Monday, September 28, 2015

experimental physics - How are the masses of unstable elementary particles measured?


I am interested in knowing how (Q1) the particle's masses are experimentally determined from accelerator observations.



What kind of particles? They must be as far as we know elementary and unstable (very short lifetime) and not subject to the strong interaction (for example, Higgs particle, Z boson, etc.) I'm not interested in neutrons (not elementary), electrons (stable) or quarks (hadron). I'm not particularly interested in neutrinos either, since I think that best constraints come from neutrino oscillations and cosmological observations.


Since the particles I'm asking about acquire their masses through the Higgs mechanism, I would like to know what is actually or more directly measured the mass or the Yukawa coupling. (Q2)


I also wonder what is actually measured the propagator's pole (this the magnitude reported as mass for stable leptons) or the running mass at certain energy scale (this is one of the magnitudes reported as mass for quarks). (Q3)


This question may be considered a follow-up of How Can We Measure The Mass Of Particle?


Thanks in advance.


Edit: In connection with the answers: From all your answer I deduce that the mass reported for the Higgs, W and Z is the mass (rest energy) that appears in the energy-momentum conservation law. I guest that this mass corresponds to the pole of the free propagator of the Higgs, W and Z, respectively (and not to the running mass). I also deduce that what is more directly measured is the mass of the Higgs and from that value one deduces the self-coupling of the Higgs (and not in the other way around). These were my Question 3 and 2. Do you agree with my conclusion?



Answer



For sufficiently long-lived charged particles, one measures the helix-shaped track in an external magnetic field, and gets from this the 4-momentum (and hence the masss).


For very short-lived particles, one gets complex masses from resonance measurements.


Edit: Any mass of an unstable particle is complex and defined as the pole of a propagator. The mass of a particle like Higgs is determined quite indirectly, as it takes lots of scattering experiments to reliably determine the relevant cross sections. See http://arxiv.org/pdf/1207.1347.pdf for how to determine the Higgs mass from measurements. See also http://arxiv.org/pdf/1112.3007.pdf



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