Say we have a decay of $B^0$ -meson to some charged particles. Is there a way of determining the masses of these particles?

I know that we can for example measure the momentum of the particles using a magnetic field (since the particles are charged), but I am wondering whether there is a way of experimentally determining the masses of the particles and therefore identifiying the decay products.


2 Answers 2


Measuring the energy with a calorimeter will work in principle, but you get much better resolution by identifying the final-state particles by other means and just using the known mass in your reconstruction of the original decaying particle. This only works for charged particles where you have a measurement of their momentum from tracking.

There are only a handful of long-lived charged particles: pions, kaons, protons, electrons and muons.

Muons are identified by penetrating through to the muon detectors.

Electrons are absorbed in the ECAL but also leave tracks so can be distinguished from photons.

Separating long-lived hadrons can be done using the angle of Cherenkov light to determine the velocity of the particle, which when combined with the momentum can be used to calculate mass and hence identify the hadron. This is done at LHCb and Belle.



For slow enough particles and good enough time resolution in your detector, you can measure the speed of the particles $v$, from which you can extract the mass through $p=\gamma m v$.

For faster particles, you can measure the energy of the particles in a calorimeter, and then determine the mass through $m=\sqrt{E^2-\vec{p}^2}$.

I believe Griffiths' Particle Physics book has a nice section on particle detectors.

  • $\begingroup$ Can you show me an example of a detector using timing to determine velocity of charged particles coming from collisions? I've never heard of this being one. Also what kind of B-meson decay products are slow enough for this to work? $\endgroup$
    – dukwon
    Nov 7, 2018 at 20:33

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