# Presentation: 586393

## Recent QCD Results From Run II

**Vivian O’Dell**
***for the ***
***CDF and D******Ø****** collaborations***

## Recent QCD Results From Run II

**What I will talk about:**
- Jet physics including
**Inclusive jet cross sections**
**Dijet mass cross sections**
**Underlying event studies**
** ********between jets**
** ******** ****tagged jets**
**What I *****won’t***** talk about (sorry!)**
- Diffractive physics
- Direct photon production
- Special topics (Higgs, Top, B physics, etc) covered in other talks

## The Fermilab Tevatron

**Run I (1992-95) ****~100 pb****-1**** recorded**
**Run IIa (2001-05)  ****~1 fb****-1**
**Run IIb (2006-09) ****~ 4-8 fb****-1**
**Note the energy in Run II is also slightly boosted**
**Run I CM energy was 1.8 TeV **
**Run II CM energy is 1.96 TeV**

## Tevatron

- DØ

## Experimental Performance

**CDF**

- Tevatron has delivered ~0.8 fb-1
- Both experiments have collected more than 0.6 fb-1
- Analyses presented here use from 150 to 400 pb-1

## CDF/D0 detectors

**CDF**

**DØ**

**Electrons:    ********E**** / E = 15% /******
**Jets:              ********E**** / E ~ 80% /******

**Electrons:    ********E**** / E = 13.5% /********(central)******
******E**** / E = 16% /********(plug)******
**Jets:              ********E**** / E ~ 80% /******

**Calorimetry is at the heart of these measurements**

**Lead/Iron + Scintillator**

**Uranium + Liquid Argon**

## x-Q2 Reach at the Tevatron

**x-Q****2**** Reach at the Tevatron**

**DØ’s most complete cross section measurement extends over |********| < 3.0**
**compliments  HERA x-Q****2**** range**
**Used in CTEQ6 and MRST2001 fits to determine gluon at large x**
**Enhanced gluon at large x**

- CTEQ6M comparison

**Notes:**
To fully illustrate the weight of this result
Shown here is the region of Q2 versus x phase space which is explored in this Run I measurement.
The dark purple region shows the region 
explored by both D0 and CDF using the central rapidity cross sections, 
and the lighter purple region shows the extended reach of the D0 forward cross section measurements.

The 90 data bins with their full correlations of uncertainties were
Used in CTEQ6 and MRST2001 fits to determine gluon at large x “better than ever before”
The most significant change to the pdfs was an Enhanced gluon distribution at large x

The data points shown to the right show the measurement with the CTEQ6M prediction overlayed.
And the data-theory/theory result is shown in the strip charts to the bottom 
Showing the total uncertainty for each rapidity interval as a function of jet transverse energy.

## Entering an era of precision QCD measurements at Tevatron/LHC

Studying `QCD backgrounds’ for more speculative phenomena

High statistics at CM Energy 1.96 TeV 
5 times higher cross section for jet ET>600 GeV!

- Entering an era of precision QCD measurements at Tevatron/LHC
- Studying `QCD backgrounds’ for more speculative phenomena
- High statistics at CM Energy 1.96 TeV
- 5 times higher cross section for jet E**T**>600 GeV!

**Run I vs. Run II**

## Central Inclusive Jet Cross Section at CDF

- Data currently agree with NLO prediction within errors
- Dominant uncertainty – Jet Energy Scale ~3%

## Central Inclusive Jet Cross Section at D0

- Data agree with NLO within errors
- Dominant uncertainty:
- Jet Energy Scale ~5%

## 
First corrected Run II cross section for forward jets
Important PDF information in cross section vs. rapidity
Jet Energy Scale uncertainties dominate – need to beat these down!

**First corrected Run II cross section for forward jets**
**Important PDF information in cross section vs. rapidity**
**Jet Energy Scale uncertainties dominate – need to beat these down!**

**Inclusive DØ Single Jet Cross Section vs. y**

##        Run II Dijet Production vs. Mjj

- ET = 666 GeV
-  =  0.43

- ET = 633 GeV
- = -0.19

- Calorimeter LEGO Plot

- r-view

- Highest mass di-jet event  so far:  Mjj = 1364 GeV/c2

**CDF**

## Agrees within uncertainties with NLO/CTEQ6M
Jet Energy Scale dominant error on measurement
Adding luminosity/improving Jet Energy Scale

**Jet Energy Scale dominant error on measurement**
**Adding luminosity/improving Jet Energy Scale**

**Inclusive Dijet Cross Section at DØ**

**Good probe of PDF’s**
**Place to look for new physics**
- Compositeness (high Mjj excess)
- resonances

## Quick aside on jet algorithms

**So far we have been using only cone algorithms**
**Improvements over time make them more stable against soft radiation/collinear partons**
- Still, ambiguity when 2 cone jets overlap
**Experimental prescription for “split/merge”**
**In theory, arbitrary parameter introduced to handle this **
- Not ideal
**Combining particles by their relative transverse momentum (k****T****) is much more robust between data and theory**

*(D* is a “distance” parameter)

## Inclusive CDF KT Jet Cross Section

- NLO not corrected for Hadronization
- & Underlying Event (important at low Pt)

- Good agreement Data vs Theory
- High-Pt tail?

## Inclusive CDF KT Jet Cross Section

**D=0.5**

**D=0.7**

**D=1.0**

**Low P****T**** soft contributions increase with increasing D (underlying event)**
**Deviations at high P****T**** a bit larger than with cone algorithms (?)**

## Underlying Event Studies 

** ****No hard scattering.  ****“Min-Bias” event****. **

**“****hard” parton-parton collision**** : large transverse momentum outgoing jets.**

**“****underlying event”****: everything but the two outgoing hard scattered “jets”. **

**Underlying event is not the same as a minimum bias event**
**Includes ISR/FSR/MPI – not completely independent of hard scatter**

## Charged Particle Density  Dependence

**Transverse direction sensitive to underlying event**

**“****transverse” region**** ****defined by leading jet (JetClu R = 0.7, |********| < 2) or by leading two jets (JetClu R = 0.7, |********| < 2).  **
**“****Back-to-Back”**** ****events have at least two jets nearly “back-to-back” (********12**** > 150****o****) and almost equal transverse energies (E****T****(Jet****2****)/E****T****(Jet****1****) > 0.8). **

**CDF Run II**

## “Associated” Charged Particle Density  Dependence

**Associated Charged particle density in back to back jet events**
**30 GeV < E****T****(Jet****1****) < 70 GeV **
**Find P****T****maxT, the maximum P****T**** particle transverse to Jet****1**
**Plot charge particle density********for charged particles (p****T**** > 0.5 GeV/c, |********| < 1, *****not including P******T******maxT*****) relative to P****T****maxT**

**Jet****1**

**Jet****2**

**Jet****3**

**Jet****4**

**CDF Run II**

## “Associated” Charged Particle Density  Dependence

**Associated Charged particle density in “minimum bias” data**
**30 GeV < E****T****(Jet****1****) < 70 GeV **
**Find P****T****maxT, the maximum P****T**** particle transverse to Jet****1**
**Plot charge particle density********for charged particles (p****T**** > 0.5 GeV/c, |********| < 1, *****not including P******T******maxT*****) relative to P****T****maxT**

**CDF Run II**

## Associated Charged Particle Density

**PYTHIA Tune A Vs. HERWIG (untuned)**

**Herwig (no MPI) predicts too few particles in transverse regions**

**Associated Charged Particle Density**

**CDF Run II**

## Associated Charged Particle Density

**PYTHIA Tune A Vs. HERWIG (untuned)**

**Associated Charge Density**

**Associated Charged Particle Density**

**CDF Run II**

## Using Tuned Pythia to predict LHC 

**Shows the center-of-mass energy dependence of the charged particle density, dN****chg****/d********d********dP****T****, for “Min-Bias” collisions compared with the a tuned version of PYTHIA 6.206 (****Set A****) with P****T****(hard) > 0.**

**PYTHIA Tune A predicts 1% of all “Min-Bias” events  at 1.8 TeV are a result of a hard 2-to-2 parton-parton scattering with P****T****(hard) > 10 GeV/c which increases to 12% at 14 TeV**

**LHC?**

**Using Tuned Pythia to predict LHC **

## DØ,  Decorrelation

- 12 is sensitive to jet formation without having to measure 3rd jet directly

- Looking at12  between jets at D**Ø**

##  between jets – Comparison with pQCD

****** ****as function of Jet****1**** P****T ****(Jet****2**** P****T**** > 40 GeV/c)**
**Compared to LO and NLO pQCD in 3****rd**** jet**
**NLO better than LO**
**Both fail at soft jet limit**

**Scale Variations**

**PDF uncertainties**

##  between jets – Comparison with MC

**Using parton shower MC (Pythia, Herwig) gives us the soft gluon contribution missing in pQCD calculations**
**Default Herwig does a better job than default Pythia**
**Pythia with tuned (i.e. enhanced) ISR models data extremely well**
**Another handle on MC tunes**

## Studies of Jet Fragmentation

******(r) measures the narrowness of the jet**
**Dictated by multi-gluon emission from primary parton**
**Good test of parton shower models**
**Also sensitive to underlying event**

**Pythia without MPI tuning too narrow especially at low Pt**
**Pythia Tune A describes data well (enhanced ISR+MPI tuning)**
**Clearly we’re doing something right****!**

## using  tagged jets enhances heavy flavor content
Good place to look for new physics? Compositeness? 

**Good place to look for new physics? Compositeness? **

****** ****+ jets cross section**

## Extracted b jet cross section using b tagged jets

b tagging demands displaced vertex

Compared with Pythia
No surprises?

New measurement from CDF coming soon

**b tagging demands displaced vertex**
**Compared with Pythia**
**No surprises?**
**New measurement from CDF coming soon**

**b jet cross section (CDF)**

## Conclusions

- New results from Run II on jet properties and cross sections
- No real surprises
- Expect more good input for pdf’s
- Tuned Pythia (Pythia Tune A) with enhanced ISR/FSR models data well
- NLO does well (except in limits of soft jets)
- Using tuned Pythia gives us some confidence in predicting LHC jet properties
- Both experiments will be measuring inclusive jet cross sections in the forward direction
- Better understanding/constraints of pdf’s/gluon contributions at high jet pT
- Jet Energy Scale is dominant error in inclusive cross sections
- Both experiments actively working to reduce uncertainties
- Will improve with better understanding, more luminosity
- And for the future...

## Run II Luminosity Projections

**Run II Luminosity Projections**

- (

**~2.8e32 peak**
**~8 fb****-1**** integrated**

- Peak Luminosity (x1030cm-2sec-1)

**Luminosity just keeps on coming!**
**(note “base goal” is 4fb****-1**** integrated)**

## Future RunII upgrades

**Accelerator upgrades**
**Electron cooling, accumulator stacktail upgrades**
**(basically continue to increase and use antiprotons)**
**Tevatron is on track for delivering 8 fb****-1**** by end of 2009**
- Base goal 4 fb-1
**Collider experiments upgrading to take advantage of increased luminosity**
**CDF RunIIb**
**Faster readout electronics**
**Improved tracking triggers**
**Calorimeter upgrade (being commissioned now)**
**D0 RunIIb**
**New silicon Layer 0 detector**
**Improves impact parameter resolution**
**Mitigates loss of efficiency due to radiation damage of current silicon inner layer**
**Improved calorimeter trigger, tracking triggers, and processing triggers**