The Discovery of the The Discovery of the Top QuarkTop Quark

By Ben SmithBy Ben Smith

IntroductionIntroduction

By 1977, the discovery of the bottom By 1977, the discovery of the bottom quark suggested the presence of its quark suggested the presence of its isospin partner, the Top quark.isospin partner, the Top quark.

This was needed to complete the 3This was needed to complete the 3rdrd generation of quarks.generation of quarks.

This would validate the standard model This would validate the standard model which requires the top quark.which requires the top quark.– Kobayashi & Maskawa found for CP violation to Kobayashi & Maskawa found for CP violation to

occur in electroweak-theory, a minimum of 3 occur in electroweak-theory, a minimum of 3 quark pairs were needed.quark pairs were needed.

Progress: 1977-1990Progress: 1977-1990 After the discovery of b, the mass of possible top After the discovery of b, the mass of possible top

quark estimated at 15GeV (predicted by comparing quark estimated at 15GeV (predicted by comparing intervals of other quark masses ).intervals of other quark masses ).

Successive experiments throughout 1980’s found Successive experiments throughout 1980’s found nothing at this mass, edging up the possible mass nothing at this mass, edging up the possible mass with higher and higher energy experiments.with higher and higher energy experiments.

By 1990, the CDF group using the Tevatron By 1990, the CDF group using the Tevatron accelerator at Fermilab, and UA2 group using LEP at accelerator at Fermilab, and UA2 group using LEP at CERN, set lower mass limit to top quark MCERN, set lower mass limit to top quark Mtoptop< < 91GeV (must be higher than W and Z mass as not 91GeV (must be higher than W and Z mass as not seen in their decays).seen in their decays).

But using the mass of ZBut using the mass of Z00, the standard model , the standard model constrained upper limit of top quark mass: constrained upper limit of top quark mass: MMtoptop<225GeV.<225GeV.

CDF ExperimentCDF Experiment The CDF (Collider Detection at Fermilab The CDF (Collider Detection at Fermilab

collaboration) experiment used the collaboration) experiment used the Tevatron proton-antiproton accelerator for Tevatron proton-antiproton accelerator for energies up to 900GeV for both P and energies up to 900GeV for both P and PP..

This was three times that of LEP at CERN.This was three times that of LEP at CERN. Began in 1988 after failure of lower energy Began in 1988 after failure of lower energy

experiments to find the top quark.experiments to find the top quark. Very small x-section for t production. Very small x-section for t production.

– At 175GeV, At 175GeV, sstttt=8pb; =8pb; sstotaltotal=60mb. =60mb. – At Tevatron experiment only 1 in 10At Tevatron experiment only 1 in 101010

collisions produce tcollisions produce ttt!!

CDF ExperimentCDF Experiment Construction points:Construction points:

– 4-layer 4-layer SSilicon ilicon VVertex ertex DDetector (SVD) immediately etector (SVD) immediately surrounding beam-pipe. This reconstructs event tracks surrounding beam-pipe. This reconstructs event tracks in the transverse plane.in the transverse plane.

– Surrounding this is the Surrounding this is the CCentral entral TTracking racking CChamber (CTC) hamber (CTC) contained by a 3m super-conducting solenoid magnet contained by a 3m super-conducting solenoid magnet creating a 1.4T field in the CTC. This chamber was used creating a 1.4T field in the CTC. This chamber was used to establish transverse momentum and charge of to establish transverse momentum and charge of particles.particles.

– Surrounding CTC were banks of calorimeters. These Surrounding CTC were banks of calorimeters. These measured energy of particles, thus enabling any missing measured energy of particles, thus enabling any missing energy due to neutrinos to be found.energy due to neutrinos to be found.

CDF experimentCDF experiment

Search for the top quarkSearch for the top quark Largest production cross-Largest production cross-

section of top quarks from:section of top quarks from:PPPPЃЃttttЃЃWW+ + b Wb W- - bb

– Very high energy; Very high energy; lifetime of top<10lifetime of top<10-24-24s. s. Does not form mesons.Does not form mesons.

– Standard model Standard model predicts tpredicts tЃЃWW++b.b.

The W’s then decay via The W’s then decay via different channels. This different channels. This defines different decay defines different decay modes of t.modes of t.

WWЃЃud, cs, lud, cs, l .u.u

Search for the top quarkSearch for the top quark Three main decay modes:Three main decay modes:

– tttt ЃЃ b bbb q qqq q qqq: Branching ratio : Branching ratio 36/81, 6 jets but highest 36/81, 6 jets but highest background background (Background/signal =10(Background/signal =1033).).

– tttt ЃЃ b bbb q qqq l luu: ‘Single-lepton : ‘Single-lepton channel’channel’

Branching ratio 24/81.Branching ratio 24/81. 4 jets and 1 lepton. 4 jets and 1 lepton. Background easily removed Background easily removed

by lepton-tagging.by lepton-tagging.− Also in lepton mode: Semi-Also in lepton mode: Semi-

leptonic decay;leptonic decay;bbЃЃll++uuXX. . In this case decay mode In this case decay mode

is tis ttt ЃЃbqqlbqqluu l luuX.X.

Search for the top quarkSearch for the top quark

– tttt Ѓ Ѓ bbbb l l--uu l l++ : u: u ‘dilepton ‘dilepton channel’.channel’.

Branching ratio=4/81.Branching ratio=4/81. 2 jets and 2 leptons.2 jets and 2 leptons. Easiest to separate Easiest to separate

from background.from background. CDF used Lepton and CDF used Lepton and

dilepton decay modes to dilepton decay modes to reconstruct treconstruct ttt mass and mass and identify top quark.identify top quark.

Search for the top quarkSearch for the top quark Processes used to identify Processes used to identify

single-lepton and dilepton single-lepton and dilepton decay channels:decay channels:– b-tagging; SVD allowed b-tagging; SVD allowed

reconstruction of reconstruction of transverse plane tracks; b transverse plane tracks; b decay seen in displaced decay seen in displaced vertex, thus can tag b-jets vertex, thus can tag b-jets ЃЃ SVX tagging. SVX tagging. Resolution=15Resolution=15mmm. m. Lifetime of W<10Lifetime of W<10-24-24. .

– Lepton tagging; energy Lepton tagging; energy and momentum of leptons and momentum of leptons found in calorimeter and found in calorimeter and CTC respectively, as well CTC respectively, as well as jet energy and mass.as jet energy and mass.

– Also semi-leptonic Also semi-leptonic tagging: Allowed tagging tagging: Allowed tagging of semi-leptonic decay of semi-leptonic decay modemode

The lepton channelThe lepton channel Identified lepton channel of Identified lepton channel of

top-quark decay by the top-quark decay by the following characteristics:following characteristics:– 1 lepton with high 1 lepton with high

transverse momentum, transverse momentum, (E(Eee,p,pmm>20GeV),>20GeV),

– Missing transverse Missing transverse energy due to neutrino,energy due to neutrino,

– 2 b-jets and 2 light 2 b-jets and 2 light quark jets, or 3 b-jets quark jets, or 3 b-jets and 2 leptons (SLT).and 2 leptons (SLT).

Eliminate from main Eliminate from main background:background:

– Lower lepton transverse Lower lepton transverse momentummomentum

– Different KinematicsDifferent Kinematics

The Dilepton channelThe Dilepton channel Look for:Look for:

– 2 high transverse-2 high transverse-momentum leptons momentum leptons with opposite charge,with opposite charge,

– Larger missing Larger missing energy due to 2 energy due to 2 neutrinos, E>25GeV neutrinos, E>25GeV (Conservation of (Conservation of momentum),momentum),

– 2 b-jets with 2 b-jets with E>10GeV. E>10GeV.

– Reconstruct initial Reconstruct initial mass of particles via mass of particles via decay modes to find decay modes to find the ‘missing energy’ the ‘missing energy’ and top-quark mass.and top-quark mass.

Main background from:Main background from:

Drell-Yan pair-production;Drell-Yan pair-production;– No jets,No jets,– No missing energy (no No missing energy (no

neutrinos).neutrinos).– Still, some background Still, some background

overlooked.overlooked.

Background eventsBackground events Although can distinguish many background Although can distinguish many background

channels, some slip through the net.channels, some slip through the net. Overcome using Monte Carlos simulations.Overcome using Monte Carlos simulations.

– Estimate background missed by tagging methods Estimate background missed by tagging methods from production and decay of non-top associated from production and decay of non-top associated events (ie Pevents (ie PPP ЃЃ b bbb ЃЃ l l++uull--uuXX).XX).

– Anything monitored above this background after Anything monitored above this background after false tagging is signalfalse tagging is signal..

The background estimate allowed a The background estimate allowed a statistical analysis of top quark existence.statistical analysis of top quark existence.– signal-to-background allowed the null hypothesis signal-to-background allowed the null hypothesis

(no top) to be proved or disproved; I.e. a (no top) to be proved or disproved; I.e. a ss value value between signal and background could be found.between signal and background could be found.

Top Quark existenceTop Quark existence Between 1992-1995, CDF observed:Between 1992-1995, CDF observed:

– 27 single lepton events. Expected from background 27 single lepton events. Expected from background (non-top associated mechanisms); 6.7±2.1. The (non-top associated mechanisms); 6.7±2.1. The probability the background accounts for all these events, probability the background accounts for all these events, PP11=2x10=2x10-5-5..

– 6 dilepton events. Expected background= 1.3±0.3. 6 dilepton events. Expected background= 1.3±0.3. Probability background accounts for all events, Probability background accounts for all events, PP22=3x10=3x10-3-3..

– 23 SLT events. Expected background; 15.4±2.0. 23 SLT events. Expected background; 15.4±2.0. Probability background accounts for all events, Probability background accounts for all events, PP33=6x10=6x10-2-2..

Total Probability of all events accounted for by Total Probability of all events accounted for by background=Pbackground=P11xPxP22xPxP33=1x10=1x10-6-6 (This is probability t (This is probability t doesn’t exist).doesn’t exist).– This is 4.8This is 4.8 s s from case with existence of top quark.from case with existence of top quark.

Top Quark exists!Top Quark exists!

Top Quark MassTop Quark Mass Using 19 single lepton Using 19 single lepton

events could events could kinematically kinematically reconstruct mass from reconstruct mass from decay remnants:decay remnants:– Energy and momentum Energy and momentum

from CTC and from CTC and calorimeters.calorimeters.

– Estimate mass by Estimate mass by adding neutrino energy, adding neutrino energy, lepton energy and 4 lepton energy and 4 highest energy Jets.highest energy Jets.

– Compile reconstructed Compile reconstructed mass distribution:mass distribution:

MMtoptop=176=176±8 GeV/c±8 GeV/c22..

D0 experimentD0 experiment Also running alongside CDF Also running alongside CDF

at Fermilab using Tevatron at Fermilab using Tevatron accelerator for Paccelerator for PPP collisions. collisions.– Used central tracking Used central tracking

chamber surrounded by chamber surrounded by 5m diameter calorimeter5m diameter calorimeter

– Calorimeter filled with Calorimeter filled with uranium-liquid-Argon.uranium-liquid-Argon.

– This surrounded by This surrounded by magnetised Iron and Muon magnetised Iron and Muon detectors.detectors.

Better energy resolution than Better energy resolution than CDF.CDF.

Looked at dilepton and lepton Looked at dilepton and lepton decay channels, eliminating decay channels, eliminating background as CDF.background as CDF.

Reconstructed mass Reconstructed mass distribution gave distribution gave MMtoptop=199±30GeV.=199±30GeV.

ConclusionConclusion Top quark took almost 2 decades to observe after Top quark took almost 2 decades to observe after

it’s existence became apparent with discovery of it’s existence became apparent with discovery of bottom.bottom.– Required advances in accelerator and detection Required advances in accelerator and detection

technology.technology.– Systematic increase in lower limit set by failure of Systematic increase in lower limit set by failure of

previous experiments.previous experiments. In the early 1990’s, CDF and D0 ran side-by-side In the early 1990’s, CDF and D0 ran side-by-side

at Fermilab looking for the top quark:at Fermilab looking for the top quark:– Observed lepton and dilepton decay channels of possible Observed lepton and dilepton decay channels of possible

top quark to it’s identify presence.top quark to it’s identify presence.– Found MFound Mtoptop=176=176±8 GeV/c±8 GeV/c2 2 and and 199±30GeV 199±30GeV

respectively.respectively. Prediction of standard model verified.Prediction of standard model verified.

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