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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
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
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
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
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 m h 600 gev c 2
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
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 m h 300 gev
Fitting of S and B in lower mass range (MH = 300 GeV)

Background -- Convoluted Landau and Gaussian distribution

Signal – Gaussian distibution

Discovery potential vs m h at 30 fb 1
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
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

Jet pt in different samples
Jet Pt in Different Samples

Various distributions show strong similarity between TTbar and H300

Matching quark to jet
Matching Quark to Jet

Di-jet Eff vs 

Di-jet Eff vs Pt

Matching R < 0.3

Hadronic w reconstruction
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
Mass of Di-jet System

New di-jet clustering Algorithm

Using standard di-jet for the mass reconstruction

Di quark di jet r
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.



Calibration vs w pt cut
Calibration vs W Pt Cut



Pt > 30 GeV/c





Dependencies on w pt cut
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 …