Transcript QCD and the origin of proton

The HERMES experiment
Gerard van der Steenhoven, 19 September 2004
Search the carriers of proton spin
• Three possible sources:
– quarks:
 valence quarks
 sea quarks (qq )
– gluons
– orbital momentum
• Mathematically:
½ = ½ Sq + DG + Lq
~ 10%
?
EMC (85): Sq ~ 10%
?
The experimental strategy
• Polarization of the sea quarks
• Polarization of the gluons
• Orbital angular
momentum
• Transversity:
“switch off
the gluons”
Outline of the lecture
1. The origin of proton spin?
– Polarization of quarks
– Gluon contributions
2. New developments
– Generalized parton distributions → DVCS → Lq,g
– Transverse spin → switch off the gluons
3. Other surprises
– The HERMES pentaquark signal
– Parton energy loss in nuclei
4. Outlook
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
Why HERMES?
• Original purpose (~1990):
– Measure inclusive spin structure
functions g1(x) for proton & neutron
– Measure polarization of u-, d- and
sea-quarks separately: Dqu,d,sea(x)
gluon
• What came out sofar (~2004)?
– Precise data on g1n,p(x), Dqu,d,sea(x)
– First measurements of DG(x), DVCS,
transversity, parton energy loss,…
Quark-antiquark pair
→ design a reliable multi-purpose detector system !
The HERMES experiment
EM Calorimeter
Magnet
RICH
TRD
Target area
Beam Loss Monitor
Lambda Wheels
The HERMES spectrometer
27.6
GeV
e+/e-
0.02 < x < 0.6, 1.0 < Q2 < 15 GeV2
p/p ~ 1-2%,  < 0.6 mrad
Data taking since 1996
1996-2000
2002 - 2004
Spin-dependent structure functions
• The function g1(x):
• Evaluate the integrals:
1p   g1p ( x)dx  16 F  181 D  19 S q


From hyperon decays
Total spin carried by quarks
• 1999 result:
Sq  0.2  0.1
2
Q
dependence of F2(x) and g1(x)
F2 ( x, Q 2 )
x

2
2
+
+
→ Gluons contribute to the nucleon spin !
QCD analysis of world data (’03)
• Next-to-Leading-Order analysis of g1 ( x) -data
Excellent data for x > 0.01
Polarized Parton Densities
• First moments:
– input scale
Q02  4.0 GeV2
– 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 scattering
• Hadron tags flavour of
struck quark
• Derive purity of tag from
unpolarized data
Key issue: role of sea quarks in nucleon spin
Particle identification
• Dual radiator RICH:
Aerogel
p
Detection efficiencies
K
P
C4F10
p
24.08.2004
K
Flavour decomposition: results
• The method:
Polarized
Parton
Distribution
Functions !
A1h ( x,Q2 ) 
2
2
h
2
e
D
q
(
x,Q
)
dzD
(
z,Q
)
f f f
 f
2
2
h
2
e
q
(
x,Q
)
dzD
(
z,Q
)
f f f
 f
Hadron
asymmetries
(measured)
Known quantities
(from other data)
• Conclusion: Dqsea  0
Flavour symmetry breaking
Unpolarized data:
Polarized data:
Strong breaking of
flavour symmetry
No significant breaking
of flavour symmetry.
Gluon polarization
• Photoproduction of high
pT –hadron pairs
→
• Contributing diagrams:
• Corresponding asymmetries:
A
VMD
||
0
A
DIS
||
Δq

q
A
QCDC
||
Δq PGF ΔG
A|| 

G
q
Data and plans for DG/G
• Asymmetry for high-pT hadron pairs production:
±0.18±0.03
SMC : ΔG/G  0.20  0.28  0.10
• New high-precision data →
Generalized Parton Distributions
• Consider exclusive processes:
– Deeply virtual Compton scatt.
– Exclusive vector meson prod.

initial
quark
 
final
quark
p0, r0L, g ...
t
x+
x-
N’
N
• Collins et al. proved factorization theorem (1997):
2
 excl. prod . 
 
*
m
( , z ) c mf ( x,  , Q 2 ) H fp ( x,  , t ) d
f
Distribution amplitude
(meson) final state
Hard scattering
coefficient (QCD)
Generalized Parton
Distribution (GPD)
(NASTY: x = xBj for quarks and x = -xBj for antiquarks → x  [-1,1])
The remarkable properties of GPDs
• Integration over x gives Proton Form Factors:
1
1
~
dx
H
 ( x,  , t )  GA (t )
 dxH ( x,  , t )  F1 (t ),
Dirac
1
-1
1
 dxE ( x,  , t )  F2 (t )
1
• The forward limit:
Axial vector
1
~
dx
E
 ( x,  , t )  GP (t )
Pauli
-1
Pseudoscalar
~
, 0
H q ( x,  , t )  q( x); H q ( x,  , t ) t
 Dq( x)
t , 0
• Second moment (X. Ji, PRL 1997):
1
1
2


t 0
1
x
H
(
x
,

,
t
)

E
(
x
,

,
t
)
dx



J

q
q
2 S q  Lq
 q
1
GPDs give access to Orbital Angular Momentum of Quarks
Applying the GPD framework
• GPDs enter description of different processes:
As Jq = ½Sq + Lq
information on Jq
gives data on Lq.
• Take Fourier transform of leading GPD:
q ( x, b )  2p 2  H ( x,  ,t )e
f

1
f
 ib t
dt
Spatial distribution of quarks in the perpendicular direction
A 3D-view of partons in the proton
Form Factor
Parton Density
Gen. Parton Distribution
A.V. Belitsky, D. Muller, NP A711 (2002) 118c
Experimental access to GPDs
• Exclusive meson electroproduction:
– Vector mesons (r0): H ( x,  , t ) and E ( x,  , t )
~
~
– Pseudoscalar mesons (p): H
( x,  , t ) and E ( x,  , t )
• Deeply virtual Compton scattering (DVCS):
DVCS
Bethe-Heitler
Experimental access to DVCS
• DVCS observables:
– Cross section:
dσ  |τ BH|2 |τ DVCS|2  (τ*BH τ DVCS  τ*DVCS τ BH )
– Beam charge asymmetry:
– Beam spin asymmetry:
– Longitudinal target spin asymmetry:
Key
differences
Beam charge
asymmetry
First DVCS results
Beam spin asymmetry
What is transversity?
• Three leading order quark distributions:
momentum carried by quarks
longitudinal quark spin, DS
transverse quark spin, S
• Gluons don’t contribute to h1(x), while dominant in g1(x):
 Study nucleon spin while switching off the gluons
• New QCD tests: Q2 evolution h1(x) & S > DS (lattice)
Measuring transversity
• The relevant diagram:
– helicity flip of quark & target
– chirally odd process
• Consequences:
quark flip
target flip
+
+
-
D  2
– no gluon contributions….
D  1
… & measure single-spin asymmetries:


N
(

,

)

N
1
h
h
s
h ( , s )
AUT ( , s ) 
PT N h ( , s )  N h ( , s )
Single – Spin Asymmetries
• Sivers effect:
AUT driven by
orbital motion
struck quark:
measure L
• Collins effect:
AUT driven by
fragmentation
process: measure
transversity
First data on transversity
‘Collins’:
Pp 
zM p
sin(  s ) ~ h1 ( x)  H
 (1)
1
( z)
‘Sivers’:
Pp 
zM p
sin(  s ) ~ f1T(1) ( x)  D1 ( z )
First evidence for non-zero Collins (h1) and Sivers effects (Lq)
HERMES, hep-ex/0408013
Parton Energy Loss
• Energy loss mechanisms:
– hadron-nucleon rescattering
DEhadron    A1/ 3
– quark-gluon propagation
DEparton  2  A2 / 3
(QCD: LPM effect)
• Relevance:
– Verification novel QCD effect
– Study of Quark-Gluon Plasma
in relativ. heavy-ion collisions.
DIS on heavy nuclei
• Hadron attenuation in 14N, 84Kr:
Data: EPJC 21 (2001) 599
Search for quark-gluon plasma
Dashed: X. Wang et al. (2002)
[QCD + LPM effect + tune g(x)]
Solid: Accardi et al. (2003)
[pN incl. Q2 rescaling effects]
Energy loss in hot matter
• p0 production in Au + Au
collisions at Phenix:
• Adjust energy loss to fit
data (cf. cold matter)
 ghot matter >> gcold matter 
New data on hadron attenuation
• Cronin effect:
– enhancement at
high pT2 (rescatt.)
• Attenuation for p0:
Search for quark-gluon plasma
Two-hadron attenuation
• Evaluate:
R2 h

(z ) 

2


d 2 N ( z1 , z 2 )
dN ( z1 )
A
2
d N ( z1 , z 2 )
dN ( z1 )
D
• Partonic energy loss:
R2h →1
• Hadronic energy loss:
R2h ~ (R1h)2  0.5(Kr) - 0.8(N)
Both partonic and hadronic energy loss processes are relevant
The HERMES pentaquark signal
• Quasi-real photoproduction: e+D   X
• Decay mode:
 p K0s  p pp
27.6 GeV e-beam
deuteron
gas target
Invariant
mass from
identified
decay
particles
+

Invariant mass peak
• Background: 3rd
order polynomial
• M =1528  2.6 MeV
•  = 8  2 MeV
(dominated by exp.
resolution)
• Significance:
2
 4.7
– naïve: N sig2 / N bck
– realistic: Nsig / Nsig  3.7
Gauss + 3rd
order polynom.
Background below the
• Background: MC simulation + resonances
• M = 1527  2.3 MeV
•  = 9.2  2 MeV
• Significance:
– naïve: 6.1 
– Realistic: 4.3 
Mixed event background
Pythia6 background
additional S*+ resonances (not in Pythia6)
+

Gaussian +
resonances +
background fit
Comparison of pentaquark data
nK+
• Mean: 1532.52.4
MeV
pK s0
Average of all data:
M = 1532.5  2.4 MeV
Includes latest from
- JINR (hep-ex/0403044)
- LPI (hep-ex/0404003)
Latest HERMES results on
+

• Require additional p in + mass spectrum
• Impose veto on K* and (1116)
Signal/background improves from 1:3

2 :1
Summary
• HERMES results:
– Quark sea → unpolarized
– Gluons → polarized // proton
– First data on transversity:
Quarks carry orbital momentum?
– First exploration of GPDs
– Partonic energy loss seen
– Co-discovery pentaquarks
• The future:
– End of HERA operations:
summer of 2007