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The Discovery of Top

By Billy Wang

Mar. 2, 2023

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Main Points

Introduction to top quark
The Tevatron Accelerator
The CDF detector
Decay Channels and Their Respective Background
Results

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Top quark

Discovered 1995
Mass of to quark: 172.76 GeV
Discovered by both CDF and D0 together

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The Tevatron

Ring of 6.28 km circumference
120 GeV Proton collide on nickel target to produce antiproton
Stored in the Accumulator
Collide in 1.8 TeV at C.M

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http://www.scholarpedia.org/article/The_top_quark_discovery

The tevatron is a 6.18 km ring that accelerates protons and antiprotons to 900 GeV and collides them heads on toward each other at center of mass energy of 1.8 TeV. The Protons were first accelerated to 120 GeV and than collided on a nickel target to produce antiprotons and is focused using a lithium lens and collected in the debuncher ring. In 1 million proton colliding on this target, only around 20 antiprotons will be created, so they sent $3*10^{12}$ protons per pulse on to the target. However, the antiproton produced in the collision has a wide energy spread that requires the technique of stochastic cooling pioneered by Simon Vanderveer to reduce its momentum and position spread. The antiprotons are focused using a lithium lens and directed to the accumulator until around $10^{12}$ are stored. They are then directed to the main ring and accelerated to 900 GeV to collide with the proton.

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The CDF Detector

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Immediate to the Beampipe: �Silicon Vertex Detectors�(SVX)

4 layers of silicon strips
Accurately depict the secondary vertex of decay

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https://www.sciencedirect.com/science/article/pii/0168900294914346

Immediately outside the beam pipe is the silicon vertex detector (SVX), which aims at identifying the secondary vertices from $b$ (the main way top quarks decay) and $c$. It consists of two barrel modules aligned end-to-end with each barrel containing 4 concentric cylindrical layers. The layers are only 2.7 to 7.9 cm away from the beam pipe. While the interaction point along the Tevatron beam line are distributed with $\sigma = 35 cm$, the SVX detector are 51 cm in total length. There are twelve silicon strips on each cylinder. The materials used to support them must be as less as possible because they are so close to the beampipe the multiple scatterings or particle decays in these materials must be kept at a minimum. Meanwhile they must also be mechanically hard and resistive to the high radiation from the beam. Eventually, the silicon strips are glued to a low weight Rohacell foam with carbon fiber support. There are also cooling lines through the front and back-end electronics to maintain the electronic modules at 20 degree Celsius.

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Surrounding the SVD: �Time Projection Chambers

Drift Chambers
Vertex Time Projection Chambers (just outside of SVD)
Forward tracking chamber + Central tracking chamber (middle)
Central drift tubes (Outmost)

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https://doi.org/10.1016/0168-9002(88)90298-7.

(https://www.sciencedirect.com/science/article/pii/0168900288902987)

Another component of the tracking system is the time projection chambers which surrounds the SVD. They are wired drift chambers inside a large magnetic field (1.5T). The charged particle deposits a very small amount of energy as it passes through the wires and the signal will drift toward electronics at the end of the wire. By collecting these signals, we can reconstruct the tracks of these charged particles. There are four components of the CDF TPCs, the innermost are the 8 vertex time projection chamber modules. These module consists of a total of 3072 wires and 3072 pads. The pads are used to … It is surrounded by the central tracking system, which consists of 5 axial superlayers with 12 wires each and 4 stereo superlayers with 6 wires each. They provides excellent spatial and momentum resolution for the tracking. 3 layers of drift tubes, called the central drift tubes covers, are outside the central tracking system. Central drift tubes has high resolution charge divisions to provide a correlated R-phi-Z measurement. These central chambers provide Pseudorapidity coverage of (-3.5<\eta<3.5) which is equivalent to a polar angle coverage of (3.5<\theta<176.5). There is also a forward and backward tracking chamber that is the exact same since the experiment is exactly symmetric forward-backward as shown in the figure below. The forward tracking chamber provides an additional angle coverage of 2.4<\eta<4 (2<\theta<10).

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The CDF Detector

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A solenoid coil surrounds the wire chambers.

-- between calorimeter and tracker

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Central Electromagnetic Calorimeter (CEM)

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A EM coil surrounds the drift tubes to create a strong magnetic field for tracking. Outside of the coil, a hybrid electromagnetic calorimeter depicts EM showers with good energy and position resolution. It is made up of scintillator with strip chambers. Each tower has 18 radiation lengths of depth with a segmentation of Δ𝜙=15°. The towers are made of lead interleaved with scintillators. 10 layers of lead is sandwiched by 11 layers of scintillators. A proportional strip chamber is inserted between the 8^th layer of lead and the 9^th layer of scintillator, which is where we expect the transverse shower development to be at maximum. This strip chamber identifies the position and transverse development of the shower through orthogonal strips and wires. On the two sides of each tower, we have wavelength shifter sheets that redirects the lights coming out of each scintillator toward the phototubes.

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Central and Endwall hadron calorimeter

48 layers
Covers polar angle between 30-150
2.5 cm (5 cm for endwall) steel and 1 cm scintillator every layer
Out of the Hcal there is a muon wired drift chamber

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Analysis: Production Mechanism

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The decay channel:

Decay Channel:
Each W bosons decays leptonically(30%) or hadronically (70%)
Focus on di-lepton channel (low branching fraction)�and lepton+jets analysis (branching fraction 24/81)
(SVX tag) The SVX help find the secondary vertex 🡪 b jet
(SVT tag) Additional leptons from b semileptonic decay

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https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.69.137

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Backgrounds

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https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.69.137

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Results

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Combined 4.8 standard deviations away from the expected background.

Top mass: 176 GeV ±8 ±10�estimated production cross section: 6.8pb

https://arxiv.org/abs/hep-ex/9503002

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Results

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Summary

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Back up Slides: the preselections

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Backup slides: the ttbar production rate

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