# Presentation: 126607

## Results and Plans for Transverse Spin Effects at STAR

- OUTLINE
-   Transverse single spin effects in p+p collisions at *s*=200 GeV
-   Towards understanding forward  cross sections
-   Plans for the near-term future

- L.C. Bland
- Brookhaven National Laboratory
*RBRC Workshop on Single-Spin Asymmetries*
- Brookhaven    1 June 2005

## 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.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Two Models for Transverse Single-Spin Effects

**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:

- p⁭ +p→+Х

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Questions

-   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, RBRC SSA Workshop

- 1 June 2005

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

- 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

-   *=1m operataion
-   spin rotators  longitudinal polarization
-   polarized atomic hydrogen jet target

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

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Run-5 StatusLongitudinal Polarization at STAR/PHENIXTransverse Polarization at BRAHMS

- Scheduled to run until 6/25/05
- Original STAR goals:  *P**beam* > 0.4,  L *dt *= 14 pb (long) / 4 pb (trans)

- As of 5/25/05

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## 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, RBRC SSA Workshop

- 1 June 2005

## 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

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## 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, <η> = 3.8

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Single Spin AsymmetryDefinitions

- Definition:
- dσ↑(↓) – differential cross section of  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 A**_**N****: more ********0 ****going **
**left ****to ****polarized beam**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Run 2 Published Result.

**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 P****beam****~27%**

**Run 3 Preliminary **
**   ****Backward Angle Data.**
**   ****-No significant Asymmetry **
**   ****seen.**
-   (Presented at Spin 2004: hep-ex/0502040)

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## xF and pT range of FPD data

**x****F**** and p****T**** range of FPD data**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Hard ScatteringHard scattering hadroproduction

- 

- Factorization theorems state that the inclusive cross section for 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  production cross section.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Why Consider Forward Physics at a Collider? Kinematics

- Assume:
- Initial partons are collinear
- Partonic interaction is elastic   *p**T*,  *p**T,*2

- Why Consider Forward Physics at a Collider? **Kinematics**

- 

- Can Bjorken *x* values be selected in hard scattering?

**Hard scattering hadroproduction**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Simple Kinematic Limits

**Mid-rapidity particle detection**:
- 0 and <>0
***  **x**q** **** x**g** **** x**T** = *2* p**T** / **s*
**Large-rapidity particle detection**:
- >>
* **x**q** ****  x**T** e****  x**F** *(Feynman *x*), and
-      *x**g* ***  x**F** e*(

- 1.0
- 0.8
- 0.6
- 0.4
- 0.2
- 0.0

- fraction

- 0            10           20          30

- pT,(GeV/c)

*qq*

*qg*

*gg*

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

- NLO pQCD  (Vogelsang)

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

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

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

**Two particle correlations (*****h*********)**

**Inclusive ********0 ****cross section**

**Do they work for forward rapidity?**

- 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

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

- Phys. Rev. Lett. 91, 241803 (2003)
- hep-ex/0304038

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Forward  production in hadron collider 

-  **Large rapidity ******** production ************~4******** probes asymmetric partonic collisions**
-  Mostly **high-x valence quark**** + ****low-x gluon**
* ***0.3***** < x*****q****<  0.7**** **
-  **0.001*****< x*****g ****< 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**

**                  ****NLO pQCD**
**Jaeger,Stratmann,Vogelsang,Kretzer**

**Forward**** ************ ****production in hadron collider**

**p**
**d**

**p**
**Au**

********

- q

- g

**E****N**

**x****q****p**

**x****g****p**

- 

********

**E****N**

- (collinear approx.)

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## But, do we understand forward 0 production in p + p? At s << 200 GeV, not really....

**But, do we understand forward ********0**** 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 low ********s from ISR **
-  data/pQCD appears to be function of , √s in addition to pT

- xF

- xF

********

********

********

********

********

- Ed3dp3[b/GeV3]

- Ed3dp3[b/GeV3]

**Data-pQCD difference at p****T****=1.5GeV**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Mass resolution ~ 20MeV

**Mass resolution ~ 20MeV**
**We understand gain ~2% level**
**Efficiencies is almost purely**
**geometrically determined**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## ppX cross sections at 200 GeV

- The  inclusive differential cross section for 0  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  production at midrapidity.

- ppX cross sections at 200 GeV

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

- NLO pQCD calculations by Vogelsang, et al.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## STAR -FPD 

**Preliminary**
**Cross Sections**

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

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## PYTHIA:  a guide to the physics

-  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

- g+g and
- q+g  q+g+g

- q+g

- Soft processes

**PYTHIA:  a guide to the physics**

- Forward Inclusive  Cross-Section:

- Subprocesses involved:

- STAR FPD

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Back-to-back Azimuthal Correlationswith large 

**Midrapidity h******** tracks**** ****in TPC**
**   ****-0.75 < ********< +0.75**
**Leading Charged Particle(LCP)**
**   ****p****T**** > 0.5 GeV/c**

**************LCP**

- Coicidence Probability
-  [1/radian]

- S = Probability of “correlated” event under Gaussian
- B = Probability of “un-correlated” event under constant
- s = Width of Gaussian

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

****

- Beam View

- Top View

**Trigger by forward ************ **

-  **E******** > 25 GeV**
** **************** ******** ****4**

- ]

- ]

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

##  “S” grows with <xF> and <pT,> 

- 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

**25<E********<35GeV**

**45<E********<55GeV**

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Pythia Simulation 

**Frankfurt, Guzey and Strikman, **
**J. Phys. G27 (2001) R23 [hep-ph/0010248].**

**Pythia Simulation **

-  constrain *x* value of gluon probed by high-*x* quark by *detection of second hadron* serving as jet surrogate.
-  span broad pseudorapidity range (-1<<+4) for second hadron  span broad range of *x*gluon
-  provide sensitivity to higher *p*T for forward   reduce 23 (inelastic) parton process contributions thereby reducing uncorrelated background in  correlation.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Disentangling Dynamics of Single Spin AsymmetriesSpin-dependent particle correlations

- Collins/Hepplemann mechanism requires transversity and spin-dependent fragmentation

- Sivers mechanism asymmetry is present for forward jet or 

- Large acceptance of FMS will enable disentangling dynamics of spin asymmetries

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Summary / Outlook

-  Large transverse single spin asymmetries are observed for large rapidity  production for polarized p+p collisions at *s* = 200 GeV
-  *A*N grows with increasing *x*F for *x*F>0.35
-  *A*N is zero for negative *x*F
-   Large rapidity  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  cross sections and particle correlations are suppressed in d+Au collisions at *s*NN=200 GeV, qualitatively consistent with parton saturation models.
-   Plan partial mapping of *A*N in *x*F*p*T 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 *A*N (complete upgrade by 10/06).  Discussing enhancement of forward calorimeter for RHIC run-6.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## 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 AN by >4 compared to run-3 data
-  enables measurement of AN(pT) at fixed *x*F
- Calculations of fixed-*x*F AN(pT) by U.d’Alesio and F. Murgia (private communication) based on work described in PRD **70** (2004) 074009

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## 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, RBRC SSA Workshop

- 1 June 2005

## Spin dependence of two-particle correlations

- For pT,>2.5 GeV/c, expect dominance of 22 partonic processes  
-  -*h* near-side correlations between FPD and FTPC to probe Collins contribution.

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## 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

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005

## Schematic of Existing FPD

- east-north

- east-south

-  Existing N/S/T/B detectors well suited to large rapidity inclusive  reconstruction

- Online reconstructions / run 6151012 / fill 7172,
- L. Nogach (IHEP)

- L.C.Bland, RBRC SSA Workshop

- 1 June 2005