1. Unpolarized Physics Program HERA-3 Workshop, MPI, 17-Dec-2002 A. Caldwell Physics Topics: eP, eD, eA Detector Requirements Accelerator Requirements Sources: Future Physics at HERA Workshop (95/96) EIC White Paper THERA contribution to TESLA TDR Durham Workshop Summary Discussions with many of you

2. Physics: eP Scattering HERA-1 opened a window on new and exciting physics. Let us now study it with optimized detectors. Rise of F 2 at small-x. Transition region of physics is also transition region for detectors – difficult systematics, limited acceptance Large diffractive cross section in DIS regime, with similar energy dependence to total cross section. Difficult separation from p-dissoc, small acceptance for proton spectrometer events. Exclusive processes (VM, DVCS) again have similar energy dependence (appropriate scale ?). Powerful tool for scanning proton, but need tagged p-elastic events at large t.

3. Sampling of Structure Function Data The rise of F 2 with decreasing x observed at HERA is strongly dependent on Q 2 Note: Only limited data available around Q 2 =1 GeV 2.

4. The behavior of the rise with Q 2 Below Q 2 =1 GeV 2, see same energy dependence as observed in hadron- hadron interactions Note: energy squared of γ * P system, W 2  Q 2 /x. Data have similar W range (acceptance issue).

5. Surprises 1.There is a large diffractive cross section, even in DIS 2.The diffractive and total cross sections have similar energy dependences Perturbative 2 gluon exchange, expect diffraction much steeper for Q 2  1 Diffraction as soft process, expect diffraction much shallower Diffraction is complicated process, ‘Miracle’ that has about same energy dependence as total cross section

6. Exclusive processes give more surprises Rise similar again to that seen in total cross section. Note that best measured is J/  at M 2 =10 GeV 2 Note scale ep  eVp (V= , , ,J/  ) ep  e  p (as QCD process) Summary of different Vector mesons

7. Summary of small-x Structure function rises steeply at small-x. The onset of this rise occurs near Q 2 =1 GeV 2. Similar behaviors seen in diffraction and exclusive reactions. Why this Q 2 ? Why the similarity in these different reactions ? Disappearing gluons ? Transverse distance probed  ħc/Q = 0.2 fm/Q (GeV) High Q 2, parton language works Q 2 =0, hadronic behavior of photon HERA probes both kinematic regimes, and the very interesting transition region in between. Note: we miss data just in this region because of detector acceptance problems. A primary goal of HERA-3 would be to make precision measurements in this transition region.

8. e q q b Physics Picture in Proton Rest Frame r ~ 0.2 fm/Q (0.02 – 2 fm for 100>Q 2 >0.01 GeV 2) Can also vary the space-time interval ct ~ 0.2 fm (W 2 /Q 2 ) (<1 fm to 1000‘s fm) – coherent scattering over had. And, in exclusive processes, can vary the impact parameter b ~ 0.2 fm/sqrt(t) t=(p-p‘) 2 Can control these parameters experimentally !

9. Precision eP measurements F 2 in the range 0.01 < Q 2 < 100 GeV 2. Test N k LO DGLAP fits and extraction of gluon densities. Can we see machinery breaking down ? Need for higher twist ? Gluon density known with good precision at larger Q 2. For Q 2 =1, gluons go negative. NLO, so not impossible, BUT – observables such as F L also negative ! Is language of partons breaking down in this Q 2 region ?

10. H1 data Note: deviations set in for Q 2 <10 GeV 2 F L in transition region, over maximum possible x range F L is very sensitive to gluon distribution, higher twist, saturation,...

11. Exclusive Processes (VM and DVCS)   VM VM= , , ,J/  Added scales – quark mass,t Elastic process with t-meas yields impact parameter scan of strongly interacting matter

12.   Advantage: known wfns Disadvantage: cross section Both: interference with QED process x 1, x 2 necessarily different: probe parton correlations in proton. Study as function of impact parameter. DVCS

13. High-x parton densities and  S Max x so far is 0.65, achieved at high Q 2. If can measure high x at lower Q 2, then cross sections large. PDF‘s not well constrained for x>0.7 strong sensitivity to  S novel effects (solitons...)

14. Precision eD measurements Universality of PDF‘s at small-x Is this behavior independent of the starting quark ? Over what range of x, Q 2 ? Can measure F 2 p -F 2 n w/o nuclear corrections is can tag scattered nucleon

15. Flavor dependence at high-x Behavior of F 2 p /F 2 n as x  1 SU(6) symmetry F 2 p /F 2 n = 2/3 Dominant scalar diquark F 2 p /F 2 n = 1/4 Can measure this ratio w/o need to correct for nuclear effects if can tag scattered nucleon.

16. Precision eA measurements Enhancement of possible nonlinear effects (saturation) b r At small x, the scattering is coherent over nucleus, so the diquark sees much larger # of partons: xg(x eff,Q 2 ) = A 1/3 xg(x,Q 2 ), at small-x, xg  x -, so x eff - = A 1/3 x - so x eff  xA -1/3 = xA -3 (Q 2 < 1 GeV 2 ) = xA -1 (Q 2  100 GeV 2 )

17. Interplay of saturation effects and shadowing Note: the physics appears to be quite complicated (many effects), but the data plotted span a wide range of Q 2. At a collider, can untangle many effects by measuring differentially over wide range of x,Q 2

18. Scan nucleus over impact parameter: need exclusive reactions, e.g. VM/DVCS. Diffractive cross section as function of impact parameter - do we reach the black disk limit at small impact parameters ? Many of the same measurements as for eP. What is the effective target area ? Depends on the matter distribution. E.g., very different results if partons clump around valence quarks as opposed to uniform distribution. Shadowing:

19. Parton densities in nuclei The measurement of the pdf‘s in nuclei is critical for understanding heavy ion collisions. Early RHIC data is well described by the Color Glass Condensate model, which assumes a condensation of the gluon density at a saturation scale Q S which is near (in ?) the perturbatively calculable regime. The same basic measurements (F 2, F L, dF 2 /d ln Q 2, exclusive processes) are needed to test the predictions of this model.

20. Partonic transport in nuclear matter At high x, hit a valence quark in the nucleus. Study the jet (leading particle) properties for different A

21. Detector Requirements Precision eP measurements (F 2,F L ) transition region 0.01 < Q 2 < 100 GeV 2, with maximum possible W coverage. Scattered electron energy down to 2 GeV or lower and  e down to 10 mrad or lower. Need calorimeter to identify electron (+ other techniques if aim for even lower electron energies) momentum resolution will determine how high in x we can go with the electron measurements alone. Goal ?

22. Precision eP measurements – VM production Need precision tracking also in proton and central directions. It is crucial to separate the elastic from proton dissociation events to access high t. 1.High acceptance proton spectrometer to verify elastic event and measure t 2.Proton breakup tagger but measure t with electron and charged VM decay tracks. 3.EM calorimeter to tag electrons in central, proton directions What W range is interesting (maximum !) What t range are we aiming for (t>1 GeV 2 )

23. Precision eP measurements - DVCS Need to separate photons and electrons over large angular acceptance. Need precision measurement of photon energy and angle from the calorimeter down to small energies. 1.What energy range of photons and electrons ? 2.What angular range for photons and electrons ? 3.What angular and energy resolutions needed ? As with VM production, need to guarantee elastic event.

24. Precision eP – high x measurements Electron method almost inconceivable below y=0.01. To measure high-x at moderate Q 2, need to measure the jet energy. Critical issue is jet acceptance in detector. Need forward hadronic calorimeter. 1.How high in x are we aiming for (x=0.9 ?) 2.What are feasible numbers in terms of calorimeter resolution, granularity and distance from IP ? 3.Additionally, can reduce E P. How much do we gain ?

25. Precision eD measurements – small x Need to tag the struck nucleon. Main detector as for precision eP. 1.Need to measure spectator neutron, so require zero degree calorimeter. What energy and angle resolution ? Want to measure P T with some resolution to distinguish from diffraction. Also, see that E n =E D /2. 2.Need to measure the spectator proton, so require detector for that. Tracking or calorimeter ? Where located ? What resolutions in position and energy ?

26. Precision eD measurements – high x Main detector requirements as for eP high x, and spectator tagging as for small x. Precision eA measurements – structure functions For inclusive measurements, same as precision eP. Precision eA measurements – elastic A events How do we know the event is elastic ? How to measure t ? 1.Spectrometer 2.Proton breakup veto, measure t in main detector

27. Accelerator Requirements 1.Luminosity: determined by exclusive processes, high-x for high eP, eD. For nuclei, 2 pb -1 /nucleon (95/96 Workshop) 2.Beam energies (how many, values of E e,E P for F L, F L D, high- x). 3.Beam divergence (how low in t do we want to go ? Requirements for deuteron tags ?) 4.Nuclear species (how high in A) 5.Alternating nuclear species according to bunch ? 6.How far away can machine elements be placed ? Where are windows ? Locations for possible detectors ? 7.How strong can dipole field be ? Synchrotron radiation ? 8.What about off-momentum electrons in dipole ?

28. Personal summary: Need precision tracking in electron direction to analyze electron Dipole magnet arrangement to access small scattering angles ? Need EM calorimeter over 4  for electron tagging and photon measurement in DVCS. Need moderate resolution tracking in central and proton directions for VM t measurements, diffractive mass measurement. Need forward hadron calorimeter to specifically access the high- x jets. Could also be used for BFKL jets studies (?) Need to instrument P (D,A) beamline with breakup vetoes (measure t in central detector). zero degree calorimeter to measure neutrons small calorimeter to measure protons from D breakup Workshop should aim to specify !