# Presentation: 804446

## Jet energy calibrations with - events

**Vasilii Shelkov**

## Analysis goals

- ~ 30% from (1 events/s or **75k**** ****events on tape/month** at L=1033)

- - together with Z/+ Jet sample it makes a few “physics-based”
- jet energy calibrations opportunities,
-  it would be nice to have a comprehensive analysis read to
- be able to extract Jet energy scale from ttbar events in the
- very first weeks of ATLAS running, perhaps without b-tagging
- available,
-  only signal contribution is studied for now:
-     reconstruct m(jj) and pick up the “best match” to (eta,phi,pt) of
-     “true” W if the quality of match is above certain cut:

## Analysis approach and tools

-  **18,800** generic ttbar events in generated/reconstructed in 7.0.0(~1 month),
-  CBNT NTuple format in ROOT format was used, default version of
- CombinedJetRec algorithm was used for jets reconstruction,
-  simple “jet-candidate” classes and **ttbar** analysis class were
- pre-compiled with CINT and loaded into ROOT as shared library,
-  NTuple event loop, call to analysis objects, and output into **final reduced**
**NTUPLE** is done via interactive ROOT scripts,
-  final analysis/fits are done on the reduced NTUPLE in ROOT using
-  **RooFit **package.

## W→jj max reconstruction efficiency

***W→jj max reconstruction efficiency***

**M****jj ****(GeV)**

- “best” match

- “second best”

**Eff **~ 2820/(18800*0.3) = **50%**

- - soft energy cuts: Et(jet)>5GeV

## M(jj) resolution vs. Jet Pt

## W→jj reconstruction efficiency

***W→jj reconstruction efficiency***

**Cos(jj) – jet-jet opening angle**

**pT****max****(jet)**

**pT****min****(jet)**

- GeV

-  energetic W creates a pair of “soft-vs-hard” jets with a typically small opening angle

## W→jj reconstruction efficiency

******w**

**pT of W(GeV)**

## Constraining M(jj) linehsapewith unbinned ML fit

-  **unlike the cases where ****W****→jj ****is a part of the decay chain and ****m(jj)**** can be directly constrained to central value of the ****W-mass**** (as a part of some kinematic fit), we would like to extract **_**jet-by-jet**_** *****energy***
***corrections***** which would constrain ****m(jj),** **to the expected W shape,**

- - **to do that:**
**  ****- select some appropriate function: **
**   ****- express mass(***m***), in terms of jet energies and opening angle: **
**   ****- parameterize energies as a function of (****Et************ and what parameterization works better**

- actual fit parameters

## Constraining M(jj) to expected W lineshape 

**Breit-Wigner shape of ****W**** convoluted with 4GeV Gaussian **

**Mjj, GeV**

## How fit works..

-  **unlike the cases w**

**M(jj), GeV)**

**M(jj), GeV)**

**M****w****=90GeV**

**M****w****=70GeV**

** ****to configure and run these fits ****RooFit**** package is used:**
**                ****http://roofit.sourceforge.net**

**before..**

**after..**

## Different parameterizations

**The best RMS, series-like correction function:**

**Frank’s function:**

**- both functions give very similar results!**

## Conclusion

** ****backgrounds**** and their PDFs have to be studied and added to the fit,**
**do full analysis,**
** ****check if new energy corrections **_**create artificial biases**_** in ****Et(miss)****,...**
** ****check if new energy scales help in other analyses as well,**
** ****play more with it: **
**                – ****more sophisticated functions for corrections, jet definition**
**                ****- separate corrections functions for low and high jet**
**                  ****energies*:**
** ****suggestions are most welcome!****  **

**Et>60GeV**

**Et<60GeV**

## Energy-dependent corrections

** ****order jets by their Et, allow separate parameters for Low and High Pt**
**components:**

** ****if ****Et>50GeV****  - for each jet apply “high energy” corrections, **
**otherwise low energy correction is applied,**

## Energy-dependent corrections

**Mjj, GeV**

**Mjj, GeV**

**no corr.**