Dr. Joe Incandela on the Higgs Boson

© 2012 CERN © 2012 CERN

Dr. Joe Incandela is a professor of physics at the University of California Santa Barbara. At CERN, he also leads the Compact Muon Solenoid team, which in 2012 made headlines when they announced the detection of the Higgs Boson.

On November 13, Dr. Incandela will give a talk at the Simons Foundation in New York, where he will discuss the discovery of the Higgs Boson, as well as what is to come. Register now; seats are limited!

Prior to his talk, we asked him a few questions to help us better understand the basics about this elusive particle.

What is a boson and why is the Higgs Boson special?
There are two types of particles: Bosons and Fermions. Bosons have whole integer values of intrinsic angular momentum (‘spin’). Fermions have half integer values. The difference is huge. Identical Fermions cannot be in the same state. This is a key ingredient in making the different atomic elements. It is the basis of structure. Identical bosons, on the other hand, can occupy the same state and even prefer to do so. They can work together as a coherent entity. Force fields are bosonic fields. For instance the electromagnetic field is made up of photons of spin 1.

What is special about the Higgs boson is that it is a fundamental particle with no spin (spin 0) and it is the first such particle ever discovered. Fundamental spin 0 particles are peculiar in many ways, as they can provide a force field that has no direction in space. Such a force field may be at the root of the inflationary eras of our universe. A field based on spin 0 particles could have a constant value everywhere in the universe without affecting the isotropy of space-time.

Simulated signature of a Higgs Boson as a result of a collision between two protons. © 1997 CERN

Why have some dubbed it the “God particle”?

I do not use this term, nor do any of the physicists I know at the LHC so I can only speculate as to why some people have called it this. It may be for the following reasons. Higgs particles are the basis of a special force field that permeates the universe. This force field has a constant strength everywhere and it interacts with most of the fundamental particles and engenders them with mass.

In the Standard Model of particle physics, all massive fundamental particles get their mass this way. (This is not the only source of mass. A proton or neutron gets most of its mass from the strong binding energy of the quarks.) Without the Higgs mechanism the electron would be massless, and so atoms could not be formed. There would be no stars, planets, and no people.

What is special about the Higgs boson is that it is a fundamental particle with no spin (spin 0) and it is the first such particle ever discovered.
In 1964, Peter Higgs, François Englert, Robert Brout and others predicted the existence of the Higgs Boson. Was that by experiment or pure theory?

What they did was purely theoretical. In fact it was not yet quite connecting to a specific description of nature. At that time it was already known that the two nuclear forces (known simply as the weak and strong forces) appeared to be short-distance forces, unlike electromagnetism and gravity. Short-distance forces could not easily be explained in quantum field theory.

Many people tried to solve this problem. Brout, Englert and Higgs were among the first to come up with a solution. Namely, that of a spin 0 field with a constant value in space that actually can limit the distance over which a force can act (which is equivalent to making the force carrying particles have mass). Steven Weinberg and others used this idea a few years later to successfully describe the weak nuclear force.

In July of 2012, your team detected the Higgs Boson. The main result slide showed they had attained statistical significance at “5 sigma”. What does that mean?

Dr. Joe Incandela © 2012 CERN

When we say we see ‘an excess of 5 sigma’ we mean that we see an excess of candidate Higgs events produced in high energy collisions of protons that is well above what we would expect for background processes.

We carefully estimate the number of events expected from backgrounds and the uncertainty on that number. This uncertainty in the background equals 1 sigma (equivalently 1 standard deviation). We require the excess we see for any mass value to be at least 5 times this number above the best estimate of the background. In statistics this would mean that the excess has less than about a 3 in 10 million chance of being a statistical fluctuation of the background. This is a very high standard, but it is in reality very difficult to do these experiments and it is possible to make mistakes and be fooled in a number of ways.

Later in the press conference, Rolf Heuer (CERN Director) said: “We have discovered a boson, but we have to find out which kind of boson it is”. How many kinds of Bosons do we expect are out there?

Well theoretically there can be many different kinds of bosons left to be found. Indications were pretty good that this was a Higgs boson (e.g. because of the rates at which we saw it decay to pairs of photons or pairs of Z bosons) but we were not sure if it was the simple type of boson predicted in the Standard Model or one of several that might exist in more complete theories of nature. It could also have been a more exotic spin 2 boson for instance or a boson that is a composite of other particles.

Without the Higgs mechanism the electron would be massless, and so atoms could not be formed. There would be no stars, planets, and no people.

It would be difficult to rule out these other possibilities with absolute certainty, but we have done many studies since the 4th of July announcement, with much more data, and more sophisticated analysis methods and so far everything is consistent with a spin 0 Higgs boson, like the one in the Standard Model. More work is needed however.

How will the Higgs Boson help us better understand the universe?

As I noted above, this is the first fundamental spin 0 particle ever discovered and as such, gives physicists confidence to develop theories based on spin 0 force fields that would explain the inflationary periods in which the universe expanded rapidly.

In addition, within the Standard Model of Particle physics, the newly measured mass of this particle, together with information we have on the top quark mass, allow us to assess the stability of the universe and potentially understand how long it will last before undergoing a major transition to something very different. There are in fact hundreds of ideas coming out. The discovery papers we submitted at the end of July 2012 already have more than 1800 citations giving some idea of how much interest this has generated in the field.

What will you discuss at your talk on November 13?

incandela2In my talk on November 13 at the Simons Foundation I will review the basics of particle physics and the reason for the Higgs boson to have been predicted and then I will cover all the hard work it took to make the discovery. I will then present the newer results, talk about what we will do in the near future and even further down the road to understand this new particle better.

Finally I will briefly talk about the big open questions (beyond the Higgs boson) that we hope to start to answer in coming years. This includes for instance trying to understand what is dark matter, which is more abundant than ordinary matter by about a factor of 5.

A video from PhD Comics to explain the Higgs Boson

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