The Higgs boson is an elementary particle in the Standard Model of particle physics produced by the quantum excitation of the Higgs field – one of the fields in particle physics theory. First observed a decade ago, it has zero spin, no electric charge and no color charge and is unstable, decaying into other particles almost immediately.

In the mainstream media, the Higgs boson has often been called the “God particle” from the 1993 book The God Particle by the late Nobel laureate Prof. Leon Lederman, an American experimental physicist who received the prize in 1988 along with two others for research on neutrinos. 

Researchers from Tel Aviv University (TAU) have now succeeded, for the first time, in describing a rare physical process starting with the Higgs boson and ending in decay into a pair of rare elementary particles called “charm quarks.” New observations from the particle accelerator in Switzerland helped the Israeli researchers to better understand the process. 

What actually are quarks? They are particles of a specific type with similar features. They compound, for instance, the protons and neutrons, which are in the nuclei of atoms. There are six different types of quarks, which are usually attributed to three different “generations,” with each generation containing a pair of different quarks. The first generation contains the quarks with the smallest masses, called “up” and “down”. The second generation contains the “strange” and “charm” quarks, which have greater masses, and the third generation contains the heaviest quarks, called “top” and “bottom,” the former being 175 times heavier than the proton.

For the last several decades, the Higgs boson captivated the physics community all around the world. Since its discovery in the particle accelerator, there has been a fascinating journey to find “new physics” relating to it and its secrets.

One of the paths in this journey is being led by a team of researchers from TAU. In this study, the researchers found that the rate of this decay process could be characterized more precisely and completely than what had been known until now.

The particle is named after physicist Peter Higgs, who in 1964 along with five other scientists proposed the Higgs mechanism to explain why some particles have mass. This mechanism demanded that a spinless particle known as a boson should exist with properties as described by the Higgs Mechanism theory. This particle was called the Higgs boson. 

A subatomic particle with the expected properties was discovered in 2012 by the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN near Geneva. The new particle was subsequently confirmed to match the expected properties of a Higgs boson.

Only in 2012 was it discovered in an experiment in the LHC that the Higgs boson is a real, measurable particle, and it is quite heavy when compared to other known particles. The Israeli researchers were senior partners in this discovery, and Higgs and François Englert won the Nobel Prize the following year.

In December 2013, Higgs and Englert were awarded the Nobel Prize in Physics for their theoretical predictions. Although Higgs’s name has come to be associated with this theory (the Higgs Mechanism), several researchers between about 1960 and 1972 independently developed different parts of it.

Evidence of the Higgs field and its properties has been extremely significant for many reasons. The importance of the Higgs boson is largely that it is able to be examined using existing knowledge and experimental technology, as a way to confirm and study the entire Higgs field theory; proof that the Higgs field and boson do not exist would have also been significant.

The study was conducted as part of the ATLAS experiment at the Large Hadron Collider (LHC) at CERN (Geneva) by Prof. Erez Etzion and doctoral students Guy Koren, Hadar Cohen and David Reikher from TAU’s Sackler School of Physics and AstronomyIt was conducted in collaboration with the research team of Prof. Eilam Gross from the Weizmann Institute of Science in Rehovot and others.

In the standard model of particle physics, there are particles called bosons, and their role is to be “force carriers”: the most well known is the photon – a particle that carries the electromagnetic force. Other bosons carry less-known forces such as the strong force and the weak force. Over half-a-century ago, Higgs and Prof. Francois Englert (who since 1984 has been a Sackler Fellow by special appointment in the TAU School of Physics and Astronomy), estimated that a new particle might exist whose field “provides the mass” to the elementary particles in our world.

In the particle accelerator, pairs of protons are made to collide with each other at extremely high velocities. In such energetic collisions, various interesting processes can occur, from which, one can learn about the nature of our universe. The way in which these processes are investigated is by means of a complex array of particle detectors placed around the point of collision, enabling reconstruction of the types of particles that are generated during the collision, as well as their features.

A vast range of processes can occur during the collisions, and each has its own unique “signature” in the detector. In order to extract rare events and from them to acquire new insights about the elementary particles and forces in nature, large amounts of statistical data must be collected (a very large number of collisions must be observed).

As said, the Higgs boson is a relatively heavy elementary particle, but it can be created in collision between protons, as long as the accelerator’s energy is high enough. Immediately after its creation, it decays into lighter particles. “It is interesting to investigate into which types of particles the Higgs decays, and with what frequency it decays into each type of particle,” said Koren. “To help answer that question, our group is trying to measure the rate at which the Higgs boson decays into particles called ‘charm quarks.’ ”

“The point is that this is not a simple mission at all, for two main reasons,” added Koren. “It is a very rare process – only one out of billions of collisions end with the creation of Higgs bosons, and only three percent of the Higgs bosons that do emerge will decay into charm quarks. Moreover, there are five other types of quarks, and the problem is that all of them leave similar signatures in our detectors. So that even when this process does indeed take place, it is very difficult for us to identify it”

From all the collisions that have been collected since 2012, the TAU group has not yet identified enough decays of Higgs bosons into charm quarks to measure the rate of the process with the required statistical accuracy.

Nevertheless, sufficient data has been accumulated to state what the maximal rate of the process is with respect to the theoretical predictions. A rate of decay higher than the predicted rate would constitute a first important indicator for “new” physics or expansion of the currently accepted model – the standard model of elementary particles. 

From the current measurement, the researchers conclude that there is no chance that the rate of decay of the Higgs boson into charm quark is 8.5 (or more) times higher than the theoretical predictions, otherwise enough such decays would have been observed in order to measure it. “This might not sound like such an exciting declaration,” says Koren modestly, “but this is the first time that anyone has ever succeeded in saying something important about the rate of this specific decay based on a direct measurement of it, therefore it is a very important and significant statement in our field.”

“The theory predicts that the Higgs boson’s rate of decay of the into the different particles will be proportional to the mass (squared) of the particles into which it decays,” added Etzion. “Therefore, we expect that in most cases it will decay into the heavier particles (lighter than the Higgs boson), and only rarely will it decay into the light ones. 

Meanwhile, the results from the accelerator confirm this hypothesis – that enough Higgs decays into the heavy third-generation quarks (as well as other heavy particles) were observed in order to verify their existence and measure their rate. The rate does indeed correspond to the theoretical predictions, but the game is not over, as Higgs decays into second (or first) generation quarks have not yet been observed. And, therefore, we cannot yet be sure that the same ‘rules’ apply to quarks from those generations.”

Etzion also explained the potential effects of future discoveries in this context: “If we suddenly discover that the Higgs boson decays into them at a rate that is not proportional to the square of their mass, there could be far-reaching implications for our understating of the universe, and in particular about the way in which elementary particles get their mass. This is also the reason why we are investing such great efforts to characterize the decay of Higgs bosons into charm quarks – this is the heaviest quark into which the rate of decay has not yet been measured.”