PAW ASKS: Christopher Tully *98

Mass has played a crucial role in the laws of physics for centuries, but as Princeton physics professor Christopher Tully *98 notes, “we didn’t really know where it came from.” With the discovery of the Higgs boson, physicists have new clarity on that fundamental question. Tully, a member of the research collaboration that made the discovery, explains its significance.

TULLY: The Higgs mechanism postulates that the masses of all the elementary particles come from one universal length scale, which is set by the Higgs mechanism itself. ... In physical form, it’s like we live in a big soup of Higgs field [composed of Higgs bosons], everywhere around us, and the mass of elementary particles comes from the degree to which these elementary particles stick to the Higgs field. It seems kind of outrageous that the properties of the particles that make us up depend on the space that we live in. But that seems to be the case.

It reminds me of the old days of science fiction ... where you’d put someone on another planet. Some writers didn’t care whether the oxygen had to be OK, but other people liked very much that you had to wear a suit and supply the oxygen. Now, if you wrote science fiction where you beamed into a parallel universe, if they didn’t have the Higgs field, we would blow apart. We wouldn’t be able to live in that universe. We are intrinsically tied to properties of the space we live in.

— Interview conducted/condensed by B.T.

Click here to send your particle physics questions to Christopher Tully *98. Responses to reader questions will be posted online with a future issue of PAW.

More from PAW’s conversation with Christopher Tully *98

What makes the Higgs field so difficult to detect? 

We have to produce it in a two-step process. We can’t just produce it by colliding beams directly and producing enough energy to create the Higgs boson. We actually have to first create an intermediate form of matter, which is very heavy, and accelerate it. In these particle collisions, we’re producing billions and billions of lower-energy particles. … With all of the stuff that’s produced, we have to fish through to find only those images of the Higgs boson. And because the Higgs particle is sort of a particle of the early universe, now that the universe has cooled down and particles have acquired masses, it doesn’t live very long. If you produce it, you see it just for an instant as it decays away into the mundane particles of today. … Part of the great success of the Higgs discovery is that we can build these five-story digital cameras that run at 40 million times a second and are able to image particles not once but many times in concentric layers.

What role has Princeton played in the search for the Higgs boson?

Princeton has had a group working at CERN for over 40 years. We were the first collaborators when the LHC [Large Hadron Collider] detectors were first proposed in 1994. We did testing for some of the most important new technologies that went into the Higgs discovery – technologies that are as exotic as crystals grown in Russia that have the right properties to be able to see the Higgs decaying into two photons. … The people involved include the initial group leader, Pierre Piroue, who is now emeritus. I actually have been working on this ever since I was a Princeton graduate student – I started back in ’92. We now have several faculty members working on it: Daniel Marlow, Jim Olsen, Valerie Halyo. Based at CERN, we have six graduate students, three post-docs, and two senior scientists. … Jim Olsen works on the coupling of the Higgs boson to what’s called the B quark – that’s one of the heavier quark families. … He, in fact, is going to be one of the leaders for the Higgs effort starting in the next calendar year, which is a very big responsibility.

Now that the Higgs boson has been identified, what are the next steps for researchers at the LHC?

For experimentalists, it certainly has allowed us to think of a program of experimental measurements that would take us to the next level of understanding in how the elementary laws of physics relate to each other. For instance, now that we know where the Higgs boson is, we’ve only measured just enough to discover it. Now we’re going to systematically check all of its properties to make sure that all the pieces of the puzzle really fit together. 

With the LHC, we’ve only scraped the surface. We’ve run it at a little over half the maximum energy and collected an order of magnitude less data than we intend to. The window of opportunity for discovering something big and new is still wide open.

Has the Higgs discovery changed your life as a physicist?

Oh, sure. It was always considered a wall – unless we understand this question, we can really never proceed any further. Now that wall is gone. You’re looking through it, and you’re not even really walking through the entryway yet because you’re so used to being on one side of the wall.   

In the same way that Newton was able to say that the moon is accelerating under the same laws of physics as this apple falling out of the tree, we’re able to say that all elementary particles that have masses are getting the mass for exactly the same reason – and eventually these properties will be better constrained at the LHC and measured with high precision at future colliders.