Calibration of high pt hadronic w
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Calibration of High Pt Hadronic W. Outline Introduction High Pt Hadronic W in TTbar and Higgs events Reconstruction Algorithm MC based calibration of W Pt Correlation between W Pt cut and W mass scale. Haifeng Pi 10/16/2007. Origin of the Topic.

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Calibration of High Pt Hadronic W

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Calibration of High Pt Hadronic W

  • Outline

  • Introduction

  • High Pt Hadronic W in TTbar and Higgs events

  • Reconstruction Algorithm

  • MC based calibration of W Pt

  • Correlation between W Pt cut and W mass scale

Haifeng Pi


Origin of the Topic

Di-jet resonance from W is a critical component in a number of important physics studies

TTbar analysis (Tt  b+ W (lv) + b + W (jj))

Wt (Wt  lv + b + W(jj))

Vector Boson Fusion Higgs (qqH(H WW)  qq + lv + jj)

V-V scattering (qqWW  qq + lv + jj)

Existing Issues:

The tuning jet reconstruction algorithm for hadronic W events was missing for a long time at LHC. The reconstruction for the high mass Higgs with hadonic W is largely based on the jet algorithm for single and “isolated” jets with its calibration constant.

The strong dependency between properties of reconstructed di-jet system and the selection cuts which are supposed to cover a wide Higgs mass range needs to be understood. (The selection cuts is mass-dependent)

What this analysis about: tuning of algorithm for the di-jet system, data driven strategy, basic performance …

Signal events

In the final states from qqH,HWWlvjj (we only consider e/), the number of signal events in 30 fb-1 based on Leading-Order

Generated Higgs Mass-Distribution (mH =700 GeV, =74.5 GeV)

The cross section we used is consistent with previous CMS Note(s) and recent reports from other channels (qqH, HZZlljj) within ±5%.

ATLAS cross sections is consistent with CMS in the low-mediate mass range, but 10-20% higher for MH > 700 GeV/c2. So CMS results might be more conservative in high mass.

Hadronic W mass in Higgs and TTbar

After applying lepton trigger and isolation, and basic selection cuts (at least 4 jet with ET > 25 GeV, and MET > 30 GeV)

Higgs, mH=700 GeV



Higgs mass distribution after selection (MH=600 GeV/c2)

Large signal to background ratio, visible mass peak and shape, right mass scale

Mass-dependent W pt cut

MH=300 GeV/c2

MH=700 GeV/c2

MH=400 GeV/c2

MH=900 GeV/c2

MH=500 GeV/c2

The W mass cut, Forward jet tagging cut are slightly different to maximize the discovery potential . Details are summarized in Appendix.

Fitting of S and B in lower mass range (MH = 300 GeV)

Background -- Convoluted Landau and Gaussian distribution

Signal – Gaussian distibution

Discovery potential vs MH at 30 fb-1

  • Best discovery potential near 600 GeV

  • High mass range is limited by statistics (although a higher S/B can be achieved)

  • Low mass range is limited by large W+jets background

Background Samples

Full matrix element event generator + FAMOS

Pythia + Full simulation

TTbar+ jets 3.5 million

W/Z + jets 4.5 million

Pythia + FAMOS

QCD 400 million

Hadronic W Properties



Jet Pt in Different Samples

Various distributions show strong similarity between TTbar and H300

Matching Quark to Jet

Di-jet Eff vs 

Di-jet Eff vs Pt

Matching R < 0.3

Hadronic W Reconstruction

Di-jet re-clustering algorithm:

Step 1 using standard iterative cone (IC) jet algorithm with 0.5

cone size to get the “jet-seed”

Step 2 applying gamma-jet calibration

Step 3 applying jet Et threshold 25 GeV

Step 4 using jet-seed of closest two jets in space to

reconstruction the di-jet cluster

Step 5 using E-scheme to calculate the di-jet cluster

Step 6dedicated W pt scale and improvement of energy


The data-driven analysis using TTbar events for the hadronic W mass reconstruction is under separate study

Another di-jet approach has been investigated by colleagues at Fermilab (CMS IN 2006/004). The comparison is also under way.

Mass of Di-jet System

New di-jet clustering Algorithm

Using standard di-jet for the mass reconstruction

di-quark (di-jet) R

Interested region of merged di-jets: R < 1.5

The refined analysis will adapt the R cut with the W Pt cut or targeted Higgs mass.



MC-based W Pt Calibration

Calibration vs W Pt Cut



Pt > 30 GeV/c





Dependencies on W Pt Cut

W Mass Resolution

W Mass

Nominal S/B


  • New di-jet clustering algorithm is developed to reconstruct hardonic W with better performance in mass resolution and offline selection efficiency

  • MC-based W Pt calibration based on di-jet cluster is studied. The corrected W Pt help restore the MET scale and resolution

  • Data driven calibration using TTbar events is studied based on one isolated lepton + 4 jets signature. The S/B is big enough to perform the calibration

  • The similarity between TTbar and Higgs events reduces the systematic uncertainties of applying the calibration constant to Higgs analysis

  • W+jets is the most important background channel. But the combinatory background of TTbar events is more serious in the high Pt region

  • The calibration can be achieved with 1-2 fb-1 of data for early discovery or exclusion. High precision can be achieved in 8-10 fb-1. The time scale fit well into the Higgs discovery or exclusion of VBF Higgs and V-V scattering …

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