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Scientists may have seen the first hint of CP parity violation in collisions of heavy nuclei


Front view of the STAR detector. Two beams of gold nuclei fly at 99.995% of the speed of light inside the central pipe against each other. They collide at center of the detector (enclosed), producing thousands of particles that are recorded by the detector and its electronics. Physicists discovered in 2005 that a state very similar to the early universe after the Big Bang was created in those collisions. Now, physicists may have seen, after analyzing billions of those collisions, a hint of CP parity violation that can be possibly detected only under conditions similar to the early universe.  Photo credit: Joseph Rubino, Brookhaven National Laboratory.


The observable universe is composed of matter only. Although the existence of antimatter is not excluded by physics, no antimatter has ever been observed. Physicists believe the cause of this matter-antimatter asymmetry is CP (charge-parity) violation. 

With CP violation, the physical laws governing matter and antimatter differ slightly. Both matter and antimatter started off in an equal amount from the Big Bang and this slight difference then caused the destruction of all antimatter but not all matter. Physicists have been looking for decades for evidence of CP violation in the strong interaction, the interaction that bounds atomic nuclei together, and thus all matter together including our own existence.

Purdue scientists, together with a team of over 600 scientists world-wide, may have just found hint of this CP violation in high energy collisions of heavy nuclei (ions) at Brookhaven National Laboratory (BNL) on Long Island, New York. They have recently published their findings in Physical Review Letters, titled “Search for the chiral magnetic effect via charge-dependent azimuthal correlations relative to spectator and participant planes in Au+Au collisions at 200 GeV.”

This collaborative team found a slight excess of particle pairs of opposite charge signs along a strong magnetic field that is presumably produced by the passing positively-charged nuclei at speed close to the speed of light. This charge separation is called the chiral magnetic effect (CME), a predicted signature of CP violation. CP violation causes an imbalance between left-handed quarks and right-handed quarks (quarks are the fundamental building blocks of matter, the underlying constituents of protons and neutrons that make up the nuclei and all matter around us).

According to Dr. Fuqiang Wang, Professor of Physics and Astronomy at Purdue University, “those quarks possess spins and behave just like little magnets (right-handed quarks have their spin and momentum in the same direction and left-handed quarks have their spin and momentum in opposite directions). Their polarity depends on their charge and thus fly apart along the magnetic field according to their charges. The recently published data suggest that we may have seen this ‘flying-apart’ in our experiment, called STAR, at Brookhaven.”

Physicists have been looking for this CME phenomenon in heavy ion collisions for over a decade. The signal is extremely hard to identify because of an overwhelming background of particles produced in those collisions. Wang and his team at Purdue University have found a clever way to beat down the background so that they can extract a signal that comes from the CME.

Wang explains his method with an American football reference. A heavy ion collision at an off-center impact has an interaction zone like an American football. The magnetic field direction is on average along the short axis. The protons and neutrons residing in this interaction zone, can, by chance, form a cluster looking like an American football but with tilted axes. This can be analogized as a group of people in an oval room cluster into an oval shape that is not aligned with the shape of the room. He and his team discovered that the CME and the background contribute differently to these misaligned ovals. By measuring charge separation signals along those orientations, they were able to extract a presumably CME signal that is of the order of (4.0±1.4)×10-5.

Wang says that they remain cautious about their findings. "This could be an indication of the CME caused by CP violation,” he says. “We are looking for other possible ways that could mimic such a CME signal. In addition, the signal significance is less than 3 standard deviations, or taking face value, there is still a 0.2% chance that the measured value is a random fluctuation of a null signal. (Physicists usually require at least 3 standard deviations to claim an evidence and 5 standard deviations to claim a discovery.)”

The current result comes from a data sample of more than 2 billion collisions, the total data volume STAR has accumulated over a period of 6 years. The team hopes to accumulate a factor of 10 more data by the end of 2025, owing to improved technology and computing power. By then, physicists should be able to say, for sure, whether or not they have finally unlocked the mystery for matter-antimatter asymmetry in our universe.


About the Department of Physics and Astronomy at Purdue University

Purdue Department of Physics and Astronomy has a rich and long history dating back to 1904. Our faculty and students are exploring nature at all length scales, from the subatomic to the macroscopic and everything in between. With an excellent and diverse community of faculty, postdocs, and students who are pushing new scientific frontiers, we offer a dynamic learning environment, an inclusive research community, and an engaging network of scholars.  

Physics and Astronomy is one of the seven departments within the Purdue University College of Science. World class research is performed in astrophysics, atomic and molecular optics, accelerator mass spectrometry, biophysics, condensed matter physics, quantum information science, particle and nuclear physics. Our state-of-the-art facilities are in the Physics building, but our researchers also engage in interdisciplinary work at Purdue’s Discovery Park, particularly the Birck Nanotechnology Center and the Bindley Bioscience Center.  Furthermore, we participate in global research including at the Large Hadron Collider at CERN, Argonne National Laboratory, Brookhaven National Laboratory, Fermilab, the Stanford Linear Accelerator, the James Webb telescope, and several observatories around the world. 

Last Updated: Mar 17, 2022 8:15 AM

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