# Presentation: 401162

## KT Inclusive Jet Cross Section

- Michael Strang
- University of Texas at Arlington

- July 11, 2000

- DOE Review

- UT Arlington

## Outline

-  Motivation
-  Implementation
-  Ntuple Creation
-  Cut Efficiency
-  Conclusion

## Motivation

-   KT Algorithm
- -  Faithful attempt to follow the physical 	process of fragmentation, multiple soft 	gluon emission, through the iterative 	reconstruction of “proto-jets” into full 	hadronic jets
- -  Theoretically infrared and collinear safe to 	all orders in perturbative QCD (does not 	involve arbitrary split/merge decision 	required if cones overlap)
- -  Can be applied to vectors from fixed order
- 	or resummed calculations in QCD, partons
- 	or particles from Monte Carlo, or energy
- 	deposited in a calorimeter

## Implementation

-  Precluster to redistribute low energy cells 	amongst nearby cells

-  Recombination procedure (D free parameter):

- 1. For each pair of clusters, i and j, define:

- where

- For each cluster define:

- 2. The minimum, dmin, of all di and dij is found

- 3. If *d**min* is a *d**ij* then the clusters are merged, using the 4
- vector recombination scheme, into a new cluster* k*
- with:

- 4. If *d**min* is a *d**i* then the cluster is “not mergable” and it is
- dropped from the cluster list and added to the jet list

- 5. Repeat 1-5 until all clusters have become jets

## Ntuple Creation

-  UTA contribution
-  Data Set
-  Run 1b d0fixed dst
-  Use *D* of 1.0 in KT algorithm
- + Corresponds to *R* = 0.7 in cone
- algorithm reducing renormalization
- and factorization scale dependence
- of cross section
-  Jet triggers:
- + JET_MIN, JET_30, JET_50, JET_85
- JET_MAX
- - Reduces data to ~770k events
-  Storage
-  55 separate ntuples
- + ~25 GB of space
- + chained together for analysis
-  Usage
- Used in many continuing Ph.D. theses
- + Inclusive Jet Cross Section
- + Thrust Distributions
- + Subjet Multiplicity in Q and G Jets

## Cut Efficiency

-  UTA contribution to Inclusive Jet measurement
-  Various cuts applied to insure only good jets
- enter sample. Efficiency is a measure of how well bad jets are properly rejected while keeping good jets.
- 	- Applied to data before being corrected for energy
- Scale

- _Vertex Cut_

-  Revertexing
-  For events with two vertices, vector sum of *E**T* is
- compared and vertex with lowest value is used. This value becomes the missing *E**T*
* *Cut
-  Eliminates jets that cannot be accurately
- measured in detector
-  Efficiency
-  Ratio of number of events that pass to total
- number of events
- Over entire range:  90.3 0.1 %

## Cut Efficiency (cont’d)

- _Missing E__T_
-  Two different cuts depending on leading *E**T*
-  lead *E**T* < 100. GeV: missing *E**T* < 30. GeV
-  otherwise : missing *E**T* / lead *E**T* < 0.3
-  Eliminates poorly measured jets and cuts out
- fakes caused by radiation from main ring and some hot cells
-  Efficiency
-  Fit *f*(*x*) = exp(*A*+*Bx*) in the good region that is
- well described by this function and extrapolate outside the cut
-  Ratio of number of events in good region to
- number of events in good region plus integral of fit in bad region
- For *E**T* > 150. :  98.0 0.1 %
- Otherwise: Fit to a quadratic and match

## Cut Efficiency (cont’d)

- _EMFrac_
-  Cut: 0.05 < EMFrac < 0.95
-  Eliminates fakes or misidentified photons or
- electrons
-  Efficiency
-  Fit gaussian to each tail in the good region that is
- well described by this function and extrapolate outside the cut
-  Ratio of number of events in good region to
- number of events in good region plus integral of fit in bad region
-  Efficiency: 99.84  0.02%
- _CHFrac_
-  Cut: CHFrac < 0.40
-  Eliminates deposits from main ring
-  Efficiency
-  Fit same form as missing *E**T*
-  Efficiency: 99.88 0.05%

## Conclusion

-  KT Algorithm has many nice properties that lend
- themselves to QCD studies
-  Iterative routine easily processed by computer
- but too slow to be used in triggering
-  Ntuple contains KT information for nearly all
- events from Run 1b
-  Cuts reduce data to what is considered to be
- good jets that can then be used in the full analysis. Cuts are very efficient.
-  Analysis is being continued by Sebastian
- Grinstein at FNAL