Measurement of ${\sigma }_{\mathrm{ABS}}$ and ${\sigma }_{\mathrm{CX}}$ of ${\pi }^{+}$ on carbon by the Dual Use Experiment at TRIUMF (DUET)

The DUET Collaboration reports on the measurements of the absorption (${\sigma }_{\mathrm{ABS}}$) and charge exchange (${\sigma }_{\mathrm{CX}}$) cross sections of positively charged pions on carbon nuclei for the momentum range 201.6 to 295.1 $\mathrm{MeV}/c$. The uncertainties on the absorption and charge exchange cross sections are $\sim 9.5\%$ and $\sim 18\%$, respectively. The results are in good agreement with previous experiments. A covariance matrix correlating the five ${\sigma }_{\mathrm{ABS}}$ and five ${\sigma }_{\mathrm{CX}}$ measured data points is also reported.

Figure 1

Dominant ${\pi }^{\pm }$-C interactions in the sub-GeV region. “N” represents any number of nucleons leaving the nucleus.

Figure 2

Schematic overview of the experimental apparatus.

Figure 3

Light attenuation curves in CEMBALOS for mirrored (solid) and unmirrored (dashed) fibers.

Figure 4

Charge per CEMBALOS hit distribution (in photoelectrons) of through-going muons in the $p_{\pi }=237.2 \mathrm{MeV}/c$ setting for data (circles) and MC (solid line), after the calibration procedure was applied. The statistical error bars are too small to be visible.

Figure 5

$d{\sigma }_{\mathrm{CX}}/d\mathrm{\Omega }$ as a function of the outgoing ${\pi }^0$ polar angle (with respect to the beam direction) for 265 $\mathrm{MeV}/c{\pi }^{+}$ interacting on ${}^{16}\mathrm{O}$, for Fluka (dashed line), Geant4 (solid line), and NEUT (dotted line), along with data from [12].

Figure 6

Example of a simulated CX event in the DUET detector setup. A 237.2 $\mathrm{MeV}/c{\pi }^{+}$ (red) undergoes CX in $\mathrm{PIA}\nu \mathrm{O}$ producing two protons (black) and a ${\pi }^0$ that decay into two photons (blue). The forward-going photon is identified in CEMBALOS as it produces $e^{+}-e^{-}$ pairs (purple, magenta) and hits are recorded in the scintillating material.

Figure 7

Distribution of the most upstream position of CEMBALOS hits for data and MC (broken down into topologies and listed with their corresponding percentage composition) in the $p_{\pi }=237.2\mathrm{MeV}/c$ setting after applying the $\mathrm{PIA}\nu \mathrm{O}$ upstream selection. Each bar represents an XY module. Topologies contributing less than 1% are not plotted.

Figure 8

Distribution of the number of hits in CEMBALOS for data and MC (broken down into topologies and listed with their corresponding percentage composition) in the $p_{\pi }=237.2\mathrm{MeV}/c$ setting after applying the veto cut. Topologies contributing less than 1% are not plotted.

Figure 9

Distribution of the number of hits in CEMBALOS vs charge deposited for MC in the $p_{\pi }=237.2\mathrm{MeV}/c$ setting after applying the requirement of a minimum of five hits. The blue entries are true CX events, whereas the black boxes correspond to neutron background events.

Figure 10

Selection efficiency of true CX events as a function of the outgoing ${\pi }^0$ momentum and angle, for the $p_{\pi }=201.6 \mathrm{MeV}/c$ setting.

Figure 11

Rejection probability of events where an ejected proton from ABS or quasielastic scattering fails the veto rejection criteria, as a function of its outgoing momentum and angle, for the $p_{\pi }=201.6$ $\mathrm{MeV}/c$ setting.

Figure 12

Charge distribution in the first layer of CEMBALOS for stopping protons in the 237.2 $\mathrm{MeV}/c$ setting for data (circles) and MC (filled histogram). Only statistical uncertainties are plotted. The solid and dashed lines are Gaussian fits to data and MC respectively.

Figure 13

CEMBALOS hit inefficiency for data (circles) and MC (solid line) in the $p_{\pi }=237.2\mathrm{MeV}/c$ setting. The statistical error bars are too small to appear.

Figure 14

DUET measurements of ${\sigma }_{\mathrm{ABS}}$ and ${\sigma }_{\mathrm{CX}}$ compared with previous measurements [11, 12, 15, 16] and ABS (red) and CX (black) model predictions from Geant4 (solid line), Fluka (dashed line), and Neut (dotted line).

Figure 15

Fractional covariance and correlation for the DUET measurements of ${\sigma }_{\mathrm{ABS}}$ and ${\sigma }_{\mathrm{CX}}$. The diagonal and lower triangle show the covariance $(\mathrm{sgn}\left(V_{ij}\right)\times \sqrt{V_{ij}})$, while the upper triangle of the matrix shows the correlation coefficients.