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GR9277 #25
Problem
 GREPhysics.NET Official Solution Alternate Solutions
\prob{25}
In experiments located deep underground, the two types of cosmic rays that most commonly reach the experimental apparatus are

1. alpha particles and neutrons
2. protons and electrons
3. iron nuclei and carbon nuclei
4. muons and neutrinos
5. positrons and electrons

Advanced Topics$\Rightarrow$}Particle Physics

Choice (A) and (C) involve atoms, which are quite massive. Choice (B) involves protons, which are also pretty massive. Massiveness eliminates three choices, leaving just (D) and (E).

Neutrinos are massless, but muons aren't. Both positrons and electrons have the same mass. Massiveness has lost its charm. (No pun intended.)

According to David Schaich, the Super-Kamiokande
and the Antarctic Muon and Neutrino Detector Array (AMANDA) are both located deep down underground to avoid interaction with other particles. Thus, with the hindsight of this bit of trivia, choice (D) is correct.

Alternate Solutions
 Shoshe2006-11-03 11:18:36 Neutrinos only experience the weak interaction, which makes them hard to detect. If you try to do a neutrino experiment on the Earth's surface, you'll have a horrible signal-to-noise ratio. Neutrino detectors are generally located underground because as cosmic rays go through the Earth, most of the particles other than neutrinos (pions, muons, antiprotons, etc.) will interact with "stuff" and not reach the detector. The deeper underground the detector is, the fewer false positives. In general, "underground particle experiment" = "neutrino experiment". Choice (D) is the only one that mentions neutrinos.Reply to this comment
epuma
2013-10-08 15:31:11
It also may be helpful to remember that the ground is an insulator. Seeing electrons from space pass through thick layers of earth would require ridiculous energies (hence (E) is out).
drunkphysics
2013-08-27 10:13:26
pretty simple to remember with the classic:

So a neutrino walks into a bar. The bartender says, "What'll ya have?" The neutrino says, "Oh nothing, I'm just passing through."

only answer that has neutrinos is D.

bam.
testtest
2010-09-22 20:22:52
If you ever worked on any neutrino / dark matter / cosmic ray experiment (I happened to work for one and have friends working for others), this takes only a second :)

Just remember that neutrinos go pretty far across matter!
sullx
2009-10-27 18:34:26
Yosun: "massless"? Not so quick I dare say! See neutrino oscillation.
sirius
2008-11-04 00:13:23
this problem is almost pure trivia. while one can spend time thinking about it, i'm positive ETS put it in there for trivia's sake.
HariSeldon
2008-06-29 11:30:45
You can also eliminate E with the knowledge that electrons have a relatively short interaction length, while muons travel much farther before interacting.
sharpstones
2007-03-26 17:50:04
it's worth noting as well that Positrons are antimatter particles which tend to have a short lifespan in any place with matter. (remember matter & antimatter = boom! energy) So they would no be able to penetrate the earth from outer space.
Shoshe
2006-11-03 11:18:36
Neutrinos only experience the weak interaction, which makes them hard to detect. If you try to do a neutrino experiment on the Earth's surface, you'll have a horrible signal-to-noise ratio. Neutrino detectors are generally located underground because as cosmic rays go through the Earth, most of the particles other than neutrinos (pions, muons, antiprotons, etc.) will interact with "stuff" and not reach the detector. The deeper underground the detector is, the fewer false positives. In general, "underground particle experiment" = "neutrino experiment". Choice (D) is the only one that mentions neutrinos.
 Almno102010-11-12 13:34:26 Neutrinos are never detected. In order to detect a neutrino, you look for heavier lepton with some momentum that is unaccounted for. This is because neutrinos penetrate almost anything (you can only detect something if you stop its motion). So neutrinos totally check out. And the muon mass is precisely the reason it is not stopped by electromagnetic interactions in the earth. Electrons are not heavy enough to make it through the random E/M fields in earth matter, and positions will hit and electron and annihilate before going too far.

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