The Higgs Boson - what's the point?

So it looks like the Higgs Boson has been found. Or at least, a Higgs Boson. Or something like it. I’ll try and explain the difference in a bit. Either way, potentially the biggest moment in physics for a century or so, played out live on the internet this morning for anyone and everyone to see. Some people are saying it’s up there with discovering DNA. It might well be, but it’s too early to say at this stage. But it is certainly the most famous particle yet discovered, thanks to it arriving in a storm of media interest, twitter trending and hype that just didn’t exist when JJ Thompson discovered the electron back at the end of the 19th century.  And a bit like a new reality TV celeb, there’s a lot of fuss being made about this particle despite the fact that no-one really understands the point of it.

With an ever diminishing memory of my physics degree, I can’t pretend that I understood every last word of what was said at today’s announcement, but watching it, I knew it was a momentous occasion and what it represents for humanity at large makes my knees tingle.  And I don’t just mean the scientific implications. The next steps in terms of what this means for physics and our understanding of the universe are immensely exciting, but probably mostly only for physicists and those of us already interested in such things. For everyone else, what difference will it make if the Higgs Boson is known to exist, rather than not? Will it pay the bills or, as a friend tweeted a while back, will it immediately provide us all with Back to the Future style hover boards to get around on? The answer is no. For now. Although I reckon there’s nothing wrong with keeping our imaginations ticking over on this…..

 

But meantime, in response to the inevitable question of why the Higgs Boson discovery is important, I think there’s a better answer. It is important because even if we only consider it for a second or two, it is the trigger to allow ourselves a fleeting sense of wonder that lifts us from the well trodden grooves of our daily lives, to think beyond ourselves, to the big questions that unite all of us living on this planet. And unlike art and music and love and other things that also inject us with wonder, the difference here is that it is borne of a reality that exists beyond our human imagination and interpretation. It is the fundamental, testable reality that underlies all the fizz and nonsense of life, and it is the thing that unites all of us, because by being here, made up of atoms, we are very much a part of it. I believe that the more often we all connect with this, at whatever level is meaningful to us, the better we relate to each other and the world around us.

That’s all very well, but what if the science is just too dense and difficult for the idea of the Higgs Boson to spark off that bit of wonder in your mind the first place? There are two things here. One is that there is a responsibility to explain the science as best we can, and two, is that the odd thing I’ve been observing so far today, and also since the switch on of the LHC a few years ago, is that even without deeply understanding the science, there’s a general sense of something magic going on over there in physics that is keeping people interested. What was once the realm of intensely niche and geeky scientific investigation, has now hit the mainstream in a big way. #Higgs and associated hashtags are trending globally on twitter, and references are being made in pretty much every media outlet you can imagine.  More interestingly, the ‘real’ academic science conference, not the press announcement, was made available to the general public online. And I’m very glad it was there, even if it was gibberish to most. I think it’s important and valuable to show what science looks and sounds like, as long as there’s then immediate backup to explain it to the non scientific population.  If we keep using scientific language out of earshot of everyone else, it will maintain a stigma of aloofness, of us and them. And I hope that much like when I watch the West Wing, although I only understand half of the dense political wranglings being discussed, the shape and sound of the language is what draws us in to want to find out more.  5 sigma, decay channels, giga electronvolts, gamma gamma, ZZ - it all sounds like a magical incantation, and there is an undeniable theatre to this mystical world that I remember entrancing me as a teenager - back then there were bubble chambers called Gargamelle, images of curling tracks revealing previous invisible particle visitors from space, tables of particle properties including improbable words like flavour and spin, compelling sci fi style diagrams - all of those things made me want to know more when I first discovered particle physics, and still do today.

 

Just over a decade ago, thanks to the world’s most energetic and inspirational teacher, I was lucky enough to be standing in the particle accelerator that pre-dated the LHC at CERN and asking a particle physicist if they thought the Higgs Boson would ever be discovered. Like most good scientists, his answer was that we’d have to wait and see what experiments could tell us. And so we have. We’ve waited til today, although that phrase doesn’t do justice to the incredible hard work and ingenuity of thousands of scientists across the planet to get us this far. The story has reached a major plot point, and while it is far from over, I feel a bit like the kids who’ve grown up along with Harry Potter, watching the blockbuster final film where we discover the final horcrux that makes sense of the whole long running plot. Except this science stuff is not fiction. And that blows the top off my mind. And blowing tops off minds is back to what I think the answer to the importance of the Higgs is all about. It opens up a little window on the extraordinary nature of the universe, which whether we are professional scientists or not, is surely is the ultimate thing for all of us to engage with in the time that we have on this planet. That is the point.

And if you want to look through the window and know a bit more about the science answer to why the Higgs is important, read on….

Once the big performance has worn off, to maintain popular momentum, even the most famous of celebrities need people to know a bit more about them, drip feeding drama over time, otherwise they lose the column inches. So I think the physics of the Higgs does need a bit of explaining.  There will be lots of people who can do this better than me - Prof Brian Cox is a good place to start… but in case he’s a bit tied up today and you can’t get through to him on the phone, I’ll try and answer some of the questions that have come my way on twitter and facebook today - I’ll keep updating this post as people ask stuff….

Tell me again about what the Higgs Boson is supposed to do?

It is thought to give mass to particles of matter.

So - stepping back a moment just to be clear and make sure everyone’s on board - what’s ‘matter’?

It’s everything that the universe is made of. You, me, a table, a milkshake, a star, a tree.  You might have got as far as thinking about matter being made up of atoms and that’s right, but you can break things down even further. There are subatomic particles inside an atom - electrons orbit a central nucleus, which is made of neutrons and protons. And ‘inside’ the neutrons and protons there are up and down quarks. These are a few of the fundamental particles, the ones you can’t break down any further, which are part of a theory in physics called the Standard Model. The Standard Model describes what the observable universe is made of and how it all interacts. (I say ‘observable’ here, because in fact the model is incomplete - we don’t know what 95% of the universe is made of. It could be dark matter or something as yet unimagined. But that’s another story)

And what exactly IS mass?

This is a tricky one to explain, because it’s one of those fundamental properties that is so fundamental, it’s hard to explain in terms of anything else. It’s like asking what time is. We sort of know, but we’re hard pushed to put it into a simple sentence.

Mass is not a discrete “thing” that you can point to or put separately into a bag and give to someone as a present, labelled ‘mass’. It is a property that can be assigned to describe how something behaves. One way of thinking about it is as a dimensionless quantity that tells us how much energy you need to apply to either get something to move in spacetime, or to get it to stop. That’s all to do with Newton’s laws, which we won’t go into now. But what you need to know about mass is that it isn’t just everywhere as a matter of fact. It seems that way, and I think that’s why people get stuck trying to work out why the mass -giving function of the Higgs Boson is at all amazing.  It is everywhere in everything we encounter in our daily lives, and is so obvious a part of life that we never think to ask the questions, where does it come from, and why? But that’s exactly what science is here to ask, which is why the idea of the Higgs Boson came up in the first place.  The Standard Model, actually predicts that all particles should be massless. But that’s not what we observe in experiment - we find that almost all the particles DO have mass, and they are all very different. This is in direct conflict with the laws of symmetry that describe everything else in physics, so we needed to come up with a way to get the Standard Model to fit in. What could be causing the wildly varied masses of particles to exist, instead of a nice symmetric massless world predicted by the maths? The Higgs Boson is the solution. It is the particle that breaks the symmetry and explains the observed properties of particles.

So if you remember one thing about all this, it’s that without the presence of the Higgs Boson and the associated Higgs Field in the universe, nothing would have mass. And without mass, everything just flies around at the speed of light. A universe without mass would  be pretty hard to pin down. Nothing would stick around long enough to create matter and the reality we know around us.  That’s how important the Higgs is to absolutely everything as we know it.

So how does the Higgs Boson actually do the mass-giving thing?

We don’t know the full answer to this yet - the data from CERN is pointing to the existence of a Higgs-type particle, but we will need much more analysis and experimental exploration before the mechanism by which it operates will become clear. But now we’ve found it, we can start to make measurements on it, find out how it behaves and interacts and start to answer this question.

The theory is as follows:  The ‘boson’ bit of the Higgs Boson tells us that it is a particular type of particle - a force carrying particle. For example, gravity is one of the forces in the universe, and so is the electromagnetic force. More accurately, when dealing with what we call forces, physicists will talk about ‘fields’ - the Higgs mechanism involves a Higgs Field that permeates all of space, and the Higgs Boson is an excitation of that field. The idea is that when other particles move through the field, they gain mass. The way in which they interact with the field determines just how much mass they get. So the photon doesn’t interact with it at all, and is massless, hence it can travel at the speed of light. But in comparison, the top quark or the W and Z bosons gain a lot of mass from the Higgs Field.

There are lots of analogies flying around to try and explain how the Higgs field works, but as with all analogies they are ultimately flawed in that none of our human senses actually give us direct access to experiencing the Higgs field, and so the best description is always mathematical. But given that the maths means something to such a small fraction of the population, analogy is our best substitute and this is still one of the best. [youtube=http://www.youtube.com/watch?v=C-pxHCJK7Yg]

If you get stuck halfway through this, it’s because it is hard. Really hard. And we don’t really know the proper way to explain it yet. That’s why the Higgs discovery is so exciting. It is like stumbling across the searchlight in a dark city and finally switching it on and being able to point it at stuff. It will hopefully show us the way to understanding the mechanism fully.

What’s all this 5 sigma data they keep talking about?

5 sigma is a statistical term that tells us how confident physicists are in their data. By comparing results across different experiments and calculating the potential error in measurements, they can determine how likely it is for a particular result to occur.

In the tweeted words of Prof Brian Cox, a 5 sigma result roughly means they are 99.9999% sure. It is the benchmark needed to be able to claim something a discovery.

What does it mean when the reports say the Higgs Boson was found at 126 GeV?

GeV stands for giga electron volt.  It is a measure of energy, and, since mass and energy are interchangeable, it is also a measure of mass. An electron volt is the amount of energy the charge of electron gains when it moves through an electric potential difference of 1 volt. That probably doesn’t help much, but the important thing to know is that when experiments are done in particle accelerators, it involves particles being whizzed round at nearly the speed of light before being smashed into each other. The higher the energy we can get the collisions to happen at, the more massive the particles we can produce in the crash debris. The 126GeV describes the energy at which the Higgs Boson type particle was found.

OK, so what’s all this Higgs Boson-type particle about? Has it been found or not?

I said at the top of this post that they’ve found the Higgs Boson, or at least a Higgs Boson. The thing is, the theory predicts different signature patterns for detecting the Higgs, and it sort of depends on which ones give the strongest results. The LHC data announced today shows strong evidence for the Higgs in two particular decay channels. What’s a decay channel? It is the process by which a particle decays, or breaks down into different particles. Some particles are very stable, and take a long time to decay, others are barely in existence before they break down into something else. We detect particles not by ‘seeing’ them directly, but by observing their tracks, their energy deposits, or the characteristic pattern of particles that they decay into. For a more detailed discussion on particle decay, check out this article by the excellent Lily Asquith.

There are beautiful diagrams invented by the charismatic physicist Richard Feynman that represent the behaviour of subatomic particles and these below show routes for a Standard Model Higgs decaying into two photons, denoted by the gamma signs. The other strong decay pattern detected was a Higgs decaying to two Z bosons. There are more expected decay patterns that haven’t yet shown up in the data, but these more complicated to analyse, so we don’t yet know if the Higgs signals seen in the LHC will produce the full set of particle pairs that are expected from the mathematics of the Standard Model. If these other two decay channels DON’T show up, it means the data gives evidence for a Higgs-type particle, but not one that sits happily in the Standard Model. This would be exciting, in that it would open up a whole new physics game, as a particle physicist friend on Facebook told me. Either way, the implications are enormously interesting.

 

Those Feynman diagrams are representational - here’s an image of data taken from one of the LHC experiments, showing what a Higgs to photon pair decay very possibly looks like.

 

So when we all get excited about the implications of this discovery, what sort of thing are we talking about? It’s less to do with immediate material developments and more to do with the range of possibilities that open up when we are able to cut such a deep new path into the fabric of reality. If we understand the mechanism that gives everything mass, we might then get to grips with gravity, which is to do with general relativity, which is resolutely not yet meshing in with quantum mechanics. Another step towards a grand unified theory.  We also won’t have a fully working Standard Model, because neutrinos are still a bit of a puzzle with their tiny masses, and the embarrassing fact is that the Higgs mechanism will only account for about 1 % of the mass in the universe. Most of the universe is still missing, in the form of dark matter or something else.  And beyond that, I suspect that the Higgs Boson will lead us to developments far beyond our current models of science. When JJ Thompson discovered the electron, he wondered what it could possibly be good for. There’s no way anyone back in the 19th Century could have imagined the impossible gadgetry of the 21st century, that would not exist without an understanding of the electron. What is the equivalent, as yet unimagined, outcome of the Higgs? I can’t wait to find out.

Thanks to the friends and colleagues on twitter and elsewhere whose thoughts and capacity for discussion have inspired and improved this post. Please keep asking questions and crossly pointing out when things don’t make sense. We have a responsibility to make this stuff as accessible as we can. It’s as vital as doing the science itself, and it’s almost as hard!