I had a short look at this for run 7469, where the charge is also reported as being -1.
I plotted the (x, y) positions of the first 25 particles, changing the colour of the blips for each station. I've uploaded the plots as muon_charge_direction.pdf. Colours are:
- Station 5 (most upstream) = red
- Station 4 = green
- Station 4 = cyan
- Station 2 = blue
- Station 1 (most downstream) = black
On a few of them I've drawn arrows indicating their direction of travel from upstream to downstream, but as best as I can tell, being naive, they all travel clockwise from up- to downstream.
About a month before run 7469 was taken, Mark Palmer recorded the field direction of SSU. I uploaded a picture of his logbook to https://micewww.pp.rl.ac.uk/elog/SSU/70 He used a 3-axis Hall probe, whose instructions claim that it reports a positive value when the field enters the probe. From his notes, I think that the Bz component of the field also travels from upstream to downstream.
So, if we're looking from Station 5 to Station 1, up- to downstream, I get particles going clockwise. The field is going "into the page", upstream to downstream. The only way that works is if the muon is negative (so the track fit returns the right charge if all of the above is correct).
- The beamline was, and still is (I've looked), in positive polarity. So I don't believe we actually had negative muons.
- I believe we can write down the number we see on a gadget correctly...
- But (!) I'm going to repeat this again when SSU gets powered at 5A tonight and see what I see
- And (!) Craig has ordered me a pretty bar magnet with defined North/South to it so that we can confirm the Hall probe works the way we think it works.
- ???
So, the most reasonable conclusion is that muon_charge_direction.pdf is actually the wrong way around -- and OK, it's pretty obvious, looking up- to downstream, positive x increases towards the MLCR (right handed co-ordinate system), so positive x is "left", whereas in muon_charge_direction.pdf positive x is "right". [Is this the assumption patt rec uses when it assigns a charge?] So the assumption that I'm looking up- to downstream in muon_charge_direction.pdf is very probably wrong.
To confirm, I made a 3D plot of the first particle (so the top left in muon_charge_direction.pdf). These are the plots "muon_directionX.pdf", where X = {1, 2, 3, 4}
- muon_direction2.pdf shows the same type of view as in muon_charge_direction.pdf, where the helix direction looks "clockwise".
- muon_direction4.pdf shows what I think an upstream to downstream view looks like, and here the helix is anticlockwise, as a good positive particle should be. The hits proceed in z increasing away from you.
- muon_direction(1,3).pdf are there to help you orient yourselves...
So, moving to a 3d plot, yes.. we have positive muons moving in a field that does go from up- to downstream. But if you were just to do a straightforward (x, y) plot you'd get it wrong.
But to cover all bases, this is what I'm doing:
- Check 3-axis Hall probe using a bar magnet to determine the direction of "positive" field
- Measure SSU at 5A (with photographic evidence of the probe orientation)
- Measure SSU at 3T (with a reading written in pencil you'll just have to trust)
- Check direction of muons with next data...
- (And make sure the BLOCs haven't just taken someones word for it that the beamline is positive. Someone could've snuck in there between now, when I just checked, and next Monday!)
Will update once I have some proof of the direction SSU's field is currently facing in (so we don't make two mistakes that magically cancel out)
(Sure, this is all the same as just looking at the phase increase, but it does help me visualise it this way and is hopefully helpful! :) )
