# Presentation: 264320

## Spin Physics With Jets at STAR

***Spin Physics With Jets at STAR***

*S. Vigdor, BNL, Ides of March 2007*

**RHIC Spin Goals**
**RHIC Spin Infrastructure**
**STAR Jet Infrastructure**
**Inclusive Jet Results 2003-5:  Constraining the Gluon Contribution to Proton Spin**
**Single-Spin Asymmetry for Di-jets 2006:  Looking for Parton Orbital Momentum Effects**
**Next Stages of Jet Program**

## Nucleon Structure Goal:  How are Various Degrees of Freedom Interrelated?

***Nucleon Structure Goal:  How are Various Degrees of Freedom Interrelated?***

***Three constituent quarks in s-states ************ good account for baryon magnetic moments -- attributes nucleon spin to quark spin alignment in rest-frame wave function.***

##  Access to spin observables in hard partonic scattering processes treatable via perturbative QCD 

********* ******probe non-perturbative nucleon spin structure via partonic degrees of freedom:***
*** ******Do gluons account for “missing” proton spin?***
***    ******Gluons provide ~ half of proton’s mass, momentum; do they also play important role in spin?  If not ************g, then orbital contributions!***
*** ******Transverse quark motion and spin preferences in transversely polarized nucleon?***
-      ***Transverse motion ************ orbiting mesons? Transverse spin decoupled from gluons, relativistically distinct from helicity preferences.***
*** ******Gluon splitting vs. Goldstone bosons and flavor-dependent sea quark polarization?***

***Jet production studies illuminate first two major questions!***

## RHIC Spin Infrastructure

## RHIC  Riken-BNL Helicity-Identified Collider

**Polarization survival has been demonstrated to 250 GeV beam momentum (from 24 GeV injection)**

***RHIC ************ ******R******iken-BNL ******H******elicity-******I******dentified ******C******ollider***

***RHIC uses super-conducting helical dipole magnets for Snakes and spin rotators***

***Pioneering accelerator development ************ unique spin capabilities!***

## Steady Improvements in Polarized Beam Performance 

***2003 ************ 2006  ************  > 2 orders of magni-tude improvement in P ******4******L******  relevant to 2-spin asym-metries!***

***Factor ~ 5--6 remains to reach “enhanced design” goals***

***STAR ************ s = 200 GeV pp Sampled Luminosities***

***2006 delivered pp ************ ******L****** dt***

***May 2006***

***May 2006***

## The Beam Polarization Monitoring Chain

| Physics process | Type of polarimeter | Accuracy, 
 limitations  |
| --- | --- | --- |
| AN in pp elastic scattering in CNI region | Polarized hydrogen gas jet target (calibrate Pbeam against Ptarget) | Absolute (~6%) calib.,
Some dedicated fills
1 value per year |
| p+12C CNI elastic scattering 
 | Carbon -ribbon target in RHIC beam,
Recoil carbon detection in Si | Absolute ~20% (so far) 
Rel. ~3% + spin orient’n
~1 minute, affects beam,
few  per RHIC fill |
| p+p  hadrons at forward rapidity | Beam-Beam counters @ STAR
Min-bias trigger | relative 10%, ~20 minutes every STAR run,
Tune spin rotators |
| Polarization pattern of bunches | 3 bits encode +,-,0
1bit : filled/empty
for every beam | 8bit word in every event
Sometimes bits get stuck or lose sync |
| Finding bunch #1
 | Localize abort gap | Needs ~5 minutes 
Sometimes ‘filled’ bunches are empty |

- Jet pol

- CNI pol

- BBC pol

***The Beam Polarization Monitoring Chain***

## STAR Jet Infrastructure

##   Detector

-   ***Detector***

****=0

**Forward Pion Detector**

**Endcap EM Calorimeter**

**Beam-Beam Counters**

**Time Projection Chamber**
**-2<****η****< 2**

**1<****η****< 2**

**-4.1<****η****< -3.3**

**2<|****η****|< 5**

**Solenoidal Magnetic**
**Field 5kG**

****=2

****= -1

**Tracking**

**Lum. Monitor **
**Local Polarim.**

**Triggering**

**Triggering**

-  = - ln(tan(****/2)

**WEST**

**EAST**

## EMC’s Essential for the Spin Program and Jet Detection

***EMC’s Essential for the Spin Program and Jet Detection***

***STAR barrel (completed 2006), endcap (completed 2005) and forward (upgraded 2006) em calorimeters permit triggering/reconstruc-tion for jets, ************, ************0******, ...***

**p+p Di-jet event in BEMC**

**STAR central Au+Au**

**STAR Barrel EMC (Wayne State U. + ...)**

**STAR Endcap EMC (Indiana U. + ANL + ...)**

**STAR Forward Pion Detector (BNL + ...)**

## Jet Energy Scale Relies on EMC Calibration

***Jet Energy Scale Relies on EMC Calibration***

***Relative gain of every tower (4800 for BEMC, 720 for EEMC) determined from MIP response. Identify MIP’s via tracks for BEMC, via SMD + preshower + postshower response for EEMC***

***Absolute E scale set by comparison to track momentum for identified electrons***

***Cross-check from reconstructed   *********** 0***** invariant mass.***

***Overall gain precision = ************ 5% presently.  Aim for 2% in near future.***

## Jet Triggering in STAR

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

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

***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***

***Correction philosophy:***
*** ******PYTHIA+GEANT ************ detector response***
*** ******Use to correct for ******“particle” ************ “detector” ******effects of resolution, efficiency, bias***
*** ******PYTHIA also ************ study ******“parton” ************ “particle”****** effects of hadronization, event energy***

## Simulations Use Same EMC Status Tables as Data Analysis to Reproduce Details of Response

***Simulations Use Same EMC Status Tables as Data Analysis to Reproduce Details of Response***

- Efficiency dips between jet patches, sometimes enhanced by FEE hardware problems, reflected in status tables

- Observed jet shapes well reproduced

## Subtleties of Jet Analysis:  Jet Energies

* **p**T* (measured)

***Subtleties of Jet Analysis:  Jet Energies***

***Steep p******T****** spectrum + crude jet energy resolution ************ detector jets spawned, on average, by lower-p******T****** particle jets (despite loss of neutral hadrons w/o hadron calorimeter).***

***Simulation + di-jet data analysis ************ resolution ************/p******T ************ 25%***

- Log yield

*** ******p******T****** shift incorporated to date by cross section corrections, A******LL****** syst. errors***
*** ******Future pubs. to correct E scale, with uncertainties in horizontal errors***
*** “******Particle” ************ “detector” jet shift ************  same for gg vs. qg vs. qq***
*** “******Parton” ************ “particle” shifts from hadronization (E loss) and underlying event (E gain) are subprocess-dependent, to be incorporated in syst. errors***

## Subtleties of Jet Analysis:  Trigger Bias

***Subtleties of Jet Analysis:  Trigger Bias***

***Conclude:***
*** ******Cut out jets at very high or very low EMF***
*** ******Use simulations to estimate syst. errors from trigger bias ***

***High Tower and Jet Patch triggers require substantial fraction of jet energy in neutral hadrons***
***Trigger efficiency turns on slowly above nominal threshold***
*** ******Efficiency differs for quark vs. gluon jets, due to different fragmentation features***
***Simulations reproduce measured bias well, except for beam background at extreme EM energy fraction***

## Inclusive STAR Jet Results 2003-5

## Published Jet Production Cross Sections (2003+4) Rely on Simulated Correction Factors

- Jet *p**T* resolution ~ 25%
- HT efficiency changes by 2 orders of magnitude!
- Consistent results from both PYTHIA and HERWIG

***ε******jet*** : decreases *c(p**T**)*
***resolution****: *increases *c(p**T**)*
***ε******Trig***: ~1 e-2 at *p**T-jet* =   5 GeV
-              ~1 at *p**T-jet*  = 50 GeV

*“**measured”*

*“**true”*

- Simulation

***Published Jet Production Cross Sections (2003+4) Rely on Simulated Correction Factors***

## Early RHIC pp Absolute Cross Section Results

**pQCD works! *****Absolute cross sections for channels critical to gluon polariza-tion determination -- p+p ************ ************0******+X, jet + X, ************ +X at ************ s = 200 GeV*** – ***are****** ******well repro-duced in NLO pQCD calcs. down to p******T ******~ few GeV/c.***

***Low-p******T****** reach of robust pQCD account was not anticipated!***

## Current Info on Unpolarized Quark & Gluon Distrib’ns

***Current Info on Unpolarized Quark & Gluon Distrib’ns***

***g*****(*****x,Q*****2****) *****from scaling violations in F******2***

## Polarized DIS Data Are Too Sparse to Constrain Gluon Helicity Preferences Well:

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

## Theory ingredients:  pQCD factorization

***Theory ingredients:  pQCD factorization***

***+ LO + NLO pQCD 2-spin asymmetries***

## STAR 2005 Inclusive Jet ALL Results

***STAR 2005 Inclusive Jet A******LL****** Results***

***3.1 pb***********1***** sampled ************ 1.6 pb***********1***** after run selection; **********P******B******P******Y********* ******** 0.25**

***Ongoing analysis is:***
*** ******recovering more runs    ***
*** ******correcting to particle jet E scale***
*** ******reducing systematic errors ***
*** ******quantifying constraint on ************g***

***Results do not support ************g*****(1.0***** *****GeV****2****) *****much > 0.4 within GRSV shape ansatz***

## STAR 2005 Inclusive Jet ALL Results are Consistent Among Different Jet Triggers, and With 2003+4 STAR Results and 2005+6 PHENIX ALL for Inclusive  0

***STAR 2005 Inclusive Jet A******LL****** Results are Consistent Among Different Jet Triggers, and With 2003+4 STAR Results and 2005+6 PHENIX A******LL****** for Inclusive *********** 0**

## Inclusive Jet Data from 2006 Will Provide Far Greater Discriminating Power for g

***Inclusive Jet Data from 2006 Will Provide Far Greater Discriminating Power for ************g***

*** ******Inclusive channels suffer from integration over x ************ model-dependent ************G****** ******extraction.***

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

*** ******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******).***

## Single-Spin Asymmetry for Di-Jets 2006

## Sizable Transverse Single-Spin Asymmetries Seen for Forward Hadron Production

***Sizable Transverse Single-Spin Asymmetries Seen for Forward Hadron Production***

***Similar to SSA seen in FNAL E704 @ ************s = 20 GeV, but here in regime where pQCD accounts for cross section!***

## Transverse Spin Measurements Have Stimulated Rapid Development of Theory

*** ******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:***

## 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

***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***

- 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)***

## 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****** for 2 jets, or *****|sin ********** |**

***Di-jet SSA Post-dictions shrinking!***

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

*** ******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***

## Next Stages of STAR Jet Spin Program

***Next Stages of STAR Jet Spin Program***

**Transverse spin:**
*** ******Full jet reconstruction for 2006 di-jets for p******T****** - sorting, leading hadron charge determ. for flavor selectivity***
*** ******New Forward (************ = 2.5 - 4) Meson Spectrometer extends coverage to region where large inclusive A******N****** seen***
**Longitudinal spin:**

*** ******Exploit full di-jet kinematic range to enhance sensitivities:***

*** ******E.g., ***********1***** ************ ***********2***** > 1 enhances qg scattering sensitivity to ************g(low x)***

*** ******High p******T******, large |***********1***** ************ ***********2****|, **********1***** + ***********2**** ******** 0, *****HT trigger enhances qq sensitivity to test pQCD***

***to be expanded by FMS!***

## 2006-10: Coincidence Measurements to Map g(x) Fully

*** ******E.g., detect ************-jet coincidences in polarized proton collisions at  ************s = 200 and 500 GeV***
*** ******Measure two-spin asymmetry in production rates between equal vs. opposite helicities, as function of ************(jet), ************(************), p******T ******(************ )***
*** ******Assuming 2-body parton kinematics, can infer initial x values of gluon and quark***

*** ******Next-Leading-Order (NLO) pQCD analyses of data, with DIS database, can extract ************g(x) for Q******2****** ~ 100 GeV ******2***
*** ******LO pQCD analysis of simulations at right ************ STAR sensitivity for 3 diff. models of initial gluon helicity distributions***

***2006-10: Coincidence Measurements to Map ************g(x) Fully***

## Summary & Conclusions

***STAR jet spin program is well under way, fueled by ample cross sections, large STAR acceptance & efficient EMC triggering. ***
***NLO QCD describes cross section well over 7 orders of magnitude.***
***Inclusive jet A******LL****** beginning to provide best constraints on gluon polarization in the proton.  2006 data will ************ dramatic improvement.***
***Di-jet A******N******Sivers****** all consistent with zero ************ compatible with sizable A******N****** for inclusive forward hadrons and SIDIS? Add flavor enhancement to test u vs. d cancellation.***
*** ******Early successes ************ shift focus now to:***
***Improve EMC calibration and quantitative understanding of syst. errors from energy scale ambiguities, trigger/reconstruction bias***
***Extend coincidence acceptance with Forward Meson Spectrometer***
***Exploit full kinematic characterization of parton scattering in di-jet and ************ - jet coincidences to enhance interpretation of gluon polarization and transverse motion preferences in polarized proton***
***Extend measurements to 500 GeV c.m. energy ************  smaller x***

## Backup Slides

## Theory Systematics

- Changing the pdf

- Changing the scale

## Jet Energy Scale Systematics

|  Charged Track Momentum | p/p TPC | 1% |
| --- | --- | --- |
|  Charged Track Inefficiency | 10% x 60% jet | 6%* |
|  Neutral Tower Energy | Gain/Gain = 10% | 40% in yield |
|  Neutral Energy Inefficiency | ∑ET from n+K0L/S+++ | 6-10%* |
|  MIP Subtraction | 10% Correction to  Jet ET | 1% |
|  Fiducial Detector Effects | Edges/Dead regions | 2% |
|  Jet  calculation | 1 tower = =0.05 | 3% |
| Background Trigger | Background trigger +MINBIAS event | 5% in yield |
| Background Energy | Jet + underlying Background |  0 |
| Underlying Event | Work in Progress |  NA |
| Fragmentation | Comparison to HERWIG ongoing |  NA |

**** Included in Correction Factor***

## Slide 44

## Slide 45

## Slide 46