Why have we tried so hard to find the Higgs particle? How does the Higgs mechanism work? What is the difference in physics between strong evidence and a discovery? Why do physicists speak in terms of "sigmas"? Find out here!
Why have we tried so hard to find the Higgs particle?
Because it could be the answer to the question: how does Nature decide whether or not to assign mass to particles?
All the fundamental particles making up matter – the electron, the quarks, etc. – have masses. Moreover, quantum physics requires that forces are also carried by particles. The W and Z particles that carry the weak force responsible for radioactivity must also have masses, whereas the photon, the carrier of the electromagnetic force, has no mass at all. This is the root of the “Higgs problem”: how to give masses to the fundamental particles and break the symmetry between the massive W and Z and the massless photon? Just assigning masses by hand leads to an inconsistent theory and nonsensical predictions. Nature must therefore have a way of correcting this inconsistency, and the mechanism proposed by Englert, Brout and Higgs could be the answer.
How does the Higgs mechanism work?
According to the Englert-Brout-Higgs mechanism, the property that we measure as the ‘mass’ of a particle is the result of a constant interaction with a field that permeates the Universe like a sort of “ether”. The existence of this Englert-Brout-Higgs field is definitively proven by the discovery of the corresponding quantum particle - the Higgs boson.
Originally, the Englert-Brout-Higgs mechanism was put forward to explain why one of Nature’s fundamental forces has a very short range, whereas another similar force has an infinite range. The forces in question are the electromagnetic force (infinite range) – which carries light to us from the stars, drives electricity around our homes, and holds together the atoms and molecules from which we are all made – and the weak force (very short range), which is responsible for radioactivity and drives the energy-generating processes of the stars. Today we know that the electromagnetic force is carried by particles called photons, which have no mass, whereas the weak force is carried by particles called W and Z, which do have mass. Rather like people passing a ball, interacting particles exchange these force carriers. The heavier the ball, the shorter the distance it can be thrown – and the heavier the force carrier, the shorter its range. The W and Z particles were discovered in a Nobel prize winning enterprise at CERN in the 1980s, but the mechanism that gives rise to their mass had not yet been understood, and that’s where the Higgs boson comes in.
The Englert-Brout-Higgs mechanism in its basic form is the simplest theoretical model that could account for the mass difference between photons and the W and Z particles, and by extension could account for the masses of other fundamental particles. The presence of the Englert-Brout-Higgs field enables these forces to cohabit a single unified electroweak theory.
It should not be thought that the Englert-Brout-Higgs field is responsible for all the mass in the Universe. Your interaction with the field actually contributes less than 1 kg to your mass. The remainder comes mainly from the strong force binding quarks inside nucleons, with a tiny contribution from the electromagnetic force that reigns over the atomic and molecular scales.
Higgs bosons are quantum fluctuations in the Englert-Brout-Higgs field that are visible experimentally only when energy is “injected” into the field. Concentrating the right amount of energy in proton-proton collisions at the LHC excites the Englert-Brout-Higgs field, which resonates at a precise energy corresponding to the mass of the Higgs boson. The Higgs boson appears momentarily before decaying into other particles that the LHC experiments can measure. Some theories predict the existence of multiple Higgs bosons.
Is the Higgs boson the only possible answer to the “mass problem”?
No, there are other theories that predict the existence of different mechanisms to explain how Nature deals with the mass problem. For example, there are rival theories that suggest the existence of extra dimensions of space.
Also, despite the fact that we see strong evidence of its existence, we do not yet know whether the Higgs boson is an elementary particle as postulated in the Standard Model, or some more complex object. Nor do we know whether there is only one Higgs boson or if there are more of them. Further studies and analysis will have to be carried out to reply to these questions.
Why is it called the “God particle”?
The term was coined for Leon Lederman's popular science book on particle physics: “The God Particle: If the Universe Is the Answer, What Is the Question?”
Is Peter Higgs the only theorist who proposed this mechanism as a solution to the “mass conundrum”?
No. In 1964 independently and almost simultaneously, The theory of the Higgs field was proposed by three groups of physicists: François Englert and Robert Brout, Peter Higgs, and Gerald Guralnik, C. R. Hagen, and Tom Kibble. However, Peter Higgs was the only one of these who pointed out explicitly the existence of the particle that bears his name and calculated some of its properties.
What is the difference in physics between “strong evidence” and a discovery? Why do physicists speak in terms of “sigmas”?
The Higgs boson cannot be observed directly because its lifetime is too short for our apparatus. At the end of its life, the boson decays and transforms into other particles, and the detectors may detect these decay products. As an example, one of the ways a Higgs particle can decay is into two photons, which can then be detected. However, there are many other processes that also produce two photons, so researchers compare the number of so-called “two-photon events” measured with the number expected from known processes. They do this for all the possible decay modes, and only when they see a statistically significant excess of events can scientists claim a discovery.
In particle physics, people talk of 95% confidence levels, which means that a given signal, such as that for a Higgs particle decaying to two photons, has only a 5% chance of being due to a statistical fluctuation. However, 95% confidence is not enough to claim a discovery. For that, the probability of a statistical fluctuation being responsible for the measurement has to be much smaller, less than one in a million. This is what physicists call a five-sigma effect. It is considered the gold standard for significance; six sigmas correspond to one chance in half a billion that the result is a random fluke.
Why did it take so long to come to such a result?
First of all, accelerators have to be powerful enough to produce the high-energy collisions that allow any given particle to be created. The lowest energy that you need in a collision in order to create a given particle is the mass of the particle itself. However, the particle you are looking for might be produced together with other particles, in which case a higher collision energy would be needed.
In a proton-proton collider such as the LHC, the physics processes are such that the probability to produce a Higgs boson increases considerably when the energy collision is increased. As an example, the Higgs boson production rate in 2011 – when the LHC was operated at 3.5 TeV per beam – was about 27% less than the production rate in 2012, when the LHC is being operated at 4 TeV per beam.
In general, the processes associated with the observation of the Higgs boson are very rare, and therefore statistics come into play. The statistical error, i.e., the expected range of statistical fluctuations, goes down as the inverse of the square root of the data sample size. For example, to halve the error bar you must quadruple the data sample. This is why physicists always try to collect more data: to reduce the size of possible statistical fluctuations.
One might think that, once the analysis is defined, it is just a matter of passing all the newly accumulated data through those selection criteria in order to extract the type of events we want to study. However, producing new results requires an incredible number of checks and cross-checks.
The analysis technique works as follows: a theoretical model is used to predict what phenomena and particles might be seen, and experimental physicists estimate what their detector response would be to such events, using complex simulation methods. They do this first for all known processes, so that they can predict the various expected types of events that will come out of the LHC. These simulated events look just like the events collected in the detectors, except they are generated using all our knowledge of what can be produced when protons collide in the LHC.
Then the experimentalists determine a series of criteria for selecting new physics, partly defined using simulations. The selection criteria are designed for the sole purpose of spotting a needle in a field full of haystacks. For this, physicists study in detail the characteristics of possible interesting events (such as the Higgs boson), comparing these characteristics with those of known processes. At this stage, the name of the game is to isolate the signal from all other types of events, which physicists refer to as background. Most of the time, the background constitutes the bulk of all collected events.
The final step is to compare the simulations of the known processes that survive the selection criteria to the collected data set. In some cases, comparison with simulations might not be necessary, and physicists may just need to subtract potential Higgs signals from the background directly inferred from the actual data.
The more data are collected, the more precise these comparisons get, making the result more significant. In the end, the goal is to produce absolutely trustworthy results, excluding flaws, bugs and oversights.
What are the next steps?
The data recorded so far in 2012 have not been completely analysed, and the LHC is still taking data. Further analysis is needed and ongoing. Despite the strong evidence for its existence, the properties of the Higgs boson need to be explored and understood.
As the particle is identified and studied more completely, the physics models will have to be updated (also read Question 9).
In the meantime, the LHC will continue its scientific programme of which the Higgs is only one item. By exploring the world of infinitely small particles, physicists hope to provide answers to the origin and fate of our universe. What happened just after the Big Bang? Why did matter dominate over anti-matter when, in laboratory settings, they are created in equal amounts? Finding out what dark matter is made of is certainly high on the LHC agenda, even if popular models such as supersymmetry have not manifested themselves yet, despite all our attempts at unveiling them. What would you say if you found out we do not live in a four-dimensional world (three dimensions of space and one of time), but rather one containing extra hidden dimensions? There are enough strange, puzzling questions and even stranger possible answers to blow your mind!
In particle physics as in other research fields, scientists will continue to study how the Universe works. With the Higgs, the Universe has disclosed just one of its numerous mysteries.
What is the impact of such a Higgs boson on the current description we use for the Universe?
The Higgs boson will complete our description of the visible matter in the Universe, and of the fundamental processes governing the Big Bang since it was a trillionth of a second old. The Higgs boson may have played a role in generating the matter in the Universe, and may be linked to dark matter. It may even provide a clue how the Universe inflated to its present size. On the other hand, the Higgs boson is a very different particle from the others we know, and poses almost as many questions as it answers. For example, what determines the mass of the Higgs boson and the density of dark energy? According to conventional ideas, both should be much larger than their observed values. The quest continues.