09 December 2011, 16:20 Science Text size A A A

Proton like a puck

High-energy protons appear as a black disk

Protons at speeds close to the speed of light appear as flat black disks. Scientists came to this conclusion by analyzing proton collisions in the Large Hadron Collider and in cosmic rays.

What does a proton look like? Typically, when we talk about “appearance,” we mean the image in the visible region of the spectrum. Protons are too small to scatter such wavelengths of light, so they “don’t look like anything” - this is how scientists usually answered the question. But modern experimental capabilities sometimes make it possible to debunk the most persistent “official” myths.

American physicists Martin Block and Francis Halzen published an article in the authoritative scientific journal Physical Review Letters in which they claim that

the proton, at least at high energies, appears as a black disk, most like a highly flattened hockey puck.

As we well know from a school physics textbook, protons are very small (their size is about 1.6 femtometers (10-15 meters), have a positive charge and, together with neutrons, make up the atomic nucleus. A proton consists of two u-quarks (upper) and one d-quark (lower). Three quarks are connected to each other by strong interactions, the carriers of which are other elementary particles - gluons. Life is in full swing inside the proton: quarks move, exchange gluons; particle-antiparticle pairs constantly appear from the vacuum and disappear in it. Considering These are complex processes and taking into account the mixed particle-wave nature of the proton (like other elementary particles, it combines the properties of a particle and a wave), it is clear that the task of “drawing” a proton is not an easy one.

However, thanks to relativistic effects that occur at speeds close to the speed of light, physicists have an idea of what a proton theoretically looks like when moving at this gigantic speed (the speed of light in a vacuum is one of the universal physical constants and is equal to 299,792,458 m/s) . Due to the Lorentzian (or Fitzgeraldian) contraction effect, the proton at ultra-high speeds should be compressed into a two-dimensional disk (that is, a disk that has no thickness). The effect of Lorentz contraction is that, from the point of view of an observer, objects moving relative to him have a shorter length (linear dimensions in the direction of movement) than their own length. The multiplier expressing the apparent compression of size, the more different from unity, the greater the speed of movement of the object.

In their study, Block and Halzen argue that the proton disk is black.

Their conclusions are based on an analysis of three different experiments - the Atlas LHC and CMS collaborations, as well as the world's largest observatory, the Pierre Auger Observatory ( Argentina ), which observes cosmic rays.

close 100%

The picture shows two protons colliding in the LHC. Due to the fact that their speed is close to the speed of light, they are compressed into thin disks // LHC

Physicists have been studying proton-proton collisions in special accelerators - colliders (from the English collide - to collide) for several decades. Studying the products of particle interaction in the cross section of a beam (not all protons behave like billiard balls - upon collision they do not just fly apart, but give birth to myriads of new particles) allows not only to study the internal structure of protons, but also to understand the structure and properties of a huge number of other elementary particles , which are born during collisions.

Block and Halzen in their work summarized the results of measurements at two different energies obtained from three different experiments. Using a beam with a collision energy of 7 TeV (the most powerful beam to date that can only be generated by the LHC at CERN), the LHC Atlas collaboration measured inelastic proton collisions over an area of 69.1 x 10-27 square meters. cm), and the LHC CMS collaboration, using completely different instruments, carried out independent measurements of the same beam over an area of 68 x 10-27 sq. see. The third experiment concerned protons from cosmic rays bombarding the Earth: particles with an energy of 57 TeV were recorded by the Pierre Auger Observatory, the cross-sectional area was 90 x 10-27 square meters. see For comparison, Block and Halzen conducted theoretical simulations of the corresponding experiments, and the results turned out to be close to the observed ones.

By combining their data, they developed a numerical model of the proton structure. It allows you to "see"

what happens when two protons collide

(This is what happens at the Large Hadron Collider, where scientists use a complex array of magnets to accelerate protons and then force them to intersect at specific points).

“In our model, at least in the limit (that is, when the proton energy reaches infinity), the proton will interact with any particles in two ways. Exactly half the time it will behave like a billiard ball (this corresponds to an elastic collision), and the other half it will completely absorb the particles colliding with it (a completely inelastic collision),” explains Halzen.

This model corresponds to the behavior of a black disk of zero thickness - this is exactly the shape scientists have suggested for the proton.

In addition, the behavior of protons at an energy of 57 TeV is already close to the hypothetical situation of infinitely high energy. Of course, artificially created proton beams (as at the LHC) are still very far from those created by nature in cosmic rays. However, the ratio of inelastic and elastic collisions in cosmic rays is close to 1:1, which confirms the correctness of the Block and Halzen model: at infinitely high energy, the proton becomes a passer-by on a gramophone record!

“Perhaps one day we will be able to develop equipment that will allow us to detect protons with even higher energy in cosmic rays. Then we will be able to provide a larger experimental basis for our modeling,” concluded Halzen.