Transcript Issues in verification of ALPGEN heavy flavor production

Top quark mass
For DØ collaboration
Regina Demina
University of Rochester
Wine and Cheese seminar at FNAL, 07/22/05
Outline
• Introduction
• Top quark mass measurement in Run II
–
–
–
–
–
Matrix element method description
In situ jet energy scale calibration on hadronic W-mass
Sample composition
Result
Systematics
• Tevatron combined top mass
• Top quark production
– Update on cross section in l+jets channel
– Search for resonance production
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Top Quark Mass: Motivation
• Fundamental parameter of the
Standard Model.
• Important ingredient for EW
precision analyses at the quantum
level:
t
W
CDF&D0
RUNII
H
W
W
W
b
MW  mt2
MW  ln(MH)
which were initially used to indirectly
determine mt.
After the top quark discovery, use
precision measurements of MW and
mt to constrain MH.
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Top production
At √s=1.96 TeV top is
produced in pairs via quarkantiquark annihilation 85%
of the time, gluon fusion
accounts for 15% of ttbar
production
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Top Lifetime and Decay
• Since the top lifetime
the top quark does not
top ~ 1/ M3top~10 -24 sec
hadronize. It decays as a
qcd ~ -1 ~10 -23 sec
free quark!
• BR(tWb) 
e-e
(1/81)
– Both W’s decay via W l
mu-mu (1/81)
• final state: llbb
-
tau-tau (1/81)
DILEPTON
e -mu (2/81)
– One W decays via
Wl
•
final state: lqq bb
e -tau (2/81)
mu-tau (2/81)
-
LEPTON+JETS
– Both W’s decay via Wqq
• final state: qq qq bb
e+jets (12/81)
mu+jets(12/81)
tau+jets(12/81)
jets
ALL HADRONIC
(36/81)
Lepton provides a good trigger, all jets are tough
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Top ID in “lepton+jets”
channel
• 2 b-jets
pp  tt
• Lepton: electron or muon
t  bW
• Neutrino (from energy
imbalance)
W  qq'or  W  l
• 2 q’s – transform to jets of
particles
• Note that these two jets
come from a decay of a
particle with well measured
mass – W-boson – built-in
thermometer for jet
energies
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DØ detector
• Electrons are identified as
clusters of energy in EM section
of the calorimeter with tracks
pointing to them
• Muons are identified as
particles passing through entire
detector volume and leaving
track stubs in muon chambers.
Track in the central tracking
system (silicon+SciFi) is
matched to track in muon
system
• Jets are reconstructed as
clusters of energy in calorimeter
using cone algorithm DR<0.5
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Top mass using matrix
element method in Run I
• Method developed by DØ (F. Canelli, J. Estrada, G.
Gutierrez) in Run I
Single most precise measurement
of top mass in Run I
Mt =180.1±3.6(stat) ±4.0(syst)
GeV/c2
Systematic error dominated by JES
3.3 GeV/c2
With more statistics it is possible to
use additional constraint on JES based
on hadronic W mass in top events – in
situ calibration
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Matrix element method
• Goal: measure top quark mass
• Observables: measured momenta of jets and leptons
• Question: for an observed set of kinematic variables x
what is the most probable top mass
• Method: start with an observed set of events of given
kinematics and find maximum of the likelihood, which
provides the best measurement of top quark mass
• Our sample is a mixture of signal and background
Pevt ( x, mt )  f top  Psgn ( x, mt )  (1  f top )  Pbkg ( x)
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Matrix Element Method
probability to observe a
set of kinematic variables
x for a given top mass
dnσ is the differential
cross section
Contains matrix element
squared
W(x,y) is the probability
that a parton
level set of variables y
will be measured
as a set of variables x
1
n
Psgn ( x; mt ) 
d
 ( y; mt ) dq1 dq2 f (q1 ) f (q2 ) W ( x, y )

 (mt )
f(q) is the probability distribution than
a parton will have a momentum q
Normalization depends on mt
Includes acceptance effects
Integrate over unknown q1,q2, y
q
b
q’
t
t
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Transfer functions
(partonjet)
• Partons (quarks produced as a result of
hard collision) realize themselves as jets
seen by detectors
– Due to strong interaction partons turn into
parton jets
– Each quark hardonizes into particles
(mostly p and K’s)
– Energy of these particles is absorbed by
calorimeter
– Clustered into calorimeter jet using cone
algorithm
• Jet energy is not exactly equal to parton
energy
– Particles can get out of cone
– Some energy due to underlying event (and
detector noise) can get added
– Detector response has its resolution
• Transfer functions W(x,y) are used to
relate parton energy y to observed jet
energy x
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h Dependence of JES
 hdependence of JES is derived on g+jet data, but the
overall scale is allowed to move to optimize MW
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JES in Matrix Element
• All jets are corrected by standard
DØ Jet energy scale (pT, h)
• Overall JES is a free parameter in
the fit – it is constrained in situ by
mass of W decaying hadronically
• JES enters into transfer functions:
W ( E j , E p , JES ) 
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W(
Ej
, Ep )
JES
JES
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Normalization
1
n
Psgn ( x; mt ) 
d
 ( y; mt ) dq1 dq2 f (q1 ) f (q2 ) W ( x, y )

 (mt )
e+jets
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μ+jets
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Signal Integration
• Set of observables – momenta of jets and leptons: x
• Integrate over unknown
– Kinematic variables of initial (q1,q2) and final state partons (y: 6 x3 p) = 20
variables
– Integral contains 15 (14) -functions for e(m)+jets
• total energy-momentum conservation: 4
• angles are considered to be measured perfectly: 2x4 jet +2 lepton
• Electron momentum is also considered perfectly measured, not true for muon
momentum: 1(0)
– 5(6) dimensional integration is carried out by Vegas
– The correspondence between parton level variables and jets is established
by transfer functions W(x,y) derived on MC
• for light jets (from hadronic W decay)
• for b-jets with b-hadron decaying semi-muonically
• for other b-jets
• Approximations
– LO matrix element
– qqtt process only (no gluon fusion – 15%)
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Background integration
• W+jets is the dominant background process
• Kinematics of W+jets is used as a representation
for overall background (admixture of multijet
background is a source of systematic
uncertainty)
– Contribution of a large number of diagrams makes
analytical calculation prohibitively complex
– Use Vecbos
• Evaluate MEwjjjj in N points selected according to the
transfer functions over phase space
• Pbkg- average over points
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Sample composition
Lepton+jets sample
–
–
–
–
Isolated e (PT>20GeV/c, |h|<1.1)
Isolated m (PT>20GeV/c, |h|<2.0)
Missing ET>20 GeV
Exactly four jets PT>20GeV/c,
|h|<2.5 (jet energies corrected to
particle level)
Use “low-bias” discriminant to fit
sample composition
– Used for ensemble testing and
normalization of the background
probability.
– Final fraction of ttbar events is fit
together with mass
e+jets
m+jets
# of events
70
80
Signal fraction
45±12%
29±10%
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Calibration on Full MC
lepton+jets
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Mt=169.5±4.4 GeV/c2
JES=1.034±0.034
calibrated
calibrated
DØ RunII Preliminary
expected: 36.4%
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Systematics summary
Source of uncertainty
Effect on top mass (GeV/c2)
B-jet energy scale
+1.32-1.25
Signal modeling (gluons rad)
0.34
Background modeling
0.32
Signal fraction
+0.5-0.17
QCD contribution
0.67
MC calibration
0.38
trigger
0.08
PDF’s
0.07
Total
+1.7-1.6
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●
B-jet energy scale
Relative data/MC b/light jet energy scale ratio
•fragmentation: +-0.71 GeV/c2
 different amounts of p0, different p+ momentum spectrum
 fragmentation uncertainties lead to uncertainty in b/light JES
ratio
compare MC samples with different fragmentation models:
Peterson fragmentation with eb=0.00191
Bowler fragmentation with rt=0.69
•calorimeter response: +0.85 -0.75 GeV/c2
uncertainties in the h/e response ratio
+ charged hadron energy fraction of b jets > that of light jets
 corresponding uncertainty in the b/light JES ratio
•Difference in pT spectrum of b-jets and jets from W-decay: 0.7
GeV/c2
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Gluon radiation
• The effect is reduced by
– Requiring four and only four jets in the final state
– High PT cut on jets
• Yet in ~20% of the events there is at least one jet that is
not matched (DR(parton-jet)<0.5) to top decay products
– These events are interpreted as background by ME method
• We study this systematic by examining ALPGEN ttj
sample and varying its relative fraction between 0 and
30% (verified on our data by examining the fraction of
events with the 5th jet)
• Final effect on top mass 0.34 GeV/c2
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Signal/Background Modeling
●
QCD background: +-0.67 GeV/c2
Rederive calibration including QCD events from data (lepton anti-isolation)
(note: sample statistics limited) can be reduced in the future
●
W+jets modeling: +-0.32 GeV/c2
study effect of a different factorization scale for W+jets events
(<pT,j>2 instead of mW2 + SpT,j2)
PDF uncertainty: +-0.07 GeV/c2
●
CTEQ6M provides systematic variations of the PDFs
reweight ensembles to compare CTEQ6M with its systematic variations
(by default the measurement uses CTEQ5L throughout:
use a LO matrix element, and for consistency with simulation)
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●
Signal fraction
Signal fraction: +0.50 -0.17 GeV/c2
Fitted top mass depends slightly
on true signal fraction (if signal
fraction is smaller than expected):
=> Vary signal fraction within
uncertainties
from topological likelihood fit
- Note: ftop fit yields identical result
with factor √2 smaller uncertainties
Cross check on data: cut
on log10(pbkg)<-13
Ftop=31%46±6%
Mtop=170.2±4.1 GeV/c2
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Systematics summary
Source of uncertainty
Effect on top mass (GeV/c2)
B-jet energy scale
+1.32-1.25
Signal modeling (gluons rad)
0.34
Background modeling
0.32
Signal fraction
+0.5-0.17
QCD contribution
0.67
MC calibration
0.38
trigger
0.08
PDF’s
0.07
Total
+1.7-1.6
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Result and cross checks
• Run II top quark mass based on lepton+jets sample:
Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2
• JES contribution to (stat+JES) 3.3 GeV/c2
• Break down by lepton flavor
– Mt(e+jets)=168.8 ±6.0(stat+JES) GeV/c2
– Mt(m+jets)=172.3 ±9.6(stat+JES)GeV/c2
• Cross check W-mass
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Summary of DØ Mt
measurements
• Statistical
uncertainties are
partially
correlated for all
l+jets Run II
results
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DØ Run II preliminary
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Projection for uncertainty on
top quark mass
Assumptions:
• only lepton+jets channel considered
• statistical uncertainty normalized at
L=318 pb-1 to performance of current
analyses.
• dominant JES systematic is handled
ONLY via in-situ calibration making use
of MW in ttbar events.
• remaining systematic uncertainties:
include b-JES, signal and background
modeling, etc (fully correlated between
experiments) Normalized to 1.7 GeV at
L=318 pb-1.
• Since most of these systematic
uncertainties are of theoretical nature,
assume that we can use the large data
sets to constrain some of the model
parameters and ultimately reduce it to 1
GeV after 8 fb-1.
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Combination of Tevatron
results
JES is treated as
a part of
systematic
uncertainty,
taken out of stat
error
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Combination
• Mt=172.7±2.9 GeV/c2
• Stat uncertainty:
1.7GeV/c2
• Syst uncertainty:
2.4GeV/c2
• hep-ex/0507091
• Top quark Yukawa
coupling to Higgs boson
• gt=Mt√2/vev=0.993±0.017
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What does it do to Higgs?
MW,GeV/c2
68% CL
MH,GeV/c2
Mt,GeV/c2
• MH=91+45-32GeV/c2
• MH<186 GeV/c2 @95%CL
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ttbar cross section in l+jets
with b-tag
DØ RunII Preliminary, 363pb-1
• Isolated lepton
– pT>20 GeV/c, |he|<1.1, |hm|<2.0
• Missing ET>20GeV
• Four or more jets
– pT>15 GeV/c, |h|<2.5
=8.1+1.3-1.2(stat+syst)±0.5(lumi) pb
≥4j, 1t
≥4j, 2t
Expect bkg
21.8±3.0 1.9±0.5
Observe
88
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Cross section summary
DØ RunII Preliminary
Submitted for
publication
Updates
 ( pp  tt ), pb
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ttbar resonances in l+jets with
b-tag
• Check ttbar invariant
mass for possible
resonance production
DØ RunII Preliminary, 363pb-1
NNLO(tt)=6.77±0.42
• Events are kinematically
constrained
– mT=175GeV/c2
– Leptonic and hadronic W
masses
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Regina Demina, Joint Theoretical and Experimental Seminar at FNAL
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ttbar resonances in l+jets with
b-tag
• Limit M(Z’)>680 GeV/c2 with G=1.2%MZ’ at
95%CL
DØ RunII Preliminary, 363pb-1
*
*R. Harris, C. Hill, S. Parke hep-ph/9911288
Run I limit 560 GeV/c2
Run II limit 680 GeV/c2
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Conclusion
• First DØ RunII top mass measurement in l+jets channel to
surpass Run I precision
– Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2
• Developed method for in situ jet energy scale calibration
using hadronic W-mass constraint
• Combined Tevatron top mass measurement reaches a
precision of 1.7%
• ttbar production cross sections updated for l+jets channel
• Invariant mass of ttbar system probed for resonance
production, exclusion limit for M(Z’)>680 GeV/c2 at
95%CL
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Backup slides
Parton Level Tests
Text
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L+jets sample composition
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Kinematics in l+jets sample
DØ RunII Preliminary, 363pb-1
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