New measurement of particle’s lifetime intrigues physicists

A proton–proton collision event detected by LHCb earlier this year. (Image: LHCb/CERN)

A new measurement of one of a particle’s properties can sometimes throw up a value that, intriguingly, is very different from previous values. In a paper posted online and submitted to the journal Physical Review Letters, the LHCb collaboration at CERN reports precisely that for the lifetime of the so-called charmed omega. Using data from proton–proton collisions, the LHCb researchers have obtained a value for the particle’s lifetime that is nearly four times larger than previous measurements. New studies are already being planned to unravel this intriguing discrepancy, at LHCb and other experiments.

The charmed omega belongs to a family of particles known as baryons. These particles, of which protons and neutrons are examples, comprise three smaller particles called quarks. But unlike protons, which contain three light quarks and are stable, the charmed omega contains two relatively light quarks and a heavier charm quark (the third heaviest of the six known types of quark), and eventually decays into other particles. Measurements of the lifetimes of charmed particles, and more generally of particles containing heavy quarks, are important because they test models of quantum chromodynamics – the theory that describes how quarks are stuck together by gluons.

The lifetime of the charmed omega was measured more than a decade ago by the E687 and FOCUS collaborations at Fermilab in the US and by the WA89 collaboration at CERN. These collaborations measured the lifetime of the charmed omega by examining some dozens of charmed-omega decays in experiments in which a beam of particles strikes the nuclei in a fixed target. The average of the values measured by these experiments, which are all relatively close to one another, is 69 ± 12 femtoseconds (one femtosecond is a millionth of a billionth of a second).

The new LHCb measurement is based on proton–proton collision data comprising about 1000 charmed-omega decays. The LHCb researchers determined the particle’s lifetime by comparing these decays with those of another particle whose lifetime is known very precisely; a similar approach was recently used by the team to determine the lifetime of a “doubly charmed” particle. The charmed-omega result – a lifetime of 268 ± 26 femtoseconds – is much larger than the average of the older values.

However, none of these measurements contradicts the theoretical estimates of the charmed omega’s lifetime, which rely on subtle calculations based on quantum chromodynamics and include predictions ranging from 60 to 520 femtoseconds. The jury is therefore out on whether the older values or the new one will stand, but the discrepancy between the values will no doubt prompt researchers to make new measurements and revise the theoretical estimates.