Question: When did physicists develop the particle accelerator?
About the article "Electron", section "Particle accelerators".

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In the circular accelerator, particles move in a circle until they reach sufficient energy.
The particle track is typically bent into a circle using electromagnets.
The advantage of circular accelerators over linear accelerators ("linacs") is that the ring topology allows continuous acceleration, as the particle can transit indefinitely.
Another advantage is that a circular accelerator is smaller than a linear accelerator of comparable power (i.e.
a linac would have to be extremely long to have the equivalent power of a circular accelerator).
Depending on the energy and the particle being accelerated, circular accelerators suffer a disadvantage in that the particles emit synchrotron radiation.
When any charged particle is accelerated, it emits electromagnetic radiation and secondary emissions.
As a particle traveling in a circle is always accelerating towards the center of the circle, it continuously radiates towards the tangent of the circle.
This radiation is called synchrotron light and depends highly on the mass of the accelerating particle.
For this reason, many high energy electron accelerators are linacs.
Certain accelerators (synchrotrons) are however built specially for producing synchrotron light (X-rays).
Since the special theory of relativity requires that matter always travels slower than the speed of light in a vacuum, in high-energy accelerators, as the energy increases the particle speed approaches the speed of light as a limit, but never attains it.
Therefore particle physicists do not generally think in terms of speed, but rather in terms of a particle's energy or momentum, usually measured in electron volts (eV).
An important principle for circular accelerators, and particle beams in general, is that the curvature of the particle trajectory is proportional to the particle charge and to the magnetic field, but inversely proportional to the (typically relativistic) momentum. 
The Belle experiment is a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers investigating CP-violation effects at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibaraki Prefecture, Japan. 
On 10 September 2008, PAEC, along with the Universities of Pakistan, send a large team of physicists and engineers to CERN.
Pakistani scientists were heavily involved and participated in Large Hadron Collider, the world's largest and highest-energy particle accelerator. 
Oops-Leon is the name given by particle physicists to what was thought to be a new subatomic particle "discovered" at Fermilab in 1976.
The E288 collaboration, a group of physicists led by Leon Lederman who worked on the E288 particle detector, announced that a particle with a mass of about 6.0 GeV, which decayed into an electron and a positron, was being produced by the Fermilab particle accelerator.
The particle's initial name was the greek letter Upsilon (formula_1).
After taking further data, the group discovered that this particle did not actually exist, and the "discovery" was named "Oops-Leon" as a pun on the original name (mispronounced ) and the first name of the E288 collaboration leader.
The original publication was based on an apparent peak (resonance) in a histogram of the invariant mass of electron-positron pairs produced by protons colliding with a stationary beryllium target, implying the existence of a particle with a mass of 6 GeV which was being produced and decaying into two leptons.
An analysis showed that there was "less than one chance in fifty" that the apparent resonance was simply the result of a coincidence.
Subsequent data collected by the same experiment in 1977 revealed that the resonance had been such a coincidence after all.
However, a new resonance at 9.5 GeV was discovered using the same basic logic and greater statistical certainty, and the name was reused (see Upsilon particle).
Today's commonly accepted standard for announcing the discovery of a particle is that the number of observed events is 5 standard deviations (σ) above the expected level of the background.
Since for a normal distribution of data, the measured number of events will fall within 5σ over 99.9999% of the time, this means a less than one in a million chance that a statistical fluctuation would cause the apparent resonance.
Using this standard, the Oops-Leon "discovery" would never have been published. 
The Alternating Gradient Synchrotron (AGS) is a particle accelerator located at the Brookhaven National Laboratory in Long Island, New York, USA.
The Alternating Gradient Synchrotron was built on the innovative concept of the alternating gradient, or strong-focusing principle, developed by Brookhaven physicists.
This breakthrough concept in accelerator design allowed scientists to accelerate protons to energies that would have been otherwise unachievable.
The AGS became the world's premiere accelerator when it reached its design energy of 33 billion electron volts (GeV) on July 29, 1960.
Until 1968, the AGS was the highest energy accelerator in the world, slightly higher than its 28 GeV sister machine, the Proton Synchrotron at CERN, the European laboratory for high-energy physics.
While today's newest accelerators can reach energies in the trillion electron volt region, the AGS has earned researchers three Nobel Prizes and today serves as the injector for Brookhaven's Relativistic Heavy Ion Collider; it remains the world's highest intensity high-energy proton accelerator.
The AGS Booster, constructed in 1991, further augments the capabilities of the AGS, enabling it to accelerate more intense proton beams and heavy ions such as Gold.
Brookhaven's linear accelerator (LINAC) provides 200 million electron volt (MeV) protons to the AGS Booster, and the Tandem Van de Graaff accelerator provides other ions to the AGS Booster.
The AGS Booster then accelerates these particles for injection into the AGS.
The AGS Booster also provides particle beams to the NASA Space Radiation Laboratory.