4.3 Candidate proton sample selection
4.3.5 Trigger and Charge selection
The trigger requirement
The selected events are required to activate the high energy trigger (HET). Fig. 4.5 illustrates this request: the figures in the top part show one event that does not pass the HET, while the figures on the bottom part correspond to an event that pass it. For particles interacting only in the calorimeter, the energy deposition threshold required on the first four layers of BGO (that is∼5 GeV) is not low enough to activate this trigger.
STK charge cut
The charge in the first point of STK is defined as:
STKFP= ADCMax (4.5)
where ADCMax is the maximum cluster charge in ADC counts among the coordinates x and y of the first point of STK (associated to the selected track). We applied a very loose cut requiring that the signal is lower than 600 ADC counts both in data and proton MC. Several distributions for various energy bins are shown in Fig. 4.7 for proton MC scaled to proton data both selected in PSD. The distributions using only FTFP MC are used in the comparisons, since the STK response is similar for the two type of MC.
CHAPTER 4. MEASUREMENT OF THE PROTON FLUX
Figure 4.5: Event display in the XZ and YZ view of a particle that released an energy of 5 TeV (top figures) and 12 TeV (bottom figures) in the calorimeter. The units on the x and y axis are mm. The particle comes from the bottom and goes to the top encountering first the PSD, then the STK and finally the BGO. The energy in the BGO and the PSD is in MeV. The stars in the STK represent the clusters used by the tracker finding algorithm. The dashed green lines in each view show the reconstructed BGO track extrapolated to the STK.
PSD difference and charge selection cut
The PSD measurement associated to the selected track on XZ and YZ is defined in this analysis as:
PSDx(y) = sdE
dxPSD hit x(y)×L (4.6)
where dEdx PSD hit x(y) is the energy deposited by the particle in the PSD layer that reads the x (y) coordinate, and L is the path that the cosmic ray crossed in the PSD bar.
The distributions of PSDx−PSDy are shown as a function of the reconstructed energy in the BGO in Fig. 4.8 and Fig. 4.9 for data compared to the two proton MC studied respectively. The area of the histograms are normalized to unity, but the data sample is not composed at this stage of the selection only by protons. There is still the noise contribution in PSD and the presence of heavier nuclei. At higher energy, the PSD is less affected by eventual noise and the flux of nuclei heavier than protons is smaller,
96
MC True Energy [GeV]
102 103 104 105
sr]×2Effective Acceptance [m
0 0.1 0.2 0.3 0.4 0.5 0.6
BGO Acceptance < 0.35 /Etot max bar
E
BGO max lateral n.tracks > 0 track selection PSD Fiducial No PSD bar fired
Figure 4.6: Effective acceptance as a function of the true energy of the particle for the various cuts of the first three groups, obtained from proton MC FTFP.
[ADC]
STKFP
0 100 200 300 400 500 600 700 800 900
10 102 103 104
< 100.00 GeV for 56.23 GeV < Erec
ADC in STKFP
p MC p Data
[ADC]
STKFP
0 100 200 300 400 500 600 700 800 900
1 10 102 103
< 1000.00 GeV for 562.34 GeV < Erec
ADC in STKFP
p MC p Data
[ADC]
STKFP
0 100 200 300 400 500 600 700 800 900
1 10 102
< 3162.28 GeV for 1778.28 GeV < Erec
ADC in STKFP
p MC p Data
[ADC]
STKFP
0 100 200 300 400 500 600 700 800 900
1 10
< 10000.00 GeV for 5623.41 GeV < Erec
ADC in STKFP
p MC p Data
Figure 4.7: Distributions of the cluster charge in the first point of the STK for protons selected in data (red) and MC (green). The dashed black line represents in each plot the chosen cut value.
CHAPTER 4. MEASUREMENT OF THE PROTON FLUX
therefore the agreement between data and MC is much better. We applied a cut on PSDx−PSDy for all the reconstructed energies of the particles requiring a difference contained in [−1,1.3]√
MeV (dashed green lines in Fig. 4.8 and 4.9). The two MC, as shown, behave almost the same way compared to data so that the same cut could be adopted. In the future, in this case, we will show only one MC. This cut allows to select
]
Figure 4.8: Distributions of PSDx−PSDy in different energy bins for data (red) and proton MC QGSP (blue).
events with fewer reconstructed tracks and as example two selected events are shown in Fig. 4.10. To remove low energy noise in PSD, we ask additionally that PSDx,yis greater than 1√
MeV. After this cut, the information on both views of the PSD was combined to a variable:
PSDglobal= PSDx+ PSDy
2 . (4.7)
The PSDglobal distribution is shown in Fig. 4.11 integrated on all the energies. The proton MC using the FTFP list corresponds to a combination of the FTFP MC sample until 100 TeV and the CRCM sample from 100 TeV until 1000 TeV. We will refer to it simply as FTFP+CRMC sample. The PSDglobal distribution was fitted in energy bins with a Landau function as shown in several examples in Fig. 4.12, using as MC the FTFP+CRMC sample. The MPV is shown as a function of the reconstructed energy in Fig. 4.13 using both the MC samples compared to data. The green curve in Fig. 4.13 on the right, corresponds to fsel, chosen in order to select a proton sample less affected by the helium background at all the energies (see section 4.3.8). In the left part of Fig. 4.13, the green curve overlaps with the MC function which is not shown for this reason. The two MC samples were generated with different statistics and the data samples used in
98
]
Figure 4.9: Distributions of PSDx−PSDy in different energy bins for data (red) and proton MC FTFP (green).
this comparisons is also different, but nevertheless FTFP MC appears to agree better with the data. More data allowed to parametrize better the rise of the MPV after 10 TeV in reconstructed energy using a log polynomial function of order four instead of three like in the case of lower statistics. After these considerations, the requirement chosen for the candidate proton selection is the following:
PSDglobal<(fsel(Erec) + 0.65). (4.8)
Shower development contained in the BGO
The energy in the first two layers of BGO is required to be smaller than the sum relative to the third and the fourth. Layers are counted from zero, starting from the layer closest to the STK:
EL0+ EL1<EL2+ EL3. (4.9) The behavior of the variable (EL0 + EL1)/(EL2+ EL3) is shown in Fig. 4.14 for several reconstructed energy intervals. Fig. 4.14 compares the data sample with the proton MC QFSP and the data with the proton MC FTFP, considering the same PSD selection for protons. We clearly see a striking difference between the two types of MC at low energy:
the proton MC using the list FTFP seems to describe the behavior of the data much better. After 250 GeV both the MC show the same deviation from the data.
CHAPTER 4. MEASUREMENT OF THE PROTON FLUX
Figure 4.10: Event display in the XZ and YZ view of a particle that released an energy of 1 TeV (top figures) and 12.7 TeV (bottom figures) in the calorimeter. The events are selected before the final charge selection, but after the PSD difference cut. The units on the x and y axis are mm. The particle comes from the bottom and goes to the top encountering first the PSD, then the STK and finally the BGO. The energy in the BGO and the PSD is in MeV. The stars in the STK represent the clusters used by the tracker finding algorithm. The dashed green lines in each view show the reconstructed BGO track extrapolated to the STK.
MeV PSD global
0 5 10 15 20 25 30
Figure 4.11: PSDglobal distribution integrated over all the energies.
100
/ ndf PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4 PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4
Constant 0.0002012 ± 0.0000027 MPV 1.853 ± 0.004 Sigma 0.1913 ± 0.0024
] MeV PSD global [
0 0.5 1 1.5 2 2.5 3 3.5 4
Constant 0.0002012 ± 0.0000027 MPV 1.853 ± 0.004
Figure 4.12: PSDglobal distributions for data (with a Landau fit in red) and proton MC FTFP (blue Landau fit) for various energy intervals.
CHAPTER 4. MEASUREMENT OF THE PROTON FLUX
MPV as a function of Energy
BGO Energy [GeV]
102 103 104 105
MPV as a function of Energy
Figure 4.13: MPV of the Landau fit on the PSDglobal distribution as a function of the recon-structed energy: left for data in red and proton MC QGSP in blue; right for data in red and proton MC FTFP+CRMC in blue. The data sample is different between the two plots (left 22 months, right 30 months of data acquisition). The green line in the plot on the right represents the proton selection function which is the same for both analysis periods.