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A century of proton

In 1907, a new Zealander named Ernest Rutherford moved from McGill University in Canada to the University of Manchester. There he conducted a series of experiments in which he fired alpha particles in different materials. When he discovered that the rays deviate by about 2º when fired through the air, he found that the atomic components in the air would have to have electric fields as strong as 100 million volts per cm to explain the effect. In the next decade, Rutherford – together with the help of Hans Geiger and Ernest Marsden – would carry out more experiments that ultimately resulted in a very important result in the history of physics: that the atom was not indivisible after all.

During the last year of the 19th century and the first year of the 20th century, Rutherford and Paul Villard had independently isolated and classified radiation in three types: alpha, beta and gamma. Their deeper elements (which we know of today) were not known until much later, and Rutherford played an important role in determining what they were. By 1911 he had determined that the atom had a nucleus that occupied 0.1% of the total volume but contained all positive charge – known today as the known Rutherford model of the atom. In 1914, he returned to Australia on a lecture tour and did not return to the United Kingdom until 1915, after the beginning of the First World War. Our wartime activities would delay the studies for two years and he could devote his attention to the atom once again in 1917.

That year, he found that when he bombed various materials with alpha particles, some long-lasting rewind particles called "H-particles" (a term coined by Marsden in 1913) were produced, more when nitrogen gas was also present. This result led him to conclude that an alpha article could have penetrated the nucleus of a nitrogen atom and knocked out a hydrogen core, which in turn supported the notion that the nuclei of larger atoms also included hydrogen nuclei. The hydrogen core is nothing but the proton. Rutherford was unable to publish his paper on this discovery until 1919, after the war had ended. He would move on to the coin term "proton" in 1920.

Interestingly, in 1901, Rutherford had participated in a debate, and spoke of the possibility that the atom was made up of smaller things, a controversial topic at that time. (His opponent was Frederick Soddy, the chemist who noted the existence of isotopes and with which Rutherford enjoyed a short but productive collaboration.) It is highly unlikely he could have expected that only three or so decades later people would start to suspect that the proton itself consisted of smaller particles.

In the early 1960s, studies of cosmic rays and their interactions with matter showed that the universe was made of much more than just the basic subatomic bits. In fact, there was such a large number of particles that the idea that there could be a hitherto unknown organizational principle consisting of fewer smaller particles was tempting, albeit just for some. In 1964, Murray Gell-Mann and George Zweig independently proposed such a system and claimed that many particles could actually consist of smaller units called quarks. By 1965, and 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 that strengthened their case.

Then, researchers at the Stanford Linear Accelerator Center (now SLAC National Accelerator Laboratory) began in a series of experimental experiments that began in the late 1960s do what Rutherford had done half a century earlier: crush a smaller particle to a larger one with enough energy for the latter to reveal their secrets. Specifically, physicists used the linear accelerator at SLAC to activate electrons to approximately 21 times the energy containing a proton at rest and crushed them in protons. The results were particularly surprising.

A popular way to study particles, now and then, has been to radiate a smaller particle at a larger and examine the collision for information about the larger particle. In this setting, physicists expect greater energy of the probing particle, the greater the resolution of the larger particle. However, this relationship fails with protons due to a function called scaling: electrons at higher and higher energies do not reveal more and more about the proton. This is because at energies beyond a certain threshold, the proton begins to resemble a collection of dot-like units and the electron interaction with the proton is reduced to its interaction with these units, regardless of their own energy.

The SLAC experiments thus revealed that the proton actually consisted of smaller units called quarks, of two types – or flavors – called up and down. Gell-Mann and Zweig had proposed the existence of up, down and weird quarks, and Glashow and Bjorken off charm cottage cheese. By the 1970s, other physicists had suggested the existence of bottom and top quarks, discovered in 1977 and 1995 respectively. With this, the quark model was completed. Even more important to our story, it also made a complete mess of the proton – literally.

In the 1970s, researchers began using neutrinos as probing particles to elicit information on the distribution of quarks in protons, supported by more accurate data from other tests with electrons in the US and Germany and with muones at CERN. They found that a proton actually contained three free quarks in a real lake of quark antique pairs, and that the sum of its entire momentum did not contribute to a proton's total momentum. This suggested in the presence of another then unknown particle that they called the gluone (which is its own mess).

During the decade, particle physicists began to build the theoretical framework called quantum chromodynamics (QCD), to explain the lives and effects of the six flavors of quarks and antiques and eight gluons – all of the particles controlled by the strong nuclear power.

Nineteen years after Rutherford announced the proton discovery by shooting alpha particles through slices of mica and columns of air, scientists switched the world's largest physical experiment – the Large Hadron Collider – to studying the basic elements of reality by crushing protons in other protons. Using it, they have shown that Higgs boson is real and has studied intricate processes of insight into the very early universe and has pursued responses to questions that continue to baffle physicists.

Through this, researchers have tried to improve our understanding of QCD, especially by studying how quarks, antiques, and gluons interact during a collision, knowledge that is crucial to determining the presence of new particles and deepening our understanding of the subatomic world. Physicists have also used collider experiments to investigate the properties of exotic forms of matter, such as vitreous, quark-gluon plasma, and colorless condensate; limit the search for proposed particles to explain some basic differences in the standard particle physics model; make precision measurements of proton properties for its impact on other particles (such as this and this); and explore unresolved problems with the proton (like the spin crisis).

And completely – just once – 100 years after the proton was first knocked out, particle physics looks very different from how it did in Rutherford's time, and much of the transformation can be attributed, in one way or another, to the proton. Today, physicists pursue others, very different particles, dreaming of building even larger proton smashing machines and are busy bringing together theories that describe a world much smaller than the quark and gluons. It is another world of different mysteries, as it should be, but it is also good that there are mysteries at all.

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