# Presentation: 319177

## Precision QCD at the Tevatron

- Markus Wobisch, Fermilab

- for the CDF and DØ Collaborations

**Notes:**
25+5 minutes

##      Fermilab Tevatron - Run II

**     ****Fermilab Tevatron - Run II**

**DØ**

**CDF**

**Chicago**

-  Ecm: 1.8  1.96 TeV
-  more Bunches 6  36
-  Bunch Crossing 3500  396ns
-  Run II started in March 2001
-  Peak Luminosity:
-                  1.7E32 cm-2 sec-1
-  Run I delivered: 0.26 fb-1
-  Run IIa delivered:
-         >1.5 fb-1
-  Run IIb Goal: 8 fb-1 before 2009

**Tevatron**

**Main Injector**
**& Recycler**

**Booster**

**p-bar source**

##    Run II Detectors

- Multi-Purpose Detectors:
-  Tracking
-  Calorimeter
-  Muon System

- New in D0 for Run IIb:
- Innermost “Layer 0” Silicon

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Hadron-Hadron Collisions

## Outline

**W-Asymmetry**
**Photon Production**
**Jet Production**
**Jets beyond 2********2**
**Vectorboson + Jets**
**Heavy Flavor Jets**

## .

** ****W-Asymmetry**

**  ****PDFs  **

## W-Asymmetry

- Lepton-Asymmetry

- Valence-Quarks
- Dominate

## W Charge Asymmetry

- CDF (170pb-1): Electron Channel
- In two Electron-ET Bins
- (Different Sensitivity)

- PRD 71, 051104 (2005)

- D0 prel. (230pb-1): Muon Channel
- Limited by Statistics
- Potential for 1fb-1!  (Run IIb)

- CTEQ6.1M

- MRST02

- Important Input in Future PDF Fits!

## .

** ****Photons **

**  ****Fixed-Order: NLO  **
**Resummation**
**PDFs**

## Direct Photon Production

**Direct Photon Production**

- Direct Photons come unaltered from the Hard Subprocess
-           Direct Probe of the Hard Scattering Dynamics
-  Sensitivity to PDFs

##            Isolated Photon Cross Sect.

**           ****Isolated Photon Cross Sect.**

- |y|<0.9

**p****T******** (GeV)**

- L = 326 pb-1

**p****T******** (GeV)**

**p****T******** (GeV)**

-  PDF sensitivity requires:
-   Reduced exp. uncertainties – dominated by purity uncertainty
-   Improved theory (resummation / NNLO)

- data/theory (NLO: JETPHOX): good agreement over 23<pT<300GeV

- DØ  hep-ex/0511054, submitted to PRL

##       Di-Photon Cross Section

-  **DIPHOX**:
-   - NLO prompt di-photons
-   - NLO fragmentation (1 or 2 ****)
-   - NNLO gg**** diagram
-  **ResBos**:
-  - NLO prompt di-photons
-  - LO fragmentation contribution
-  - Resummed initial state gluon
-     radiation (important for qT)
-  **PYTHIA** (increased by factor 2)

- CDF Collab., Phys. Rev. Lett. 95, 022003, 2005. (207pb-1)

-  Pseudorapidity < 0.9
-  Photon pT> 13 & 14 GeV

**M******** (GeV/c****2****)**

##       Di-Photon Cross Section

**      ****Di-Photon Cross Section**

- NLO fragmentation contribution
-     - only in DIPHOX
-      at high qT, low ****, low mass
- Resummed initial-state gluon radiation – only in ResBos  at low qT

**************(rad)**

- _Important:_
- Combined Calculation with
- NLO Fragmentation
- & Initial State Resummation

## .

** ****Jets **

**PDFs**
**  ****New Physics?  **

## Biggest Misconception:

- “A Jet Represents a Parton
- from the LO 2 n Process.”
- “The Jet Algorithm should find
- this Parton with high Efficiency.”

**Nonsense!**

## Parton-, Hadron-, Detector- “Jets”

**Parton-, Hadron-, Detector- “Jets”**

- Use Jet Definition to relate Observables
-     defined on Partons, Particles, Detector
- Direct Observation:
-     Energy Deposits / Tracks
- Stable Particles (=True Observable)
- Idealized: Parton-Jets
-     no Observable (color confinement)
-     But only Quantity that can
-     be predicted in pQCD
- Jet Algorithm:
-     required to be IR- / Collinear Safe

## Inclusive Jet Cross Section

- Theory @NLO is reliable (±10%)
-       Sensitivity to PDFs
-       Unique: High-x Gluon
- Run II: Increased x5 at pT=600GeV
-       Sensitive to New Physics:
-          Compositeness, LED, ...(?)...

- x2

- x5

- DØ   - midpoint cone algorithm
- CDF  - Run I cone algorithm
-         - modified midpoint cone algorithm
-         - kT algorithm

## From Particle to Parton Level

- Measurement:     pp-bar  Jet Cross Section  (on “Particle-Level”)
-      Corrected for Experimental Effects (Efficiencies, Resolution, ...)

- Use Models to Study Effects
- of Non-Perturbative Processes
- (PYTHIA, HERWIG)
- Here (CDF Study):
- Hadronization Correction
- Underlying Event Correction
- for central Jet Cross Section

- At pT>150GeV: Non-Perturbative Effects are below 10%
-  Apply this Correction to the pQCD Calculation

- New in Run II !!!

##           Inclusive Jets            Cone and kT Algorithms

- In 2005: published both central cone and kT jets with 400pb-1
- Here: new preliminary results with full rapidity coverage for 1fb-1

- Midpoint Searchcone Algorithm

- kT Algorithm

##           Inclusive Jets            Cone and kT Algorithms

- Data are well-described by NLO pQCD
- Experimental Uncertainties: Smaller than PDF Uncertainties!!
- (only shown for kT Algorithm)

- Midpoint Searchcone Algorithm

- kT Algorithm

##             Inclusive Jet Cross Section

**            ****Inclusive Jet Cross Section**

- DØ preliminary (800pb-1)

- Compare to NLO pQCD + 2-loop
- Threshold Corrections (Kidonakis, Owens)

-  Sensitive to PDFs!!

##            Run I and Run II so far...

- Good agreement of theory and data over large pT range 20-700 GeV

## .

** ****Vector Boson + Jets**

**  ****Fixed-order: NLO**
**  ****NLO + Parton Shower  **
**  ****Matched Tree-Level + PS  **
**(CKKW/MLM)**

## W+n Jets                  W+2 Jets

**W+n Jets                  W+2 Jets**

- Cross Section for Restricted W Phase Space
-  Avoid Model-Dependent Acceptance Corr.

- CDF preliminary (320pb-1)

- Shape-Comparison with LO Alpgen+PYTHIA:
- Reasonable Agreement!

## Z+n Jets

**Z+n Jets**

- Smaller Cross Section than (W+Jets) but cleaner exp. Signature

- Data are described by:
- MCFM: NLO for Z+jet, Z+2-jet
- Madgraph:  ME+PS (Z+ <= 3 parton) tree-level  + PYTHIA parton shower
- ALPGEN (pT Dependence)

- D0 preliminary (340pb-1)

- Ratios to total inclusive Zee

- (normalized at n=1)

- (normalized)

## Z + Jets

- Comparison on Detector-Level: Data vs. PYTHIA and SHERPA

- PYTHIA does not describe
- Higher Jet Multiplicities

- SHERPA is pretty good!

- D0 preliminary (950pb-1)

## Z + Jets

**Z + Jets**

- Comparison on Detector-Level: Data vs. PYTHIA and SHERPA

- PYTHIA does not describe
- Leading Jet pT Spectrum

- SHERPA is pretty good!

- D0 preliminary (950pb-1)

## .

** ****Heavy Flavor Jets**

**Heavy Flavor PDFs  **
**Fixed-Order: NLO  **
**  ****LO + Parton Shower  **

##         Inclusive b-Jet Cross Section

- Reconstruct Secondary Vertex from B Hadron
- Dominant Syst. Uncertainties
-      - Jet Energy Calibration
-      - Purity of Tagged B-Jet

- Agreement of Data / NLO Theory
-          Within Uncertainties
- Huge Scale Dependence @NLO

- CDF preliminary (300pb-1)

##         Inclusive b-Jet Cross Section

- Comparison with PYTHIA and HERWIG
- Both low (Factor 1.5-2)
- PYTHIA:  pT dependent
- HERWIG:  more flat

- CDF preliminary (300pb-1)

## Z + b-jet

- Cross Section Ratio (Z+b-jet)/(Z+jet)      pT-jet>20GeV, |eta-jet|<2.5

- D0 (180pb-1), Phys. Rev Lett. 94, 161801 (2005)

-            Ratio = 0.023±0.004(stat) +0.002/-0.003(syst)
- NLO (MCFM) = 0.018±0.004 (CTEQ6.1M)

## Z + b-jet

**Z + b-jet**

- Z+b-jet production:       pT-jet>20GeV, |eta-jet|<1.5

- CDF (335pb-1), submitted to PRD, hep-ex/0605099

-                  sigma = 0.96±0.32(stat)±0.14(syst) pb           NLO: 0.48pb
- ratio over (Z+jet) = 0.0237±0.0078(stat) ±0.0033(syst)    NLO: 0.018±0.004

## .

** ****Jets beyond 2********2**

**  ****Underlying Event**
**Parton Shower**
**  ****Matched Predictions  **
**3-Jet NLO **

- Internal Jet Structure

- Dijet Azimuthal Decorrelation

##        Internal Jet Structure

- Integrated Jet Shape:
- Fractional pT in Subcone vs.(r/R)

- CDF, PRD, hep-ex/0505013 (170pb-1)

- Sensitive to Soft and
- Hard Radiation – and UE

- Well-Described by (tuned) MCs

##        Internal Jet Structure

- At fixed r=0.3   (38<pT<400GeV)
- Study pT Dependence of predicted
- Psi(r/R) for Quark- & Gluon-Jets
-  Significant Difference

- Quark- & Gluon-Jet Mixture in
- PYTHIA gives Perfect Description
- of Data

##           Dijet Azimuthal Decorrelation

- Idea: Dijet Azimuthal Angle is
- Sensitive to Soft & Hard Emissions:
- Test Parton-Shower
- Test 3-Jet NLO

##           Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2pi/3
-        & Divergence towards pi

##           Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2pi/3
-        & Divergence towards pi
- NLO is very good – down to pi/2
-        & better towards pi
-        ... still: resummation needed

##           Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2pi/3
-        & Divergence towards pi
- NLO is very good – down to pi/2
-        & better towards pi
-        ... still: resummation needed
- HERWIG is perfect “out-the-box”
- PYTHIA is too low in tail ...

##           Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2pi/3
-        & Divergence towards pi
- NLO is very good – down to pi/2
-        & better towards pi
-        ... still: resummation needed
- HERWIG is perfect “out-the-box”
- PYTHIA is too low in tail ...
-      ... but it can be tuned
-      (“tune A” is too high!)

##           Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2pi/3
-        & Divergence towards pi
- NLO is very good – down to pi/2
-        & better towards pi
-        ... still: resummation needed
- HERWIG is perfect “out-the-box”
- PYTHIA is too low in tail ...
-      ... but it can be tuned
-      (“tune A” is too high!)
- SHERPA is great
- ALPGEN looks good – but low
-     efficiency  large stat. fluctuations

## Summary

- Tevatron is more than “the Place to Develop Tools for the LHC”
- This Presentation: Broad Spectrum of Processes
-          Jets, Photons, W-Asymmetry, Vector-Boson + Jets,
-          Heavy-Flavor Jets, Jet Production at higher Orders
- “Bread-and-Butter Physics”:
-          Precision Measurements of Fundamental Observables @2TeV
- Successfully Testing various Approximations of QCD
- Significant Impact in Constraining Proton PDFs

**Run IIb has just started!!**