Getting to the bottom of the Higgs boson

In high-energy particle collisions we study the smallest known constituents of matter. According to our best knowledge of physics, these constituents have mass only because of the way they interact with a unique quantity which permeates all of space. This quantity, like practically everything else in the strange world of the very small, is a quantum field.

That is not what makes this quantity unique. Quantum fields are all over the place. The light by which you are reading this text is a wiggling quantum field (an electromagnetic field, in that case). What makes the field involved in giving mass to particles unique is precisely the fact that it exists everywhere. It is present even in the lowest energy, emptiest vacuum. In fact, unlike any other quantum field, if you wanted to get rid of this field in a region of space, you would have to add energy, not remove it.

The wiggles in this mass-endowing field are called Higgs bosons, after one of the three theorists (Brout and Englert being the other two) who first postulated this theory, in the early 1960s. Five years ago, in 2012, the Higgs boson was discovered in high-energy collisions of protons in the Large Hadron Collider (LHC) at CERN, demonstrating that the essentials of this theory of mass were correct.

So much for the recap. Last week we learned something new about the Higgs boson.

When a Higgs boson is made, it very rapidly falls apart; that is, it decays. The fact that it was ever there at all is determined by measuring the fragments emitted in these decays. The boson has various decay options open to it, and for any individual Higgs, the option it takes is decided by the rolling dice of quantum mechanics. Our best theory – the Standard Model of particle physics – predicts the probability of each choice.

The prediction is that the most popular choice will be a decay to bottom quarks. Quarks are the particles which make up protons and neutrons, which make up the nucleus of every atom. The bottom quark is the most massive quark to which the Higgs can decay – which, because of the role the Higgs plays in mass, is the reason for its popularity. The theory predicts that 58% of Higgs bosons will produce a bottom quark and anti-quark when they decay.

Determining whether or not this prediction is correct is a very important task, and a surprisingly difficult one. It is important in that the answer would either provide another pillar of support for the Standard Model understanding of particle masses, or fatally undermine it. It is difficult because there are many, many ways of producing bottom quarks at the LHC which have nothing much to do with the Higgs. The background ‘noise’ is huge, which is why the Higgs was initially discovered via other decay products, which are rarer but which have less background noise.     

Read more https://www.theguardian.com/science/life-and-physics/2017/jul/17/getting-to-the-bottom-of-the-higgs-boson

Courtesy of Guardian News & Media Ltd