# Presentation: 391232

## Jet Production in Polarized pp Collisions at RHIC

**Carl A. Gagliardi**
- Texas A&M University
**for the                Collaboration**
**Outline**

-   Introduction
-   2005 results
-   2006 results
-   Looking ahead

## Origin of the proton spin?

- Polarized DIS:  0.2~0.3

- Poorly Constrained

- A primary goal of the RHIC Spin program is to determine the **gluon polarization distribution**

- Three recent fits of equal quality:
- ΔG = 0.13 ± 0.16
- ΔG ~ 0.006
- ΔG = -0.20 ± 0.41
- all at *Q*2 = 1 GeV2
- Leader et al, PRD 75, 074027

## Polarized pp collisions at RHIC

- For most RHIC kinematics, ***gg*** and ***qg*** dominate, making *A**LL* for jets sensitive to **gluon polarization**.

- Partonic fractions in jet production at 200 GeV

- 0

## RHIC:  the world’s first polarized hadron collider

- “Thousands of millions”

- Spin varies from rf bucket to rf bucket (9.4 MHz)
- Spin pattern changes from fill to fill
- Spin rotators provide choice of spin orientation
- “Billions” of spin reversals during a fill with little if any depolarization

## STAR detector

**     ****E-M Calorimeter**

**Time of          Flight**

**Projection               Chamber**

***STAR*** detector

## Jet reconstruction in STAR

- (Adapted from Tevatron II - hep-ex/0005012)
-   Seed energy = 0.5 GeV
-   Cone radius in -
-  R=0.4 with  0.2 <  < 0.8 (2005)
-  R=0.7 with -0.7 <  < 0.9 (2006)
-   Splitting/merging fraction f=0.5

- Jet reconstruction in *STAR*

**Detector**

**GEANT**

**PYTHIA**

**Data jets                                     MC jets**

**Particle**

- Use **PYTHIA** + **GEANT** to quantify detector response

## Jet production at RHIC

- PRL 97, 252001

- Jet structure at 200 GeV is well understood
- Mid-rapidity jet cross section is well described by pQCD over 7 orders of magnitude

## Reconstruction bias: combination of steeply falling spectrum and ~25% jet ET resolution
Trigger bias: non-uniform sampling of gg, qg, and qq processes

Will illustrate impact on ALL with 2006 data

- Trigger bias: non-uniform sampling of *gg*, *qg*, and *qq* processes
- Will illustrate impact on ALL with 2006 data

**Neutral Energy Fraction**

- 2005 inclusive jets ALL systematics

| ALL systematics | (x 10 -3) |
| --- | --- |
| Reconstruction + Trigger Bias | 2-5 (pT dep) |
| Non-longitudinal Polarization | 0.1-0.8 (pT dep) |
| Relative Luminosity | 0.94 |
| Backgrounds | 0.70 |

| pT systematic | [-5.4%,+6.7%] |
| --- | --- |

## 2005 inclusive jets ALL

- Comparison to predictions within the GRSV framework with various input values of ΔG demonstrate sensitivity

- hep-ex  arXiv:0710.2048

- Model calcs from:
- Jager et al, PRD 70, 034010

**200 GeV**
**0.2 < ****η**** < 0.8**

## Limits on ΔG from 2005 jet results

- Limits on ΔG from 2005 jet results

- Significant new constraints on ΔG when compared to predictions derived from one global fit to DIS data

- hep-ex  arXiv:0710.2048

## Other global analyses

- There are many other global analyses of the polarized DIS data

***x*****Δ*****g*****(*****x*****) at**
***Q*****2**** = 10 GeV****2**

## Additional ALL predictions for 2005

- Additional ALL predictions for 2005

- Many predictions for ALL vs. pT
- Is the sensitivity unique to GRSV?

## Comparison to other global analyses

- The *STAR* data **exclude a broad range of models** that have ΔG larger than that in GRSV-std
- The counterexample is GS-C, which is negative at large *x*, has a node near *x* ~ 0.1, and has a large first moment at small *x*

## 2006 data

**Improved figure of merit**
- Luminosity :			2  4.7 pb-1
- Polarization: 			50%  60% (online polarization)
- Barrel EMC  coverage: 	[0,1]  [-1,1]
**In addition**
- Jet cone radius: 0.4  0.7
- -0.7 < jet axis < 0.9
- Neutral energy fraction < 0.85
- Increased trigger thresholds
- Inclusion of Endcap EMC towers
- Improved tracking at large ||
- Shielding from backgrounds

## Jet reconstruction and trigger bias for 2006 data

**Step 1:  Use PYTHIA+GEANT to estimate the p****T**** shift from detector jets to particle jets**

**Step 2: Simulate the difference in A****LL**** between particle and detector jets for various gluon polarization scenarios**

**Particle jets**
**Shifted detector jets**
**Detector jets**

- Maximum deviation determines ALL systematic
- Confidence level calculations take account of any residual

## 2006 inclusive jets ALL

- Statistical uncertainties are 3-4 times smaller than in ’05 data for pT > 13 GeV/c

| ALL systematics | (x 10 -3) |
| --- | --- |
| Reconstruction + Trigger Bias | [-1,+3]
 (pT dep) |
| Non-longitudinal Polarization | ~ 0.03
 (pT dep) |
| Relative Luminosity | 0.94 |
| Backgrounds | 1st bin ~ 0.5
else ~ 0.1 |

| pT systematic | 6.7% |
| --- | --- |

- Model curves calculated with cone radius 0.7 and -0.7 <  < 0.9

**200 GeV**
**-0.7 < ******** < 0.9**

## Limits on ΔG from 2006 jet results

- Within the GRSV framework:
- GRSV-std excluded with 99% CL
- ΔG < -0.7 excluded with 90% CL

## Looking beyond inclusive ALL measurements

- Inclusive ALL measurements at fixed pT average over a **broad x range**.
- Need a global analysis to determine the implications
- Can hide considerable structure if Δ*g*(*x*) has a node

## DSSV – first global analysis with polarized jets

- The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing
- Finds a node in the gluon distribution near *x* ~ 0.1, but with the opposite phase from GS-C

- de Florian et al., arXiv:0804.0422 [hep-ph]

## The next few years: di-jets and Δg(x)

- The next few years: di-jets and Δ*g*(*x*)

- Di-jets provide direct access to parton kinematics at LO

- Mass

- Rapidity

- |cos(*)|

**Ratio**

- 2005 preliminary di-jet distributions

## Projected di-jet sensitivity for the 2009 run

- Assumes 50 pb-1 of 200 GeV pp collisions with 60% polarization

**GS-C**

***x******1***** ~ 0.45**
***x******2***** ~ 0.16**

***x******1***** ~ 0.27**
***x******2***** ~ 0.27**

**Rel FOM ~ 70%**

***x******1***** ~ 0.33**
***x******2***** ~ 0.044**

## Conclusions

*STAR* inclusive jet ALL measurements  have **excluded extreme gluon polarization scenarios**
- Over the next few years, *STAR* **di-jet measurements** will provide **direct information about ****Δ*****g*****(*****x*****)**
*STAR* will also obtain complementary information about **Δ*****g*****(*****x*****) from ****γ**** + jet**
**Stay tuned!**

## Slide 23

## Double longitudinal spin asymmetry, ALL

- N :  Spin dependent yields (# of reconstructed jets)
- P :  Beam polarization (measured by RHIC Polarimeter)
- R :  Relative luminosities between different spin states

- Double longitudinal spin asymmetry, ALL

## STAR detectors for jet measurements

- Time Projection Chamber
- 	 ||<1.4 tracking
- Barrel EM Calorimeter
- 	||<1 triggering & calorimetry
- Endcap EM Calorimeter
- 	1.09<<2 triggering & calorimetry
- Beam-Beam Counters
- 	3.4<||<5 triggering, luminosity, local polarimetry

## Other global analyses

***x*****Δ*****g*****(*****x*****) at**
***Q*****2**** = 10 GeV****2**

## Additional ALL predictions for 2005

- Additional ALL predictions for 2005

**A****LL**

## Estimating the non-jet background

**Charged Energy Fraction**

- Data

- Monte Carlo

- Non-jet backgrounds stand out in the data as single-jet events that have a small charged energy fraction
- Isolate the background in the ratio of mono-jet to di-jet events vs. charged energy fraction

- Data

**Charged Energy Fraction**

**Charged Energy Fraction**

## ALL vs. run index for 2006 data

- ALL vs. run index for 2006 data

**STAR Preliminary**

## False asymmetries in 2006 data

**STAR Preliminary**

**STAR Preliminary**

- Longitudinal single-spin (parity-violating) asymmetries are consistent with zero, as expected at this precision

## Inclusive jet ALL for fixed values of ΔG

- Inclusive jet ALL for fixed values of ΔG

- Calculations by Stratmann and Vogelsang within the GRSV framework