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The mysterious case of Piz Daint and the proton spin puzzle

Speed read
  • 30-year-old proton angular momentum enigma solved 
  • Lattice theory of quantum chromodynamics used to complete calculations
  • Piz Daint and pre-optimized algorithms made project possible

Nucleons — protons and neutrons — are the principal constituents of the atomic nuclei. Those particles in turn are made up of yet smaller elementary particles: Their constituent quarks and gluons.

Each nucleon has its own intrinsic angular momentum, or spin. Knowing the spin of elementary particles is important for understanding physical and chemical processes. University of Cyprus researchers may have solved the proton spin puzzle – with a little help from the Piz Daint supercomputer.

Proton spin crisis

Spin is responsible for a material’s fundamental properties, such as phase changes in non-conducting materials that suddenly turn them into superconductors at very low temperatures.

<strong>Inside job.</strong> Artist's impression of what the proton is made of. The quarks and gluons contribute to give exactly half the spin of the proton. The question of how is it done and how much each contributes has been a puzzle since 1987. Courtesy Brookhaven National Laboratory.

Theoretical models originally assumed that the spin of the nucleon came only from its constituent quarks. But then in 1987, high-energy physics experiments conducted by the European Muon Collaboration precipitated what came to be known as the 'proton spin crisis': experiments performed at European Organization for Nuclear Research (CERN), Deutsches Elektronen-Synchrotron (DESY) and Stanford Linear Accelerator Center (SLAC) showed that quarks contribute only 30 percent of the proton spin.

Since then, it has been unclear what other effects are contributing to the spin, and to what extent. Furhter high-energy physics studies suggested that quark-antiquark pairs, with their short-lived intermediate states might be in play here – in other words, purely relativistic quantum effects.

Thirty years later, these mysterious effects have finally been accounted for in the calculations performed on CSCS supercomputer Piz Daint by a research group led by Constantia Alexandrou of the Computation-based Science and Technology Research Center of the Cyprus Institute and the Physics Department of the University of Cyprus in Nicosia. That group also included researchers from DESY-Zeuthen, Germany, and from the University of Utah and Temple University in the US.

For the first time, researchers could calculate the quantitative contributions from constituent quarks, gluons, and sea quarks –– to nucleon spin. (Sea quarks are a short-lived intermediate state of quark-antiquark pairs inside the nucleon). With their calculations, the group made a crucial step towards solving the puzzle that brought on the proton spin crisis.

To calculate the spin of the different particles in their simulations, the researchers consider the true physical mass of the quarks.

“This is a numerically challenging task, but of essential importance for making sure that the values of the used parameters in the simulations correspond to reality,” says Karl Jansen, lead scientist at DESY-Zeuthen and project co-author.

The strong force acting here, which is transmitted by the gluons, is one of the four fundamental forces of physics. The strong force is indeed strong enough to prevent the removal of a quark from a proton. This property, known as confinement, results in huge binding energy that ultimately holds together the nucleon constituents.

The researchers used the mass of the pion, a so-called meson, consisting of one up and one down antiquark –the ‘light quarks’ – to fix the mass of the up and down quarks to the physical quark mass entering in the simulations. If the mass of the pion calculated from the simulation corresponds with the experimentally determined value, then the researchers consider that the simulation is done with the actual physical values for the quark mass.

And that is exactly what Alexandrou and her team have achieved in their recently published research.

Their simulations also took into account the valence quarks (constituent quarks), sea quarks, and gluons. The researchers used the lattice theory of quantum chromodynamics (lattice QCD) to calculate this sea of particles and their QCD interactions. 

Elaborate conversion to physical values

The biggest challenge with the simulations was to reduce statistical errors in calculating the ‘spin contributions’ from sea quarks and gluons, says Alexandrou. “In addition, a significant part was to carry out the renormalisation of these quantities.”

<strong>Spin cycle. </strong> Composition of the proton spin among the constituent quarks (blue and purple columns with the lines), sea quarks (blue, purple, and red solid columns) and gluons (green column). The errors are shown by the bars. Courtesy Constantia Alexandrou.

In other words, they had to convert the dimensionless values determined by the simulations into a physical value that can be measured experimentally – such as the spin carried by the constituent and sea quarks and the gluons that the researchers were seeking.

Alexandrou’s team is the first to have achieved this computation including gluons, whereby they had to calculate millions of the ‘propagators’ describing how quarks move between two points in space-time.

“Making powerful supercomputers like Piz Daint open and available across Europe is extremely important for European science,” notes Jansen.

“Simulations as elaborate as this were possible only thanks to the power of Piz Daint,” adds Alexandrou.

Read the original article on the Swiss National Supercomputing Center site here. 

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