Momentum Measurement of Charged Particles in FASER's Emulsion Detector at the LHC

Abstract

We present a momentum measurement method based on multiple Coulomb scattering (MCS) in the FASER$ν$ emulsion detector. The measurement of charged-particle momenta is essential for studying neutrino interactions in the TeV energy range at the FASER experiment. This method exploits the sub-micron spatial resolution and long tracking length of the FASER$ν$ detector, enabling momentum determination from a few GeV up to a few TeV. The performance was evaluated using Geant4-based Monte Carlo simulations and validated with muon test beam data in the momentum range 100-300 GeV. As a first probe of the method for higher momentum muons, background muons recorded by the FASER$ν$ detector were examined, showing reconstructed momenta consistent with expectations from their angular spread.

Figures (13)

• Figure 1: Left: The second difference in $n_{\mathrm{cell}}$. Right: The schematic view of $n_{cell}$ = 1-3 and the shift of 1 plate when calculating the second difference (dotted line).
• Figure 2: Examples of $s^{\mathrm{RMS}}$ measurements and momentum fitting for single tracks. Left: $p_{\mathrm{true}}$ = 100 GeV. Right: $p_{\mathrm{true}}$ = 1000 GeV. The black points are measured $s^{\mathrm{RMS}}$ (µm) for each cell length and red line is the fit of the function in \ref{['eq:srms']}. The black bars represent the effective statistical weights. In the fitting procedure, $1/w(n_{\mathrm{cell}})$ is used for each data point. $p_{\mathrm{rec}}$ = 102 GeV for $p_{\mathrm{true}}$ = 100 GeV, and 1002 GeV for 1000 GeV. For this example, the tracks are generated with Geant4 Geant4:2003, following the same setup as in \ref{['sec:Evaluation']}, are used. A total of 100 plates are considered, with a position smearing of $\sigma_{\mathrm{pos}}$ = 0.3µm and a base-track reconstruction efficiency of 90%.
• Figure 3: Left: reconstructed momentum ($p_{\mathrm{rec}}$) distribution. Right: $1/p_{\mathrm{rec}}$ distribution. Both are for a single-energy MC sample with a true momentum of 2000 GeV. If the reconstructed momentum exceeded 7 TeV, it was set to 7 TeV. The red line in the right panel indicates a Gaussian fit performed within a range of the mean $\pm$ 1.5 RMS of the distribution, excluding tracks with $p_{\mathrm{rec}} = 7$ TeV.
• Figure 4: The relative offset (left), resolution (center), and success rate (right) as a function of $p_{\mathrm{true}}$. Black circles, blue squares, and red triangles correspond to $n_{\mathrm{cell}}^{\mathrm{max}}$ = 16, 24, and 32, respectively. The relative offset is defined as $(p_{\mathrm{center}} - p_{\mathrm{true}})/p_{\mathrm{true}}$, and the success rate represents the fraction of tracks reconstructed with momenta below 7 TeV for each $p_{\mathrm{true}}$.
• Figure 5: $p_{\mathrm{rec}}$ as a function of $p_{\mathrm{true}}$ using $n_{\mathrm{cell}}^{\mathrm{max}}$ = 24. The MC simulation assumes a flat momentum distribution from 1 GeV to 3000 GeV. The colour scale represents the number of entries in each bin. The black line indicates $p_{\mathrm{rec}} = p_{\mathrm{true}}$, and the red profile histograms show the $p_{\mathrm{center}}$ values with 1$\sigma$ uncertainties for each 100 GeV bin.