A decade of Higgs: what have we discovered, what is hidden and why is particle physics not dead?

The new launch of the hadron accelerator in Geneva marks a decade since the discovery of the Higgs and opens a new decade of equally fascinating research. In this article we will review what we discovered and what questions remain open.

About a decade ago, on the Fourth of July 2012, scientists around the world celebrated the discovery of the Higgs, a boson particle that was then considered the missing element in the standard model. The elementary particle was predicted in a series of papers in the sixties and was discovered after four decades in the Large Hadron Collider in two parallel experiments - ATLAS and CMS. The discovery is the pinnacle of success for the European Center for Particle Research, which also included Israel at the time as an observer country (today as a full member of the organization).

And despite the successes, while the accelerator begins the third run (out of five runs), voices are heard in the scientific community that this event signifies its end. The reason? In the last decade, no discovery was announced that deviated from the standard model (we should point out that some interesting discoveries were published in recent years, but they did not meet the standards that define a "discovery" in particle physics, i.e. five sigma certainty). To remind you, the model developed in the fifties and completed in the seventies does not include quantum gravity, dark matter, dark energy, it does not explain the asymmetry between matter and antimatter and neutrino oscillations.

The high expectations from the accelerator are due to the natural tendency to add new particles to explain deviations from the standard model - some of them are even in the energy range of the existing accelerators, but they have not yet been observed. Due to the lack of experimental clues, critics argue that there is no reason to waste billions of dollars more of the public's money on blind exploration without a clear goal. Of course this is an understandable criticism, but at the same time it should be remembered that particle physics does not focus only on the discovery of new particles, but also on the study of the behavior of existing particles, including the Higgs boson. Since its discovery in 2012, physicists have debated whether the new particle does match the predictions of the Standard Model. The celebrations of the decade for the discovery of the Higgs open up a good opportunity for us to summarize the five key discoveries in the field and the five questions that remain open.

The Higgs mass is 125 billion electron volts

Physicists expected to find the Higgs sooner or later, but did not know exactly when. In the 2009s, researchers hypothesized that the Higgs field could explain why the photon (the particle of light) has no mass or why the W and Z bosons, which mediate the weak force, are so heavy. Although the proposed theory predicts the mass of the bosons, it is unable to explain the Higgs mass. For this reason physicists were not sure where the Higgs was hiding. To the surprise of many, the Higgs appeared earlier than expected - already in the first run in XNUMX, evidence of the elusive particle began to accumulate and three years later it was announced as a discovery. The speed at which the Higgs was announced is mainly due to the accelerator's sensitivity to the energy range in which the particle is found and the accelerator's ability to detect its many decays.

Higgs spin zero

The Higgs is the first elementary particle with zero spin to be discovered, and in fact the only one. The rest of the particles in nature have half spin or one spin. Spin is a quantum property that has no classical analogue. Mostly particles with half spin are likened to small magnets but of course the similarity between the two is not exact and cannot be generalized for particles with full spin. For us, spin can be thought of as a "charge" like an electric charge that any particle can carry. Charges in nature affect the interactions between the particles and the products that can be formed from their dispersion. At the time of its discovery in 2012, the Higgs spin was not verified and remained a mystery. In 2013, researchers examined the distribution of photons emitted from the decays of the Higgs particle and confirmed with high certainty that the new boson has zero spin. Before the spin was verified, researchers hesitated to name the new particle the Higgs boson because the theoretical mechanism requires a particle with zero spin.

Higgs rejected theories that extend the standard model

Some of the theories trying to explain the asymmetry between matter and antimatter were not consistent with the Higgs mass. In fact, there are quite a few theories that the Higgs mass places them in the gray area. For these reasons it is difficult to disprove or prove which theories extend the standard model and which theories are inconsistent with nature based on the mass of the particle alone.

The Higgs reacts with other particles according to the standard model

Based on the decay rate, it is possible to estimate the strength of the interaction of the Higgs with the rest of nature's particles. In other words, physicists count how many times the Higgs decayed into one or another particle and from the distribution they deduced that the Higgs decays into heavier particles more frequently. That is, the mass is proportional to the strength of the interaction between the particles, just as predicted in the standard model about 60 years ago.

The universe is stable, more or less

Calculations performed in the past showed that the Higgs field is not necessarily at its global minimum value. That is, in the unknown future it is possible that the Higgs field will become energetic and change the structure of the universe. The reason for this is that its minimum value determines the mass of the particles and their interaction with the Higgs field. A change in the strength of the interaction or the mass of the particles may trigger a catastrophic chain reaction. In the scientific literature this case is referred to as "vacuum decay". The decay does not happen immediately, but spreads like a bubble at a tremendous speed. Fortunately for us, in order for such a change to happen immediately, enormous energy is required, the probability of which will be created in the universe is zero by any measure.

What questions remain open?

Can we measure the Higgs with higher precision?

The last runs showed a match of about 90 percent in the properties of the Higgs with respect to the standard model. Although it is a high accuracy, an error of 10 percent is not negligible and is not sensitive to corrections of the model. The new runs should increase the number of collisions and thus increase the accuracy of the measurements. So far, the accelerator has collected only five percent of the data it is expected to collect throughout its lifetime, so the accuracy should improve significantly in the coming years. The Saren researchers claim that new physics is more likely to be discovered through precise measurements than from a new particle found in an experiment.

Does the Higgs react to light particles?

So far we have observed the interactions of the Higgs with heavy particles, for example the decay of the Higgs into a heavy quark. Physicists believe that the Higgs should decay similarly to lighter quarks. In 2020 it was announced that the accelerator detected a relatively rare decay of the Higgs into muons, a cousin of the electron, similar in properties but heavier. Although the experiment confirms the connection between the mass of the particle and the strength of the interaction with the Higgs field, the uncertainty is still great.

Does the Higgs react to itself?

Since the Higgs mass is different from zero, the particle should interact with itself. The decays of energetic Higgs particles into less energetic Higgs particles are extremely rare and difficult to detect. At the moment we do not expect to see such interactions, perhaps in the fourth run in 2026 after a further upgrade in the ATLAS and CMS experiments. The self-interactions are very important for the structure of the universe - from the rate at which the Higgs reacts to itself, physicists can deduce how the potential energy of the Higgs changes at the minimum point where it is. This discovery may explain the dynamics of the early universe and possibly also the asymmetry between matter and antimatter.

The lifetime of the Higgs boson

Physicists are interested in the lifetime of the Higgs, that is, how long it takes on average for the Higgs to decay from the moment of its formation. The reason this parameter can be interesting is because a deviation from its expected value may indicate the presence of new particles. The method for calculating the life time takes into account the energy distribution of the Higgs in all the collisions created in the accelerator, where the width of the energy distribution is proportional to the life time of the particle. Last year the life time calculated is 2.1 tenths of a billionth billionth billionth second. The result that was discovered corresponds to the standard model, but perhaps in the near future, deviations from this result will be discovered that may hint at particles that cannot be directly detected, such as dark matter.

Exotic prophecies

Theorists involved in expanding the Standard Model have previously proposed several corrections to the Higgs - some hypothesize that the Higgs is not an elementary particle but is made up of smaller bodies, just as the proton is made up of quarks. Others claim that there are other particles similar to the Higgs but with a different spin or electric charge. In the following lectures, the great accelerator will examine these hypotheses and also parallel disintegrations that are forbidden according to the standard model.

The latest upgrade accelerates the exploration of the Higgs into new regions. As I have already mentioned, particle physics is not only based on the direct discovery of new particles, but they are also very important.

The article is based on a published article in nature

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• CERN tour notes, part 21: Prof. Elam Gross: The discovery of the Higgs is the beginning of the new science of the XNUMXst century
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