Lessons from HERA A. Caldwell, Max-Planck-Institut f. Physik

March 17, 2004 A. Caldwell, MPI f. Physik 1

Lessons from HERAA. Caldwell, Max-Planck-Institut f. Physik

Overview of main results

A detector for small/large-x


ZEUS detector

Main features:• Solenoid• Focus on central rapidities• Excellent hadronic resolution• general purpose detector


e±L,R p

H1 detector: better EM calorimetry, hadronic calorimeter less precise.


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

Proton collection of ‘frozen’ partons on time scale of interaction. Variations of cross section with x, Q2 attributed to details of proton structure. Sensible at small-x ?

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

small x corresponds to scattering of dipole with proton. At large Q2, dipole small, reduced radiation, color transparency. At small Q2, highly evolved dipole, hadron-hadron scattering.

What about a longitudinal distance scale ?


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

In dipole model, photon qq wfn extracted from T,L


Hadron-hadronscattering crosssection versusCM energy

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

s0.08 vs W2 0.08

Higher energy – more time for evolution of wfns - short lived fluctuations become visible.


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)


In general, NLO pQCD (DGLAP) fits do a good job of reproducing the data over the full measurement range.


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 !


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.


Fractional error

There are also significant uncertainties at large x


From 1 fb-1 with Ep=920 GeV. 15% measurement at x>0.7 . Will be a first measurement of this quantity.

Statistics at large-x from HERA II


Detector acceptance, performance: Key issue for detector builders

Main detector acceptance

Shifted vertex run

Beamline CalThe Q2=1 GeV2

region not covered continuously. Very interesting region focus of new experiment.

Large-x also has limited coverage in DIS regime.


Detector Acceptance and Kinematics

Small-x: scattered particles in electron direction. Scattered electron energy+angle determines kinematics

Large-x: scattered jet in proton direction. Electron angle depends on Q2. Jet energy gives x. Jet acceptance critical.

e

e

P

P


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 ?

Key detector issues:• Need to guarantee proton intact. • Cover full W range• Good MX resolution

Experience: measuring scattered proton gives cleanest measurements, but acceptance limited in PT,xL. Tag/reject proton breakup gives full acceptance, but need to work on PT resolution.


• Exclusive Processes (VM and DVCS)

VM

Clean process, but• was it really elastic ?• limited W coverage – depends on rapidity coverage of tracking system.


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.


Forward jet and particle production

• Select energetic forward pions, with kT ~ Q2, in low x DIS events.

• Simple DGLAP description fails.• BFKL reasonable agreement.• CCFM fails.• Jet results similar.• Parton radiation at low x not well

understood!


In the end, we need to understand QCD radiation and to solve QCD at large distance scales – non-perturbative physics !

track CAL


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. Best is full acceptance detector for scattered electrons at small angles.

• Measure FL over widest possible kinematical range, as this is a crucial observable for testing our understanding of radiation processes in QCD. Need large y small electron energies. Start with maximum possible electron beam energy, good e/ separation, electron resolution.

• 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. Need to verify scattering elastic. Maximum rapidity coverage of tracking, good PT resolution.


• 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. Need calorimetry and tracking at high rapidity.

• Measure structure functions at high-x. Need large luminosities, calorimetry at high rapidity.

• And, do it all with nuclei !

Open Questions – Next Steps


HERA II 2003-2007

Spin Rotators successfully implementedNew physics possibilities due to big increase in data sets, polarization of the beam


EW Physics

helicitysuppression

Measure CC cross sections as a function of the lepton charge and polarization. Sensitive to right-handed weak currents (up to WR=400 GeV).

Classic test of EW interaction.


Deeply Inelastic eP scattering:

Neutral Current

Charged Current e- W- , W- scatters on u,c,d, se+ W+ , W+ scatters on d,s,u,c

Measure distribution of quarks and gluons in the proton (structure functions) as a function of a transverse resolution scale (Q – 4 momentum transferred) and a longitudinal momentum scale (x – fraction

of proton momentum).


Large-x

_ _--- (e-p) ~ x (u+c) + (1-y2) x (d+s) _ _--- (e+p) ~ x (u+c) + (1-y2) x (d+s)

Charged Current interactions allow to constrain separately u and d valence quark density at high x and Q2

Put emphasis on eR+ to get max

sensitivity to d(x),s(x) [u(x) primarily from F2

NC]. Require >500 pb-1 for large-x measurements.


HERA-III InitiativeTwo 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 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


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

FL Measurement Range


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

Cross sections calculated from ALLM

High x, large W – unmeasured region


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


Quote from DESY summary to HGF 5 Year Planning


Physics/Detector studies for eRHIC

2x14 Si tracking stations


1. EIC/HERA III 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 kinematical 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 should be greatly improved.7. Higher energies smaller x (important for CGC type issues).

Preliminary summary of eRHIC detector/physics studies:

Documents

Lessons from HERA A. Caldwell, Max-Planck-Institut f. Physik