# Presentation: 043887

## Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV

**DIS 2007**
**April 16-20, 2007**
**Munich, Germany **

- Measurement of Sivers asymmetry for di-jets at STAR in polarized p+p collisions at 200GeV

- Outline
-  Motivation
-  Theory predictions
-  Principle of measurement, L2 trigger
-  Fast M-C model of Sivers spin asymmetry
-  Measured SSA
-  Conclusions

**Notes:**
Understand the origin of spin of proton
Proton consist of valence q., sea q-qbar, and gluons – each carrying its own spin and angular momentum –yielding the total proton spin of 1⁄2
STAR measurements focus on polarization of gluons and sea-q. inside proton   

##   1⁄2  =  1⁄2   +  G  +  Lq  +  Lg

- Where does the proton’s spin come from?

- p

-   1⁄2  =  1⁄2   +  G  +  Lq  +  Lg

- p is made of 2 u and 1d quark

- S  =  1⁄2  =    Sq

- Explains magnetic moment of baryon octet

- BUT partons have an x distribution and there are sea quarks and gluons

- Check via electron scattering and find quarks carry only ~1/3 of the proton’s spin!

**Jets,pions,A****LL**

**Di-jets, Sivers A****N**

## 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 q vs. g Sivers effects. ***
*** ******Do we observe q Sivers consistent                                              with HERMES?  Tests universality.***
*** ******First direct measurement of gluon                                            Sivers effects.***

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

## History of Predictions of Sivers AN for Di-jets @ STAR

## Detector

****=0

**Endcap EM Calorimeter**

**Beam-Beam Counters**

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

**1<****η****< 2**

**2<|****η****|< 5**

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

****=2

****= -1

**Tracking**

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

**Triggering**

**Triggering**

-  = - ln(tan(****/2)

**+z beam**

**-z beam**

## L0 Jet Trigger

**Luminosity 3*10****31****/cm****2****/s**
**Allocated  Jet Rate **
**to tape only 15 Hz**

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

##  STAR Di-Jet Software Trigger (developed in 2006)

-  STAR Di-Jet Software Trigger (developed in 2006)

**Rates @ STAR **
-  9.4 MHz :  RHIC bunches collide
-  300 kHz : min-bias collisions trigger BBCs
-  150 Hz : L0 single jet exceeds 4 GeV ET
- in a **hardwired 1x1** ( x ) EMC patch
-  **10 Hz : L2 di-jet** trigger with ET > 3.5 GeV
- for a pair of back-to-back **sliding 0.6x0.6** patches
- ~**80 muSec decision time**

**Notes:**
Presented data only from d-jet L2 trigger, one of several contributiong to STAr di-jets

## STAR  Di-Jets Acquired in 2006 run

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

- Sivers SSA

## Fast M-C  Captures Essence of Sivers SSA

- Back-to-back partonic collision yields a di-jet
- intrinsic kT1,2 smearing
- 2D *Gauss*(0,kTx, kTy)
-  sum kT from both IS
- kT=kT1+kT2
-  sum:
-    partonic pT + intrinsic kT

**Model Generator**:

**Notes:**
Spent lot of time explaining:
- back-to-back  peak in zeta
Green circles  width of zeta
kTy does NOT depend on spin
kTx DOES depend on spin

## Fast M-C Tuned to Real Di-jets

****

- Data

- Fast M-C

- Data

- Fast M-C

****

## Fast M-C: shift in   equivalent to Sivers AN 

- Fast M-C: shift in  ****** **equivalent to Sivers AN

- Displaced  peak
-  for pol=+ and -

- SSA AN results
- naturally as
- the difference

**Assume non-zero Sivers  <k****T****X****>=100 MeV/c **

## STAR Di-Jet Results Integrated Over Pseudorapidity

***STAR Di-Jet Results Integrated Over Pseudorapidity***

*** ******Sivers asymmetries consistent with zero with stat. unc. = *********0.002**
** *****Fast M-C model ************ sensitivity to Sivers **********k******T******x********* *****offset ************ few MeV/c ************ 0.004 ************(k******T******x******)***
*** ******Systematic uncertainties smaller than statistics***
*** ******All null tests, including forbidden 2-spin asym. ************ cos(************bisector******), consistent with zero***
*** ******Validity of spin-sorting confirmed by reproducing known non-zero A******N****** for inclusive forward charged-particle production (STAR BBC’s)***

## STAR Results vs. Di-Jet Pseudorapidity Sum

- Emphasizes (50%+ )
**quark **Sivers

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

***V&Y calcs. include:***
*** ******no hadronization***
*** ******5 < p******T******parton****** < 10 GeV/c***
*** ******STAR ************ acceptance***
*** ******HERMES-fitted q Sivers ***
*** ******no gluon Sivers fcns.***
***They predict:***
*** ******A******N****** ~ A******N******HERMES****** where q dominates, if only ISI included (******à****** la Drell-Yan)***
*** ******A******N****** sign reversal, factor ~2 reduction when gauge links include FSI as well***
***Reverse V&Y signs here to obey Madison convention!***

- Emphasizes (80%+)  **gluon** Sivers

## AN  , yields weighted w/ |sin()|

**Bacchetta, **_**Bomhof, Mulders**_**, Pijlman**

**A****N  ****, **yields weighted w/ |sin()|

- ISI

- SPIN **2006**

**Bomhof, Mulders, Vogelsang, Yuan,**
**hep-ph/0701277**
- 31 Jan **2007 **

- ISI+FSI

- u-quark

- d-quark

**u+d **

- Theory Few Months Later

- Delicate cancellations of Sivers fcn for:
-  u and d quarks  and
-  ISI and FSI
- reconciles theory & experiment

- ISI+FSI

## STAR Results : weighted AN

***STAR Results : weighted A******N***

- AN

- Di-jet yield

******=180 deg**

- Di-jet yield
- W=|sin()|

- Theory calculation for AN more robust,
- better characterization of initial state kT.
- Describes forward inclusive pi0's and
- analogous k_T-weighted SIDIS asymmetries moderately well

- AN
- (weighted)

****

## Summary & Outlook

-  STAR acquired 2.6M di-jet events w/ transversely polarized proton beams at sqrt(s)=200 GeV
- Experimental values of Sivers SSA for quarks & gluons is consistent w/ zero +/-0.002
- Early large predictions tuned to non-zero HERMES quark Sivers SSA has been reduced due to accounting for cancellation between ISI & FSI and u- and d-quarks Sivers fcns.
- More detailed analysis is ongoing ....

- Backup Slides

## CNI  Measured On-line  60%+ Polarization at RHIC in 2006 

**CNI**  Measured On-line  60%+ Polarization at RHIC in 2006

- CNI
- pol

-  Year 2006

-  Year 2002

## 2006 pp200 Transverse Run @ RHIC

- Yellow bXing (uses revTick)

- STAR event count

- Yellow
- Abort gap

- Blue
- Abort gap

**spin8 state**

- STAR spin sorting (uses V124)

- Imperfections : few ppm

- Available STAR events:
-  both TPC & EMC active
-  few M 1-jet trig, ET>8.0 GeV
-  3M di-jet trig, 2xET>3.7 GeV
-  few M di-jet w/o TPC
-  (crude run QA)

- Example of di-jet events for one of 4
-  RHIC fill polarization patterns { \\ , \\+1, // , //+1 }

-  other value
**B –****Y –  ****51**
**B –****Y + **** 53**
**B +****Y –  ****83**
**B +****Y + **** 85**

## Is the Di-jet Spin Sorting Correct? ( Find MinB BBC SSA)

- Off-Line

- On-Line

- West-Blue
- 5.6 

- West-Yellow
- 0.3 

- East-Blue
- 0.1 

- East-Yellow
- 5.8 

- Pol patt 1,2,3,4

**YES !**

- 12M MinB evens

## Theory - exp’t discrepancy raises questions!

***Are observed di-jet Sivers SSA much smaller than predictions because:***
*** ******ISI & FSI  both important in pp ************ jets and tend to cancel?***
***Need q Sivers or different q Sivers x, k******T****** - shapes in HERMES fits?***
-  ***Contributions from Collins fragmentation asymmetries in incompletely reconstructed jets conspire to cancel Sivers asymmetries for quarks, while gluon Sivers functions naturally small?***
***If ISI / FSI cancel at mid-rapidity, does their balance change at high ************ to yield sizable Sivers contribution to observed pp ************ ************ *****0*****X SSA?***