Q&A: Expecting the Unexpected

Physics 11, 77
Particle physicist Matthew McCullough is hopeful about finding physics beyond the standard model in data from the Large Hadron Collider.
Matthew McCullough/CERN; adapted by APS/Katherine Wright

Matthew McCullough is not panicking. The “new physics” that many physicists assumed would be seen at the Large Hadron Collider (LHC)—in the form of new particles—hasn’t shown up. But McCullough, a theoretical physicist working at CERN, sees opportunity in being jilted by Nature. He thinks that this perceived setback could spur a revolution, with physicists scratching some of their most cherished assumptions about the Universe and starting over. In the end, they may find a new fundamental theory that works like “clockwork.” Physics spoke to McCullough to find out whether he thinks new physics might still be uncovered at the LHC and, if so, where in the data it might be hiding.

–Michael Schirber

What attracted you to particle physics?

Particle physics appeals to me because it gives answers to fundamental questions about Nature and why it works the way it does. But what’s also important to me is that these answers can always change. Physicists never have “the” absolute theory of everything. We have the best theory we can come up with. And then some unexpected experimental result comes along, and we realize that there is another layer of reality that we hadn’t conceived of before.

Your work seems to strive to find that new layer. Is it time to come up with a new “best theory”?

At the LHC, the only new fundamental particle that’s been seen is the Higgs boson. Finding the Higgs was an incredible result. But other expected particles, like those predicted by supersymmetry, haven’t yet turned up. That has led many particle physicists, including me, to revisit theoretical assumptions.

I am interested in whether we’re missing something because of how we look at the data. Normally, when searching for new particles, we look for a single bump—like that seen for the Higgs. But what if the LHC produces lots and lots of particles that individually only make little bumps? The only way to see that scenario would be to look for the bumps collectively, as an oscillation or “wiggle” pattern in the data.

Are there theories that predict such a wiggle pattern?

Yes. There are currently two general possibilities. The first one is the “linear dilaton model,” which involves extra dimensions in spacetime. The second option relates to a class of models known as clockwork models that were proposed in 2015. My colleague Gian Giudice and I have worked on extending clockwork models to a more general setting.

What is a clockwork model?

The original clockwork model ties together a set of symmetries, called U(1) symmetries. These symmetries describe fields that are invariant to a phase rotation. Such a field could be associated with a force, like those in electromagnetism. Imagine having many of these fields and linking them together such that a phase rotation of 2𝜋 for one field causes a phase rotation of 6𝜋 for the next field, 18𝜋 for the one after that, and so on. As a result, a tiny phase rotation at one end of this so-called clockwork chain becomes an enormous phase rotation at the other end. It’s like a bike with two gears, one large and one small: turn the large gear around once, and the smaller gear goes around several times.

What are these models good for?

These models could explain the existence of exponentially weak forces in a variety of settings. For example, they could help us understand why gravity is so much weaker than all the other forces. There are various ways this could work, but one way is to assume that there are N phases and to associate each phase with a force. If each step in the chain increases the phase rotation by a factor of 3—like in the previous example I gave—then the force at one end will be a factor of 3N times weaker than the force at the other end.

And where do the wiggle patterns come in?

Assume each field of the clockwork is associated with a particle. If there are ten fields, then that implies that there are ten particles. The clockwork model predicts that those ten particles might show up in collider data as a set of closely spaced bumps.

You seem to enjoy considering alternative models. Is it good that the LHC hasn’t found evidence of supersymmetry?

I wouldn’t go that far. There were compelling reasons for postulating supersymmetry, and not seeing its particles challenges those reasons. That’s neither good nor bad; we just have to take what Nature gives us.

Moments like this one can be revolutionary. A previous example would be the search for the aether. Not finding the aether was—despite perceptions—a tremendous success because the effort to explain that nondetection eventually led to the development of relativity. I feel like we are in an “aether moment” now, where scientists expect something to show up and it hasn’t. The absence of supersymmetry is probably telling us something quite profound; we just have to figure out what.

If you went into work tomorrow, and someone told you that the LHC found something big, what would you wish that discovery to be?

I’d wish for something that entirely throws our picture of the world on its head, something so far beyond current ideas that we’d have to start from scratch.

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

Know a physicist with a knack for explaining their research to others? Write to physics@aps.org. All interviews are edited for brevity and clarity.

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