Bug #1884

240 MeV/c beams - lobe structure in X-Y distribution

Added by Liu, Ao over 7 years ago. Updated over 7 years ago.

Start date:
31 October 2016
Due date:
% Done:


Estimated time:


For runs #8451 8456 8460 8462 8467 (240 MeV/c with absorber without diffuser), a weird structure in X-Y distribution at TKU station 5 was observed. (See attached figure)

Left plot: # of particles on X-Y plane, average momentum distribution on X-Y plane;
No PID cut was applied in the plots.

Things to check:
1. Individual runs, does this pattern exist for each run?
Status: done. Yes it does. Also, today Durga, Paolo and myself looked at the OnRec and the lobes were clear in each Scifi X-Y plot;

2. Was it because of bad tracks (i.e. not enough tracks to make the PR fit)?
Status: done. Required 4 station hits in each scifi_track, the pattern still persists.

3. Could it be because of dispersion and bad PID cut, so that when pions are separated from muons by momentum, the lobes will be formed?
Status: partially done. First I doubt so because as can be seen, average momentum was almost uniform within the r<=100 mm cut made by the diffuser irises; second, after a preliminary PID cut the lobes still exist for muons only. I'll make some plots for the PID cut.

4. Does G4BL see this feature?
According to Paolo it should not. But TOF1 also sees this lobe structure.

5. Does DS or SSU on/off status change the structure?


lobes_and_avrgP.png (245 KB) lobes_and_avrgP.png Liu, Ao, 01 November 2016 10:47
momentum_TOF01_PID.png (115 KB) momentum_TOF01_PID.png Liu, Ao, 01 November 2016 10:47
PID_cut_robustish.png (281 KB) PID_cut_robustish.png Liu, Ao, 01 November 2016 11:59
PID_cut_robustish.png (245 KB) PID_cut_robustish.png Liu, Ao, 01 November 2016 14:30
spacepoints_lobes.png (168 KB) spacepoints_lobes.png Liu, Ao, 02 November 2016 00:44
spacepoints_muons_TKUstn5_multi.png (45.6 KB) spacepoints_muons_TKUstn5_multi.png Liu, Ao, 02 November 2016 11:41
spacepoints_muons_TKUstn5_noAbs.png (35.8 KB) spacepoints_muons_TKUstn5_noAbs.png Liu, Ao, 02 November 2016 13:00
upstream_x_upstream_y_cuts-True.png (10.4 KB) upstream_x_upstream_y_cuts-True.png Rogers, Chris, 02 November 2016 14:18
upstream_x_upstream_px_cuts-True.png (10.2 KB) upstream_x_upstream_px_cuts-True.png Rogers, Chris, 02 November 2016 14:18
upstream_y_upstream_py_cuts-True.png (9.75 KB) upstream_y_upstream_py_cuts-True.png Rogers, Chris, 02 November 2016 14:18
XY_muon_Pslices.png (152 KB) XY_muon_Pslices.png Liu, Ao, 02 November 2016 16:53
XPx_muon_Pslices.png (122 KB) XPx_muon_Pslices.png Liu, Ao, 02 November 2016 16:53

Updated by Franchini, Paolo over 7 years ago

I think you still need to attach the plots.


Updated by Liu, Ao over 7 years ago

(Attached plots) 3rd bullet: PID cut was done for one of the runs. Attached is the plot of momentum v.s. TOF01 before and after the muon cut.

First of all, one sees this clear momentum lobes on the P-TOF plot, which corresponds to a lower peak momentum of ~150 MeV/c. Now if one sees the avrg. momentum plot on X-Y, the green/yellow colors show up at the edge of the irises (r>=100 mm).

Secondly, after the muon cut, the lobes within r<=100 mm still exist. They have ~ 240 MeV/c momentum and correct TOF01, so the lobes are not caused by pion-muon mixing. Of course, the cut also eats in part of the questionable low P pion-like region, which could be also iris-slowed pions, but again, they do not form the tubes.

Therefore, my guess is that the beam, for some reason between TOF0 and tracker (very likely before TOF1) was collimated by something, then starts to possess two lobes and lobes rotate in X-Y during transporting. Durga discovered that the beam is wide in Y at TOF1 already, so could be before then.


Updated by Rajaram, Durga over 7 years ago

What is the difference between the 2 plots? Both have same x & y variables. Is one after pid cuts?


Updated by Liu, Ao over 7 years ago

For lobes_and_avrgP.png:
left: # of particles on XY plane, right: avrg P on XY plane;

For momentum_TOF01_PID.png:
left: P v.s. TOF01 for all, right: P v.s. TOF01 for muons.


Updated by Blackmore, Victoria over 7 years ago

It looks to me (from the tof-momentum plot) like you've still got an awful lot of junk in what you're called 'muons'. What's dictating that cut?


Updated by Liu, Ao over 7 years ago

Durga stopped by and suggested more robust cuts too, so attached. Previous PID cuts were done "automatically" based on a possible range of TOF01 assuming the particle is muon at each momentum. It was not robust and will keep all the ambiguous ones.

Attached are after the new cuts.
-->First, cut only based on TOF01 with a slice.
lobes still exist, particles outside r=100 mm still exist;
-->Second, cut out only the Gaussian part of the muon peak, which should be robust enough.
lobes still exist, particles outside r=100 mm disappear.

The above proves:
Lobes not formed by different PID or momentum;
Particles with r>=100 mm should be those slowed by irises, which have lower momentum.


Updated by Blackmore, Victoria over 7 years ago

Can you do a profile plot for the latter set? I'm not convinced that the "lobes" are there anywhere near as strongly with the tighter PID cut. It looks more flat (though diagonal) with a bit of a tendency towards the right. (I think if it had been the first plot I'd seen, I wouldn't be imagining it as a split distribution)


Updated by Liu, Ao over 7 years ago

That's a fair point. I attached plot from all the 240 MeV/c runs. Again I think the momentum cut will effectively remove the ones slowed by irises (r>=100 mm)

Victoria and Durga doubted whether the hole was created by low P_t particles that were not reconstructed (similar to straight tracks). So checking spacepoints will either confirm or reject this speculation. To be continued.


Updated by Liu, Ao over 7 years ago

I have plotted spacepoints rather than reconstructed trackpoints at TKU station 5.

I did a TOF01 cut to select out the muon peak. Results are shown below.

Notice that the previous colormap had "lognormal" scale, so color difference was milder. The newly attached plot uses a linear colormap.


Updated by Liu, Ao over 7 years ago

Plot with more statistics from runs
8451, 8456, 8460, 8462, 8467
(muons, spacepoints @ TKU STN 5)


Updated by Liu, Ao over 7 years ago

Bear with me... Just consistency check -

240 MeV/c runs without absorber in early Oct. showed the same thing.

Plotted are muon-generated spacepoints at TKU STN 5.


Updated by Rogers, Chris over 7 years ago

Just to note that I also see the behaviour, only looking at a single run (8451). I also see it in transverse momentum plots.

  • If you slice into momentum bins, do you still see the behaviour?

Updated by Liu, Ao over 7 years ago

X-Y and X-Px plots attached with momentum slices. Interesting to see how the beam moves with different momenta and avoids the center.

One of the speculations from Chris was that the beam was separated by their momenta and thus different rotation frequency in a solenoid. That can be checked by back-tracking the beam upstream until it does not see the solenoidal field.

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