A New Round of Experiments for HERA

A. Caldwell, April 26, 2003

• Motivation• Letters of intent

e

eWhat’s in the bubble ??

Radiation patternsMass generationConfinementChiral condensateGluon condensateInstantons,solitons,…

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.

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 Q2

• 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• FL

• eA• Polarized protons (deuterons) • …

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,Q2 parton densities, heavy quark physics, measurement of EW parameters, high PT processes, and searches for new physics.• The H1 and ZEUS detectors were designed for this physics !

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)

Precision Structure Function Measurements

• F2,F2D,FL,FL

D in the range 0.1 < Q2 < 100 GeV2. Test NkLO DGLAP fits and extraction of gluon densities. Can we see machinery breaking down ?

Gluon densities not known at higher order, low Q2. Need more precise measurements, additional observables (e.g., FL)Additionally, current measurement depends critically on assumptions for PDF’s !

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

xd – xu with constraint xd=xu for x0

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

Transition region: end of partonic behavior, start of hadronic behavior ‘see’ formation of constituent quarks ?

Q2=0.5 GeV2 corresponds to a transverse distance scale of about 0.3 fm. Pin this down with precision measurements in the range 0.1 < Q2 < 10 GeV2. Study over maximum possible x-range. Do we see the same behavior in nuclei ?

DL

• 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

Advantage: known wfns

Disadvantage: cross section

Both: interference with QED process

x1, x2 necessarily different: probe parton correlations in proton. Study as function of impact parameter.

DVCS

Jet and Particle Production Studies over wide Rapidity Range

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

Forward jet cross sectionsHERA-I results indicate that NLO DGLAP alone not enough to describe forward jet production

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)

• 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 Q2. At a collider, can untangle many effects by measuring differentially over wide range of x,Q2

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:

• 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 QS which is near (in ?) the perturbatively calculable regime.

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

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 F2

p/F2n = 2/3

Dominant 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

Mapping Nucleon Spin

g1(F2/2x)(A/p e)(y2+2(1-y))/(y(1-y))

Deuterons with spectator tagging even better option

A New Detector for Strong Interactions

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

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.

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 !