Why is Muon Charge Ratio Positive?

[dt19]

Hi,
Could anyone explain to me why it is that cosmic radiation supplies slightly more positive than negative muons? I know that negative muons disappear a little faster than the positive ones as they can decay or be captured, but I don't know if this would have any impact on what I'm asking.

Thanks for any help!

 

[Bob S]

Here is a paper I found on Google:
http://www.cosmic-ray.org/papers/icrc733-1833.pdf

There are several possible reasons for a small charge asymmetry:
1) Charge of incident cosmic ray (proton or alpha) is overwhelmingly positive.
2) Does the Earth's magnetospheric field polarity matter? Probably will affect the east-west ratio (Lorentz force qv x B).
3) Negative muons rarely do not decay, even in muonic nitrogen and oxygen. They ~never form muonic atoms before stopping. For negative muons captured in iron or higher. most muons are absorbed.
4) CP violating K-decay modes?
5) There is a very very slight dE/dx Bethe-Bloch range asymmetry in stopping +/- muons.
I am puzzled if there is a significant effect. Hadronic showers create muons mostly via pion or kaon decay.
Bob S

[added] See Phys. Rev. D 74, 082006 (2006) [8 pages]
The charge ratio of the atmospheric muons at low energy
The effect is attributed to the geomagnetic field.

 

[Astronuc]

The charge ratio obtained for muons coming from pious produced in proton
interactions was Zpπ+ it /Zpπ- = 1.46. Muons from kaons tend to increase the charge ratio,
and hadronic cascades in the atmosphere tend to decrease it, as do neutrons present in
primary nuclei.

from paper cited by Bob S.

I would expect it has to do with the overwhelmingly positive charge of incident cosmic rays (protons) which strike ordinary matter (protons and neutrons). It would seem to factor production of π+ and K⁺. That seems to be a conclusion in the paper.

 

[Bob S]

Here is another newer measurement:
http://prd.aps.org/abstract/PRD/v76/i5/e052003
indicating a μ⁺/μ^- charge ratio ≈1.37, consistent with kaon decay asymmetry predictions at very high energies.
Phys. Rev. D 76, 052003 (2007) [16 pages]
Measurement of the atmospheric muon charge ratio at TeV energies with the MINOS detector
Abstract
P. Adamson et al. MINOS Collaboration

The 5.4 kton MINOS far detector has been taking charge-separated cosmic ray muon data since the beginning of August, 2003 at a depth of 2070 m.w.e. in the Soudan Underground Laboratory, Minnesota, USA. The data with both forward and reversed magnetic field running configurations were combined to minimize systematic errors in the determination of the underground muon charge ratio. When averaged, two independent analyses find the charge ratio underground to be Nμ+/Nμ-=1.374±0.004(stat)-0.010+0.012(sys). Using the map of the Soudan rock overburden, the muon momenta as measured underground were projected to the corresponding values at the surface in the energy range 1–7 TeV. Within this range of energies at the surface, the MINOS data are consistent with the charge ratio being energy independent at the 2 standard deviation level. When the MINOS results are compared with measurements at lower energies, a clear rise in the charge ratio in the energy range 0.3–1.0 TeV is apparent. A qualitative model shows that the rise is consistent with an increasing contribution of kaon decays to the muon charge ratio.

Bob S

 

[Bob S]

Monte-Carlo multi-particle-transport-simulation-code FLUKA simulations of cosmic ray showers show that the μ⁺/μ^- ratio >1.3 is due largely to the production ratio of strangeness +1 K⁺ to strangeness -1 K^-, which decay (usually by charged pions) to μ⁺ and μ^- respectively. See Fig. 2 in
http://www.srl.utu.fi/AuxDOC/kocharov/ICRC2009/pdf/icrc0177.pdf
Bob S