A. Gray's Web Page: Proton - AntiProton Collisions

Introduction

Fermilab is based in Batavia, IL, and operates the world's highest-energy particle accelerator, the Tevatron. The Tevatron is a synchrotron p-pbar collider. Run 1 collisions produced 1.8 TeV, which was sufficient for identifying the top quark; current Run 2 collisions occur at 1.96 TeV. [1]

The process that starts with hydrogen atoms and ends with top quarks comprises six stages:

Cockroft-Walton accelerator

Linac

Booster

Main Injector

Tevatron

DZero

CDF

[2]

Isolating Protons and Anti-protons

"The particle beam beings with a bottle of pure hudrogen" gas, which is ionized by a magnetron inside the dome of the Cockroft-Walton accelerator. [1] Ionization creates negative ions formed of one proton and two electrons. The ions are accelerated to 750 keV and handed off to the Linac, where they are accelerated to 400 MeV. Once up to speed, the ions are sent through a carbon foil, which strips off the electrons, leaving the bare protons to continue on to the third stage (the Booster).

Antiprotons are generated in the "Antiproton Source," a stationary target accelerator. 120 GeV protons from the Main Injector are sent to the Antiproton Source, where they are smashed into a nickel target. Antiprotons are collected from the collision, focused, and sent to the Antiproton Accumulator (storage) Ring. [2]

Accellerating Protons and Anti-protons

The Cockroft-Walton Accelerator is a 750 kV DC voltage source . This electric field gives the ions one good tug towards the Linac. [3]

The Linac is an Alvarez-type linear accelerator, about 130 m long with copper drift tubes [1] through which the beam of ions is directed. An electric field is produced by radio frequency power and polarized in the same direction in all the gaps between the drift tubes: this accelerates the ions forward. The drift tubes shield the ions from the parts of the electric field where the polarity is reversed (otherwise the ions would slow down again). [3]

The Booster is a synchroton with a circumference of 475 meters. The beam is curved by a magnetic field, and accelerated by RF cavities that create an electric field of 500 kVolts around every turn. The protons are accelerated from 400 MeV to 8 GeV in a period of 0.033 seconds. [3]

The Main Injector is a synchrotron accelerator with a circumference of 3319.419 m [4] that replaced the Main Ring in 1998. In addition to accelerating protons to 150 GeV for use in the Tevatron, it operates simultaneously in fixed target and antiproton production modes [5], generating a 120 GeV beam for the production of antiprotons or for use in the NuMI project.

The Tevatron is a superconducting synchrotron with a circumference of 1.25 miles. Protons are accelerated clockwise around the ring and antiprotons are directed counterclockwise by magnets that are kept at 4.3 Kelvin (for collider operation at 980 GeV). [6]

Collisions

The proton and antiproton beams cross at two points: the CDF and DZero detectors. The detectors are roughly 5000 tons each [7], an "intricate array of tightly packed devices... with the beam pipe of the Tevatron running through [the] center." [8] The beam pipe is surrounded by silicon devices and gas-filled tracking chambers; as particles pass through these devices, they dislodge electrons which in turn creates an electric impulse, allowing a particle's path can thus be measured to a small fraction of a millimeter. The next layer of the detector is composed of calorimeters, which measure the particles' energies. Muon detection chambers are located outside the calorimeters. [8]

Because the Top quark is so heavy, there is a small probability of producing one in a collision. The identification of the Top quark in 1994 was based on 20 t-tbar events out of roughly 50 million collisions. The D0 Detector was upgraded between runs I and II, and is expected to deliver 15 - 20 times the integrated luminosity (the rate of top pair production events per cross-section, integrated over time and expressed in pb^-1) of run I.