1. light jet energy scale from Wjj
See
D. pallin: Tatra workshop ATLAS phys workshop (Rome) Top meetings
J schwindling Top meetings

2. Wjj sample in lepton+jets events
Select an enriched W sample from ttbar (lepton+jets) events
Selection with 1 or 2 btag
typically W/fb-1 (2btag, _btag=0.6)
x2 if 1 btag
( 80%-90%) purity (1 or 2 btag)
Different Selection if 1btag
W mass shift linked to JES
Jets energies between 40 and 400 GeV j1 j2
l(e,m) n t1 t2 W1 W2 b1 b2
Rome AOD 500pb-1

3. Method 1 (D.Pallin) Principle to obtain Constraint 1 Mjj=MWPDG
Constraint 2 the calibration function is universal
No hypothesis on function , no MC inputs
build W mass distributions in bin of jet energies
extract the peak values for each bins (n bins)
Deduce (E) from MW(E)

4. Method 1 W mass distributionsMW
Ejet (GeV)
Rome AOD ‘cone 04’

5. Method 1: Deducing (E) from MW(E)
Extraction of n values (Ebin)
Several possible methods
chi2 fit
Iterative method
(equalisation of MC/DATA distribution)
Results (R) reproduce well the initial
calibration (theo) as function of E
1% precision with 1fb-1
EPart / E E

6. Method 1: BUT
the calibration function as function of E is not the true calibration
E_jet / E_part % E with MC calib=1
Bias is within 1% above 40 GeV
But need to be corrected
huge effect below 40 GeV
EPart / E
Eparton
E recons
How to correct?
origine of bias : purely statistical from :
Ejet shape+ Ejet resolution
negligeable if resolution(E)<<E
important when resolution(E)~E
easy to compute if shape and resolution known
Shape given since W sample is 80% pure
Resolution could be computed from W width (or obtain from Z,+jets samples)

7. Method :1 ongoing work
extraction of jet energy resolution from W width
evaluations of systematics
transportation of calibration to other samples
JES from 20 GeV
Increase size of initial sample
Note in preparation

8. Method 2: templates (J Schwindling)
h: 297 / √E  11 mrad
Idea more or less « à la CDF »
Will be presented in jet/ETmiss meeting in Trigger/Physics week end of May
Idea: generate mjj template histograms by smearing W  qq quarks with:
Expected angular resolution
b x expected energy resolution
energy correlation between jets
energy scale a
Resolution (mrad)
j: 224 / √E  10 mrad
Expected angular resolution
Fitted on Rome ttbar sample 4100
Equark (GeV)
Expected Energy resolution
s(Ej – Eq) (GeV)
s(E) = 3.8 GeV xE
Eq (GeV)
Jet - quark
s(E1)
10.7
s(E2)
9.1
s(E1)  s(E2)
14.0
s(E1+E2)
16.5
(Error bars
~ 0.2 GeV)
+ 18 %

9. Meth2 :Expected performance
Fit both energy scale a and resolution b
With enough statisitics, can do this as a function of energy (and/or h ?)
Example: apply templates on independent set of smeared quarks with:
Different energy scale(s)
Different energy resolution
Results (templates  smeared quarks):
Can fit average energy scale to ~0.4% with 1fb-1
With more statistics (10 fb-1), could fit energy scale as a function of E
In principle allows also to check energy resolution as a function of E
Injected a 1%
Reconstructed a
Injected resolution xE
Fitted resolution
Default resolution xE

10. Meth 2: Towards real data
PT cut = 40 GeV
All jj combinations
Only 2 light jets < mjjb < 200
Only 2 light jets
Try to fit the templates (smeared PYTHIA quarks) on jj pairs from + Geant 4 + reconstruction (Rome sample 4100)
Event selection (tt  jjb lnb):
1 isolated lepton with PT > 20 GeV
ETmiss > 20 GeV
2 b-tagged jets with PT > 40 GeV
2 and only 2 light jets with PT > 40 GeV
150 GeV < mjjb < 200 GeV
 purity ~ 83 %, ~ 1200 jj pairs for 500 pb-1
Expect
energy scale (Ejet / Eq) ~ 0.96 – 0.97
b = 1

11. Meth 2: Conclusion / questions
Results (templates  sample 4100)
a = ± 0.005
b = 1.07 ± 0.05
Fit of a seems very stable: ~ independent of qq smearing, ~ independent of mtop (± 0.4 % uncertainty on Escale for Dmtop = ± 5 GeV), independent of combinatorial background
b sensitive to angular resolution, correlation between jet energies … cannot claim to measure energy resolution
We need to understand better:
Origin of energy correlation between jets
Angular correlation ?
Influence of underlying event
E scale versus pileup