ATLAS Top Mass reconstruction in ttbar events (lepton+jets channel)

Regina Moles-Valls Maria Moreno Llácer Carlos Escobar Ibañez Vicente Lacuesta Miquel Andreas Wildauer IFIC – Universitat de València

ATLAS Top Mass reconstruction in ttbar

events (lepton+jets channel)


Top
Mass
Physics
Mo-va-ons


o 
Top
quark
mass
is
a
fundamental
parameter
of
the
SM
par-cle
physics
 o 
Top
quark
was
discovered
by
CDF
&
D0
in
1995.




Top Physics field was opened !!


o 
Top
proper-es:


o 
Charge=
(2/3)e
,

Spin
=
1/2
 o 
Large
mass
(m_top
>
35m_boPom)


o 
Present
world
average
value
measured
by
CDF

 



&
D0
with
L_int=5.8S^‐1
,
is:


m_top
=
(173.2
±
0.56_stat±
0.76_syst)
GeV


 
(arXiv:1107.5255v1
[hep‐ex]
26
Jul
2011


TOP
QUARK
MASS
INTERESTING
FIELDS:



• 
Top
quark
mass
is
an
important
parameter
for
the

 

EW
precision
measurement
due
to:


• 
Large
contribu-ons
in
radia-ve
correc-ons:


• 
Constraint
the
Higgs
Boson
Mass


• 
Increase
the
sensi-vity
to
physics
beyond
SM



Top
Quark
at
LHC


85%
at
LHC
 Gluon‐gluon
fusion


15%
at
LHC


Quark‐AnVQuark
scaXering


Top
quark
Produc/on



o Top
quark
is
produced
predominantly
in
pairs:


o 
LHC
at
√s
=
7TeV







σ_top=
165⁺¹¹_‐16pb
(theore-cal
NLO+NNLO)
 o 
Precision
measurements
and
new
searches


Top
quark
Decay



o 
Top
quark
decays
take
place
almost

 



exclusively

t





W+b


Dileptonic

 (11%)


• 
2
leptons


• 
2
neutrinos


• 
2bbar



Semileptonic

 (44%)


• 
1
lepton


• 
1
neutrino


• 
2
light
jets


• 
2bbar


FullHadronic

 (44%)


• 
4
light
jets


• 
2bbar


b‐jet
always
hadronize




W
decay?


TRIG




lep+E_tmiss





lep+E_tmiss





mulV‐jets BKG



Top
Mass
Analysis
in
ATLAS
(lepton+jets)


The
most
precise
top
mass
measurements
can
be
achieved
using
semi‐leptonic
channel:


B‐jet


E_T^miss


Muon


Electron:
Energy
deposits
in
EM
calorimeter
with
 associated
well
reconstructed
track
in
the
ID



Muon:
Track
segments
in
different
layers
of
muon
 chambers
are
combined
and
matched
with
ID
track


Jets: An-‐kt4
algorithm
is
used
star-ng
from
 energy
cluster
of
adjacent
calorimeters
jets.


Calibra-on
is
done
using
EM+HAD
calorimeter
 informa-on.
b‐jet
tagger
used
ID
in
forma-on
too


Trigger:
Trigger
is
based
on
signatures
of
electron
 muons
and
jets,
as
well
as
missing
transverse
 energy.


Full
Detector
Working
!!!



Top
Mass
Analysis
in
ATLAS
(lepton+jets)


The
main
methodology
used
to
determine
m_top
at
hadron
collider
consists
in
measuring
 the
invariant
mass
of
the
decay
products
of
the
top‐quark
candidate
(m_top^reco)
and


reconstruc-ng
top
mass
using
different
methods.


EVENT
SELECTION:


o 
Single
electron
or
muon
trigger
fired
 o 
Only
one
reconstructed
lepton:


• 
electron:
p_T>25GeV



• 
muon:
p_T>30GeV
 o 
Transverse
energy:


• 
electron:
E_t^miss>35GeV

&
m_T(W)>25GeV


• 
muon:
E_T^miss>20GeV

&
E_T^miss+m_T(W)>60GeV
 o 
At
least
4
jets
with
p_T>25GeV
|η|<2.5


o 
One
of
these
jets
tagged
as
b‐jet
(SV0>5.85)


Background:


o 
The
background
is
under
control


Hadronic
top
mass
reconstrucVon:


o 
How
the
jet
associaVon
is
done?


• 

Jet
triplet
with
high
p_T
and
at
least
1
b‐jet


• 

ΔR
constraint
between
two
light
jets



• 
W
mass
window
cut
to
improve
the
purity
of
the
 


jet
triplet


o 
 Agreement
 Data/MC
 is
 important
 to
 understand
 the
 combinatorial
background



o 
Precise
knowledge
of
the
Jet
Energy
Scale
is
needed
to
 forbid
possible
ships
in
the
m_top^reco:


• 
jet
re‐calibra-on



o 
Op-miza-on
of
the
es-mator
to
limit
systema-c
errors



Top
Mass
Analysis
in
ATLAS
(lepton+jets)


Muon
channel


M_W
cut
=
±
30


Muon
channel



Top
Mass
Analysis
in
ATLAS
(lepton+jets)


Leptonic
Top
mass
reconstrucVon:


o 
The
most
difficult
challenge
comes
from
the
kinema-c
of
the
neutrino
 o E_T^miss
comes
from
undetected
neutrino:
E_T^miss=
p_T^ν

o 
p_z^ν
chosen
using
W
boson
mass
 o 
b‐jet
assignment


o 
How
the
b‐jet
associaVon
is
done?


• 
Jet
triplet
with
high
p_T





(usually
p_T^max
method
used)


• ΔR
distance
between
b‐jet
and
W
bosson





(closer
b‐jet)

Muon
channel



Top
Mass
Analysis
in
ATLAS
(lepton+jets)


o 
Last
ATLAS
top
mass
results:


o 
2‐dimensional
template
analysis

 o L_int=
0.7S^‐1

Electron
Channel


Muon
Channel


o 
Systema-c
errors



(electron)


m_top
=
(173.9
±
1.2_stat±
3.1_syst)
GeV





 (muon)






m_top
=
(175.9
±
1.0_stat±
2.7_syst)
GeV


o 
ATLAS
has
rediscovered
and
re‐measured
the
top
quark
mass


• 
2010
with
a
L_int=35.3pb^‐1:

(electron)


m_top
=
(173.8
±
6.7_stat±
4.8_syst)
GeV






 (muon)






m_top
=
(166.7
±
5.0_stat±
5.0_syst)
GeV



• 2011
with
a
L_int=0.7S^‐1:


(electron)


m_top
=
(173.9
±
1.2_stat±
3.1_syst)
GeV





 (muon)






m_top
=
(175.9
±
1.0_stat±
2.7_syst)
GeV



Summary
of
the
Top
Mass
Analysis
in
ATLAS



m_top=
(175
±
0.9_stat±
2.7_syst)
GeV


The
uncertainty
of
the
results
is
domi‐


nated
by
the
systema-c
uncertain-es:


• 
JES
Uncertainty
for
b‐jets
and
light‐jets


• 
Ini-al
and
final
state
radia-on


• 
Some
aspects
of
the
MC
modelling
 Sta-s-c
is
increasing


Thank
you
very
much


o 
This
method
determines
simultaneously

m_top
and
Jet
Energy
Scale
Factors
 o 
JSF
based
on
observed
differences
between
m_w^reco
m_w^predicted(η,p_T)
 o 
χ²
fit
used
to
obtain
parton
scale
factors
(α)


o 
Signal
template
for
m_top,
m_w
and
background
are
built
for
different
mass
points.


o Template
hostograms
fiPed
by
(sum
of)
Gaussian/Landau
func-ons


o 
Parameters
of
the
Gaussian/Landau
func-ons
fiPed
as
a
func-on
of
(m_top,JES)
 o For
any
set
of
(m_top,m_W)
a
Likelihood
can
be
maximize
with
respect
to
m_top,
JSF
 and
frac-on
background



2‐D
Template
Method