Physics + Math

15
Mar

The Higgs Boson…or a Higgs Boson?

Yesterday, on March 14, 2013—what would have been Albert Einstein’s 134th birthday—scientists announced another milestone in the hunt for the Higgs boson. After analyzing all the data from the Large Hadron Collider, they have strong evidence that a Higgs boson—and maybe the Higgs boson—has been found. First hypothesized in 1964, the Higgs boson is thought to be the phenomenon that gives mass to all fundamental subatomic particles. Without mass, we would live in a very different place. Electrons, protons, and neutrons wouldn’t combine to form atoms, and you, me, and just about everything else simply wouldn’t exist. This latest announcement is just the next chapter in a story that began eight months ago, on July 4, 2012.

LHC-CMS experiment result
Illustration of a result from the CMS experiment at the LHC, gathered on May 27, 2012.

For Americans, Independence Day is a time for fireworks, family, and barbecues. However last year, the really interesting pyrotechnics were of a scientific nature. They didn’t take place in parks across America, either, but at the CERN laboratory just outside Geneva, Switzerland. There, physicists announced that they had discovered a new particle in data collected using the LHC. As a member of one of the two teams making the announcement, my colleagues and I were thrilled.

Apparently, we weren’t the only ones. Discovering a new particle doesn’t always draw the attention of the international media, but this breakthrough appeared on the home pages of CNN and the BBC and garnered front page, above the fold coverage in the New York Times, the Washington Post, and more. After a search nearly half a century in the making, scientists claimed that they may have found the elusive Higgs boson.

Go behind the scenes at CERN to learn more about the historic July 4 announcement.

The “may” in that last sentence is important. Eight months ago, what scientists had really announced was that a new particle had been discovered. The particle had some of the expected properties of the Higgs boson, but not all of them had been studied. The discovered particle looked and smelled like the Higgs boson, so to speak, but nobody had been able to touch, taste, or listen to it yet. To be sure that the new particle was the Higgs boson, more work was needed. After the announcement last July, the LHC continued to run. Since then, the amount of data it has produced—and allowed us to analyze—has more than doubled.

The Nature of Discovery

In order to understand the significance of Thursday’s press release, we need to discuss the discovery process in a little more detail. There are two types of discoveries in science. The first kind is when you encounter something totally unexpected, like a leprechaun on the beaches of Maine. The second is when someone conjures up some theory which makes specific predictions. Until these predictions are confirmed by experiments, they are just unsubstantiated musings. The discovery of the Higgs boson is a discovery of the second type.

The Higgs boson is named after English physicist Peter Higgs. He wrote a paper in 1964 that predicted an energy field in the universe that gives fundamental subatomic particles their mass. His theory also said that if such an energy field existed, there must be a particle that creates it. That particle is the Higgs boson. The Higgs theory predicted all of the boson’s attributes, except its mass.

Learn how Peter Higgs' ideas about matter transformed our understanding of space.

That’s what we’ve been looking for with the LHC. Discovering the Higgs boson’s mass is the key to unlocking more information about the particle, including its production and decay properties. In addition, the Higgs boson was predicted to be electrically neutral, have no other subatomic particles inside it, and have a quantum mechanical spin of zero. Spin is a distinguishing feature of subatomic particles, just like electric charge. Typically, fundamental particles have a spin of ½ or 1, but the Higgs boson is unique in having a predicted value of zero.

Fruits of More Data

Last July, when we announced we found a new particle, we only were only able to measure some of the ways in which the particle decayed and the rate at which the various decays occurred. It was for this reason that we claimed that we had found a particle which was “consistent with being a Higgs boson.” We simply had not made enough measurements to be sure. It was entirely possible that we had discovered something else.

Because of the additional data we’ve analyzed since then, along with improved analysis techniques, we’ve been able to make more precise and a greater range of measurements. It is these advanced studies that have given us far more confidence that we’ve found the Higgs boson. So what have we learned about it?

Our analysis has told us that the particle has a mass of about 125 times the mass of the proton and no electric charge. The rates at which the particle decays into pairs of other subatomic particles—photons, Z bosons, W bosons, tau leptons, and bottom quarks—are statistically consistent with the predictions of Higgs theory. These are simply improvements beyond what was presented eight months ago.

However, the really new result comes from studies of the subatomic spin and parity of the discovered Higgs particle. We knew that, given the ways it decayed, the particle discovered in July 2012 had either a spin of zero or two, though we didn’t know which. We also didn’t know the particle’s parity. But we do now.

The Higgs boson has zero quantum mechanical spin and specific parity properties meaning that if you reflected the particle in a mirror, it would look exactly the same in the reflection. This property is tricky to visualize, but you can understand it by looking at a mirror with both hands raised in the air. The reflection also has both hands in the air. In contrast, if you had your right hand raised and left hand lowered, in the reflection you would have your left hand raised and right hand lowered. The Higgs boson acts like both hands are raised. Technically, we call a particle with this property as having “positive parity.”

We have also been able to establish, with good precision, that the particle has a positive parity—exactly as predicted by the Higgs theory. When we combine this with the more precise measurements of the decay and production rates, it is now very clear that if this particle isn’t the Higgs boson, it would be a really bizarre coincidence.

Could There Be More Than One?

But if the observed particle isn’t the Higgs boson, what could it be? There are several possible answers. For instance, the Higgs theory proposed in 1964 had, as a result, a single Higgs boson, hence why people refer to it as the Higgs boson. However we know that the Standard Model, which describes the behavior of known subatomic particles, is incomplete. This model doesn’t explain everything. So theoretical physicists have invented new and (they hope) improved explanations to describe a broader amount of phenomena, including why there may be multiple Higgs bosons. One of these is supersymmetry.

Supersymmetry isn’t a theory, as is often assumed by non-experts. Rather, it’s a property of a theory. Technically, a theory is supersymmetric if the equation looks the same when you, in every instance, swap certain terms (those that represent the fermions with spins of ½ and terms that represent the integer-spin bosons, to be specific). There are lots of equations that are supersymmetric.

The simplest of the many supersymmetric theories is called the Minimal Supersymmetric Standard Model, known as MSSM. The MSSM simply adds the absolute minimum number of terms to the regular Standard Model equation to make it supersymmetric. In MSSM, the Higgs theory predicts not one Higgs boson, but five! One of those five has the properties of the Standard Model Higgs boson, meaning that the recently discovered particle may be a Higgs boson rather than the Higgs boson. My colleagues and I are now working to try to figure out which is the case. The answer to that question will have profound implications for our understanding of the universe.

Suppose that what we’ve discovered is the Higgs boson. What have we learned? Well, several things. It suggests that the universe is in a metastable state. That’s a fancy way of saying “pretty stable.” If the universe were in a stable state, it would never change. If it were in an unstable state, it would change quickly. A metastable state is one that persists for a long while, but eventually changes.

If the universe is indeed in a metastable state, then in the far future it might transition to a more stable state. When that happens, a small bubble of stability will appear somewhere in the universe and sweep across the cosmos at the speed of light. Where the new stable state exists, the laws of physics would change. Matter as we understand it would no longer exist. I wouldn’t worry about it anytime soon—the Milky Way will be burned out before that happens. But it is a prediction of the Higgs theory and the observed mass.

The other thing we’ve learned is that the mass of the Higgs boson—if it is the Higgs boson—is relatively small, only about a hundred times heavier than the proton. That’s not exactly what the Standard Model predicts. Instead of several hundred times heavier than a proton, the Standard Model Higgs boson should be more than a trillion, trillion, trillion times heavier. It will take us quite a while to solve this huge disagreement between measurement and theoretical prediction. But when we do, will have a better idea of the ultimate rules that make up the cosmos.

Live To Collide Another Day

Fortunately, we’ll have a few years to mull over our theories and adjust them to the new data. The LHC shut down in December of 2012 for refurbishment and upgrades that are expected to take two years to complete. In late 2014 or early 2015, we expect operations will resume at even higher energy and with much brighter particle beams. My colleagues and I can’t wait. There are a lot of mysteries we hope to solve by studying what is now looking like a—if not the—Higgs boson.