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Physics

Primordial soup

Brazilians help analyze unexplained phenomena in liquid created from particle collisions

Infographics Ana Paula Campos Illustrations Fabio Otubo The most famous discovery using the Large Hadron Collider (LHC) was undoubtedly the detection of the elementary particle known as the Higgs boson. Announced on July 4, 2012, this discovery resulted in the award of the 2013 Nobel Prize in Physics to Peter Higgs and François Englert, two of the theoretical physicists who proposed that it existed in 1960. But that boson, which would explain the origin of the mass of all elementary particles, is not the only interesting thing that has been appearing in the collisions produced since 2009 by the most powerful particle accelerator ever built, installed on France’s border with Switzerland and coordinated by the European Organization for Nuclear Research (CERN).

While the Higgs boson was discovered by analyzing the result of collisions between protons, some of the physicists involved in the LHC experiments, including researchers from the University of São Paulo (USP), the Universidade Estadual Paulista (Unesp), the University of Campinas (Unicamp) and the Federal University of the ABC (UFABC), are more interested in using energy from the accelerator to make the atomic nuclei of lead, with 82 protons and 126 neutrons, collide with each other. The energy of these collisions breaks up the protons and neutrons (composite particles) into their elementary components, indivisible particles called quarks and gluons.

For a very brief instant after the collision, the quarks and gluons form something that behaves like a liquid: the quark-gluon plasma, a little-known state of  material that is denser than the matter in the nuclei of atoms and about 250,000 times hotter than the interior the Sun. The Brazilian physicists collaborated on analyses published this year that revealed the completely unexpected behavior of this plasma, for which there is still no convincing explanation.

“The extreme energy of these collisions recreates the same conditions nuclear matter underwent in the early universe,” explains Jun Takahashi, a Unicamp physicist who is a member of the Alice (A Large Ion Collider Experiment) team, which collects data using the only LHC detector—there are four—designed to observe lead collisions. Takahashi and others researchers from the state of São Paulo presented the results of some recent LHC experiments at a workshop held in August at FAPESP.

Physicists believe that, up to about 10 millionths of a second after the Big Bang, the explosion that created the cosmos 13.8 billion years ago, the universe was filled with a sea of quarks and gluons—some call this condition the primordial soup—which, on cooling, formed protons and neutrons. The fact that the matter in the universe is now agglomerated into stars and planets, and not scattered through space as a uniform cloud of gas and dust, is at least partly a result of ripples in this soup. “Studying the collective behavior of quarks and gluons helps us understand how the universe evolved,” concludes Takahashi.

Another mystery that involves the interaction between quarks and gluons is the origin of mass. The Higgs boson only explains the mass of elementary particles such as electrons, muons and the six known types of quarks, plus their corresponding antiparticles (identical, but with opposite electrical charges). Since electrons are extremely light, the mass of atoms is almost all in the nucleus, made of protons and neutrons. These particles are composite: they are made up of trios of quarks bound by the strong nuclear force, transmitted by massless particles, the gluons, emitted and absorbed by quarks. The sum of the mass of the quarks in a proton or a neutron is only 1% of its total mass. The remaining 99% comes from the energy of interaction between its quarks and gluons.

Collective behavior
Since the 1970s, physicists believe they have found the general laws that describe the strong nuclear force, but no one properly understands the details of the collective motion of quarks and gluons. “It’s like water,” Takahashi compares. “We know it is made of H2O molecules, but knowing this does not tell us how water turns into steam, which is a result of the collective molecular behavior.”

In the current universe, quarks and gluons are never isolated. Both quarks and their antiparticles (antiquarks) are always together in composite particles called hadrons—these hadrons can be composed of trios of quarks (baryons), like protons and neutrons, or quark-antiquark pairs (mesons). The reason is that, unlike the other fundamental forces of nature that lose strength with distance, the strong nuclear force increases as two quarks move away from each other. “Think of two balls connected by a rubber band,” explains physicist David Chinellato, of Unicamp, who also participates in the Alice project. “When one moves away from the other, the tension in the elastic increases, and when they are close enough, the tension disappears and the balls move freely.”

The objective of the collisions between heavy nuclei is to compress protons and neutrons until their quarks and gluons break free for a moment. The energy from the collision also creates new pairs of quarks and antiquarks, in addition to other elementary particles. Then the temperature and density at the collision point begin to decline and the quarks recombine, forming thousands of new hadrons whose trajectories are recorded by detectors.

Infographics Ana Paula Campos Illustrations Fabio Otubo Evidence that quarks could break free from hadrons and mesons began to emerge in the 1980s and 1990s at the Super Proton Synchrotron accelerator at CERN. But the quark-gluon plasma was only discovered in 2005, when researchers at the Relativistic Heavy Ion Collider (RHIC) in the United States announced that they had sufficient evidence that collisions of gold nuclei had produced a state in which quarks and gluons were not trapped inside hadrons, but were also not totally free like the molecules in an ideal gas. To everyone’s surprise, the quarks and gluons seemed to form a drop of liquid that can flow perfectly, almost without viscosity.

In November 2010, the LHC stopped colliding isolated protons and started colliding lead nuclei for the first time for a period of a month. The energy involved was about 14 times greater than the energy of the collisions at RHIC. More lead collisions were carried out in November 2011 and in 2013. Some theoretical models predicted that, at this level of energy, quarks and gluons would behave like a gas, but a state with characteristics like a liquid, similar to that recorded at RHIC, was observed. It is estimated that the droplets of quark-gluon plasma produced in the LHC are twice as large as those produced in the RHIC, and that their temperature reached 7 trillion degrees (250,000 times the core temperature of the Sun).

Lead collisions in the LHC are being studied by nearly 1,200 researchers from 36 countries working on the Alice detector. The Brazilian contribution to the experiment is coordinated by physicist Alejandro Szanto de Toledo, who worked at RHIC until 2006. He and his colleagues, Alexandre Suaide and Marcelo Munhoz, all at USP, are studying hadrons made of charm and bottom type quarks, thousands of times heavier than the up and down quarks that make up protons and neutrons. “What’s interesting is that these quarks need a lot of energy to form,” says Munhoz. “They appear very early in the collision and can therefore tell the entire story of the interaction, because they have time to interact with everything that is subsequently formed.”

The stone and the boulder
The researchers had thought that hadrons made of heavier quarks would lose less energy than lighter quarks when crossing the plasma, like a giant stone is less affected by the current of a river than a boulder. “This was not seen at RHIC nor at LHC,” says Munhoz. “Or we do not quite understand how quarks lose energy, or we do not understand the properties of the plasma.”

Takahashi and Chinellato focus on examining hadrons made of lighter quarks, produced in greater quantities in the collisions. Chinellato coordinates the work of 80 researchers who study hadrons containing the strange quark, about 100 times heavier than the up and down quarks. As discussed in an article published in July in the electronic repository ArXiv, the Alice researchers found that in a certain range of momenta (a quantity that gives an idea of the energy of the particles), collisions of lead particles tend to produce more baryons (trios) containing strange quarks than mesons (pairs) of strange quarks, an effect expected by some theories. However, unexpectedly, Alice also noted a similar effect, to a lesser extent, in collisions between lead nuclei and protons, in which, in principle, a plasma should not be formed. “There are several physical mechanisms that could explain this,” says Takahashi. “We’re trying to understand which one makes the most sense.”

New phenomena involving heavy nuclei are also being discovered by the team using another detector in the LHC, the Compact Muon Solenoid (CMS), which is used by 3,000 researchers from 40 countries. Among them is a group coordinated by Sergio Novaes at Unesp and at UFABC. At Unesp, physicist Sandra Padula develops and applies techniques to combine the trajectories of the particles produced in the collisions and thereby estimate the size of the system formed, the collective motion of the particles and other properties of the medium from which they came. One of the effects observed in collisions between gold nuclei at the RHIC and between lead nuclei at the LHC was the emergence of a structure that resembles a ridge, which generated a number of theoretical attempts at explanations. “One of them suggests that this structure arises because the plasma resembles a liquid flowing without viscosity,” Padula says. “And that the particles formed reflect this collective behavior.”

The problem is that a similar version of this effect was also observed using the CMS, in collisions between protons and between protons and lead nuclei—two situations in which one would not expect a plasma to be formed.

Collisions at the LHC have been suspended since February. The accelerator was shut down to undergo improvements that should increase the energy of collisions and instrument sensitivity. Experiments will resume in 2015, and it is expected that the energy will be twice the current level by 2018. “Simulations are being developed in order to analyze what can happen at this level of energy,” says Padula, “but I think the unexpected is the most interesting.”

In 2018, the LHC is expected to remain idle again for more improvements. Szanto’s group, in partnership with engineer Wilhelmus Van Noije’s team at the USP Polytechnic School, is expected to contribute to the construction of microelectronic components to enhance Alice’s detection system. Novaes and his team, in turn, plan to contribute to the creation of microelectronic components to improve the CMS’s detection capability.

Projects
1. High energy nuclear physics at the RHIC and the LHC (No. 2012/04583-8); Grant Mechanism Thematic Project; Coord. Alejando Szanto de Toledo, IF/USP; Investment R$2,789,509.20 (FAPESP).
2. Hadronic experimental physics from RHIC to LHC (No. 2012/02895-2); Grant Mechanism Regular Line of Research Project Award; Coord. Jun Takahashi, IF/Unicamp; Investment R$104,995.95 (FAPESP).
3. São Paulo Regional Analysis Center: participation in the DZero and CMS experiments (No. 2008/02799-8); Grant Mechanism Thematic Project; Coord. Sergio Ferraz Novaes, IFT/Unesp; Investment R$2,026,797.78 (FAPESP).
4. Design of a signal acquisition and digital processing Asic for the Time Projection Chamber of the ALICE experiment (No. 2013/06885-4); Grant Mechanism Regular Line of Research Project Award; Coord. Wilhelmus Van Noije, Polytechnic School/USP; Investment R$858,978.38 (FAPESP)

Scientific articles
ALICE Collaboration. Multiplicity Dependence of Pion, Kaon, Proton and Lambda Production in p-Pb Collisions at sqrt (sNN) = 5.02 TeV. eprint arXiv:1307.6796. Jul. 2013.
CMS Collaboration. Multiplicity and transverse-momentum dependence of two-and four-particle correlations in p-Pb and Pb-Pb collisions. Physics Letters B. v. 724, n. 213. May 2013.

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