Why is the proton rotating

The earth revolves around itself - every child knows that today. What is less well known, however, is that tiny particles of matter also rotate around their own axis - for example atoms, but also neutrons and protons, the building blocks of atomic nuclei. This intrinsic twist, known as "spin", has been causing headaches for physicists for years. Thanks to new results from the Hamburg particle accelerator “Hera”, it is now gradually becoming clear how protons and neutrons get their spin.

Twelve years ago, the cause of the protons spinning seemed very simple; so-called quarks, i.e. the building blocks of protons and neutrons, had to contribute to this: "One naively imagined that the proton consists of three quarks, each of which rotates around itself," explains Michael Düren, particle physicist at the University of Nuremberg Gain. "Two quarks rotate in one direction, the third in the other, and their total spin adds to the spin of the proton."

But this simple formula turned out to be wrong. The EMC (European Muon Collaboration) experiment carried out at the European Laboratory for Particle Physics CERN in Geneva in 1987 had shown that the sum of the quark spins was only a fraction of the expected value for the proton spin. What contributed the lion's share was a mystery to the experts - the "spin crisis" spread.

Some particle researchers even questioned the widely accepted quark model. Only new experimental set-ups could illuminate. This is how the “Hermes” experiment was created at the “Desy” German electron synchrotron in Hamburg. It obtains its data at the “Hera” large accelerator. The principle: electrons that are almost as fast as light hit a gas cloud of helium or hydrogen atoms with full force. A special detector then picks up the fast electrons deflected by the gas, but also the elementary particles knocked out of the gas atomic nucleus.

"If you imagine the Proton as a ballet stage on which the quarks run back and forth as dancers and turn their pirouettes, then Hermes can precisely measure how these dancers move and turn," explains Düren. In this picture, the electron injected into the proton corresponds to a medicine ball that is thrown onto the stage in a “burning ball” fashion in order to transport one of the dancers into the auditorium. From the way the ballet artist falls from the boards, one can infer the sense of rotation of his pirouettes in the rows of spectators. In relation to Hermes this means: The particle detector measures the deflected electrons as well as the particles knocked out of the atomic nucleus.

With the help of complex computer calculations, the physicists can use the measurement data to reconstruct how the quarks move inside the proton and how strong their intrinsic spin is. The Hermes physicists recently presented revealing interim results: According to them, in addition to quarks, so-called gluons also contribute to proton spin. As “glue particles”, gluons ensure that the three quarks stick together by rushing back and forth between them immeasurably and as messengers conveying information about the force field. Occasionally the glue pieces can even materialize: For a tiny moment they turn into quarks, the "sea" quarks. The three actual quarks (valence quarks) swim in a shimmering, swirling lake consisting of gluons and sea quarks.

Transferred to the ballet stage, this means that the dancers are connected to one another by rubber bands. “The glue particles hold the quarks together like ribbons,” explains Düren. “The dancers can not only rotate around their own axis, but also rotate around each other with a rubber band in their center.” In physical terms: In addition to the spin of the individual particles, there are also the “orbital angular impulses” of the respective relative movements - a much more complex picture than the physicists had drawn it earlier. According to the latest Hermes data, the quarks only contribute around 28 percent to the proton's spin. By carefully analyzing the collision processes, the physicists even managed to break down the spin contributions of the various “quark types”. The “up quarks” rotate in the same direction as the proton, while the “down quarks” rotate in the opposite direction.

The extremely short-lived sea quarks, on the other hand, are unlikely to make any contribution. The gluons seem to be responsible for the lion's share of the proton rotating worm - both through their own spin and their swirling interplay with the quarks. The physicists are hoping for an exact picture of the complex particle carousel in a few years. Hermes will then receive further measurement data and have also received support from a new experiment: from next year, “Compass” (Common Muon Proton Apparatus for Structure and Spectroscope) at CERN will measure the gluons' rotational movements in detail.
FRANK GROTELÜSCHEN