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 3 rd 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:  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 M top < 91GeV (must be higher than W and Z mass as not seen in their decays).  But using the mass of Z 0, the standard model constrained upper limit of top quark mass: M top <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,  tt =8pb;  total =60mb. –At Tevatron experiment only 1 in 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  Largest production cross- section of top quarks from: PP Ѓ tt Ѓ W + b W - b –Very high energy; lifetime of top< s. Does not form mesons. –Standard model predicts t Ѓ W + b.  The W’s then decay via different channels. This defines different decay modes of t.  W Ѓ ud, cs, l

Search for the top quark  Three main decay modes: –tt Ѓ bb qq qq: Branching ratio 36/81, 6 jets but highest background (Background/signal =10 3 ). –tt Ѓ bb qq l: ‘Single-lepton channel’ Branching ratio 24/81. Branching ratio 24/81. 4 jets and 1 lepton. 4 jets and 1 lepton. Background easily removed by lepton-tagging. Background easily removed by lepton-tagging. − Also in lepton mode: Semi- leptonic decay; b Ѓ l +  X In this case decay mode is tt Ѓ bqql lX.

Search for the top quark –tt Ѓ bb l -  l + ‘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 Ѓ SVX tagging. Resolution=15m. Lifetime of W< –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, (E e,p  >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 transverse- momentum 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.  Main background from:  Drell-Yan pair-production; –No jets, –No missing energy (no neutrinos). –Still, some background overlooked.

Background events  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 Ѓ bb Ѓ l + l - XX). –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  value between signal and background could be found.

Top Quark existence  Between , 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, P 1 =2x –6 dilepton events. Expected background= 1.3±0.3. Probability background accounts for all events, P 2 =3x –23 SLT events. Expected background; 15.4±2.0. Probability background accounts for all events, P 3 =6x  Total Probability of all events accounted for by background=P 1 xP 2 xP 3 =1x10 -6 (This is probability t doesn’t exist). –This is 4.8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:  M top =176 ±8 GeV/c 2.

D0 experiment  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 M top =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 M top =176 ±8 GeV/c 2 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,  The discovery of the Top Quark, M. Liss & P. Tipton,  F. Abe et al, Observation of Top Quark Production in pp Collisions with the Collider Detector at Fermilab, Phys. Rev. Lett, 74 (1995),  S. Abachi et al, Observation of the Top Quark, Phys. Rev. Lett, 74 (1995), 2632.