Heavy anti-particle

International group including Brazilians observes and measures anti-helium 4 nuclei for the first time. It is the heaviest anti-matter ever detected in a laboratory. The study was published in Nature.

By Fábio de Castro

Agência FAPESP – In 1911, New Zealand scientist Ernest Rutherford (1871-1937) used nuclei from helium-4 atoms—the so-called alpha particles—to show that the atoms’ positive charge is concentrated in a small nucleus. The discovery was the kickoff to nuclear physics.

Exactly 100 years after the creation of Rutherford’s atomic model, an international group of scientists, including Brazilians, describes for the first time the observation and measurement of antiparticles in the nuclei of helium-4. It is the heaviest antimatter produced and measured in a laboratory to date.

According to the authors, this has important consequences for future antimatter observation in the Universe. According to them, the antiparticle study is fundamental for the advance of knowledge in fundamental aspects of nuclear physics, astrophysics and cosmology.

The experiment, performed by the Star Collaboration—which brings together 584 scientists from 54 institutions in 12 nations—was produced in the Relativistic Heavy Ion Collider (RHIC) in the United States. The results were published this Sunday (April 24) in Nature magazine’s Letters section.

The Brazilian co-authors are Alexandre Suaide, Alejandro Szanto Toledo and Marcelo Munhoz – all professors at Universidade de São Paulo’s (USP) Nuclear Physics Department in the Physics Institute–, Jun Takahashi, professor at the Gleb Wataghin Physics Institute (IFGW) at the Universidade Estadual de Campinas (Unicamp) and his doctoral students, Rafael Derradi de Souza and Geraldo Vasconcelos.

The same group had already produced the first experimental evidence of an anti-hypernucleus   a year ago: this means that the scientists had obtained a nucleus that was not part of the periodical table. The study was published in the March 2010 edition of Science magazine.

According to Suaide, in the 2010 work, the antiparticles were submitted to coalescence—a process similar to condensation—aggregating two antineutrons and one antiproton, forming an antitritium—in other words, an antimatter nucleus corresponding to the tritium atom--, the hydrogen isotope that has two neutrons and one proton.

“In observing the anti-helium—an antimatter alpha particle—for the first time, we took one more step toward building a new periodical table. It was a difficult task, because the chances of having coalescence decreases as the complexity of the antinucleus increases,” Suaide told Agência FAPESP.

In the STAR experiment, the scientist said that nuclear collisions were performed with gold atoms at a speed near to that of light, an extremely high temperature creating an energy density similar to that which existed a few microseconds after the Big Bang. Both in the lab and at the beginning of the Universe, the collisions resulted in the formation of equal quantities of matter and antimatter.

“Theoretically, we believe that the Big Bang came about because of a large concentration of energy in one singularity and, based on models, we concluded that this process must have produced a great deal of antimatter. However, when we look at the Universe we hardly find any antimatter at all. The experiment may help us to understand what happened in these initial moments,” said Suaide.

Behind antimatter

In order to detect the antiparticles, the scientists used sophisticated detectors—the largest of which is 10 meters wide and 15 meters long—that measure the trajectory of the particles and, based on this information, try to identify the type of particle observed.

The storage and analysis of the immense quantity of data produced, according to Suaide, demand the involvement of dozens of institutions the world over and a powerful computing infrastructure. “In the experiment we produced something around 1 billion collisions with the gold nuclei. Each of these produces millions of different particles. Of all these trillions of particles, we managed to find 18 anti-helium nuclei. The complexity of the task explains why the anti-alpha particles had never been observed before, even though the alpha particle was identified a century ago,” he said.

Scientists hope that laboratory antiparticle studies will allow them to create the conditions necessary to observe antimatter in the cosmos in the future. “These studies help us to know what to expect to observe. When measuring anti-helium 4, we are contributing to the possibility of cosmologists making predictions about how and where to look for antimatter,” he explained.

Suaide says that the Brazilian group was part of the STAR experiment 15 years ago and played an important role in the development and construction of the main detectors. 

“The Brazilian researchers were actively involved in the experiment, very involved in coordination of the subgroups between which the work is divided. In this last project, we participated directly in the revision of the article,” he affirmed. 

 The article Observation of the antimatter helium-4 nucleus (DOI: 10.1038/nature10079) of the STAR Collaboration, can be read by Nature subscribers at www.nature.com/nature/journal/v473/n7347/full/nature10079.html.