Light jet energy scale determination with Top events
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Transcript of Light jet energy scale determination with Top events
Light jet energy scale determination with Top events
E. Cogneras, D. Pallin, S. Binet
D. Pallin Rome workshop June 2005
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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’
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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)
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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
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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
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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
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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
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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
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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
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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
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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)
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W selection Selection for optimal mass measurement
Comb. BKG
AOD
ATLFAST
3 days at L=1033Mw
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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
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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
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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
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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
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Method2 Results
EPa
rt /
E
E
ATLFAST
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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
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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
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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