Transcript PowerPoint Presentation - The Discovery of the Top Quark

The Discovery of the
Top Quark
By Ben Smith
Introduction
By 1977, the discovery of the bottom
quark suggested the presence of its
isospin partner, the Top quark.
 This was needed to complete the 3rd
generation of quarks.
 This would validate the standard model
which requires the top quark.

– Kobayashi & Maskawa found for CP violation to
occur in electroweak-theory, a minimum of 3
quark pairs were needed.
Progress: 1977-1990

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
After the discovery of b, the mass of possible top
quark estimated at 15GeV (predicted by comparing
intervals of other quark masses ).
Successive experiments throughout 1980’s found
nothing at this mass, edging up the possible mass
with higher and higher energy experiments.
By 1990, the CDF group using the Tevatron
accelerator at Fermilab, and UA2 group using LEP at
CERN, set lower mass limit to top quark Mtop<
91GeV (must be higher than W and Z mass as not
seen in their decays).
But using the mass of Z0, the standard model
constrained upper limit of top quark mass:
Mtop<225GeV.
CDF Experiment
The CDF (Collider Detection at Fermilab
collaboration) experiment used the
Tevatron proton-antiproton accelerator for
energies up to 900GeV for both P and P.
 This was three times that of LEP at CERN.
 Began in 1988 after failure of lower
energy experiments to find the top quark.
 Very small x-section for t production.

– At 175GeV, stt=8pb; stotal=60mb.
– At Tevatron experiment only 1 in 1010
collisions produce tt!
CDF Experiment

Construction points:
– 4-layer Silicon Vertex Detector (SVD) immediately
surrounding beam-pipe. This reconstructs event tracks
in the transverse plane.
– Surrounding this is the Central Tracking Chamber (CTC)
contained by a 3m super-conducting solenoid magnet
creating a 1.4T field in the CTC. This chamber was used
to establish transverse momentum and charge of
particles.
– Surrounding CTC were banks of calorimeters. These
measured energy of particles, thus enabling any missing
energy due to neutrinos to be found.
CDF experiment
Search for the top quark

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
Largest production crosssection of top quarks from:
PPttW+ b W- b
– Very high energy;
lifetime of top<10-24s.
Does not form mesons.
– Standard model
predicts tW+b.
The W’s then decay via
different channels. This
defines different decay
modes of t.
Wud, cs, lu.
Search for the top quark

Three main decay modes:
– tt  bb qq qq: Branching ratio
36/81, 6 jets but highest
background
(Background/signal =103).
– tt  bb qq lu: ‘Single-lepton
channel’
Branching ratio 24/81.
4 jets and 1 lepton.
Background easily removed
by lepton-tagging.
− Also in lepton mode: Semileptonic decay;
bl+uX. In this case decay mode
is tt bqqlu luX.
Search for the top quark

– tt  bb l-u l+u: ‘dilepton
channel’.
Branching ratio=4/81.
2 jets and 2 leptons.
Easiest to separate
from background.
CDF used Lepton and
dilepton decay modes to
reconstruct tt mass and
identify top quark.
Search for the top quark

Processes used to identify
single-lepton and dilepton
decay channels:
– b-tagging; SVD allowed
reconstruction of
transverse plane tracks; b
decay seen in displaced
vertex, thus can tag b-jets
 SVX tagging.
Resolution=15mm. Lifetime
of W<10-24.
– Lepton tagging; energy
and momentum of leptons
found in calorimeter and
CTC respectively, as well
as jet energy and mass.
– Also semi-leptonic
tagging: Allowed tagging
of semi-leptonic decay
mode
The lepton channel

Identified lepton channel of
top-quark decay by the
following characteristics:
– 1 lepton with high
transverse momentum,
(Ee,pm>20GeV),
– Missing transverse
energy due to neutrino,
– 2 b-jets and 2 light
quark jets, or 3 b-jets
and 2 leptons (SLT).

Eliminate from main
background:
– Lower lepton transverse
momentum
– Different Kinematics
The Dilepton channel

Look for:
– 2 high transversemomentum leptons
with opposite charge,
– Larger missing
energy due to 2
neutrinos, E>25GeV
(Conservation of
momentum),
– 2 b-jets with
E>10GeV.
– Reconstruct initial
mass of particles via
decay modes to find
the ‘missing energy’
and top-quark mass.
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Main background from:
Drell-Yan pair-production;
– No jets,
– No missing energy (no
neutrinos).
– Still, some background
overlooked.
Background events
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Although can distinguish many background
channels, some slip through the net.
Overcome using Monte Carlos simulations.
– Estimate background missed by tagging methods
from production and decay of non-top associated
events (ie PP  bb  l+ul-uXX).
– Anything monitored above this background after
false tagging is signal.

The background estimate allowed a
statistical analysis of top quark existence.
– signal-to-background allowed the null hypothesis
(no top) to be proved or disproved; I.e. a s value
between signal and background could be found.
Top Quark existence

Between 1992-1995, CDF observed:
– 27 single lepton events. Expected from background
(non-top associated mechanisms); 6.7±2.1. The
probability the background accounts for all these events,
P1=2x10-5.
– 6 dilepton events. Expected background= 1.3±0.3.
Probability background accounts for all events,
P2=3x10-3.
– 23 SLT events. Expected background; 15.4±2.0.
Probability background accounts for all events,
P3=6x10-2.

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Total Probability of all events accounted for by
background=P1xP2xP3=1x10-6 (This is probability t
doesn’t exist).
– This is 4.8s from case with existence of top quark.
Top Quark exists!
Top Quark Mass

Using 19 single lepton
events could
kinematically
reconstruct mass from
decay remnants:
– Energy and momentum
from CTC and
calorimeters.
– Estimate mass by
adding neutrino energy,
lepton energy and 4
highest energy Jets.
– Compile reconstructed
mass distribution:

Mtop=176±8 GeV/c2.
D0 experiment
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Also running alongside CDF
at Fermilab using Tevatron
accelerator for PP collisions.
– Used central tracking
chamber surrounded by
5m diameter calorimeter
– Calorimeter filled with
uranium-liquid-Argon.
– This surrounded by
magnetised Iron and Muon
detectors.
Better energy resolution than
CDF.
Looked at dilepton and lepton
decay channels, eliminating
background as CDF.
Reconstructed mass
distribution gave
Mtop=199±30GeV.
Conclusion

Top quark took almost 2 decades to observe after
it’s existence became apparent with discovery of
bottom.
– Required advances in accelerator and detection
technology.
– Systematic increase in lower limit set by failure of
previous experiments.

In the early 1990’s, CDF and D0 ran side-by-side
at Fermilab looking for the top quark:
– Observed lepton and dilepton decay channels of possible
top quark to it’s identify presence.
– Found Mtop=176±8 GeV/c2 and 199±30GeV
respectively.

Prediction of standard model verified.
Bibliography
G. Fraser, The particle century, Institute of
physics publishing,1998.
 The discovery of the Top Quark, L.Han,
www.hep.man.ac.uk.
 The discovery of the Top Quark, M. Liss &
P. Tipton, www.pas.rochester.edu.
 F. Abe et al, Observation of Top Quark
Production in pp Collisions with the
Collider Detector at Fermilab, Phys. Rev.
Lett, 74 (1995), 2626.
 S. Abachi et al, Observation of the Top
Quark, Phys. Rev. Lett, 74 (1995), 2632.
