Re: [fluka-discuss]: Question about neutrino production in FLUKA

From: Andrea Celentano <>
Date: Thu, 10 Dec 2020 13:52:20 +0100

Dear Paola,
thanks for your answer. I agree that, if a mu- decays, then it produces both a nu_mu and a nu_e_bar, while if it is captured by a nuclei, only a nu_mu is produced. However, in case of a capture:

mu- p -> n nu_mu

The energy of the emitted nu_mu is ~ 100 MeV, with an enlargement due to the Fermi motion of the proton inside the nucleus. This is visible in the spectrum that I obtained.

Instead, in case of a decay, mu- -> e- nu_mu nu_e_bar, the two neutrinos have energy less than ~ mu/2 ~ 50 MeV . Therefore, I was expecting to have the same number of nu_mu and nu_e_bar in this region of the spectrum.


> On Dec 10, 2020, at 13:18, Paola Sala <> wrote:
> Dera Andrea,
> sorry for the late answer, maybe you found already -)
> My impression is that you start with a largely different number of
> positive and negative pions decaying at rest, that are then originating
> low energy mu+ and mu- . This can bee seen by the monochrmatic peak at
> 30 MeV in the nu-mu and nu-mu-bar, the numu (pion+ -> mu+ numu) being
> almost two orders of magnitude more than the num-bar (pion->mu- numubar).
> This large difference reflects in the lfact that you get a lot of numubar
> and nue fom the positive muon decay.
> The few negative muons, all of them produce a nu_mu, either from decay or
> when captured, two orders of magnitude less than numubar and nue from
> muon+.
> Then, only the non-captured fraction produces a nuebar. From the plot, the
> capture probability seems to be around 50%
> Check it...
> ah yes, the negative pions are preferentially interacting or being
> captured by nuclei when at rest
> Paola
>> Dear all,
>> I am using FLUKA to sample the neutrino production by a 11 GeV electron
>> beam impinging on a thick target.
>> I have a custom mgdraw.f code that samples (via entry BXDRAW) each
>> neutrino passing through a plane, perpendicular to the beam axis,
>> downstream the target.
>> The following plot shows the neutrino current, in neutrinos / GeV /
>> impigning_electron, where each color refers to a different neutrino
>> species.
>> In the low energy part of the spectrum (E < ~ 50 MeV), the continuous part
>> of the distribution is due to the decay at rest of muons:
>> mu- -> e- nu_mu nu_e_bar
>> mu+ -> e+ nu_mu_bar nu_e
>> I observe that nu_e and nu_mu_bar are produced with higher intensity than
>> nu_mu and nu_e_bar, due to the fact that mu- can be captured by nuclei and
>> give rise to mu- X(Z,A) -> nu_mu X(Z-1,A) (with the large peak in the
>> nu_mu distribution at ~ 100 MeV).
>> However, while the contribution of nu_e and nu_mu_bar is the same at low
>> energy, this is not true for nu_mu and nu_e_bar.
>> I do not understand why, since any time a mu- decays at rest, it will
>> produce a nu_mu and a nu_e_bar. Here it seems there is a factor x2
>> difference.
>> Note that I am considering phase-space decays, hence the energy spectra
>> for decay-at-rest are all the same.
>> Thanks,
>> Bests,
>> Andrea
> Paola Sala
> INFN Milano
> tel. Milano +39-0250317374
> tel. CERN +41-227679148

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