1. A New Round of Experiments for HERA A. Caldwell, April 26, 2003 Motivation Letters of intent e e What’s in the bubble ?? Radiation patterns Mass generation Confinement Chiral condensate Gluon condensate Instantons,solitons,…

2. Physics Motivation– 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, chiral symmetry breaking, … 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, …) Can we observe the saturated gluon state (color glass condensate). This would be a universal state of matter.

3. To improve our understanding, we need to: Follow-up on interesting effects observed in data from HERA-I disappearing gluons in NLO DGLAP at small Q 2 transition from partonic to hadronic behavior of cross sections forward particle (jet) production & disagreement with NLO DGLAP behavior of diffractive cross sections (inclusive, exclusive) With improved/new detectors, and Make new measurements which will qualitatively change our understanding F L eA Polarized protons (deuterons) …

4. What about HERA-2 EW unification in one plot – measured with HERA-1. Few K CC events. The higher luminosity, and different polarizations, will yield precise tests of EW and flavor dependent valence quark densities. The goal of HERA-2 is to deliver 1 fb -1 /expt, divided into e -,e + and L,R handed lepton polarization. The physics goal is the extraction of high-x,Q 2 parton densities, heavy quark physics, measurement of EW parameters, high P T processes, and searches for new physics. The H1 and ZEUS detectors were designed for this physics !

5. 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 u v /d v at large-x diffraction on n, D Discuss also interest in: eA, F 2, F L at small Q 2, 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)

6. Precision Structure Function Measurements F 2,F 2 D,F L, F L D in the range 0.1 < Q 2 < 100 GeV 2. Test N k LO DGLAP fits and extraction of gluon densities. Can we see machinery breaking down ? Gluon densities not known at higher order, low Q 2. Need more precise measurements, additional observables (e.g., F L ) Additionally, current measurement depends critically on assumptions for PDF’s !

7. F L shows tremendous variations when attempt to calculate at different orders. But F L is an observable – unique result.

8. xd – xu with constraint xd=xu for x  0 Constraint removed. Uncertainties on the parton densities much larger if don’t assume isospin symmetry - should be tested. Symmetry of the sea at small-x

9. Transition region: end of partonic behavior, start of hadronic behavior  ‘see’ formation of constituent quarks ? Q 2 =0.5 GeV 2 corresponds to a transverse distance scale of about 0.3 fm. Pin this down with precision measurements in the range 0.1 < Q 2 < 10 GeV 2. Study over maximum possible x- range. Do we see the same behavior in nuclei ? DL

10. Exclusive Processes (VM and DVCS)   VM Elastic process with t-meas yields impact parameter scan of strongly interacting matter. Need high-t elastic process, max W range Small b is high t

11.   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

12. Jet and Particle Production Studies over wide Rapidity Range

13. Large effects are expected in forward jet cross sections at high rapidities (also for forward particle production: strange, charm, …) Forward jet cross sections HERA-I results indicate that NLO DGLAP alone not enough to describe forward jet production

14. 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 )

15. 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

16. 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:

17. 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.

18. Spin of the proton Open question: how do we understand the spin of the proton ? Quark model, What we measure: Ang Momentum from quarks only 0.3 h/2 Rest in gluons ? Orbital ang mom ? Polarized p beam would allow look at gluon spin

19. Possible upgrades of H1 detector Upgrade in electron direction for low Q 2 F 2 and F L measurements Upgrade in proton direction for forward particle, jets measurements + very forward proton, neutron detectors

20. Precision eD measurements Universality of PDF‘s at small-x, isospin symmetry Can measure F 2 p -F 2 n w/o nuclear corrections if can tag scattered nucleon

21. 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. Simulation with H1 based on 50 pb -1

22. Mapping Nucleon Spin g 1  (F 2 /2x)(A  / p e )(y 2 +2(1-y))/(y(1-y)) Deuterons with spectator tagging even better option

23. A New Detector for Strong Interactions e p Hadronic Calorimeter EM Calorimeter Si tracking stations Compact – fits in dipole magnet with inner radius of 80 cm. Long - |z|  5 m

24. The focus of the detector is on providing complete acceptance in the low Q 2 region where we want to probe the transition between partons and more complicated objects. W=315 GeVW=0 Q 2 =1 Q 2 =0.1 Q 2 =10 Q 2 =100 Tracking acceptance

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

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

27. Electron only Q 2 =2E E`(1-cos  ) y = 1-E`/2E(1+cos  ) x = Q 2 /sy dx/x  1/y dp/p Jet method: Jet energy yields x via x = E jet /E P Mixed method in between – needs studies  E` e Nominal beam energies

28. F L can be measured precisely in the region of maximum interest. This will be a strong test of our understanding of QCD radiation. d 2  /dxdQ 2 =2  2 /xQ 4 [ (1+(1-y) 2 )F 2 (x,Q 2 ) - y 2 F L (x,Q 2 ) ] Fix x, Q 2. Use different beam energies to vary y. Critical issue: e/  separation

29. Very forward calorimeter allows measurement of high energy, forward jets, and access to high-x events at moderate Q 2 Cross sections calculated from ALLM Integral of F 2 (x,Q 2 ) up to x=1 known from electron information

30. Forward jet cross sections: see almost full cross section ! Range covered by H1, ZEUS New region

31. 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. HERA-1 HERA-3 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

32. Comments HERA is unique. With experiments dedicated to strong interaction physics studies, we can make substantial progress in understanding QCD on different distance scales. This is of fundamental importance – QCD is at the heart of matter ! There is a distinct possibility of a paradigm shift in our conception of nature. Additional benefits: parton densities for particle, astroparticle and nuclear high energy physics experiments. Crucial for cross section calculations. Need to show there is a strong community of particle physicists interested in HERA physics. High energy physics needs this type of program: only huge efforts on extremely long time scales is bad for the field.

33. Next steps Letters of intent submitted to DESY PRC. You can pick up a copy here, or find them on the web: H1 LoI: http://www-h1.desy.de/h1/www/publications/H1eDLoI.ps.gz http://www-h1.desy.de/h1/www/publications/H1eDLoI.pdf New det: http://wwwhera-b.mppmu.mpg.de/hera-3/index.html Interested individuals welcome ! DESY PRC meeting May 7,8 DESY scientific council meeting May 21,22 Many important reasons to make HERA-III a reality !