Physics - QCD in the high energy limit

December 4, 2003 A. Caldwell, MPI fuer Physik 1

A New Round of DIS Experiments – HERA III/eRHIC

A. Caldwell, Max-Planck-Institut f. Physik /Columbia University

Physics - QCD in the high energy limit

New Detector for HERA/eRHICH1 Upgraded Detector


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 (color glass condensate) ? Expected to be a universal state of matter.


s=(k+P)2 = (320 GeV)2 CM energy squaredQ2=-(k-k`)2 virtualiyW2=(q+P)2 *P CM energy squared

Transverse distance scale probed: r hc/Q

McAllister, Hofstadter Ee=188 MeV rmin=0.4 fmBloom et al. 10 GeV 0.05 fmCERN, FNAL fixed target 500 GeV 0.007 fmHERA 50 TeV 0.0007

fm

HERA Kinematics

Ee=27.5 GeVEP=920 GeV

*

/

r


Proton momentum frame Proton rest frame

x=Q2/2P q fraction of P momentum carried by struck quark

= E/Q2 Lifetime of hadronic = W2/2MPQ2 fluctuations of photon = 1/2Mpxx=10-4 1000 fm, x>0.1 <1 fm

Radiation cloud surrounds both photon, proton universal property of nature

Structure attributed to proton

Structure attributed to photon at small x


In the end, it boils down to understanding QCD virtual radiation


Proton rest frame

d2/dxdQ2=22/xQ4[(1+(1-y)2)F2 - y2FL]

F2 = f e2f x {q(x,Q2) + q(x,Q2) }

ef is quark chargeq(x,Q2) is quark density

FL = 0 in LO (QPM), non-zero after gluonradiation. Key test of our understanding

d2/dW dQ2 = (T + L)

is flux of photonsT,L are cross sections for transversely, longitudinally polarized photons to scatter from proton is the relative flux

F2 = Q2/42 (T + L)

Rutherford

Proton momentum frame


Physics Picture in Proton Rest Frame

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 !

r

b*


Hadron-hadronscattering crosssection versusCM energy

*P scattering cross section versus CM energy (Q20). Same energy dependence observed

s0.08 vs W2 0.08

Don’t see partons


The rise of F2 with decreasing x observed at HERA is strongly dependent on Q2

Cross sections as a function of Q2

Equivalently, strongly rising *P cross section with W at high Q2


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)


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.

Analysis of F2 in terms of parton densities (quarks and gluons)


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 !


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.

Gluon densities not known at higher order, low Q2. Need more precise measurements, additional observables (e.g., FL)

Thorne


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.


Diffractive Surprises

‘Standard DIS event’

Detector activity in proton direction

Diffractive event

No activity in proton direction


Diffraction1. There is a large diffractive cross section, even in DIS (ca. 20

%)2. The diffractive and total cross sections have similar energy

dependences. Data suggests simple physics – what is it ?


• Exclusive Processes (VM and DVCS)

VM


Energy dependence of exclusive processes

Rise similar again to that seen in total cross section.

epeVp (V=,,,J/) epep (as QCD process)

Summary of differentVector 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.


Golec-Biernat.Wuesthoff

Dipole Model for DIS:



More recent advances: (Kowalski, Teaney)• add gluon evolution to cross section (DGLAP)

• add impact parameter dependence

Profile function:Gaussian example


Fix parameters of T(b) from exclusive J/ production

Gluon density parameters fixed with F2 (or P)


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, … ?


E. Iancu, K. Itakura, S. Munier hep-ph/0310338Saturation 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)


Investigate this region

Large effects are expected inForward jet cross sections at high rapidities (also for forward particle production (strange, charm, …)

More detailed tests of radiation in QCD: forward jets


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 !


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(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)


HERA-III ProgramTwo 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 CollaborationOptimized detector for precision structure function measurements, forward jets and particle production over alarge rapidity interval (eP, eD, eA)


Possible upgrades of H1 detector

Upgrade in electron direction for low Q2 F2 and FL measurements

Upgrade in proton direction for forward particle, jets measurements

+ very forward proton, neutron detectors


Precision eD measurements

• Universality of PDF‘s at small-x, isospin symmetry

Can measure F2p-F2

n w/o nuclear corrections if can tag scattered nucleon


• Flavor dependence at high-x

Behavior of F2p/F2

n as x 1

SU(6) symmetry F2p/F2

n = 2/3Dominant scalar diquark F2

p/F2n = 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


A new detector to study strong interaction physics

e

p

HadronicCalorimeter

EM CalorimeterSi tracking stations

Compact – fits in dipole magnet with inner radius of 80 cm.Long - |z|5 m


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.

W=315 GeVW=0

Q2=1

Q2=0.1

Q2=10

Q2=100

Tracking acceptance


Tracking acceptance in protondirection

This region covered bycalorimetry

Huge increase in trackingacceptance compared to H1And ZEUS. Very important for forward jet, particle production, particle correlation studies.

Accepted 4 Si stations crossed.

ZEUS,H1


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.


Electron onlyQ2=2E E`(1-cos )y = 1-E`/2E(1+cos )x = Q2/sy dx/x 1/y dp/p

Jet method:Jet energy yields x via x = Ejet/EP

Mixed method in between – needs studies

E`

e

Nominal beam energies


FL can be measured precisely in the region of maximum interest. This will be a strong test of our understanding of QCD radiation.

d2/dxdQ2=22/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


Very forward calorimeter allows measurement of high energy, forward jets, and access to high-x events at moderate Q2

Cross sections calculated from ALLM

Integral of F2(x,Q2) up to x=1 known from electron information


Forward jet cross sections: see almost full cross section !

Range covered by H1, ZEUS

New region


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

W=0 50 100 150 200 250 300 GeV

0

5

10

15

20


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.


Quote from DESY summary to HGF 5 Year Planning


Physics/Detector studies for eRHIC

2x14 Si tracking stations



Max W for 10x200

Gap in tracking, but covered by calorimeter


Electron only

Jet for large x

Mixed methods



W2>5 GeV2

Ejet>50 GeV

Cross sections in pb


Tracking in proton direction


1. eRHIC would allow precision measurements in the transition region observed at HERA at small-x (<10-2)

2. FL measurements would be possible in an interesting kinematic range .

3. The structure functions could be measured at large x over a wide range of Q2. New territory.

4. Particle production/correlations over a wide rapidity range could be explored.

5. eA collisions would provide a different angle on studying systems with a large gluon density.

6. Performance for exclusive states not yet studied.7. Detector likely not optimum for spin studies since luminosity

will likely be too low.8. Higher energies smaller x (important for CGC type issues).

Preliminary summary of eRHIC detector/physics studies:

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Physics - QCD in the high energy limit