Scientists working in the CMS collaboration reported the probable discovery of an unknown particle decaying into muons with a total mass of 28 GeV. Currently, no theoretical model predicts the existence of this particle, but scientists hope that this anomaly is not the result of a statistical error. The observation preprint is available in the arXiv.org repository. We will tell you in detail about the study, which can turn out to be both a breakthrough discovery and another puff.
Hellish coil
The Compact Muon Solenoid, or CMS (Compact Muon Solenoid), is a large particle detector located at the Large Hadron Collider (LHC). This giant device with a diameter of 15 meters and a weight of 15 thousand tons is designed to search for New Physics - physics beyond the Standard Model. If the Standard Model describes the properties of all known elementary particles (and some have not yet been confirmed), then hypotheses within the framework of New Physics try to explain various phenomena that still remain a mystery to scientists.
According to one of the hypotheses - supersymmetry - each known elementary particle corresponds to a superpartner with a heavier mass. For example, the partner of the electron, which is the fermion, is the selectron boson, and the partner of the gluon (which is the boson) is the gluino fermion. However, the lack of results to confirm supersymmetry has led to the fact that this model is abandoned by more and more scientists.
Proton-proton collisions take place inside the detector. Each proton is made up of three quarks that are held together by the gluon field. At a high speed, comparable to the speed of light, the gluon field turns into a "soup" of particles - gluons. In a head-on collision of protons, only a few quarks or gluons interact with each other, the rest of the particles fly by unhindered. Reactions take place that produce many short-lived particles, and various CMS detectors record their decay products, including muons. Muons resemble electrons, but 200 times more massive.
With the help of detectors located outside the solenoid, scientists are able to track the trajectories of muons with high accuracy and determine what exactly caused the appearance of a particular particle. A large number of proton-proton collisions are required to increase the chances of producing a rare particle that disintegrates into muons. This generates an astronomical amount of data (about 40 terabytes per second), and in order to quickly find something unusual in them, a special trigger system is used, which decides what information to record.
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The ghost inside
The CMS, together with the ATLAS detector, also located at the LHC, was used to search for the Higgs boson predicted by the Standard Model. This particle is responsible for the mass of the W- and Z-bosons (carriers of the weak interaction) and the lack of mass in the photon and gluon. In 2012, the Higgs boson with a mass of 125 GeV was discovered. However, scientists believe there may be other lower-mass Higgs bosons outside the Standard Model. They are predicted by the two-doublet Higgs model and the NMSSM (next-to-minimal supersymmetric Standard Model). Despite all the experimental tests, scientists have still not been able to prove or disprove these hypotheses.
Scientists at the CMS are looking for other light exotic particles. These include, for example, dark photons - carriers of a completely new fundamental interaction, reminiscent of the electromagnetic, and which are analogous to photons for dark matter. Another hypothetical particle is the dark analogue of the Z-boson.
Physicists have conducted an experiment to find evidence for the existence of a light boson, which is emitted by a pair of pretty quarks (b-quarks) and decays into a muon and an anti-muon. During the experiment in proton-proton collisions at an energy in the center of mass system (a system in which particles have equal and oppositely directed momenta) equal to 8 TeV, a number of events were recorded that are probably associated with a hypothetical boson.
The first type of events includes the appearance of a jet of b-quarks in the center of the detector and its front part, and the second - the appearance of two jets in the center and not a single jet in the front part. In both cases, an excess of the emerging pairs of muons was observed, and the mass of the pairs, as shown by subsequent analysis, reached 28 GeV. The difference in the number of muon pairs from background values for events of the first kind is 4.2 standard deviation (sigma), and for events of the second kind it is 2.9 sigma.
Death of physics
In particle physics, a five sigma difference indicates a certain existence of an anomaly that could not have arisen by chance. However, if the difference lies in the 3-5 sigma range, then physicists say that this only indicates the existence of a new particle. In the latter case, it is necessary to obtain much more data to confirm (or refute) the result, in order to exclude errors in data processing and interpretation. If everything is confirmed, then we can say that muons arise due to the decay of a particle of New Physics.
This is not the first time that a phenomenon has been observed at the LHC that does not fit into the Standard Model. In 2016, physicists announced the discovery of signs of the existence of a resonance corresponding to a massive short-lived particle. It was registered in 2015 as an excess of pairs of photons with a total mass of 750 GeV, into which this particle supposedly decays. In other words, this particle should have been six times more massive than the Higgs boson. However, analysis of the data collected at the collider later did not confirm this result.
Until now, physicists have not found any reliable traces of the existence of New Physics. However, there is no doubt that it should exist, because the Standard Model is not able to explain such phenomena as the problem of the hierarchy of fermion masses (a hypothetical Goldstone boson is introduced to solve it), the existence of mass in neutrinos, the asymmetry of matter and antimatter, the origin of dark energy, and others. The very presence of dark matter in the Universe presupposes a whole class of hypothetical particles with exotic properties that make up it. Paradoxically, all that scientists have been able to do so far is to experimentally confirm the exhausted Standard Model.
Some scientists suggest that if it is possible to prove the New Physics, then this should be done in the very near future, within the next few years. Otherwise, it will be possible to seriously fear that humanity will no longer be able to make significant discoveries. It is encouraging that more and more anomalies have been found on accelerators lately, hinting that scientists are on the verge of something completely new.
Alexander Enikeev