We are building a particle accelerator that will tell us what holds matter together

The Electron-ion collider should be ready by the end of this decade: it will help us understand one of the fundamental forces of nature

(Photo: Electron-ion collider) Over a year ago, on January 9, 2020, the US Department of Energy announced to the world that it had selected the Brookhaven national laboratory in Upton, New York, as the site of a "new research facility for nuclear physics". This is the so-called Electron-ion collider (Eic), a huge particle accelerator whose cost is estimated between 1.6 and 2.6 billion dollars and which, if all goes well, will be completed by the end of this decade.

Today we know something more, also thanks to the examination just published in The Conversation by Daria Sokhan, experimental particle physics who works, precisely, on the design of Eic. In short: the Electron-ion collider will be an instrument that, by crashing electrons against protons and atomic nuclei of heavy elements, will try to tell us something more about the so-called strong force, one of the four fundamental interactions of nature (the others are the force weak, electromagnetic force and gravitational force) that holds protons and neutrons together in the nuclei of atoms. What we hope, in essence, is that Eic will help us understand how one of the "glues" that holds matter together and, in fact, guarantees its existence, works.

The Electron-ion collider scheme

A long history

Eic's is a history that has roots in the 1950s. In particular in 1956, when the team of physicist Robert Hofstadter (awarded the Nobel in 1961) used the Stanford Linear Accelerator, a linear particle accelerator (i.e. straight, unlike, for example, the Large Hadron Collider of CERN, which is circular), to send a beam of highly energetic electrons against a small sample of hydrogen. Until then it was thought that protons and neutrons (the particles present in the nuclei of atoms) were "fundamental", that is, they could not be broken down into even more elementary entities: Hofstadter and colleagues, in particular, observed a small deviation in the "rebound" of the electrons hitting the nuclei, which suggested, precisely, that protons and neutrons were by no means "dimensionless points", but had a rather complex internal structure.

Today we know that protons and neutrons ( which belong to the class of hadrons, the name given to heavy particles - those as light as electrons instead belong to the class of leptons) are composed of quarks, the fundamental building blocks of matter, which are held together thanks to the so-called strong force, a mediated interaction from the so-called gluons (the previous choice of the word "glue", glue in English, was not strange). The theory that describes this type of interaction is called quantum chromodynamics and, while allowing us to perfectly predict the behavior of quarks and gluons, it does not allow us to analytically calculate the properties of protons: "It is not the fault of theoretical physicists, nor of computers - explains Sokhan -. The equations of quantum chromodynamics simply cannot be solved ".

Under scrutiny

This is where experimental physicists, and in particular those who use particle accelerators, come to their aid. instruments capable of inducing very strong collisions between particles, breaking them, and thus allow us to look at them in depth, in their most elementary components. When examining a proton with a so-called collider (a type of accelerator in which two beams of particles are made to run), what one observes critically depends on the characteristics of the collision and the setup of the experiment.

Sometimes the proton reveals itself in the three quarks that compose it, at other times it appears as an "ocean" of gluons, at other times it manifests itself in the form of a set of quark-antiquark pairs (antiquarks are the antiparticles of quarks, identical to them except for the electric charge and other quantum properties, which are opposite) that are created, destroyed and recreated in infinitesimal times. Hence the fundamental questions: how and why do quarks appear in hadrons? And why do protons have mass, given that quarks are almost devoid of it?

The idea of ​​building the Electron-ion collider was born precisely to answer these questions. The instrument, which should be operational from 2032, will be a sort of enormous "microscope" that will allow the protons to be magnified to excess and to show precisely what is happening inside them. As its name suggests, the Electron-ion collider will use a beam of very high energy electrons to "dissect" protons and atomic nuclei, making barely perceptible phenomena manifest, such as scattering events so rare that only one time every billion collisions. “By studying these processes - says Sokhan - my colleagues and I will be able to reveal the structure of protons and neutrons, how they change when they are bound by the strong force and how new hadrons are created. Furthermore, we will be able to find out if there is a type of matter made up of gluons alone, something that we have never seen so far. "

The Electron-ion collider will also allow you to select precisely the energy of the electron beam, which Continuing the analogy with the microscope, it is a bit like changing the magnification level: the higher the energy, the deeper you look inside the proton or neutron. Over a thousand physicists from all over the world will work on the project, including about ninety researchers from the Italian National Institute of Nuclear Physics.


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