ2=22/xQ4[ (1+(1-y)2)F2(x,Q2) - y2FL(x,Q2)] Fix x, Q2. Use different beam energies to vary y. Critical issue: e/ separation FL can be measured precisely in the region of maximum interest. This will be a strong test of our understanding of QCD radiation. A. Caldwell, MPI f. Physik

38. High x, large W – unmeasured region Very forward calorimeter allows measurement of high energy, forward jets, and access to high-x events at moderate Q2 Cross sections calculated from ALLM A. Caldwell, MPI f. Physik

39. Forward jet cross sections: see almost full cross section ! New region Range covered by H1, ZEUS A. Caldwell, MPI f. Physik

40. HERA-1 Very large gain also for vector meson, DVCS studies. Can measure cross sections at small, large W, get much more precise determination of the energy dependence. 0 5 10 15 20 HERA-3 W=0 50 100 150 200 250 300 GeV Can also get rid of proton dissociation background by good choice of tagger: FHD- hadron CAL around proton pipe at z=20m FNC-neutron CAL at z=100m A. Caldwell, MPI f. Physik

41. Quote from DESY summary to HGF 5 Year Planning A. Caldwell, MPI f. Physik

42. Physics/Detector studies for eRHIC 2x14 Si tracking stations A. Caldwell, MPI f. Physik

43. Preliminary summary of eRHIC detector/physics studies: • EIC/HERA III would allow precision measurements in the transition region observed at HERA at small-x (<10-2) • FL measurements would be possible in an interesting kinematical range. • The structure functions could be measured at large x over a wide range of Q2. New territory. • Particle production/correlations over a wide rapidity range could be explored. • eA collisions would provide a different angle on studying systems with a large gluon density. • Performance for exclusive states should be greatly improved. • Higher energies  smaller x (important for CGC type issues). A. Caldwell, MPI f. Physik