Measurement of the Top Quark Mass at D Ø Zhenyu Ye/Fermilab USTC Hefei China, 2012/3/16.
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Transcript of Measurement of the Top Quark Mass at D Ø Zhenyu Ye/Fermilab USTC Hefei China, 2012/3/16.
Measurement of the Top Quark Mass at DØ
Zhenyu Ye/Fermilab USTC Hefei China, 2012/3/16
USTC Hefei China, Z. Ye2 2012/3/16
Z. Ye3 2012/2/27
DØ
Chicago
Tevatron6 km circumference
collide protons with antiprotons every 396 ns
at √s=1.96 TeV
Fermilab Tevatron Collider
Z. Ye4
The Standard Model of Particle Physics
2012/2/27
almost no
USTC Hefei China, Z. Ye5
Top Quark in the Standard Model
2012/3/16
Weak isospin partner of b quark, spin=1/2, charge=+2/3.
Heaviest (~175 GeV) fundamental particle discovered so far.
Yukawa coupling 0.996±0.006. A special role in EWSB ?
Lifetime (~10-24s) <<1/ΛQCD, decays before hadronization.
t
USTC Hefei China, Z. Ye6
Top Quark in the Standard Model
2012/3/16
Weak isospin partner of b quark, spin=1/2, charge=+2/3.
Heaviest (~175 GeV) fundamental particle discovered so far.
Yukawa coupling 0.996±0.006. A special role in EWSB ?
Lifetime (~10-24s) <<1/ΛQCD, decays before hadronization.
tIs the top quark “standard”?
USTC Hefei China, Z. Ye7
A Bit History on Top Quark
2012/3/16
• 1976: discovery of the bottom quark at Fermilab suggested the existence of the top quark
• 1984: PETRA mt>23.3 GeV• 1988: UA1 mt>44 GeV• 1990: TRISTAN mt>30.2 GeV SLC mt>40.7 GeV LEP mt>45.8 GeV UA1 mt>60 GeV UA2 mt>69 GeV • 1992: CDF mt>91 GeV• 1994: DØ mt>128 GeV• 1994: evidence of top quark from CDF
C.Quigg
EW FitCDF D0Tevatron
USTC Hefei China, Z. Ye8
A Bit History on Top Quark
2012/3/16
1995 Feb --- discovery of top quark reported by CDF & DØ in Tevatron Run I (1992-1996).
• CDF(67 pb-1) : mt=176±13 GeV, σ=6.8+3.6
-2.4 pb, observed 19 events, expected 6.9 bkg, bkg-only hypothesis rejected at 4.8σ
• D0 (50 pb-1): mt=199±30 GeV, σ=6.4±2.2 pb, observed 17 events, expected 3.8 bkg, bkg-only hypothesis rejected at 4.6σ
USTC Hefei China, Z. Ye9
Where Are We Now?
2012/3/16
1200 top events in this channel alone
Tevatron Run II 2001-2011
l+jetsno b-tag
this talk
USTC Hefei China, Z. Ye10
Top Quark Physics
2012/3/16
t
t
b
b
W+
W-
q
vq’
l-
q
vq’
l+
branching ratioCKM |Vtb|
rare decay (FCNC)
anamolus couplingsmass, width,
chargemass difference
search for t’
production cross sectionforward-backward
asymmetryspin correlationttγcross sectionsearch for tt Hsearch for Z’
W-helicitysearch for W’
pp
USTC Hefei China, Z. Ye11
Top Quark Physics
2012/3/16
… …
USTC Hefei China, Z. Ye12
Outline
2012/3/16
Introduction
Precision measurement of the top-quark mass lepton+jets channel dilepton channel
Top-antitop quark mass difference
Summary and outlook
USTC Hefei China, Z. Ye13
Top-Quark Mass
2012/3/16
(1-r) radiative correction
• New physics might affect the top-quark mass measurement differently in different top quark final states.
• Top mass is a free parameter in the SM.
• Top and Higgs masses enter in the radiative correction to the W mass. Knowing top and W masses constrains Higgs mass.
USTC Hefei China, Z. Ye14
Top-Quark Production and Decay
2012/3/16
Tevatron RunII: σ(mt=173 GeV) ~7.5 pb, 85% qqbar annihilation, 15% gg fusion
Standard Model: Γt~1.4 GeV, BR(tW+b)~100%
USTC Hefei China, Z. Ye15
Top-Quark Production and Decay
2012/3/16
lepton+jets:
dilepton:
all hadronic: (lepton refers to electron and muon.)
USTC Hefei China, Z. Ye16
Top-Quark Final States
2012/3/16
Final state may include: electron, muon, neutrino, and jets.
jet: narrow cone (R=0.5 for these analyses) of hadrons and other particles produced by quarks or gluons through parton showering and fragmentation.
USTC Hefei China, Z. Ye17
DØ Detector
2012/3/16
Tracker (2T Solenoid): • silicon microstrip
tracker• scintillator fibre
tracker
Calorimeter:• liquid Ar+Uranium
Muon system (1.8T Toroid):• drift tubes• scintilation counters
Tracker
USTC Hefei China, Z. Ye18
DØ Calorimeter
2012/3/16
Based on the fact that interactions should be symmetric in Φ.
Using Ze+e- for EM layers (absoluate scale) and dijet events for hadronic layers.
Injecting known signal into pre-amplifiers and equalize readout response.
Electronics Calibration
Inter-Φ Calibration Inter-η Calibration
USTC Hefei China, Z. Ye19
Jet Energy Scale (I)
2012/3/16
• Jet energy scale is determined from photon+jet and dijet events at D0.
• absolute scale set by the photon energy which is well measured by the EM calorimeter.
• relative scale determined from transverse momentum balance.
• The energy of each reconstructed calorimeter jet is corrected from the raw energy to the energy of the corresponding particle jet:
• EO: contribution for pile-up, noise and MPI
• Rjet: calorimeter energy response to particle jet
• Sjet: energy leaking into/out of the jet cone
USTC Hefei China, Z. Ye20
Jet Energy Scale (II)
2012/3/16
• Typical jet energy scale uncertainty is about 2-3%. More on this later.
USTC Hefei China, Z. Ye21
Outline
2012/3/16
Introduction
Precision measurement of the top-quark mass lepton+jets channel dilepton channel
Top-antitop quark mass difference
Summary and outlook
USTC Hefei China, Z. Ye22
Lepton+Jets Channel
2012/3/16
• One e/μ, four jets (two of them are b-quark jets), missing transverse momentum pT due to theν;
• Reasonable branching ratio: ~30%• Modest background level: W+jets, multijet
Golden channel for the top-quark mass measurement!ttbar W+jets multijet
USTC Hefei China, Z. Ye23
Event Selection (I)
2012/3/16
Event pre-selection cuts (minimizing bias on the measured top-quark mass): single lepton or lepton+jets trigger; exactly one isolated lepton with pT>20 GeV;
exactly four jets with pT>20GeV, leading one> 40GeV, and |η|<2.5;
missing transverse momentum pT>20 (25) GeV for e (μ)+jets.ttbar (MC) W+jets (MC) multijet
(data)
USTC Hefei China, Z. Ye24
Event Selection (II)
2012/3/16
Event pre-selection cuts (minimizing bias on the measured top-quark mass): single lepton or lepton+jets trigger; exactly one isolated lepton with pT>20 GeV;
exactly four jets with pT>20GeV, leading one> 40GeV, and |η|<2.5;
missing transverse momentum pT>20 (25) GeV for e (μ)+jets.
USTC Hefei China, Z. Ye25
Event Selection (III)
2012/3/16
Additional event selection cut: By requiring at least one jet tagged as
a b-quark jet, the signal purity can be increased from ~35% to ~70%, while losing only 1/4 of signal events.
DØ
USTC Hefei China, Z. Ye26
How to Measure mt - Template Method
2012/3/16
Build MC templates for a quantity that is sensitive to mt ;Compare data to MC templates to extract mt from data.
DØ MC
DØ MC
DØ MC
DØ MC
DØ MC
DØ MC
DØ MC
DØ MC
DØ MC
USTC Hefei China, Z. Ye27
Matrix Element Method (I)
2012/3/16
Matrix element method is based on the calculation of event probability densities estimated from differential cross section and detector resolutions.
Transfer functions encode detector resolutions and provide mappings from the parton kinematic y to the measured one x. Diracδfunction for lepton and jet angular resolution; Gaussian functions for lepton energy resolution; Double Gaussian functions for jet energy resolution
(see next slides).
Parton densities
LO matrix element
Transfer functions
USTC Hefei China, Z. Ye28
Jet Energy Transfer Function (I)
2012/3/16
Jet TFs determined with MC and parametrized as
USTC Hefei China, Z. Ye29
Jet Energy Transfer Function (II)
2012/3/16
Verify that TFs describe well the detector resolutions.
USTC Hefei China, Z. Ye30
Matrix Element Method (II)
2012/3/16
Similarly we write down the probability for background. And the probability to observe an event can be written as:
where f is the fraction of the signal events.
ME methods use the full event kinematic information and allow signal-like events contribute more to the result, thus usually yield better results in terms of precision than template methods.
USTC Hefei China, Z. Ye31
Matrix Element Method (III)
2012/3/16
minm
Event nEvent n-1Event 3Event 2Event 1
From these we buildthe likelihood function
The best estimate of the top mass is then determined
by minimizing: );...(ln 1 topn mxxL
);();...(1
1 top
n
iitopn mxPmxx
L
0.5
And the statistical error can beestimated from:
5.0)(ln)(ln minmin mm LL
USTC Hefei China, Z. Ye32
Jet Energy Scale (II)
2012/3/16
• Typical jet energy scale uncertainty is about 2-3%, which can lead to an uncertainty in top-quark mass of as large as 2 GeV.
USTC Hefei China, Z. Ye33
In-situ JES Calibration
2012/3/16
It is possible to calibrate the jet energy scale in-situ by using the two jets from the hadronic decayed W and the well-known
W mass, and obtain a better knowledge on jet energy scale and thus on top quark mass.
kJES is a global multiplicative factor for jet energy scale. Uncertainty on kJES can be much smaller than 2%, leading to an much reduced uncertainty in the measured top-quark mass.
USTC Hefei China, Z. Ye34
Calibration of the Method (II)
2012/3/16
We perform MC pseudo-experiments to estimate biases on the measured masses and uncertainty.
USTC Hefei China, Z. Ye35
Correction for MC-Data Difference (I)
2012/3/16
Default MC
DØ data
USTC Hefei China, Z. Ye36
Correction for MC-Data Difference (II)
2012/3/16
Need MC to well represent the data: use the particle jet matched to a reco-level jet in MC to estimate a MC-to-data correction factor
where i sums over all the particles in the particle jet, R is the single particle response in data and MC (depend on particle type, energy and η).
Then we apply the above correction factor to the reco jet to correct for the MC-data difference
Corrected MC (points) compared to data (dotted line)
USTC Hefei China, Z. Ye37
Correction for MC-Data Difference (II)
2012/3/16 pT (GeV)
correction for u, d, s, c quark jets
correction for gluon jets
correction of b quark jets
correction factorstatisticalsystematic
USTC Hefei China, Z. Ye38
Calibration of the Method (I)
2012/3/16
• In order to estimate and correct for biases on the measured top-quark mass and uncertainty, we perform MC pseudo-experiments. Each pseudo-experiment consists of MC events randomly drawn from signal and background MC samples according to the signal fraction measured in data.
USTC Hefei China, Z. Ye39
Lepton+Jets Result (I)
2012/3/16
Preliminary
USTC Hefei China, Z. Ye40
Lepton+Jets Result (II)
2012/3/16
PRD 84, 032004 (2011)
with the updated 1.0fb-1 RunIIa result using the BLUE method
we obtain a result corresponding to 3.6 fb-1 of data:
Combine the new 2.6fb-1 RunIIb result
relative uncertainty 0.9%
USTC Hefei China, Z. Ye41
Systematic Uncertainties (I)
2012/3/16
Physics modeling: 0.80 GeV
Detector modeling: 0.57 GeV
Method: 0.26 GeV
Total systematic: 1.02 GeV
USTC Hefei China, Z. Ye42
Outline
2012/3/16
Introduction
Precision measurements of the top-quark mass lepton+jets channel dilepton channel
Top-antitop quark mass difference
Summary and outlook
USTC Hefei China, Z. Ye43
Dilepton Channel
2012/3/16
• Two oppositely charged leptons, two b-quark jets, missing transverse momentum pT due to the two v’s
• Small branching ratio: ~5%• Small background: Z/γ* + jets• No hadronically decaying W can’t do in-situ
JESttbar (MC) Z+jets (MC)
Z. Ye44
Dilepton Channel
2012/2/27
Small branching fraction: ~5%, small background contribution: Z+jets.
Signal event characteristics: two oppositely charged lepton with large transverse
momentum; large missing transverse momentum; two b-quark jets.
No hadronic decayed W boson. Can not perform in-situ JES calibration. Use the knowledge from the lepton+jets channel kJES=1.013±0.008.
ttbar Z+jets
Z. Ye45
Dilepton Channel Result
2012/2/27
World’s most precise result in this channel!
Submitted to Phys.Rev.Lett.
relative uncertainty 1.6%
Z. Ye46
Summary of Top-Quark Mass Results
2012/2/27
• Results are consistent across all channels. No sign of new physics.
• The precision reaches 0.6%, dominated by the systematic uncertainty.
• Achieved with great efforts from both experimentalists and theorists.
USTC Hefei China, Z. Ye47
Outline
2012/3/16
Introduction
Precision measurements of the top-quark mass lepton+jets channel dilepton channel
Top-antitop quark mass difference
Summary and outlook
USTC Hefei China, Z. Ye48
Top-Antitop Quark Mass Difference
2012/3/16
Because of the very short life time, the top (and antitop) quark decays before hadronizing.
This allows direct measurements of top and antitop masses and to examine the CPT invariance theorem.
The first result from DØ (1 fb-1) in 2009:
The first result from CDF (5.6 fb-1) in 2010:
PRL 103, 132001 (2009)
PRL 106, 152001 (2011)
2σeffect ?!
USTC Hefei China, Z. Ye49
Data Analysis
2012/3/16
• Same data and event selection as the top-quark mass measurement in the lepton+jets channel.
• Using a matrix element method:
independently measured the masses of the top and antitop quarks, also extracted the average top-antitop mass as a cross-check.• We used the lepton charge to tell whether the leptonic
decayed W was from top or antitop, and measured the top and antitop quark masses in both the leptonic as well as (mainly) hadronic channels.
• Took into account all possible differences in the detector response between particles and antiparticles (lepton charge ID, jet energy scale difference between b and bbar or c and cbar quark jet, …).
USTC Hefei China, Z. Ye50
Calibration of the Method
2012/3/16
We perform MC pseudo-experiments to estimate biases on the measured mass difference and uncertainty.
USTC Hefei China, Z. Ye51
Top-Antitop Quark Mass Difference
2012/3/16
PRD 84, 052005 (2011)
USTC Hefei China, Z. Ye52
Summary and Outlook
2012/3/16
The precision of the top-quark mass measurement has reached 0.6%, and is dominated by systematic uncertainty.
With 1-2 times more data on tape, we expect that statistical and JES-related uncertainties will be significantly reduced.
We are also working hard to improve our understandings of systematic effects in order to further improve the precision of the measurement: detectror modeling ISR/FSR parton showering color reconnection …
USTC Hefei China, Z. Ye53
Systematic Uncertainties (II)
2012/3/16
Physics modeling: 0.80 GeV
Detector modeling: 0.57 GeV
Method: 0.26 GeV
Total systematic: 1.02 GeV
was about1 GeV
USTC Hefei China, Z. Ye54
Systematic Uncertainties (II)
2012/3/16
Physics modeling: 0.80 GeV
Detector modeling: 0.57 GeV
Method: 0.26 GeV
Total systematic: 1.02 GeV
CDF
55
The precision of the top-quark mass measurement has reached 0.6%. With ~2 times more data from the Tevatron to be analyzed, we expect that the uncertainty will be further reduced.
Summary and Outlook
2012/2/27Z. Ye
Z. Ye56
Top Quark Physics
2012/2/27
t
t
b
b
W+
W-
q
vq’
l-
q
vq’
l+
branching ratioCKM |Vtb|
rare decay (FCNC)
anamolus couplingsMass,lifetime,
chargemass difference
search for t’
production cross sectionforward-backward
asymmetryspin correlationttγcross sectionsearch for tt Hsearch for Z’
W-helicitysearch for W’
pp
Z. Ye57
Summary and Outlook
2012/2/27
The large top quark mass suggests a special role in Electroweak symmetry breaking. The top quark sector could be the place to find new physics at the Large Hadron Collider.
Z. Ye58 2012/2/27
Large Hadron Collider
Z. Ye59
Top Electrical Charge
2012/2/27
Use jet charge algorithm to determine b-quark jet charge
Use kinematic fitter to reconstruct event, group lepton and b-quark jets, and determine the top electrical charge.
Exclude exotic model > 5σ C.L.
Work in Progress
USTC Hefei China, Z. Ye60
Top Quark Mass From Cross Section
2012/3/16
Top quark carries color, thus its pole mass can only be defined with ambiguity ~ΛQCD.
Its definition in field theory depend on renormalization scheme, often used is scheme.
Direct measurements use LO or NLO matrix element MC generators with parton showering to extract the result. It is believed that the mass from such direct measurements is close to the top quark pole mass but there is no precise answer yet. Theorists are working on this issue.
Presented in W&C seminar “Three Tails of Two Tops” on 2011/2/11 by Shabnam Jabeen
USTC Hefei China, Z. Ye61
Top Quark Mass From Cross Section
2012/3/16
• MC is used to estimate the acceptance for the cross section measurement.Therefore the measured cross section only weakly depends on the value of the top quark mass in MC.
• A constraint on the top quark pole mass is obtained by combining the experimental and theoretical inputs.
• The result is insensitive to the interpretation of the top quark mass in MC.
PLB 703, 422 (2011)
USTC Hefei China, Z. Ye62
Top Quark Discovery
2012/3/16
Announcement of top quark discovery in public seminar at Fermilab on March 2nd, 1995