# Presentation: 889444

## QCD: Approaching True Precisionor, Latest Jet Results from the TeVatron

-  Experimental Details
-  SubJets and* *Event Quantities
-  Cross Sections

- Presented by
- John Krane
- Iowa State University

- Moriond 2001

## Changing Paradigms in QCD

- Inclusion of error estimates in the PDFs
- Progress toward NNLO predictions
- More rigorous treatment of experimental errors
- More consistent ET calculations between experiments

- covariance
- matrices

- jet algorithms
- workshop

- calculation of virtual corrections

- hope for
- better underlying event

- Moriond 2001

**Notes:**
scientific paradigms: "accepted examples of actual scientific practice-- providing models for coherent traditions of scientific research. 
[People] whose research is based on shared paradigms are committed to the same rules and standards for scientific practice.

I don’t mention all this because it is central to my talk, but because it is background over which I paint the picture of these experimental results.

## Jet Definition

- Cone DefinitionR=0.7 in merge/split
- KT Definition
- cells/clusters are combined if their relative kT2 is “small”(D=1.0 or 0.5 -- scaling parameter)
- For subjets, also define “large”
-  	(ycut, =10-3)

- Iteration vs ratcheting

- 0.5 vs 0.75

- Centroid found with4-vector addition vs
- Snowmass addition

*D** uses Ellis and Soper’s definition*

- Moriond 2001

**Notes:**
For D=1 and deltaR << 1, dij is the relative transverse momentum squared between vectors i and j .

If smallest d is dii, then it is a jet, remove it from list.

## Jet Cuts

- Central Tracking

- EM Calorimeter

- Hadronic
- Calorimeter

- Muon A-Layer

- Magnet

- Muon B&C

*e*

- 

- jet

- 

- noise

**Jet Cuts**

- Typical cuts on EM fraction, hotcells, missing ET, vertex position, etc.
- > 97% efficient
- > 99% pure

- Moriond 2001

## Jet Energy Corrections

- Response functions
- Noise and underlying event
- “Showering”
- ResolutionsEstimated with dijet balancing or simulation

- the symmetric part

**   **d2
- dET d

- ET

**   **d2
- dET d

- ET

- no distinction between jetsof different kinds

- Moriond 2001

## Jet and Event Quantities

- Subjet Multiplicity
- Underlying event structure
- Low ET Multijet studies

-         New !

- Moriond 2001

## Subjet Multiplicity

-         1        2       3       4

- 0.5
- 0.4
- 0.3
- 0.2
- 0.1

**Subjet Multiplicity**

- Linear Combination:
***<M>****** = ******f******g****** ******M******g****** + (1-******f******g******) M******Q***
- Assume ***M******g******, M******Q*** independent of ***√s***
- Measure M at two ***√s*** energies andextract the g and Q components
- Largest uncertainty comes fromthe gluon fractions in the PDFs

***Mean Jet Multiplicity***

***Quark Jet Fraction***

***Gluon Jet Fraction***

- Subjet Multiplicity *M*

- D0 Preliminary

*Coming soon as a PRD*

- no attempt to developa likelihood function

**HEWIG ****prediction ****=1.86****±****0.08(stat)**

*Using the k**T** algorithm*

- Moriond 2001

**Notes:**
One day useful to identify event topologies for search physics

## Underlying event structure

**Underlying event ****structure**

- Counting charged particles in azimuthal sectors
- Improve modeling of underlying event

- Moriond 2001

## Low ET Multijet events

- At high-ET, QCD does quite well.
- But try counting jets at low-ET...compare to Pythia

- Each jet’s ET>20 GeV.
- For 2 jets or more,    normalization is off, so    correct to >40 GeV
-     spectrum.

- Moriond 2001

**Notes:**
Pythia is put through detector simulation.  Data is not unsmeared.

## Low ET Multijet events

- Strong pT ordering in DGLAP suppresses “spectator jets”
- BFKL has diffusion in log(pT)

- Looking also at Jetrad and Herwig

- Moriond 2001

**Notes:**
The strong ordering of Pt in DGLAP reduces the numbers of “spectator jets”, perhaps too much.

I am not in a big hurry to associate BFKL with the observed effect, but there is an effect, and it is statistically significant.

Studying phi-dependence of extra jets

## Cross Sections

- R32
- 630/1800 ratio of jet cross sections
- Forward cross sections
- Two PRDs
- KT central inclusive

- Published!
- Published!
- Published!
- Submitted
-         New !

- 2 analysis

- Moriond 2001

## Inclusive R32

- A study of soft jet emission rate
- Ratios exploit error correlations
- We observe lack of PDF sensitivity
- Investigate R scale sensitivity with Jetrad

- Moriond 2001

## Inclusive R32

- several formulae for R
- R not always same for jets in the same event (yet agreement not improved)
- single-scales seem better than mixed-scales,
-  0.3*HT is most robust
- (As minimum ET threshold climbs, the “hardness” of the event is less well-represented by ETmax?)

- PRL 86, 1955 (2001)

- Moriond 2001

**Notes:**
We do not find mu-scale work particularly compelling, 

but Jetrad is LO 3jet and NLO 2jet, so we can’t find alpha_s

## Inclusive Ratio

**Inclusive Ratio**

- Published yesterday in PRL
- 2 probabilities fall between 30% and 60%
- D has tried a “normalization-only” test, which yields generally poor agreement (<1%), except for 2*ET (7%) and 0.25*ET (23%).

- PDF Variations

- fR Variations

- Moriond 2001

**Notes:**
Would like NNLO to see if the normalization differs (alpha-s also?)

## A few words on 2

**A few words on ********2**

- 10 MC simulations of systematic errors

- 100 trials

- The calculations work great,root-diagonal elements alonenot sufficient for plots

- Moriond 2001

**Notes:**
In the future we propose not to use the root-diagonal elements of error matrices in these plots.

Rather one should find the 1 sigma of the *distribution* and plot that.

## Rapidity-dependent Inclusive

*******d******2************ dE******T ******d*************** (fb/GeV)**

***E******T***** (GeV)**

**Rapidity-dependent Inclusive**

- D’s most complete cross section measurement
- Uncertainty in theory is larger than uncertainty is data!

- PRL 86, 1707 (2001)

- Moriond 2001

**Notes:**
NNLO anyone?

Sensitive to PDF, sensitive to mu.

## Jets PRD #1:  CDF’s Run 1b inclusive jet

- Has discussion of 2 technique.

- Includes comparisons to Run 1a dataD****’s run 1b data

- Moriond 2001

## Jets PRD #2:  D’s Run 1b jet results

- Tour de Force:
-  1800 central inclusive,dijet mass,dijet angular distribution,630/1800 inclusive ratio

- Moriond 2001

## KT Inclusive Jet Cross Section

- -0.5 <  < 0.5
- D = 1.0
- Predictions IR and UV safe
- Merging behavior well-defined for both exp. and theory

- D Preliminary

- Moriond 2001

**Notes:**
D=1 to match cone 0.7

The first hadron collider measurement of the Kt inclusive jet

## KT Comparison

**K****T**** Comparison**

- Normalization differs by 20% or more
- pT dependence
- D has an error matrix  expect 2 numbers soon
- “No significant deviations of predictions from data”

- D Preliminary

- Moriond 2001

**Notes:**
Deviation of last point not significant: only 2 units of chi2

Details of hadronization effects appear to be important for Kt jets but not for cone.

Jetrad is a parton-level generator, and that we know the neglected hadronization has an effect in Kt predictions 
that "goes the right way"

## Summary

- TeVatron stopped running in 1996 but several results are still in the queue
- Jet substructure
- Event structure
- Cross sections
- Growing sophistication in results
- Greater consistency between experiments
- Error matrices
- Better corrections

- Moriond 2001

**Notes:**
Q differs from g
Underlying event progress
Too few multijets in Pythia

Lots of new results in the public domain now!

Repeated need for NNLO and new PDFs (error matrices)