# Presentation: 558140

## Transverse Spin Asymmetries at RHICIf Not Gluon Polarization, then orbital angular momentum?

- L.C. Bland
- Brookhaven National Laboratory
- 5 June 2007

## Transverse Single Spin Asymmetry (SSA)Definitions:  how and what to measure in an experiment

- Definition:
- dσ↑(↓) – differential cross section of  when incoming proton has spin up(down)

- _One way to measure:_
- Single arm detector:
-     R      – relative luminosity
-    **P***Beam* – beam polarization

_**positive A**_**N****: more ********0 ****from ****spin up(******** than from spin down(******
**Accuracy,** AN *N**N*1⁄2**            ******** many events required for good accuracy**

- In general, *A**N* is a function of both *x*F and *p*T

## Expectations from Theory

- Expectations from Theory

- What would we see from this gedanken experiment?

*F*0 as *m**q*0 in vector gauge theories, so *A**N* ~ *m**q*/*p**T*
- or,*A**N* ~ 0.001 for *p**T* ~ 2 GeV/c
- Kane, Pumplin and Repko  PRL **41** (1978) 1689

- If seen by experiment, what is the pT dependence?

## A Brief History...

- s=20 GeV, pT=0.5-2.0 GeV/c

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

-   QCD theory expects very small (AN~10-3) transverse SSA for particles produced by hard scattering.

-   The FermiLab E-704 experiment found strikingly large transverse single-spin effects in p+p fixed-target collisions with 200 GeV polarized proton beam (s = 20 GeV).

- What is the pT dependence?

## Two of the Explanations for Large Transverse SSA

- Require experimental separation of Collins and Sivers contributions

## Transverse Single-Spin AsymmetriesWorld-wide experimental and theoretical efforts

- Transverse single-spin asymmetries are observed in semi-inclusive deep inelastic scattering with transversely polarized proton targets
-   HERMES (*e*); COMPASS (); and planned at JLab
- Transverse single spin asymmetries are observed in hadron-pair production in *e*+*e* collisions (BELLE)
- Intense theory activity underway

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

**√****s=23.3GeV**

**√****s=52.8GeV**

**Do we understand forward ************ production in p + p?** **At ********s < 200 GeV, not really...**

- 2 NLO collinear calculations  with different scale:
- pT and  pT/2
**Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results**

******data****/********pQCD**** appears to be function of ********, √s in addition to p****T**
**   ****NLO pQCD calculations underpredict the data at ********s < 200 GeV**** **

*x*F

********

********

- Ed3dp3[b/GeV3]

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

**Notes:**
Need journal reference to BS paper.  Dependence on h,s^-1/2 through structure functions?

## Does pQCD describe particle production at RHIC?Compare cross sections measured for p+p +X at s=200 GeV to next-to-leading order pQCD calculations 

- Does pQCD describe particle production at RHIC?Compare cross sections measured for p+p +X at s=200 GeV to next-to-leading order pQCD calculations

- S.S. Adler *et al.* (PHENIX), PRL **91** (2003) 241803

- J. Adams *et al.* (STAR), PRL **92** (2004) 171801; and PRL **97** (2006) 152302

- Cross sections agree with NLO pQCD down to pT~2 GeV/c over a wide range, 0 <  < 3.8, of pseudorapidity ( = -ln tan /2) at s = 200 GeV.

## Relativistic Heavy Ion Collider

**3**** ****Spin**** Experiments**
-  PHENIX
-  STAR
-  BRAHMS
-  PHOBOS (heavy-ion)

**Characteristics**
- 2 counter-circulating rings
- 3.8 km in circumference
- Top Energies (each beam):
- 100GeV / Au-Au
- 250GeV / p-p
- Mixed Species (d+Au)

**Notes:**
The experiment is taking place at Brookhaven National Laboratory at RHIC. RHIC is composed by 2 counter circulating rings - blue and yellow- of 3.8 Km in circumference. RHIC has the ability to accelerate ions at speeds close to the speed of light, and particularly Au Au at 100GeV, pp at 250 Gev or even mixed species, and collide them at 4 different crossing points. STAR is located at 6 o’clock. STAR stands for Solenoidal Tracker at RHIC and it is composed by several detectors that have the ability to track particles produced by collisions and measure their energy.  

## First Transverse SSA at RHICprior to RHIC run 6

- S.S. Adler *et al*. Phys. Rev Lett. **95** (2005) 202001

- Particle production at ~90 (midrapidity) relative to the colliding beams have zero tranverse single spin asymmetry.

- p+p collisions , s = 200 GeV

## Spin Effects in the Forward Direction

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

- Can be described by several models available as predictions:
_**Sivers**_: spin correlated k in the proton (orbital angular momentum)
_**Collins/Heppelmann**_: spin and k correlation in quark   fragmentation
_**Qiu/Sterman**__ (initial state) / __**Koike**__ (final state)_: twist-3 pQCD   multi-parton correlations

- √s=200 GeV, <η> = 3.8

- D. Morozov, for STAR [hep-ex/0512013]

- Spin effects initially observed in RHIC run 2 confirmed by measurements in runs 3,5.

-  Transverse SSA persist at large *x**F* at RHIC energies

## Overview of transverse spin runs at STAR with forward calorimetry: 2001→2006  

|  | Run2 | Run3 | Run5 | Run6 |
| --- | --- | --- | --- | --- |
| detector | EEMC 
and FPD
prototypes | 6 matrices 
of FPD | full FPD
(8 matrices) | East  FPD
West FPD++ |
|  | ~15 | ~30 | ~45 | ~60 |
|  | 0.15 | 0.25 | 0.1 | 6.8 |
| 
 | 3.8 | ±3.3/±4.0 | ±3.7/±4.0 | -3.7/3.3 |

- sampled

- FOM (P2L) in Run 6 is ~50 times larger than from all the
- previous STAR runs, and ~ 725 times larger than for Run 2

## STAR Results vs. Di-Jet Pseudorapidity Sum

***STAR Results vs. Di-Jet Pseudorapidity Sum***
***Run-6 Result***

## π0 AN at √s=200 GeV – xF-dependence 

-  AN at positive xF grows with
-   increasing xF
-  AN at negative xF is consistent
-   with zero
-  Run 6 data at <η>=3.7 are
-   consistent with the existing
-   measurements

-  Small errors of the data points
-    allow quantitative comparison
-    with theory predictions

- What is the pT dependence?

- hep-ex/0612030

## AN(pT) in xF-bins 

- Combined data from three runs at <η>=3.3, 3.7 and 4.0
- In each xF bin, <xF> does not significantly changes with pT
- Measured AN is not a smooth decreasing function of  pT as predicted by multiple theoretical models

- D’Alesio & Murgia  PRD **70** (2004) 074009
- Kouvaris, Qiu, Vogelsang, Yuan PRD **74** (2006) 114013

## Slide 16

## PHENIX Goes ForwardFirst results with muon piston calorimeter from run 6p+p+X, s = 62 GeV

- PHENIX Goes ForwardFirst results with muon piston calorimeter from run 6p+p+X, s = 62 GeV

- Transverse SSA persists with similar characteristics over a broad range of collision energy (20 < s < 200 GeV)

## SummaryFrom RHIC run 6 (“Renaissance Run”)

- Firmly established that large transverse single spin asymmetries are observed at s = 200 GeV, where generally cross sections agree with pQCD calculations.
- Large transverse single spin asymmetries are observed only at large *x**F*; midrapidity asymmetries are small.
- Large *x**F* spin asymmetries show the same pattern for 20  s  200 GeV
- First observation of pT dependence, enabled by the run-6 luminosity/performance
-  Some aspects of the theory are still not understood
- Intense theory activity is underway to understand these spin effects.  Most theorists agree the Sivers mechanism is responsible for the dynamics
-  evidence for partonic orbital angular momentum?