The Higgs Boson

On 4^th July 2012, Rolf Heuer, director of CERN, announced that the Higgs boson had been discovered. The packed auditorium of scientists and reporters erupted in applause, cameras flashed and history was made.

A few hours later, Googling “Higgs boson found” gave 262 million results. Replacing “Higgs boson” by “God particle” gave many million more.

So, what is the Higgs boson, what does it mean for physics, and where does God come in?

The Layman’s Picture  

To start the story, let’s roll back 80 years.

In 1932, the discovery of the neutron completed what I call the Layman’s Picture of the universe familiar nowadays to anyone with a high school education (if they haven’t slept through all their science classes, that is…).

In this picture, all the “stuff” in the universe, aka “matter”, is made up of atoms, which are themselves composed of protons, neutrons and electrons.

All matter is subject to the force of gravity which shapes the stars and galaxies.

If, in addition, the matter has a charge, it also feels the forces of electricity and magnetism.

Nearly 70 years before our story begins, James Clerk Maxwell had shown that electricity and magnetism were two aspects of the same force, henceforth called electromagnetism. This was the first of the so called “unifications of physics” and as a stunning consequence was that light was revealed to be an electromagnetic wave – a disturbance of the electromagnetic field that propagates through space like waves through water.

The three decades, 1900 to 1930, had witnessed possibly the most spectacular advances ever in theoretical physics.

On the one hand we had Einstein’s theories of Relativity. Special Relativity reconciled Maxwell’s theory with mechanics in a revolutionary way – among the implications was the equivalence of mass and energy (yes, E = mc²). General Relativity subsumed special relativity and revealed gravity an effect of the curvature of space and time in the presence of matter and energy.

On the other hand, the theory of Quantum Mechanics, developed by a group of physicists in the 1920’s had incredible success explaining the world of atoms and molecules. This theory came with its own baggage of bizarreness – wave-particle duality, the Uncertainty Principle and more.

Many intellectuals believed that our picture of the world was essentially complete. All that remained was to unify gravity with electromagnetism and Einstein was already working on it.

The truth would turn out to be far stranger…

The Standard Model

Fast forward to the late 1940’s.

The terrifying potential of nuclear power had engendered  interest in peering even deeper into the fundamental constituents of matter – after all who knew what mightier sources of energy were waiting to be tapped ?

This was the age of the great particle accelerators – huge machines that accelerate atoms to extremely high velocities and smash them together.

The hope was to reveal even simpler ingredients underlying protons and electrons.

What ensued instead was complete chaos. Instead of a few simple components, the accelerators turned out a profusion of literally hundreds of new particles at an astonishing rate!!

Physicists despaired of ever making sense of the mess. Some speculated that rather than getting simpler, maybe matter gets ever more complex as one goes further down…

Then as the 50’s rolled into the 60’s, a picture began to emerge, and here it is:

Welcome to the Standard Model. This is the best picture we have so far of the fundamental constituents of our universe.

To the left of the picture are the basic building blocks of matter.

You will notice that protons and neutrons are nowhere in the picture. That’s because they themselves are made of even more basic constituents – the quarks. Quarks come in 6 varieties – up, down, charmed, strange, top and bottom.

Electrons, however, are in the picture.

The “e” in the upper left corner of the group called “leptons” is the electron.

Along with them in the same row are the muon and the tau particle.

The bottom row on the left shows the 3 types of neutrino – strange, ghostlike particles than can pass through entire planets without leaving a trace.

Why these twelve? Why no more or fewer? Nobody knows.

The right side shows the “force particles”. Come again?

Well, the theoretical framework underlying the forces of nature is Quantum Field Theory (QFT).

In QFT, each force is represented by a “quantum field” pervading all of space. Associated with each such quantum field is a characteristic particle.

In the parlance of QFT, a matter particle “feels” a force if it is able to “interact with” or “couple to” the particle of the corresponding quantum field.

Just to make things fun, the force particles can even interact with each other, or even themselves. (This comes up later)

Clear? So now, the “γ” at bottom right is the photon, which is the particle corresponding to the electromagnetic force.

As for the rest – two new forces were discovered via all the particle smashing.

The strong nuclear force binds quarks together into protons and neutrons. Its particle is the gluon, which is the “g” on top right.

The weak nuclear force is responsible for radioactive decay and its particles are the W and Z in middle right.

[A diversion:  You will notice that the “force particles” are called bosons. This is because all force particles share some commonalities that were elucidated in the early 1920’s by Bengali physicist Satyendranath Bose. So, all Bengalis reading this – enjoy your 2 seconds of reflected glory.]

But talking of forces, where’s gravity? Welcome to the biggest gap in the current foundations of theoretical physics. Despite decades of effort, nobody has come up with a successful QFT for gravity. Anybody who succeeds will usher in a new era of physics.

But we digress.

Another force particle is missing in this picture – missing because until very recently, nobody was sure it even existed. Enter the Higgs boson.

The Higgs Field(s)

Remember Maxwell joining electricity and magnetism together?

The quest to unite the forces of nature under a single description has been a prime motivator for theoretical physics over the last century.

Starting with Paul Dirac in the 1920’s, physicists managed to merge Maxwell’s theory with quantum mechanics to develop the spectacularly successful theory of Quantum Electrodynamics – the QFT for the electromagnetic field over the next several decades.

 (Heard of Richard Feynman? This was his biggest work.)

Why stop with electromagnetism? From the 1960’s onwards an ongoing effort was made to incorporate both the strong and weak nuclear force into the same framework.

But there was a problem.

The photon and gluon have zero mass while the W and Z particles of the weak force have mass.

Why is that a problem?

The reasons are extremely technical (here’s where a PhD in physics comes in handy).

Suffice it to say that the theoretical framework constrains the particles of all three forces to have zero mass. So, how to reconcile theory with reality?

But what if there’s another “force” – another quantum field pervading the world? Maybe the reason why the W and Z particles seem to have mass is because they feel this strange new force.

To make a very crude analogy, suppose you are doing experiments with marbles on a very smooth floor. You flick all the marbles with the same force. Some of them shoot off, while others roll away slowly and then come to a stop. How to explain this?

One possibility – they have different weights and respond differently to your flicking.

But here’s another – they all weight the same, but some of the marbles are made of iron, and there’s a powerful magnet behind you.

The iron marbles feel the magnetic field and the others don’t, hence the different reactions.

The explanation proposed for the masses of the W and Z is similar.

The mysterious field, analogous to the magnetic field of our example, is the Higgs field – first proposed by Peter Higgs in 1964.

The idea of a Higgs field was used to unify electromagnetism with the weak force by Sheldon Glashow, Abdus Salam and Steven Weinberg – a feat which won them a Nobel prize in 1979.

But now the bonus:

If interaction with the Higgs field can “give mass” to the W and Z particles, could it be the case that all particles derive their mass from the same mechanism?!

If true, this would mean that mass is not a fundamental property of matter – just a consequence of some particles “feeling the Higgs field”.

So, does this really happen?  Yes. If you believe the Standard Model

Why do different particles have different masses?

Nobody knows. The model says that a particle’s mass is proportional to how strongly it interacts with the Higgs, but that just pushes the question one step back.

Is there just one Higgs field or many? Nobody knows.

In fact, until a week ago, nobody was sure that there was even a single Higgs field.

A number of alternative theories had been proposed to explain the particle masses without any need for a Higgs – although all these theories have a bunch of side-effects.

So, how would one know if the Higgs field was just a figment of the imagination?

Answer: As mentioned above, every quantum field has an associated particle. The particle associated with the Higgs field is the Higgs boson. (Finally, we get to the title of the post!)

Detect the Higgs boson and you know the field exists.

And that’s what we did on July 4, 2012!

(Note: Strictly speaking I should say we detected a Higgs boson, and so there is at least one Higgs field. For all we know, there could be dozens of them.)

Stagnation

Okay, so if you’ve followed so far, we just verified a theory proposed nearly 50 years ago.

What’s the big deal? And how did we do it?

Answering the first question requires a bit of scientific history.

The late 70’s were a heady time for physicists.

The past six decades had been a period of unprecedented progress, leading us deeper than ever into the secrets of Nature.

The constant stream of insights and breakthroughs, it was felt, could only have one end – the Theory of Everything, unifying space, time, matter, energy and forces into one stupendously grand overarching framework.

Many – including the very outspoken Stephen Hawking – believed that this would happen by the turn of the millennium, bringing the Century of Physics to a supremely triumphant end.

What followed instead was three decades of massive stagnation.

String Theory, hailed as the most promising candidate for unification, degenerated into a thicket of wild speculation and unverifiable hypotheses. Currently, supporters claim that they have made great “conceptual progress”, while detractors argue that the theory is “not even wrong”, i.e. it can be tweaked arbitrarily to fit any observation.

On the experimental side, things ground to a halt with Congress refusing funding for more powerful particle accelerators.

Only the astronomers maintained an iota of progress, indicating that not all was well. Evidence steadily mounted, especially in the last decade, that the matter described by the Standard Model only constitutes about 15% of all the matter there is. Meanwhile, cosmology yielded a huge surprise – the expansion of the universe is accelerating and nobody knows what’s causing it.

But alas, no help was forthcoming from the theorists to explain any of this, lost as they were in the wild goose chase of strings…

For those not enamored of string-world, the easiest path beyond the Standard Model lay in the investigation of the Higgs field(s). This was the only feature of the theory which remained somewhat speculative, but concrete experimental data was needed to make headway.

Hence, all hope focused on the Large Hadron Collider, scheduled to start operating in early 2009 at CERN in Switzerland.

Discovery

The Large Hadron Collider (LHC) is the most powerful particle accelerator ever built.

Magnets of tremendous power accelerate beams of protons are along a circular tunnel 27 km long and smash them into each other with savage force. Hundreds of detectors track the debris that erupts and record its characterestics.

How does this help?

Well, quantum theory predicts that collisions of particles don’t just give you constituents of the things colliding. Completely new particles can arise, born out of the conversion of energy into mass. The higher the energy of collision, the higher the mass of the particle that may appear. (Remember, E = mc²).

[This, to my mind is one of the weirdest aspects of the quantum world. Imagine a situation where you smash two stones hard enough and a butterfly pops out and flies off. That is pretty much what goes on in these collisions!]

The hope was that the collision energy of the LHC would be sufficient to generate a Higgs boson.

But that would just be the beginning, because the Higgs could not be detected directly.

Instead, theory predicts that it would quickly decay into other types of particles.

So, the hope was to spot the relics of the Higgs rather than the particle itself.

To make matters even worse, the collisions at the LHC would generate zillions of other particles.

Thus, a Higgs that forms and quickly decays would only manifest as a slight excess of particles spotted at certain energies.

And finally, the Standard Model did not predict what the mass/energy of the Higgs boson would be. So, the experimenters would have to comb through a vast range of energy bands, looking for tiny excesses in the number of particles of certain types.

See the problem? Seeking a needle in a haystack is trivial by comparison.

But this is what was achieved!

In Dec 2011, after the LHC has run for about 2 years, researchers reported a slight excess of particles in the 120 to 130 GeV range.

(Gev stands for “Giga-Electron-Volts” and is a unit of energy. The mass of a proton is about            1 GeV, which is about 1.8×10⁻²⁷ kg)

But statistical techniques indicated this was only a “two sigma” result – there is a 2% chance of seeing something like this purely by chance. More work needed to be done.

Over the next 6 months, a massive effort was undertaken to scrutinize the mountains of data that had accumulated. Towards the last weeks of June, excitement rose in the scientific blogosphere. Rumors circulated that this was indeed the real thing. And so it was.

On July 4^th, after months of speculation, the announcement was finally made.

Two separate sets of experiments had verified the existence of a particle with a mass of about 125 GeV.

In each case, the probability of a chance occurrence was “five sigma”– less than one in a million.

The Higgs boson had been found.

Dreams and Nightmares

So what does this mean for the future of physics?

Very different things, depending on whom you ask.

Optimists like cosmologist Sean Carroll believe that the Higgs only heralds the beginning of a series of discoveries by the LHC, ushering in a new dawn for physics.

But not everyone agrees.

Detractors argue that all the properties of the Higgs discovered are completely consistent with the Standard Model which is nearly 40 years old. There is no hint yet of anything inexplicable, no trace of any new particles or phenomena, nothing to suggest the next step forward.

Maybe the only new physics occurs at much higher energies, way beyond what the LHC can probe and way beyond our technological capabilities.

This would be a physicist’s nightmare.

So, what shall it be? Dream or nightmare? Progress at last or stagnation without end?

Only time – and more data – will tell.

------x------

Afterword on the “God particle”:

This nickname for the Higgs boson was used by physicist Leon Lederman in a popular book on the subject. It remains, to date, the most egregious example of “God-mongering” to sell a popular science book, closely followed by Stephen Hawking’s “Brief History of Time”.

So just to be clear:

No, the Higgs boson has no religious significance or divine powers, and its discovery does not prove the existence of God.