Transcript Eun

Transverse Spin Physics at STAR
Len K. Eun
For
STAR Collaboration
26th Winter Workshop on Nuclear Dynamics
Ochos Rios, Jamaica, January 2010
2
STAR
Transverse Spin Physics at STAR
Large Transverse Single Spin
Asymmetry(SSA) in forward
meson production persists up to
RHIC energy.
• Collins effect: asymmetry comes from the transversity
and the spin dependence of jet fragmentation.
SP
p
No asymmetry for the jet axis
PRL 92, 171801 (2004)
p
Sq
kT,π
• Sivers effect: asymmetry comes from spin-correlated
kT in the initial parton distribution
SP
kT,q
Di-jet, photon-jet not exactly back to back
p
p
√s=200 GeV, <η> = 3.8
Photons have asymmetry
Jet vs. Photon sign flip predicted
3
STAR
STAR Forward Pion Detector (FPD)
Run6 Configuration
• STAR forward calorimeters have gone through
significant upgrades since run3.
• In run6, the original FPD remained in the east,
while the west FPD was expanded to FPD++.
• The east FPD consists of two 7X7 Pb-glass
modules, EN and ES. During run6, it was placed at
the “far” position. (x-offset~30cm, <h>~3.7)
4
STAR
Forward 0 Single Spin Asymmetry(SSA)
At √s=200GeV, 0 cross-section measured by STAR FPD is
consistent with the NLO pQCD calculation. Results at <h>=3.3
and <h>=3.8 have been included in the DSS global pion
fragmentation function analysis. (Phys.Rev.D75(2007) 114010)








d

d
1
N
S

S
N
A


N






d

d P
N
S

S
N
Phys.Rev.Lett.101:222001,2008
Black Points: Phys.Rev.Lett.101:222001,2008
4
From Spin2008 talk by J.Drachenberg,
arXiv:0901.2763
5
STAR
pT Dependence of AN
For Fixed XF, the asymmetry AN does not fall with pT as predicted by models
and perhaps expected on very general grounds.
 NLO PQCD does describe
the size and shape of this
forward 0 cross section.
 Model calculations (Sivers,
Collins or twist-3) can explain
the XF dependence of AN.
X Flat or increasing dependence
of AN on pT is very difficult
to understand within any of
these frameworks!
Phys.Rev.Lett.101:222001,2008
STAR
Previous Observation of Transverse SSA Forward
Production of Eta Meson by FNAL Exp 704
1) Nominally (perhaps not significantly) larger asymmetry for Eta than Pi0.
2) Large Uncertainty in Eta AN.
p  p  M X
p  p  M X
proton & proton
s  19.4GeV
d   d 
AN 
d   d  
proton & anti-proton
pT ~ 1GeV / c
6
7
STAR
Run6 FPD Acceptance for 0 and Eta
Fast Simulator
The ratio of N(reconstructed particles) to
N(generated particles with CoM within FPD)
m  Etot 1  ( Z  ) Sin
Separation(FPD cell)
2

z  
2
E 1  E 2
E 1  E 2
Eta
8
6
4
2
0
0.8
0.6
0.4
20
40
0.2
60
80
• 7x7 FPD has limited acceptance for Eta mesons. At 40GeV, a
symmetrically decaying Eta needs to point to the center of the
FPD to fit in. Acceptance improves greatly at higher energy.
• 0 reconstruction efficiency starts to drop over 60GeV, where
the separation between two photons for symmetric decay
becomes on average less than 1 cell width.
0
Separation(FPD cell)
100
0
8
6
4
2
0
0.8
0.6
0.4
20
40
0.2
60
80
100
0
8
STAR
Event Selection for Run6 FPD Eta Analysis
Di-Photon Center of Mass (EN&ES)
h pion  3.691
0 mass region with Center Cut in black
Event Cuts
• 2 photon events
• Etotal>25GeV
• Hardware threshold nominally at 25GeV
• “Center Cut” for 2 CoM defined as
(h  3.65) 2  Tan( ) 2  (0.15) 2
heta  3.664
h mass region with Center Cut in red
m  Etot 1  ( Z  ) 2 Sin

2
• Etot: Detector summed energy
• Z and photon separation: Fitted photon
energy/locations
• Reconstructs on the entire FPD
• Vertex set at zero for all events
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STAR
Eta Signal in Run6 FPD
Di-Photon Invariant Mass Spectra in 3 Energy Bins
• Center Cut
• 3 columns for 3
energy bins
• Each column
shows a single plot
in log and linear
scale.
0 Mass Cut
.085 GeV  M   .185 GeV
Eta Mass Cut
.48 GeV  M   .62 GeV
AN(xF) will be reported for di-photon events in these two shaded mass regions. We will
not separate contributions from backgrounds under the Eta and 0 peaks.
10
STAR
Mass Dependence of AN
p  p  M X
M   
s  200 GeV
STAR 2006 PRELIMINARY
1.
2.
3.
4.
Nphoton = 2
Etotal > 40GeV
No Center Cut
Average Beam
Polarization = 56%
• Forward asymmetry clearly
reveals the shape of two mass
resonances.
• There is an “asymmetry
valley” in between 0 and Eta
mass regions.
11
STAR
AN(xF) in 0 and Eta Mass Regions
p  p  M X
M   
s  200 GeV
1.
2.
3.
4.
Nphoton = 2
Center Cut (h and )
Pi0 or Eta mass cuts
Average Beam
Polarization = 56%
.55  X F  .75
AN
h
AN

 0.361  0.064
 0.078  0.018
For .55  X F  .75 , the
asymmetry in the Eta mass
region is greater than 5 sigma
above zero, and about 4 sigma
above the asymmetry in the 0
mass region.
12
STAR
Should AN be larger for Eta than π0 ?
• Gluons or η has Isospin I=0.
• u quark has Isospin I=1/2
• π0 has Isospin I=1.
• But we expect both mesons to come
from fragmentation of quark jets.
I 0
I 1

𝜂=
1
𝜂′ =
𝜋
0
=
𝑢𝑢 + 𝑑𝑑 − 𝑠𝑠
3
1
6
1
𝑢𝑢 + 𝑑𝑑 + 2𝑠𝑠
2
*Assume h ,h ' mixing angle:
𝑢𝑢 − 𝑑𝑑
P ~19.5
• For Sivers Effect: Asymmetry is in the jet and should not depend on the
details of fragmentation.
• For Collins Effect: Asymmetry reflects fragmentation of the quark jet into
a leading η or π0 meson. Differences in fragmentation could relate to:
• Mass differences?
• Isospin differences?
• Role of Strangeness?
• But Collins Effect Should be suppressed when Z~1
13
STAR
STAR Forward Meson Spectrometer (FMS)
New for Run 8:
FMS
• Stack of 1264
lead glass cells,
roughly 18 X0 in z.
• Located at far
West side of Hall,
at the opening to
RHIC tunnel.
Faces blue beam.
East FPD
(since runs 3)
• 7.5 meters from
interaction point
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STAR
FMS Greatly Enhances STAR EM Coverage
•Tracking
• EMC Calorimeters
TPD
bEMC -1<η<1
FTPC
eEMC 1<η<2
FMS 2.5<η<4
FPD movable
With installation of FMS, STAR EM
calorimeter coverage spans most
of the pseudo-rapidity region from
-1<h<4.
15
Forward Meson Spectrometer
Lead Glass From FNAL E831
804 cells of 5.8cm5.8cm60cm
Schott F2 lead glass
Small Cell PSU Type
224 of 476
Cockcroft-Walton HV
bases with computer
control through USB.
Designed/built in
house for FEU-84.
Designed and built at Penn State University
QT board
Readout of 1264
channels of FMS
provided by QT boards.
Each board has
• 32 analog inputs
• 5-bit TDC / channel
• Five FPGA for data and
trigger
• Operates at 9.38 MHz
and higher harmonics
• Produces 32 bits for
each RHIC crossing for
trigger
• 12-bit ADC / channel
Designed and built at UC Berkeley/SSL
15
16
STAR
Preliminary Run8 FMS π0 AN
Azimuthal Angle Dependence of AN
AN vs. xF
 Consistent with previous measurements
STAR Preliminary
stat.errors only
Estimate tot. ≤ 1.2 stat.
From SPIN08 talk by N. Poljack, arXiv:0901.2828
17
STAR
pT-dependence of π0 AN
• Large solid angle of FMS allows
simultaneous mapping of xF vs pT with
greater statistics
Run3+5+6
Run8
F.o.M. was smaller in run8 than in run6
 More statistics needed
Black: Phys.Rev.Lett.101:222001,2008
Blue: From Spin2008 talk by J.Drachenberg,
arXiv:0901.2763
18
STAR
Sivers Effect with Mid-Rapidity Di-Jets
spin
• The Sivers effect predicts spin dependent left/right
bias in kT due to parton orbital angular momentum.
1
open  180
kTx
di-jet
bisector
2
• For di-jets at mid-rapidity, this initial state kT
asymmetry leads to a spin dependent shift in the
azimuthal opening angle for the jets.
• In contrast, di-jet measurements are insensitive to
the Collins effect, which produces an asymmetry
within a jet.
19
STAR
Sivers Effect with Mid-Rapidity Di-Jets
PRL 99 (2007) 142003
• The observed AN is an order of magnitude smaller than what was seen in SIDIS by
HERMES
• Might be due to cancellations between initial state and final state, u and d quark,
and/or very small gluon Sivers effect.
20
STAR
Summary
1. The STAR Forward Pion Detectors (FPD) at RHIC measured cross-section for 0 meson in
<h>=3.3~4.0 region during √s=200GeV p+p collision. It was found to be consistent with
pQCD calcuations.
2. From RHIC run3 to run8, the FPD measured large forward single spin asymmetry, AN, for
0. The xF dependence of AN was qualitatively consistent with theoretical predictions. pT
dependence, however, differed significantly from predictions based on all currently existing
models
3. In addition to 0, Eta mesons were observed in the east FPD during RHIC run6. We
measured the single spin asymmetry in the 0 and the Eta mass regions, at <h>~3.65 and
xF above 0.4. We found the AN in Eta mass region to be ~4 standard deviation greater than
the AN in 0 mass region from 55GeV to 75GeV. (xF=0.55~0.75)
4. Forward Meson Spectrometer (FMS), commissioned in RHIC run8, greatly enhances EM
coverage of STAR. The preliminary results from run8 show that 0 AN(xF) is consistent with
previous measurement, while the azimuthal angle dependence of AN is as expected. With
additional transverse running, we can also significantly improve our measurement of AN vs.
pT utilizing much improved acceptance of the FMS
Len Eun
STAR
Back Up
STAR
Pattern from Previous Transverse SSA Measurements of
Forward Pion / Eta Production with High Energy Polarized
Proton / Antiproton Beams
1. Majority valence quark
in polarized proton.
• u for proton
• u for antiproton.
2. Minority valence quark
• d for proton
• d for antiproton.
1. Pion containing only
majority quarks: large
positive AN.
2. Pion containing only
minority quarks: large
negative AN.
3. Pion containing both
majority and minority
quarks: intermediate
positive AN.
6
9
STAR
Eta and 0 Energy Sharing (Z) Distribution
z  
E 1  E 2
E 1  E 2
0 mass region with Center Cut
Nevents
Eta mass region with Center Cut
Z
Z
7
STAR
Mass Dependence of AN
p  p  M X
M   
s  200 GeV
STAR 2006 PRELIMINARY
1.
2.
3.
4.
5.
Nphoton = 2
No energy cut
With Center Cut
Z < 0.85
Average Yellow Beam
Polarization = 56%
• Yellow beam asymmetry
clearly reveals the shape of two
mass resonances.
• There is an “asymmetry
valley” in between 0 and Eta
mass regions.
8
STAR
AN(xF) in 0 and Eta Mass Regions
p  p  M X
M   
s  200 GeV
1.
2.
3.
4.
5.
Nphoton = 2
Center Cut (h and )
Pi0 or Eta mass cuts
Z < 0.85
Average Yellow Beam
Polarization = 56%
.55  X F  .75
AN
h
AN
0
 0.36  0.065
 0.078  0.018
For .55  X F  .75 , the
asymmetry in the Eta mass
region is greater than 5 sigma
above zero, and about 4 sigma
above the asymmetry in the 0
mass region.
20
STAR
pT-dependence of π0 AN for Negative xF
Negative xF consistent with zero
Run8 FPD east result : arXiv:0901.2763 (James Drachenberg– SPIN08)