(An advanced particle physics topic today…)
There have been various intellectual wars over string theory since before I was a graduate student. (Many people in my generation got caught in the crossfire.) But I’ve always taken the point of view that string theory is first and foremost a tool for understanding the universe, and it should be applied just like any other tool: as best as one can, to the widest variety of situations in which it is applicable.
And it is a powerful tool, one that most certainly makes experimental predictions… even ones for the Large Hadron Collider (LHC).
These predictions have nothing to do with whether string theory will someday turn out to be the “theory of everything.” (That’s a grandiose term that means something far less grand, namely a “complete set of equations that captures the behavior of spacetime and all its types of particles and fields,” or something like that; it’s certainly not a theory of biology or economics, or even of semiconductors or proteins.) Such a theory would, presumably, resolve the conceptual divide between quantum physics and general relativity, Einstein’s theory of gravity, and explain a number of other features of the world. But to focus only on this possible application of string theory is to take an unjustifiably narrow view of its value and role.
The issue for today involves the behavior of particles in an unfamiliar context, one which might someday show up (or may already have shown up and been missed) at the LHC or elsewhere. It’s a context that, until 1998 or so, no one had ever thought to ask about, and even if someone had, they’d have been stymied because traditional methods are useless. But then string theory drew our attention to this regime, and showed us that it has unusual features. There are entirely unexpected phenomena that occur there, ones that we can look for in experiments.
Forces that are Weak and/or Strong
The strong nuclear force, despite the name, isn’t always strong. (“Strong” is meant in a relative sense; it is strong compared to the typical force at the same distance, as described here.) The strong nuclear force is strong when measured at distances comparable to or larger than the size of a proton (a millionth of a billionth of a meter) and in subatomic processes with energy much less than the E=mc2 mass-energy of a proton. It’s in this context that the strong nuclear force traps (“confines”) quarks, anti-quarks and gluons inside protons, neutrons and other “hadrons”, such as pions. (A hadron is any particle made from quarks, anti-quarks and gluons.) But when collisions occur at much shorter distances and higher energies than this, the strong nuclear force becomes relatively weak: only a few dozen times stronger than electromagnetism at the same energy scale.
This feature, called “asymptotic freedom,” makes the force (relatively) weak enough that we can think of quarks and gluons in high-energy collisions as nearly free particles that scatter off each other only occasionally, much the way we typically think of electrons and photons. The 1973 discovery of asymptotic freedom in the theory of quarks and gluons won three of the physicists involved the 2006 Nobel Prize. The discovery of quarks themselves was made around 1970 (following suggestions of James Bjorken) under the assumption that asymptotic freedom was true. That discovery won the 1990 Nobel Prize; Bjorken didn’t win a Nobel, but was awarded the Dirac medal and a couple of other prizes.
Thus the strong nuclear force, as for any similar force that we might someday uncover in nature, shows different behavior in two distinct regimes:
- At high energy and short distance, the force between particles is weak and the particles do not act as though they are confined.
- At low energy and long distance, the force between particles is strong and the particles act as though they are confined.
Now, you might guess this is the general pattern; weak forces mean no confinement, and strong forces mean confinement. It turns out isn’t actually the case. As was learned over the two decades following 1973, there can be strong forces without confinement. For instance, a force could be strong and non-confining at high energies and short distances, while also being strong and causing confinement at low energies and long distances.
We haven’t discovered such a force so far. But the forces we know today may be nowhere near the complete list, so we need, in our experiments, to be on the lookout for new forces, including ones that might have unfamiliar behavior. We need to keep our eyes out for a force of this type, whose quark-like and gluon-like states may still have strong forces even at high energy, and may never act at all like free particles that scatter occasionally — they may never be “asymptotically free.”
To illustrate this in the crudest way possible, I’ve drawn a cartoon below of these two possibilities. Again, only one appears in the real world so far, but the other type of force might show up some day.
How would we know we’d found such a force experimentally? A key feature of real-world quarks and gluons is that a high-energy quark turns into a jet of hadrons, as I described here. This jet is narrow; we see such jets all the time in experiments. Now suppose we found a new force of nature, with its own quark-like, gluon-like and hadron-like particles. If it is similar to the strong nuclear interactions, its hadron-like particles would appear in narrow jets. But if it were strong at high-energy, then the corresponding jets of hadron-like particles would be more numerous and much wider than the ones we get from the strong nuclear force. Exactly how wide and numerous these jets would be is hard to calculate, but we could try to make a rough estimate by extrapolating traditional methods.
All of this is stuff that was known as far back as the mid-1980s, and was applied to certain theories of the Higgs field in the context of “Walking Technicolor” (though I’m not sure when the experimental implications for jets were first emphasized; we were first thinking about the issues in 2001-2006.) As far as I know, no one suspected there were any other possibilities. But then in 1997, something new came onto the scene, emerging from mathematical realms of string theory.
Enter String Theory
In 1997, Juan Maldacena proposed that strings, when moving around on certain curved shapes with nine space dimensions and one time dimension, can be exactly the same as (that is, indistinguishable from) certain combinations of particles moving around in a familiar flat space with three space dimensions and one time dimension.
I’ve let Maldacena’s proposal be its own paragraph, because the incredible strangeness of his claim needs to resonate in the air for a bit. We’re comparing two different types of objects moving around in completely different contexts. The equations that describe them look utterly unrelated. The strings come with gravity; the particles they’re equivalent to feel no gravity. Even the number of dimensions is different. And yet, it is claimed, there’s a translation table between the two. You can take any physical question in the string context and translate that question into the particle context, and vice versa. And if you answer the string version of the question on the one hand, and answer the particle version of the question on the other, you will get exactly the same answer, even though none of the steps of the two calculations will look at all the same until the very last step.
This proposal, often referred to as “gauge/string duality”, is still not strictly proven. But there is an enormous amount of evidence now, and many cases in which the implications of the proposal are known to be qualitatively or even quantitively true.
A practical implication of Maldacena’s proposal is that there are certain extremely challenging questions about strings that can easily be answered by “quantum field theory” (the math we use for describing particles.) The reverse is also true: there are issues that arise in quantum field theory, and may even be seen in particle collisions at the LHC, for which the only path to an answer is through the math of string theory.
In exploring Maldacena’s proposal, scientists learned that among forces that have confinement, there is a third, qualitatively different class from the first two I mentioned. We might refer to these as “ultra-strong forces”; I’ve added it to the cartoon below. It’s ultra-strong forces which are most directly reflective of the physics of string theory.
[The technical distinction between these three regimes involves the ‘t Hooft coupling αN, where α is the standard coupling strength, and N is the number of “colors” (i.e. versions) of each type of quark. In the three regimes, the ‘t Hooft coupling is much less than 1 (weak), close to 1 (strong), or much greater than 1 [but still less than N] (ultra-strong). Prior to Maldacena’s work it was more or less assumed that strong and ultra-strong forces were qualitatively similar.]
The Experimental Prediction
A prediction of string theory is that there is a third regime for confining theories, which I’ll call the ultra-strong regime. Production of particles in that regime (as well as the structure of hadron-like objects) is different from the other two. For a force that is ultra-strong, high-energy quark-like and gluon-like particles do not lead to jets, narrow or wide. Instead, their production leads to nearly spherical sprays, with no hint of jetty structure.
(I gave an intuitive argument for this, and discussed its experimental implications briefly, here. The argument was based on work I did in 2002 with the late Joe Polchinski, on hadronic structure in ultra-strong confining theories. Around the same time, Hofman and Maldacena made a similar but more confident claim, with a decisive proof that relies on gauge/string duality. Another closely related paper appeared at the same time by Hatta, Iancu and Mueller.)
Nowadays people call this kind of spherical spray by the cute acronym SUEP, for Soft (meaning lots of low-energy particles) Uncorrelated (meaning they go in random directions, forming a near-spherical spray) Energy Pattern.] We don’t understand nearly as much about SUEP theoretically as we would like — there are lots of subtleties to consider in realistic situations — so predictions for experiment are still quite crude.
But experimenters at the LHC have this on their radar screen, and rightly so. What’s really important is that this type of phenomenon, depending on its precise realization,
- could be thrown away by the trigger system and lost permanently, or
- could be very difficult to look for because of the way LHC data is stored, or
- could be buried unseen within the huge amount of LHC data.
So even though a new force of this type, through its SUEPy signature (and variants of this signature, which I haven’t time to mention here), is just one of many possibilities that experimenters need to be looking for, it’s vitally important that they think about it carefully in advance. Otherwise they may inadvertently make it unnecessarily difficult or impossible to find, because of their assumptions on how best to gather, store, and analyze data. (An important theoretical paper looking at the trigger issues, by Knapen, Pagan Griso, Papucci and Robinson, appeared in 2016. Much more work remains to be done.)
The experimenters at the LHC have an interesting but tough job. There are dozens of possible novel phenomena that they need to be looking for in their data. Any one of them is a long shot; SUEP is no exception. But if it shows up someday, we’ll have string theorists like Maldacena (and many others, such as Igor Klebanov, Sasha Polyakov, Steve Gubser, and Ed Witten) to thank. Without the years spent developing string theory’s mathematical underpinnings — without all that fancy supersymmetry and supergravity and extra dimensions and extremal black holes — those of us who make direct predictions for experiments wouldn’t even have known that SUEP was on the menu.
An apology: this is immensely oversimplified, for the sake of relative (!) brevity. If enough readers are interested I can try to go into more depth at a later time, though this would require quite a few posts, not only on SUEP but on Hidden Valleys more generally.