By Don Lincoln, Fermilab
The Higgs field is a uniformly distributed energy field. It is a field that some particles interact with and some don’t. When the field interacts with a particle, it gives it mass, which effectively slows it down and also makes it massive. Those particles that don’t interact are massless. That is the essence of the Higgs field and the Higgs boson.
Modern Electroweak Theory
It was 1964 when Peter Higgs and his five compadres published three influential papers on the subject. Actually, Higgs published two and it was his second paper in which he predicted what we now call the Higgs boson.
The story is a little complicated, as there were several other scientists who contributed to the development of a modern electroweak theory, which abstractly unifies electromagnetism and the weak force with massless force carrying particle and then, with the addition of the Higgs field, transforms into the modern world in which the weak nuclear force and electromagnetism act very differently.
In the 1960s, the whole thing was simply theoretical. There was no experimental evidence that confirmed the theory, beyond the simple fact that the weak nuclear force was very weak and had a very short range. But the predictions of theory were quite clear. There should be a massless photon, which scientists have known about for over a century, but there should also be a massive neutral particle called the Z boson, two massive electrically charged particles called the W bosons, and finally a massive and neutral particle called the Higgs boson.
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W and Z Bosons
The exact properties of these particles weren’t completely known. Their electrical charge was known, as was their subatomic spin, but the masses weren’t known, at least not precisely. However, the range and strength of the weak force gave a hint of the mass of the W and Z bosons. They should be about 100 times heavier than the proton. And that’s kind of a crazy thing when one thinks about it. The W and Z bosons shouldn’t have any internal structure and each of them should have the mass in the ballpark of an entire silver atom, which has 47 protons and 60 neutrons.
Naturally, scientists went looking for them and it took a long time. Technology wasn’t ready. It took quite a few years until a particle accelerator could collide beams of particles together with enough energy to have a shot at making W and Z bosons.
S-p-pbar-S Accelerator
It was in 1981 that a particle accelerator operating at the CERN laboratory in Switzerland began operations. It was called the S-p-pbar-S. The name tells us something about the accelerator.
It collided a beam of protons and antimatter protons together at very high energy; at an energy of 540 GeV. That number is equivalent to the mass energy of 575 protons, or a fair bit more than the energy it would take to make two uranium atoms. For the era, it was a staggering achievement.
The S-p-pbar-S accelerator only accelerated the particles. Researchers needed detectors to inspect the collisions for the signature that W and Z bosons were created. And they actually built two, called UA1 and UA2. UA is just an acronym meaning ‘underground area’.
UA1 and UA2: The Two Detectors
In particle physics, there are often two experiments built at an accelerator. There are several reasons. First, the two detectors are made by different groups, using different technologies. Having such different detectors protects against the weaknesses of any specific design. Second, having the two experiments taking data at the same time pushes them to work fast and smart. In high stakes science, there is first and not-first. Second never wins the glory and so nobody wants to be second.
A third reason to have two detectors is so that whoever is able to see something, second can confirm what the first experiment saw. Science requires confirmation. And finally, the two experiments are there for safety. If one experiment has some sort of accident (say a fire or something), the accelerator can continue to run and make discoveries.
So, UA1 and UA2 were competitors, hot on the trail of the W and Z boson.
Decay Patterns
Both experiments wanted the other experiment to do well, but not quite as well as they did. They both were looking for specific signatures of the W and Z bosons.
W bosons should decay into an electron and an electron neutrino, or a muon and a muon neutrino. This decay pattern is because the W bosons have electrical charge, so the daughters must also have electrical charge. And the W boson transmits the weak force, so it can make neutrinos. There are other ways in which it can decay, but those are the easiest ones to see and understand.
The neutrino doesn’t interact very often, so the neutrino escapes the detector undetected. Thus, the experimental signature of a W boson is an electron or muon on one side of the detector and nothing on the other. That nothing results in an energy imbalance, which is easy to see.
In contrast, the Z boson would decay into an electron and positron, or a muon and antimatter muon, or again, a number of other ways that are harder to see and aren’t part of the discovery story. The Z boson is electrically neutral, so it has to decay into a positive and negative particle and the electron and muon decay chains are just two that are easiest to detect. Note that in this case there is no neutrino involved and therefore no missing energy.
Common Questions about Higgs Field and the Search for W and Z Bosons
In 1981, a particle accelerator operating at the CERN laboratory in Switzerland began operations. It was called the S-p-pbar-S.
S-p-pbar-S collided a beam of protons and antimatter protons together at very high energy; at an energy of 540 GeV. That number is equivalent to the mass energy of 575 protons, or a fair bit more than the energy it would take to make two uranium atoms.
W bosons should decay into an electron and an electron neutrino, or a muon and a muon neutrino. This decay pattern is because the W bosons have electrical charge, so the daughters must also have electrical charge. And the W boson transmits the weak force, so it can make neutrinos.
