Measurement of absorption and charge exchange of ${\pi }^{+}$ on carbon

The combined cross section for absorption and charge exchange interactions of positively charged pions with carbon nuclei for the momentum range 200 MeV/$c$ to 300 MeV/$c$ have been measured with the DUET experiment at TRIUMF. The uncertainty is reduced by nearly half compared with previous experiments. This result will be a valuable input to existing models to constrain pion interactions with nuclei.

Figure 1

Pion interactions on nuclei. “N” represents any number of nucleons emitted after interactions.

Figure 2

Apparatus layout. A detailed description is given in the text.

Figure 3

Cherenkov light in ADC counts vs TOF [ns] for the beam particle at $p_{\pi }=237.2$ MeV/$c$ setting. The broken line corresponds to the threshold to distinguish pions from muons and electrons.

Figure 4

Front view of fiber-tracker detector.

Figure 5

Charge distribution (in photoelectrons) of through-going pions for data and MC, for $p_{\pi }=150$ and 300 MeV/$c$ data set. The hits below the 2.5-p.e. threshold are not shown in the plots. The fits closely follow each other.

Figure 6

CEMBALOS charge distribution (in photoelectrons) of through-going muons for $p_{\pi }=237.2$ MeV/$c$ setting.

Figure 7

Beam-position distribution in $X$, for the data set with $p_{\pi }=237.2$ MeV/$c$ setting. The black (red) histogram shows the distribution for data (MC).

Figure 8

Beam angular distribution in $X$ projection, for the data set with $p_{\pi }=237.2$ MeV/$c$ setting. The variable $\theta$ is the angle from horizontal line $\left(X=0\right)$. The black (red) histogram shows the distribution for data (MC).

Figure 9

Example of ABS candidate event in data $(p_{\pi }=237.2$ MeV/$c)$. The filled circles (red) correspond to the large hits $(>20$ p.e.), the crosses correspond to the hits identified as crosstalk hits and the thick lines (blue, green, and red) correspond to reconstructed tracks.

Figure 10

Difference between the true and reconstructed vertex position in (a) U, (b) V, (c) $Z$ (c), and (d) the true and reconstructed angles.

Figure 11

Example of pion scattering candidate event in data $(p_{\pi }=237.2$ MeV/$c)$. The track (blue) in the upstream side is identified as the incident pion track, and the track in the downstream side (green) is identified as a scattered pion track.

Figure 12

Example of CX candidate event in data $(p_{\pi }=237.2$ MeV/$c)$. The track in the upstream side (blue) is assumed to be the incident pion track, and the track in the downstream side (green) is assumed to be a proton track from the CX interaction.

Figure 13

Illustration of the Good incident ${\pi }^{+}$ cut requirement. The broken line represents the boundary of the fiducial volume.

Figure 14

The $X\text{−}Y$ view of incident beam-position distribution. The white broken line represents the boundary of the fiducial volume.

Figure 15

$dQ/dx$ distribution (in photoelectrons per mm) in six different angular regions for $p_{\pi }=237.2$ MeV/$c$ for data and MC. The dotted vertical lines represent the threshold to distinguish pions (left of the line) and protons. For multiple track events, only the smallest value of $dQ/dx$ among the tracks is filled in the histogram. The events in the “Others” category is mainly from events with pions decaying in flight and Coulomb scattering events.

Figure 16

Comparison of elastic inclusive cross section between the previous experiments (summarized in Table 4) and the default geant4. The cross sections are plotted as a function of pion kinetic energy.

Figure 17

Comparison of inelastic inclusive cross sections between the previous experiment [8] and the default geant4. The cross sections are plotted as a function of pion momentum.

Figure 18

The number of reconstructed tracks for data, and the MC before and after tuning, for $p_{\pi }=237.2$ MeV/$c$ setting. The No final ${\pi }^{+}$ cut is not applied.

Figure 19

The angular distribution of reconstructed tracks for MC before and after tuning, and for data, for $p_{\pi }=237.2$ MeV/$c$ data set. The No final ${\pi }^{+}$ cut is not applied.

Figure 20

Angular distribution of the reconstructed tracks in the final state for $p_{\pi }=201.6$ (left), 237.2 (center), and 295.1 (right) MeV/$c$ data, before (top) and after (bottom) applying No final ${\pi }^{+}$ cut. When the true track angle is close to 90 degrees, the track reconstruction algorithm tends to reconstruct the track exactly at 90 degrees, so the number of events in the bin corresponding to 90 degrees is larger than the neighboring bins.

Figure 21

Distribution of number of reconstructed tracks in the final state, for 201.6 (left), 237.2 (center), and 295.1 (right) MeV/$c$ data sets, before (top) and after (bottom) applying No final ${\pi }^{+}$ cut.

Figure 22

Angular distribution of the backward-going proton-like track for data and MC, for ABS and CX events, for $p_{\pi }=237.2$ MeV/$c$ setting. For each event, a proton track with largest angle is selected and filled into the histogram. The $\text{ABS}+\text{CX}$ event selection is applied for this plot. The background component (SCAT) is subtracted according to the prediction in MC, and the histograms are normalized by the number of events after subtraction.

Figure 23

Angular distribution of the backward-going proton-like track for nominal and reweighted MC, for ABS and CX events, for $p_{\pi }=237.2$ MeV/$c$ setting.

Figure 24

Example of $dQ/dx$ (in photoelectrons per mm) distribution for ${30}^{\circ }<\theta <{60}^{\circ }$ after event selection, for the projection which was not used for calculating $dQ/dx$ in No final ${\pi }^{+}$ cut, for the data set with $p_{\pi }=237.2$ MeV/$c$ setting. The broken line shows the threshold to distinguish pion-like tracks and proton-like tracks.

Figure 25

The predicted momentum distribution of protons from ABS events, for geant4 (black) and neut (red), for $p_{\pi }=295.1$ MeV/$c$. The histograms are normalized by the number of ABS events.

Figure 26

Angular distribution of $\pi$-like tracks for data and MC with $p_{\pi }=237.2$ MeV/$c$ setting. The histograms are normalized by number of incident pions in the data.

Figure 27

Example of $dQ/dx$ distribution (in photoelectrons per mm) for incident track for the data set with $p_{\pi }=237.2$ MeV/$c$ setting, after requiring $\pi$-like track in only one of the two projections. The histograms are normalized by the number of incident pions.

Figure 28

Predicted momentum distribution of pions from inelastic scattering event, for geant4 (black) and neut (red), for $p_{\pi }=201.6$ MeV/$c$. The histograms are normalized by area.

Figure 29

Result of ${\sigma }_{\mathrm{ABS}+\mathrm{CX}}$ vs pion momentum compared with the results from previous experiments.