Cooling at CERN

No, I am not referring to my cooling off following a full day of service at the CERN Open Days in September 2019.

This blog refers to beam cooling, or, more precisely, stochastic cooling of particle beams in storage rings. "Cooling" will narrow the transverse momentum distribution of a bunch of charged particles by detecting momentum fluctuations and applying an appropriate correction.

View of the AA ring. Slide from Simon van der Meer's Nobel Prize lecture:
Stochastic Cooling and the Accumulation of Antiprotons

In 2025, the Museumsgesellschaft chose CERN as the plat de résistance for its traditional summer tour. During our visit to the site, we saw a special cooling facility. The building we entered is familiar to Red Baron. Here, he helped to build the AA (Antiproton Accumulator) in the late 1970s.

Simon van der Meer invented and developed the technique of stochastic cooling at CERN in the Initial Cooling Experiment (ICE).

The principle of Stochastic Cooling.
Slide from Simon van der Meer's Nobel Prize lecture

Stochastic cooling uses the electrical signals produced by particles in a "bunch "to drive an electric kicker. This will kick the bunch of particles, reducing their transverse momentum.

The circulating particles (nearly) and the electrical signal travel at the speed of light. So the correction sent along the chord reaches the kicker well in time to apply the necessary kick to the bunch of particles.
When individual kicks are applied continuously over an extended period, the average tendency of the particles to exhibit wayward momenta is reduced.

 

Note the shrinking of the beam profile with time.

Cooling times range from a second to several minutes, depending on the required cooling depth.

Many accelerators had to work together.
Slide from Simon van der Meer's Nobel Prize lecture

The technique was used to collect and cool antiprotons* in the Antiproton Accumulator (AA). They were then injected into a modification of the Super Proton Synchrotron (SPS), i.e., the Proton-Antiproton Collider (PAC), to collide with counter-rotating protons.

*Noted as p in the graphic

While Carlo Rubbia proposed and pushed for the PAC, Simon van der Meer's stochastic cooling provided the necessary tool for its success. In 1984, Carlo and Simon were awarded the Nobel Prize in Physics for the discovery of the W and Z bosons that carry the weak nuclear force.

 
The ICE ring was also instrumental in evaluating electron cooling, a technique introduced by Gersh Budker.

Electron Beam Cooling is a technique used primarily in particle accelerators and storage rings to reduce the emittance (the measure of the spread of particle positions and momenta) of a beam of charged particles, typically protons or heavy ions. Dense, parallel beams of quasi-monoenergetic electrons travel with the heavier particles over the distance of the cooling section. The protons or ions undergo Coulomb scattering in the electron "gas" and exchange momentum with it, thus reducing the space coordinates and the angles of the heavier particles.

Click graphics to enlarge.

CERN operates electron coolers in its Antimatter Decelerator (AD) and in the smaller Extra Low Energy Antiproton (ELENA) ring.

Looking down on ELENA

This is where it gets exciting, because ELENA produces antiprotons of nearly zero energy that capture a positron, forming antihydrogen atoms. This opens research on antimatter.

Does a hydrogen atom fall in a gravitational field just as quickly as an antihydrogen atom? Are there differences in the emitted spectral lines of the two species? With the current measurement accuracies, no differences have been detected so far.

The CERN computer center was the other interesting part of our visit. Here, we were impressed by the volume of data from the experiments to be processed.

On the way out, Red Baron was abruptly confronted with his past.

On Display was the first paper by Tim Berners-Lee and Robert Cailliau, which marks the beginning of the World Wide Web, the precursor to the Internet. Even today, the sequence of letters HTTP (HyperText Transfer Protocol) for calling up websites is a reminder of the HyperText Project proposed by the two CERN computer freaks, who had no idea at the time that their proposal would change the world.

Click to enlarge

I had many a cup of coffee with Robert, a wide-awake colleague who was always up for a joke. CERN's superiors looked favorably on this coffee-drinking while leaning on bar tables, as new ideas were discussed with colleagues from outside their own field.

It was a thoroughly nostalgic visit to CERN.

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