From HERA to eRHIC A' Caldwell, MaxPlanckInstitut f' Physik

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From HERA to eRHICA. Caldwell,
Max-Planck-Institut f. Physik
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eRHIC vs. Other DIS Facilities
eRHIC
DIS
eRHIC would cover a kinematic range which has
already been measured
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Special eRHIC feature
Physics

• Wide range of A
• High luminosity
• Polarized beams
• Full acceptance det

• 3D structure of nuclear matter
• Spin structure
• QCD dynamics in much greater detail

Detector feature
The physics program is broad and extremely
interesting
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Optimized detector design will make a big
difference in the physics which can be accessed.
Towards Bjorkens FAD.
See I.Abt, A.Caldwell, X.Liu and
J.Sutiak, arXivhep-ex/0407053
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Selected HERA results

• There are many physics topics addressed by HERA
  which I will not cover
• electroweak
• searches for BSM physics
• precision tests of QCD (?S, heavy quark
  production, jet rates, )
• all the results from HERMES and HERA-B
• Instead, I will focus on the unique aspects of
  HERA collider physics, which is the small-x
  physics, and also discuss the high-x end. We
  will see what eRHIC could add

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Inclusive cross sections
Cross sections measured over wide kinematic range
! Precise determination of PDFs in context of NLO
DGLAP.
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ZEUS 1997 F2 measurement
Dominated by statistical uncertainty
y1
y0.01
Dominated by syst uncertainty
Red total error gt 4
Blue total error lt 4
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Structure function data used to parametrize
parton densities in proton. Impressive precision
achieved.
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But all is not well
From Pumplin, DIS05
There are signs that DGLAP (Q2 evolution) may be
in trouble at small x (negative gluons, high ?2
for fits). How well do we understand the small-x
physics ? So far, no theory which predicts the
x-dependence of cross sections (PDFs). Guided by
data.
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High y cross sections
Note the turn-over of the cross section with
decreasing x at small x in the H1 data.
The data can be fit consistently with NLO DGLAP
by H1 assuming no gluon saturation. The
turn-over is due the negative contribution from
FL. MRST, CTEQ have trouble fitting the H1 low Q2
data consistently at NLO DGLAP.
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In QPM hadron is made up of quarks with zero PT
Helicity conservation
?L0
QCD radiation introduces quark PT, ?L?0
LO pQCD
We will make an FL measurement at HERA in 2007
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Measuring FL
Small Q2, ignore F3
For best sensitivity, maximize lever arm (y-range)
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Expected precision on FL HERA
eRHIC cannot push the small-x limit, but should
provide much more accurate FL measurements.
Could be crucial in understanding the physics of
gluons.
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FL eRHIC vs. Other DIS Facilities
FL measurement from eRHICHERA
eRHIC
FL measurement from eRHICfixed target
DIS
eRHIC is in an optimal energy range to extract FL
via cross section comparisons to previous
experiments.
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xlt0.01 measure universal structure of QCD
radiation.
xgt0.1 measure hadronic structure.
Non-perturbative boundary conditions. Eventually
get these from the lattice ?
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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 constituent quarks in
data. Distance scale ? 0.3 fm ??
eRHIC could probe this region with high precision
(with the right detector)
Hadron-hadron scattering energy dependence
(Donnachie-Landshoff)
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Probing the parton-hadron transition
? eRHIC
Extremely precise measurements possible in this
interesting region
ALLM parametrization
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Diffraction - the big surprise
Large diffractive cross section came as a
surprise
Still no understanding from a pQCD approach.
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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 ?

• 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.

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Notes on diffraction
Measurements have been performed with and without
measuring the outgoing proton

• Without proton
• no t measurement
• proton dissociation into low mass state
  difficult to estimate
• large acceptance

• With proton
• can measure t
• clean elastic sample
• small acceptance ()

Ideal measure the outgoing proton with large
acceptance. Will be easier at eRHIC because of
the smaller EP, but needs detailed discussions
with accelerator experts !
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• Exclusive Processes (VM and DVCS)

?VM
??
Clean process - has been measured for many
different vector mesons differentially in many
variables - wealth of information
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Curves from dipole model analysis of Kowalski,
Motyka, Watt Hep-ph 0606272
BG4 GeV-2 corresponds to an rms impact parameter
of 0.56 fm. smaller than the proton charge
radius of 0.870 PDG ? eRHIC should aim to
perform these measurements with the best possible
precision (detector requirements)
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Small-x is not the only frontier
There is limited data on cross sections at high-x
and high Q2
BCDMS has measured F2 up to x0.75 H1, ZEUS have
measured F2 up to x0.65
Q2
x
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The PDFs are poorly determined at high-x.
Sizeable differences despite the fact that all
fitters use the same parametrization
xq?(1-x)?. Is it possible to check this ?
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HERA high-x

• At high Q2, scattered electron seen with ?100
  acceptance
• For not too high x, measure x from jet
• For xgtxEdge, measure

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HERA Kinematics
Jet found
No jet found
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Results
99-00 eP
Red line is expectation from CTEQ6D
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99-00 eP
Good agreement with CTEQ6D in previously measured
region. Data tend to lie above expectations at
highest x. With the right detector, eRHIC could
make precision measurements at high-x !
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eRHIC with 100 pb-1 and FAD
Limited by MC generator
Measurements close to x1 possible with good
precision !
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HERA ? eRHIC

• Precise scan of the transition region between
  partonic hadronic behavior. Something changes
  there - can we understand it ? Need acceptance in
  electron direction.
• Make precision measurements at high-x to
  understand the valence quarks. Need acceptance in
  proton direction.
• Make FL a highlight of the program - much more
  direct access to gluon density than via F2
  scaling violations. Needs high precision
  measurements - good resolution, small
  systematics. Note FL can also be derived from
  comparison with HERA, fixed target.
• Focus on clean, high acceptance diffractive and
  elastic scattering measurements. Needs high
  efficiency rejection of proton dissociation and
  high acceptance proton spectrometer.

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Physics Picture
r 0.2 fm/Q (0.02 2 fm for 100gtQ2gt0.01 GeV2)
transverse size of probe. Scan across transition
from partons to hadrons !
ct 0.2 fm (1/2MPx) (lt1 fm to 1000s fm). For
xlt0.01, ctgt10 fm. Study universal features of
QCD radiation (short time scale fluctuations).
For xgt0.1, ctlt1 fm. Study proton structure
(long times).
b 0.2 fm/sqrt(t) t(p-p)2 Exclusive
processes yield matter profile of hadron.
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Big picture
eRHIC would allow a precise 3D mapping of nuclear
structure at different distance scales,
permitting the study of the transition from
partonic constituents to hadrons. The short
time-scale fluctuations of QCD which became
visible at HERA could be studied in much greater
detail, and with different targets. More input
needed for theoretical understanding. Both
topics are fundamental, and need new data for a
deeper understanding.