Top mass measurements at the Tevatron and the standard model fits

1
Top mass measurements at the Tevatronand the
standard model fits

• Michael Wang, Fermilab
• For the DØ and CDF collaborations
• 42nd Recontres de Moriond
• March 17-24, 2007

2
Outline

• Significance of top mass

• 3 decay channels used in measuring mass

• Different measurement techniques

• New results from DØ and CDF

• Brand new world average and SM fits

• Conclusions

3
Why measure the top mass?

• Why is there so much interest in the top mass?

• To see why, consider the mass of the W boson

Radiative corrections

• mt enters quadratically while mh enters
  logarithmically, so a precise knowledge of the W
  and Top masses will constrain the Higgs mass,
  providing a guide to the Higgs search.

4
Decay channel (1) All jets
ljets 29

• All jets
• Largest branching fraction
• High background levels
• QCD multijet
• Benefits from in-situ jet energy calibration
  using hadronic W

5
Decay channel (2) Dilepton
dilepton 5
b
W
t
p
p
t
W-
b

• Dilepton
• Low bkg levels
• diboson
• Zjets
• Low branching fraction

6
Decay channel (3) Leptonjets
dilepton 5
ljets 29
TauX 22
b
W
t
p
p
t

• Lepton jets
• Reasonable branching fraction
• Medium bkg levels
• Wjets
• QCD multijet
• Benefits from in-situ jet energy calibration
  using hadronic W
• Has traditionally yielded the best results

W-
b
7
Measurement challenge
Primary interaction vertex
p
p

• In general, dont know which jet comes from which
  parton
• In the ljets case e.g., detector sees 4 jets, a
  lepton, missing ET, and an interaction vertex
• No displaced vertices to isolate signal from
  background
• Must try all permutations
• No clean and sharp mass peaks

8
Template methods

• Identify variable x sensitive to Mtop.

• Using MC, generate distributions (templates) in x
  as a function of input Mtop.

• Parameterize templates in terms of probability
  density function (p.d.f) in x, Mtop.

• Construct likelihood L based on p.d.fs
• Compare data x distributions with the MC
  templates using L
• Maximize L (minimize -ln(L)) to extract top mass

Probability density function ? P(xMtop)
9
Matrix Element methods

• In the M.E. method, probabilities are calculated
  directly for each event. For instance, with
  signal and background contributions

• Probabilities are taken to be the differential
  cross sections for the process in question. For
  example, the signal probability is given by

where
10
Matrix Element methods
To extract from a sample of
events, probabilities are calculated for each
individual event as a function of
11
Ideogram method

• Like the M.E. methods, the Ideogram method
  constructs an analytic likelihood for each event.

• The portion of the signal probability that is
  sensitive to the top mass is of the form

• The main feature of this technique is the use of
  a constrained kinematic fit to extract the mass
  information xfit consisting of the fitted mass
  mi, estimated uncertainty s2, and goodness of fit
  ?2 (contained in wi).

• This method which was first applied to the W mass
  at LEP aims to achieve similar statistical
  uncertainties as the M.E. method but without the
  burden of huge computational requirements.

12
CDF leptonjets
systematics
highlights

• Matrix Element method
• In-situ jet energy calibration
• Current world best

Result
0.94 fb-1
170.9 2.2 (statJES) 1.4 (syst) GeV/c2
13
DØ leptonjets
Only ejets channel shown
highlights

• Matrix Element method
• In-situ jet energy calibration

Result
0.9 fb-1
170.5 2.4 (statJES) 1.2 (syst) GeV/c2
14
DØ leptonjets (ideogram)
highlights

• Ideogram method
• In-situ jet energy calibration

Result
0.4 fb-1
173.7 4.4(stat.JES)2.1 -2.0(syst.) GeV/c2
15
CDF dilepton
systematics
highlights

• Matrix Element method
• Best dilepton measurement

Result
1 fb-1
164.5 3.9 (stat.) 3.9 (syst.) GeV/c2
16
DØ dilepton
Expected error distribution
Mass calibration curve
highlights

• Template method
• ? weighting technique
• eµ, ee, and µµ channels

Results
1 fb-1
172.5 5.8 (stat.) 5.5 (syst.) GeV/c2
17
CDF all jets
highlights

• Matrix Element Template
• In-situ jet energy calibration

Result
1 fb-1
171.1 3.7 (statJES) 2.1 (syst) GeV/c2
18
New world average
Combining the new results from the previous
slides shown above with past results from CDF and
DØ yields a NEW world average for the top mass
170.9 1.1 (stat) 1.5 (syst) GeV/c2
mtop now known to an uncertainty of 1.1 !
19
New SM fit
NEW!
March 2007, LEP EW WG

• Preferred value mH 76 GeV at minimum
• Upper limit mH lt 144 GeV

20
Summary and conclusions

• Great interest in top mass due to the Higgs
• Challenging measurement but various techniques
  make a precise measurement possible
• New measurements from D0 and CDF yielding a new
  world average for the top mass with uncertainty
  of 1.1
• Using the new top mass in the SM fits yields new
  limits on the Higgs mass
• The top mass measurements still dominated by the
  Leptonjets results
• Best results in two other channels are very
  impressive, hope to see results competitive with
  ljets in the future
• Something to look forward to since different
  channels dominated by different systematics
• By the end of Tevatron run (8fb-1), the
  statistical uncertainty of the top mass lt 1 GeV
  and the total uncertainty dominated by the
  systematic uncertainty.

21
End
22

• I have just described three measurement
  techniques to extract the top mass.
• Although the three methods differ from each other
  substantially, they all share the common need for
  validation and calibration in order to determine
  that the extracted top mass corresponds to the
  true value and that the estimated errors are
  reliable
• Since the procedure takes up a significant
  portion of any top mass analysis, I will briefly
  describe how it is done in the next slide
• Although I will use the M.E. method as an
  example, the procedure should be very similar if
  not identical for the other two measurement
  techniques

23
Ensemble tests
From a large pool of M monte carlo events, we
perform ensemble tests by randomly drawing n
events N number of times to form N
pseudo-experiments
1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18
19 20 . . . . . . . . . . . . . . . . . . .
. . M
A. The error is estimated for each experiment and
entered into the Mass Error histogram
B. The mass at the minimum for each experiment is
entered into the Top Mass histogram
C. The pulls are calculated for each experiment
by dividing the deviation of the mass at the
minimum from the mean of this mass for all
experiments by the estimated error