LHCb measures tiny mass difference between particles


Illustration of the mass difference between the D1 and D2 mesons
The LHCb collaboration has measured the tiny mass difference between the D1 and D2 mesons, which are a manifestation of the quantum superposition of the D0 particle and its antiparticle. This mass difference controls the speed of the D0 oscillation into its antiparticle and back.
(Image: CERN)

The LHCb collaboration has measured a difference in mass between two particles of 0.00000000000000000000000000000000000001 grams – or, in scientific notation, 10-38 g. The result, reported in a paper just submitted for publication in the journal Physical Review Letters and presented today at a CERN seminar, marks a milestone in the study of how a particle known as a D0 meson changes from matter into antimatter and back.

The D0 meson is one of only four particles in the Standard Model of particle physics that can turn, or “oscillate”, into their antimatter particles, which are identical to their matter counterparts in most ways. The other three are the K0 meson and two types of B mesons.

Mesons are part of the large class of particles made up of fundamental particles called quarks, and contain one quark and one antimatter quark. The D0 meson consists of a charm quark and an up antiquark, while its antiparticle, the anti-D0, consists of a charm antiquark and an up quark.

In the strange world of quantum physics, just as Schrödinger’s notorious cat can be dead and alive at the same time, the D0 particle can be itself and its antiparticle at once. This quantum “superposition” results in two particles, each with their own mass – a lighter and a heavier D meson (known technically as D1 and D2). It is this superposition that allows the D0 to oscillate into its antiparticle and back.

The D0 particles are produced in proton-proton collisions at the Large Hadron Collider (LHC), and they travel on average only a few millimetres before transforming, or “decaying”, into other particles. By comparing the D0 particles that decay after travelling a short distance with those that travel a little further, the LHCb collaboration has measured the key quantity that controls the speed of the D0 oscillation into anti-D0 – the difference in mass between the heavier and lighter D particles.

The result, 10-38 g, crosses the “five sigma” level of statistical significance that is required to claim an observation in particle physics.

“To put this incredibly small mass difference in context, it is still a small number even when compared with the mass of the D0 particle – the same as the mass of a snowball compared to the mass of the entire Mont Blanc, the highest peak in Europe, standing at over 4800 metres,” says LHCb spokesperson Chris Parkes. “And it’s a big step in the study of the oscillatory behaviour of the D0 particles.”

With the tiny mass difference now observed, a new phase of particle exploration can begin. Researchers can make further measurements of the D0 decays to obtain a more precise mass difference and look for the effect on the D0 oscillation of unknown particles not predicted by the Standard Model.

Such new particles could increase the average speed of the oscillation or the difference between the speed of the matter-to-antimatter oscillation and that of the antimatter-to-matter oscillation. If observed, such a difference could shed light on why the universe is made up entirely of matter, even though matter and antimatter should have been created in equal amounts during the Big Bang.

LHCb spokesperson Chris Parkes explains the new result. (Video: CERN)

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