# Presentation: 472049

## Probing the Proton’s Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results

**Probing the Proton’s Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results**

- W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.

## Topics: “Slice” of STAR Spin Physics Status & Overview

- Introduction

- STAR longitudinal spin - Recent results

***Topics: “***_***Slice***_***” of STAR Spin Physics Status & Overview***

- STAR Transverse spin di-jets - Recent results

- STAR EMC Calorimetry!

- Summary

- Look at near future and outlook

## Spin Problem & DIS Gluon Helicity Preference Constraints

**World data (2005) on *****g******1******p****** = 1⁄2 ************e******i******2 *****[***********q******i ******(x,Q******2******) *****+***** ************q******i ******(x,Q******2******)*****]**** **

***limited info on scaling violations, ************ on shape or integral of gluon helicity preference ************g(x,Q******2******).***

**Only valence quarks are strongly polarized**

## Prefer dominant/”clean” reaction mechanism w/ large “aLL” ( -jet)

***Theory ingredients:  pQCD factorization***

***+ large parton-level 2-spin sensitivity***

- Prefer dominant/”clean” reaction mechanism w/ large “**a****LL**” (****** -jet)
- But jet and 0 rates are sufficient to give significant G constrain in initial RHIC polarized p data

## Inclusive cross-section 

***Inclusive cross-section ***
***(jets, ******************, and ************±******)***

***Good agreement with NLO pQCD over many orders of magnitude***

***Hadron inclusive production***

***1******st****** RHIC inclusive Jet x-sec***

***pQCD works!***

***also direct ***_***photon ***_***incl. @ PHENIX; ***
*** ***_***forward***_*** incl. ****************** ******@ STAR & incl chg. hadrons @ BRAHMS***

-  ***jets***

-  *********±***

## Relativistic Heavy Ion Collider  

-  100 GeV beam proton beams
-   Each bunch filled with a distinct polarization state
-   Spin Rotators at STAR IR allow for transverse and longitudinal spin orientation
-   Bunch Xings every 100-200ns
-   CNI polarimeters + Hydrogen Jet target provide run by run & absolute polarization
-  HJT calibr to ~********* 5% ***goal in progress

- ...world’s 1st       Collider

| pp Run Year     FOM=P4L | 2002  | 2003 | 2004 | 2005                                                           |  2006 |  |
| --- | --- | --- | --- | --- | --- | --- |
| < Polarization> % | 15      | 30 | 40-45 | 45-50 | 60 |  |
| Lmax [ 1030 s-1cm-2 ] | 2 |  6 | 6 | 16 | 30 |  |
| Lint    [pb-1 ] at STAR (L/T) | 0 / 0.3  | 0.3 / 0.25 | 0.4 / 0 | 3.1 /  0.1  | 8.5 / 3.4,6.8  |  |

## Level 0 High Tower & Jet Triggering in STAR

**Allocated Jet Rate **
**to tape: ~15 Hz**

- 2006 rate ~ 2.5 Hz, sent to tape without prescaling

***Level 0 High Tower & Jet Triggering in STAR***

- 2006 rate ~ 150 Hz, combine with L2 trigger to fit in limited bandwidth

***Trigger either on***
***HT******: ************ 1 (of 4800 BEMC or 720 EEMC) tower E******T****** > thresh - ******Or***
***JP******: ************ 1*** ***(of 12 BEMC or 6 EEMC) hard-wired jet patch  ********* *****E******T****** > thresh.***
***[note: also a minbias cond]***

**BARREL EMC**

**ENDCAP  EMC**

**Notes:**
Message:
Key for spin : TPC+EMC
Key for rel lum : BBC=minB triggers

## Inclusive ALL measurements (, ±, and jets)

* **Predicted A**LL** sensitivity for different ****G scenarios*

- Inclusive ALL measurements (, ±, and jets)

-  ************

-  *********±***

-  ***jets***

******

## Neutral pions (in Jets); Inclusive 0 ALL

- Neutral pions (in Jets); Inclusive 0 ALL

- /ndf compared to NLO calculations
- (ignoring systematic errors):
-  GRSV Std: 	0.8
-  GRSV Max: 	2.5
-  GRSV Min: 	0.8
-  GRSV Zero: 0.4

-  GRSV max scenario disfavored

- use  invariant mass spect. (data from HT trig)
- MC 0 line shape
- low inv. mass bkgrd
- comb. bkgrd & residual fit
-  assoc w/ full Jets if within a Jet cone (0.4 in , ) but typ. ~ 5 deg.

- 2005 data

## Jet Reconstruction in STAR

***Jet Reconstruction in STAR***

***Search over all possible seeds (p******T******seed****** > 0.5 GeV) for stable groupings***
*** ******Check midpoints between jet-jet pairs for stable groupings***
*** ******Split/merge jets based on E******overlap***
*** ******Add all track/tower 4-momenta***

*** ******Data well described by MC***

## Inclusive Jet ALL and x-sec Analysis Issues     

- Inclusive Jet ALL and x-sec Analysis Issues

- Use Simulation (MC) to provide correction to RAW jet yield                   - trigger and jet inefficiencies               - jet resolution & bin migration            - undetected particles (n  + )            - PYTHIA 6.205 CDF Tune A            - GEANT (Geisha)
- Verification of DATA/MC agreement  essential

**The shape of the  Fraction of Neutral Energy in the Jet (EMF) is sensitive to the trigger bias as well as contributions from beam background.**

**Minbias**
**JP2**
**HT2**

*STAR Preliminary*

**Notes:**
Remove?

## Effects of Jet Resolution

-  Systematic offsets in pT cause dilution of the jet asymmetry which depend on the size of the asymmetry!

- Trigger Bias

- The trigger biases jets toward higher neutral energy. This may change the ratios of  qq+qg+gg and therefore change the asymmetries

**Trigger Bias: **
- JP << HT
- ALL/ALL   larger at low pT
**Total Systematic:**

**Notes:**
For our best guess of the proton structure, we compute how the average Pythia jet is reconstructed in our detector.  The 25% jet energy resolution dominates, causing the more numerous low energy jets to be reconstructed in a higher energy bin, so the jets we measure in the detector on average have to be shifted approximately 25% lower in pT.  The ERROR on this shift is given as the systematic error: since the shift is slightly subprocess dependent, we determine the shift independently for each subprocess, then take the largest deviation from the combined shift, for EACH of the assumed gluon structure functions (except GRSV MAX, which is ruled out.)

## 2005 Inclusive jet ALL at mid-rapidity

- g = -g (min)
- g = 0
- GRSV-STD

*** ******Error bars are statistical***
*** ******Systematic band includes 25% scale error from current polarization uncertainties due to online values***
*** ******Results in good agreement with ’03/’04 A******LL ******data******* ******in region of overlap but ~ 4 times more precise and p******T****** range nearly doubled [* PRL 97,252001 (2006)]***
***	******	***

| Systematic | (x 10 -3) |
| --- | --- |
| False Asymmetries | <6.5 
(pT dep’t) |
| Reconstruction + Trigger Bias | 2-12 
(pT dep’t) |
| Non-long Polarization | 3 |
| Relative Luminosity | 2 |
| Backgrounds | <1 |

***2005 Inclusive jet A******LL****** at mid-rapidity***

## Constraint on G

*Significant new constraints on ****G when compared to predictions derived from one global fit to DIS data*

***GRSV DIS best fit=0.24***
***1************ = -0.45 to 0.7***
***PRD 63, 094005 (2001)***

***Constraint on ************G***

## Inclusive Jet () Data from 2006 -> Greater Discr’m Power for g

***Inclusive Jet (******************)****** ******Data from 2006 -> Greater Discr’m Power for ************g***

*** ******High-statistics (esp. at high p******T******) inclusive jet and ***********0**** *****A******LL****** data from 2006 will select among ************g models, ******assuming****** a shape of ************g(x,Q******2******).***
*** ******Need global analysis including these A******LL ******results!  ***

- Significant increase in sampled luminosity
- Polarization typically ~60%
- acceptance in BEMC increased by a factor of 2

- significant increase in figure of merit!

************

-  also analysis w/ Endcap EMC

-  + vs. - analysis in Barrel

## Di-Jet Asymmetry Measurements with STAR; Sivers Transverse Spin Asymmetries

*** ******Needs ISI and/or FSI to evade time-reversal violation***
*** ******Assuming QCD factorization, subsume ISI/FSI contributions in gauge-invariant kT -dependent parton distribution fcns.***

*** ******Characterize by Sivers effect:***
*** ******Sensitive to parton orbital angular momentum. ***

**Do quarks and/or gluons have transverse motion preferences in a proton polarized transverse to its momentum?**

## Motivation for pp  Di-Jet Measurement

*** ******Do we observe q Sivers consistent                                              w/ HERMES, after inclusion of proper                                                pQCD-calculable ISI/FSI gauge link factors                                          for pp ************ jets?  Tests limited TMD “universality”.***
*** ******First direct measurement of gluon Sivers effects.  ***
*** ******Expect Collins and Boer-Mulders asy’m contributions                           small: no charge-sign bias from EMC-only L2 trigger jet reconstruction & small role of q-q scattering at probed p******T***
***2006 p + p run at RHIC; 3 wks transverse spin @ STAR***

***Motivation for pp ************ Di-Jet Measurement***

*** ******HERMES transverse spin SIDIS asymmetries ************ u and d quark Sivers functions of opposite sign, different magnitude.***
***Sivers effect in pp ************ spin-dependent sideways boost to di-jets, suggested by Boer & Vogelsang (PRD 69, 094025 (2004))***

***Both beams polarized, x ******+z****** ************ x ************z****** ************ can                                distinguish high-x vs. low-x (primarily                                               gluon) Sivers effects. ***

## STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in 2006

-  =+2

-  = -1

- TPC

**EMC Barrel**

**EMC **
**Endcap**

- BBC East

- BBC West

- Yellow (-z) beam

- Blue (+z)

***STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in 2006***

- Endcap
- essential
- for hi-x vs.
- lo-x Sivers
- distinction

- Signed azimuthal opening angle 

**Broad ********1,2**** coverage**

**Full, symmetric ********1,2**** coverage**

## Fast MC Simulations Illustrate Di-Jet Sivers Effects

***Fast MC Simulations Illustrate Di-Jet Sivers Effects***

*** ******2-parton events, transverse plane***
*** ******match full jet reco. p******T****** distribution***
*** ******Gaussian + exp’l tail k******T****** distribution fits ************ distribution***

*** ******random k******T******x,y****** (rms = 1.27 GeV/c) for each parton***
*** ******Sivers spin-dep. k******T******x****** offset ************ ************ shift, L-R di-jet bisector asymetry***
*** ******1-spin effects vary linearly with k******T******x****** offset***

## STAR Results Integrated Over Pseudorapidity

- effective  beam pol’n for each slice =  Pbeam |cos(bisector)|

-  rotation samples kTy, parity-violating sp•kT correlation

***STAR Results Integrated Over Pseudorapidity***

*** ******Sivers asymmetries consistent with zero with stat. unc. = *********0.002**
** *****Fast MC ************ sensitivity to Sivers **********k******T******x********* *****offset ************ few MeV/c ************ 0.002 *********(*****k******T******x*****)****2********1/2**
*** ******Systematic uncertainties smaller than statistics***
*** ******All null tests, including forbidden 2-spin asym. ************ cos(************bisector******), consistent with zero, as are physics asymmetries for all polarization fill patterns***
*** ******Note: P_beam from online CNI analysis, with ************20% calibr. uncertainty***

- STAR data

## What Did We Expect? Constraints from SIDIS Results

***Fits to HERMES SIDIS Sivers asym constrain u and d quark Sivers functions, for use in pp ************ dijet + X predictions. ***

- Bomhof, Mulders, Vogelsang & Yuan, *hep-ph*/0701277

*** ******5 < pTparton < 10 GeV/c***
***Initial State  Inter’ns only (à la Drell-Yan)***
***Trento sign conv. (opposite Madison)  ***

***Bacchetta et al. *****[PRD 72, 034030 (2005)]***** deduce gauge link struct for pp ************ jets, hadrons:***
***         ************ ******A******N *****(ISI+FSI) ******** ******** 0.5 *****A******N *****(ISI)**
*** ******Gauge links more robust for SSA wt’ed by ************ p******T****** or *****|sin ********** |, *****due to k******T****** - factorization breakdown (Collins & Qiu, arXiv:0705.2141)***

***models (2) of Sivers fcn. x-dep***

## STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum I

***STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum I***

**Trans Spin Asym**

**Blue beam**

**Yellow beam**

- Extract analyzing powers averaged over ** and b w/ fit to asym’s in |cos b|  using cross ratio:

- Typical xT ~ 0.05 - 0.10;
- 1+2 range  0.01 < xBj < 0.4

## STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum

***STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum***

**Blue beam**

**Yellow beam**

*** ******All calcs. for STAR ************ acceptance***
*** ******Reverse calc. A******N****** signs for Madison convention***
*** ******Scale Bomhof calcs by *****1/********|sin **********|******** ******** 3.0 *****to get A******N****** of unit max. magnitude***
*** ******u vs d ******and ******FSI vs ISI ******cancellations ************ sizable SSA in inclusive fwd. h prod’n and SIDIS (weighted SSA) compatible with small weighted di-jet SSA -- ******test via LCP flavor select***

## Near Term future, and RHIC run 8 & 9

***Near Term future, and RHIC run 8 & 9***

*** ******Inclusive channels for longitudinal spin prog suffer from broad integration over x ************ model-dep. ************G****** ******extraction as well as other systematic issues***

*** ******With improved beam & detector performance, focus will now shift to ******jet-jet and ************ -jet coincidences****** for event-by-event constraints on colliding parton x******1,2****** .***

*** ******Sivers -> Ongoing analysis incorporates TPC tracks for full jet reconstruction ************ allowance for cuts on jet pT , u vs. d filtering via leading hadron charge sign, etc.  (w/o cuts consistent w/ EMC only)***

*** ******Presently in detailed planning stages for  RHIC run 8 (accelerator cool-down ~1 Nov. 2007) ... as per the Beam Use Request (BUR) there will be a significant amount of  polarized pp running (divided among longitudinal and transverse spin orientations) ... ******as we also work on run 6 analysis!***
***Similarly we expect a signifcant amount of polarized pp beam in run 9***

## 2008-12: Coincidence Measurements to Map g(x) Fully

***2008-12: Coincidence Measurements to Map ************g(x) Fully***

***For example, simulations (L. Bland) of STAR capabilities for ************ - jet coincidences give rough indication of ************g discriminating power for various models of input gluon polarization.***

*** ******Simplified LO analysis used for simulations here to illustrate sensitivity***
*** ************ ******- jet and di-jet measurements @ ************s = 200 & 500 GeV, will map  ************g(x************ 0.01-0.3,high Q******2******), when included in NLO treatments of entire spin structure database.***

## Slide 26

## Summary

** ****NLO pQCD describes hadron **_**x-sections**_** at RHIC for inclusive *****jets******,***
*********************and ************±******) *****allowing spin program to access ********G directly**
_**Longitudinal spin**_**: ***** 2005 inclusive jet data provide significant new ***
***constraint on ************G when compared to predictions derived from one***
***global fit to DIS data ******(******GRSV-max scenario ruled out w/ ‘03/’04 data);***
***2006 data should provide sizable add’l constraint (as will global fits!)***
**STAR longitudinal spin program entering phase of correlation and**
**direct ********measurements, while continuing to expand the p****T**** reach of the **
**incl. channels; *****2008-09, focus on A******LL****** for ******di-jet and ************+jet production****** ***

*** ***_***Transverse spin***_***: spin asym’s for ******pp*** ***di-jet production ************ Sivers asym’s consistent w/ zero, whether dominated by valence or sea partons ... data will constrain unified theoretical accounts of SSA in hard trans spin pQCD, and connection to parton orbital momentum.***
*** ******Present ******pQCD calcs. reconcile small observed asym’s with larger effects seen in SIDIS (& pp ************ forward hadron), via cancelling ******ISI ******vs. ******FSI ******and ******u****** vs. ******d****** contr. ***

*** *****p+p in pQCD regime viable complement to DIS *********** ***_***more data coming***_***!***

## BACKUPS 

## 2005 Inclusive Jet ALL         		

- _GRSV curves_*
- G = G
- G = -G
- G = 0
- G = STD

*Phys.Rev.Lett 97 252001 (2006)*

- 2005 ALL is consistent with previous 2003/2004 results.

## EMC-Only Information OK For 1st Dijet Sivers Asymmetry

***EMC-Only Information OK For 1******st****** Dijet Sivers Asymmetry***

**Full offline di-jet reconstruction for ~2% of all runs shows triggered jet *****p******T****** *****spectrum:  **

- Typical xT ~ 0.05 - 0.10;
- 1+2 range  0.01 < xBj < 0.4

**and ******** angle resolution loss @ L2 OK:  **

- [()=5.0, ()=0.10] full reco. jet vs. parton angles

- [()=3.9, ()=5.8] L2 vs. full jet << observed()  20, mostly from *k**T*

## Distinguishing Sivers from Collins Asymmetries

***Distinguishing Sivers from Collins Asymmetries***

***In SIDIS, can distinguish transverse motion preferences in PDF’s (Sivers) vs. in fragmentation fcns. (Collins) via asym. dependence on 2 azimuthal angles:***

***HERMES results ************ both non-zero, but ************ +****** vs. ************ ******–****** difference suggests Sivers functions opposite for ******u ******and ******d****** quarks.***

## Theory of Transverse SSA Developing Very Rapidly!

***Bacchetta, Bomhof, Mulders & Pijlman *****[PRD 72, 034030 (2005)]***** deduce gauge link structure for pp ************ jets, hadrons:***
***         ************ ******A******N *****(ISI+FSI) ******** ******** 0.5 *****A******N *****(ISI)**
****** *****Gauge links more robust for SSA weighted by ************ p******T****** or *****|sin ********** |, *****due to k******T****** - factorization breakdown (Collins & Qiu, arXiv:0705.2141)***

##         Detector at RHIC

-  = -ln[tan(/2)]

| 
TPC
||<1.4
 | 
Charged particle momentum |
| --- | --- |
| 
BEMC
||<1.0
 | 
Neutral Energy
High pT Trigger |
| 
EEMC
1<<2
 | 
Neutral Energy
High pT Trigger |
| 
BBC
3.4<<5
 | 
MinBias Trigger
Relative Lumi
(also ZDC) |

- 

## STAR Calorimeter Coverage

***FMS, EEMC and BEMC provides nearly complete EM coverage from -1 ************ ******η****** ************ +4***