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CERN’s LHCb breakthrough will reveal a lot about the universe’s origins


What happened at the beginning of the universe, in the very first moments? The truth is, we don’t really know because it takes huge amounts of energy and precision to recreate and understand the cosmos on such short timescales in the lab. But scientists at the Large Hadron Collider (LHC) at CERN, Switzerland aren’t giving up.

Now our LHCb experiment has measured one of the smallest differences in mass between two particles ever, which will allow us to discover much more about our enigmatic cosmic origins.

The Standard Model of particle physics describes the fundamental particles which make up the universe and the forces that act between them. The elementary particles include quarks, of which there are six – up, down, strange, charm, top, and bottom. Similarly, there are six “leptons” which include the electron, a heavier cousin called the muon, and the still heavier tau, each of which has an associated neutrino. There are also “antimatter partners” of all quarks and leptons which are identical particles apart from an opposite charge.

The Standard Model is experimentally verified to an incredible degree of accuracy but has some significant shortcomings. 13.8 billion years ago, the universe was created in the Big Bang. The theory suggests this event should have produced equal amounts of matter and “antimatter”. Yet today, the universe is almost entirely made up of matter. And that’s lucky because antimatter and matter annihilate in a flash of energy when they meet.

One of the biggest open questions in physics today is why is there more matter than antimatter. Were there processes at play in the early universe that favored matter over antimatter? To get closer to the answer, we have studied a process where matter transforms into antimatter and vice versa.

Quarks are bound together to form particles called baryons – including the protons and neutrons that make up the atomic nucleus – or mesons, which consist of quark-antiquark pairs. Mesons with zero electric charge continually undergo a phenomenon called mixing by which they spontaneously change into their antimatter particle, and vice versa. In this process, the quark turns into an anti-quark and the anti-quark turns into a quark.

It can do this because of quantum mechanics, which governs the universe on the tiniest of scales. According to this counter-intuitive theory, particles can be in many different states at the same time, essentially being a mix of many different particles – a feature called superposition. It is only when you measure its state that it “picks” one of them. A type of meson called D0, for example, which contains charm quarks, is in a superposition of two normal matter particles called D1 and D2. The rate at which the D0 meson turns into its anti-particle and back again, an oscillation, depends on the difference in masses of D1 and D2.

Tiny masses

It is difficult to measure mixing in D0 mesons, but it was done for the first time in 2007. However, until now, nobody has reliably measured the mass difference between D1 and D2 that determines how quickly the D0 oscillates into its antiparticle.