A century of proton

In 1907, a New Zealander named Ernest Rutherford was transferred from McGill University to Canada to the University of Manchester. There, he conducted a series of experiments where he shot alpha particles on different materials. When he found that the beams deviated about 2 ° when fired into the air, he understood that the atomic constituents of the air should have electric fields reaching 100 million. volts per cm to explain the effect. Over the next decade, Rutherford – with the help of Hans Geiger and Ernest Marsden – will lead more experiments that will result in a very important result in the history of physics: the fact that Atom is not indivisible.

During the last year of the 19th century and the first year of the 20th century, Rutherford and Paul Villard independently isolated and classified the radiations into three types: alpha, beta and gamma. Their deepest constituents (as we know them today) were only known much later, and Rutherford played an important role in establishing who they were. By 1911, he had determined that the nucleus of the atom occupied 0.1% of the total volume, but contained all the positive charge – known today as the famous Rutherford model of the 'R & B'. atom. In 1914, he returned to Australia for a lecture tour and returned to the UK only in 1915, after the start of the First World War. Wartime activities would delay his studies for another two years, and he could devote himself to the atom. again only in 1917.

That year, he discovered that when he had bombarded different materials with alpha particles, some long-range recoil particles called "H particles" (a term coined by Marsden in 1913) had been produced, even more so when Nitrogen was also present. This discovery led to the conclusion that an alpha particle could have penetrated the nucleus of a nitrogen atom and destroyed a nucleus of hydrogen, thus confirming that the nuclei of Larger atoms also included hydrogen nuclei. The nucleus of hydrogen is nothing but the proton. Rutherford was able to publish his articles on this discovery only in 1919, after the end of the war. He invented the term "proton" in 1920.

It is interesting to note that in 1901, Rutherford had participated in a debate, in favor of the possibility that the atom be made up of smaller elements, a controversial topic at the time. (His "opponent" was Frederick Soddy, the chemist who had verified the existence of isotopes and with whom Rutherford had had a short but productive collaboration.) It is highly unlikely that he could have anticipated that only three or three decades later, people would begin to suspect that the proton itself was composed of smaller particles.

In the early 1960s, cosmic ray studies and their interactions with matter indicated that the universe consisted of much more than just subatomic elements. In fact, the number of particles was so abundant that it was tempting to think that there might be an organizational principle hitherto unknown, composed of fewer smaller particles. In 1964, Murray Gell-Mann and George Zweig independently proposed such a system, stating that many of the particles could actually be composed of smaller entities called quarks. In 1965, with the help of Sheldon Glashow and James Bjorken, the quark model could explain the existence of a variety of particles as well as some other physical phenomena, thus reinforcing their case.

Then, in a series of pioneering experiments begun in the late 1960s, scientists at the Stanford Linear Accelerator Center (now the SLAC National Accelerator Laboratory) began doing what Rutherford had done half a century ago: breaking a smaller particle into a larger particle. with enough energy for him to reveal his secrets. Specifically, physicists have used the SLAC linear accelerator to excite electrons at about 21 times the energy contained by a proton at rest and to crush them into protons. The results were particularly surprising.

A popular way to study particles has been, as at present, to transmit a smaller particle to a larger particle and to examine the collision for information on the larger particle. In this configuration, physicists expect that the higher the energy of the sounding particle, the larger the resolution at which the larger particle will be probed. However, this relationship fails with protons due to a feature called scaling: electrons at higher and higher energies do not reveal the proton more and more. At energies above a certain threshold, the proton begins to resemble a set of point features, and the interaction of the electron with the proton is reduced to its interactions with the proton. these entities independent of its own energy.

The SLAC experiments revealed that the proton was indeed made up of smaller entities called quarks, of two types – or flavors – called up and down. Gell-Mann and Zweig had proposed the existence of up, down and strange quarks, and Glashow and Bjorken from charm Quark. In the 1970s, other physicists had proposed the existence of low and top quarks, discovered in 1977 and 1995, respectively. With that, the quark model was complete. More importantly for our story, it also completely destroyed the proton.

In the 1970s, use neutrinos as sounding particles to obtain information on the distribution of quarks in protons. They discovered that a proton actually contained three free quarks in a real lake of quark-antiquark couples, and that the sum of all their moments was not added to the total momentum of a proton. This hinted at the presence of another particle then unknown that they called the gluon (which is its own clutter). During this decade, particle physicists began to develop the theoretical framework called Quantum Chromodynamics (QCD), which explained the life and functioning of the six quark and antiquark flavors and eight gluons – all particles subject to the powerful nuclear force.

Ninety years after Rutherford announced the discovery of the proton by projecting alpha particles through slices of mica and air columns, scientists have rocked the world's largest physics experiment – the Large Hadron Collider – to study the fundamental constituents of reality by breaking other protons. . Using it, they proved that the Higgs boson was real, they studied complex processes to better understand the ancient universe and sought answers to questions that continue to confuse physicists.

All the while, scientists have been trying to improve our understanding of QCD, including studying the interactions between quarks, antiquarks and gluons during a collision. This knowledge is essential for determining the existence of new particles and deepening our understanding of the subatomic world. Physicists have also used collider experiments to examine the properties of exotic forms of matter, such as glasma, quark-gluon plasma and colored glass condensates; Refine the search for proposed particles to explain some of the basic discrepancies in the standard model of particle physics; make precision measurements of the properties of the proton for its implications on other particles (such as this one and this one); and explore unresolved proton problems (such as the spin crisis).

100 years after the first appearance of the proton, particle physics is very different from what it was in the Rutherford era, and much of the transformation can be attributed, in a way or from another, to the proton. . Today, physicists study other very different particles, dream of building even larger machines to break protons and develop theories that describe a world much smaller than that of quarks and gluons. It is a different world of different mysteries, as it should be, but it is as wonderful as there are mysteries.