probing sea quarks and gluons: the electron-ion...
TRANSCRIPT
Tanja Horn
International Conference on the Structure of Baryons, BARYONS 2013
Probing Sea Quarks and Gluons: The Electron-Ion Collider Project
Glasgow, Scotland, 24-28 June 2013
/ JLab
Nuclear Science: to discover, explore, and understand all forms of
nuclear matter and its benefits to our society
QCD: A fundamental theory for the dynamics of quarks and gluons
It describes the formation of all forms of nuclear matter
the nucleus
the nucleons
the quarks and gluons
Accounting for essentially all of the mass of the visible universe
Nuclear matter:
Rutherford exp’t SLAC “Rutherford”
1911 1968 Revolutionized our view
of atomic structure Open the gateway
to nucleon structure
1972
Discovery of QCD
Understanding QCD is a fundamental & compelling goal of Nuclear Science
QCD and the Origin of Mass
98% of the mass of visible matter is
dynamically generated by self-
generating gluon fields
– Higgs mechanism has no role here.
u + u + d = proton 0.003 + 0.003 +0.006 ≠ 0.938 GeV
The similarity of mass between the
proton and neutron arises from the
fact that the gluon dynamics are the
same
– Quarks contribute almost nothing.
How do gluons create energy/mass?
Hadron mass spectrum
from lattice QCD
At low energy:
At high energy:
Measure e-p at 0.3 TeV (HERA)
Predict p-p and p-p at 0.2, 1.96, and 7 TeV
Asymptotic freedom
+ perturbative QCD
Successes of QCD
Mysteries of the Strong Force
Gluon dynamics plays a large role in proton spin
– Spin=intrinsic (parton spin) + motion (orbital angular momentum)
3D structure of the nucleon is non-trivial
– Far more than the simple valence quark structure
– Need to understand confinement to know how
proton properties arise from constituents
Why do quarks contribute so little (~30%) to proton spin?
How do gluons bind quarks and themselves into 3D hadrons?
Dynamical balance between gluon radiation and
recombination
– How does the unitarity bound of the hadronic cross
section survive if soft gluons in a proton or nucleus
continue to grow in numbers?
Saturation of gluons?
Map the spin and spatial structure of quarks and gluons in nucleons
Discover the collective effects of gluons in atomic nuclei
Understand the emergence of hadronic matter from color charge
Opportunities for fundamental symmetry measurements?
• How much spin is carried by gluons?
• Does orbital motion of sea quarks contribute to spin?
• What do the partons reveal in transverse momentum and coordinate space
• What is the distribution of glue in nuclei?
• Are there modifications as for quarks?
• Can we observe gluon saturation effects?
• How do color charges evolve in space and time?
• How do partons propagate in nuclear matter?
Physics Program of an EIC
o Generalized Parton Distribution (GPDs)
o Transverse Momentum Distributions (TMDs)
• Can nuclei help reveal the dynamics of fragmentation?
Needs high luminosity
and range of energies
Unified view of nucleon structure
Wigner distributions:
EIC – 3D imaging of sea and gluons:
TMDs – confined motion in a nucleon (semi-inclusive DIS)
GPDs – Spatial imaging of quarks and gluons (exclusive DIS)
5D
3D
1D
JLab12 COMPASS
for Valence
HERMES JLab12
COMPASS
Only a small subset
of the (x,Q2)
landscape has been
mapped here:
terra incognita
Gray band: present
“knowledge”
Purple band: EIC (2 )
u u
An EIC with good
luminosity & high
transverse polarization
will be the optimal tool
3D
1D
Imaging the Transverse Momentum of the Quarks
No other machine in the world can do this!
Transverse Spatial imaging of sea quarks and gluons
GPDs Exclusive processes – DVCS or meson production:
Quark GPDs and its orbital contribution to proton’s spin:
First meaningful constraint
on quark orbital motion to
proton spin by combining the
sea from the EIC and
valence region from JLab 12
Fourier transform of the t-dep
Spatial imaging of glue density
t-dep
J/Ψ, Φ,.. Q
Example: Gluon Imaging from J/
Only possible at the EIC
Weiss et al, INT 10-3
(J/Ψ, φ)
ep → e'π+n ep → e'K+Λ
Horn et al. 08+, INT10-3
(DVCS)
Spatial Imaging through meson production
Are radii of quarks
and gluons different
at a given x? –
DVCS and J/
Are strange and non-
strange (light sea)
quark sizes different
at given x? - and K
production
Full image of the proton can be obtained by mapping t-distributions for different processes.
Hints from HERA:
Area (q + q) > Area (g)
Dynamical models predict difference: pion cloud, constituent quark picture
-
• Q2 > 10 GeV2
for factorization
• Statistics hungry at high Q2!
x = 10-3
• DVCS asymmetries with a transversely polarized target
are sensitive to the GPD E, and open opportunities to
study spin-orbit correlations
F. Sabatie
Ei(x, ξ, t) = κi(t) Hi(x, ξ, t)
Model:
• Colliders provide an very good Figure-Of-Merit (FOM)
– Unpolarized FOM = Rate = Cross section x Luminosity x Acceptance
– Polarized FOM = Rate x (Polarization)2 x (Target dilution)2
Error bars shown only for κsea = +1.5
DVCS with a polarized “target”
(Lattice Calculation of the IP
density of up quark,
QCDSF/UKQCD Coll., 2006)
Proton Spin and Hadron Structure
Lq? Jg?
Measure g - Explore the “full” gluon and sea quark contribution
Measure TMD and GPDs - quantify the role of orbital motion
Two complementary approaches to resolve proton spin puzzle
Proton spin:
Over 20 years effort (following EMC discovery)
Quark (valence + sea) helicity (world DIS): of proton spin
Gluon helicity (RHIC+DIS): small?
It is more than the number ½! It is the interplay between the
intrinsic properties and interactions of quarks and gluons
current data
w/ EIC data
Q2 = 10 GeV2 Helicity PDFs at an EIC
G
Many models predict
u > 0, d < 0
Can constrain light quark sea polarization
(needs intermediate s and high luminosity)
and gluon polarization at small and large x. √
Precision measurement of Δg:
o Extend to smaller x regime
Gluons in nuclei
• HERA measured the longitudinal gluon distributions in the nucleon
– FL and dF
2/dln(Q2)
• Very little is known about gluons in nuclei
• Knowledge of gluon PDF
essential for quantitative
studies of onset of saturation
• EIC: access gluons through
FL (needs variable energy)
and dF2/dln(Q2)
Ra
tio
of g
luo
ns in
le
ad
to
de
ute
riu
m
pT
q
h
g* e
e
tproduction
>RA
pT2 vs. Q2
Hadronization – parton propagation in matter
tproduction
< RA
pT
q g*
e
e
h
• pT broadening strongly constrains theory
Accardi, Dupre
• Large range in υ at a collider allows
– Isolation of pQCD energy loss (large υ)
– Study of (pre)hadronization (smaller υ)
large υ smaller υ large υ
• Heavy flavors: B, D mesons, J/Ψ ...
• Jets above s = 1000 GeV2
– „real“ pQCD, IR safe
pT2 = pT
2(A) – pT2(2H)
Z. Kang
„If one could tag neutron, it typically
leads to larger asymmetries“
C. Hyde
• MEIC will provide longitudinal and transverse polarization for d, 3He, and other light ions
• Measurements of exclusive reactions like DVCS also greatly benefit from polarized neutron “targets“
Sivers asymmetry
EIC kinematics
• Polarized neutrons are important for probing d-quarks through SIDIS
Spectator Tagging with Polarized Deuterium
DpnnXLDRD for polarized light nuclei at the EIC
+
EIC forward detection design allows for separating the virtual photon
(current) and fragmentation regions in the final state
Final transverse momentum of the detected pion Pt arises from convolution
of the struck quark transverse momentum kt with the transverse momentum
generated during the fragmentation pt.
Du+(z,pt)
Rap
idity
Current
Target
Corr
ela
tions?
Color neutralization – a correlated 3D problem
o May reveal dynamical pair correlations as expected by
dynamical breaking of chiral symmetry in QCD vacuum
Pt
kt
pt
Pt
P’t
US based EIC: Kinematic reach and machine properties
For e-N collisions at the EIC:
First polarized e-p collider
Polarized beams: e, p, d/3He
Variable center of mass energy
Luminosity Lep ~ 1033-34 cm-2s-1
HERA luminosity ~ 5x1031 cm-2 s-1
For e-A collisions at the EIC:
First e-A collider
Wide range in nuclei
Variable center of mass energy
Luminosity per nucleon same as e-p
s (GeV2)
Q2 ~ ysx
EIC – staging at BNL and JLab
Stage I Stage II eRHIC @ BNL
MEIC / EIC @ JLab √s = 13 – 70 GeV
Ee = 3 – 12 GeV
Ep = 15 – 100 GeV
EPb
= up to 40 GeV/A
√s = 34 – 71 GeV
Ee = 3 – 5 (10 ?) GeV
Ep = 100 – 255 GeV
EPb
= up to 100 GeV/A
√s = up to ~180 GeV
Ee = up to ~30 GeV
Ep = up to 275 GeV
EPb
= up to 110 GeV/A
√s = up to ~140 GeV
Ee = up to 20 GeV
Ep = up to at least 250 GeV
EPb
= up to at least 100 GeV/A
(EIC) (MEIC)
MEIC/EIC Layout
Pre-
booster
Ion linac
IP1 IP2
High Energy Ring
(Stage-II EIC)
CE
BA
F
Electron
Injection
Interaction Region
ultra forward
hadron detection low-Q2
electron detection large aperture
electron quads
small diameter
electron quads
central detector
with endcaps
ion quads
50 mrad beam
(crab) crossing angle
n,γ
e p
p
small angle
hadron detection
60 mrad bend
In general, e-p and e-A colliders have a large fraction of their science related to the detection of
what happens to the ion beams… spectator quark or struck nucleus remnants will go in the
forward (ion) direction this drives the integrated detector/interaction region design
e
Hadron detection in
three stages
Endcap with 50 mrad
crossing angle
Small dipole covering
angles to a few degrees
Ultra-forward up to one
degree, for particles passing
the accelerator quads
Roman pots Thin exit
windows
Fixed
trackers in
vacuum?
• Good acceptance for ion fragments (different rigidity from beam)
– Large downstream magnet apertures
– Small downstream magnet gradients (realistic peak fields)
– Roman pots not needed
• Good acceptance for recoil baryons (rigidity similar to beam)
– Small beam size at second focus (to get close to the beam)
– Large dispersion (to separate scattered particles from the beam)
– Roman pots important
• Good momentum and angular resolution
– Large dispersion (e.g., 60 mrad bending dipole)
– Long, instrumented magnet-free drift space
• Sufficient separation between beam lines (~1m)
Ultra-forward Hadron detection requirements
DpnnXDVCS on the proton
e
p
n
Ultra-forward detection concept
• Momentum resolution < 3x10-4
– 15 MeV resolution for 50 GeV/A deuterium beam
• Excellent acceptance for all ion fragments
• Neutron detection in a 25 mrad cone down to zero degrees
• Recoil baryon acceptance
– up to 99.5% of beam energy for all angles
– down to 2-3 mrad for all momenta
• 100 GeV maximum ion energy allows using large-aperture magnets with achievable field strengths
20 Tm dipole 2 Tm dipole
solenoid
n p e
The central detector
Similar solutions for both designs: interesting dual-solenoid idea for JLab (à la ILC4)
eRHIC
GEANT4 modeling has started
BNL
JLAB
Summary
• An EIC is required to fully understand nucleon structure and the role of gluons in nuclei
EIC is the ultimate tool for studying sea quarks and gluons
EIC offers a fully integrated interaction region
• Straightforward detection of recoil baryons, spectators, and target fragments
EIC is a maturing project
• Designs ongoing at JLab and BNL
• Funds for joint detector R&D projects and accelerator R&D have been allocated
• Promising collaboration on central detector with HEP
• Physics effort for polarized light nuclei and forward tagging ongoing