# Presentation: 752531

## Recent QCD Results from CDF

*Stefano Lami*
- The Rockefeller University
-       Inclusive Jet Cross Section
-             W + n Jets
-                    Diffractive Dijet Production
-                           Underlying Event Studies

- From high to lower Q2

##        Run 2 High ET Jets

- ET = 666 GeV
-  =  0.43

- Highest mass di-jet event  so far
- (corrected mass = 1364 GeV):

- Entering an era of precision QCD measurements at Tevatron/LHC
- Studying `QCD backgrounds’ for more speculative phenomena
- Probing ~10-19 m scale, best place to look for new physics
- High statistics at                 TeV; x5 higher cross section E**T****J**>600 GeV

- ET = 633 GeV
- = -0.19

- Calorimeter LEGO Plot

- r-view

## Inclusive Jet Cross Section

- 177 pb**-1** collected Feb 2002 - Summer 2003
-            Extending Run 1 reach by ~150 GeV

## Inclusive Jet Cross Section (2)

- Excitement about Run 1 high E**T** excess
- SM explanation: gluon PDF not well constrained at high x

- Data currently agree with NLO (EKS) prediction within errors

## Inclusive Jet Cross Section (3)

- Theoretical error dominated by PDFs
- Experimental systematic error dominated by energy scale

- Working on:
- Reducing systematic error
- Mid-point, K**T** algorithms
- Forward jets

## W    e + >= n jet cross section

- W    e + >= n jet cross section

- Test QCD predictions at large Q2  for fundamental channel for Top/Higgs Compared to  LO  ALPGEN + Herwig
-                                                                            

**Systematic uncertainty (10% in ********1**** to 40% in ********4****) limits the measurement sensitivity **

**Backgrounds: Top dominates for 4jets, QCD important in all bins**

## W    e + >= n jet cross section (2)

- W    e + >= n jet cross section (2)

**From top: highest E****T**** jet in W+>=1 jet, second highest E****T**** jet in W+>=2 jet, etc**

- The ratio **R****n/(n-1)** measures the decrease in the cross section with the addition of 1 jet. It depends on ******s **

-      Differential cross section as a function of jet E**T** , compared to theory:

## Run 2 Forward Detectors

** ****MiniPlug Calorimeters  (3.5<********<5.5)**
**  ****Beam Shower Counters (5.5<********<7.5)**
**  ****Antiproton Roman Pot Spectrometer**

- MiniPlug

- Hard diffraction process, signature in CDF: leading antiproton + rapidity gap + 2 high ET jets

- Diffractive Dijet Production

## Diffractive Dijet Production (2)

- fraction) from calorimeter info

- 	Compare diffractive events to ND
-      Measure diffractive structure function    from **R****SD/ND** vs  **x ****Bj**

- overlap events

- RP+J5 trigger

- parton xBj     ************

## Diffractive Dijet Structure Function

**No ******** dependence observed within 0.03<******** <0.1**
**CONFIRMS RUN I RESULT**

- Ratio of SD to ND dijet event rates
- as a function of **x****Bj**
- compared with Run I data

## Underlying Event Studies 

- Underlying Event Studies** **

**The ****Underlying Event**** ****is everything but the two outgoing**** ****Jets****, including:**** ****initial/final gluon radiation, beam-beam remnants and possible multiple parton interactions**

- Central Tracking to study low momenta

-  Direction of leading jet to define 3 ****** **regions

**Min-Bias**
**0.25 per unit ********-******

**Charged Particle ******** Correlations **
**p****T**** > 0.5 GeV/c |********| < 1**

**     ****UE unavoidable background****  to be removed from the jets before comparing to NLO QCD predictions**

-  Run 2

##  Underlying Event Studies (2)

- Shows the _**average charged particle density**_, dN/d****d****, in the “transverse” region (pT > 0.5 GeV/c, |****| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events compared with PYTHIA Tune A and HERWIG after CDFSIM.

**“****Leading Jet”**

**“****Back-to-Back”**
**(********12****>150****o****,E****T****j2****/E****T****j1****>0.8)**

**Min-Bias**
**0.25 per unit ********-******

- Interplay between pQCD and non-pQCD physics.
- Current MC with default parameters fail to reproduce data.
- In Run 1 PYTHIA 6.206 tuned adding multi-parton interactions.

-  Run 2 data:

##  Underlying Event Studies (3)

-  Underlying Event Studies (3)

-  Run 2 data:

**“****Leading Jet”**

**“****Back-to-Back”**
**(********12****>150****o****,E****T****j2****/E****T****j1****>0.8)**

- PYTHIA tuned (on Run 1 data) reproduces well Run 2 data.
- HERWIG (with no multi-parton interaction) works only at high E**T****j1**.

**Min-Bias**
**0.24 GeV/c per unit ********-******

_**Average charged PTsum density**_, dPT/d****d****, in the “transverse” region
-      (pT > 0.5 GeV/c, |****| < 1) versus ET(jet#1) for “Leading Jet” and “Back-to-Back” events compared with PYTHIA Tune A and HERWIG after CDFSIM.

## Conclusions

**CDF is now accumulating large samples of interesting events**
**Run 2 data will enable the measurement of ********s**** and  PDFs**
**Measured inclusive jet cross section agrees with NLO QCD (CTEQ 6.1) **
** ****Work on reducing systematic uncertainty (calorimeter jet energy scale 3%) **
** ****and include Forward Jets and Run 2 jet algorithms K****T**** and Mid-Point**
**Study of W+n jets is important to test QCD predictions at large Q****2**
**  ****It is a gold channel for Top/Higgs/SUSY physics**
**Exciting Diffractive physics program with dedicated forward detector/triggers**
** ****Preliminary Dijet results show no Q****2 ****dependence in SD/ND**
**Modeling the Underlying Event is important for precise Jet measurements**
** ****A tuned version of PYTHIA reproduces well CDF data**
**Very exciting and important QCD Physics program at CDF**