Higgs boson and its decay width

zaman786 · Aug 12, 2023

hi, I noticed that with higher mass decay width also go higher - but for higgs boson its mass is higher that W and Z boson but its decay width is lower , why?

Vanadium 50 · Aug 12, 2023

Why should mass determine decay width? The rho and omega have the same mass and very different widths. The tau is heavier than both and much, much narrower.

zaman786 · Aug 13, 2023

i thought decay width is proportional to the mass of particle by seeing data for W and Z Boson

vanhees71 · Aug 13, 2023

I guess you refer to phase space, i.e., the larger the mass the larger the phase space the particle can decay to. That's correct, but of course the decay rate also depends on to which other particles the particle in question can decay and the coupling strength to this particle to the possible decay products.

mfb · Aug 13, 2023

A given particle generally has a larger decay width if its mass is larger. That was relevant before we knew the Higgs mass (so people made predictions about its width for all possible masses), but that doesn't mean anything if you compare different particles with different decays.

Vanadium 50 · Aug 13, 2023

zaman786 said:

i thought decay width is proportional to the mass of particle

First, that's untrue. I gave examples with the rho and omega.

Second, the statement that if particles X were twice as heavy it would decay twice as fast might or might not be true, but in any case describes a counteractual universe and is a different statement than that two different particles whose mass differs by two also have widths differ by two.

In this set of counterfactual universes, the Higgs width is proportional to m³, not m.

ohwilleke · Aug 15, 2023

zaman786 said:

TL;DR Summary: Higgs Boson mass and its decay width

hi, I noticed that with higher mass decay width also go higher - but for higgs boson its mass is higher that W and Z boson but its decay width is lower , why?

The question is asked at an advanced level and at that level of analysis, the decay width of a particle is a purely derived quantity in the Standard Model for both fundamental particles and composite particles.

You list every possible decay of the particle, you determine the decay width for each possible decay (which can be calculated in the case of the Higgs boson from the mass of the Higgs boson, the Standard Model properties of the Higgs boson, and the other experimentally measured parameters of the Standard Model like the relevant coupling constants), and you add them up to get the total decay width of the particle.

Different particles have the decay widths that they do because when you do the math, that is how it turns out.

You might expect these particles to have similar decay widths, because the list of possible Higgs boson decays and the list of possible Z boson decays is quite similar, even if they aren't exactly identical, and because they have masses of the same order of magnitude.

But, heuristically, basically what is going on that makes the Higgs boson decay width smaller than the W and Z boson decay width is that the Higgs boson has very low probabilities of decays into lower mass particles. This is because the Higgs field couplings are proportional to the rest mass of the particle, which vary over a great range of rest masses. In contrast, Z boson decays are "democratic" (a.k.a. "universal"), which is to say that the probability that a Z boson decays into any possible particle (treating quarks of different colors as different particles) is identical. So, Higgs bosons are less likely to decay to lighter fundamental particles than Z bosons.

W bosons have a different set of possible decays than Z bosons do, but the number of possible decays isn't wildly different and W bosons, like Z bosons, also couple "democratically" (a.k.a. "universally") to particles that interact via the weak force. And, of course, the value of the weak force coupling constant is identical for both the W boson and the Z boson.

This isn't a rigorous explanation. One can imagine a counterfactual world where frequent Higgs boson decays to heavy particles completely balance out their infrequent decays to lighter particles resulting in nearly identical widths of the Higgs boson and the Z boson, or even the opposite result. But this explanation provides some intuitive sense of what is going on beyond merely saying that if you do the math, that's how it comes out (which is the most true answer).

zaman786 · Dec 6, 2023

ohwilleke said:

The question is asked at an advanced level and at that level of analysis, the decay width of a particle is a purely derived quantity in the Standard Model for both fundamental particles and composite particles.

You list every possible decay of the particle, you determine the decay width for each possible decay (which can be calculated in the case of the Higgs boson from the mass of the Higgs boson, the Standard Model properties of the Higgs boson, and the other experimentally measured parameters of the Standard Model like the relevant coupling constants), and you add them up to get the total decay width of the particle.

Different particles have the decay widths that they do because when you do the math, that is how it turns out.

You might expect these particles to have similar decay widths, because the list of possible Higgs boson decays and the list of possible Z boson decays is quite similar, even if they aren't exactly identical, and because they have masses of the same order of magnitude.

But, heuristically, basically what is going on that makes the Higgs boson decay width smaller than the W and Z boson decay width is that the Higgs boson has very low probabilities of decays into lower mass particles. This is because the Higgs field couplings are proportional to the rest mass of the particle, which vary over a great range of rest masses. In contrast, Z boson decays are "democratic" (a.k.a. "universal"), which is to say that the probability that a Z boson decays into any possible particle (treating quarks of different colors as different particles) is identical. So, Higgs bosons are less likely to decay to lighter fundamental particles than Z bosons.

W bosons have a different set of possible decays than Z bosons do, but the number of possible decays isn't wildly different and W bosons, like Z bosons, also couple "democratically" (a.k.a. "universally") to particles that interact via the weak force. And, of course, the value of the weak force coupling constant is identical for both the W boson and the Z boson.

This isn't a rigorous explanation. One can imagine a counterfactual world where frequent Higgs boson decays to heavy particles completely balance out their infrequent decays to lighter particles resulting in nearly identical widths of the Higgs boson and the Z boson, or even the opposite result. But this explanation provides some intuitive sense of what is going on beyond merely saying that if you do the math, that's how it comes out (which is the most true answer).

thanks - got it

Higgs boson and its decay width

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