CERN experiments announce initial evidence of rare Higgs boson decay into muons 

The problem with detecting this decay is that muon pairs are also created in other ways and it is difficult to separate the noise from the signal, explains Prof. Eliezer Rabinowitz from the Rakach Institute of Physics at the Hebrew University and chairman of the High Energy Committee of the Academy of Sciences and the Ministry of Science

Experiments at CERN announce the first indications of a rare decay of the Higgs boson into muons. The problem with detecting this decay is that muon pairs are also created in other ways and it is difficult to separate the noise from the signal, explains Prof. Eliezer Rabinowitz from the Rakah Institute of Physics at the Hebrew University, and the chairman of the High Energy Committee of the National Academy of Sciences and the Ministry of Science and the senior scientific representative of Israel Basran, and the responsible On behalf of Israel for the regional cooperation in the SESAMI accelerator in Jordan.

The 40th ICHEP high energy physics conference was recently held (by digital means, as is customary in the days of the Corona virus). One of the important announcements at the conference was of new findings by the researchers working in the accelerator according to which evidence was discovered for the first time that the Higgs boson decays into two muons. A muon is a heavier double of the electron, one of the elementary particles that make up the matter in the universe. While electrons are classified as first generation particles, muons belong to the second generation. The physics process of a Higgs boson decaying into a pair of muons (a muon and an anti-muon) is a rare phenomenon since only about one Higgs boson in 5000 decays into muons. These findings are of crucial importance to fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

Physicists at CERN have been studying the Higgs boson since it was discovered in 2012 in order to study the properties of this special particle. The Higgs boson, produced by proton collisions at the Large Hadron Collider, decays almost instantly into other particles. One of the main methods for studying the properties of the Higgs boson is the analysis of the way in which it decays into the various elementary particles and the decay rate.

As is well known, two different experiments operate at Saran - large facilities filled with equipment containing many sensors - CMS and Atlas (where the Israeli scientists are also concentrated and where the detectors manufactured in Israel also operate). The CMS experiment obtained evidence of this decay with 3 sigma, which means that the chance of seeing the Higgs boson decay into a pair of muons is less than one in 700. At the Atlas facility, which, according to Prof. It only reached sigma - 2. However, when you take the data from the two experiments over the years and combine them in one big data array, you get a 1:40 chance, a combination that gives a sigma much greater than three, and provides strong evidence of the decay of the Higgs boson into two muons.

The Higgs boson is the quantum expression of the Higgs field, which gives mass to the elementary particles with which it interacts, through the Brot-Englet-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the decay rate for a given particle, the stronger its interaction with the field. So far the Atlas and CMS experiments have observed the decay of the Higgs boson into various types of bosons such as W and Z, and heavier fermions such as the tau lepton. The interaction of the Higgs boson with the heaviest top and bottom quarks was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Thus, interactions between the Higgs boson and muons have not previously been seen at the LHC.

What makes the studies even more challenging is that at the LHC, for every predicted Higgs boson that decays into two muons, there are thousands of muon pairs produced by other processes that mimic the expected experimental signature. The characteristic signature of the decay of a Higgs boson to muons is a small excess of events clustering near the mass of a pair of muons around 125 GeV, which is the mass of the Higgs boson. Isolating the interaction between a Higgs boson and two muons is no easy feat. To this end, both experiments measure the energy, momentum and angles of the candidate particles for the pair of muons from the decay of the Higgs boson. In addition, the sensitivity of the analyzes was improved through machine learning. Atlas scientists divided these events into 20 categories that focused on the possible products of the Higgs decay.

The results, which so far match the predictions of the Standard Model, used the full data set collected from the second run of the LHC. With more data to be recorded from the next run of the particle accelerator and with the LHC High-Luminosity, the Atlas and CMS collaborations scientists expect to reach the 5-sigma sensitivity necessary to establish the discovery of the decay of the Higgs boson into two ions and constrain the possibility of possible theories of physics beyond the model the standard that will affect this state of decay of the Higgs boson.

"According to the standard model, the Higgs particle plays a central role in imparting non-zero mass to many particles, including the electrons, protons and neutrons (and the quarks that make them up). In doing so, it largely determines the nature of the atoms we are made of." Prof. Rabinowitz explains. Sometimes you can find in articles that the Higgs is responsible for all the mass in the universe. That is not accurate. Even if the Higgs particle were shut down or simply did not exist, many particles would acquire a non-zero mass. But without the Higgs, the mass was much smaller and the nature of the atoms was very different from what we see in nature. In the standard model there is an explicit quantitative expression for this qualitative diagnosis. The more mass a particle has, the stronger the Higgs particle sticks to it. One of the experimental ways to confirm this claim is to measure the rate at which the Higgs particle decays into lighter particles. The stronger the Higgs sticks to a particle, the greater its tendency to decay into it. If the degree of attachment is indeed characteristic of the mass of the particle, then it is possible to predict that the Higgs will decay directly into a particle called tau, which is the heaviest in the family of electron complements, than into a muon, which is lighter than the tau but heavier than the electron. From this it can also be concluded that the number of direct decays to the electron, the lightest of all in its family, will be the smallest.

When the giant accelerator and the various detectors were designed, it was expected that the number of decays of the Higgs to tau would be large enough so that with a supreme experimental effort it would be possible to observe this decay. The decline was indeed observed and at the rate predicted by the standard model. Only the most optimistic thought that the experimenters would actually be able to isolate and observe the decay of the Higgs directly into the muons. Thanks to the creativity of the experimental physicists, the evidence for this was almost certainly found, and indeed at the rate expected for a lighter particle. The possibility of detecting an even rarer decay of the Higgs into the lightest particle of the electron family, which is the electron itself, remains very small in the current accelerator"

Along with the successes, Prof. Rabinovitch added that "we would be happy to discover phenomena that do not correspond to the standard model in order to find a tip for new physics, but the verification of the model is also of great importance and a symbol of the victory of human knowledge."

The Israelis in the field

And Prof. Giora Mikenberg adds: This is a preliminary finding, which still needs to be verified, but if it is true, it strengthens the proof of the connection between the Higgs boson and the mass of the particles. First they managed to prove this on very heavy particles, and now also on lighter particles such as the muons. The problem with the experiment is that the background noise is very loud. That's why it took a long time before they were able to separate the two. We were able to do this because we collected data from a huge number of collisions, which allowed us to see this. Both CMS and Atlas are very sophisticated experiments. Atlas, the device that detects the muonoids is a facility that I was in charge of for nine years. The experiments look at slightly different things. CMS, for example, has a magnetic field twice as strong as that of Atlas, so the emphasis is on ions that have medium energy. Atlas discovers muons with very high energy and therefore Atlas is less good when it comes to the Higgs boson. In the case of the spectrometer used to detect ions in the Atlas, we are talking about a monster of 45 x 25 x 25 meters with 24 magnets on conductors (parts of which were built in Israel AB). We know where each and every detector is to a hair's breadth. The parts of the machine that are constantly exposed to high energies, function at a level of 99%. This is thanks to engineers who dedicate their whole lives to maintain them. "

The Technion has a senior postdoctoral fellow whose job it is to repair defective detectors made in Israel.

Prof. Ehud Duchovani, also from the Weizmann Institute, adds: "The Technion team led by Prof. Shlomit has not yet made a significant contribution to the identification and reconstruction of muons. The sensors transmit an electrical signal when a particle hits them. According to the heat map and other clues, it is recognized whether the particle that hit is a muon, which is a complicated step. The next step: identifying the momentum of the particle is much more challenging."

The Technion has a senior postdoctoral fellow whose job it is to repair defective detectors made in Israel.

More of the topic in Hayadan:

• Prof. Francois Engler, winner of the Nobel Prize in Physics (and Prof. Higgs' prize partner) is also a researcher at the Tel Aviv School of Physics * Prof. Rabinovitch's response from the Hebrew on the Nobel: Justice has been done
• The founders of SESAME, including Prof. Eliezer Rabinowitz, won the Science Diplomacy Award of the AAAS
• CERN tour notes, part 3: Will we only sail if we know there's an America across the ocean? Prof. Eliezer Rabinovitch from the Hebrew University on the importance of basic science (Gallery - LHC)