1. Measurement of the Top Quark Mass with the Matrix Element Method at D0
Petra Haefner
Ludwig-Maximilians-Universität München

2. Why measure the Top Mass?
because it is SO HEAVY (~ 172 GeV) !
only (known) fermion with mass near electro-weak scale
indirect constraints on mass of Higgs boson possible
decay time shorter than hadronisation timescale
allows direct mass measurement based on decay products
Petra Haefner
Scientific American, June 2003

3. How is Top produced? mainly in top-antitop pairs (strong interaction)
85 % annihilation ( 5 LHC)
15 % gg fusion LHC)
cross section ~ 7pb (800 LHC)
single top production
(electroweak interaction)
several production channels
(s-, t-, associated t+W)
larger backgrounds
lower cross section (~ ½ stt)
not observed yet
Petra Haefner

4. How does Top decay?
branching ratio t  Wb ~ 100 % as |Vtb| >> |Vts|,|Vtd|
topology determined by W decays
branching ratio W  light q ~ 2/3, W  l+n ~ 1/3
t difficult to identify
consider only decays to l= e,m
alljets (44%)
largest branching fraction
huge backgrounds
lepton+jets (30%)
lower backgrounds
good statistics
dilepton (5%)
cleanest signature
very low statistics
Petra Haefner

5. The Tevatron Collider proton-antiproton collisions Run I (1992-1996)
ECM = 1.8 TeV
 L dt  100 pb-1
Run IIa ( )
ECM = 1.96 TeV
 L dt  1.2 fb-1
Run IIb (2006-?)
just started in June
 L dt  fb-1 (expected)
Petra Haefner

6. The D0 Detector “standard“ collider detector configuration
silicon microvertex &
tracking detector
within solenoid (2 T)
LAr calorimeter
high granularity
excellent resolution
muon chambers
large coverage (h < 2.0)
three trigger levels
Level kHz
Level Hz
Level Hz
coordinate system:
pseudorapidity h =  ln (tan /2)
radius r
polar angle j
(track) distance
Petra Haefner

7. The Tracking System two tracking subdetectors
Silicon Microstrip Tracker
measurement of charged particle origin
distinguish primary vertices (important at high luminosity)
identify b jets, t decays (secondary vertices)
reject cosmic muon background
Central Fiber Tracker
measurement of charged particle momentum & direction
dca resolution
Petra Haefner

8. The Calorimeter sampling calorimeter energy resolution contributions
uranium as absorber
liquid argon as active material
high granularity
longitudinal shower shape
divided into 3 cryostats
energy resolution contributions
stochastic term
jet fragmentation
sampling fluctuations
EM fraction fluctuations
noise term
electronic noise
multiple interactions
constant term
dead material
magnetic field
non-compensation
Petra Haefner

9. The Muon System consists of 3 layers (A,B,C) of
Proportional Drift Tubes
accompanied by
scintillation trigger counters
(Pixel detectors)
toroidal magnet
between layer A and B
1.9 T magnetic field
field perpendicular to beam axis
muons only charged particles
likely to penetrate both the
tracking system and calorimeter
reconstruct muon track outside calorimeter
magnetic field allows stand-alone momentum measurement
match muon to central track
use better resolution of tracking detector for pt measurement
Petra Haefner

10. What do we measure?
lepton+jets signature
1 energetic, isolated lepton (e / m)
4 energetic jets (2 b jets, 2 light jets)
missing transverse energy (1 n)
event kinematics
2 solutions for n pz (along beam axis)
otherwise fully determined kinematics
24 possible jet combinations (fewer with b tagging)
no need to distinguish light jets from W
12 jet combinations
whole detector needs to be well understood, calibrated and all subdetectors need to work for the top reconstruction!
Petra Haefner

11. Comparison of Methods Template Method (classical approach)
take all 2x12 combinations into account
choose kinematic fit with smallest 2
fill histogram with reconstructed top mass
compare to simulated distributions of different mtop
correct combination only in ~40% of events!
all events are weighted equally
Matrix Element Method
(developed at D0 Run I)
calculate probability Pi(mtop) for every event to be a top decay
take information from all 24 combinations into account
combine probabilities of all combinations & events
get total probability P(mtop) for the whole sample
measure mtop as most likely one
all combinations contribute
events are weighted according to their information content
Petra Haefner

12. Probability Calculation
probability for a given combination x in one event to be a top decay
parton density function (PDF) for all flavor compositions of the colliding quark and antiquark
leading order matrix element used for cross section calculation
leading order PDF (CTEQ5L)
jet / m resolutions taken into account by transfer functions
jet / lepton angles & electron energy assumed to be perfectly measured
simultaneous fit to mtop & JES factor S
minimisation of total uncertainty due to JES
Petra Haefner

13. Improving the method
simultaneous mtop / JES fit allows in situ calibration of jets ( constraint W mass in the matrix element)
include b-tagging information
obtain more pure samples
use as constraint on jet combinations
include muon transfer functions to describe resolutions
improve agreement between MC and data
improve JES / jet resolution
direct improvement on largest uncertainty
Petra Haefner

14. Jet Energy Scale top mass reconstructed from measured jet energies
jet resolution dominant effect on top mass resolution
need to calibrate detector to jets  Jet Energy Scale
“classical approach“:
calibrate EM calorimeter with Z ee
assume e  g
calibrate jet response with g + jet
only EM calorimeter calibrated well
hadronic calorimeter only indirectly calibrated
complicated procedure  large systematic uncertainties
Petra Haefner

15. Hadronic Calibration DiEM events very clean event signature
easy to calibrate mass fit
high statistics
DiJet events
hadronic W, Z decays
swamped by QCD
no mass constraint
study single hadrons
Petra Haefner
Single Hadron Response
need to find isolated particles
backgrounds from neutral particle decays
very low statistics (especially at higher E)

16. Eflow Algorithm improved Track-Jet- Algorithm
improve jet energy resolution by improving absolute energy scale
propagate tracks to CAL surface
calculate h, j of path at each layer
subtract expected energy deposit in CAL from Ejet
(Single Hadron Response)
add Etrack instead
correct Ejet for in-out /out-in tracks
cancels sources of fluctuations
10 % jet resolution improvement
in g+jet data (before JES corrections)
20 % improvement on Z  bb mass resolution
(important for Higgs search)
10 % improvement on W jj mass resolution
(important for top mass)
see IMPRS Seminar Talk
Petra Haefner

17. Muon Resolution Studies
study differences between MC / data resolutions
derive smearing factors for MC
improve agreement between MC / data resolutions
study effects of track quality / run periods on resolution
derive muon transfer functions
better description of muon resolution
include transfer functions into top mass probability calculation
Petra Haefner

18. Latest Top Mass Results
CDF:
l+jets, Template, 680 pb –1
l+jets, ME method, 940 pb –1
di-lep, ME method, 680 pb –1
all-had, Ideogram method, 310 pb –1
D0:
l+jets, ME method, 370 pb-1
l+jets, Ideogram method, 370 pb-1
di-lep, Matrix weighting, 370 pb-1
di-lep, Neutrino weighting, 370 pb-1
Winter 2006
Summer 2005
There‘s more to come!
Petra Haefner

19. Outlook Run IIa finished with 1.2 fb-1recorded, Run IIb just started
triple the statistics of the last published analysis on tape
being analysed right now
first results expected soon
b-tagging included in the next analyses
publication in preparation
new JES expected within the next months
improvements on jet resolution
smaller systematic errors
collaboration wide effort of data reprocessing and new analysis format
large improvements on all sectors from data reconstruction to MC generation, to efficiencies, scale factors, b-tagging, ...
Petra Haefner