Why lower limits on proton decay lifetimes depend on channel

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Discussion Overview

The discussion revolves around the varying lower limits on proton decay lifetimes as set by experiments like Super-Kamiokande, focusing on the different decay channels (e.g., proton decaying into kaons, muons, or electrons). Participants explore the reasons behind these differences, considering both theoretical predictions and experimental factors.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the differences in lower limits on proton decay lifetimes are due to experimental effects, including detection efficiencies and background rates for each decay channel.
  • One participant notes that if a fixed number of protons are observed, the expected decay rates to different products (e.g., kaons versus muons) will influence the limits set on those decay channels.
  • Another participant questions the specific factors affecting detection in a water Cherenkov detector, such as the quality of Cherenkov rings produced by different particles.
  • It is mentioned that kaons, being slower, may produce wider and less bright Cherenkov cones, while muons might create clearer cones but could be affected by background noise from neutrino events.
  • Participants emphasize the importance of consulting publications for detailed information on detection efficiencies and background levels.

Areas of Agreement / Disagreement

Participants generally agree that experimental factors play a significant role in the differences observed in proton decay lifetime limits across various channels. However, there is no consensus on the specific reasons or mechanisms behind these differences, and some questions remain unresolved.

Contextual Notes

Limitations include the dependence on specific experimental conditions, the need for precise definitions of detection efficiencies, and the unresolved nature of background noise effects on different decay channels.

Anchovy
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If I look at the lower limits on the proton decay lifetime \tau set by, say, Super-Kamiokande, I'll see different lower limits depending on what the proton could decay into, eg. \tau_{min}(p \rightarrow K^{+} \overline{\nu}) < \tau_{min}(p \rightarrow \mu^{+} \pi^{0}) < \tau_{min}(p \rightarrow e^{+} \pi^{0}).

What is the reason for this? Is it to do with what the theoretical prediction for each decay channel's decay rate is? Or is experimental, ie. how good the detector is at picking up each observable decay product? Or perhaps to do with different background rates? Or something else?
 
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It is an experimental effect. If you observe a fixed amount of protons, and expect to see 50% of all decays to kaons but only 5% of all decays to muons, you'll probably set better limits on the kaon decay. On the other hand, if you have a lot of background for some decay channel, your limit can be much worse.

Assuming protons decay at all, those three decay channels will have different partial lifetimes (= different branching fractions = different partial decay widths).

The 50% and 5% numbers are made up, didn't check the actual numbers.
 
mfb said:
It is an experimental effect. If you observe a fixed amount of protons, and expect to see 50% of all decays to kaons but only 5% of all decays to muons, you'll probably set better limits on the kaon decay..

OK, so what would cause such difference in a water Cerenkov? (I think in your example it'd be the other way round, I think Super K has set a higher lower-limit for the muon channel than a kaon channel). Do kaons produce worse-quality rings at the PMTs? Or would more muons be energetic enough exceed the threshold at which they can radiate Cerenkov light? Or something else?
 
Kaons are slower, their Cherenkov cones (assuming they are fast enough at all) will be wider and not as bright. They decay to other particles which can make the event look messy.

Muons lead to a nicer Cherenkov cone, but they could suffer from more background from neutrino-induced events. Same for electrons.

The publications should give the detection efficiencies and the background levels.
 
mfb said:
Kaons are slower, their Cherenkov cones (assuming they are fast enough at all) will be wider and not as bright. They decay to other particles which can make the event look messy.

Muons lead to a nicer Cherenkov cone, but they could suffer from more background from neutrino-induced events. Same for electrons.

The publications should give the detection efficiencies and the background levels.

OK, thanks.
 

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