Dec 17 2019
During the LHCb experiment at the Large Hadron Collider, one of several products of collisions seen are electrons accompanied by their “colleagues”—other leptons. Theorists say it is possible to create some of these particles in processes that extend beyond standard physics. These predictions have been verified in the latest study.
Dr Jihyun Bhom from the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow against the background of the LHCb detector at CERN. Image Credit: IFJ PAN.
Are the so-far unidentified particles that fall outside the currently well-tested and valid Standard Model hidden by the anomalies seen in the LHCb experiment in the decay of B mesons? To find a solution to this question, physicists are searching not just for further indications of the occurrence of new particles, but also for traces of the phenomena that might exist along with them.
The breaking of the principle of lepton number preservation is one of the processes put forward by theoreticians looking beyond the realm of known physics.
This hypothetical phenomenon was the main interest of an international team of scientists, including representatives of the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow, Technische Universität in Dortmund (TUD), and Centre National de la Recherche Scientifique (CNRS) in Paris.
The researchers specifically focused on the analysis of data gathered in 2011–2012 from proton collisions carried out during the LHCb experiment at the Large Hadron Collider at CERN near Geneva. The outcomes have been reported in the renowned journal Physical Review Letters.
Several decades of experiments and measurements by cosmic ray researchers and nuclear physicists have revealed that particles of matter are categorized into two totally independent families—quarks and leptons (including their anti-matter counterparts). Quarks (up, down, top, bottom, charm, and strange) always occur in groups.
Two-quark systems are termed as mesons, while three-quark systems are called baryons. Protons and neutrons—the particles making up atomic nuclei—belong to the latter type. On the other hand, leptons include electrons, muons, tau particles, and their respective neutrinos.
The properties of leptons and quarks differ fundamentally. As a result, both groups of particles are described using sets of different numbers, called quantum numbers. One of the quantum numbers used to describe leptons is the lepton number. For example, each electron has an electron number of 1. In turn, antimatter counterparts of electrons, i.e. positrons, have an electron number of −1.
Dr Jihyun Bhom, Study Main Author, IFJ PAN
Dr Bhom continued, “That’s how we come to the key phenomenon to explain the meaning of our work. Under the Standard Model, the principle of preserving the lepton number applies. It says that the sum of lepton numbers of particles at the beginning and end of the process must always be the same.”
The need to maintain the lepton number implies that if, for instance, two electrons that have a total electron number of two take part in an interaction, when the process ends, this number will also be two. In this example, according to the Standard Model, two electrons, along with four electrons and two positrons, etc. can be synthesized.
It is possible to divide both leptons and quarks into three groups known as generations. The occurrence of the same number of generations of leptons and quarks motivated theorists to propose that with adequately high energy, leptons and quarks could “weld together” into leptoquarks—hypothetical particles that have the properties of both leptons and quarks.
If leptoquarks did exist, they should be unstable particles that have extremely high masses, equivalent even to the mass of a whole lead nucleus.
In processes involving leptoquarks, lepton numbers do not be preserved. The detection of traces of phenomena where the principle of preserving the lepton number was violated would therefore be a significant step on the road to the detection of particles outside the Standard Model.
Dr Jihyun Bhom, Study Main Author, IFJ PAN
Dr Bhom added “In particular, it would make it easier for us to interpret the nature of the anomalies that have recently been more and more clearly visible in data from the decay of B mesons, i.e. particles containing the down quark and the bottom quark,” says Dr Bhom.
In the most recent statistical analyses, the use of artificial intelligence was observed to be essential—moreover, not just one.
“We were interested in the B meson decays leading to the formation of the K meson, a muon and an electron. However, it just so happens that under the Standard Model, a significant proportion of B meson decays lead to exactly the same products with addition of neutrinos (the latter cannot be recorded),” stated Dr Bhom.
This huge background had to be eliminated very precisely from the collected data. One artificial intelligence was responsible for this task. The second proved necessary to get rid of background residues that passed through the first.
Dr Jihyun Bhom, Study Main Author, IFJ PAN
In spite of using advanced mathematical tools, scientists from IFJ PAN, TUD, and CNRS could not identify traces of phenomena that break the lepton number preservation. But there is a positive side to any problem.
“With a certainty of up to 95% we’ve improved the existing restrictions on the solutions presented by theoreticians to explain the presence of anomalies in the decay of B mesons by a whole order of magnitude. As a result, we are the first to have significantly narrowed the area of searching for theories explaining the existence of these anomalies using new physics,” reiterated Dr Bhom.
If these anomalies do occur, processes breaking the principle of lepton number preservation evidently take place with very less frequency than could be estimated by the widely known extensions of the Standard Model including leptoquarks. Furthermore, anomalies in the B meson decay, as such, do not have to be associated with new particles.
The probability can still not be ruled out that they are artifacts of the mathematical tools employed, measurement methods, or the outcome of not considering certain phenomena existing within presently known physics.
It can only be hoped that further, already launched analyses, which consider the most recent data gathered from the LHC, will eventually eliminate doubts related to the occurrence of physics beyond the Standard Model in the near future.