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We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs boson. Read the previous posts: “What is the Higgs boson?”, “Why is the Higgs boson called the ‘god particle’?”, “Is the particle recently discovered at CERN’s LHC the Higgs boson?”, and “How does the Higgs mechanism create mass?”

By Jim Baggott

The 4 July discovery announcement makes it clear that the new particle is consistent with the long-sought Higgs boson. The next step is therefore reasonably obvious. Physicists involved in the ATLAS and CMS detector collaborations at the LHC will be keen to push ahead and fully characterize the new particle. They will want to know if this is indeed the Higgs boson.

How can they tell?

I mentioned in the third post in this series that the physicists at Fermilab’s Tevatron and CERN’s LHC have been searching for the Higgs boson by looking for the tell-tale products of its different predicted decay pathways. The current standard model of particle physics is used to predict the rates of production of the Higgs boson in high-energy particle collisions and the rates of its various decay modes. After subtracting the ‘background’ that arises from all the other ways in which the decay products can be produced, the physicists are left with an excess of events that can be ascribed to Higgs boson decays.

Now that we know the new particle has a mass of between 125-126 billion electron-volts (equivalent to the mass of about 134 protons), both the calculations and the experiments can be focused tightly on this specific mass value.

So far, excess events have been observed for three important decay pathways. These involve the decay of the Higgs boson to two photons ( H → γγ), two Z bosons (H → ZZ → ι⁺ι^-ι⁺ι^-) and two W particles (H → W⁺W^- → ι⁺υ ι^-υ). You will notice that these pathways all involve the production of bosons. This should come as no real surprise, as the Higgs field is responsible for breaking the symmetry between the weak and electromagnetic forces, giving mass to the W and Z particles and leaving the photon massless.

The decay rates to these three pathways are broadly as predicted by the standard model. There is an observed enhancement in the rate of decay to two photons compared to predictions, but this may be the result of statistical fluctuations. Further data on this pathway will determine whether or not there’s a problem (or maybe a clue to some new physics) in this channel.

But the Higgs field is also involved in giving mass to fermions (matter particles, such as electrons and quarks). The Higgs boson is therefore also predicted to decay into fermions, specifically very large massive fermions such as bottom and anti-bottom quarks, and tau and anti-tau leptons. Bottom quarks and tau leptons (heavy versions of the electron) are third-generation matter particles with masses respectively of about 4.2 billion electron volts (about 4 and a half proton masses) and 1.8 billion electron volts (about 1.9 proton masses).

These decay pathways are a little more problematic. The backgrounds from other processes are more significant and considerably more data are required to discriminate the background from genuine Higgs decay events. The decay to bottom and anti-bottom quarks was studied at the Tevatron before it was shut down earlier this year. But the collider had insufficient collision energy and luminosity (a measure of the number of collisions that the particle beams can produce) to enable independent discovery of the Higgs boson.

ATLAS physicist Jon Butterworth, who writes a blog for the British newspaper The Guardian, recently gave his assessment:

If and when we see the Higgs decaying in these two [fermion] channels at roughly the predicted rates, I will probably start calling this new boson the Higgs rather than a Higgs. It won’t prove it is exactly the Standard Model Higgs boson of course, and looking for subtle differences will be very interesting. But it will be close enough to justify [calling it] the definite article.

When will this happen? This is hard to judge, but perhaps we will have an answer by the end of this year.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog posts.

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs. Read the previous posts: “What is the Higgs boson?”,“Why is the Higgs boson called the ‘god particle’?”, “Is the particle recently discovered at CERN’s LHC the Higgs boson?”, and “How does the Higgs mechanism create mass?”

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Subscribe to only physics and chemistry articles on the OUPblog via email or RSS.
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We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs boson. Read the previous posts: “What is the Higgs boson?”, “Why is the Higgs boson called the ‘god particle’?”, and “Is the particle recently discovered at CERN’s LHC the Higgs boson?”

By Jim Baggott

Through thousands of years of speculative philosophy and hundreds of years of hard empirical science, we have tended to think of mass as an innate property (a ‘primary quality’) of material substance. We figured that, whatever they might be, the basic building blocks of matter would surely consist of microscopic lumps of some kind of ‘stuff’.

But this is not quite how it has worked out. There was a clue in the title of one of Albert Einstein’s most famous research papers, published in 1905: ‘Does the inertia of a body depend on its energy content?’ This was the paper in which Einstein suggested that there was a deep connection between mass and energy, through what would subsequently become the world’s most famous equation, E = mc².

We experience the mass of an object as inertia (the object’s resistance to acceleration) and Einstein was suggesting that the latter is determined not by mass as a primary quality, but rather by the energy that the object contains.

So, when an otherwise massless particle travelling at the speed of light interacts with the Higgs field, it is slowed down. The field ‘drags’ on it, as though the particle were moving through molasses. In other words, the energy of the interaction is manifested as a resistance to acceleration. The particle acquires inertia, and we think of this inertia in terms of the particle’s ‘mass’.

In the Higgs mechanism, mass loses its status as a primary quality. It becomes secondary — the result of massless particles interacting with the Higgs field.

So, does the Higgs mechanism explain all mass? Including the mass of me, you, and all the objects in the visible universe? No, it doesn’t. To see why, let’s just take a quick look at the origin of the mass of the heavy paperweight that sits on my desk in front of me.

The paperweight is made of glass. It has a complex molecular structure consisting primarily of a network of silicon and oxygen atoms bonded together. Obviously, we can trace its mass to the protons and neutrons which account for 99% of the mass of every silicon and oxygen atom in this structure.

According to the standard model, protons and neutrons are made of quarks. So, we might be tempted to conclude that the mass of the paperweight resides in the masses of the quarks from which the protons and neutrons are composed. But we’d be wrong again. Although it’s quite difficult to determine precisely the masses of the quarks, they are substantially smaller and lighter than the protons and neutrons that they comprise. We would estimate that the masses of the quarks, derived through their interaction with the Higgs field, account for only about 1% of the mass of a proton, for example.

But if 99% of the mass of a proton is not to be found in its constituent quarks, then where is it? The answer is that the rest of the proton’s mass resides in the energy of the massless gluons — the carriers of the strong nuclear force — that pass between the quarks and bind them together inside the proton.

What the standard model of particle physics tells us is quite bizarre. There appear to be ultimate building blocks which do have characteristic physical properties, but mass isn’t really one of them. Instead of mass we have interactions between elementary particles that would otherwise be massless and the Higgs field. These interactions slow the particles down, giving rise to inertia which we interpret as mass. As these elementary particles combine, the energy of the massless force particles passing between them builds, adding greatly to the impression of solidity and substance.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog posts.

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs. Read the previous posts: “What is the Higgs boson?”, “Why is the Higgs boson called the ‘god particle’?”, and “Is the particle recently discovered at CERN’s LHC the Higgs boson?”

Subscribe to the OUPblog via email or RSS.
Subscribe to only physics and chemistry articles on the OUPblog via email or RSS.
View more about this book on the  

We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs boson. Read the previous posts: “What is the Higgs boson?” and “Why is the Higgs boson called the ‘god particle’?”

By Jim Baggott

Experimental physicists are by nature very cautious people, often reluctant to speculate beyond the boundaries defined by the evidence at hand.

Although the Higgs mechanism is responsible for the acquisition of mass, the theory does not give a precise prediction for the mass of the Higgs boson itself. The search for the Higgs boson, both at Fermilab’s Tevatron collider and CERN’s Large Hadron Collider (LHC), has therefore involved elaborate calculations of all the different ways a Higgs boson might be created in high-energy particle collisions, and all the different ways it may decay into other elementary particles.

At CERN, the attentions of physicists working in the two main detector collaborations, ATLAS and CMS, have been drawn to Higgs decay pathways involving the production of two photons (which we write as H → γγ), a pathway leading to two Z bosons and thence four leptons (particles such as electrons and positrons, written H → ZZ → ι⁺ι^-ι⁺ι^-) and a pathway leading to two W particles and thence to two leptons and two neutrinos (H → W⁺W^- → ι⁺υ ι^-υ).

Finding the Higgs boson is then a matter of looking for its decay products — in this case the photons and leptons that result — at all the different masses that the Higgs may in theory possess. Just to make life more difficult, at the particle collision energies available at the LHC, there are lots of other processes that can produce photons and leptons, and this background must be calculated and subtracted from the observed decay events. Any events above background that produce two photons, four leptons or two leptons (and ‘missing’ energy, as neutrinos cannot be detected) then contribute to the evidence for the Higgs boson.

What the CERN scientists announced on 4 July was a statistically significant excess of decay events consistent with a Higgs boson with a mass between 125-126 billion electron volts, about 134 times the mass of a proton. This is definitely a new boson, one that decays very much like a Higgs boson is expected to decay. But, until the scientists can gather more data on its physical properties, they can’t say for sure precisely what kind of boson it is.

It’s also important to note that although the Higgs boson is predicted by the standard model of particle physics, there are theories that also predict the existence of a Higgs boson (actually, they predict many Higgs bosons). Until the scientists gather more data, they can’t be sure the new particle is precisely the particle predicted by the standard model.

We just need to be patient and stay tuned.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog posts.

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the week to explain some of the mysteries of the Higgs. Read the previous posts: “What is the Higgs boson?” and “Why is the Higgs boson called the ‘god particle’?”

Subscribe to the OUPblog via email or RSS.
Subscribe to only physics and chemistry articles on the OUPblog via email or RSS.
View more about this book on the  

We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the next week to explain some of the mysteries of the Higgs boson. Read the previous post: “What is the Higgs boson?”

By Jim Baggott

The Higgs field was invented to explain how otherwise massless force particles could acquire mass, and was used by Weinberg and Salam to develop a theory of the combined ‘electro-weak’ force and predict the masses of the W and Z bosons. However, it soon became apparent that something very similar is responsible for the masses of the matter particles, too.

The way the Higgs field interacts with otherwise massless boson fields and the way it interacts with massless fermion fields is not the same (the latter is called a Yukawa interaction, named for Japanese physicist Hideki Yukawa). Nevertheless, the Higgs field clearly has a fundamentally important role to play. Without it, both matter and force particles would have no mass. Mass could not be constructed and nothing in our visible universe could be.

In his popular book The God Particle: If the Universe is the Answer, What is the Question?, first published in 1993, American physicist Leon Lederman (writing with Dick Teresi) explained why he’d chosen this title:

This boson is so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive, that I have given it a nickname: the God Particle. Why God Particle? Two reasons. One, the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing. And two, there is a connection, of sorts, to another book, a much older one…

Lederman went on to quote a passage from the Book of Genesis.

This is a nickname that has stuck. Most physicists seem to dislike it, as they believe it exaggerates the importance of the Higgs boson. Higgs himself doesn’t seem to mind.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog post “Putting the Higgs particle in perspective.”

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the next week to explain some of the mysteries of the Higgs. Read the previous post: “What is the Higgs boson?”

Subscribe to the OUPblog via email or RSS.
Subscribe to only physics and chemistry articles on the OUPblog via email or RSS.
View more about this book on the  

On 4 July 2012, scientists at CERN’s Large Hadron Collider (LHC) facility in Geneva announced the discovery of a new elementary particle they believe is consistent with the long-sought Higgs boson, or ‘god particle’. Our understanding of the fundamental nature of matter — everything in our visible universe and everything we are — is about to take a giant leap forward. So, what is the Higgs boson and why is it so important? What role does it play in the structure of material substance? We’re celebrating the release of Higgs: The Invention and Discovery of the ‘God Particle’ with a series of posts by science writer Jim Baggott over the next week to explain some of the mysteries of the Higgs.

By Jim Baggott

We know that the physical universe is constructed from elementary matter particles (such as electrons and quarks) and the particles that transmit forces between them (such as photons). Matter particles have physical characteristics that we classify as fermions. Force particles are bosons.

In quantum field theory, these particles are represented in terms of invisible energy ‘fields’ that extend through space. Think of your childhood experiences playing with magnets. As you push the north poles of two bar magnets together, you feel the resistance between them grow in strength. This is the result of the interaction of two invisible, but nevertheless very real, magnetic fields. The force of resistance you experience as you push the magnets together is carried by invisible (or ‘virtual’) photons passing between them.

Matter and force particles are then interpreted as fundamental disturbances of these different kinds of fields. We say that these disturbances are the ‘quanta’ of the fields. The electron is the quantum of the electron field. The photon is the quantum of the electromagnetic field, and so on.

In the mid-1960s, quantum field theories were relatively unpopular among theorists. These theories seemed to suggest that force carriers should all be massless particles. This made little sense. Such a conclusion is fine for the photon, which carries the force of electromagnetism and is indeed massless. But it was believed that the carriers of the weak nuclear force, responsible for certain kinds of radioactivity, had to be large, massive particles. Where then did the mass of these particles come from?

In 1964, four research papers appeared proposing a solution. What if, these papers suggested, the universe is pervaded by a different kind of energy field, one that points (it imposes a direction in space) but doesn’t push or pull? Certain kinds of force particle might then interact with this field, thereby gaining mass. Photons would zip through the field, unaffected.

One of these papers, by English theorist Peter Higgs, included a footnote suggesting that such a field could also be expected to have a fundamental disturbance — a quantum of the field. In 1967 Steven Weinberg (and subsequently Abdus Salam) used this mechanism to devise a theory which combined the electromagnetic and weak nuclear forces. Weinberg was able to predict the masses of the carriers of the weak nuclear force: the W and Z bosons. These particles were found at CERN about 16 years later, with masses very close to Weinberg’s original predictions.

By about 1972, the new field was being referred to by most physicists as the Higgs field, and its field quantum was called the Higgs boson. The ‘Higgs mechanism’ became a key ingredient in what was to become known as the standard model of particle physics.

Jim Baggott is author of Higgs: The Invention and Discovery of the ‘God Particle’ and a freelance science writer. He was a lecturer in chemistry at the University of Reading but left to pursue a business career, where he first worked with Shell International Petroleum Company and then as an independent business consultant and trainer. His many books include Atomic: The First War of Physics (Icon, 2009), Beyond Measure: Modern Physics, Philosophy and the Meaning of Quantum Theory (OUP, 2003), A Beginner’s Guide to Reality (Penguin, 2005), and A Quantum Story: A History in 40 Moments (OUP, 2010). Read his previous blog post “Putting the Higgs particle in perspective.”

Subscribe to the OUPblog via email or RSS.
Subscribe to only physics and chemistry articles on the OUPblog via email or RSS.
View more about this book on the