# Presentation: 572007

## Measurement of Transverse spin effects

**Measurement of Transverse spin effects**
** ****with the Forward Pion detector at STAR **
**in Polarized p+p Collisions at 200GeV**
-  **Steve Heppelmann **
**Penn State University**
**(for the STAR collaboration)**

- Forward
-      Detected

## Overview: The QCD Challenge

- STAR has shown that the measured cross sections at large rapidity 3<<4 and modest transverse momentum PT>1 GeV/c for
- are in good agreement with NLO PQCD calculations.

- Confirmation of these calculations validates the long held prejudice that events in this kinematic region involve the collisions between a large x quark in one proton and a soft parton (gluon) in the other proton.

- Single Transverse Spin asymmetry “AN” for this process require the interference between the real part of a quark helicity non-flip amplitude and the imaginary part of a helicity flip amplitude.  Such  “T odd, helicity flip”  terms are not present in the colinear parton leading twist calculations that now well describe the _**spin averaged cross section.**_

**A QCD based description of the **_**un-summed cross sections**_** leading to ****A****N**** ****.......**** ****will impact “****next generation****” questions about perturbative QCD, probing the ultimate limit to the intuitive factorized parton distribution picture.**

## Pproton

**S**

**Transverse Single Spin Asymmetry for Small Angle **
**                                     **
***(100 GeV/c on 100 Gev/c)  Proton Collisions @RHIC*****  **

- Pproton

- P

- Pion Detected

- The difference between
- spin **up **and **down**
**(depends on Up vs Down **
**Luminosity)**

- or the difference between **left** and** right **scattering
**(Depends upon Left vs Right acceptance).**

- or the cross ratio
**(Insensitive to Left/Right Acceptance or  to  Up/Down Luminosity).**

## Transverse Polarization at RHIC

**55 bunches**  of protons in each RHIC Ring
**Polarization of each bunch** prepared independently at injection.
- Half of the bunches are filled with spin up protons and half with spin down protons at injection.
- The polarization is kept transverse as beam circulates in RHIC.
- Bunch collisions every ~100 nS
**Each bunch collision involves different but known spin combinations.**  Repeats after 110 collisions.

** ****spin  up x up**

**spin up x down **

## Slide 5

## Single Spin Asymmetries (the historical context)

_**Polarized Partons (transversity):**_  At large x (parton momentum fraction) ...  up /down quarks have their transverse spins aligned/opposite the spin of the proton.
** **_**Parton-parton sub-process**_: involves scattering between a
- hard up quark and soft parton for + or 0
- hard down quark and soft parton for  -
** **_**Parton scattering amplitued:**_ Proportional to interference between real non-helicity flip and complex helicity flip amplitudes (properties that exclude the leading and simplest terms of PQCD calculations).
_**Asymmetry vs X**__**f**__** thought to be related to**_ initial-final state (Sivers), final state Collins, effects or higher twist effects.

- The _naïve (but perhaps correct)_ interpretation of many single spin polarization effects in hadronic interaction is:

## pp  π0 + X cross sections at 200 GeV

- nucl-ex/0602011

- Consistent with NLO pQCD calculations at 3.3 < η < 4.0. NLO pQCD calculations by Vogelsang, et al.
- Data at low pT trend from KKP fragmentation functions toward Kretzer.  PHENIX observed similar behavior at mid-rapidity.

- PRL 92, 171801 (2004)

- √s=200 GeV, <η> = 3.8

- Asymmetry revealed at lower energies
- persists at √s=200 GeV

## Pions identified in this region with severe xF and pT dependence (measured here by STAR in forward region)  tend to carry most of the of the jet momentum (<z>~ 60% to 80%).  

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

## Pt Dependence in Calculations of AN

- Pt Dependence in Calculations of AN

**Sivers Effect**** / ****Collins Effect** these types of models involve:
- _initial state parton distribution_  (Sivers)
- or _final state fragmentation_ distributions  (Collins)
- that introduce transverse spin dependent  _offsets in transverse_ momentum .... independent of the hard scattering (definition of factorization).
- “±” depending on the sign of proton transverse spin direction. _**Using our **__**(STAR)**__** measured cross section form:**_

**Higher Twist Effects**: Off diagonal initial state parton density for hard quark
- Qui and Sterman
- Kouvaris et. al. **Phys.Rev.D74:114013,2006**.
- Fall as 1/PT  as required by definition of higher twist.

- kT shift effect on measured
- cross section.

***All of these models***
***lead to***
***A******N****** ~************ 1/P******T***

## Traditional Factorization in PQCD seems to require AN falling with PTWhat about other approaches?

**Boer,  Dumitru, Hayashigaki**
**Small x gluon modification for unpolarized proton **
**Color Glass Condensate**

- Early STAR data and theory: Highly correlated XF and PT acceptance.

**??If plotted for fixed X****F****, **
**still seems to be falling **
**with P****T****??**

**Needs to be analyzed **
**to separate X****F**** and P****T**** **
**Dependence.**

## FPD                             

- FPD

- West end of the ***STAR*** interaction region

-  FPD++ for Run 6 (2006)

## Run 2 Published Result.

**Caveats:**
**   ****-RHIC CNI Absolute polarization ****    still preliminary.**
**   ****-Result Averaged over azimuthal **
**    ****acceptance of detectors.**
**   ****-Positive X****F**** (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 **
**   ****seen.**
-       ( Presented at Spin 2004:
-                      hep-ex/0502040)

**Run 3 + Run 5 Preliminary  <********>=3.7,4.0**
**(Presented SPIN 2005  Dubna Sept 27-Oct 1)**

## STAR Run 6 Detector Layout

**TPC:  -1.0 < ******** < 1.0**
**FTPC: 2.8 < ******** < 3.8**
**BBC :  2.2 < ******** < 5.0**
**EEMC: 1 < ******** < 2**
**BEMC: -1 < ******** < 1**
**FPD++/FPD: **
**               ******** ****~ 3.3/-3.7**

-  Run 6 (2006) – FPD++

## Detector acceptance

-  Strong correlation between xF
-   and pT in the individual detectors
-  Rapid change in number of
-    events for either increasing pT
-    at fixed xF or increasing xF
-    at fixed pT
-  Broader pT range in xF bins
-    when combining  data at
-    <η>=3.3 and 3.7

## 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

**Figure of Merit **
** ****(P****BEAM****2 ****x L) in Run 6 is ~50 times larger** than from previous STAR runs combined

-  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

-  Theory expects the reverse
-   dependence on η

- π0 AN at √s=200 GeV – xF-dependence Run 6

## AN(pT) at xF > 0.4 

- AN(pT) at xF > 0.4

**Run3+Run5 data (hep-ex/0512013):**

**Run 6 data:**
***consistent with the previous runs in the overlapping  p******T****** region***
***complicated dependence on p******T****** (not 1/p******T******)***
***more precise measurements***

-  Online calibration of CNI
-   polarimeter
-  Hint of AN decrease with
-   increasing pT at pT~1-2 GeV/c

**Phys.Rev.D74:114013,2006**.

**Phys.Rev.D74:114013,2006**

*Preliminary*

## AN(pT) in xF-bins 

- 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 theoretical
-   models

*Preliminary*

## Near full EM coverage  -1<<4
Pairs of Forward Pions       same side correlations (Fragmentation – Collins)
Event by event  “x” measurement from two jets.
Opposite side correlated pions (dijets) 
 Sivers effect
d-Au (Gluon saturation in Nuclei)
Other future objectives
Forward Lepton pairs
Charm

**FMS  1⁄2 Wall  Pb. Glass **

**FMS Wall**

**FMS for Run 7**
**NOW!! **

- Near full EM coverage  -1<<4
- Pairs of Forward Pions       same side correlations (Fragmentation – Collins)
- Event by event  “x” measurement from two jets.
- Opposite side correlated pions (dijets)
-  Sivers effect
- d-Au (Gluon saturation in Nuclei)
- Other future objectives
- Forward Lepton pairs
- Charm

**PHYSICS OBJECTIVES**

## Conclusions

-  **STAR**** collected 6.8 pb****-1**** of data for p+p collisions at √s=200 GeV with transversely polarized beam. With average polarization ~60%, this represents 50 fold increase in analyzing power sensitivity in comparison to all previous RHIC runs.**** **
**We present the P****T ****dependence of A****N**** for restricted bins of X****F**** in a kinematic region where the cross sections is well described by NLO PQCD.**** **
**In contrast to predictions of conventional calculations, the asymmetry is “NOT” falling with P****T**** for 1GeV/c<P****T****<3GeV/c and may indeed be rising.**

## STAR Forward EM Calorimetry History

- STAR Forward EM Calorimetry History

- Prototype FPD
- Run 2 polarized proton data  (published 2004  spin asymmetry and cross section)

- Prototype FMS
- Run 6 2006 (FPD++)

- FMS Forward Meson
- Spectrometer
- Run 7 2007
-  (**hep-ex/0502040).**

- FPD
- Run 3-5 data
- pp, dAu
- (Preliminary An Results)