W.What happened at the beginning of the universe, in the very first moments? The truth is, we don’t really know because it takes enormous amounts of energy and precision to recreate and understand the cosmos on such short time scales in the laboratory. But the scientists at the Large Hadron Collider (LHC) at Cern in Switzerland are not giving up.
Now ours LHCb experiment has ever measured one of the smallest mass differences between two particles, which will allow us to learn much more about our enigmatic cosmic origins.
The standard model of particle physics describes the fundamental particles that make up the universe and the forces that work between them. Elementary particles include quarks, of which there are six – up, down, strange, charm, top and bottom. Similarly, there are six “leptons,” including the electron, a heavier cousin called the muon, and the even heavier tau, each with a neutrino associated with it. There are also “antimatter partners” of all quarks and leptons, which are identical particles except for one opposite charge.
The Standard Model is experimentally verified with incredible accuracy, but it has some significant shortcomings. The universe was created in the Big Bang about 13.8 billion years ago. The theory is that this event should have produced equal amounts of matter and “antimatter”. Yet today is the universe consists almost entirely of matter. And that’s a godsend, because antimatter and matter annihilate each other in a flash of energy when they meet.
One of the biggest open questions in physics today is why there is more matter than antimatter. Were there processes in the early universe that favored matter over antimatter? To get closer to the answer, we examined a process in which matter turns into antimatter and vice versa.
Quarks are linked into particles called baryons – including the protons and neutrons that make up the nucleus – or mesons, which are made up of quark-antiquark pairs. Mesons with no electrical charge are constantly going through a phenomenon called mixing, in which they spontaneously transform into their antimatter particles and vice versa. The quark becomes an anti-quark and the anti-quark becomes a quark.
It can do this because of quantum mechanics that rules the universe on the smallest scales. According to this counterintuitive theory, particles can be in many different states at the same time, essentially a mixture of many different particles – a property called superposition. Only when you measure his condition will he “choose” one of them. For example, a meson named D0, which contains charm quarks, is located in a superposition of two normal matter particles named D1 and D2. The speed at which the D0 meson changes into its antiparticle and back again, an oscillation, depends on the mass difference between D1 and D2.
It is not easy to measure mixing in D0 mesons, however it was done for the first time in 2007. So far, however, nobody has reliably measured the difference in mass between D1 and D2, which determines how fast D0 oscillates in its antiparticle.
Our newest discovery, announced at the Charm conference, changes this. We measured a parameter that was a mass difference of 6.4×10. corresponds to-6 Electron volts (a measure of energy) or 10-38 Gram – one of the smallest mass differences ever measured between two particles.
We then calculated that the oscillation between the D0 and its antimatter partner lasts about 630 picoseconds (1ps = 1 millionth of a millionth of a second). This may seem quick, but the D0 meson doesn’t live long – it is not stable in the laboratory and decays (decays) into other particles after only 0.4 picoseconds. Therefore, it will typically go away long before this oscillation occurs, which is a serious experimental challenge.
The key is precision. We know from theory that these oscillations follow a known wave type (sinusoidal). If we measure the beginning of the wave very precisely, we can deduce its full period since we know its shape. The measurement therefore had to achieve record precision on several fronts. This is made possible by the unprecedented amount of magic particles produced at the LHC.
But why is that important? To understand why the universe produces less antimatter than matter, we need to learn more about the asymmetry in the production of the two, a process known as CP violation. It has already been shown that some unstable particles decay differently than their corresponding antimatter particle. That may have contributed to it to the abundance of matter in the universe – With earlier discoveries of which leads to Nobel Prizes.
We also want to find CP violations during the mixing process. If we start with millions of D0 particles and millions of D0 antiparticles, will we have more D0 normal matter particles after a while? Knowing the rate of oscillation is an important step towards this goal. Although we did not find any asymmetry this time, our result and further precision measurements can help us find it in the future.
Next year the LHC will turn on after a long shutdown, and the updated LHCb detector will be collecting much more data, further increasing the sensitivity of these measurements. Theoretical physicists are now working on new calculations to interpret this result. The LHCb physics program is supplemented by the Belle II experiment in Japan. These are exciting perspectives for the investigation of matter-antimatter asymmetry and the vibrations of mesons.
While we cannot fully solve the mysteries of the universe just yet, our latest discovery has laid the next piece of the puzzle. The improved LHCb detector will open the door to an era of precision measurements with the potential to uncover as-yet-unknown phenomena – and perhaps beyond the Standard Model’s physics.
Martha Hilton is a PhD student in particle physics at the University of Manchester. Nathan Jurik is a Particle Physics Research Fellow at Syracuse University. Sascha Stahl is a research associate at CERN. This article first appeared on The conversation