eic white paper introduction & executive summary...
TRANSCRIPT
EIC White Paper
Introduction & Executive Summary
Physics Overview
Abhay Deshpande Zein-Eddine Meziani
Jianwei Qiu
Writing committee meeting, February 27, 2012 Room 2-78, Physics, BNL
Eternal Questions
Where did we come from?
People have long asked
The Big Bang theory
What is the world made of?
Basic building blocks
Fundamental forces to hold them together
Where are we going to?
The future?
The little Bang experiments
The Electron-Ion Collider
Where did we come from?
BNL - RHIC
Gold - Gold
CERN - LHC
Lead - Lead
Expansion of the universe
Create a matter (QGP) with similar temperature and energy density
Little Bang in the Laboratory
Relativistic heavy-ion collisions
q Artist’s view of the collision (Virtual Journal on QCD Matter):
Lorentz contraction
Near collision
Quark-gluon plasma Hadronization Freeze-out
Seen in the detector
² How observed particles were formed (after collision)?
² Does the initial condition matter (before collision)?
Properties of
visible matter
q Questions:
q Discoveries – Properties of QGP:
² A nearly perfect quantum fluid – NOT a gas!
at 4 trillion degrees Celsius, Not, at 10-5 K like 6Li
What the world is made of?
q Evolving in time – a long history:
q The budget of our universe today: Visible matter
Dark energy
Dark matter
The visible matter
Seen by the light, interacting with the photon
q Basic building blocks:
Crystal Molecule
Atom
q Why the visible matter is so rich in varieties?
Limited number of atoms, but, so many ways to put atoms together!
q What hold them together?
² The force between electric charges (neglecting gravity for now)
² BUT, the building block – the atom is charge neutral!
The internal structure of the building blocks – the atoms determines the richness of the visible matter
The atom
q Rutherford’s experiment (100 years ago):
Large angle events!
p
p’
Momentum transfer square: (p-p’ )2 < 0, large! – Deep Inelastic Scattering
(DIS) q Evolution of our knowledge:
J.J. Thomson’s plum-pudding model
Rutherford’s planetary model
Bohr’s model Hydrogen atom
Modern model Quantum orbitals
q The structure:
² Tiny nucleus (localized charge and mass!) – 104 times smaller in size
² Vast space for electrons zooming in quantum orbitals
The nucleus
q Building blocks – nucleon:
q Not too many – the period table:
Quarks: u, d, … Gluons
Proton (1919) Neutron (1931)
q What hold them together?
² The force between color charges – short-range confining force
² BUT, the building block – the nucleon is color neutral!
The internal structure of the building blocks – the nucleons determines the properties of nuclei
The nucleon
q SLAC’s “Rutherford” experiment (60 years later – 1969):
θ e
e Proton
parton p
P’
Electron-proton deep inelastic scattering (DIS)
xB , Q2
q The structure:
Naïve quark model Modern picture Quantitative structure
q Building blocks: Quarks – u, d, …
q Fundamental force: QCD: color force – gluons (are dark!)
Q2 = −(p− p�)2 � 1 fm−2
1
Q� 1 fm
² Two variables:
² Localized probe:
² Very light quarks ( < the 100th proton mass) – zooming relativistically
² No localized color and mass centers – energy for nucleon mass
Challenge of strong interaction
q Hadron properties in terms of dynamics of quarks and gluons:
Hadron properties
Charge, Mass,
Spin,
Magnetic moment,
…
QCD
Quarks Color,
Flavor,
Charge,
Mass,
Spin, …
Gluons Color,
Spin,
…
+
q Lattice QCD:
Could calculate all hadron properties in principle! Has done an excellent job in reproducing the hadron mass spectrum
q But,
It does not reveal the space-time distribution of partons inside a hadron, details of interactions, reasons of confinement, …
Probes for quarks and gluons
q Asymptotic freedom:
q Hard probe in high energy collision (large momentum transfer)
² Advantage: systematic application of perturbative QCD (pQCD)
² Disadvantage: not sensitive to dynamics at confinement scale
² Hadron structure?
q QCD is “right”, but,
Go beyond 1-D local momentum distribution
Parton distributions:
Helicity distributions:
q(x,Q), ...
∆q(x,Q), ...
² Only tested in the most trivial regime of its dynamics
The asymptotic regime: < 1/10 fm (~ 2 GeV) p
xp
Where are we going?
q Future “Rutherford” experiments (10 years later?):
@ an Electron-Ion Collider (EIC)
² First (might be the only) polarized electron-proton collider in the world ² First electron-nucleus (various species) collider in the world
q Two possible options:
ELIC (Jlab) eRHIC (BNL)
Intensity frontier: at least 100 times higher in luminosity than HERA
What EIC can do or measure?
q Inclusive (1 hard scale):
1-D momentum distributions
q(x,Q), G(x,Q)
∆q(x,Q),∆G(x,Q)
Nulceus A q Semi-Inclusive (2 scales):
3-D momentum distributions
q(x, kT , Q), g(x, kT , Q), ...
Hadronization/fluctuation
q Exclusive (1 hard + 1 soft scale):
1+2D imaging - GPDs
H(x, ξ, t, Q), E(x, ξ, t, Q)
H(x, ξ, t, Q), E(x, ξ, t, Q)
Quark/gluon total angular momentum to proton’s spin
S(µ) =�
f
�P, S|Jzf (µ)|P, S� =
1
2≡ Jq(µ) + Jg(µ)
q in QCD – complexity of the proton state:
q Proton spin structure:
Intrinsic spin vs. dynamical motion of quarks and gluons
=1
2Σ(µ) + Lq(µ) + Jg(µ)
S(µ) =1
2 µ ⇒ ∞
Proton spin
Hadron properties in terms of dynamics of quarks and gluons
q Fixed target DIS measurements:
q RHIC collider measurements:
Total measured quark spin contribution ~ 25-30%
Total measured gluon spin contribution ~ 0%
Δq ΔG
Lq Lg=Jg-ΔG
² Need a wider range of momentum fraction x ² Transverse motion of quarks and gluons
What EIC can do to ΔG?
INT report q Decisive measurement:
No other machine in the world can have this kind of kinematic reach and accuracy on ΔG!
1+2D confined motion in a nucleon
q Quark TMD distributions – production of pion, …
q Motion at the confining scale (<< Q) – partonic structure:
pxp, k⊥
q Gluon TMD distributions, …
Production of quarkonium, two-photon, …
² Two scale observables
² SIDIS – Q, pT
² Hadron-parton correlations
sidebar
EIC is ideal for probing TMDs
q Two scattering plans – Separation of various TMDs:
q Two scales – SIDIS has the natural kinematics:
Natural event structure: high Q - localized probe
Low pT - sensitive to confining scale
�(se) + p(sp) → �+ h(sh) +X
1( , )
sin( ) sin( )
sin(3 )
l lUT h S
h SSiverCollins
PretzelosiUT
tyU
sUT h S
h ST
N NAP N
AA
NA
ϕ ϕ
φ φ φ φ
φ φ
↑ ↓
↑ ↓
−=
+
= + + −
+ −
1
1 1
1
1 1
sin( )
sin(3 )
sin( )Co
PretzelosityU
SiversUT
llins
T h S T
h S
UT
UT h S
TU
UT
TA
H
f
A
D
A h H
h
φ
φ
φ
φ φ
φ
⊥
⊥ ⊥
⊥
∝
∝ −
+
∝
⊗
− ∝ ⊗
⊗
∝
∝
Lepton plane – hadron plane
Power of angular modulation
Example: transverse single-spin asymmetry (SSA)
q Left-right asymmetry:
A(pA, s↑) +B(pB) → π(p) +X
A direct probe for parton’s transverse motion (low pT)
A direct probe of QCD quantum interference (high pT)
q Vanish without parton’s transverse motion:
q Inclusive DIS has no SSAs!
Time-reversal invariance (Christ and Lee, 1965 )
Our knowledge on TMDs
q Sivers function from SIDIS:
q NO TMD factorization for hadron production in p+p collisions
Collins and Qiu, 2007, Vogelsang and Yuan, 2007, Mulders and Rogers, 2010, …
Correlation of proton’s transverse spin and quark’s transverse motion
What EIC can do to Sivers function?
Unpolarized quark inside a transversely polarized proton
Up quark Down quark
Transition from low pT to high pT
q TMD factorization to collinear factorization:
TMD Collinear Factorization
Two factorization are consistent in the overlap
region where
ΛQCD � pT � Q
AN (Q2, pT )
pT
pT ∼ QpT � Q
∼ Qs
q Quantum interference – high pT region (integrate over all kT):
T (3)(x, x) ∝
Non-probabilistic quark-gluon quantum correlation
Single quark state quark-gluon composite state
−1
2p
1
2p
(Spin flip)
interfere with
Imaging quarks and gluons
q The “big” question:
How color is distributed inside a hadron? (clue for color confinement?)
q Electric charge distribution:
Elastic electric form factor Charge distributions
q
p'p
G.A. Miller (2007)
q But, NO color elastic nucleon form factor!
Hadron is colorless and gluon carries color
1+2D spatial imaging of parton density
q Partonic structure – spatial distributions of quarks and gluons:
² Need a localized probe
² Scan in transverse direction
² Partonic structure
q Exclusive process – DVCS:
EIC at high energy can provide large enough Q, phase-space for t = (p’-p)2 !
, . . . dσ
dxBdQ2dt
pxp
1
Q
q Generalized parton distributions (GPDs) of quarks and gluons:
Evolution in Q
– gluon GPDs P P �
Hq(x, ξ, t, Q), Eq(x, ξ, t, Q),
Hq(x, ξ, t, Q), Eq(x, ξ, t, Q)
q Imaging ( ): ξ → 0
q(x, b⊥, Q) =
�d2∆⊥e
−i∆⊥·b⊥Hq(x, ξ = 0, t = −∆2⊥, Q)
Exchange of colorless object
What EIC can do?
q Imaging – effect of transverse polarization:
q QCD quantum evolution in probing scale – Q:
Cross check with diffractive J/ψ production @ EIC
No QCD factorization for diffractive scattering in pp
q Total quark’s orbital contribution to proton’s spin:
Jq =1
2limt→0
�dx x [Hq(x, ξ, t) + Eq(x, ξ, t)] =
1
2∆q + Lq Should this be consistent
with Lattice QCD?
Transverse spin
Lattice calculation on parton orbital motion
q Moments of GPDs on lattice: Negele et al
q Contribution from orbital motion:
q Both Lu and Ld large:
But, Lu + Ld ~ 0
Mainly valence contribution
Role of sea and gluon?
Huge gluon density at small x
q Scaling violation of F2 – HERA measurement:
∂F2(x,Q2)
∂ lnQ2∝ G(x,Q2)
q Gluon recombination and saturation ought to be there: QCD non-linear dynamics
Probe
Soft gluons at small-x
High density glue at small x should interact – recombine – reach saturation?
q QCD color fluctuation taken place at various time scales:
dP ∼ αsdk2Tk2T
dx
x
² Radiation:
² Change of distributions – evolution:
dx
x→ d log(1/x)
dk2Tk2T
→ d log(Q2)DGLAP
BFKL
Reaching the saturation region
q HERA(ep):
Only hope to see saturation in ep: LHeC?
for all values of x
Q2 > Q2s
Q2 > Q2s
If
Q2s ≤ 1 GeV2
q Local probe:
whole HERA kinematics is outside the saturation region
Q2 ≥ 1 GeV2,
q Need a super “ep”: √s > TeV,
for some range of Q
q eA helps! 1
Q∼ 1
xp> 2R
�m
p
�or equivalently x < xc =
1
2mR∼ 0.1
Localized in transverse, quantum coherence in longitudinal direction
Reaching the saturation with eA
q small-x probe interacts with partons from all nucleons at a given impact parameter:
Q2s(eA) ∝ Q2
s(ep) A1/3
Di-hadron angular distributions in eA
Dominguez, Xiao, Yuan (2010) q Strong suppression of dihadron correlation in eA:
² Never be measured!
² Directly probe Weizsacker-Williams (saturated) gluon distribution
in a large nucleus
² A factor of 2 suppression of away-side hadron-correlation!
ϕ12
ep
eA
QCD in nuclei
q How a nucleus look like if we only see quarks and gluons?
Know how a molecule or crystal looks like if see only nuclei and electrons
q EIC can map out nuclear distributions of quarks and gluons:
Marquet@EICAC
FL measurement at EIC
Energy: 5 GeV + 100 (250, 325) GeV – stage one INT report
q Direct access to gluons (HERA relies on Q2-evolution):
Diffractive vector meson production
Toll and Ullrich (2011)
q Diffractive (coherent) – “gluonic” form factor:
Spatial gluon density (Gluon GPD’s)
² Never be measured
² Fourier transform of t dependence:
² 1+2D gluon density imaging of a nucleus – in saturation region?
Could be very useful tool to probe the short-range correlations!
Q = 10 GeV
In-medium hadronization
q Unprecedented range of photon energy ν at EIC: ν =Q2
2mx
² Smallν- in medium hadronization:
Stages of hadronization: parton, pre-hadron, hadron
² Largeν- parton multiple scattering: Parton energy loss – cold nuclear matter q
q Heavy quark and quarkonium production:
Filter for production mechanism!
² Plot: Ratio of SIDIS vs zh for various A
Density distribution – Fluctuation
q SIDIS – transverse momentum broadening:
�∆p2T �AN = �p2T �A − �p2T �N
�p2T (φ)�A ≡�
dp2T p2TdσeA
dxBdQ2dp2T dφ
�dσeA
dxBdQ2
q Azimuthal asymmetry:
² Classical expectation:
Any distribution seen in Carbon should be washed out in heavier nuclei
² Surprise (EIC can cover a wider range of xB and Q2):
Quantum fluctuation probed by transverse momentum broadening!
Electroweak physics at EIC
q Mixing angle of weak interaction – high luminosity:
Fill the region never
be measured
The three most important goals of EIC
q Extract the confined motion of quarks and gluons in a nucleon with and without polarization, and in a nucleus – STAGE ONE
² Possible clue for color confinement, hadron – parton correlations, … ² Ultimate solution of proton spin – hadron property in QCD
² Naturally measured at EIC, not easy, if not impossible, at other machines
q Measure the confined spatial distribution of quarks and gluons in a nucleon with and without polarization, and in a nucleus
² Complementary to the motion measurement ² Sum rule for proton spin – hadron property in QCD
² EIC has the “sufficient” kinematic reach for reliable imaging
q Discover clear evidences of QCD’s manybody non-linear dynamics and the range of color coherence – STAGE ONE
² Saturation scale – consequence of QCD non-linear dynamics ² Range of color coherence – nuclear property in QCD
² EIC, like RHIC for heavy ion, can pioneer the search of non-linear
dynamics
Summary
q Many aspects of hadron’s partonic structure can be naturally addressed by EIC, but, not other machines: e+e-, pp, pA, AA
q After almost 40 years, we have learned a lot of QCD dynamics, but, only in its most trivial asymptotic regime (less than 0.1 fm), and very limited information on nucleon/nuclear structure
q EIC with polarization provides a new program to explore new frontier research of QCD dynamics – key to the visible matter
Rutherford exp’t SLAC “Rutherford” Future “Rutherford”
1911 1969 2020? Nucleus
Atomic structure Parton
Collinear PDFs Manybody non-linear dynamics
Confined partonic structure
Backup slices
Di-hadron angular distributions in pA
Marquet@EICAC q Di-hadron correlation:
QCD at a sub-femtometer scale
q Scale to apply for pQCD:
Q > 2 GeV ∆r < 10−1 fm αs(Q) < 1/3
q Scattering cross sections with identified hadron(s):
Dynamics at the typical hadron scale ~ 1/fm is not perturbative!
ke(l)
h(p)e(l�)
Parton in a hadron femtometer probe Cross section
�1/fm
Q
�n
² Hard scattering is localized in space-time to 1/Q
Dynamics at hadronic scale is effectively frozen
² Quantum interference between two scales is suppressed by
q QCD factorization:
ke(l)
h(p)e(l�)
2
≈e(l)
e(l�) 2h(p)k
2
⊗xp
Breakdown of leading power QCD
Visible failure only
when Q < 1 GeV
and x is small
DGLAP works
well
if Q > 2 GeV
Can large nuclei
Help?
Electroweak physics at EIC
q Mixing angle of weak interaction – high luminosity:
q Parity-violating single longitudinal asymmetries:
Flavor separation of
helicity distributions
Fill the region never
be measured