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 decay to bottom quarks. Quarks are the particles that 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.
Digging out a signal for Higgs bosons decaying to bottom quarks, therefore, requires applying complex selection filters on the millions of collisions measured at the LHC. Every one of these filters could go wrong, could bias your result, so the analyzers practice “in the dark” for a long time, using simulated data, and real data from similar collisions, to cross-check and calibrate without looking at the place they actually expect the signal to be; this is a technique called a “blinded analysis”.
As you might imagine, the moment at which you decide your analysis is as good as you can get it, and “unblind” yourself to see whether the signal is there, is quite tense. Tim Scanlon at UCL is one of the leaders of the analysis team. As he puts it:
After 5 years of searching for H->bb, I spent an anxious couple of hours waiting for the results after asking one of our key analysers (Andy Bell, a PhD student) to unblind the result. Would we see what we’ve been waiting for over the past five years? Would we be unlucky and just fall short? Or despite all our best efforts to ensure the analysis was correct, could we have missed something critical and have produced a seriously flawed result? Even if you are confident that your analysis is solid, you are still relying upon the roll of a dice when on the threshold of discovery.
The result, shown in the plot, was positive – significant enough evidence to supersede the hints from previous experiments and to cross the (arbitrary but important) “three sigma” threshold, meaning that if the data were just random noise, without a Higgs, they would produce a result like this (or stronger) less than 0.23% of the time¹. As Tim puts it
This is the accumulation of the work of a lot of people over the last six years, where each new result has built upon the last.
Important physics at the LHC continues, long after the big splash of the first Higgs discovery. Step by step, we are learning more about how the universe we inhabit operates.
¹ This was phrased incorrectly in the first version, see comment below. Also note that for a “discovery” the threshold is conventionally five sigma. Three sigma we conventionally call “evidence”.
guardian.co.uk © Guardian News & Media Limited 2010
This article titled “Getting to the bottom of the Higgs boson” was written by Jon Butterworth, for theguardian.com on Monday 17th July 2017 13.11 UTC
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