## Mirjam Cvetic on String Theory and Supersymmetry

#### Special Topic of Supersymmetry Interview, March 2012

Cvetic has been associated with the University of Pennsylvania since her days as a research associate in 1987, having risen to full professor of physics in 1999. Her concentration is high-energy theoretical physics. In 2004, Cvetic discussed her work on black holes for the Special Topics feature devoted to that subject: http://esi-topics.com/blackholes/interviews/MirjamCvetic.html |

In the interview below, Cvetic talks to Gary Taubes about her highly cited work on supersymmetry.

**How did you first get started working on supersymmetry, and what was it about the notion that enticed you?**

I started working on supersymmetry back in the early-to-mid 1980s. I came to the U.S. from Slovenia, which used to be part of the former Yugoslavia. In my home country, studying nuclear physics was the tradition. Since I wanted to study the unification of forces—to study Einstein’s dream of unifying the four different forces of nature and finding underlying principles—I came to the University of Maryland for my graduate work. I stayed in this country, and this became my research career. At the time, there was a very early explanation about how to relate supersymmetry to what we were seeing in particle physics. As a graduate student, that was my first exposure to supersymmetry, and my doctoral thesis was on how supersymmetry can constrain or make more concrete predictions about various features of particle physics. My thesis was on the so-called “left-right symmetric models,” which are slightly enhanced versions of the standard model of particle interactions that include supersymmetry. In the mid-to-late 1980s, when the first string theory revolution took place, I was a post-doc at the Stanford Linear Accelerator and got involved in studying the connection of string to supersymmetry and particle physics.

**Your two most highly cited papers both date to 2001—one in Nuclear Physics B (see #8 on this Special Topic’s 10-year list of highly cited papers)and the other in Physical Review Letters (10-year list, #13). Were these two papers related? And, if so, what made these papers so influential?**

They’re related. Those two papers were trying to describe particle physics by studying consistent four-dimensional string theories using what were then relatively new objects called “D-branes.” This was the beginning of what I would like to call modern string theory, namely, describing string theory with the presence of these D-branes, which have a very beautiful geometric interpretation.

In those two papers, we obtained the first four-dimensional solutions of string theory with D-branes that actually describe the particle physics of the standard model with three families of quarks and leptons and with supersymmetry. Those were the very first consistent constructions from string theory that are stable, self-consistent solutions, and because they also included supersymmetry, these two papers basically launched the effort to describe different particle physics properties from string theory with D-branes. They generated a lot of activity.

The paper published in *Nuclear Physics B*, the longer of the two, also included an important formal development, a mathematical development, which related this four-dimensional string theory with D-branes to a description of what we call bigger M theory and a description of the solutions in terms of curved space time. This was an important connection, and so these papers had a lot of impact. Not only did they start this model-building of particle physics with D-branes, but they also provided a more formal understanding of how string theory works.

**How has your work evolved in the decade since those papers?**

We’ve continued to pursue the construction of four-dimensional solutions of superstring theory with D-branes. What we’ve been trying to understand is not just the type of particles we get from this string theory construction, but also how to derive the particle interactions—namely their couplings, which in turn determines the masses of the particles. For example, how strongly do quarks interact with the Higgs particle, which we hope to find at the LHC, the Large Hadron Collider? We have developed techniques to do this and we can calculate those couplings for specific solutions of string theory. But because each four-dimensional solution has different strengths of these couplings, we still have problems connecting these properties of solutions to what the experiments tell us.

What we can do well is describe consistent four-dimensional solutions of the theory. Our problem is that we have many consistent four-dimensional solutions. We don’t have a unique prediction for a solution that would be a description of our world. We have many of them. So we want to test these solutions against experimental data. Neutrino masses in particular cause serious problems for us—the predictions don’t match the data.

We’ve recently introduced new types of what are called “non-perturbative effects” that could be responsible for small neutrino masses. If we include this new effect, we can explain the origins of neutrino masses for this type of string solution. This effect, however, turns out not to have analogs in standard particle-physics descriptions. It’s coming from new insights into string theory, from insights that couldn’t have been dreamt of just by studying particle physics.

That has been a major effort for me in the past few years. I’ve also been working to describe string theory at genuinely finite coupling. This is what we call “F theory.” Our techniques here are much more limited, and a lot of very important issues still have to be developed. My goal is to explore that further and see if this helps us describe particle physics in F theory formulation where the coupling is not very small. Again, I’m working from a mathematical perspective to understand the theory more deeply and to develop techniques that will help us describe effects at these finite couplings, and also to see how far we can go to connect this particular insight to particle physics.

"If supersymmetry is pushed to very high energies because we can’t find supersymmetric particles at the LHC, it’s not going be a very happy situation for us and our theories."

**How do you approach this work when you have little or perhaps no idea if it is really relevant to our universe or not?**

It’s not necessary to believe that our world is described from these theories. The important thing is that we have to explore this possible connection. That’s part of the drive that brought me into the field: to seek the underlying principle of the unification of the forces. We don’t have proof that the world of particles we see is really uniquely described from string theory, but we’ve reached a lot of beautiful insights nonetheless. My view is that there is such beauty in the geometry and the self-consistency of the theory that, one way or the other, this will be very important to our understanding of nature.

**With the Large Hadron Collider (LHC) up and generating data at CERN, what’s the result you’d most like to see in an ideal world?**

As I said, string theory clearly does not make unique predictions for particle physics, so we’re in a difficult situation. What we do have, though, is a large number of consistent solutions that can produce three families of quarks and leptons, namely the elementary particles found in nature. These solutions often produce additional light particles as well—additional gauge bosons, for example. They’re often referred to as “exotics.” So I’m hoping that the LHC will produce some additional particle physics beyond what we already know. These exotics could have potential implications for dark matter and other cosmological questions. Again, there’s no unique prediction from string theory, but if we could discover something that looked like these additional exotics, that would be very encouraging.

I also want to say that we know the bounds of the masses of supersymmetric particles in many supersymmetric theories of particle physics. Those bounds have already been pushed very high by the LHC. If the simplest supersymmetric extension of standard particle physics is valid, the LHC would have found those particles by now. So I think that, as theoretical physicists working on particle-physics models with supersymmetry expected, that supersymmetry would be discovered at the LHC, and it would certainly be disappointing if supersymmetry is not found after the LHC has run for an extended period. In other words, if supersymmetry is pushed to very high energies because we can’t find supersymmetric particles at the LHC, it’s not going be a very happy situation for us and our theories.

**Should this happen, and nothing beyond the standard model shows up, how do you think theoretical physicists will typically respond?**

I think a typical reaction would be that we can still deal with that, and we just need to adjust the theories. But we have to admit at some point that if our ideas are not being experimentally confirmed, maybe we’re just not on the right track. We need experimental feedback, and without it, theoretical research becomes isolated. On the other hand, it could well happen that the LHC finds completely new physics that has nothing to do with supersymmetry. In that case, we’ll have to focus on alternatives that may not involve supersymmetry or string theory at all.

**Are there viable alternatives that don’t involve supersymmetry?**

There are alternatives. They may involve very strong interactions underlying new composite structures of elementary particles. The problem is that our techniques are not developed enough to deal with these kinds of strong interactions. There are a lot of open questions about describing such strong interactions with effective compositeness of matter. This is what some people refer to as the “onion structure” of nature—quarks or electrons are built of still-smaller objects. If this is the case, as we go higher and higher in energy, we’ll find more and more new structures. But that involves descriptions in terms of these very strong forces, and we don’t have reliable techniques to describe these phenomena. It may well happen, though, that nature chose this path. At the moment, we don’t see evidence of that type of substructure in particles, but who knows?

Other alternatives are motivated by string theory. For instance, if we had extra dimensions to our universe, these might be probed at the LHC. There could be dramatic signals that would indicate the appearance of new particles associated with these not-so-small extra dimensions. These effects could be very dramatic and not necessarily tied to supersymmetry

**Which do you think would be the more exciting or satisfying discovery: supersymmetric particles, or evidence of compositeness or something unexpected entirely?**

If supersymmetry is discovered, even if the supersymmetric particles turn out to have very large masses, that would be more satisfying, because we have techniques to deal with it, and it fits very naturally into string theory. If, however, completely new, unexpected things were to show up, there would be brainstorming and huge activity trying to develop new ideas from a theoretical-physics perspective that would describe these phenomena. This outcome might not be as satisfying, but it would be extremely exciting.

**Are the multitude of supersymmetric models sufficiently well-constrained that if a new particle was discovered, we would know for certain whether it was or wasn’t evidence of supersymmetry?**

In certain cases, yes, because supersymmetry has very constrained couplings and interactions. A simple example is that supersymmetry has put such constraints on interactions and masses that it will be a real problem if the Higgs particle turns out to have a mass greater than 150 GeV. It would be extremely hard to accommodate that within supersymmetry. The same could be true with new particles and the way they interact, the way they’re created, and in what channels, etc. These may not fit particular supersymmetric descriptions. But I have to admit that I’m not myself very heavily involved in this kind of collider-physics phenomenology. There are topnotch experts that could analyze in detail the phenomenological properties of the LHC signals and unambiguously determine whether this can or cannot happen in a supersymmetric description of the theory.

**Mirjam Cvetic, Ph.D.
University of Pennsylvania
Philadelphia, PA, USA**