Physics: The discovery of the Higgs Boson

Note: The following article was written by William Fawcett L6C (20FawcettW@students.watfordboys.org)

What is the Higgs Boson?

In the early 1960s, physicists developed the Standard Model of Particle Physics which sought to explain what the elementary particles - particles that cannot be broken up into constituent parts - were and how they interacted to explain the forces we see in life, successfully explaining most experimental results. However, it couldn’t explain why some particles had mass and others didn’t. A theory proposed by Peter Higgs, Robert Brout and François Englert in 1964 stated that there was a field throughout all space which gave particles mass when they interacted with it, called the Higgs Field. This was developed from the Quantum Field Theory which states that all particles are fluctuations in fields that span across all space, explaining why particles behave like waves. The discovery of the Higgs Boson in 2012 at CERN gave proof of the Higgs Field, finally completing the Standard Model of Particle Physics.

How was it discovered?

The Higgs Boson only lasts for 1.6 x 10-22  seconds, due to it being a very high energy fluctuation in the Higgs Field and hence being very unstable. This makes it incredibly difficult to measure, not helped by the fact that it can only be created through particle collisions in the Large Hadron Collider near the speed of light in order to provoke that high energy fluctuation. The lifetime is about a billion times too short to travel a detectable distance while being too long to have a large enough mass variation by the Heisenberg uncertainty principle, which is inversely proportional to the lifetime. Therefore, scientists had to look at the decay chains and see if there were any distinguishable features that didn’t occur normally. The below figure shows all the different decay channels and the frequency with which they occur for a standard 125.5 GeV Higgs Boson:

Unfortunately, the most likely bottom/anti-bottom quark decay channel is extremely hard to detect due to the billions of quarks being produced in the Large Hadron Collider in the billions of proton collisions. The next most likely W + and W - decay instantly decays into two leptons and two neutrinos (H → WW(*) → ℓνℓν). Neutrinos are near impossible to detect as they interact incredibly rarely so are essentially seen as lost energy and momentum by the detectors in the Large Hadron Collider. This missing energy would provide evidence of the Higgs Boson if it could be detected. 

ZZ* decay was a promising decay chain as the two Z bosons decayed into two pairs of oppositely charged leptons (H → ZZ* → ℓℓ ℓℓ ). These four leptons decayed from a 125.5 GeV Higgs Boson so must have a total invariant mass of 125.5 GeV providing a clean and distinct signal through the “noise” of all the other particles, which have a total mass randomly distributed. This H → ZZ* → ℓℓ ℓℓ   decay channel was later called the “Golden channel” due to it being so distinct. 

Another promising decay chain was the decay into two photons (H → γγ) channel, with the two photons having an invariant mass of 125.5 GeV between them, again differing from the randomly distributed mass of other photons from background collisions. Scientists therefore tried to detect these distinct signals from the background noise when the Large Hadron Collider was active.

How does the Large Hadron Collider detect these decay chains?

The Large Hadron Collider worked by speeding up two bunches of protons (about 100 billion protons in each bunch) to 99.9999991% of the speed of light. It had them go in opposite directions so that the bunches of protons collide as they go around the 27 kilometre ring of superconducting magnets. This caused 20-30 collisions per crossing between the bunches and around 11,200 crossings happened every second. This gives 280,000 collisions per second. 

The Large Hadron Collider had nine detectors inside it that could detect the different particles and photons produced from these collisions and how much energy they had. The two used to discover the Higgs Boson were the CMS (Compact Muon Solenoid) and the ATLAS (A Toroidal LHC ApparatuS) detectors. 

The CMS works by using very strong magnets to bend particles sharply over short distances and see how they interact through different materials. It has layers of different materials that interact with particles without affecting them in ways that computers can measure. For example, when charged particles fly through the second layer, the silicon layer, they disturb electrons in the silicon element which computers can use to reconstruct the particle's path. Using other such materials, computers can measure the energy, momentum and type of particle flying through. The ATLAS works differently with less strong magnets allowing the particles to bend over a longer distance (it is about two times as long as the CMS detector).

These two differing detection methods with different physicists controlling the experiment cross-referenced allow physicists to conclude data is accurate and enough to conclude findings. Given that the Higgs Boson is only formed once in every billion collisions, you have to run the Large Hadron Collider approximately 4,000 seconds to get just one Higgs Boson to form. To then detect this singular Higgs Boson and its decay chain through the billion other interactions occurring is impossible. Therefore physicists had to use statistical analysis to conclusively prove that the one in a billion signals weren’t anomalies caused by simple chance (all quantum physics comes down to probabilities) and were in fact the Higgs Boson.

The H → ZZ* → ℓℓ ℓℓ signal

This graph provides evidence for the existence of the Higgs Boson through the “Golden Channel” decay chain. It shows the number of events on the y-axis and mass in GeV on the x-axis for two pairs of oppositely charged lepton pairs with the same flavour. The blue and green areas of the graph show well documented background interactions. If the Higgs Boson didn’t exist, the black data points (recorded collisions with the lines being error bars) would have followed the normal path shown by the blue. The red peak clearly shows that this doesn’t happen, which is explained by the theoretical decay of a Higgs Boson, providing crucial evidence for the existence of the Higgs Boson.

The H → γγ  signal

This graph provides evidence for the Higgs Boson through the Diphoton decay chain. At the top, it shows the number of events on the y-axis against invariant mass in GeV on the x-axis for two photons detected together. The red line shows the experimental data while the blue dotted line shows the background curve. The peak at 125.5 GeV can only be explained by the decay of a Higgs Boson providing further evidence. The bottom part of the graph shows the graph after the background has been subtracted from the line, clearly showing the peak at the Higgs Boson’s invariant mass.

Why is this data conclusive?

The work from CMS and ATLAS together both show a peak at 125.5 GeV for two different decay chains provides cross-validated evidence for the existence of the Higgs Boson. The two differing ways of detecting particles both show the same results, giving conclusive evidence that these interactions happen in the same way and it is not a quirk of one detection method. As mentioned previously, the bump in the curve detected in both results could have occurred naturally by pure chance due to all quantum interactions being probabilistic. Therefore physicists used the rule of when a discovery is less likely to occur than five standard deviations from the expected value - about 1 in 3.5 million - which has been coined five sigma.

On the 4th July 2012, CMS and ATLAS together reached this five sigma threshold (i.e the chance of the results they saw occurring naturally being less likely than 1 in 3.5 million) providing conclusive evidence for the discovery of a Higgs-like Boson. They couldn’t conclude it was the Higgs Boson until 2013 when they could say with no uncertainty that the particle they discovered had a spin of 0. They also couldn’t detect all of the decay channels until much later due to how much background noise there was for quark decay channels, particularly the bottom anti-bottom quark decay channel.

This discovery proved the existence of the Higgs Field, a field that particles interact with to gain mass. The more strongly a particle interacts with the Higgs Field, the more massive it is. This discovery is so influential because it defines how particles have mass and it also is the discovery of an entirely unique particle. The Higgs Boson is the only particle with zero spin so physicists are using it as a tool to try and discover other unique branches of physics, like dark matter. Ultimately, this discovery explains how we have mass, and is still being used today to discover further mysteries of our universe.