What gives some particles mass?

A major breakthrough in particle physics came in the 1970s when physicists realized that there are very close ties between two of the four fundamental forces – namely, the weak force and the electromagnetic force. The two forces can be described within the same theory, which, along with Quantum Chromodynamics (a theory of the strong force), form the basis of the Standard Model. This ‘unification’ implies that electricity, magnetism, light and some types of radioactivity are all manifestations of a single underlying force called, unsurprisingly, the electroweak force. But in order for this unification to work mathematically, it required the force-carrying particles to have no mass. We know from experiments that this is not true, so physicist Peter Higgs and others came up with a solution to solve this conundrum.

They suggested that all particles were massless just after the Big Bang. As the Universe cooled and the temperature fell below a critical value, an invisible force field called the ‘Higgs field’ was formed together with the associated ‘Higgs boson’. The field prevails throughout the cosmos: any particles that interact with it are given a mass via the Higgs boson. The more they interact, the heavier they become, whereas particles that never interact, like the photon, are left with no mass at all.

This idea provided a satisfactory solution and fitted well with established theories and phenomena. The problem is that no one has ever observed the Higgs boson in an experiment to confirm the theory. Finding this particle would give an insight into why particles have specific masses, and help to develop subsequent physics. The technical problem is that we do not know the mass of the Higgs boson itself, which makes it more difficult to identify. Physicists have to look for it by systematically searching a range of mass within which it is predicted to exist. The as yet unexplored range is accessible using the Large Hadron Collider, which will determine the existence of the Higgs boson. If it turns out that we cannot find it, this will leave the field wide open for physicists to develop a completely new theory to explain the origin of particle mass.

Finding the Higgs at CMS

Finding the Higgs boson through experiment would prove that the Higgs field exists, and this is one of the main goals of the CMS experiment.

If the Higgs boson is formed in an LHC collision, it will be relatively heavy and will decay very quickly to other particles. What it decays into, the telltale ‘signature’ it leaves behind, will depend on its mass. From previous LEP experiments, we know that the Higgs boson must be heavier than around 100GeV (where GeV is a unit of energy or mass used in particle physics – a proton has a mass of about 1GeV), but since the theory does not say exactly how massive it is, any experiment must look carefully at a range of energies.

Different signatures are more or less likely within the different energy ranges and CMS is designed to find the most easily identifiably signatures within each range.

If the Higgs boson is relatively light, below about 140 GeV, it is likely to be identified first by its decay into two photons, detected in the electromagnetic calorimeter. In the first image we see the detector head-on with the electromagnetic calorimeter (ECAL) in green. Green tracks show these photons, whilst the blue tracks show other particles emerging from the collision with the red marks showing hits in the hadron calorimeter (HCAL). The ECAL is able to tell the mass of the particle to better than 1% in this range.

The most distinctive signature of the Higgs in the range from around 150 GeV to 180 GeV is the decay to two W bosons, which then decay into two leptons and two neutrinos. Detecting leptons requires excellent performance not only from the ECAL but also from the muon chambers and tracking detectors. Neutrinos however are neutral, weakly interacting particles that the detector cannot stop or detect. But still their presence can be inferred. If we see particles from a single collision shoot out one side of the detector but not the other, we can add up the energies of all the emerging particles and deduce if some is “missing”. With a “hermetic” hadron calorimeter that totally surrounds the collision point and stops all detectable particles, we can infer that the “missing energy” must be due to an “invisible” particle, in this case, a neutrino.

Another possible signature, that is likely from 140 GeV and up to 600 GeV and above, is the decay into two Z bosons, which in turn decay into four leptons, particles like electrons or muons.

A typical event where each Z has produced two muons is shown below. In this zoomed-out image the muons chambers are marked in red and the electromagnetic calorimeter is the green ring near the centre.

Finally, if the Higgs boson is very heavy, with a mass in excess of 500 GeV, there are yet more ways in which it might decay that become useful in trying to detect it. One possibility is shown here, where the Higgs decays into two Z bosons, which in this example then decay into two electrons and two quarks. The electrons produce clean tracks whilst the quarks produce sprays of particles known as jets.

Supersymmetric models suggest that there are in fact five or more new Higgs bosons to be found. These would have a range of signatures with some of the more complicated signatures becoming more probable, for instance the decay to specific types of short-lived quarks, which we would see through the particle jets they produce.

Why haven’t we found the Higgs before?

CMS has a range of sub-detectors to identify and measure all the possible signatures. But this is no easy task and the Higgs boson has remained hidden until now because no one has been able to produce the required combination of a high-energy, high-intensity accelerator with detectors sharp enough to spot it. The particles produced will be in very small, specific bands of energy within a sea of other particles, and if our measurements are not accurate enough, and if we do not produce enough of them, then a peak in a tiny range will be averaged out and lost in all the background noise. But the LHC can produce these particles at a significant rate and as CMS is able to measure energy and momentum very precisely, if the Higgs boson is there, we will find it. If no LHC experiment finds the elusive particle, physicists will have to think again about how the world is given its substance.