Our universe was believed to have been created with equal quantities of matter and antimatter, only for antimatter to completely disappear over time. We know that matter and antimatter can annihilate each other but we don’t know how matter came to gain an upper hand and survive to this day, creating, stars, planets, and – of course – us.
In the theories that physicists have to explain the universe, they believe that the matter-antimatter asymmetry is the result of two natural symmetries being violated. These are the charge and parity symmetries. The charge (C) symmetry is that the universe would work the same way if we replaced all the positive charges with negative charges and vice versa. The parity (P) symmetry refers to the handedness of a particle. For example, based on which way an electron is spinning, it’s said to be right- or left-handed. All the fundamental forces that act between particles preserve their handedness except the weak nuclear force.
According to most particle physicists, matter won the war against antimatter through some process that violated both C and P symmetries. Proof of CP symmetry violation is one of modern physics’s most important unsolved problems.
In 1964, physicists discovered that the weak nuclear force is capable of violating C and P symmetries together when it acts on a particle called a K meson. In the 2000s, a different group of physicists found more evidence of CP symmetry violation in particles called B mesons. These discoveries proved that CP symmetry violation is actually possible, but they didn’t bring us much closer to understanding why matter dominated antimatter. This is because of particles called quarks.
Quarks are the smallest known constituent of the universe’s matter particles. They combine to form different types of bigger particles. For example, all mesons have two quarks each. All the matter that we’re familiar with are instead made of atoms, which are in turn made of protons, neutrons, and electrons. Protons and neutrons have three quarks each – they’re baryons. Electrons are not made of quarks; instead, they belong to a group called leptons.
To explain the matter-antimatter asymmetry in the universe, physicists need to find evidence of CP symmetry violation in baryons, and this hasn’t happened so far.
On December 7, a group of researchers from China published a paper in the journal Physical Review D in which they proposed one place where physicists could look to find the answer: the decay of a particle called a lambda-b baryon to a D-meson and a neutron.
Quarks come in six types, or flavours. They are up, down, charm, strange, top, and bottom. A lambda-b baryon is the name for a bundle containing one up quark, one down quark, and one strange quark. A D-meson is any meson that contains a charm quark. In the process the researchers have proposed, the D-meson exists in a superposition of two states: a charm quark + an up anti-quark (D0 meson) and a charm anti-quark and an up quark (D0 anti-meson).
The researchers have proposed that the probability of a lambda-b baryon decaying to a D0 meson versus a D0 anti-meson could be significantly different as a result of CP symmetry violation.
The proposal is notable because the researchers have tailored their prediction to an existing experiment that, once it’s upgraded in future, will collect data that can be used to look for just such a discrepancy. This experiment is called the LHCb – ‘LHC’ for Large Hadron Collider and ‘b’ for beauty.
The LHCb is a detector on the LHC, the famous particle-smasher in Europe that slams energetic beams of protons together to pry them open. The detectors then study the particles in the detritus and their properties. LHCb in particular tracks the signatures of different types of quarks. Physicists at CERN are planning to upgrade LHCb to a second avatar that’s expected to begin operating in the mid-2030s. Among other features, it will have a 7.5-times higher peak luminosity – a measure of the number of particles the detector can detect.
If the lambda-b baryon’s decay discrepancy exists in the new LHCb’s observed data, the decay proposed in the new study will be one way to explain it, and pave the way for proof of CP symmetry violation in baryons.