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Physics - QCD in the high energy limit

A New Round of DIS Experiments – HERA III/eRHIC A. Caldwell, Max-Planck-Institut f. Physik /Columbia University. New Detector for HERA/eRHIC H1 Upgraded Detector. Physics - QCD in the high energy limit. Physics Motivation– Study Strong Interactions.

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Physics - QCD in the high energy limit

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  1. A New Round of DIS Experiments – HERA III/eRHICA. Caldwell, Max-Planck-Institut f. Physik /Columbia University New Detector for HERA/eRHIC H1 Upgraded Detector Physics - QCD in the high energy limit A. Caldwell, MPI fuer Physik

  2. Physics Motivation– Study Strong Interactions • QCD is the most complex of the forces operating in the microworld  expect many beautiful and strange effects • QCD is fundamental to the understanding of our universe: source of mass (relation to gravity ?), confinement of color, … • We need to understand radiation processes in QCD, both at small distance scales and large. • small distance scales: understand parton splitting (DGLAP, BFKL, CCFM, …) • larger distance scales: suppression of radiation, transition to non-perturbative regime (constituent quarks, …) • Observation of the saturated gluon state (colorglass condensate) ? Expected to be a universal state of matter. A. Caldwell, MPI fuer Physik

  3. 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 fuer Physik

  4. 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 Structure attributed to proton Structure attributed to photon at small x Radiation cloud surrounds both photon, proton  universal property of nature A. Caldwell, MPI fuer Physik

  5. In the end, it boils down to understanding QCD virtual radiation A. Caldwell, MPI fuer Physik

  6. 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

  7. 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

  8. 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 Don’t see partons A. Caldwell, MPI fuer Physik

  9. 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

  10. 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

  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 fuer 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 fuer Physik

  13. We need to test NkLO DGLAP fits and extraction of gluon densities. Crucial, since DGLAP is our standard tool for calculating PDF‘s in unmeasured regions. Thorne Gluon densities not known at higher order, low Q2. Need more precise measurements, additional observables (e.g., FL) A. Caldwell, MPI fuer Physik

  14. 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

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

  16. 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 ? A. Caldwell, MPI fuer Physik

  17. Exclusive Processes (VM and DVCS) VM    A. Caldwell, MPI fuer Physik

  18. 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 fuer Physik

  19. Dipole Model for DIS: Golec-Biernat. Wuesthoff A. Caldwell, MPI fuer Physik

  20. A. Caldwell, MPI fuer Physik

  21. More recent advances: (Kowalski, Teaney) • add gluon evolution to cross section (DGLAP) • add impact parameter dependence Profile function: Gaussian example A. Caldwell, MPI fuer Physik

  22. Fix parameters of T(b) from exclusive J/ production Gluon density parameters fixed with F2 (or P) A. Caldwell, MPI fuer Physik

  23. Model able to reproduce inclusive, diffractive, and exclusive differential cross sections with relatively few parameters. Some examples: The model is in many respects quite simple but quite successful at reproducing the data. Can we understand relationship of this approach to NkLO DGLAP, BFKL, CCFM, … ? A. Caldwell, MPI fuer Physik

  24. E. Iancu, K. Itakura, S. Munier hep-ph/0310338 Saturation and BFKL dynamics in the HERA data at small x Excellent fit, 3 free parameters. Energy dependence in agreement with NLO BFKL with saturation (Triantyfillopolous, Mueller) A. Caldwell, MPI fuer Physik

  25. More detailed tests of radiation in QCD: forward jets Investigate this region Large effects are expected in Forward jet cross sections at high rapidities (also for forward particle production (strange, charm, …) A. Caldwell, MPI fuer Physik

  26. 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. • Measure FL over widest possible kinematic range, as this is a crucial observable for testing our understanding of radiation processes in QCD. • 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. • 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. • And, do it all with nuclei ! • And possibly spin as well ! A. Caldwell, MPI fuer Physik

  27. 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), at small-x, xg  x- , so xeff- = A1/3x-  so xeff  xA-1/3  = xA-3 (Q2< 1 GeV2) = xA-1 (Q2  100 GeV2) A. Caldwell, MPI fuer Physik

  28. HERA-III Program • Two letters of intent were submitted to the DESY PRC (May 7,8) • 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

  29. 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

  30. 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

  31. 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 fuer Physik

  32. 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

  33. 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 fuer Physik

  34. 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 fuer Physik

  35. e/ separation study Tracking detector: very wide rapidity acceptance, few % momentum resolution in ‘standard design’ over most of rapidity range. Aim for 2 GeV electron ID A. Caldwell, MPI fuer Physik

  36. Nominal beam energies Jet method: Jet energy yields x via x = Ejet/EP E`  e Electron only Q2=2E E`(1-cos ) y = 1-E`/2E(1+cos ) x = Q2/sy dx/x 1/y dp/p Mixed method in between – needs studies A. Caldwell, MPI fuer Physik

  37. 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

  38. Very forward calorimeter allows measurement of high energy, forward jets, and access to high-x events at moderate Q2 Integral of F2(x,Q2) up to x=1 known from electron information Cross sections calculated from ALLM A. Caldwell, MPI fuer Physik

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

  41. 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 transistion 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

  42. Quote from DESY summary to HGF 5 Year Planning A. Caldwell, MPI fuer Physik

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

  44. A. Caldwell, MPI fuer Physik

  45. Gap in tracking, but covered by calorimeter Max W for 10x200 A. Caldwell, MPI fuer Physik

  46. Jet for large x Electron only Mixed methods A. Caldwell, MPI fuer Physik

  47. A. Caldwell, MPI fuer Physik

  48. Cross sections in pb W2>5 GeV2 Ejet>50 GeV A. Caldwell, MPI fuer Physik

  49. Tracking in proton direction A. Caldwell, MPI fuer Physik

  50. Preliminary summary of eRHIC detector/physics studies: • eRHIC would allow precision measurements in the transition region observed at HERA at small-x (<10-2) • FL measurements would be possible in an interesting kinematic 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 not yet studied. • Detector likely not optimum for spin studies since luminosity will likely be too low. • Higher energies  smaller x (important for CGC type issues). A. Caldwell, MPI fuer Physik

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