The importance of being Higgs
On 4 July 2012 the CERN Director General, Rolf Dieter Heuer, made an announcement during a seminar that will go down in history: “We have taken a fundamental step forward in our understanding of nature. The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies that require larger statistics and that will allow us to discover the new particle's properties, probably shedding light on the mysteries of the Universe”. Sitting in the audience, listening quietly, there was also Mr Peter Higgs in person, the one who started this epic scientific adventure and who surely did not expect to see it fulfilled during his lifetime. The irony is that in the world of subnuclear physics particles are often evanescent, lasting a very short time, small fractions of a second, while their discovery often takes more than a lifetime. The discovery of the Higgs boson is sensational because it allows us to give a coherent explanation as to why all things have a mass. Without it the Universe would appear to be very different and rather extravagant. In particular, atoms would not exist and there would not be any form of life. We would not have matter, nor stars and planets, nor people who raise their eyes to the skies to admire the wonders of the Universe.
A story that began 60 years ago
The adventure began in 1964. Mr. Higgs came up with his theory or, in his words, with “a great idea” while walking in the Scottish countryside. Back at the laboratory, together with fellow scientists, François Engler and Robert Brou, he postulated the “Higgs field”. In addition to this, he postulated the existence of a new particle in an article that was initially refused by the journal Physics Letters but then accepted and published by Physical Review Letters. In the sixty years that have passed since when the article was first published, physics has taken giant steps forward in the exploration of the realm of the infinitely small. In the Seventies, the quark model, proposed in 1963 by the physicists Murray Gell-Mann and George Zweig, was completed. In 1983, Carlo Rubbia and Simon van der Meer detected the W+- and Z0 bosons at the CERN. In 2007, again at the CERN, the Large Hadron Collider (LHC) started operating. This accelerator is able to reach an energy of 7 Tev (7000 GeV*) per proton beam, thus recreating the conditions necessary for the detection of the Higgs boson.
*The mass of a particle
The mass of a particle is indicated as a multiple of the electronvolt (eV), the unit of measure that is used in the realm of the infinitely small. An electronvolt represents the kinetic energy acquired by a single electron (q) when it is accelerated through a potential difference of 1 volt (V). According to the equation Ecin= qV, it follows that the electronvolt is equivalent to 1.6 x10-19 joules.
The mass that is equivalent to the energy of 1 MeV (mega-electronvolt, equivalent to one million electronvolts) can be written, using Einstein's equation E=Mc2, in the form 1 MeV / c2 = 1.78 x10-27 g (grams); it follows that the electron mass, me, is equal to 0.5 MeV / c2. As far as the proton is concerned, its mass is two thousand times the mass of an electron, i.e. 938.1 MeV/ c2. The mass of the hypothesised Higgs boson observed by the ATLAS and CMS experiments is around 125 GeV/c2.
What is a boson?
Bosons are one of the two fundamental classes into which particles are subdivided. All particles which obey Bose-Einstein statistics and have integer spin are defined bosons. They differ from the other class, the fermions, which have half-integer spin and obey the Pauli exclusion principle according to which only one fermion can occupy a particular quantum state. On the contrary, any number of bosons can occupy the same quantum state.
Photons, gluons, gravitons and mesons are bosons while quarks and leptons, such as electrons and neutrinos, are fermions.
The boson and the Higgs field
“There is, we believe, a wraithlike presence throughout the Universe that is keeping us from understanding the true nature of matter. It is as if something, or someone, wants to prevent us from attaining the ultimate knowledge. The invisible barrier that keeps us from knowing the truth is called the Higgs field. Its icy tentacles reach into every corner of the Universe, and its scientific and philosophic implications raise large goose bumps on the skin of a physicist. The Higgs field works its black magic through a particle. This particle goes by the name of the Higgs boson”. (Leon Lederman, 1988 Nobel Prize in Physics)
The Standard Model is a theory that describes both matter and all the forces of the Universe (excluding gravity). At first glance it appears very simple and refined because it manages to explain the existence of hundreds of particles and complex interactions through just a few particles and fundamental interactions.
It is based on two fundamental ideas. The first claims that there are particles that mediate forces, i.e. any type of fundamental interaction can take place thanks to the mediation of a particular particle. For example, the electromagnetic force is mediated thanks to the exchange of a photon.
The second idea explains the existence of matter through the two most fundamental types of particles, quarks and leptons (the electron, for example). Also, there are two kinds of particles: particles that make up matter (such as electrons, protons, neutrons and quarks) and force-mediating particles (such as photons, gluons, gravitons and bosons).
The Standard Model works well, except for the fact that, before Peter Higgs' “great idea” this theory could not explain how elementary particles gain their mass. Higgs postulated that there could be a field, the Higgs field, that could confer mass on elementary particles. The Higgs field permeates space and makes all particles “gain weight”. Paraphrasing an analogy used by the physicist John Ellis, the Higgs field is like a field of snow as big as the entire Universe. This snow field is made up of snowflakes, little quanta: the Higgs bosons.
Let's imagine a skier crossing this snow field. We see him skimming across the top without interacting with the field, just like a particle with no mass travelling at the speed of light.
If that same skier walked with snow shoes, he would sink into the snow and would move slower, exactly like a particle with mass interacting with the field.
The stronger the interaction a particle has with this field, the greater the mass of the particle.
The Higgs boson hunters
The hypothesised Higgs boson was discovered in the LHC accelerator at CERN in Geneva. But what is an accelerator and how does it work?
A particle accelerator is a machine that can produce beams of ions or subatomic particles (for example, protons) with tremendous kinetic energy. The particles are accelerated using a combination of electrical and magnetic fields. The former supplies the energy to accelerate the particles while the latter causes their trajectory to arc.
One simple rule holds true: the higher the energy, the greater the mass of the particles that can be produced and fewer are the dimensions that can be explored.
In 2007 the LHC accelerated proton beams to an energy of 7 TeV.
The accelerator is housed in a 27 km-long circular tunnel that is located 100 metres underground. Two separate beams of particles are accelerated in opposite directions and made to collide in four points along a ring. Where the beams collide there are caverns which house enormous experimental halls (See the Diagram of the LHC (Large Hadron Collider) accelerator at CERN in Geneva)
Currently in these experimental halls there are four particle detectors: ATLAS (A Toroidal LHC ApparatuS), CMS (Compact Muon Solenoid), LHCb and ALICE (A Large Ion Collider Experiment). Each of these detectors uses different collision detection methods.
The ATLAS and CMS experiments
The CERN physicists have been searching for the Higgs boson in a wide energy range (from 115 GeV to 1 TeV). This is because the Standard Model does not provide indications regarding the mass of the Higgs boson. The lower limit of 115 GeV was already well-known, while the 1 TeV upper limit was chosen because according to the Standard Model, above this value the existence of this particle is highly improbable.
So, when the beams collide, will we observe the production of a boson and be able to measure its mass? Not at all: what can be measured are the products a boson decays into, or rather, the products of one of several possible ways in which a boson can decay.
The ATLAS and CMS experiments have observed the decay of a boson into a pair of photons. However, it is not sufficient to detect two photons to discover a boson. Unfortunately, when protons collide a very large number of pairs of photons are generated, which are not connected to the Higgs boson but which create the so-called 'background'. In other words, it is like looking for a needle in a haystack.
However, the ATLAS e CMS experiments have observed a small 'bump', a signal above this background made by the photons generated by the proton-proton collisions. (See the graph showing the events observed by CMS and ATLAS in the two-photon decay channel)
Moreover, both the detectors also observed the boson decay into four leptons and even in this case there is a good chance that the results obtained were not just background fluctuations.(See the graph showing the events observed by CMS and ATLAS in the 4-lepton decay channel)
We can conclude that something was detected! In fact, the results of both the ATLAS and CMS detectors, measured in different decay channels, show a peak of events around 125 GeV that can be attributed to the presence of a boson. But are we sure that it is a Higgs boson? Couldn't it be something else? An argument in favour of the Higgs boson is that this particle decays into two photons that have spin 1. Since spin is conserved during a particle decay, the particle must have spin 0 or 2 and in either case it should be a boson since it has integer spin. However, only after having analysed more data it will be clear if it is the Higgs boson or a particle that resembles it closely, which would open the door to new theories and modifications of the Standard Model. Whichever of two it turns out to be, it will definitely be the starting point for more research.
Edited by Simona Romaniello
Astrophysicist and science populariser, Ms Romaniello is responsible for formation and development and the installation of museum exhibits for the Turin Planetarium.