The Copernican

To decay or not to decay

Why do physicists study decay processes? Because they give us the pieces of a puzzle we can't fully reconstruct.

In particle physics, every particle dreams of losing weight and turning into a bunch of lighter particles. There are no exceptions. This process is called decaying.

Even the Higgs boson, that elusive goddamned residual particle of the Higgs field, decays into W bosons, Z bosons, leptons, and photons. In fact, each Z boson then decays to two leptons.

These processes are important because they allow physicists to reconstruct objects that exist for too short a period for them to be captured and studied. Instead, physicists study what the results of the decay process are and then piece together what must have come before.

Going by observations: The heavier the particle, the stronger the desire to decay into lighter particles. This is evinced by the particle’s lifetime.

The most massive elementary particle is the top quark, one of six kinds, or >flavours , of quarks. It weighs a whopping 172.9 ± 1.5 GeV/c 2, which is almost as heavy as an atom of tungsten! Its lifetime, however, is a pitiful five trillion-trillionths of one second. (Here, GeV/c 2 is a unit of mass: Einstein's famous equation states mass and energy are related as E = mc 2, 'c' being the speed of light. So, m = E/c 2.)

One of the least massive elementary particles, on the other hand, is the electron. It weighs a decent 0.511 MeV/c 2, is perfectly stable, and never decays.

Decay to what and when

Because of their propensity for decaying, heavier particles will not only decay faster but also in more combinations of lighter particles. This is because the heavier you are, the more options there are of particles lighter than you to choose from. Of course, there are limits to how often one combination of particles is chosen to decay through over another.

Moreover, heavier particles seldom come together to make up even heavier particles. The top quark, for example, doesn’t even last long enough to pair up with other quarks to form hadrons like protons and neutrons.

However, in the off-chance that two heavy particles have come together, the composite particle is going to have a far shorter lifetime than either of the constituents, and is going to decay through literally an abundance of combinations. One example of such a particle that’s been in the news is the B_c meson, first discovered by the Tevatron in 1995.

Mesons are particles that contain one quark and one antiquark. Unfortunately, the B_c meson contains two of the heaviest flavours of quarks (after the top quark) known – bottom and charm – and so its lifetime is abysmal…

But not abysmal enough for the Large Hadron Collider (LHC).

The B_c meson decays

The LHCb detector on the LHC is specialised to study the bottom quark, which weighs around 4.2 GeV/c 2. The other particle in the B_c meson, the charm antiquark, weighs 1.3 GeV/c 2.

Note that these masses are approximates; a strange quantum mechanical principle called >colour confinement has kept us from accurately measuring their masses.

Anyway, the B_c meson has access to a whopping >66 decay modes (PDF). However, only a few have been observed experimentally, such as the following:

1. B_c ±J/ ψ (meson) + l ± (lepton) + ν (neutrino) ( >link)

2. B_c ±J/ ψ + π ± (pion) ( >link)

3. B_c ±J/ ψ + K ± (kaon)

The B_c’s decay to a J/ ψ (pronounced “jay psi”) meson is favoured by experimental physicists because this particle consequently decays into two µ-mesons, i.e. muons, which are easy to detect and measure.

And via a paper submitted to the arXiv pre-print server >on March 7, 2013, the LHCb collaboration announced another decay mode that it had spotted: B_c +ψ(2S) + π +. Here, ψ(2S), also known as ψ(3686), is an excited state of the J/ ψ meson.

The paper also revealed that a B_c’s decay to an excited J/ ψ meson instead of a “normal” J/ ψ meson happened fully one-fourth of the time. It also noted the emergence of another decay mode: B_c +J/ ψ + π + + π + + π -.

LHCb data showing spikes for two decay processes of the B_c meson. The height of each spike denotes the strength of the signal while its narrowness brackets off the B_c meson's mass-range.

The mechanism of these decays is through what’s called the weak interaction because it transpires through the exchange of W ± and Z bosons. What happens is one quark decays while the other remains spectator.

Why are these measurements important? As I stated earlier, the colour confinement principle, which prevents quarks from being spotted in isolation, keeps physicists from measuring their masses accurately. By extension, the B_c meson’s mass also eludes capture.

But when they decay, physicists can zero in on those elusive masses by noting how the decay process progresses. They use their knowledge of the properties of lighter particles to piece together the properties of the heavier particles.

For example, the decay mode B_c ±J/ ψ + l ± + ν was >used in 1998 by scientists at Fermilab to determine the B_c + meson’s mass to be 6.40 ± 0.39 (stat. error) ± 0.13 (sys. error) GeV/c 2 and its lifetime to be around 0.46 picoseconds (i.e., 0.46 of one trillionth of one second).

These are important numbers because 1) they validate some hypotheses and invalidate some others, 2) they indicate by how much our calculations were off, and 3) they let us give values to things and find a way to accommodate them in our formulae.

In fact, this is what most of experimental physics has to give theoretical particle physics, and side by side, keep our curiosity well-equipped to keep moving.

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