Recent Spin and FMS Results at STAR

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Recent Spin and FMS Results at STAR
STAR
Andrew Gordon Brookhaven National
Laboratory Moriond-QCD March 14-21, 2009
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Colliding-Beam Spin Physics Only at RHIC
1) Longitudinal spin program at STAR
Inclusive Jets
?s200 GeV
Strong constraint on the size of ?g from RHIC
data for 0.05ltxlt0.2.
STAR data contributes strongly to global fits as
in D. deFlorian et al., PRL 101 072001, 2008.
Run 9 currently running polarized protons at
?s500 GeV for the first time. This will allow
start of measurements of W bosons.
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Colliding-Beam Spin Physics Only at RHIC
2) Transverse Spin program at STAR
Run 8 (2008) and earlier runs show large
asymmetries in the forward region.
New detector will allow higher kinematic reach
and extensions beyond inclusive ?0 data.
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STAR Detector
New for Run 8 Forward Meson Spectrometer (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.
FPD (runs 3)
7.5 meters from interaction point.
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FMS provides nearly 20x the coverage of previous
forward detectors at STAR
FMS newly commissioned for Run 8
Run 5 FPD
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Bunch-by-bunch polarization from colliding beams
at STAR
East BBC (yellow)
Expected spin up
Expected spin down
West BBC (blue)
3.5 s (statistical) measurement of polarization
per bunch per hour
See also J. Kiryluk (STAR) ArXivhep-ex/0501072v1,
28 Jan 2005
Statistical uncertainties only
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Why high XF at a Collider?
High rapidity ps (hp4) from asymmetric partonic
collisions
pp p0, hp3.8, vs200GeV
ltzgt
Mostly high-x valence quark on low-x gluon
Fragmentation z nearly constant and high 0.7
0.8
ltxqgt
NLO pQCD Jaeger, Stratmann, Vogelsang, Kretzer
ltxggt
Which beam is polarized (not averaged over)
probes different regions
p0
Detector
p
p
If Polarization here XFgt0 Valence quark
spin effects
If Polarization here XFlt0 Low x gluons
and other partons
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Runs 2, 3, 5, and 6 forward p0 asymmetry from FPD
Distributions steeply falling with PT and XF.
PT vs XF coverage of data
Cross sections well described within pQCD
framework at vs200 GeV
FPD set at different distances from beam for the
different lthgt ranges.
Suggests that asymmetry data can be described
within the context of pQCD.
Forward Neutral Pion Transverse Single Spin
Asymmetries in pp Collisions at sqrt(s)200
GeV, PRL 101 (2008) 222001
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Run 3, 5, and 6 asymmetry data Theory can
predict XF dependence based on Sivers function
fits to p/p- asymmetries
Run 6 Data
L6.8 pb-1
Yellow beam polarization 56?2.6
ds? - ds?
AN
ds? ds?
ds?(?)differential cross section when proton has
spin up (down).
Data B.I. Abelev et al. (STAR), PRL 101 (2008)
222001 Theory (red) M. Boglione, U. DAlesio,
F. Murgia arXivhep-ph/0712.4240 Theory (blue)
C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD
74 (2006) 114013
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but rising PT dependence is not predicted by the
same fits
XFgt0.4
B.I. Abelev et al. (STAR) PRL 101 (2008) 222001
Data broken out in XF bins
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FMS Acceptance allows azimuthal dependence to be
measured
Run 8 data
Run 8 data
P(blue)45.53.3
L6.2 pb-1 in plot
AN versus ltcos fgt for positive (blue beam) and
negative (yellow beam) XF
AN as a function of xF integrated over the FMS
acceptance.
Important confirmation of previous data
Plots from Nikola Poljak, for STAR collaboration,
Spin-dependent Forward Particle Correlations in
pp Collisions at ?s 200 GeV,
hep-ex/0901.2828, to be published as Spin 2008
conference proceedings.
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Jet-like events in the FMS can potentially
allow integration over fragmentation products to
measure Sivers effect.
Collins effect asymmetry in fragmentation
Sivers effect asymmetry from initial state
KT-spin correlations
STAR Preliminary
Inclusive p0 asymmetries can not distinguish
these.
Caveat for Run 8 FMS data was acquired with a
high tower trigger, which creates a bias
towards jets that derive from a small number of
high-z initial fragments
Jet-shape distribution of energy within
jet-like objects in the FMS as a function of
distance from the jet axis.
Simulation and Data agree well for jet-like
events
Plot from hep-ex/0901.2828.
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Heavier mesons also accessible at high XF
Di-photons in FPD with E(pair)gt40 GeV
No center cut (requirement that two-photon
system point at middle of an FPD module)
With center cut and Zgglt0.85
Average Yellow Beam Polarization56
S. Heppelmann, PANIC 2008
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Measurement of vector mesons at large xF
Inclusive p0 asymmetries include feed-down from
higher mass decays.
One source of p0s is the spin 1 w (782 MeV).
w decays are accessible in the FMS through their
p0g (BR9) decay channel.
These events are closer to the original
fragmentation product.
Spin-1 production is potentially interesting, and
explictly occurs in some models of string
fragmentation that provide an intuitive picture
of asymmetry in fragmentation.
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w Selection
Look at all triplets of clusters with Egt6 GeV.
Apply fiducial cuts of 1/2 cell from all module
boundaries
Associate two of clusters with p0 from the ?
decay and one with g
Choose p0 to be pair with mass closest to 0.135
GeV, and photon from ? decay to be the third
cluster. A fast, photons-only simulation
indicates that this produces the correct
identification for nearly 100 of ??p0? decays
Require that M(p0) be within 0.1 GeV of 0.135
GeV.
STAR Preliminary
Run 8 data
Pair mass for all triples

• Kinematic cuts to reduce QCD background (real p0
  decays with a third EM-rich hadronic cluster in
  the FMS)
• PT(triplet)gt2.5 GeV
• E(triplet)gt30 GeV
• PT(photon cluster)gt1.5 GeV
• PT(p0)gt1 GeV.

All cuts applied (see below)
M(p0 clusters) (GeV/c2)
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Mass distribution of all triples
STAR Preliminary
For this plot simulation is background only
Run 8 data
PythiaGEANT (background only) (simulation
generation ongoing)
Fit is gaussian cubic polynomial
 µ0.7840.008 GeV s0.0870.009
GeV Scale1339135 Events
Comparison to Run 8 dAu data may be interesting.
Significant (10s) w?p0g signal seen in the data.
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Conclusions
Large-acceptance FMS is online
jet-like events hold the promise of a direct
measurement of the Sivers effect.
Data should allow asymmetry measurements beyond
inclusive p0s
Outlook
Run 8 measurements of AN vs PT for inclusive p0s
at high XF.
Run 8 pp and dAu comparisons.
Run 9 (running now) should allow STAR to extend
constraints on DG and also to begin W
measurements.
Possibility of future direct-g AN measurements
with FMS.
Possibility of future Drell-Yan AN with FMS.
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End
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• Kinematic cuts to reduce QCD background (real p0
  decays with a third EM-rich hadronic cluster in
  the FMS)
• PT(triplet)gt2.5 GeV
• E(triplet)gt30 GeV
• PT(photon cluster)gt1.5 GeV
• PT(p0)gt1 GeV.

Comparison of data (black) to simulation (red)
STAR Preliminary
STAR Preliminary
Run 8 data
Run 8 data
PythiaGEANT
PythiaGEANT
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Sizeable asymmetries can be used to measure beam
polarization
STAR BBC
Inner tiles cover 3.5lthlt5
Can use large asymmetries to measure (relative)
bunch-by-bunch polarization
See also J. Kiryluk (STAR) ArXivhep-ex/0501072v1,
28 Jan 2005
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Inner tiles of BBC accumulated every clock cycle

Scaler Boards

Discriminated phototube outputs
24-bit word is histogrammed every clock cycle

East-West Coincidence

Bunch Crossing (7-bits)
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Inclusive Jets
Jets at each pT are a different mix
of subprocesses
Smeared x-range for jets at a few pTs
Phys. Rev. Lett. 97 (2006) 252001