LHC detects production of large quantities of strange particles in proton collisions

Until now, the phenomenon had been observed only in collisions of heavy nuclei, resulting from the formation of quark-gluon plasma. A Brazilian researcher contributed decisively to this study.

By José Tadeu Arantes  |  Agência FAPESP – In an article published on April 24 in Nature Physics, the ALICE (A Large Ion Collider Experiment) international collaboration reported an abundant production of hadrons with strange quarks in proton-proton collisions performed at the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, located on the Franco-Swiss border. This is the first time these objects, observed with increasing frequency in collisions of heavy nuclei (lead-lead, gold-gold), have been detected in such great abundance in collisions of particles as light as protons.

Brazilian researchers contributed decisively to the study that resulted in the published article, “Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions”, especially David Dobrigkeit Chinellato at the Gleb Wataghin Physics Institute of the University of Campinas (UNICAMP) in São Paulo State. Chinellato acts as the international coordinator for Light Flavor, one of the ALICE physics working groups, and is supported by a grant from FAPESP for the project “Strangeness production in Pb-Pb collisions at 5.02 TEV in ALICE”.

Enhanced production of hadrons with strange quarks is considered a signature of quark-gluon plasma, an extremely hot, dense state of matter that is believed to have been predominant in the universe for a tiny fraction of a second after the Big Bang and is now being recreated in the LHC and the world’s only other hadron accelerator, the Relativistic Heavy Ion Collider (RHIC), located in the United States.

“What’s really new here is that, for the first time, enhanced production of hadrons with strange quarks was observed in collisions of systems as small as protons,” said physicist Alexandre Alarcon do Passo Suaide, who is affiliated with the University of São Paulo’s Physics Institute (IF-USP). Suaide is a co-principal investigator for the Thematic Project “High energy nuclear physics at RHIC and LHC”, through which FAPESP supports participation in ALICE by scientists at research institutions in São Paulo State.

“The phenomena that characterize quark-gluon plasma are being observed in collisions of ever-smaller systems,” he added. “A few years ago, we had no idea this could happen.”

The evidence for hadrons with strange quarks in proton-proton collisions reported by the ALICE collaboration suggests that quark-gluon plasma may also be produced when these very small particles collide, not just by the collisions of heavy nuclei that have already been observed, such as lead-lead in the LHC or gold-gold in the RHIC. However, the researchers believe it is too soon to say so categorically. “More detailed measurements need to be performed to link hadrons with strange quarks to other observables resulting from the collision,” Suaide said. “We’ll gradually add new pieces of the puzzle until we eventually get the complete picture.”

This caution is justified by the fact that, among other things, quark-gluon plasma cannot be observed directly because it is extremely ephemeral. In the experiments performed at the LHC and RHIC, its assumed duration is on the order of 10^-23s, which is much too short for direct observation. What the researchers do in fact observe are the objects that form after the quarks and gluons cease to move about freely in the plasma when they are confined in hadrons again.

Brazilian participation

These new findings reported by the ALICE collaboration owe a great deal to the contributions made by the young Brazilian researcher David Dobrigkeit Chinellato. 

“I started thinking about this measurement in 2010, when I was in my fourth year of PhD research. In 2012, 2013 and 2014, several colleagues and myself spent a long time studying how to make the measurements and the technical procedures we’d have to use in order to avoid detection biases that could vitiate the results. Analysis of the data was finally completed in 2015. We then began the publication process. The article has only four pages but took a lot of time, people and hard work to produce,” he told Agência FAPESP.

“From data collection to publication there’s a long chain that requires the participation of many people. In fact, there are about 1,500 scientists from dozens of countries on the ALICE team.”

Chinellato explained that the experiment is operated from a computerized control room located above ground, while the collider itself, with a circumference of 27 km and four detectors (ATLAS, CMS and LHCb, in addition to ALICE), is located 175 m below ground. He took part in data collection during the experimental phase and analysis of the data, as well as helping to write the article. 

What are strange hadrons?

The concept of “strangeness” was proposed in the 1950s by Murray Gell-Mann, Abraham Pais and Kazuhiko Nishijima to characterize the property that led certain particles to survive longer than expected. Symbolized by a capital S, strangeness is a physical property expressed as a quantum number.

The idea of quarks came later, in the 1960s, when it was proposed independently by Gell-Mann and George Zweig. Several kinds of quarks have been discovered over the years. One was called strange because its existence offers an explanation for the property of strangeness. The strange quark, symbolized by a small s, is one of six quarks recognized by the standard model of particle physics: up [u], down [d], charm [c], strange [s], top [t], and bottom [b]. Its mass is several times greater than the masses of up and down quarks, from which protons and neutrons are made.

Strange hadrons are larger particles and are so named because they contain at least one strange quark. They are transient objects, with names such as Kaon, Lambda, Xi [pronounced Ksai to rhyme with “eye”] and Omega, and have become familiar from experiments involving collisions of heavy nuclei, such as lead-lead and gold-gold. The study published in Nature Physics reported  that these strange hadrons were found in unexpectedly large quantities following proton-proton collisions that produced a great many particles.

“Since the 1980s, the relative abundance of strange hadrons has been considered a possible signature of the formation of quark-gluon plasma in central collisions of heavy nuclei. The new study shows these objects are also produced abundantly in proton-proton collisions when large quantities of particles are formed. The large number of particles formed is an indication of the high level of energy reached in the collision, approaching the level seen in central nucleus-nucleus collisions,” said physicist Marcelo Gameiro Munhoz, principal investigator for the Thematic Project “High energy nuclear physics at RHIC and LHC”.

“The formation of quark-gluon plasma creates mechanisms that facilitate the subsequent production of strange hadrons,” Munhoz added. “These mechanisms wouldn’t be present without the plasma. So the detection of strange hadrons can be considered a sign, a signature, of the prior formation of quark-gluon plasma. However, there could be another explanation, unrelated to quark-gluon plasma, for this increase in the number of strange particles. If so, we’d even have to reinterpret what happens in nucleus-nucleus collisions.”