Light jet energy scale determination with Top events

E. Cogneras, D. Pallin, S. Binet

D. Pallin Rome workshop June 2005

2

Purpose The absolute energie scale will be know at the level

of few percent at LHC start The calibration function varies with E and => Get the light jet energy scale such that

E jet E parton

Calibration function (Ejet) = Eparton / Ejet

AOD

Cone 04

EPa

rt /

E E

An example: Rome AOD (Ejet) = 1. to 1.1 ‘cone 04’

D. Pallin Rome workshop June 2005

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Purpose Absolute jet scale essential for the Top mass measurement A 1% miscalib induces a 0.9 GeV shift on the Top mass => in-situ calibration for Top calibration usable outside Top studies?

Source of uncertainty

Hadronic Mtop (GeV)

Fitted Mtop (GeV)

Light jet scale 0.9 0.2b-jet scale 0.7 0.7b-quark fragm 0.1 0.1ISR 0.1 0.1FSR 1.9 0.5Comb bkg 0.4 0.1Total 2.3 0.9 ATLFAST 1 year @ 1033

M(jjb) M(lb)

D. Pallin Rome workshop June 2005

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How Use the well defined W mass to rescale the jet energies from

the W decay Wj1 j2 such that

MW=MWPDG

Dont use any hypothesis for the calibration function Dont rely on MC difficulty : 2 jets with different energy, 1 # 2

Select an enriched W sample from events

jeti

parti

iWPDGW E

EwithMM 21

j1

j2l(e,

t1 t2

W1 W2

b1 b2

tt

D. Pallin Rome workshop June 2005

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Reconstructed W (cone 04)

MW ~ 76 GeV , ~ 7 GeV 40<E<500 GeV AOD ~ ATLFAST except cosjj

AOD ATLFAST

E MW

Cosjj

D. Pallin Rome workshop June 2005

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(1-costh)

(1-cosjj)

Reconstructed W (cone 04)

AOD : <M jet>~ 10 GeV ATLFAST: jets are massless AOD and ATLFAST: jets are miscalibrated

deviation for cosjj>0

AOD ATLFAST

Mj Cosjj

1 2

1 2

p pp p

jjcos E

Eth /

E

D. Pallin Rome workshop June 2005

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(1-costh)

(1-cosm)

Reconstructed W (cone 04)

AOD=

Angle between the 2 jets recovered, shift only due to E miscalib. ATLFAST: jets are massless, angle is not accurately defined => ~2 GeV shift on MW

AOD ATLFAST

Cosm2 2

1 2

2 21 2 1 2 1 2

2 (1 cos )

(2 cos ) / 2jj m m i i

m jj

E E E p

p p M M E E

Wcos cos in M

cos

D. Pallin Rome workshop June 2005

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Method 1 to extract the E scale

compute R for k bins in E apply k factors on R and recompute R n times =>

jeti

parti

iWPDGW E

EwithMMR 21/

1 2k j j True nk k

n

R

E

EPa

rt /

E

E

AODW Recons.

No comb BKG

D. Pallin Rome workshop June 2005

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Method1 Results after recalibration

Function found with any

‘a priori’ hypothesis

EPa

rt /

E

E

before

after

AOD4000 W Recons.No comb BKG

Mw

D. Pallin Rome workshop June 2005

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Method1 Results on Z+jets Use the AOD Top sample to correct the jet energies of the

AOD Z+jet sample TOP 12000 jets, Z+jet 8000 jets Apply same cuts on jets energies => Top light jet scale works for all light jets

Top Z+jets

++

After calib ‘Top’

EPa

rt /

E

E E

EPa

rt /

E

D. Pallin Rome workshop June 2005

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W selection

Selection for optimal mass measurement Clean W sample with

2 b tag Cuts on Mt or Mt-Mw

Selection for commissioning No b tag, cut on Mjlv ( see ivo van Vulpen talk) Reduced b tag (with lower efficiency or 1btag only), cut on angles

=> W sample ~85% purity (see D Pallin talk Slovakia)

W sample ~85% purity

(see D Pallin talk Slovakia)

D. Pallin Rome workshop June 2005

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W selection Selection for optimal mass measurement

Comb. BKG

AOD

ATLFAST

3 days at L=1033Mw

D. Pallin Rome workshop June 2005

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Method1 with comb. BKG

Preliminary result with 1000 W

+bkg evts Calibration marginaly sensible

to BKG (purity=85%)

EPa

rt /

E E

AOD

3 days at L=1033

Very preliminary

To reach the 1% precision on the E scale, 10000 W should be enough = 1month data taking

D. Pallin Rome workshop June 2005

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Method2

If the angle between the 2 jets is not well determined (see ATLFAST example), Method 1 is not usable

Mw calibrated with Meth 1 not constant in function of cos

Mw

corr

/Mw

cosjj

ATLFAST

D. Pallin Rome workshop June 2005

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Method2 Need to

To disentangle the contributions from cosjj and E To evaluate separately the correction E1 & E2

perform a fit to constraint MW to MwPDG

evt by evt, compute 1E and 2

E for the two jetsand deduce E =f( E)

Allow E, , to move within detector resolution

2

,,

2,1

2

2χ

EX

W

Wjj

X

iXiEi

XMm

D. Pallin Rome workshop June 2005

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Method2

(E=Ei)

=E

p/Ej

E (GeV)

Deduced function

= Eparton / Ejet =f(E)

FSR contrib

Extract PEAK values for various energies

D. Pallin Rome workshop June 2005

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Method2 Results

EPa

rt /

E

E

ATLFAST

D. Pallin Rome workshop June 2005

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Method3 Perform also a fit to constraint MW to Mw

PDG

Mix meth 1 and 2 Constraints not given through the detector

resolutions define E corrections factor per bins in E constraint cosjj =1 for cosjj =-1 Mw corrected by E or cosjj in window [Mw : Mwpdg]

1 2 1 2 cos2 (1 cos )jjE E mE E WM

D. Pallin Rome workshop June 2005

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Method3 Results same results as Meth 2 works on TOY MC No cosjj correction found by the fit to AOD data

EPa

rt /

E

EPa

rt /

E

E E

After calib

D. Pallin Rome workshop June 2005

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Conclusions New methods investigated to get in-situ the light jet

calibration for Top mass measurement Preliminary results from AOD

AOD and ATLFAST comparable for top studies 1% precision on the jet scale seems achievable in 1-2 month of

data taking at 1033

Top in-situ Light jet scale seems usable for all jet (if calibration done for jets with same selection)

Futur Run all AOD files +physical BKG Detailed look to jet algorithms