Signs of a "new physics" appeared in two major experiments. The Tevatron Hadron Collider recorded particles where they should not be, and the PAMELA space experiment found traces of the decay of dark matter particles. Both facts fit well into the theory that the "dark force" exists
While the Large Hadron Collider (LHC) is preparing for repairs after a major September accident, the American Tevatron, which has survived the last months as the most powerful accelerator on the planet, has presented physicists with an unexpected surprise. Late last week, CDF collaborators working on the giant Tevatron particle detector of the same name published a preprint describing something that goes beyond the almost sacred standard model of elementary particles for physicists.
If this signal turns out to be not some unaccounted for background effect, this discovery will be the first earthly evidence of the limitations of the standard model.
Terrestrial in the sense that astrophysicists have long known dark matter and dark energy, which also do not fit into the Standard Model. True, practically nothing is known about the properties of the particles that make up dark matter.
Tevatron and extra muons
With the CDF detector, physicists study the particles produced by the collision of protons - positively charged particles that make up all atomic nuclei, and antiprotons - their negatively charged antipodes. In the Tevatron accelerator, as its name suggests, these particles are accelerated to energies of almost 1 TeV, or 1000 GeV - a thousand billion electron-volts, and the collision energy is, accordingly, almost 2000 GeV, which makes it possible to create a variety of, even very massive elementary particles.
However, it is not even possible to simply fix the existence of most of the particles of interest. As a rule, they are unstable and turn into several lighter particles in a tiny fraction of a second. It is the properties of the decay products that are measured by the detector, and physicists then, in accordance with the well-known metaphor, "try to restore the structure of the clockwork, examining the fragments of clock gears that collided at near-light speed."
One of the most popular "gears" of this kind is the muon. In terms of their properties, muons are very similar to ordinary electrons orbiting atomic nuclei. However, muons are much more massive, and therefore are of particular value for experimental physicists. Firstly, it is more difficult to "mislead" them when they encounter the protons and electrons of the detector, and secondly, in the collisions themselves, fewer of them are born, and it is easier to make out their traces in the detector than the entangled trajectories of numerous electrons.
One of the particles that has been actively studied using muons is the so-called B-meson, which includes a heavy b-quark (or antiquark).
And here muons for a long time led the experimenters by the nose.
The theory of the structure and interaction of quarks - quantum chromodynamics - allows you to calculate the probability of the production of B-mesons and their participation in various interactions. Hence, it is possible to estimate the number of muons that will be born during the decay of these particles. However, in the experiment, much more muons were produced than planned. Moreover, another method of measuring the properties of B-mesons showed results that are in better and better agreement with theory. So the experimenters had less and less reason to accuse theorists of not knowing how to count (and calculations in quantum chromodynamics are extremely difficult).
The reason for these discrepancies remained a mystery for a long time, until scientists found out that some of the muons, which physicists for a long time took for the decay products of B-mesons, in fact had nothing to do with them. The fact is that the B-meson lives for a very short time and, having been born in the collision of protons and antiprotons, manages to fly away from the axis of the vacuum tube, where the collisions occur, only by 1–2 mm. Here it decays into muons. When scientists figured out where the muons that their detector detected, the problem of B-mesons was solved: as it turned out, some of them arose much further from the axis, and the contribution of these "extra muons" to the final result exactly explained the discrepancy with the theory.
But where do those "extra" muons come from?
Some of them originate at 3 mm from the axis, at five, and at seven; some are completely outside the vacuum tube, which really does not fit into any gate.
The nascent physical "sensation" is connected with these particles. This word, rare for venerable science, actually characterizes the excitement of theorists and experimenters in the best possible way. Discussions about the reality of the signals found by the CDF collaboration are already raging on the professional blogs of physicists, and on the website of electronic preprints at Cornell University for the third day in a row, more and more theoretical explanations for what they saw appear.
New particles?
In principle, there can be a great variety of reasons for the appearance of unnecessary, or, as physicists say, "background" particles, and most of the article by the CDF collaboration is devoted to the analysis of possible reasons for the appearance of a signal that does not appeal to the "new physics" beyond the standard models. Maybe we did not take into account some other particles from which muons are born - for example, cosmic rays, or maybe we take for muons other decay products of particles produced in the Tevatron? Finally, maybe the signals in the detector themselves, which we take for traces of muons, are not such - noise, statistical fluctuations, artifacts of furious methods of mathematical processing of experimental results?
Promotional video:
According to the authors of the last work, they failed to find a "standard" explanation.
It should be noted that almost a third of the collaboration - about 200 out of 600 people - refused to put their signatures on the article, which had been undergoing an "internal audit" for almost six months. By…
Everything looks as if they managed to find signs of the existence of some new particle that lives much longer than the B-meson, and it has no place in the physics we know. However, scientists still refrain from such a direct statement: the experience of a whole generation of physicists, over and over again convinced of the applicability of the standard model to seemingly completely inexplicable phenomena, makes itself felt. But it is impossible to simply ignore almost 100 thousand events recorded by one of the best instruments of the still most powerful accelerator on Earth.
The properties of "extra" muons are amazing in and of themselves. One of the most striking is that they were very often born in "packs" - not one particle at a time, but two, three, even eight at a time. In addition, as a rule, from the point at which they were born, they did not fly out in all directions, but in approximately the same direction - scientists even use the term "muon jet". And the characteristic energy of a new unknown particle - if it really exists - is several GeV. In other words, the "new physics" - if we really begin to distinguish it in the muon fog - begins at energies not in the thousands of GeV, at which monsters like the LHC are directed, but much earlier.
And these properties strikingly approximate the results from the terrestrial accelerator with the data published just a few days earlier from the space antiparticle detector PAMELA.
Positron fraction as a function of energy // PAMELA Group, arXiv.org
Results of the PAMELA experiment The
international research vehicle PAMELA on board the Russian artificial satellite Resurs-DK1 has reliably recorded an excess of high-energy positrons in a stream of charged space …
According to many astrophysicists, the excess of high-energy positrons (antiparticles to electrons) in cosmic rays arises from the decay or annihilation of mysterious dark matter particles. This is another element of physics beyond the standard model, the existence of which (and even domination by mass) astronomers have long known, but cannot say anything worthwhile: that is why it is dark matter, that it is not visible, and its presence gives out only through gravity.
Dark Power
As it turned out, the quartet of theorists from Princeton, Harvard and New York already have an explanation of the PAMELA results, which came in handy with the new data from the Tevatron. According to Nima Arkanihamed and his colleagues, within the framework of their supersymmetric model, a unified and natural explanation is obtained for the excess of positrons reliably measured by the PAMELA apparatus, a barely discernible excess of gamma rays coming seemingly from nowhere, and the foggy glow of the center of the galaxy in gamma and radio beams recorded by other astrophysical satellites.
In accordance with the model, dark matter particles have a mass of about 1000 GeV and do not participate in the interactions we know. However, they act on each other with the help of a short-range "dark" force, which is carried by another dark particle with a mass of about 1 GeV. In other words, to the three usual types of interaction, acting only on ordinary matter (electromagnetic and nuclear, weak and strong), one more is added, which acts only in the world of dark matter. Gravity, as usual, stands apart, linking both worlds.
Theorists needed the "dark" force to bind the dark matter particles into a kind of "atoms", in which one of the dark particles has a negative "dark charge", and the other has a positive "dark charge". Only the formation of "atoms" allows dark matter to annihilate intensely enough to explain the results of astrophysical observations (this is the so-called Sommerfeld mechanism).
However, the particle that carries the "dark" force can already decay directly with the emission of ordinary particles, and it is this particle, according to Arkanihamed and his colleagues, that may be responsible for the appearance of "extra" muons.
Moreover, the decay of dark particles charged with a dark charge naturally proceeds in a cascade until it hits the lightest stable dark particle, which has nothing to decay into. Each step of this cascade involves a particle - a carrier of dark force, and therefore an extra muon may appear at each step. So much for muons in "packs". Well, the fact that they all fly out in the same direction is simply due to the fact that the decaying particle is moving quickly - so the charges of the festive fireworks, exploding before reaching the highest point of their trajectory, throw out whole fountains of bright lights forward. So much for the "jet".
However, the publication of data by the CDF and PAMELA collaborations will undoubtedly lead to the emergence of dozens, if not hundreds, of possible explanations in the coming months. So it might not be worth dwelling on Arkanihamed's model. So far, she is distinguished only by the fact that she turned out to be at the court when interpreting both those and other data.
Of course, it is possible that both experimental results will receive more trivial explanations. "Extra muons" may turn out to be nothing more than an unaccounted for instrumental effect of the giant CDF installation, and "extra positrons" may be generated in the vicinity of neutron stars in our Galaxy.
But the prospects are intriguing. In the world of dark matter, which until recently seemed like a formless turbidity behind which astronomers hide their misunderstanding of the structure of the world, a structure began to emerge - some kind of interactions, "dark charges", "dark atoms". Maybe physics is not over, and new generations of scientists will have something to study in the "dark world".
