# Presentation: 673585

## Di-Jet Asymmetry Measurements with STAR

*S. Vigdor for STAR Collaboration, RHIC/AGS Users Meeting, June 2007*

## Level 0 Jet Triggering in STAR

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

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

***Level 0 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.***

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

## 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***

***Trigger-level EMC-only L2 partial jet reconstruction:***
***  ******Search for any EMC ************ ************ ************ = 0.6 ************ 0.6 section with summed EMC E******T****** > threshold***
***  ******Extract (************,************) of jet axis from E******T******-weighted EMC centroid locations***

## Transverse Spin Measurements Have Stimulated Rapid Development of Theory

***Hard hadronic d************  =  PDF’s ************  ******hard partonic d************  ************  ******fragment’n fcn.***

***Non-pert.***

***Non-pert.***

***pQCD***

**Factorization (valid?):**

******

*** ******Observed A******N****** values orders of magnitude too large to arise from explicit chiral-symmetry breaking quark mass terms in QCD Lagrangian.***
*** ******Steep p******T****** -dependence of d************ ************ sensitivity to spin-correlated transverse momentum preferences in non-perturbative factors:***

## Origin of Transverse Spin SSA, continued

***c) Boer-Mulders effect in the ******UN******polarized proton:***
***Can contribute to A******N****** in combination with transversity in polarized proton, via pQCD transverse spin correlation      A******TT****** ************ 0 in quark-quark scattering contributions.***

***ISI and FSI tend to enter with opposite phase ************ predicted sign change of Sivers fcn. between SIDIS and Drell-Yan:  ******non-universality!  ******Hadron, jet production in p+p have ******both****** ISI and FSI  QCD effects.***

## 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.***

## Motivation for pp  Di-Jet Measurement

*** ******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. ***
*** ******Do we observe q Sivers consistent                                              with 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.                                           ***
*** ******Collins and Boer-Mulders asymmetry contributions                           expected to be very small, due to absence of charge-                            sign bias in hadrons omitted from EMC-only jet                reconstruction and small role of q-q scattering at probed p******T****** .***

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

## 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**

## 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*

## 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 asym.***
*** ******1-spin effects vary linearly with k******T******x****** offset***

## STAR Results Integrated Over Pseudorapidity

***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***
*** ******Validity of spin-sorting confirmed by reproducing known non-zero A******N****** for inclusive forward charged-particle production (STAR BBC’s)***

- Pbeam from online CNI analysis, with 20% calib. uncert’y

## What Did We Expect? Constraints from SIDIS Results

***What Did We Expect? Constraints from SIDIS Results***

**W. Vogelsang and F. Yuan, PRD 72, 054028 (2005).**

***E.g., Vogelsang & Yuan use two different models of Sivers fcn. x-dependence:***

***Dijet calcs. include:***
*** ******no hadronization***
*** ******no gluon Sivers fcns.***
***5 < p******T******parton****** < 10 GeV/c***
***Initial-state interactions only (******à la Drell-Yan)***
***Trento sign convention (opposite Madison)  ***

*********1 jet forward, expected *****|*****A******N*****|***** ~ 0.1, little p******T****** - dep.***

## 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)***

## 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***

## Summary and Ongoing Analysis

*** ******First measurement of spin asymmetries for di-jet production in pp collisions ************ Sivers asymmetries consistent with zero, whether dominated by partons in the valence or sea regions.***
*** ******pQCD calculations reconcile small observed asymmetries with larger effects seen in SIDIS and in pp ************ forward hadron, via cancelling ******ISI ******vs. ******FSI ******and ******u****** vs. ******d****** contributions.***
*** ******Ongoing analysis incorporates more triggers + TPC tracks for full jet reconstruction ************ allowance for cuts on jet p******T****** , u vs. d filtering via leading hadron charge sign, etc.  Without adding cuts or events, full reconstruction results consistent with EMC-only analysis.***
*** ******Results constrain unified theoretical accounts for transverse SSA in hard pQCD processes, and connection to parton orbital momentum.***
*** ******STAR plans to make heavy use of di-jets as well in upcoming A******LL****** analyses to begin constraining ************g(x).  Also, use FMS to extend transverse asymmetry measurements to di-jets at large rapidity, where inclusive A******N****** is sizable.***

## Backup Slides

## Slide 17

## Slide 18

## Inferred kT Distribution Width ~Consistent with World Data for pp Collisions

## Comparison of Jet Properties from L2 vs. Full Offline Reconstruction:  Angle Information

**EMC-only ******** resol’n << k****T****-induced ******** width**

**Little correl’n of ******** resol’n with neutral E****T ****fraction**

**EMC-only resol’n on detector ******** ~ same as on jet ******

**Extra ********1****+ ********2 ****  smearing from vertex spread**

**Compares offline jet ******** to L2 detector ******

**Compares offline jet ******** to L2 detector ******

## Comparison of Reconstructed Jet Angles to PYTHIA-Generated Parton Angles

*** ******Full reconstruction of simulated jets (2005 JP2 trigger and status tables) ************ jet angle resolutions, with respect to PYTHIA partons:***
*** ******These resolutions should be added in quadrature to those for the difference between full and L2 EMC-only reconstructions:***
*** ******Net di-jet ************ resolution vs. parton level is then ************ (************ ) ************ 9.0************, compared to intrinsic ************ width ~ 20************ from k******T****** distribution.  Measurement resolution may be a bit better in 2006 with fewer EMC “holes”.***

## 2006 p+p run  STAR measurement of Sivers transverse single-spin asymmetry for di-jets -- shows smaller effects than predicted for observable sensitive to orbital components of parton motion in proton.

**Blue beam asymmetry**

**Yellow beam asymmetry**

**2006 p+p run ******** STAR measurement of Sivers transverse single-spin asymmetry for di-jets -- shows smaller effects than predicted for observable sensitive to orbital components of parton motion in proton.**