Podcast: How particle accelerators were born

Like Brookhaven, CERN was founded shortly after World War II. Its mission was to unite European scientists and share the growing costs of nuclear physics facilities. Particle accelerators are big projects; they cannot be achieved by small teams of scientists working alone; they need big teams, lots of money and government support. Ernest Lawrence realized this when he was building cyclotrons. Lawrence is often called the “father of great science”. He was not the only scientist to campaign for government support for big science projects, but he did so loudly and effectively, says science and technology historian Catherine Westfall:

Catherine Westfall: “Lawrence himself was very, very — and I say this in a positive way — really a promoter and the bigger the better. He got out and connected, first with the industry , then with the government, so he was a great promoter, which is very good in a field like particle physics, in which to advance you constantly need bigger and therefore more expensive equipment.

What kind of advances were made then, to justify all the expenses? To understand the importance of particle accelerators for particle physics, let’s visit another “big science” institution: the Stanford Linear Accelerator Center, known as SLAC. Michael Peskin is Professor of Particle Physics and Astrophysics at SLAC. He told me about his beginnings in the 1960s.

Michael Peskin“People were trying to figure out what the structure of the proton was by shooting protons at each other and seeing what was going on. And the people who founded SLAC had this vision, that what you should be doing is d to get an electron beam and make it so intense that you could actually see, like an electron microscope, inside the proton and find out what was there.

Thus, at SLAC, they built a machine 3 kilometers long to accelerate electrons to a speed close to light. It was the longest straight structure in the world.

Michael Peskin“And so, literally, you inject electrons, let them do whatever they do to the proton, watch them come out, and try to deduce what the structure of the proton is. And they found that there were things like hard steel balls inside the proton, which are the quarks. And so that was absolutely a revolution in particle physics. It changed the way everybody thought about strong interactions and the structure of protons.

After finding quarks, using particle accelerators, scientists discovered more fundamental particles, such as W and Z bosons. These discoveries helped them understand the most basic forces known, including “l ‘strong interaction’ which binds the quarks together and the ‘weak interaction’ carried by the W and Z bosons. Despite these successes, not everyone thought the expense was worth it. Catherine Westfall explains that some thought “big science” was a bad thing:

Catherine Westfall“That it was getting out of control, and it would push other kinds of science to the side, it was expensive, it was esoteric. In the American history of science, there’s always been this tension between what’s exciting and cutting-edge and maybe splashy, and what’s practical. And so some leaders in the scientific community, and some in government, feared that the money was wasted on something esoteric that could have been used for more practical purposes.

In 1993, the most ambitious accelerator project to date, the Superconducting Super Collider, was canceled by the US government despite its partial construction. Ernest Courant, writing in the Annual Review of Nuclear and Particle Scienceremember :

A partially dug tunnel remained. 2000 scientists, engineers, technicians and support staff needed new jobs. 2 billion dollars had been spent for nothing.

It was a low point for particle physics. Catherine Westfall explains that after the Cold War, particle physics sort of went out of fashion in the United States and particle accelerators were used for other types of work.

Catherine Westfall: “When the cold war ended and the superconducting super collider was cancelled, there was another group of scientists who used accelerators, the most exciting of which were light sources that accelerate synchronous light so you can actually to create a picture of the material to be studied. So it’s not just the tiny constituents of matter, it’s really a way of understanding a variety of materials. And these people were very different from the physicists who came before them; they were interested in finding something much more convenient.

The light sources are synchrotrons that accelerate electrons to high speeds, like the linear accelerator at SLAC, but in a circle rather than a straight line. When electrons whirl around the synchrotron ring, they produce synchrotron light, also called synchrotron radiation. This radiation includes powerful X-rays that can be used to probe the structure of a wide variety of materials, from proteins to insect wings to ancient artifacts. Today, there are dozens of light sources around the world that are not used by particle physicists but by biologists, materials scientists and archaeologists. Catherine Westfall calls this the “new big science”.

But that was not the end of the trend to build bigger machines for particle physics. The action moved to Europe. In an underground tunnel 27 kilometers in circumference and crossing two countries (France and Switzerland), CERN has installed a new machine: the Large Hadron Collider. The LHC is the largest and most powerful particle accelerator ever built. It was started in 2008. As its name suggests, it is a kind of particle accelerator called a “collider”.

Physicists realized that instead of accelerating particles towards a stationary target, if you had two beams of fast particles moving in opposite directions around a synchrotron, you could collide with them and get a lot of energy interaction higher. It’s like crashing a car. Here is Paul Collier to explain:

Paul Collier: “So it’s like driving your car into a brick wall, a lot of the energy is wasted trying to move the brick wall. In the car scenario, you would have a lot more for your money if you bang the cars together instead of ramming them into a wall.

Hopes were high for CERN’s new collider. Each beam was designed to accelerate protons to 3.5 tera-electron volts, creating head-on collisions of 7 tera-electron volts. It is enormous! For comparison, Collier says that a car battery produces an accelerating voltage of about 12 electron-volts. The first cyclotrons built by Lawrence were aimed at 1 mega-that is, one million electron volts. The Cosmotron could accelerate protons to 3 giga-electron-volts. And in 2010, the LHC was at 7 tera-electron Volts. The first collisions were almost four times more energetic than the previous world record. Would it lead to new discoveries?

Rolf Heuer (Director General, CERN): “Today is also a special day because we had two presentations from the two experiments, ATLAS and CMS, on their update for the search for a certain particle.”

In 2012, CERN made an important announcement.

Joe Incandela (particle physicist, CERN): “And we conclude by saying that we have observed the new boson, with a mass of 125.3 plus or minus 0.6 GeV at 4.9 standard deviations. Thanks.”

Scientists working at the LHC had found evidence for the existence of the Higgs boson, a particle whose existence was predicted by theory but which, until now, had not been seen. Michael Peskin remembers watching the ad.

Michael Peskin“I gasped. I certainly did not expect the discovery to be so striking. It’s really beautiful to see the experiment align with the theory.

The Higgs is important because, according to the standard model of particle physics, it is the particle that gives mass to all other particles. Paul Collier is one of the many people who made the discovery possible. He joined CERN in the 1980s as an engineer and worked on other machines before joining the LHC and driving the first beams around the ring.

Paul Collier: “So to actually find this missing particle, it took 60 years from the first conception of the Higgs particle to its discovery, it was very emotional, very important – a fantastic experience, yeah. You know, for many of us, work on a machine is a life. There are generations of physicists and engineers who live from the construction, improvement, operation and maintenance of this type of machine. facilities. It really gets in your blood because it’s been there for so long.

The LHC is not only vast, it is also a very delicate and complex machine located 100 meters underground.

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