F 2 /F L with HERA III/eRHIC A. Caldwell, Max-Planck-Institut f. Physik

1. F2/FL with HERA III/eRHICA. Caldwell, Max-Planck-Institut f. Physik New Detector for HERA/eRHIC Physics - QCD in the high energy limit A. Caldwell, MPI fuer Physik

2. Kinematics Ee=10 - 27.5 GeV EP=250 - 920 GeV r * s=(k+P)2 = (100-320 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 EIC 5 TeV 0.002 fm HERA 50 TeV 0.0007 fm / A. Caldwell, MPI fuer Physik

3. Proton  momentum frame Proton rest frame Rutherford • 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 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) A. Caldwell, MPI fuer Physik

4. Track Cal Structure function measurements – measure distribution of quarks at the end of the radiation chain. Try to understand result with various QCD based evolution approaches. A. Caldwell, MPI fuer Physik

5. Physics Picture in Proton Rest Frame r * b r~ 0.2 fm/Q (0.02 – 2 fm for 100>Q2>0.01 GeV2) transverse size of probe ct ~ 0.2 fm (W2/2MPQ2) (<1 fm to 1000‘s fm) – scale over which photon fluctuations survive And, in exclusive processes, can vary the impact parameter b~ 0.2 fm/sqrt(t) t=(p-p‘)2 Can control these parameters experimentally ! A. Caldwell, MPI fuer Physik

6. 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 fuer Physik

7. 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 fuer Physik

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

9. 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 (deuterons, FL). A. Caldwell, MPI fuer Physik

10. 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 fuer Physik

11. 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 fuer Physik

12. Large-x • Motivation: • No data on parton densities for x>0.75 in DIS regime. • Phenomenology uses standard parametrizations for large x, q(x)  (1-x)b , but not well motivated. Could have surprises at large-x. At very high-x, novel QCD effects expected (ends in ‘on’). • Parton densities need to be well understood for searches for new physics. • Requirements: • Max possible luminosity • e+, e- for valence quark flavor separation • low EP running for added sensitivity to large-x A. Caldwell, MPI fuer Physik

13. Large-x Uncertainties in parton densities from ZEUS NLO fit: includes available fixed target data. Parametrization uncertainty not included. Fractional error A. Caldwell, MPI fuer Physik

14. Statistics at large-x (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 fuer Physik

15. Precision eA measurements • Enhancement of possible nonlinear effects (saturation) r b At small x, the scattering is coherent over nucleus, so the diquark sees much larger # of partons: xg(xeff,Q2)  A1/3 xg(x,Q2) This should enhance the chances to see saturated gluon state (CGC) in nuclei. Need quantitativepredictionsfor behavior of structure functions with x in heavy nuclei. A. Caldwell, MPI fuer Physik

16. 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 fuer Physik

17. 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 fuer Physik

18. 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 fuer Physik

19. 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. 0.1 1. A. Caldwell, MPI fuer Physik

20. 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 fuer Physik

21. 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 fuer Physik

22. EIC kinematics A. Caldwell, MPI fuer Physik

23. Summary • Existing data (F2 fits, forward jets,+…)show limitations of pQCD calculations. Transition region observed. • Exciting theoretical developments over the past few years. We are approaching a much deeper understanding of the high energy limit of QCD. • Measure with more precision, over wider kinematical range, to see where/how breakdown takes place (high rapidities, high-t exclusive processes, expanded W, MX range for diffraction, full coverage of transition region) • Precision FL measurement: key observable for pinning down pQCD. Large differences in predictions at LO, NLO, NNLO, dipole model. • eD, eA measurements to probe high density gluon state, parton densities for nuclei. • Additional benefits: parton densities for particle, astroparticle and nuclear high energy physics experiments. Crucial for cross section calculations. A. Caldwell, MPI fuer Physik