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STAR. Forward Physics at RHIC Transverse Spin Effects and Probing Low- x Gluons. OUTLINE Transverse single spin effects in p+p collisions at  s =200 GeV Towards understanding forward p 0 cross sections Probing low-x gluon densities Plans for the future. L.C. Bland

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forward physics at rhic transverse spin effects and probing low x gluons


Forward Physics at RHICTransverse Spin Effects andProbing Low-x Gluons
  • Transverse single spin effects in p+p collisions at s=200 GeV
  • Towards understanding forward p0 cross sections
  • Probing low-x gluon densities
  • Plans for the future

L.C. Bland

Brookhaven National Laboratory

The Partonic Structure of Hadrons

ECT*, Trento 9 May 2005

transverse spin effects
Transverse Spin Effects

L.C.Bland, ECT* Workshop

a brief history

s=20 GeV, pT=0.5-2.0 GeV/c

  • 0 – E704, PLB261 (1991) 201.
  • +/- - E704, PLB264 (1991) 462.
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.
  • 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.

L.C.Bland, ECT* Workshop

two models for transverse single spin effects
Two Models for Transverse Single-Spin Effects

p +p→p0+Х

  • 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:

L.C.Bland, ECT* Workshop

  • Do transverse single spin effects persist to RHIC energies (200<s<500 GeV)?
  • Do we understand the unpolarized cross section where transverse single spin effects are large?
  • Can we disentangle the dynamics?

L.C.Bland, ECT* Workshop


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

L.C.Bland, ECT* Workshop

run 5 status longitudinal polarization at star phenix transverse polarization at brahms
Run-5 StatusLongitudinal Polarization at STAR/PHENIXTransverse Polarization at BRAHMS

L dt = 0.8 pb-1

Scheduled to run until 6/25/05

Original STAR goals: Pbeam > 0.4, L dt = 14 pb-1 (long) / 4 pb-1 (trans)

L.C.Bland, ECT* Workshop

star detector layout
STAR detector layout
  • TPC: -1.0 <  < 1.0
  • FTPC: 2.8 <  < 3.8
  • BBC : 2.2 <  < 5.0
  • EEMC:1 <  < 2
  • BEMC:0 <  < 1
  • FPD: || ~ 4.0 & ~3.7

L.C.Bland, ECT* Workshop

star forward calorimetry recent history and plans
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).

L.C.Bland, ECT* Workshop

first a n measurement at star prototype fpd results
First AN Measurement at STARprototype FPD results

STAR collaboration

Phys. Rev. Lett. 92 (2004) 171801

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

√s=200 GeV, <η> = 3.8

L.C.Bland, ECT* Workshop

single spin asymmetry definitions

dσ↑(↓) – 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


π0, xF<0

π0, xF>0




Single Spin AsymmetryDefinitions

positive AN: more p0 going

left to polarized beam

L.C.Bland, ECT* Workshop



-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.

Run 3 Preliminary

Backward Angle Data.

-No significant Asymmetry


(Presented at Spin 2004: hep-ex/0502040)

L.C.Bland, ECT* Workshop



xF and pT range of FPD data

L.C.Bland, ECT* Workshop

forward p 0 cross sections at rhic
Forward p0 Cross Sections at RHIC

L.C.Bland, ECT* Workshop

hard scattering hard scattering hadroproduction
Hard ScatteringHard scattering hadroproduction


Factorization theorems state that the inclusive cross section for p+p  p +X can be computed in perturbative QCD using universal PDF and fragmentation functions, and perturbatively calculated hard-scattering cross sections,

, for partonic process a+bc. All such processes are summed over to yield the inclusive p production cross section.

L.C.Bland, ECT* Workshop

why consider forward physics at a collider kinematics

Deep inelastic scattering

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.

L.C.Bland, ECT* Workshop

simple kinematic limits

p+p  p0+X, s = 200 GeV, h=0

Simple Kinematic Limits
  • Mid-rapidity particle detection:
    • h10 and <h2>0
    •  xq  xg  xT = 2 pT / s
  • Large-rapidity particle detection:
    • h1>>h2
    • xq  xT eh1 xF(Feynman x), and

xg xF e-(h1+h2)

NLO pQCD (Vogelsang)











0 10 20 30


 Large rapidity particle production and correlations involving large rapidity particle probes low-x parton distributions using valence quarks

L.C.Bland, ECT* Workshop



How can one infer the dynamics of particle production?Particle production and correlations near h0 in p+p collisions at s = 200 GeV

Inclusive p0 cross section

Two particle correlations (h)

STAR, Phys. Rev. Lett. 90 (2003), nucl-ex/0210033

At √s = 200GeV and mid-rapidity, both NLO pQCD and PYTHIA explains p+p data well, down to pT~1GeV/c, consistent with partonic origin

Do they work for forward rapidity?

Phys. Rev. Lett. 91, 241803 (2003)


L.C.Bland, ECT* Workshop

forward p 0 production in hadron collider




Forwardp0production in hadron collider














(collinear approx.)

  • Large rapidity p production (hp~4) probes asymmetric partonic collisions
  • Mostly high-x valence quark + low-x gluon
    • 0.3 < xq< 0.7
    • 0.001< xg < 0.1
  • <z> 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



L.C.Bland, ECT* Workshop

but do we understand f orward p 0 production in p p at s 200 gev not really



But, do we understand forward p0 production in p + p?At s << 200 GeV, not really….

Data-pQCD difference at pT=1.5GeV

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 low s from ISR

sdata/spQCD appears to be function of q, √s in addition to pT

L.C.Bland, ECT* Workshop


Di-photon Mass Reconstruction and calibrationPb-glass reconstruction with STAR FPD

Time/luminosity dependent gain shift corrections

PMT Gain Matching

p0 reconstructionefficiency

Cluster categorization

MC & Data comparison

FTPC-FPD matchingPhoton conversion in beam pipe

Track in


  • Clustering and moment analysis
  • Fitting with parametrized shower shape
  • Number of photons found >= 2
  • Fiducial volume > 1/2 cell width from edge
  • Energy sharing zgg=|E1-E2| / (E1+E2) < 0.7
  • Absolute gain determined from  peak position for each tower
  • Energy dependent gain correction
  • Run/luminosity dependent gain correction
  • Checking with MC (PYTHIA+GEANT)

2 photon cluster example

p + p (+ X)   (+ )  e+ e-

Hit in



vs PMT gain

Beam pipe

Limit with zgg<0.5 cut

Try both


1g Cluster

Gain stability (before correction)

from reconstruction



Geometrical limit



2g Cluster

  • FPD position known relative to STAR


Gain stability (after correction)

2nd moment of cluster (long axis)

High tower sorted mass distributions

Mass resolution ~ 20MeV

We understand gain ~2% level

Efficiencies is almost purely

geometrically determined

L.C.Bland, ECT* Workshop

pp p 0 x cross sections at 200 gev
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 < 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 p0 production at midrapidity.

D. Morozov (IHEP),

XXXXth Rencontres de Moriond - QCD,

March 12 - 19, 2005

NLO pQCD calculations by Vogelsang, et al.

L.C.Bland, ECT* Workshop




Cross Sections

Similar to ISR analysis

J. Singh, et al Nucl. Phys.

B140 (1978) 189.

L.C.Bland, ECT* Workshop

pythia a guide to the physics








PYTHIA: a guide to the physics

Subprocesses involved:

Forward Inclusive Cross-Section:


g+g and

q+g  q+g+g


Soft processes

  • PYTHIA predictionagrees 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

L.C.Bland, ECT* Workshop

Probing low-x gluon densitiesForward inclusive particle production in p+p and d+AuParticle correlations in p+p and d+Au

L.C.Bland, ECT* Workshop

parton densities in the proton

Deep inelastic scattering

Parton Densities in the Proton

Deep inelastic scattering (DIS) of electrons and muons is the primary source of information about the quark and gluon structure of the proton.

Kinematics defined for electron(muon) scattering from a fixed proton target.

Global analyses use world data from DIS, neutrino scattering, Drell-Yan,… to determine parton distribution functions (PDF).

L.C.Bland, ECT* Workshop

determining the gluon density
Determining the gluon density

The gluon density is determined by applying QCD evolution equations to account for the Q2 dependence (scaling violations) of structure functions measured in DIS.

At low-x, the full QCD evolution equations can be simplified to approximate the gluon distribution by

i.e., determine g(2x) by measuring the lnQ2 slope of F2(x,Q2) at fixed x.

K. Prytz, Phys. Lett. B311 (1993) 286

L.C.Bland, ECT* Workshop

gluons in the proton
Gluons in the Proton
  • DIS results from HERA ep collider provide accurate determination of xg(x) for the proton in the range 0.001<x<0.2
  • the low-x gluon density is large and continues to increase as x0 over the measured range

J. Pumplin, D.R. Stump, J. Huston, H.L. Lai, P. Nadolsky, W.K. Tung JHEP 0207 (2002) 012.

L.C.Bland, ECT* Workshop

nuclear gluon density
Nuclear Gluon Density

e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) 044905

and data references therein

World data on nuclear DIS constrains nuclear modifications to gluon density only for xgluon > 0.02

L.C.Bland, ECT* Workshop


Figure 3 Diagram showing the boundary between possible “phase” regions in the t=ln(1/x) vs plane


New Physics at high gluon density

  • Shadowing. Gluons hidingbehind other gluons. Modificationof g(x) in nuclei. Modified distributionsneeded by codes that hope to calculateenergy density after heavy ion collision.
  • Saturation Physics. New phenomena associated with large gluon density.
    • Coherent gluon contributions.
    • Macroscopic gluon fields.
    • Higher twist effects.
    • “Color Glass Condensate”

Edmond Iancu and Raju Venugopalan, review for Quark Gluon Plasma 3,

R.C. Hwa and X.-N. Wang (eds.), World Scientific, 2003 [hep-ph/0303204].

L.C.Bland, ECT* Workshop


FPD Detector and º reconstruction

  • robust di-photon reconstructions with FPD in d+Au collisions on deuteron beam side.
  • average number of photons reconstructed increases by 0.5 compared to p+p data.

L.C.Bland, ECT* Workshop

dependence of r dau


As y grows

Kharzeev, Kovchegov, and Tuchin, Phys. Rev. D 68 , 094013 (2003)

 Dependence of RdAu

G. Rakness (Penn State/BNL),

XXXXth Rencontres de Moriond - QCD,

March 12 - 19, 2005

See also J. Jalilian-Marian, Nucl. Phys. A739, 319 (2004)

  • From isospin considerations, p + p  h is expected to be suppressed relative to d + nucleon  h at large [Guzey, Strikman and Vogelsang, Phys. Lett. B 603, 173 (2004)]
  • Observe significant rapidity dependence similar to expectations from a “toy model” of RpA within the Color Glass Condensate framework.

L.C.Bland, ECT* Workshop

constraining the x values probed in hadronic scattering

For 22 processes



Barrel EMC






Constraining the x-values probed in hadronic scattering

Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 (2004).


Collinear partons:

  • x+ = pT/s (e+h1 + e+h2)
  • x = pT/s (eh1 + eh2)
  • FPD: ||  4.0
  • TPC and Barrel EMC: || < 1.0
  • Endcap EMC: 1.0 <  < 2.0
  • FTPC: 2.8 <  < 3.8

CONCLUSION: Measure two particles in the final state to constrain the x-values probed

L.C.Bland, ECT* Workshop

back to back azimuthal correlations with large
Back-to-back Azimuthal Correlationswith large 

Fit LCP normalized distributions and with Gaussian+constant

Beam View

Top View

Trigger by forward 



  • E > 25 GeV
  •  4


Coicidence Probability


Midrapidity h tracksin TPC

  • -0.75 < < +0.75

Leading Charged Particle(LCP)

  • pT > 0.5 GeV/c


S = Probability of “correlated” event under Gaussian

B = Probability of “un-correlated” event under constant

s = Width of Gaussian

L.C.Bland, ECT* Workshop



STAR Preliminary

STAR Preliminary

PYTHIA (with detector effects) predicts

  • “S” grows with<xF> and <pT,>
  • “s” decrease with <xF> and <pT,>

PYTHIA prediction agrees with p+p data

Larger intrinsic kT required to fit data



Statistical errors only

L.C.Bland, ECT* Workshop


Expectation from HIJING (PYTHIA+nuclear effects)

X.N.Wang and M Gyulassy, PR D44(1991) 3501

with detector effects

  • HIJING predicts clear correlation in d+Au
  • Small difference in “S” and “s” between p+p and d+Au
  • “B” is bigger in d+Au due to increased particle multiplicity at midrapidity



L.C.Bland, ECT* Workshop

dau correlations probing low x



PT is balanced by many gluons

Dilute parton system (deuteron)

Dense gluon field (Au)


Beam View

Top View



  • E > 25 GeV
  •  4

STAR Preliminary


Statistical errors only

dAu Correlations: probing low x

L.C.Bland, ECT* Workshop


dAu Correlations: probing low x

Large 0+h± correlations

  • Suppressed at small <xF> , <pT,>

Consistent with CGC picture

  • Consistent in d+Au and p+p at larger <xF> and <pT,>

More data are needed…


Fixed as

E & pT grows

STAR Preliminary


Statistical errors only

L.C.Bland, ECT* Workshop

plans for the future
Plans for the Future

L.C.Bland, ECT* Workshop

star forward meson spectrometer
STAR Forward Meson Spectrometer
  • NSF Major Research Initiative (MRI) Proposal
  • submitted January 2005
  • [hep-ex/0502040]

L.C.Bland, ECT* Workshop

three highlighted objectives in fms proposal not exclusive
Three Highlighted Objectives In FMS Proposal(not exclusive)
  • A d(p)+Aup0p0+X measurement of the parton model gluon density distributions xg(x) in gold nucleifor0.001< x <0.1. For 0.01<x<0.1, this measurement tests the universality of the gluon distribution.
  • Characterization of correlated pion cross sections as a function of Q2 (pT2) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au)
  • Measurements withtransversely polarized protonsthat are expected toresolve the origin of the large transverse spin asymmetriesin reactions for forward  production. (polarized pp)

L.C.Bland, ECT* Workshop


FMS Design

  • FMS increases areal coverage of forward EMC from 0.2 m2 to 4 m2
  • FMS to be mounted at roughly the same distance from the center of the STAR interaction region as the FPD, and would face the Blue beam
  • Addition of FMS to STAR provides nearly continuous EMC from -1<h<+4

FPD Calorimeters

L.C.Bland, ECT* Workshop

star detector layout with fms

FMS: 2.5<< 4.0

STAR detector layout with FMS

TPC: -1.0 <  < 1.0

FTPC: 2.8 <  < 3.8

BBC : 2.2 <  < 5.0

EEMC:1 <  < 2

BEMC:-1 <  < 1

FPD: || ~ 4.0 & ~3.7

L.C.Bland, ECT* Workshop

fms mri proposal details
FMS MRI Proposal Details
  • Full azimuthal EM coverage 2.5<<4.0
    • Extending STAR coverage to -1<<4.0
  • 684 3.8 cm  3.8 cm  45 cm lead glass inner cells (IHEP, Protvino).
  • 756 5.8 cm  5.8 cm  60 cm Schott F2 lead glass outer cells (FNAL-E831).
  • New Photinis XP2202 (outer cells)
  • Cockroft Walton Bases.
  • Readout Electronics

L.C.Bland, ECT* Workshop


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 (-1<h<+4) 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.

L.C.Bland, ECT* Workshop

Pythia Simulation


d+Au  p0+p0+X, pseudorapidity correlations with forward p0

HIJIING 1.381 Simulations

  • increased pT for forward p0 over run-3 results is expected to reduce the background in Df correlation
  • detection of p0 in interval -1<h<+1 correlated with forward p0 (3<h<4) is expected to probe 0.01<xgluon<0.1  provides a universality test of nuclear gluon distribution determined from DIS
  • detection of p0 in interval 1<h<4 correlated with forward p0 (3<h<4) is expected to probe 0.001<xgluon<0.01  smallest x range until eRHIC
  • at d+Au interaction rates achieved at the end of run-3 (Rint~30 kHz), expect 9,700200 (5,600140) p0-p0 coincident events that probe 0.001<xgluon<0.01 for “no shadowing” (“shadowing”) scenarios.

L.C.Bland, ECT* Workshop

disentangling dynamics of single spin asymmetries spin dependent particle correlations
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

L.C.Bland, ECT* Workshop

timeline completion by fall 2006
TimelineCompletionByFall 2006

L.C.Bland, ECT* Workshop

other possible applications of fms
Other Possible Applications of FMS
  • forward p0/g reconstruction in heavy-ion collisions
  • direct photon detection at large rapidity
  • reconstruction of other mesons decaying to g or e produced in p+p or d(p)+Au (and heavy-ion?) collisions
    • h  gg
    • w  p0g
    • Kshort p0p0  4g
    • h gg ?
    • J/ e+e- ?

Reconstruction of HIJING/GSTAR simulations

Limited sample of events obtained in Cu+Cu run with good view of interaction region

L.C.Bland, ECT* Workshop

summary outlook
Summary / Outlook
  • Large transverse single spin asymmetries are observed for large rapidity p0 production for polarized p+p collisions at s = 200 GeV
    • AN grows with increasing xF for xF>0.35
    • AN is zero for negative xF
  • Large rapidity p0 cross sections for p+p 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 d+Au collisions at sNN=200 GeV, qualitatively consistent with parton saturation models.
  • Plan partial mapping of AN in xF-pT plane in RHIC run-5
  • Propose increase in forward calorimetry in STAR to probe low-x gluon densities and establish dynamical origin of AN (complete upgrade by 10/06).

L.C.Bland, ECT* Workshop


New FMS Calorimeter

Lead Glass From FNAL E831

Loaded On a Rental Truck for Trip To BNL

L.C.Bland, ECT* Workshop