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The quark-gluon structure of hadrons
Gerard van der Steenhoven
8 October 2003
Santorini, Greece
Outline
GSI
MAMI
DAFNE
few
nucleons
COMPASS,
HERMES
(*)
heavy
nuclei
ALICE
quarks
gluons
neutrinos
NESTOR, GRAN
SASSO, ANTARES
(*) Honestly stolen and ‘EUadapted’ from James Symons,
NSAC chair 2000 - 2002, US
•
•
•
•
•
•
Introduction
Hadron spectroscopy
Hadron spin structure
Quark-gluon correlations
Quark-gluon propagation
Outlook
Introduction
• The physics of hadrons:
Hadrons: complex multiquark-gluon systems.
gluon
Quark-antiquark pair
• What we would like to understand: quark-gluon structure, global
properties, origin of spin, confinement, interactions………
Elastic Scattering
• Proton Form Factor ratio
GEp(Q2)/GMp(Q2)
• Surprising JLab data:
 Discrepancy with Rosenbluth L/T separation
 New data?
 Radiative corrections
Is the shape of the proton
what we think it is ??
Deep Inelastic Scattering
• The proton structure
function F2(x,Q2):
• HERA data:
High gluon density in
in the proton!
The problem of hadron physics
• Extrapolate s to the size
of the proton, 10-15 m:
• If s  1 perturbative
expansions fail…
Enter:
Non-perturbative QCD:
– Proton structure & spin
– Confinement
– Hadron interactions
l  rproton   s  1
– Hadron spectroscopy
How to make progress?
• Theory:
lattice QCD
chiral perturbation theory
QCD inspired models (chiral soliton)
 Anthony Thomas
• Experiment: searches for novel q-g structures
polarization degrees of freedom
exclusive reactions (DVCS)
the nucleus as a femtometer probe
 Present talk
Hadron spectroscopy
• Allowed states in QCD:

qq
states  mesons

qqq states  baryons

qqqqq
states  pentaquarks?
Discovery
θ  (1540)
Harvest 2002 – 2003:
*
Discovery D sJ ( 2317)
Discovery

cc
Ξ (3520)
u
u
d
d
u
d
d
s
s
a) Five quarks in a sstate configuration.
u
d
d
u
b) Five quarks in a K+ -n
molecular configuration.
d
u
s
c) Five quarks in a strong
diquark correlation state.
u
ss
d
u
d) Collective excitation of
a multiquark configuration.
Observation of narrow DsJ-states
• BaBar studied decay
D*sJ  D*s (2112)  0
with D*s (2112)  K  K  
• Two new s c mesons ?
• The K+K-+-spectrum:
0+ @ 2.32 GeV
1+ @ 2.46 GeV
New charmed baryons
• SELEX experiment at
FermiLab (E781)
– 600 GeV/c π/Σ beam
– Decay schematic:
– Discoveries:


Ξcc
(3460), Ξcc
(3520)
A new narrow S=+1 state
• Predictions:
– Bag models (Jaffe ’77; De
Swart ’80): 1.8-1.9 GeV
– Skyrme model (Praszalowicz ’87, Walisser ’92):
1.3 – 1.8 GeV
– Chiral-Soliton Model (see
talk by M. Polyakov, ‘97):
E = 1.53 GeV and narrow!
Spring8 data, hep-ex/0301020:
• Discovery:
– n K-K+n: +(1540)
(see talk by T. Nakano)
 (1520)
0
θ  (1540)
Pentaquark observations?
JLab
ITEP
 See talk
D. Tedeschi
ELSA
HERMES
 See talk
A. Airapetian
Why has it taken so long?
• Experimentally:
– Lack of particle ID (RICH)
– Large combinatorial backgrounds at hadronic beams
– Lack of luminosity at photon
and electron beams
• Theoretically:
– Nobody realized that the
+(1540) would be narrow…
– Is the - - (1700) also narrow?
Hadron spin structure
• Possible carriers of
proton spin:
 quark-antiquarks
 gluons
 orbital momentum
• Mathematically:
½ = ½ Sq + DG + Lq
~ 25%
>0?
???
Even they were puzzled…
N. Bohr
W. Pauli

… but this is purely classic!
How to probe the quark polarization?
Polarized
deep
inelastic
electron
scattering
Parallel electron & quark spins
Anti-parallel electron & quark spins
• Measure yield asymmetry:
1 N   N 
A1 
DPT PB N   N 
• In the Quark-Parton Model:
A1 
g1 ( x)
1
2

e

f Dq f ( x )
F1 ( x) F1 ( x) f
Spin-dependent Structure Function
Spin-dependent structure functions
• NLO QCD analysis of all existing
Excellent data for x > 0.01
g1 ( x) data:
Polarized Parton Densities
• First moments:
– input scale
Q02  4.0 GeV 2
– pol. singlet density:
DSq  0.167  0.169 (stat)
 0.133 (exp)  0.070 (th)
– pol. gluon density:
DG  0.616  0.388 (stat)
 0.175 (exp)  0.424 (th)
There must be other sources of angular momentum in the proton
Flavour decomposition of spin
• Semi-inclusive deep
inelastic lepton scattering
• Hadron tags flavour of
struck quark
• Derive purity of tag from
unpolarized data
Key issue: role of sea quarks in nucleon spin
 See talk
B. Maiheu
Sea quark polarization
• Up and down quarks are
oppositely polarized
• Sea quarks: unpolarized...
• First data on xDu  Dd  :
(HERMES, hep-ex/0307064)
Chiral Quark Soliton Model
Gluon polarization
• Photon-gluon fusion:
– COMPASS (see F. Kunne):
• Open charm production:
D 0 ( D 0 )  K   ( K   )
D *  D 0 s
• High pT –pairs (> 1 GeV)
– E161: D or C  
• Prompt photons (RHIC):
 or 
q g  q
photon
 or 
 g  cc
The COMPASS experiment
Beam: 160 GeV µ+
2 . 108 µ/spill (4.8s/16.2s)
Muon filter 2
MWPCs
ECal2 & Hcal2
~50m
SM2
Muon
filter 1
ECal1 & Hcal1
RICH
GEM &
MWPCs
SciFi
SM1
GEM & MWPCs
Silicon
SciFi
Scintillating
fibers
GEM & Straws
Micromegas &Drift chambers
Polarized
target
Polarization:
• Beam: ~80%
• Target:<50%>
First COMPASS results
π
μ,e
K
• Data being collected for:
–
–
–
–
Vector Meson production
Long. Polarized  DG
Trans. Polarized  h1(x)
Hyperon production
Polarized Protons at RHIC
Absolute Polarimeter (H jet)
RHIC pC CNI Polarimeters
BRAHMS
PHOBOS
RHIC
s = 50 - 500 GeV
PHENIX
STAR
Siberian Snakes
Spin Rotators



y








y
y








y
y




y




y
1
0
0
1
0
E
x
p
e
r
i
m
e
n
td
a
t
a
(
1
9
9
7
)
S
i
m
u
l
a
t
i
o
n
(
1
9
9
7
)
9
0
Partial Solenoid Snake
Pol. Source
500 A, 300 s
Partial Helical Snake
AGS
8
0
AGS pC CNI Polarimeter
AGS Quasi-Elastic Polarimeter
200 MeV Polarimeter
Rf Dipoles
E
x
p
e
r
i
m
e
n
td
a
t
a
(
2
0
0
2
)
S
i
m
u
l
a
t
i
o
n
(
2
0
0
2
)
7
0
 Vertical Polarization 
LINAC
BOOSTER
9
0
E
x
p
e
r
i
m
e
n
td
a
t
a
(
2
0
0
0
)
S
i
m
u
l
a
t
i
o
n
(
2
0
0
0
)
S
i
m
u
l
a
t
i
o
n
(
2
0
0
3
)
8
0
7
0
6
0
6
0
5
0
5
0
4
0
4
0
3
0
3
0
2
0
2
0
1
0
1
0
0
5
1
0
1
5
2
0
2
5
3
0
G

3
5
4
0
4
5
0
5
0
First RHIC results
• Measure forward 0 production at STAR:
• Single spin-asymmetry in
p  p   X

0
• Relevance: transverse spin
• Red curve: Collins effect
(~ transversity)
• Blue curve: Sivers effect
(~ p dependence of PDF)
• Green curve: Twist-3
Transverse spin
• Leading order quark distributions:
momentum carried by quarks
longitudinal quark spin, DS
lattice QCD: DS  . 
transverse quark spin, dS
lattice QCD: dS  .5 9
• Helicity conservation: gluons do not contribute to h1(x)
• Novel testable QCD predictions:
– Large value of the nucleon tensor charge dS
– Very weak Q2 evolution of h1(x)
Measuring transverse asymmetries
• Semi-inclusive DIS
with a transversely
polarized H target:
Transverse Target Magnet at HERMES
• Evaluate the azimuthal
asymmetry wrt Starget:
1 N h ( , s )  N h ( , s )
A ( , s ) 
PT N h ( , s )  N h ( , s )
h
UT
First data on transversity
 See talk
N. Bianchi
“Collins”:
Ph
Mh
sin(   s ) ~ h1 ( x)  H1(1) ( z )
“Sivers”:
Ph
Mh
sin(   s ) ~ f1T(1) ( x)  D1 ( z )
Quark-Gluon Correlations
• Four independent Generalized Parton Distributions:
~
~
H ( x,  , t ), H ( x,  , t ), E ( x,  , t ), and E ( x,  , t )
• GPDs enter description of many different processes:
GPDs
DVCS handbag diagram
 See talks by K. Goeke, M. Garcon, ...
The remarkable properties of GPDs
• Integrate GPDs over x:
1
1
~
dx
H
 ( x, , t )  GA (t )
 dxH ( x, , t )  F1 (t ),
1
-1
1
1
~
 dxE ( x, , t )  GP (t )
 dxE ( x, , t )  F (t )
2
1
-1
• Second moment (X. Ji – 1997):
1
 x H ( x, , t )  E ( x, , t ) dx  A(t )  B(t )
1
And take the limit t  0: J  A (0)  B
1
•
q,g
2
q, g
q,g
(0)

GPDs give access to Orbital Angular Momentum of Quarks
A 3D view of quarks in the proton
A.V. Belitsky et al, NP A711 (2002) 118c
A.V. Belitsky et al, hep-ph/0307383
x=0.01
x=0.4
z
z
r
r
r
r
Experimental access to GPDs
• Exclusive meson electroproduction:
– Vector mesons (0): H ( x,  , t ) and E ( x,  , t )
– Pseudoscalar mesons (): H~ ( x,  , t ) and E~ ( x,  , t )
• Deeply virtual Compton scattering:
– Beam charge asymmetry:
– Beam spin asymmetry:
– Longitudinal target spin asymmetry:
Key
differences
Selected DVCS results
• Azimuthal dependence
beam-spin asymmetry:
1 N  ( )  N  ( )
ALU ( ) 
PT N  ( )  N  ( )
• Beam-charge and target
spin asymmetries……..
Quark-Gluon Propagation
• Energy loss mechanisms:
– hadron-nucleon rescattering
DEhadron    A1/ 3
– quark-gluon propagation
DE parton   2  A2 / 3
(QCD: LPM effect)
• Relevance:
– Verification novel QCD effect
– Study of Quark-Gluon Plasma
in relativ. heavy-ion collisions.
Parton energy loss in DIS
• Hadron attenuation in 14N and 84Kr:
Data: see P. Di Nezza
Search for quark-gluon plasma
Dashed: see X. Wang
[incl. LPM effect + tune g(x)]
Solid: see A. Accardi
[incl. Q2 rescaling effects]
Energy loss in hot matter
• 0 production in Au + Au
collisions at Phenix:
• Adjust energy loss to fit
data (cf. cold matter)
 g hot matter  g cold matter 
Prospects: short-term future
• The origin of proton spin:
– Gluon polarization, transversity and orbital motion
• First measurements of DG and h1(x)
Prospects: mid-term future
Search for glueballs (ggg)
and hybrid particles (qqg)
(US: JLab 12 GeV upgrade)
Prospects: long-term future
• Precise verification of novel QCD predictions requires
high-energy (> 50 GeV) & high-luminosity (> 1033 /s)
• Proposed new lepton scattering facilities in the US:
ELIC @ JLab with e-A coll
at 4 x 65 GeV2 & 1035 cm2/s
EIC @ BNL
e-p coll at 10 x 250 GeV2 &1033 cm2/s
Outlook
• QCD hadron physics in 2003:
–
–
–
–
–
discovery pentaquarks
first data on transverse spin
exploration of GPDs
quadratic parton energy loss?
…………..
• The future:
– many new data in coming years
– new facilities under design
Santorini ‘03 changes
our perspective !
SPARE SLIDES FOLLOW …….
Orbital angular momentum
• The origin of proton spin:
½ = ½ Sq + DG + Lq
Flavour Decomposition: 0.3
High pT pairs: 1.0
Orb. ang. mom.: -0.65 ?
• A new idea: azimuthal asymmetry in 0 production
Ju = S u + L u
Gluon Polarization at RHIC
• Longitudinal double spin asymmetry in p+p:
d  d
ALL 
d  d
• Dominant processes:
 or 
 or 
photon
 or 
Direct photon production
(heavy flavor)
 or 
Di-jet production
Dominance of qg  q
• LO QCD calculations for 2 jet production (left) and
direct photon production (right)
Estimate of the asymmetries
• Direct photoproduction:
ALL ( pT ) 
Dg ( xg )
g ( xg )
 A1 ( xq )  aˆ LL
p
From inclusive DIS data:
A1p is large for x > 0.2
QCD: approaches unity
if  is emitted forward
• Two jet-production:
(Complication: disentangle contribution
from various partonic processes)
Sensitivity to Gluon Polarization
• First evaluation of first moment of DG(x), i.e. .
• Other relevant channels: inclusive , inclusive jets, J/y
production, heavy flavor production, etc.
Anticipated improvement in xDG(x)
• Present QCD
analysis
• With new RHIC
data from STAR
M. Hirai, H.Kobayashi, M. Miyama et al.- preliminary
Selected DVCS results
• Azimuthal dependence
beam-spin asymmetry:
• Target-spin asymmetry:
• Nuclear dependences
Beam spin asymmetry for +
• Measure BSA for semi-inclusive + production in DIS:
• BSA ~ interference between L and T parts of X-section:
sin
LU
A
H
'
LT
Electron-Light Ion Collider
Ion Source
Snake
IR Solenoid
IR
5 GeV electrons
Snake
50-100 GeV light ions
Injector
CEBAF with Energy Recovery
Beam Dump
s  20  45 GeV
L  1034-35 cm2s -1
Polarized beams
RHIC at BNL