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Transcript of Forward Particle Production and Transverse Single Spin 2006. 2. 27.¢  Forward Particle...

  • Forward Particle Production and Transverse Single Spin Asymmetries

    OUTLINE • Transverse single spin effects in p+p collisions at √s=200 GeV

    • Towards understanding forward π0 cross sections

    • Plans for the future

    L.C. Bland Brookhaven National Laboratory RBRC Workshop on Parton Orbital Angular Momentum Albuquerque 25 February 2006

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 2

    Developments for runs 2 (1/02), 3 (3/03 → 5/03) and 4 (4/04 → 5/04)

    • Helical dipole snake magnets • CNI polarimeters in RHIC,AGS

    fast feedback

    • β*=1m operataion • spin rotators longitudinal polarization • polarized atomic hydrogen jet target

    Installed and commissioned during run 4 To be commissioned Installed/commissioned in run 5

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 3

    RHIC Spin Physics Program

    • Direct measurement of polarized gluon distribution using multiple probes

    • Direct measurement of anti-quark polarization using parity violating production of W±

    • Transverse spin: Transversity & transverse spin effects: possible connections to orbital angular momentum?

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 4

    STAR detector layout

    • TPC: -1.0 < η < 1.0

    • FTPC: 2.8 < |η| < 3.8

    • BBC : 2.2 < |η| < 5.0

    • EEMC: 1 < η < 2

    • BEMC: -1 < η < 1

    • FPD: |η| ~ 4.0 & ~3.7

    STAR characterized by azimuthally complete acceptance over broad range of pseudorapidity.

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 5

    Single Spin Asymmetry Definitions

    • Definition:

    • dσ↑(↓) – differential cross section of π0 when 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

    ↓↑

    ↓↑

    + −

    = σσ σσ

    dd ddAN

    ↓↑

    ↓↑

    =⎟⎟ ⎠

    ⎞ ⎜⎜ ⎝

    + −

    ⋅= L LR

    RNN RNN

    P A

    Beam N

    1

    ⎟ ⎟

    ⎜ ⎜

    ⋅+⋅

    ⋅−⋅ ⋅=

    ↓↑↓↑

    ↓↑↓↑

    LRRL

    LRRL

    Beam N

    NNNN

    NNNN P

    A 1 π0, xF0

    Left

    Right

    p  p

    positive AN: more π0 going left to polarized beam

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 6

    First AN Measurement at STAR prototype FPD results

    STAR collaboration Phys. Rev. Lett. 92 (2004) 171801

    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

    Can be described by several models available as predictions:

    Similar to result from E704 experiment (√s=20 GeV, 0.5 < pT < 2.0 GeV/c)

    √s=200 GeV, = 3.8

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 7

    Dynamical Origins of Transverse SSA

    • Sivers effect [Phys Rev D41 (1990) 83; 43 (1991) 261]: Flavor dependent correlation between the proton spin (Sp), momentum (Pp) and transverse momentum (kT) of the unpolarized partons inside:

    • Collins effect [Nucl Phys B396 (1993) 161]: Correlation between the quark spin (sq), momentum (pq) and transverse momentum (kT) of the pion. The fragmentation function of transversely polarized quark q takes the form:

    ⊥ ⊥⊥⊥ ×⋅+=

    qPP

    qpP qq

    N qqqPqq

    )( )k(x,ƒΔ

    2 1)k(x,ƒ),(x,ƒ

    kPS kPS

    Sk

    ⊥ ⊥⊥⊥

    ×

    ×⋅ Δ+=

    πq

    πqqN qπ kp

    kps sk

    )( )k(z,D)k(z,D),(z,D /q/q/q πππππ 2

    p +p→π0+Х

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 8

    Run 3 Preliminary Result:Run 3 Preliminary Result: --more Forward anglesmore Forward angles --final FPD Detectorsfinal FPD Detectors

    Run 3 Preliminary Backward Angle Data:Run 3 Preliminary Backward Angle Data: --No significant asymmetry seen.No significant asymmetry seen.

    A. Ogawa, for STAR: [hep-ex/0502040]

    Run 3 + Run 5 Preliminary Run 3 + Run 5 Preliminary =3.7,4.0 D. D. MorozovMorozov, for STAR [hep, for STAR [hep--ex/0512013]ex/0512013]

    Present Status Uses online beam polarization values

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 9

    STARSTAR

    xF and pT range of FPD data

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 10

    AN(pT) from run3+run5 at √s=200 GeV

    • Combined statistics from run3 and run5 with xF>0.4

    • There is evidence that analyzing power at xF>0.4 decreases with increasing pT

    • To do: systematics study

    Uses online beam polarization values

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 11

    • Large rapidity π production (ηπ>4) probes asymmetric partonic collisions

    • Mostly high-x valence quark + low-x gluon

    • 0.3 < xq< 0.7

    • 0.001< xg < 0.1

    • nearly constant and high 0.7 ~ 0.8

    • Large-x quark polarization is known to be large from DIS

    • Directly couple to gluons ⇒ probe of low x gluons

    NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer

    p + p → π 0,ηπ = 3.8, s = 200GeV

    Forward π0 production in hadron collider

    p d p

    Au

    π0 θq

    θg

    Q2 ~ pT 2

    s = 2EN

    η = −ln(tan(θ 2

    ))

    xq ≈ xF / zEN xqp xgp

    θπ

    Επ xF ≈

    2Eπ s

    z = Eπ Eq

    xg ≈ pT

    s e−η g

    EN

    (collinear approx.)

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 12

    √s=23.3GeV √s=52.8GeV

    But, do we understand forward π0 production in p + p? At √s < 200 GeV, not really…

    2 NLO collinear

    calculations with different

    scale:

    pT and pT/2

    Bourrely and Soffer (hep-ph/0311110, Data references therein): NLO pQCD calculations underpredict the data at √s < 200 GeV

    (ISR and fixed target)

    σdata/σpQCD appears to be function of θ, √s in addition to pT

    xF xF θ=22ο

    θ=15ο

    θ=6ο

    θ=10ο

    θ=53ο

    E d3

    σ/ dp

    3 [μ b/

    G eV

    3 ]

    E d3

    σ/ dp

    3 [μ b/

    G eV

    3 ]

    Data-pQCD difference at pT=1.5GeV

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 13

    The error bars are point-to-point systematic and statistical errors added in quadrature

    The inclusive differential cross section for π0 production is consistent with NLO pQCD calculations at 3.3 < η < 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 π0 production at midrapidity.

    pp→π0X cross sections at 200 GeV

    D. Morozov (IHEP), XXXXth Rencontres de Moriond - QCD, March 12 - 19, 2005

    NLO pQCD calculations by Vogelsang, et al.

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 14

    STARSTAR--FPD FPD Cross SectionsCross Sections

    Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189.

    ( )

    6 5

    13 3

    ≈ ≈

    −∝ −

    B C

    px dp dE BT

    C F

    σ

    Expect QCD scaling of form:

    ( ) ( ) ( ) anBpxspxx dp dE anT

    C F

    an T

    C F

    a T +=⇒−=−∝

    −−−− 12/13 3σ

    ⇒ Require √s dependence to disentangle pT and xT dependence

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 15

    • PYTHIA prediction agrees well with the inclusive π0 cross section at η∼3-4 • Dominant sources of large xF π0 production from:

    ● q + g → q + g (2→2) → π0 + X

    ● q + g → q + g + g (2→3) → π0 + X

    g+g and q+g → q+g+g

    q+g

    Soft processes

    Subprocesses involved: PYTHIA: a guide to the physics

    Forward Inclusive π0 Cross-Section:

    q π0

    g g

    q g

    π0

    STAR FPD

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 16

    Plans for the Future • STAR Forward Pion Detector upgrade (FPD++) planned as an engineering test of the FMS during RHIC run 6

    • STAR Forward Meson Spectrometer (FMS) planned for installation by RHIC run 7

    ⇒Disentangle the dynamical origins to transverse SSA in p+p collisions via measurements of AN for

    jet-like events

    direct photon production

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 17

    FPD++ Physics for Run6

    Run-5 FPD

    The center annulus of the run-6 FPD++ is similar to arrays used to measure forward π0 SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct γ events.

    We intend to stage a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc.

    Run-6 FPD++

  • 2/25/2006 L.C.Bland, RBRC Parton OAM 18

    STAR Configuration for Run 6