CERN collider reveals major clue to universe's bias against antimatter
A key clue comes from something called CP violation — a difference in the behaviour of matter and antimatter.
While CP violation has been observed in certain types of particles called mesons, it has never been reported in baryons, which are the particles (like protons and neutrons) that make up most of the matter around us.
Based on new data, the LHCb collaboration in Europe has now reported the first-ever observation of CP violation in baryon decays, specifically in a particle called the Λb⁰ baryon (pronounced 'lambda bee-zero baryon').
Their findings were published in Nature on July 16.
'For the first time, we have clear evidence of CP violation in baryons,' Xueting Yang, the corresponding author of the study, a member of the LHCb team, and a PhD student at Peking University in Beijing, told The Hindu. 'The matter-antimatter asymmetry in the universe requires CP violation in baryons, such that the discovery is a key step forward.'
Looking for the signal
In CP, 'C' stands for charge conjugation, which means the action of swapping a particle with its antiparticle. 'P' stands for parity, which is the action of flipping the spatial coordinates, like looking in a mirror. CP symmetry stipulates that if you swap particles for antiparticles and look in a mirror, the laws of physics should be the same.
CP violation thus means this symmetry is broken and that the laws of physics are slightly different for matter and antimatter. This is important because CP violation is a necessary ingredient to explain why the universe is made mostly of matter.
The Λb⁰ baryon is made up of three smaller particles: an up quark, a down quark, and a bottom quark. The antiparticle of the Λb⁰ baryon is called the Λb⁰-bar.
The newly reported result focuses on a specific decay of the Λb⁰ baryon: into a proton, a negatively charged kaon, a positively charged pion, and a negatively charged pion. This is denoted: Λb⁰ → p K⁻ π⁺ π⁻.
The collaboration also studied the same decay for the antiparticle, Λb⁰-bar, but with all charges reversed.
The experiment used data from the Large Hadron Collider at CERN, specifically from the LHCb detector on the machine.
The LHCb team collected data between 2011 and 2018, corresponding to a very large number of collisions between beams of protons accelerated to nearly the speed of light.
In these collisions, Λb⁰ and Λb⁰-bar baryons are produced and then rapidly decay. The LHCb researchers looked for events where the decay products matched p K⁻ π⁺ π⁻.
To reduce background noise — in the form of random combinations of particles that mimic the signal — they used machine learning to distinguish real decays from fake ones. They also used particle identification tools on computers that could tell protons, kaons, and pions apart.
The main quantity they measured was the CP asymmetry. It compares the number of Λb⁰ decays to the number of Λb⁰-bar decays: if there is no CP violation, the value of CP asymmetry should be zero. In practice, they measured the yield asymmetry, which is the difference in the number of decays observed for Λb⁰ and Λb⁰-bar.
There are some effects that can mimic CP violation. For example, the proton-proton collisions may produce more Λb⁰ than Λb⁰-bar to begin with. For another, the LHCb detector on the Large Hadron Collider might have been slightly better at detecting one charge over another.
To correct for these possible biases, the researchers used a control channel — a similar decay where no CP violation is expected. Here, an Λb⁰ baryon decays to a positively charged Λc baryon, and a negatively charged pion: Λb⁰ → Λc⁺ π⁻.
Any asymmetry seen in this control channel was considered a nuisance and subtracted from the main measurement.
Mesons, then baryons
The researchers used statistical methods to determine how many real Λb⁰ baryon and Λb⁰-bar antiparticle decays the detector recorded. Then they checked their results for consistency across different data-taking periods, detector settings, and analysis methods.
Thus, the team found a significant difference in the decay rates: about 2.45%.
According to the paper, this result is 5.2 standard deviations away from zero, which is well above the statistical threshold required for physicists to claim a discovery in particle physics.
'It was expected that the LHCb group had enough data. They are reporting it now,' theoretical physicist, University of Hawai'i affiliate graduate faculty, and Chennai's Institute of Mathematical Sciences retired professor Rahul Sinha told The Hindu.
This is the first time CP violation has been observed in baryon decays. Previously, physicists had reported CP violation only in mesons, particles which are made of a quark and an antiquark, and not baryons, which are made of three quarks.
The result matches the predictions of the Standard Model, the main theory of particle physics, which says CP violation comes from the way quarks mix and decay.
However, the amount of CP violation in the Standard Model is not enough to explain the matter-antimatter imbalance in the universe.
'The observation of CP violation in baryons still doesn't settle the mystery of the universe's missing antimatter,' Prof. Sinha said. 'The Standard Model predicts a rate of disappearance of antimatter that doesn't match what we're seeing in the universe.'
The new announcement opens new ways to search for 'new physics', the name for hitherto unknown effects or particles beyond what the Model predicts, and which physicists believe will reveal the 'complete' theory of subatomic particles.
Mind the phase
According to Prof. Sinha, the new paper reports observing CP violation in baryons but doesn't say whether the amount of violation is higher or lower than that predicted by the Standard Model. Ascertaining that requires researchers to determine the complex phase.
In the context of CP violation, the complex phase is a combination of variables present in the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a mathematical tool physicists use to understand how the quarks in a baryon interact with each other.
If the complex phase has a non-zero value, it means the laws of physics are not identical for matter and antimatter, leading to observable differences in their behaviour.
The Standard Model predicts specific values for the amount of CP violation, which are determined by the magnitude and phase of the variables in the CKM matrix. By measuring the phase associated with CP violation in baryon decays, physicists can compare the observed amount of violation to the Standard Model's predictions.
In their paper, the LHCb researchers have reported that the complex phase information proved too difficult to extract from the data collected by the detector.
'Until we measure the phase, we can't say if the rate of antimatter's disappearance is too high or too low compared to the Model's prediction,' Prof. Sinha said.
The same technique to measure the phase for mesons can't be used for baryons. To this end, Prof. Sinha added that in 2022, he and his peers Shibasis Roy and N.G. Deshpande described a new way to measure the complex phase for baryons. It was published in Physical Review Letters.
Observing CP violation in baryons is important because the visible matter around us today is made of baryons. Some baryons like protons and neutrons are very stable and don't decay for a long time. Others, like Λb⁰, decay in around 1.5 picoseconds. The point is what is true for one baryon should be true for all baryons.
'To definitively resolve the asymmetry problem, both experimental and theoretical progress are needed,' Dr. Yang said.
'Experimentally, more precise and comprehensive measurements across different particle systems are required to build a coherent and consistent picture of CP violation. Theoretically, improved calculations and refined models are essential to connect these experimental observations with the fundamental physics driving the matter-antimatter asymmetry.'
The Sakharov conditions
How did matter gain an overwhelming upper hand over antimatter in the universe? CP violation in baryons is an important piece of this puzzle — but also only one piece.
In 1967, the Soviet physicist and later political dissident Andrei Sakharov said three conditions will have to be met for the universe to be made predominantly of only matter. They are:
(i) Baryon number violation: physical processes must exist that create an imbalance between the number of baryons and the number of antibaryons.
(ii) CP violation in baryons
(iii) Departure from thermal equilibrium: to prevent processes from balancing baryon and antibaryon production, interactions must occur out of equilibrium.
The observation of CP violation in baryon decays provides a 'source' that adds to CP violation among mesons. The complex phase of the mesons' violation has been measured whereas that of the baryons is pending. Once the latter is known physicists will be able to compare it to that predicted by the Standard Model.
If they match, it will mean the Standard Model is right — but at the same time leave a gap between the predicted matter-antimatter asymmetry and that observed in the universe.
If the values don't match, it could be a sign of 'new physics', which physicists will have to explain using new theories and experiments.
Overall, the newly reported observation is a milestone showing that the laws of physics treat matter and antimatter differently not just in mesons but also in baryons — the building blocks of the visible universe.
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