TAU RESEARCHERS FURTHER UNDERSTANDING OF ‘GOD PARTICLE’ AT CERN

Researchers from Tel Aviv University have found new insights into the behavior of the Higgs boson particle (commonly known as the “God Particle”) in a new study, the university announced on Sunday.

The Higgs boson is a particle that is theorized to be responsible for allowing particles to clump together to form stars, planets, and other bodies. The researchers are investigating the decay of the Higgs boson into a pair of elementary particles called charm quarks.

The study was conducted as part of the ATLAS experiment at the Large Hadron Collider (LHC) at the CERN research center by Prof. Erez Etzion and doctoral students Guy Koren, Hadar Cohen, and David Reikher from the Raymond and Beverly Sackler School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, at Tel Aviv University. Prof. Eilam Gross from the Weizmann Institute of Science collaborated with the research team.

The “charm” is one of the six “flavors” or types of quarks in the Standard Model of particle physics. Quarks are split into three different “generations.” The first generation contains quarks with the smallest masses: “up” and “down.” The second generation, with greater masses, contains the “charm” and “strange” quarks. The third generation contains the heaviest ones, the “top” (truth) and “bottom” (beauty) quarks.

The Higgs boson is a relatively heavy elementary particle and can be created in collisions between protons, as long as the accelerator’s energy is high enough. “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 in a press release. “To help answer that question, our group is trying to measure the rate at which the Higgs boson decays into particles called ‘charm quarks.’”

Koren stressed that this isn’t a simple mission. “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,” said Koren. “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.”

The researchers have not yet identified enough decays of the Higgs bosons into charm quarks to measure the rate of the process with the required statistical accuracy, but have found sufficient data to state what the maximal rate of the process is with respect to the theoretical predictions.

The gold standard of particle physics is five standard deviations, also known as five sigma, meaning there is about a 1 in 3.5 million chance that the measurement is a statistical coincidence.

If the rate of decay is found to be higher than the predicted rate, it could constitute an important indicator for “new” physics or expansions of the Standard Model. The researchers have concluded with a well-defined statistical certainty that there is “no chance” that the rate of decay is higher than 8.5 times the theoretical predictions, as enough such decays would have been observed to measure it if this was the case.

“This might not sound like such an exciting declaration, 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,” said Koren.

Etzion explained in the press release that the Higgs boson’s rate of decay is predicted to be proportional to the mass (squared) of the particles into which it decays. “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.”

The results the team found confirm this prediction, according to Etzion, with enough Higgs decays into the heavy third-generation quarks 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,” added Etzion.

“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,” said Etzion. “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.”

The new study is the latest in a series of groundbreaking research which has been published at CERN in recent months.

In July, the Large Hadron Collider beauty (LHCb) experiment at CERN presented the discovery of a new particle which is the longest-lived exotic matter ever discovered, labeled as Tcc+, a tetraquark, an exotic hadron containing two quarks and two antiquarks. The particle is also the first to contain two heavy quarks and two light antiquarks.

Hadrons are formed from quarks. Tcc+ contains two charm quarks and an up and a down antiquark. Charm quarks (second-generation quarks) are heavier than up and down quarks (first generation quarks). This is called “double open charm.” While particles with a charm quark and a charm antiquark have a charm quantum number that adds up to zero (known as “hidden charm), this particle has a charm quantum number that adds up to two.

The discovery of the new particle paves the way for the search for heavier particles of the same type, with one or two charm quarks replaced by bottom quarks, which could have a much longer lifetime than any previously observed exotic hadron.

In March, physicists from the Universities of Cambridge, Bristol, and Imperial College London taking part in the LHCb experiment at CERN published a paper stating that data from the LHC suggested a violation of the Standard Model, which may point to the existence of new particles or a new force of nature. The paper has not yet been peer-reviewed.

The scientists found evidence that “beauty” quarks do not decay in the way they should following the Standard Model.

Beauty quarks, particles similar to but heavier than electrons, interact with all forces in the same way, so they should decay into muons and electrons at the same rate.

However, the data collected by the LHCb seems to show that these quarks are decaying into muons less often than they decay into electrons, which should only be possible if unknown particles are interfering and making them more likely to decay into electrons.

While the Standard Model doesn’t explain about 95% of what the universe is made of, it is the current central theory of particle physics. If the results are confirmed further, it could open a whole new area of physics to discover.

The LHC is the world’s largest and most powerful particle accelerator, measuring 27-kilometers long. Two high-energy particle beams travel at close to the speed of light inside the accelerator until they collide, forming new particles and allowing physicists to study particles that are unstable and cannot be directly observed.