Sven Heinemeyer on Supersymmetry at the Large Hadron Collider

Special Topic of Supersymmetry Interview, February 2012

Sven Heinemeyer

Our Special Topics analysis of papers published 2001 to 2011 dealing with supersymmetry and the search for the Higgs boson has identified Professor Sven Heinemeyer of the Spanish National Research Council (CSIC) as a highly cited theoretical physicist in the field of supersymmetry. In the last decade he has contributed to 88 of the papers identified for this topic, gaining 3,135 citations, earning the #3 spot in the author ranking by papers, and the #6 spot by total citations.  

Heinemeyer is currently on the research staff at the Instituto de Fisica de Cantabria (IFCA-CSIC), Santander, Spain.

Supersymmetry arises from attempts to extend what’s known as the standard model of particle physics. The postulated symmetry of the theory proposes that (speaking simply) every elementary particle has a “superpartner” that differs by one-half unit of spin, all other quantum numbers being identical. In other words, for every type of boson, there exists a fermion, and vice versa. Theorists have worked on supersymmetry for over 30 years because it answers many problems in particle physics that are intractable in the standard model.

To learn more about Heinemeyer's Highly Cited papers in this area, European correspondent Simon Mitton spoke to him in his office in Santander.

SW: To outsiders, theoretical particle physics seems to be a field of awesome complexity and difficulty. I know that fundamentally this field is about finding out how the world works, but it is a quantum world that, superficially at any rate, appears detached from reality. So what got you into this great intellectual puzzle?

Television! When I was about 16 I was fascinated by a presentation of the standard model on German TV. I found it absolutely thrilling that scientists really understood how the world works. So at university I studied physics. I kept particle physics as my ultimate goal, and it was the topic of my doctoral thesis in Karlsruhe.

SW: When did you decide to go beyond the standard model and work on supersymmetry, and where have you carried out your research?

My doctoral adviser in Karlsruhe proposed the topic to me, and I liked it and stuck to it. Then in 1998 I started research on it as a postdoc at DESY Hamburg, the largest particle physics research center in Germany. After that I had a fellowship in the theory division at the Brookhaven National Laboratory, then I went back to Munich, and finally I went to CERN Geneva, home of the Large Hadron Collider (LHC). Here in Santander I was the first theorist to be hired, and I now have a small group of postdocs and doctoral students all working on supersymmetry.

SW: Can you give me an easy introduction to the Higgs boson and the LHC?

We have a model of nature that explains all of the experimental results obtained with colliders over past decades. It’s rather simple and is known as the standard model of particle physics. All components of this model have been seen experimentally apart from one: the famous Higgs particle. This particle is needed within the theory in order to make it mathematically consistent. For that reason many people consider that the particle must exist. That’s why so much effort has been spent on looking for it, so far without success. But we now have the LHC up and running at CERN, and it is capable of probing the energy levels at which the Higgs boson will have to show up—if it exists.

SW: So finding the Higgs particle would be a great prize, a triumph for theoretical physics and the standard model. But why do we need the complexity of supersymmetry? Can you give me an example from my own field, astronomy?

Yes. The standard model cannot explain the cold dark matter found by astrophysical experiments. Observations of galaxies and their dynamics show that there must be more matter in the universe than that which is visible in stars. The existence of dark matter is now proven beyond reasonable doubt. Supersymmetry offers a candidate for cold dark matter: When theorists first developed supersymmetric models, they realized that the lightest supersymmetric particle could be a dark-matter candidate. This particle has the right properties to account for the dark-matter content of the universe. It's one of the big successes of the theory. Admittedly we still lack direct evidence, but there are experiments looking for such particles, although we have not seen any clear signal so far.

SW:  I'm aware that theorists feel the standard model is unsatisfactory from an aesthetic point of view because there are three parameters that are measured experimentally, but how they’re connected is a mystery.

Three forces in nature feature in the standard model: the electromagnetic, strong, and weak forces. They dictate how particles and fields behave. We’ve measured the coupling strengths of these three forces, but we do not know why they are what they are. We want a theory that unifies these forces, particularly at the high energies that existed in the early universe. That concept, unification, has been successful in the past, for example with the unification of the electricity and magnetism forces. Unfortunately, in the standard model this does not work!

SW:  How does research on supersymmetry fix that?

In supersymmetry the unification of the three forces just falls out, because all three forces automatically unify at very high energies, something that does not work out in the standard model.

The concept of supersymmetry arises because in nature we have two classes of fundamental particles: the fermions with half-integer spin (internal angular momentum) and the bosons with whole-integer spin. Supersymmetry achieves symmetry between fermions and bosons. As a physicist, it is almost inconceivable to me that this symmetry does not exist. We don’t know at what energy the symmetry might be realized, but it could be at energy levels accessible with the LHC. Most of the papers in this Special Topics analysis are concerned with the search for supersymmetric particles.

"The results of the Higgs searches at the LHC announced in December 2011 were extremely encouraging. Indeed, both large LHC experiments, ATLAS and CMS, are seeing some hints of a Higgs particle, exactly in the mass range predicted by our supersymmetric models."

SW:  Before we dive into the details, would you like to give me an overview of the content of your highly cited papers?

At a general level, these papers are all about trying to discover supersymmetric particles, including the famous Higgs boson. Do note, however, that in supersymmetry there are five or more Higgs particles, all as a natural outcome of the model. My high-impact papers focus on the potential for the planned searches, or the indirect evidence that supports the existence of these particles.

SW:  Most of the papers in the list were published before the LHC started. They suggest what experimenters should look for. Can we look at your most cited? (Allanbach BC, et al, “The Snowmass Points and Slopes: Benchmarks for SUSY searches,” Eur. Phys. J. C  25[1]: 113-23, 2002).  This has an intriguing title—is there a story behind it?

The “Snowmass Points and Slopes” in the title are a set of benchmark points and parameter lines in the parameter space of the Minimal Supersymmetric extension of the Standard Model, or MSSM for short. These benchmarks were agreed upon at a workshop held in 2001 in Snowmass, Colorado. We defined about 10 supersymmetric model points that were used over the last decade to analyze the potential of the LHC. We did simulations of the behavior of particles. We looked at how the experiments would respond to our different models, by running simulations on the 10 test points. In effect that paper says: “If this point is realized in nature, then we would expect to see the following behavior in LHC experiments.”

SW:  The LHC has now run for more than one year searching for supersymmetry, but so far without finding anything. So what did paper #1 actually achieve?

Most of our test points have been ruled out by LHC. This is not alarming; in collider physics you make predictions from your models and if you do not see anything, then the point is excluded. That does not mean the model as such is excluded. It simply means that a specific point in parameter space is ruled out. Paper #1 got so many citations because so many people were looking at the test points, or benchmarks.

SW:  Your next-most-cited paper for this Special Topic stands out as a rarity in your publication list because it's experimental. (Schael S, et al., “Search for neutral MSSM Higgs bosons at LEP,” Eur. Phys. J. C 47[3]: 547-87, 2006.) What was your contribution?

The paper is about searching for the Higgs boson in the supersymmetric models, using an earlier experiment at CERN called LEP, or Large Electron Positron collider. The result was negative, but we nevertheless established lower bounds on the masses of these supersymmetric Higgs particles. I’m pleased with this paper: its authors amount to 1,000 experimentalists and just three theorists! It’s a splendid example of theorists and experimentalists working hand in hand in a great team effort.

SW: Elsewhere on your listing I've noticed a quartet of related papers (Degrassi G, et al., Eur. Phys. J. C 28[1]: 133-43, 2003; Heinemeyer S, Eur Phys J. C 22[3]: 521-34, 2001; Frank M, et al., J. High Energy Phys 2: No. 047, 2007; Heinemeyer S, et al., Eur. J. Phys. C 39[4]: 465-81, 2005.). Why was so much effort expended on these high-precision predictions for the mass of the lightest Higgs boson?

In the standard model, the mass of the Higgs particle is a free parameter, but in supersymmetry we can predict the mass in terms of other parameters. If we could measure the parameters of all the other particles, we could predict rather precisely the mass of the lightest Higgs particle, and then we could see if the predicted and measured masses agree. Such an agreement would be a profound test of supersymmetry.

This quartet of papers is devoted to the second-order corrections or 1% tweaking, which roughly agrees with the precision we expect in the experiments. My colleagues and I are putting a lot of effort into making precise calculations at the 1% level. These four papers explain what kinds of corrections we made. They are highly cited because many people are searching for the Higgs boson, and they demand accurate predictions.

SW:  When the Higgs boson is found, is that the end of the quest to understand the physical forces?

The popular notion that the LHC will find the Higgs particle, and that’ll be the end of the story, is far too simple. We may find a new particle, but before we can say “That’s it!” we must be sure that its behavior corresponds exactly to the predictions.

Paper #6 deals with measurements of the couplings of the Higgs boson, which means we know how it decays to particles of lesser energy. The question is whether the outcomes of the decays can be measured at the LHC. This paper was the first to address the question, and achieved to a level of 10-20% the probabilities of how the Higgs boson of the standard model decays to other particles.

SW:  Finally, when can we look forward to some dramatic news from LHC?

The results of the Higgs searches at the LHC announced in December 2011 were extremely encouraging. Indeed, both large LHC experiments, ATLAS and CMS, are seeing some hints of a Higgs particle, exactly in the mass range predicted by our supersymmetric models. However, not enough data could be collected in 2011 to give a definite answer. We expect to know more by mid-2012 or by the end of this year at the latest. Then we will know whether there is a new particle behaving more or less like a standard model Higgs—which would be in perfect agreement with our supersymmetric predictions—or whether the current standard model of particle physics has finally cracked. But I’m very optimistic that our predictions will turn out to be correct. 2012 could be our “annus mirabilis.”End

Sven Heinemeyer
Instituto de Fisica de Cantabria IFCA-CSIC
Edificio Juan Jorda, Avda. de Los Castros s/n
39005 Santander, Spain


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