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Physics

Fleeting matter

Study details the interaction of particles that are born and die at every moment in the atomic nucleus

EDUARDO CESAREssential difference: Vacuum is the state of minimum energy and matter (bubbles) of higher energyEDUARDO CESAR

There is intense agitation in the atomic nucleus. The protons and the neutrons, elements that make up the nucleus, are surrounded by clouds of other particles that arise and disappear at every moment. Looking in detail at the inside of the protons and neutrons, we find the basic particles of matter, the quarks. But inside each proton and neutron there are not only three quarks – as the textbooks show – but many, forming pairs of particles and antiparticles, with opposite charges, so that they annul each other and in the end only three are left over. The quarks move at almost the speed of light, collide amongst themselves and with the inner walls of the protons and neutrons.

Accordingly, more fleeting particles are created, which also disappear without warning. “It’s a mess”, Marina Nielsen recognizes. Her work and that of other physicists from the University of São Paulo (USP), in conjunction with specialists from the São Paulo State University (Unesp), is helping to understand better the birth and death of the particles that make the protons and neutrons wax and wane at a hallucinating speed, based on theories that modify the concept of matter and of vacuum, no longer regarded as something empty, but rather as something full.

The results obtained by this group are due to the intense exploration of the ramifications of a theory created almost 30 years ago, quantum chromodynamics (QCD), which explains the interaction of the quarks – very different if the particles are to be found in normal situations, those called low energy, or in the particle accelerators, tunnels kilometers long, in which atomic nuclei collide, at levels of energy millions of times higher. In the routine situations, it is mainly the pions that appear – particles made up of pairs of quark and antiquark, identical to a quark, except for the opposite charge.

Discovered in 1947 by a group of physicists that included the Brazilian César Lattes, pions scatter in all directions and form the cloud around the protons and neutrons. It is the pions that keep protons and neutrons united in the inside of the nucleus and guarantee the stability both of the most simple nucleus, the deuteron, made up of just one proton and one neutron, and more complex nuclei, like the nucleus of gold, made up of 79 protons and 118 neutrons.

To study the low energy phenomena, the researchers from USP and Unesp adopted an approach called chiral symmetry, analogous to the one that explains the similarity of the hand (cheir, in Greek): in front of a mirror, the image of the left hand looks like that of the right hand, and vice-versa. In the same way, the world of the quarks practically does not change when reflected in an imaginary mirror. “Chiral symmetry forms the basis of the strict treatment of the interaction between pions and other particles”, comments the coordinator of the project, Manoel Robilotta. The physicists from São Paulo have shown that this approach may also be adopted to understand the behavior of particles at high energies.

In the inside of the accelerators, atomic nuclei collide amongst themselves and produce thousands of different particles, which are also impressive for the diversity of sizes and shapes and behaviors. To distinguish what has been formed, the physicists are looking for a special kind of particle, called J- psi, made up of a quark and antiquark pair. The quantity of iota J-psi may be an important indicator of the creation of a kind of matter that may have existed only right after the Big Bang, the explosion that, it is believed, originated the Universe. This state is a plasma, a very hot soup, made of quarks and gluons (gluons work like a sort of spring or elastic, which joins the quarks together inside the protons and neutrons). It is so hot as much as 10 trillion degrees Celsius, that this plasma should dissolve iota psi.

Clues on J-psi are being sought, because this particle works like a sort of thermometer: when it is found among the fragments of a collision, it is a sign that the temperature rose to an extremely high level and that the primordial plasma was recreated. Or, in the opposite direction, when it is found among the particles produced in the collision, it is almost certain that no plasma was formed. “Is this really a good thermometer?” asks Fernando Navarra, another member of the group. “In principle, it is, but there is a problem: J-psi may disappear in another way, interacting with the pions that arise in abundance in the collisions”.

This possibility has already been studied, but without any conclusion. The problem was born again in 1998, when the European Laboratory for Particle Physics (CERN) recorded this extremely strange phenomenon, called anomalous suppression of j-psi, which could indicate that the plasma had finally appeared. It was a transitory episode, verified when the equipment reached its maximum point of energy, moments before being disabled and replaced by the current one.

The physicists from USP, in collaboration with Gastão Krein, from Unesp, refined their calculations and reached the conclusion that the probability of j-psi being destroyed in interactions with pions is half what used to be thought. So what CERN could in fact have produced was plasma of quarks and gluons, and hence a sort of baby of Big Bang. “What was seen at CERN was more probably plasma than normal matter”, says Navarra. Published last year in the Physical Review C and this year in Physics Letters B, the results are adding fuel to the flames, suggesting another form of identifying plasma, and, if confirmed, may enrich research into the atomic world. In July, Marina Nielsen and Fernando Navarra presented their conclusions at a meeting of specialists in the area, Quark Matter, held in Nantes, France, with over 600 participants. They were heard with interest.

The experiments that seek to generate J-psis and plasma are going ahead at the Relativistic Heavy Ion Collider (RHIC), built in the United States, with a collision energy ten times greater than at CERN. At the RHIC, heavy nuclei like those of gold reach almost the speed of light and become as flat as a pizza, shrinking 100 times in diameter, moments before they collide. But only in some two years will it be possible to know if it really is possible to make up plasma with quarks and gluons.

At the Federal University of Rio Grande do Sul (UFRGS), Cesar Vasconcellos is trying to unveil the process of forming plasma by means of an alternative manner, which does away with colliders: studying the pulsars, compact celestial objects, with one and a half of the mass of the sun condensed into a radius of just 10 kilometers, predominantly made up of neutrons. Also at work in this area, which is called hadron physics (a hadron is any particle made up of quarks), are teams from the Federal Universities of Rio de Janeiro (UFRJ) and of Santa Catarina (UFSC), besides the Brazilian Center for Research in Physics (CBPF).

Whether by means of colliders or by means of the stars, for the physicists in this area – where the leading lights are Frank Wilczek, from the Massachusetts Institute of Technology (MIT), in the United States, and Gerard’t Hooft, from the Institute for Theoretical Physics, of Utrecht University, in Holland -, the revelations that may emerge from the interactions between the particles constitute the hope of finally understanding the mass of particles, and, in the ultimate instance, of the Universe itself.

It is believed that this interaction may create mass, since the particles constitute very little of the existing mass. Each quark has a mass of from 5 to 10 MeV (millions of electron-Volts, the unit of mass of atomic particles), but the three quarks of each proton or neutron correspond to only 5 thousandths of the mass of each one of these particles. “The major part of the mass of the protons and the neutrons comes from the interaction of the quarks between themselves and with the vacuum that surrounds them”, comments Celso Luiz Lima, a physicist from the group. “When they interact, quarks and antiquarks activate a mass generating mechanism, creating pions”.

To wipe out the impression that the pions emerge from nothing, we have to go into the essence of the work of this and of other groups that are studying the behavior of the particles inside the atomic nucleus. It is a complete inversion of the concept of matter. “Matter is not what exists, but what is missing”, announces Robilotta. “And a vacuum is not empty, but full”. The atomic nucleus is then a defect in the vacuum – living bubbles surrounded by a dense vacuum, made of particles, like the bubbles in mineral water or hair gel. What makes them different is just the level of energy: vacuum is the state of minimal possible energy, while the bubbles – matter – represent a state of higher energy.

“If we believe in this idea, we manage to find out what energy is spent for three quarks to dig a hole in the vacuum”, says Robilotta, who has calculated this energy: it is 46 MeV, close to the value in the experiment. This full vacuum approach, launched by English physicist, Paul Dirac (1902-1984), makes it possible to state: of the total mass of a proton or a neutron, it is estimated that 90% is due to potential and kinetic energy (integration with the vacuum and the continuous creation of particles), 7% to the fact of having dug a hole in the vacuum (those 46 MeV), and 3% to the mass of the quarks themselves.

Now, the major part of the mass of the protons and the neutrons can be attributed to the interaction of the quarks with the vacuum. “Isolated quarks do not exist, except inside holes in the vacuum”, Lima informs. When quarks join up with antiquarks, they may form a meson, or they may originate a condensed state, the characteristics of which are similar to superconductivity, a phenomenon that occurs with certain substances at low temperatures. But there is another possibility: “When a quark joins up with another two, which have also not been able to couple themselves with antiquarks, they push the vacuum, forming protons or neutrons whose insides are almost the emptiness that really is empty”, says Lima. “Banging on the inside walls of this bubble, the quarks disturb the vacuum on the outside and create the pions, the particles of the clouds that surround the protons and neutrons”.

As in balancing the books of a company, the accounts have themselves out. It does not matter if in the bubble, which represents a proton or a neutron, there are three quarks, four quarks and an antiquark, or five quarks and two antiquarks: the final result has to have a surplus of three quarks, as in a simple sum, done and redone at every moment in a world that looks like an immense empty space: if atoms measured 10 kilometers, the nucleus would measure 1 kilometer, the protons and neutrons, 10 centimeters, and each quark, 10,000 times smaller, would measure 1 hundredth of a millimeter, equivalent to an ameba. There, like people, protons and neutrons gain and lose weight, feeding on particles.

The Project
Hadron Physics; Modality Thematic Project; Coordinator Manoel Roberto Robilotta – Institute of Physics at USP; Investment R$ 141,660.00

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