Transcript Proton spin structure from longitudinally polarized pp

Proton spin structure from
longitudinally polarized pp
collisions from PHENIXat RHIC
Alexander Bazilevsky
BNL
The 6th Circum-Pan-Pacific Symposium on
High Energy Spin Physics
July 30 – August 2, 2007
Vancouver BC, Canada
Nucleon Spin Structure
Naïve parton model:
1 1
 uv  d v 
2 2
 Gluons are polarized (G)
 Sea quarks are polarized:
1989 EMC (CERN):
=0.120.090.14
1 1
 q  q   G
2 2
  u  d  s  u  d  s
 Spin Crisis
For complete description
include parton orbital
angular momentum LZ:
1 1
 q  q   G  LZ
2 2
Determination of G and q-bar is the main goal of
longitudinal spin program at RHIC
Parton Distribution Functions (PDF)
• Quark Distribution
unpolarised distribution
q(x,Q2)=
=
helicity distribution
q(x,Q2)=
transversity distribution
• Gluon Distributions
dq(x,Q2)=
g(x,Q2)=
g(x,Q2)=
No Transverse Gluon Distribution in 1/2
Polarized PDF from DIS
Asymmetry Analysis Collaboration
M. Hirai, S. Kumano and N. Saito, PRD (2004)
• Valence distributions
well determined
• Sea Distribution
poorly constrained
• Gluon can be either
positive, 0, negative!
… To polarized pp collider
Utilizes strongly interacting probes
 Probes gluon directly
 Higher s  clean pQCD interpretation
 Elegant way to explore guark and antiquark polarizations through W production
Polarized Gluon Distribution Measurements (G(x)):
Use a variety of probes
Access to different gluon momentum fraction x
Different probes – different systematics
Use different energies s
Access to different gluon momentum fraction x
RHIC as polarized proton collider
Absolute Polarimeter
(H jet)
RHIC pC Polarimeters
BRAHMS & PP2PP (p)
Lmax  2 1032 s 1cm 2
70% Polarizati on
PHENIX (p)
50 
s  500 GeV
STAR (p)
Siberian Snakes
Spin Rotators
Partial Siberian Snake
LINAC
BOOSTER
Pol. Proton Source
500 mA, 300 ms
200 MeV Polarimeter
2  1011 Pol. Protons / Bunch
e = 20 p mm mrad
AGS
AGS Internal Polarimeter
Rf Dipoles
PHENIX for Spin
Philosophy (initial design):
 High rate capability & granularity
 Good mass resolution & particle ID
 Sacrifice acceptance
p0/g/h
Electromagnetic Calorimeter
p/p
Drift Chamber
Ring Imaging Cherenkov Counter
J/y
Muon Id/Muon Tracker
Relative Luminosity
Beam Beam Counter (BBC)
Zero Degree Calorimeter (ZDC)
Local Polarimetry - ZDC
PHENIX Long. Spin runs
Year
2003 (Run 3)
2004 (Run 4)
s [GeV]
200
200
Recorded L
.35 pb-1
.12 pb-1
Pol [%]
32
45
2005 (Run 5)
2006 (Run 6)
2006 (Run 6)
200
200
62.4
3.4 pb-1
7.5 pb-1
.08 pb-1 **
50
60
48
FOM
(P4L)
3.7 nb-1
4.9 nb-1
200 nb-1
1000 nb-1
4.2 nb-1 **
Unpol. Cross Section in pp
ppp0 X : hep-ex-0704.3599
|h|<0.35
ppg X: PRL 98, 012002
Good agreement between NLO pQCD
calculations and data  confirmation
that pQCD can be used to extract spin
dependent pdf’s from RHIC data.
• Same comparison fails at lower
energies
Probing G in pol. pp collisions
pp  hX
ALL 
d  d



d  d


 f
a ,b
a
 f b  dˆ f a f b  fX  aˆ LLf a f b  fX  D hf
f
a
 f b  dˆ f a f b  fX  D hf
a ,b
Double longitudinal spin
asymmetry ALL is
sensitive to G
Measuring ALL
ALL 
d    d  
1 N    RN 

;
d    d   | P1P2 | N    RN 
R
L 
L 
(N) Yield
(R) Relative Luminosity
 BBC vs ZDC
(P) Polarization
 RHIC Polarimeter (at 12 o’clock)
 Local Polarimeters (SMD&ZDC)
 Bunch spin configuration alternates every 106 ns
 Data for all bunch spin configurations are collected at the same time
 Possibility for false asymmetries are greatly reduced
ALL: p0
PHENIX Preliminary Run6 (s=200 GeV)
5
10
pT(GeV)
GRSV model:
“G = 0”: G(Q2=1GeV2)=0.1
“G = std”: G(Q2=1GeV2)=0.4
Stat. uncertainties are on level to
distinguish “std” and “0” scenarios? …
Run3,4,5: PRL 93, 202002; PRD 73, 091102;
hep-ex-0704.3599
From soft to hard
hep-ex-0704.3599
exponential
fit
Exponent (e-pT) describes our pion
cross section data perfectly well at
pT<1 GeV/c (dominated by soft
physics):
=5.560.02 (GeV/c)-1
2/NDF=6.2/3
Assume that exponent describes soft
physics contribution also at higher
pTs  soft physics contribution at
pT>2 GeV/c is <10%
For G constrain use pi0 ALL data at
pT>2 GeV/c
From pT to xgluon
NLO pQCD: p0 pT=29 GeV/c  xgluon=0.020.3
 GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 )
Each pT bin corresponds to a wide range in xgluon, heavily
overlapping with other pT bins
 These data is not much sensitive to variation of G(xgluon) within
our x range
 Any quantitative analysis should assume some G(xgluon) shape
Log10(xgluon)
From ALL to G (with GRSV)
Calc. by W.Vogelsang and M.Stratmann

“std” scenario, G(Q2=1GeV2)=0.4, is
excluded by data on >3 sigma level:
2(std)2min>9
 Only exp. stat. uncertainties are included
(the effect of syst. uncertainties is
expected to be small in the final results)
 Theoretical uncertainties are not included
Extending x range is crucial!
Gehrmann-Stirling models
GSC:
G(xgluon= 01) = 1
G(xgluon= 0.020.3) ~ 0
GRSV-0: G(xgluon= 01) = 0
G(xgluon= 0.020.3) ~ 0
GRSV-std: G(xgluon= 01) = 0.4
G(xgluon= 0.020.3) ~ 0.25
Current data is sensitive to G for
xgluon= 0.020.3
G: what’s next

Improve exp. (stat.) uncertainties and
move to higher pT


More precise G constrain in probed x range
Probe higher x and constrain G vs x
 Different
channels
 Different systematics
 Different x
 gqgg sensitive to G sign

Different s
 Different x
gq  g g
gg  QQ

q G
q G

G G
G G
Improve exp. uncertainties
Need more FOM=P4 L (stat. uncertainty ~ FOM)
p0: expectations from Run-8
Higher pT measurements  probe higher x  constrain G vs x
Different channels
Need more FOM=P4 L (stat. uncertainty ~ FOM)
 
p  p p   X
 
p  p p   X
 Different sensitivities of charged pions to u and d provide more
sensitivity to sign of G through qg scattering
 Predictions are sensitive to fragmentation functions
Different channels
Need more FOM=P4 L (stat. uncertainty ~ FOM)
 
p  p h  X
 Complementary to p0 measurements
 Need h fragmentation functions
 
p  p  J /y  X
Probe G with heavy quarks
Open charm will come soon
Need more theoretical input
pp  g + jet
PHENIX Projection
 Theoretically clean (no fragmentation at LO)
 Gluon Compton dominates  sensitive to sign of G
 Requires substantial FOM=P4 L
Different s
s=62 GeV p0 cross section
described by NLO pQCD within
theoretical uncertainties
Sensitivity of Run6 s=62 GeV
data collected in one week is
comparable to Run5 s=200
GeV data collected in two
months, for the same xT=2pT/s
s=500 GeV will give access
to lower x; starts in 2009
Flavor decomposition
d  u  W 
u  d  W 
d  u  W 
u  d  W 
Measured through longitudinal single
spin asymmetry AL in W production at
s=500 GeV
First data expected in 2009-2010
Other measurements
Helicity correlated kT
from PHENIX
May be sensitive to orbital
angular momentum
PHENIX Upgrades
See talk by I.Nakagawa
Silicon Tracking
VTX (barrel) by 2009
FVTX (forward) by 2011
Electromagnetic Calorimetry
NCC by 2011
MPC, already installed!
Muon trigger upgrade
By 2009
Momentum selectivity in the LVL-1
trigger
G from heavy flavor, photon-tagged jets
Expanded reach in x
Flavor separation of spin asymmetries
W physics at 500GeV
Transverse Spin Physics (see talk by M.Liu)
rapidity
Summary

RHIC is the world’s first and the only facility which provides collisions of
high energy polarized protons
 Allows to directly use strongly interacting probes (parton collisions)
 High s  NLO pQCD is applicable

Inclusive p0 accumulated data for ALL has reached high statistical
significance to constrain G in the limited x range (~0.020.3)
 G is consistent with zero
 Theoretical uncertainties might be significant

Extending x coverage is crucial
 Other channels from high luminosity and polarization
 Different s

PHENIX upgrades strengthen its capability in nucleon spin structure study
 Larger x-range and new channels (e.g. heavy flavor)
 W measurements for flavor decomposition
Polarimetry
Utilizes small angle elastic scattering in the
coulomb-nuclear interference (CNI) region

Fast relative polarization measurements with
proton-Carbon polarimeter
Single measurement for a few seconds

(Relatively) slow absolute polarization
measurements with polarized atomic
hydrogen jet target polarimeter
p
Pbeam  
eN 
p or C12
eN
ANpC
NL  NR
NL  NR
e beam  AN  Pbeam

 Pbeam   e beam  Ptarget
e target   AN  Ptarget
e target
Used to normalize pC measurements
Beam polarization in Run6: P ~ 60-65%
Polarization measurements in Run5:
P/P~6% and (PBPY)/(PBPY)~9%
Backup: Rel. Lum. In PHENIX
Year
[GeV]
dR
dALL
2005 *
200
1.0e-4
2.3e-4
2006 *
200
3.9e-4
5.4e-4
2006 *
62.4
1.3e-3
2.8e-3
Backup: s=62 vs 200 GeV
Partonic Orbital Angular Momentum
: Beam momenta
• Partonic orbital angular
momentum leads to
rotation of partons
Jet 1
correlated with the
proton spin vector
• This leads to different pT
imbalances (pT-kicks) of
jet pairs in semiclassical
models
• Can be measured by
measuring helicity
Jet 1
dependence of <kT2>
Jet 2 w/<kT>=0
Like helicities
Peripheral Collisions
Jet 2
Larger
kT2
Integrate over b, left
with some residual kT
Net pT kick
Jet 2 w/<kT>=0
Jet 2
Central Collisions
Smaller
kT2
Partonic Orbital Angular Momentum
: Beam momenta
• Partonic orbital angular
momentum leads to
rotation of partons
Jet 1
correlated with the
proton spin vector
• This leads to different pT
imbalances (pT-kicks) of
jet pairs in semiclassical
models
• Can be measured by
measuring helicity
Jet 1
dependence of <kT2>
Jet 2 w/<kT>=0
Jet 2
Unlike helicities
Peripheral Collisions
Smaller
kT2
Integrate over b, left
with different residual kT
Net pT kick
Jet 2 w/<kT>=0
Jet 2
Central Collisions
Larger
kT2
Backup: W
W production
» Produced in parity violating V-A process
— Chirality / helicity of quarks defined
» Couples to weak charge
— Flavor almost fixed
A
W  m  m
L

u(xa )d(xb )  d(xa )u(xb )
u(xa )d(xb )  d(xa )u(xb )
xa>>xb: AL(W+) → u/u(x)
- +
xb>>xa: AL(W ) → d/d(x)
Backup: SIDIS for G
HERMES preliminary
Backup: G
From M. Stratmann
Backup: from G to ALL
By Marco&Werner
GRSV: G(Q2=1GeV2)= 1.76  +1.89