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Marcela Carena

Beyond the Standard Model

Fermilab’s director of international relations talks about the challenges of new theories that attempt to expand our understanding of particle physics

The Argentinian physicist Marcela Carena: new theories to try to explain events for which the standard model has no satisfactory answers

Léo RamosThe Argentinian physicist Marcela Carena: new theories to try to explain events for which the standard model has no satisfactory answersLéo Ramos

In November 2015, theoretical physicist Marcela Carena became the first person to hold a recently created position: that of Fermilab international relations director. The laboratory is the premier particle physics laboratory in the United States, located in Batavia, near Chicago. There is no lack of academic honors qualifying this nice, 53-year-old Argentinian woman for her position as the laboratory’s ambassador. She is the head of the Theoretical Physics Department and Fermilab and professor at the University of Chicago. She has already worked on three continents and speaks six languages. “I remember my first trip to São Paulo, when I was 25, and I learned how to ask, in Portuguese, if the bus station was near or far,” says Carena, who has Italian and US citizenships, in addition to Argentinian.

During her frequent trips abroad, the physicist now spends most of her time seeking international partners willing to participate in a scientific megaproject that is being developed by Fermilab: the Deep Underground Neutrino Experiment (Dune), a billion dollar undertaking that will attempt to discover new properties of neutrinos, one of the most difficult elementary particles to detect (see interview with Nigel Lockyer, Fermilab director, in Pesquisa FAPESP Issue nº 235). But there is always room in her schedule for events in which the researcher in her is more prominent than the international relations director. One of these occasions was in early February 2016, when she was in the city of São Paulo to participate in a seminar at the South-American Institute for Basic Research (ICTP-SAIFR), based in the Theoretical Physics Institute of São Paulo State University (IFT-Unesp).

At the event, Carena spoke of the outlook for physics research attempting to fill in the gaps not covered by what is known as the Standard Model, a theory that has been increasingly refined since the 1960s and tries to explain how particles and forces have been interacting since the origin of the Universe. The Standard Model predicts the existence of two major types of particles: fermions and bosons. Fermions are particles of matter (electron, muon, tau, three types of neutrino and six types of quark). Bosons are particles that transmit electromagnetic forces, the strong nuclear force and the weak nuclear force, and are absorbed or emitted by fermions. They include the photon, the gluon, and the Z and W bosons. In 2012, the last particle predicted by the model, the Higgs boson, was discovered. It provides the mass for the other elementary particles. The model was complete, but was, and still is, insufficient to explain the cosmos. In this interview, Carena addresses some of the limitations of the model and mentions theories, such as those based on the concept of supersymmetry, that try to provide hints for the questions still unanswered.

Why are people searching for theories to correct or extend the Standard Model?
The model works incredibly well. Now that we have found the Higgs Boson and calculated its mass, we see that this particle fits it well enough.  But this does not mean that we have nothing left to do. The model explains everything very well for the energy levels that we can access. But there are some things for which it does not provide satisfactory responses. It does not explain dark matter, for example [actually, the model provides answers for only 4% of the composition of the known Universe and says nothing about the origin of its dark matter (23%) and dark energy (73%)]. Nor does it explain the asymmetry between the amount of matter and antimatter observed in the Universe [hypothetically, after the Big Bang, the amount of matter and antimatter was balanced but, so far, astrophysicists have found mainly particles, and almost no antiparticles.

Proton collision at LHC that could have generated the Higgs boson: particle mass is very sensitive

CMS/CERN Proton collision at LHC that could have generated the Higgs boson: particle mass is very sensitiveCMS/CERN

But when the model was proposed, dark matter had not yet been discovered.
Yes, of course. In the Standard Model, neutrinos are part of dark matter, but a small part. There must be something more. In truth, we do not have a clear idea of what dark matter could be made of. It could be made up of one particle, many particles, or massive particles that interact weakly, called Wimps [weakly interacting massive particles, for now just a theoretical proposal]. It could be made up of axions [hypothetical elementary particles] that would exist to explain some chromodynamic quantum problems in the Standard Model. Basically, we do not know what dark matter is made of. We just know that it is there. If it were not, we would not know how to explain much of what we see in astrophysics.

Are neutrinos a big problem for the Standard Model?
We can say that they are more or less within the scope of the Standard Model. They are predicted, but without mass. Since we now know that they have a very small mass, perhaps this mass, and only the mass of neutrinos, does not come entirely from the Higgs boson. The mass of all particles comes from the Higgs boson, but maybe another mechanism contributes to the mass of neutrinos. In addition, according to the Standard Model, neutrinos could only be left-handed. But today we know that neutrinos have mass and, in order to oscillate, there must also be right-handed ones. Neutrinos are called left-handed when the spin and propagation are in opposite directions, and right-handed when they are the same.

Does this idea that there could be other ways to give particles mass apply to all neutrino types?
Let’s assume that there are only the three neutrinos we know about today: the electron, the muon and the tau. It is possible that the mass of these three neutrinos is partially due to the Higgs mechanism and partially due to something else. There is a reason to think this. Possibly, the right-handed neutrinos, instead of being very light like we believe, are very heavy. In this case, a mechanism other than that of the Higgs boson would be needed to generate this mass. The idea of heavy, right-handed neutrinos is already a topic beyond the Standard Model.

Does the mass of the Higgs boson also create problems in and of itself?
There is a conceptual problem: the mass of the Higgs boson [about 125 gigaelectron-volts (GeV)] is very sensitive to any new physics that is relevant at scales much smaller than we have proven. Because of this, we have to adjust huge numbers for the Standard Model to work. From a theoretical point of view, this solution is a bit uncomfortable. It is not very elegant. We also do not know why a particle like the neutrino is, comparatively, the size of an ant, while another, like the top quark, is the size of a blue whale. And all of the fermions are between these two extremes. What generates such a large difference? There must be a way to explain this, something that the Standard Model does not do. We could be totally wrong, but these issues led us to think of supersymmetry theories and other proposals that go beyond the Standard Model.

Simulation of dark matter (in blue) and gas (orange): unexplained by the Standard Model

Illustris CollaborationSimulation of dark matter (in blue) and gas (orange): unexplained by the Standard ModelIllustris Collaboration

What are these theories?
Since my doctorate, more than 25 years ago, I have been working on supersymmetry theories. There are two large branches of supersymmetry theories. One of them hopes to extend the symmetries between the bosons and fermions [each known fermion would have a hypothetical boson as a superpartner, with the same mass and other characteristics, and each boson already discovered would be complemented by a corresponding fermion]. It is a very elegant idea and fits well with string theory [this theory argues that all elementary particles are really small strings that vibrate and can have up to 26 space-time dimensions and multiple universes]. But, for all of this to work well in it, we should have found some supersymmetric particles at energies not much higher than those produced by the Large Hadron Collider (LCH) [located at the European Organization for Nuclear Research (CERN)]. Ten years ago I began working in another branch, parallel to supersymmetry, known as composite models.  In these theories, everything works according to the Standard Model until a certain energy level. Above that point, a thousand times greater than the mass of the Higgs boson, there are strong interactions that change everything. In this case, the Higgs boson, instead of being a fundamental scalar particle, would be composed of other particles.

Would there then be more particles like the Higgs boson?
Yes, there would be sister particles. Actually, they are called additional Higgs bosons. In supersymmetry theories, several Higgs could exist, not just one. At the moment, there are many theoretical options, some better than others — some accommodate dark matter better, for example. The important thing is that now we know the mass of the Higgs boson and also how it interacts with all of the known particles in the Standard Model. With this information, we can play detective and see which theories work better. In each theory that goes beyond the Standard Model, the way in which the Higgs boson interacts with these particles differs somewhat. In this plethora of theories, some are more beautiful than others. Some are very complicated and predict many other particles. And some of these particles would have to be seen at the LHC. When choosing a theory, the way the Higgs boson interacts with other particles changes, as does the number of new particles, including that of new Higgs bosons.

Do these new Higgs bosons have to interact with these unknown particles?
Yes, and also with the known particles. All of these new particles — extra Higgs bosons, fermions and other bosons — are being searched for. The LHC has already tried to find them and has placed many restrictions on our predictions. For example, today we believe that new fermions must have energies over 1 teraelectron-volt (TeV). Whenever someone wants to extend the theories to explain the Universe, I think it is great. But, as a theoretician, I put everything I want in my model and determine what has to be observed in order to support it. If I do not find evidence, the model fails. In 1999, when I was at the LEP [Large Electron-Positron Collider, the CERN accelerator prior to the LHC], we already knew that the Higgs boson had to be between 114 and 200 GeV.

When I began, there were no women in my field that I could look to for inspiration. Today, this situation has changed. My female students already have female role models. For example, Fabiola Gianotti [an Italian physicist who became the new general director of CERN in 2016] is a friend. I think it is important to have more women in the field, but you cannot hire a researcher just because she is as woman. You have to hire someone because he or she is good. Today, many scientific committees want 30% of their members to be women. This is good but, since there are still few of us, the same women end up being called to participate in a very large number of committees.