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The nucleon structure, what an Electron-Ion Collider will teach us


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Title: The nucleon structure, what an Electron-Ion Collider will teach us

The nucleon structure, what an Electron-Ion
Collider will teach us
Physics program of an (M)EIC
  • EIC a machine, which will allow to develop a
    unified picture to describe the structure of the
    nucleus and nuclei
  • Spin an integrated part in this description
  • Requirements
  • Beam energy variability
  • mEIC a natural first stage of the full EIC
  • full acceptance crucial
  • Luminosity gt Hera / Hermes
  • need to measure un-integrated

The World of DIS in 2015
  • Unpolarized DIS
  • H1 Zeus Data will be analyzed
  • finalized results on PDFs, diffraction, exclusive
    production, .
  • data taking at JLab-12GeV starts
  • valence regime x gt 0.1
  • all DIS data on nuclei already know today
  • Polarized DIS
  • Hermes data will be fully analyzed
  • Compass will have finished the new longitudinal
    transverse polarized proton running potentially
    first data taking for DVCS
  • RHIC-spin will have taken data at 500GeV
  • polarized quark PDFs 0.001lt x lt 0.4
  • gluon very similar
  • data taking at JLab-12GeV starts
  • valence regime x gt 0.1

Measure Glue through DIS
Observation of large scaling violations
Gluon density dominates
What do we know till know
Q2 1GeV ? 0.2fm EM proton radius 0.9fm
Does the rise of F2 set in at the same Q2 for
Issues with our Current Understanding
  • Linear DGLAP evolution scheme
  • strange behavior of xG from HERA at small x and
  • G(x,Q2) lt Qsea(x,Q2) ?
  • Unexpectedly large diffractive cross-section
  • 20 energy independent
  • Linear BFKL Evolution
  • Density along with c.s. grows as a power of
    energy N s?
  • Can densities cross-section rise forever?
  • Black disk limit stotal 2pR2

Parton Saturation
  • at small x linear evolution gives
  • strongly rising g(x)
  • BK/JIMWLK non-linear evolution includes
  • recombination effects ? saturation
  • Dynamically generated scale
  • Saturation Scale Q2s(x)
  • Increases with energy or decreasing x
  • Scale with Q2/Q2s(x) instead of x and Q2

Question What is the relation between
saturation and the soft regime? Confinement?
  • HERA (ep)
  • Despite high energy range
  • F2, Gp(x, Q2) only outside the saturation regime
  • Regime where non-linear QCD (saturation
    phenomena) matter
  • (Q lt Qs) not reached, but close
  • Only way in ep is to increase vs

Beyond collinear pQCD
  • Suppose we view DIS in rest frame of target
  • ? fluctuation into quark, anti-quark (dipole)
  • w / radial separation r
  • Dipole interacts with proton/nuclei
  • Then DIS cross-section
  • Interesting physics in
  • What happens _at_ large r ?
  • In dipole picture saturates for rgtR0
  • assume
  • Use BFKL for x dependence of

Geometric Scaling ? works for proton and nuclei
x lt 0.01
FL measures glue directly
? G(x,Q2) with great precision
FL as G(x,Q2) requires vs scan Q2/xs y
Plot contains ?Ldt 4/A fb-1 (10100) GeV
4/A fb-1 (1050) GeV 2/A fb-1
(550) GeV statistical errors only
  • Syst. studies of FL(A,x,Q2)
  • xG(x,Q2) with great precision
  • Distinguish between models

Diffractive physics ep vs eA
Curves Kugeratski, Goncalves, Navarra, EPJ C46,
  • HERA/ep 20 of all events are hard diffractive
  • Diffractive cross-section sdiff/stot in eA ?
  • Predictions 25-40?
  • Diffractive structure functions
  • FLD for nuclei and p extremely sensitive
  • Exclusive Diffractive vector meson production
    ds/dt xG(x,Q2)2 !!
  • Distinguish between linear evolution and
    saturation models

VM production _at_ small x
W t dependences probe transition from soft ?
hard regime
s Wd
s e-bt
  • steep energy dependence of s in presence of the
    hard scale
  • universality of b-slope parameter point-like
    configurations dominate

Measure the Gluon Form Factor
RA 1.2A1/3fm Elastic scattering on full
nuceus ? long wavelength gluons (small t)
Expectation for 1M J/y Requirement Momentum
resolution lt 10MeV great t resolution Need to
detect nuclei break-up products
Interaction of fast probes with gluonic medium
  • nDIS
  • Clean measurement in cold nuclear matter
  • Suppression of high-pT hadrons analogous to, but
    weaker than at RHIC

RHIC AuAu _at_ 200 GeV/n
  • Fundamental question
  • When do partons get colour neutralized?

Parton energy loss vs. (pre)hadron absorption
Energy transfer in lab rest frame EIC 10 lt ? lt
1600 GeV HERMES 2-25 GeV
Charm measurements at an EIC
Charm also suppressed at RHIC - above and beyond
model predictions
  • EIC allows multi-differential measurements of
    heavy flavour
  • Covers and extends energy range of SLAC, EMC,
    HERA, and JLab
  • allowing for the study of wide range of
    formation lengths

  • From DIS at HERA
  • At small-medium Q2,
  • s(NC) gtgt s(CC)
  • For Q2 gt MZ2 and MW2,
  • s(NC) s(CC)
  • EW Unification
  • Already a textbook figure ...
  • What about on the parton scale?
  • Small-x running-coupling BFKL
  • QCD evolution predicts
  • QS approaches universal behaviour
  • for all hadrons and nuclei
  • No dependence on A!!
  • Not only functional form f(QS)
  • universal, but even QS itself
  • becomes universal
  • Radical View
  • Nuclei and all hadrons have a component of
  • their wave function with the same behaviour
  • Can this idea be tested ??

Important to understand hadron structure Spin
Is the proton spinning like this?
gluon spin
Helicity sum rule
Where do we go with solving the spin puzzle ?
angular momentum
total uds quark spin
Compass a lot of new results
LO Dq analysis
Compass new results Dq and Dubar - Ddbar
Polarized Quark Distributions
DSSV Global Analysis of world data
DSSV arXiv0904.3821
The Gluon Polarization
  • Dg(x) very small at medium x
  • best fit has a node at x 0.1
  • huge uncertainties at small x

Need to enlarge x-range
Dg(x) small !?
How to measure DS and DG
  • DG Indirect from scaling violation

Beyond form factors and quark distributions
Generalized Parton Distributions
Correlated quark momentum and helicity
distributions in transverse space - GPDs
How to access GPDs?
quantum number of final state selects different
p0 2DuDd
h 2Du-Dd
?0 2ud, 9g/4
? 2u-d, 3g/4
f s, g
? u-d
J/? g
Proton Tomography
Allows for Transverse Imaging
Proton Tomography
M. Burkardt, M. Diehl 2002
FT (GPD) momentum space ? impact parameter
polarized nucleon
from lattice
Results from Theory
K. Kumericki D. Mueller arXiv 0904.0458
First hints for a small Jq ? Lq What about the
Gluons ?
  • dominated by gluon contributions
  • Need wide x and Q2 range to extract GPDs
  • Need sufficient luminosity to bin in

DVCS on Nuclear Targets
  • How does the nuclear environment modify
    parton-parton correlations?
  • How do nucleon properties change in the nuclear

deuterium, spin-1, 300 pb-1
DVCS at HERMES Nuclear mass dependence
Select for each target two samples
(t-cutoffs) ? coherent enriched ( 65 coherent
fraction) ? incoherent enriched ( 60
incoherent fraction)
No nuclear mass dependence of BCA and BSA
observed within uncertainties ? no enhancement
of tDVCS
More insights to the proton - TMDs
Explore spin orbit correlations
Single Spin Asymmetries
Unpolarized distribution function q(x), G(x)
Transversity distribution function dq(x)
First data on proton and deuterium targets from
Hermes, Compass still kinematics and statistics
limited Belle results on Collins and IFF FF RHIC
AN no separation in underlying subprocesses
dominated by gluon
Helicity distribution function Dq(x), DG(x)
peculiarities of f1T chiral even naïve T-odd
DF related to parton orbital angular
momentum violates naïve universality of
PDFs QCD-prediction f1T,DY -f1T,DIS
Sivers function and OAM
Model dependent statement
M. Burkardt et al.
anomalous magnetic moment
ku 1.67
kd -2.03
Lattice P. Haegler et al. lowest moment of
distribution of unpol. q in transverse pol.
proton and of transverse pol. quarks in unpol.
Integrate TMD picture to the multi parton
correlations in CGC or other methods to extract
unintegrated PDFs
Anselmino et al. arXiv0809.2677
Summary To Do List for the next month
  • Need to get an consistent set of simulations
  • same assumptions on beam energy and luminosity
  • need to pipe simulations through a detector
  • acceptance
  • detector optimization
  • resolutions
  • Need to include raditative corrections in our
  • critical for FL and heavy nuclei
  • Need impact plots for DG and Dq to show gain
    compared to world data
  • Need to get our software/MC up to speed to allow
    people to do easily simulations/studies
  • https//
  • software /afs/
  • .

Detector Requirements from Physics
  • ep-physics
  • the detector needs to cover inclusive (ep -gt eX)
    ? semi-inclusive (ep -gt ehadron(s)X) ? exclusive
    reactions (ep -gt epp)
  • large acceptance absolutely crucial
  • particle identification (p,K,p,n) over wide
    momentum range
  • excellent vertex resolution (charm)
  • particle detection for very low scattering angle
  • around 1o in e and p/A direction
  • ? in big contradiction to high focusing quads
    close to IP
  • small systematic uncertainty for e/p polarization
  • very small systematic uncertainty for luminosity
  • eA-physics
  • requirements very similar to ep
  • most challenging get information on recoiling
    heavy ion
  • from exclusive and diffractive reactions.

First ideas for a detector concept
Solenoid (4T)
Dipol 3Tm
Dipol 3Tm
  • Dipoles needed to have good forward momentum
  • Solenoid no magnetic field _at_ r 0
  • DIRC, RICH hadron identification ? p, K, p
  • high-threshold Cerenkov ? fast trigger for
    scattered lepton
  • radiation length very critical ? low lepton

MeRHIC Detector in Geant-3
Thank you for your attention
Deep Inelastic Scattering
Measure of resolution power
Measure of inelasticity
Measure of momentum fraction of struck quark
Quark splits into gluon splits into quarks
Gluon splits into quarks
Increasing resolution higher Q2
Inclusive World data
  • Inclusive DIS-Data

not in DNS
input to the old GRSV-analysis
input to the DIS SIDIS analysis by DNS
Semi-Inclusive World Data
  • Semi-inclusive DIS-Data

not in DNS
no SIDIS data included in old GRSV-analysis
Existing Experiment Luminosities
  • Hera
  • H1 and Zeus
  • 1031cm-2s-1
  • 27GeV e/e- 920GeV p vs 320GeV
  • did measure
  • inclusive semi-inclusive DIS
  • exclusive reactions (DVCS)
  • diffractive physics
  • electro-weak
  • Hermes
  • fixed target 27GeV e/e- vs7.2GeV
  • polarised 5x1031cm-2s-1 unpol 1032-33cm-2s-1
  • did measure
  • inclusive semi-inclusive DIS
  • exclusive reactions (DVCS, pS, VM)
  • spin physics

Existing Experiment Luminosities
  • CERN
  • Compass
  • fixed target 160GeV m/m- vs17GeV
  • polarised 1031cm-2s-1 unpol 1032-33cm-2s-1
  • did measure
  • inclusive semi-inclusive DIS
  • exclusive reactions (VM)
  • spin physics
  • plan to measure
  • exclusive reactions (DVCS, PS)
  • Drell Yan
  • SMC, NMC and EMC very similar, with different
    beam energies

QCD the nearly perfect theory
  • Emergent phenomena not evident from Lagrangian
  • Asymptotic Freedom Color Confinement
  • Non-perturbative structure of QCD vacuum
  • Gluons mediator of the strong interactions
  • Determine essential features of strong
  • Dominate structure of QCD vacuum (fluctuations
    in gluon fields)
  • Responsible for gt 98 of the visible mass in
    universe (!)

How to measure DS and DG
  • DG Indirect from scaling violation

EIC one solution eRHIC _at_ BNL
p (A) e p (A) e
Energy, GeV 250 (100) 4 325 (125) 20 lt30gt
Number of bunches 111 166
Bunch intensity (u) , 1011 2.0 0.31 2.0 (3) 0.24
Bunch charge, nC 32 5 32 4
Beam current, mA 320 50 420 50 lt10gt
Normalized emittance, 1e-6 m, 95 for p / rms for e 15 73 1.2 25
Polarization, 70 80 70 80
rms bunch length, cm 20 0.2 4.9 0.2
ß, cm 50 50 25 25
Luminosity, x 1033, cm-2s-1 0.1 -gt 1 with CeC 0.1 -gt 1 with CeC 2.8 2.8
The Nuclear Enhancement Factor
  • Enhancing Saturation effects
  • Probes interact over distances L (2mnx)-1
  • For probes where L gt 2RA ( A1/3), cannot
  • distinguish between nucleons in the front or
  • back of of of the nucleus.
  • Probe interacts coherently with all nucleons.
  • Probes with transverse resolution
  • 1/Q2 (ltlt ?2QCD) 1 fm2 will see
  • large colour charge fluctuations.
  • This kick experienced in a random walk is the
    resolution scale.

Simple geometric considerations lead to
Nuclear Enhancement Factor
Enhancement of QS with A ? non-linear QCD regime
reached at
significantly lower energy in eA than in
How to measure coherent diffraction in eA ?
  • Can measure the nucleus if it is separated from
    the beam in Si (Roman Pot) beamline detectors
  • pTmin pA?min
  • For beam energies 100 GeV/n and ?min 0.08
  • These are large momentum kicks, much greater than
    the binding energy ( 8 MeV)
  • Therefore, for large A, coherently diffractive
    nucleus cannot be separated from beamline without
    breaking up

species (A) pTmin (GeV/c)
d (2) 0.02
Si (28) 0.22
Cu (64) 0.51
In (115) 0.92
Au (197) 1.58
U (238) 1.90
Large rapidity gaps at an eRHIC
  • Method
  • Use RAPGAP in diffractive and DIS modes to
    simulate ep collisions at eRHIC energies
  • Clear difference between DIS and Diffractive
    modes in most forward particle in event
  • Little change in distributions with increasing

Large rapidity gaps at an eRHIC
  • Efficiency vs Purity
  • Efficiency fraction of diffractive events out
    of all diffractive events in sample
  • Purity fraction of diffractive events out of
    all events in sample
  • Possible to place a cut to have both high
    efficiency and high purity
  • However, reduce the acceptance by 1 or 2 units of
    rapidity and these values drop significantly
  • Need hermetic detector coverage!!

Machine design options
  • two main design options for eRHIC
  • Ring-Ring
  • Linac-Ring

electron storage ring
electron linear accelerator
L x 10
Parton Propagation and Fragmentation
  • nDIS
  • Suppression of high-pT hadrons analogous but
    weaker than at RHIC
  • eRHIC Clean measurement in cold nuclear matter
  • Energy transfer in lab rest frame
  • eRHIC 10 lt ? lt 1600 GeV HERMES 2-25 GeV
  • eRHIC can measure heavy flavor energy loss
  • Work in Progress
  • Simulation with PYTHIA 6.4.19
  • 10 weeks of beam at eRHIC
  • 10100 GeV
  • Large reach in Q2 and pT
  • small ? - hadronization inside A
  • large ? - precision tests of QCD
  • parton energy loss
  • DGLAP evolution and showers

Nuclear Modification Measure
Questions to Address with the eRHIC
  • What is the nature of glue at high density?
  • How do strong fields appear in hadronic or
    nuclear wave functions at high energies?
  • Do gluon densities saturate?
  • What drives saturation, whats the underlying
  • What are the appropriate degrees of freedom
  • Does the Color Glass Condensate describe matter
    at low-x?
  • Universality of gluon dynamics energy
  • Is there a fixed point where all hadronic
    matter have a component of their wave function
    with the same behavior
  • Could a better knowledge of glue help solve the
    longstanding problem of confinement in QCD?
  • Whats the role of gluons in the nuclear

4 Key Measurements in eA Physics
  • Momentum distribution of gluons in nuclei?
  • Extract via scaling violation in F2 ?F2/?lnQ2
  • Direct Measurement FL xG(x,Q2) - requires vs
  • Inelastic vector meson production (e.g. J/?,?)
  • Diffractive vector meson production (
  • Space-time distribution of gluons in nuclei?
  • Exclusive final states (e.g. ?, J,?)
  • Deep Virtual Compton Scattering (DVCS) - s A4/3
  • F2, FL for various impact parameters
  • Role of colour-neutral (Pomeron) excitations?
  • Diffractive cross-section sdiff/stot ( 10
    HERA ep
  • 30? eRHIC eA?)
  • Diffractive structure functions and vector meson
  • Abundance and distribution of rapidity gaps
  • Interaction of fast probes with gluonic medium?
  • Hadronization, Fragmentation, Energy loss