Results and Plans for Transverse Spin Effects at STAR

1
Results and Plans for Transverse Spin Effects at
STAR

• OUTLINE
• Transverse single spin effects in pp
  collisions at ?s200 GeV
• Towards understanding forward p0 cross sections
• Plans for the near-term future

L.C. Bland Brookhaven National Laboratory RBRC
Workshop on Single-Spin Asymmetries Brookhaven
1 June 2005
2
A Brief History

• At leading twist and with collinear
  factorization, the chiral properties of QCD
  predict small analyzing powers for particle
  production with transversely polarized protons
  colliding at high energies.

• The FermiLab E-704 experiment found strikingly
  large transverse single-spin effects in p?p
  fixed-target collisions with 200 GeV polarized
  proton beam. Large effects were also observed at
  lower ?s.

• Theoretical models were developed to explain
  these effects using spin and transverse-momentum
  dependent distribution or fragmentation functions
  or higher-twist effects.

• Large transverse single-spin effects were
  observed in semi-inclusive electroproduction
  experiments.

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Two Models for Transverse Single-Spin Effects
p? p?p0?

• Sivers effect Phys Rev D41 (1990) 83 43 (1991)
  261
• Flavor dependent correlation between the
  proton spin (Sp), momentum (Pp) and transverse
  momentum (k?) of the unpolarized partons inside.
  The unpolarized parton distribution function
  fq(x,k?) is modified to
• Collins effect Nucl Phys B396 (1993) 161
• Correlation between the quark spin (sq),
  momentum (pq) and transverse momentum (k?) of the
  pion. The fragmentation function of transversely
  polarized quark q takes the form

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Questions

• Do transverse single spin effects persist to
  RHIC energies (200lt?slt500 GeV)?
• Do we understand the unpolarized cross section
  where transverse single spin effects are large?
• Can we disentangle the dynamics?

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Installed and commissioned during run 4 Planned
to be commissioned during run 5 Installed in run
5 and to be commissioned in run 5

• Developments for runs 2 (1/02), 3 (3/03 ? 5/03)
  and 4 (4/04 ? 5/03)
• Helical dipole snake magnets
• CNI polarimeters in RHIC,AGS
• ? fast feedback

• b1m operataion
• spin rotators ? longitudinal polarization
• polarized atomic hydrogen jet target

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Run-5 StatusLongitudinal Polarization at
STAR/PHENIXTransverse Polarization at BRAHMS
As of 5/25/05
Scheduled to run until 6/25/05 Original STAR
goals Pbeam gt 0.4, ?L dt 14 pb-1 (long) / 4
pb-1 (trans)
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STAR detector layout

• TPC -1.0 lt ? lt 1.0
• FTPC 2.8 lt ??? lt 3.8
• BBC 2.2 lt ??? lt 5.0
• EEMC1 lt ? lt 2
• BEMC0 lt ? lt 1
• FPD ? 4.0 3.7

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STAR Forward Calorimetry Recent History and Plans

• Prototype FPD proposal Dec 2000
• Approved March 2001
• Run 2 polarized proton data (published 2004
  spin asymmetry and cross section)
• FPD proposal June 2002
• Review July 2002
• Run 3 data pp dAu (Preliminary An Results)
• FMS Proposal Submitted Jan 2005. Near full
  Forward EM Coverage(hep-ex/0502040).
• ? See talk by Steve Heppelmann on 6/3

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First AN Measurement at STARprototype FPD results
STAR collaboration Phys. Rev. Lett. 92 (2004)
171801
Similar to result from E704 experiment (vs20
GeV, 0.5 lt pT lt 2.0 GeV/c)
Can be described by several models available as
predictions

• Sivers spin and k? correlation in parton
  distribution functions (initial state)
• Collins spin and k? correlation in fragmentation
  function (final state)
• Qiu and Sterman (initial state) / Koike (final
  state) twist-3 pQCD calculations, multi-parton
  correlations

vs200 GeV, lt?gt 3.8
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Single Spin AsymmetryDefinitions

• Definition
• ds?(?) differential cross section of p0 then
  incoming proton has spin up(down)

• Two measurements
• Single arm calorimeter
• R relative luminosity (by BBC)
• Pbeam beam polarization
• Two arms (left-right) calorimeter
• No relative luminosity needed

positive AN more p0 going left to polarized beam
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Caveats -RHIC CNI Absolute polarization
still preliminary. -Result Averaged over
azimuthal acceptance of detectors.
-Positive XF (small angle scattering of the
polarized proton).
Run 2 Published Result.
Run 3 Preliminary Result. -More Forward
angles. -FPD Detectors. - 0.25 pb-1 with
Pbeam27
Run 3 Preliminary Backward Angle Data. -No
significant Asymmetry seen. (Presented at
Spin 2004 hep-ex/0502040)
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xF and pT range of FPD data
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Hard ScatteringHard scattering hadroproduction
p
Factorization theorems state that the inclusive
cross section for pp ? p X can be computed in
perturbative QCD using universal PDF and
fragmentation functions, and
perturbatively calculated hard-scattering cross
sections, , for partonic process ab?c.
All such processes are summed over to yield the
inclusive p production cross section.
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Why Consider Forward Physics at a Collider?
Kinematics
Hard scattering hadroproduction
Can Bjorken x values be selected in hard
scattering?

• Assume
• Initial partons are collinear
• Partonic interaction is elastic ? pT,1 ? pT,2

?
Studying pseudorapidity, h-ln(tanq/2),
dependence of particle production probes parton
distributions at different Bjorken x values and
involves different admixtures of gg, qg and qq
subprocesses.
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Simple Kinematic Limits

• Mid-rapidity particle detection
• h1?0 and lth2gt?0
• ? xq ? xg ? xT 2 pT / ?s
• Large-rapidity particle detection
• h1gtgth2
• xq ? xT eh1 ? xF (Feynman x), and
• xg ? xF e-(h1h2)

NLO pQCD (Vogelsang)
1.0 0.8 0.6 0.4 0.2 0.0
qq
fraction
qg
gg
0 10 20 30
pT,p (GeV/c)
? Large rapidity particle production and
correlations involving large rapidity particle
probes low-x parton distributions using valence
quarks
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How can one infer the dynamics of particle
production?Particle production and correlations
near h?0 in pp collisions at ?s 200 GeV
Inclusive p0 cross section
Two particle correlations (h?)
STAR, Phys. Rev. Lett. 90 (2003), nucl-ex/0210033
At vs 200GeV and mid-rapidity, both NLO pQCD
and PYTHIA explains pp data well, down to
pT1GeV/c, consistent with partonic origin
Do they work for forward rapidity?
Phys. Rev. Lett. 91, 241803 (2003) hep-ex/0304038
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Forward p0 production in hadron collider
p0
Ep
p d
qq
EN
qp
p Au
xgp
xqp
qg
EN
(collinear approx.)

• Large rapidity p production (hp4) probes
  asymmetric partonic collisions
• Mostly high-x valence quark low-x gluon
• 0.3 lt xqlt 0.7
• 0.001lt xg lt 0.1
• ltzgt nearly constant and high 0.7 0.8
• Large-x quark polarization is known to be large
  from DIS
• Directly couple to gluons A probe of low x
  gluons

NLO pQCD Jaeger,Stratmann,Vogels
ang,Kretzer
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But, do we understand forward p0 production in p
p? At ?s ltlt 200 GeV, not really.
Data-pQCD difference at pT1.5GeV
2 NLO collinear calculations with different
scale pT and pT/2
Ed3s/dp3mb/GeV3
Ed3s/dp3mb/GeV3
q6o
q10o
q15o
q53o
q22o
xF
xF
Bourrely and Soffer (hep-ph/0311110, Data
references therein) NLO pQCD
calculations underpredict the data at low ?s from
ISR sdata/spQCD appears to be function of q, vs
in addition to pT
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Mass resolution 20MeV We understand gain 2
level Efficiencies is almost purely geometrically
determined
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pp?p0X cross sections at 200 GeV

• The error bars are point-to-point systematic and
  statistical errors added in quadrature
• The inclusive differential cross section for p0
  production is consistent with NLO pQCD
  calculations at 3.3 lt ? lt 4.0
• The data at low pT are more consistent with the
  Kretzer set of fragmentation functions, similar
  to what was observed by PHENIX for p0 production
  at midrapidity.

D. Morozov (IHEP), XXXXth Rencontres de Moriond
- QCD, March 12 - 19, 2005
NLO pQCD calculations by Vogelsang, et al.
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STAR -FPD Preliminary Cross Sections
Similar to ISR analysis J. Singh, et al Nucl.
Phys. B140 (1978) 189.
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PYTHIA a guide to the physics
Forward Inclusive ?? Cross-Section
Subprocesses involved
qg
gg and qg ? qgg
STAR FPD
Soft processes

• PYTHIA prediction agrees well with the inclusive
  ?0 cross section at ??3-4
• Dominant sources of large xF ?? production from
  
• q g ? q g (2?2) ? ?? X
• q g ? q g g (2?3) ? ?? X

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Back-to-back Azimuthal Correlationswith large ??
Fit ???????????LCP normalized distributions and
with Gaussianconstant
Beam View
Top View
Trigger by forward ??
??

• E? gt 25 GeV
• ???? ? 4

Coicidence Probability 1/radian

• Midrapidity h? tracks in TPC
• -0.75 lt ??lt 0.75
• Leading Charged Particle(LCP)
• pT gt 0.5 GeV/c

???????????LCP
S Probability of correlated event under
Gaussian B Probability of un-correlated event
under constant ?s Width of Gaussian
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• PYTHIA (with detector effects) predicts
• S grows with ltxFgt and ltpT,?gt
• ?s decrease with ltxFgt and ltpT,?gt
• PYTHIA prediction agrees with pp data
• Larger intrinsic kT required to fit data

25ltE?lt35GeV
45ltE?lt55GeV
Statistical errors only
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Frankfurt, Guzey and Strikman, J. Phys. G27
(2001) R23 hep-ph/0010248.

• constrain x value of gluon probed by high-x
  quark by detection of second hadron serving as
  jet surrogate.
• span broad pseudorapidity range (-1lthlt4) for
  second hadron ? span broad range of xgluon
• provide sensitivity to higher pT for forward p0
  ? reduce 2?3 (inelastic) parton process
  contributions thereby reducing uncorrelated
  background in Df correlation.

Pythia Simulation
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Disentangling Dynamics of Single Spin
AsymmetriesSpin-dependent particle correlations
Collins/Hepplemann mechanism requires
transversity and spin-dependent fragmentation
Sivers mechanism asymmetry is present for forward
jet or g
Large acceptance of FMS will enable disentangling
dynamics of spin asymmetries
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Summary / Outlook

• Large transverse single spin asymmetries are
  observed for large rapidity p0 production for
  polarized pp collisions at ?s 200 GeV
• AN grows with increasing xF for xFgt0.35
• AN is zero for negative xF
• Large rapidity p0 cross sections for pp
  collisions at ?s 200 GeV is in agreement with
  NLO pQCD, unlike at lower ?s. Particle
  correlations are consistent with expectations of
  LO pQCD ( parton showers).
• Large rapidity p0 cross sections and particle
  correlations are suppressed in dAu collisions at
  ?sNN200 GeV, qualitatively consistent with
  parton saturation models.
• Plan partial mapping of AN in xF-pT plane in
  RHIC run-5 and first measurements of
  spin-dependence of two-particle correlations.
• Propose increase in forward calorimetry in STAR
  to probe low-x gluon densities and establish
  dynamical origin of AN (complete upgrade by
  10/06). Discussing enhancement of forward
  calorimeter for RHIC run-6.

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Separating xF and pT dependence

• 1 week of transverse polarization running at
  STAR in present run is expected to yield ?L dt ?
  1pb-1 with ?50 beam polarization ? reduce
  statistical errors dAN by gt4 compared to run-3
  data
• enables measurement of AN(pT) at fixed xF
• Calculations of fixed-xF AN(pT) by U.dAlesio and
  F. Murgia (private communication) based on work
  described in PRD 70 (2004) 074009

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STAR detector layout

• TPC -1.0 lt ? lt 1.0
• FTPC 2.8 lt ??? lt 3.8
• BBC 2.2 lt ??? lt 5.0
• EEMC1 lt ? lt 2
• BEMC0 lt ? lt 1
• FPD ? 4.0 3.7

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Spin dependence of two-particle correlations

• For pT,pgt2.5 GeV/c, expect dominance of 2?2
  partonic processes ?
• p0-h? near-side correlations between FPD and
  FTPC to probe Collins contribution.

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New FMS Calorimeter Lead Glass From FNAL E831 804
cells of 5.8cm?5.8cm?60cm Schott F2 lead glass
Loaded On a Rental Truck for Trip To BNL
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Schematic of Existing FPD
Online reconstructions / run 6151012 / fill
7172, L. Nogach (IHEP)
east-north
east-south

• Existing N/S/T/B detectors well suited to large
  rapidity inclusive p0 reconstruction