by Panos Charitos. Published: 20 August 2013

Gerardo Herrera Corral is presently working at the Physics Department of CINVESTAV in Mexico. Herrera is one of the first experimental physicists in Mexico breaking with a long tradition that was giving emphasis mainly on theoretical physics. Back in the ’80s, the theorists’ community decided to support the development of experimental physics. Many young students moved abroad to work on experimental HEP. Gerardo pursued his PhD in Germany in the mid 80s and returned to Mexico in 1991 to join the CINVESTAV institute. Since then Gerardo has been very active in the field of High Energy Physics and played a key-role in the development of the HEP community both in Mexico but also in other Latin American Countries. Gerardo has been working with ALICE for the last 18 years almost since the beginning of the collaboration. We met him during the recent ALICE week and discussed with him about the open questions that ALICE seeks to explore as well as his future career steps.

What were the main reasons for the change from theoretical particle physics to experimental high energy physics?

I did my bachelor on Engineering and obtaineded my MSc degree in theoretical physics. Afterwards I pursued a PHD in experimental high energy physics. I was always attracted to technology. I worked on my doctoral thesis at the ARGUS experiment at DESY in Hamburg, Germany, before moving to Mexico. From Mexico I started working in a fixed target experiment based at FERMILAB. Later in 1995 I met Paolo Giubellino at a Conference in Brazil where we discussed the physics of ALICE as well as the possibilities offered by the project to new groups willing to join the collaboration.

Paolo convinced me that our institute would benefit greatly from becoming a full member of ALICE. We started with a formal group and made our first steps in the project, while at the same time we applied for funding to various sources. We have been collaborating with ALICE for almost 18 years. Building a team meant that we had to find a project suitable for us. In the beginning, we were involved in the Inner Tracking System; the same project in which Paolo and the INFN group were involved. We collaborated with the Torino group and worked on the silicon detectors for the ITS. At that time, many of our students spent some time in Torino, completing their PhDs.

In 2000 we were lucky to receive one of the four special funds assigned to Mexico by the World Bank’s aimed to help developing countries to support major projects.. This prize gave us the chance to build new laboratories that were much needed in order to participate in the experiment and contribute to the testing and building of new detectors.

Around that time, there was a discussion about the trigger system that would be used in ALICE. We joined the debate offering new ideas for the trigger while we also decided that part of the prize should be invested in the development of a trigger detector. Hence, we got involved in the V0 detector. We had a project, a group of scientists who would run it and the money to fund it. This was our first step as an independent group in ALICE.

Gerardo Herrera has been a long-standing collaborator of the ALICE experiment since 1995 when they met with Paolo Giubellino during a conference in Brazil.

Did you also try to get more institutes involved in ALICE? I guess this was also the mission of the prize, to a certain extent.

Yes indeed. We got in touch with Puebla (BUAP) and two institutes from the National University (UNAM) as well as with the University of Sinaloa. As you may know, Puebla was interested in Cosmic Ray Physics and, naturally, they joined ALICE, participating in the cosmic ray detector. That is how the Mexican team came to ALICE with two projects: one was the construction of the Cosmic Ray Detector and the other was to build the V0 detector.

In addition, we had many connections in other Latin American Countries such as Brazil, Argentina, Chile, and Cuba. In particular, one of my students at Fermilab, Alberto Gago moved back to Peru and we worked with him to form a group that would participate in ALICE. I felt that we had to help the new groups from the region, After all we had also received valuable help from our Italian colleagues when we first joined ALICE. Alberto Gago now at the Pontificia Universidad Catolica de Peru is doing valuable work in ALICE and I am proud to say that he is one of the most brilliant students I ever had.

Gerardo Herrera with former ALICE Spokesperson Jurgen Schukraft.

What is the most fascinating point in the physics programme of ALICE?

There are several fascinating issues in the physics programme of ALICE. First, there was the idea of exploring something new, something different but also controversial, since many people thought that it would not be possible to study something as complicated as heavy-ions collisions. As I come from the high-energy physics community, it was clear to me that studying lead-lead collisions was very complicated. However, I considered it as one of the attractive aspects of heavy-ion physics. I felt that we could explore ideas that people considered impossible. Pursuing something that was not clear how it could be observed was the most challenging aspect of ALICE.

Another very interesting aspect was the possibility to work on instrumentation. We wanted to develop this field in Mexico and participate in an experiment at the same level as with other countries. Hopefully, the atmosphere in ALICE was very encouraging and welcoming. This played an important role in our decision to join the collaboration.

Do you think that it was a challenge worth taking on? Do we learn something from heavy-ion collisions?

I believe that the physics of ALICE goes deep in the fundamentals of modern physics. The understanding of the phenomena that we observe may take some time as a result of the difficulties involved.

Creating and studying the quark-gluon plasma that behaves as a perfect liquid has profound implications for our understanding of the Universe. I firmly believe that in the future with further analysis of the data we will make a crash of what is behind the phenomenology we are looking at.

In particular, I think that the properties of the plasma (i.e. a perfect liquid with small viscosity) gives a hint of an interesting relation of quantum chromodynamics with more abstract theories that are linked through this AdS/CFT duality suggested by Maldacena; a fascinating relation that might be more profound in nature.

How would you describe this AdS/CFT relation?

In 1998 Juan Maldacena proposed an idea, known today as AdS/CFT duality. He conjectured that there is a relationship between an Anti de Sitter space in more dimensions with an holographic formalism and a field theory like Quantum Chromodynamics. The relationships apply to a Conformal Field Theory and QCD is not such a theory. However with some considerations one may apply the ideas to get some phenomenological results to compare with what has been measured in the lab. This is for the first time phenomenology from String Theory and the ideas that are being tested may be very profound in terms of our vision of the universe.

Do you think that this might merely be mathematical formalism? Will it really provide information about the Universe?

This is an open question and perhaps one of the most interesting we have to address. If this duality is really there we may be studying something related to extra dimensions and a space with a special geometry. As you already mentioned, one way of looking at it is as a mathematical tool. But, at the same time, it may represent something deeper in nature. That indicates a way to follow in order to get a better description of the world. In other words, a mathematical tool may also give some hints about the direction to take in the way to formulate a Theory of Everything.

As for, the relation between mathematics and the natural world there have always been two approaches. On the one hand, there are those who claim that mathematics only describes the world and, on the other hand, those who believe that mathematics are indeed part of the world. I think that there are many reasons to support the second statement. Perhaps, the Higgs field is a good example, it was originally proposed as a mathematical formulation to give mass to the W and Z bosons, while getting massless photons. When we looked for it we found that the Higgs was there as a manifestation of the mechanism that was purely mathematical. Maybe we are looking at phenomena that have a mathematical representation and maybe there is something in nature that is related to these phenomena. In that context/sense I believe that the physics of ALICE is very profound and will give us very interesting results in the future. Weare now observing a small part of something bigger and this is truly exciting.

There is sometimes a criticism that ALICE lacks a real theory but is only about different models. What would be your response to this line of criticism?

Well, as I come from High Energy physics I have heard this criticism many times in my life. Most of the HEP community works on the electro-weak sector where there is a clean environment and a very well understood theory. My colleagues used to say that we are doing is “pineapple physics”, an expression used in Portuguese to denote that we explore something very complicated.

I used to answer that focusing only on the electro-weak sector of the Standard Model is like losing a coin downtown and looking for it in your lab because that’s the only place where you have light and the right instruments. If you really want to find the coin you need to go and look in the dark streets of the city where you lost it.. Of course, QCD is a difficult theory but we have to keep studying all non-trivial aspects. For example, if you want to understand confinement you have to conduct experiments and ALICE is specially designed to help us understand this property of QCD.

In addition, as I often say to my colleagues, if we want to understand the problem of the mass we have to understand QCD, as it is responsible for 99.9% of the mass that we see. QCD is a difficult theory but we have to measure and understand.

In order to look for the scalar boson that gives mass to the protons when the quarks are bound together we need experimental ideas. This scalar boson has not been observed yet and the dynamic behind the proton mass is therefore not understood.

This is what ALICE is about. In ALICE, we would be able to look at the restoration of chiral symmetry. It is not easy to look at the restoration of chiral symmetry in a direct way because you have to correlate many different measurements but all these are very fundamental questions.

What is the Higgs of the QCD to which you referred?

The mechanism to produce mass is the same as the one proposed by the Higgs mechanism. It was formulated by Nambu, who proposed the spontaneous breaking of symmetries in the S.M as a mass-mechanism, a Nobel Prize winning idea. More specifically, this mechanism lies on the chiral symmetry breaking. If you simply add up the mass of the quarks you don’t get the mass of the proton and this points to the fact that something else is going on. We think that there must be a scalar boson associated to the spontaneous breaking of chiral symmetry. It is hard to say which boson because it is most probably a low mass resonance living in a region which is not easy to examine. One candidate is the F600, also known as the sigma-meson, which might be responsible for the breaking of this symmetry and this is often called the QCD-Higgs. This is known as the QCD-Higgs.

These are the core ideas behind ALICE. They describe the physics of a very complicated experiment involving many particles produced in lead-lead collisions.

At the very moment that the ions collide you get a very hot volume and the quarks deconfine. If they were in a proton or in a hadron they would have the constituent mass but when they deconfine they get again a very low mass. In a “perfect” world, they would go to mass zero but they do not.

Instead they get their bare mass (0.003 or 0.005 GeV) in contrast to the constituent mass of 0.3 – 0.5 GeV. In This means that in addition to the very small bare mass, quarks also have mass that they acquire from the breaking of symmetry.

A particle with no mass and spin has a symmetry that is called chiral. This symmetry disappears when the particle acquires mass. The point is that a zero mass particle travels at the speed of light and the projection of its spin on the flight direction is the same no matter the reference frame. For a massive particle this is different because one can always find a Lorenz Frame traveling faster than the particle. In this reference frame the projection of the spin upon the fly direction of the particle would look in the opposite direction. The symmetry is not conserved.

"QCD is a difficult theory but we have to keep studying all non-trivial aspects. For example, if you want to understand confinement you have to conduct experiments and ALICE is specially designed to help us understand this property of QCD."

When the QGP is produced the chiral symmetry should be restored. But do we see that in the lab and how can we study that?

That’s when experimental physics gets into the game. Again this is not something that can be observed directly. Different signatures of different phenomena have to be examined in order to indicate that this is happening.

In normal conditions particles containing a strange quark are not abundantly produced because the mass of the strange quark is high. The production of strange quarks is suppressed by the big mass it has.

In a plasma of quarks of gluons the mass of the strange quark would go down thereby allowing a higher production of strange quarks than one can observe experimentally.

The increase of strangeness is an indication that chiral-symmetry restoration might be taking place. Also, the formation of resonances in such an environment as the quark gluon plasma would introduce some modification on the way the quarks bound together inside. One may look at the shape of those resonances and see how this modification looks like. This may give us information about the environment where they were born. By looking at these and other phenomena all together one may gain a general frame of how things are happening.

Do you think that the recent discovery of Higgs poses new challenges for ALICE?

We know that the Higgs exists and that the mass of the heavy-quarks is given mainly by the Higgs mechanism. ALICE explores the QCD regime where there is a similar mechanism to produce mass for the light quarks. We know that the light quarks get their masses from a spontaneous breaking of a symmetry which is restored when ALICE produces a plasma of quarks and gluons in heavy ion collisions.

Which do you think would be the future steps of the HEP community in Mexico?

I think that our participation in ALICE played a role in developing HEP in Mexico and other Latin America Countries. Currently there are experimental groups in many areas of Mexico and many groups participate in several experiments around the globe.

In the future we should push forward accelerator physics in Mexico and we already have proposals on this. We also need to start training students in accelerator physics which is also part of high energy physics and I hope that in the future Mexico will further invest in these efforts.

Altogether, I believe that the community will naturally evolve and start having impact on other areas. But for this to happen we need to develop in the two above directions which will help the advancement and technology in other areas.