First Measurement of the Muon Neutrino Interaction Cross Section and Flux as a Function of Energy at the LHC with FASER

The FASER collaboration
Phys.Rev.Lett. 134 (2025) 211801, 2025.

Abstract
This letter presents the measurement of the energy-dependent neutrino-nucleon cross section in tungsten and the differential flux of muon neutrinos and anti-neutrinos. The analysis is performed using proton-proton collision data at a center-of-mass energy of $13.6 \, {\rm TeV}$ and corresponding to an integrated luminosity of $(65.6 \pm 1.4) \, \mathrm{fb^{-1}}$. Using the active electronic components of the FASER detector, $338.1 \pm 21.0$ charged current muon neutrino interaction events are identified, with backgrounds from other processes subtracted. We unfold the neutrino events into a fiducial volume corresponding to the sensitive regions of the FASER detector and interpret the results in two ways: We use the expected neutrino flux to measure the cross section, and we use the predicted cross section to measure the neutrino flux. Both results are presented in six bins of neutrino energy, achieving the first differential measurement in the TeV range. The observed distributions align with Standard Model predictions. Using this differential data, we extract the contributions of neutrinos from pion and kaon decays.

  • fig_1_numu_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t1

    The systematic uncertainties of the fitted number of neutrino interactions.

  • fig_1_numubar_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t2

    The systematic uncertainties of the fitted number of anti-neutrino interactions.

  • fig_1_nu_numubar_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t3

    The systematic uncertainties of the fitted number of neutrino and anti-neutrino interactions.

  • fig_1_numu_relative_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t4

    The relative systematic uncertainties of the fitted number of neutrino interactions.

  • fig_1_numubar_relative_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t5

    The relative systematic uncertainties of the fitted number of anti-neutrino interactions.

  • fig_1_nu_numubar_relative_systematic_uncertainties

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    10.17182/hepdata.156838.v1/t6

    The relative systematic uncertainties of the fitted number of neutrino and anti-neutrino interactions.

  • fig_2_top_observed_neutrino_candidates

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    10.17182/hepdata.156838.v1/t7

    Number of observed CC $\nu_{\mu}$ neutrino candidate events in data and simulation.

  • fig_2_top_observed_neutrino_candidates_per_inv_bin_width

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    10.17182/hepdata.156838.v1/t8

    Number of observed CC $\nu_{\mu}$ neutrino candidate events per inv. bin width.

  • fig_2_bottom_neutrino_interactions

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    10.17182/hepdata.156838.v1/t9

    Number of CC $\nu_{\mu$} neutrino interactions in the fiducial volume.

  • fig_2_bottom_numu_neutrino_interactions_per_bin_width

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    10.17182/hepdata.156838.v1/t10

    Number of CC $\nu_{\mu$} neutrino interactions in the fiducial volume per bin width.

  • fig_2_bottom_numubar_neutrino_interactions_per_bin_width

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    10.17182/hepdata.156838.v1/t11

    Number of CC $\bar{nu}_{\mu$} neutrino interactions in the fiducial volume per bin width.

  • fig_2_bottom_numu_numubar_neutrino_interactions_per_bin_width

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    10.17182/hepdata.156838.v1/t12

    Number of CC $\nu_{\mu}$ and $\bar{nu}_{\mu$} neutrino interactions in the fiducial volume per bin width.

  • fig_3_top_numu_cross_section

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    10.17182/hepdata.156838.v1/t13

    Neutrino cross section

  • fig_3_top_numubar_cross_section

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    10.17182/hepdata.156838.v1/t14

    Anti-neutrino cross section

  • fig_3_top_numu_numubar_cross_section

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    10.17182/hepdata.156838.v1/t15

    Neutrino and anti-neutrino cross section

  • fig_3_top_numu_cross_section_over_energy

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    10.17182/hepdata.156838.v1/t16

    Neutrino cross section divided by energy.

  • fig_3_top_numubar_cross_section_over_energy

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    10.17182/hepdata.156838.v1/t17

    Anti-neutrino cross section divided by energy.

  • fig_3_top_numu_numubar_cross_section_over_energy

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    10.17182/hepdata.156838.v1/t18

    Neutrino and anti-neutrino cross section divided by energy.

  • fig_3_bottom_numu_flux

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    10.17182/hepdata.156838.v1/t19

    Neutrino flux divided by the cross sectional area of the fiducial volume.

  • fig_3_bottom_numubar_flux

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    10.17182/hepdata.156838.v1/t20

    Anti-Neutrino flux divided by the cross sectional area of the fiducial volume.

  • fig_3_bottom_numu_numubar_flux

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    10.17182/hepdata.156838.v1/t21

    Neutrino and anti-Neutrino flux divided by the cross sectional area of the fiducial volume.

  • fig_3_bottom_numu_flux_per_bin_width

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    10.17182/hepdata.156838.v1/t22

    Neutrino flux per bin width divided by the cross sectional area of the fiducial volume.

  • fig_3_bottom_numubar_flux_per_bin_width

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    10.17182/hepdata.156838.v1/t23

    Anti-neutrino flux per bin width divided by the cross sectional area of the fiducial volume.

  • fig_3_bottom_numu_numubar_flux_per_bin_width

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    10.17182/hepdata.156838.v1/t24

    Neutrino and anti-neutrino flux per bin width divided by the cross sectional area of the fiducial volume.

  • fig_4_num_nu_production_modes

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    10.17182/hepdata.156838.v1/t25

    Number of neutrino interactions from pion and kaon decays.

  • fig_4_covariance_matrix

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    10.17182/hepdata.156838.v1/t26

    Covariance matrix of the fitted number of neutrinos from pion and kaon decays.

  • fig_6_response_matrix

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    10.17182/hepdata.156838.v1/t27

    Response matrix to unfold the reconstructed, calibrated muon momentum $q/p^{'}_{\mu}$ to the true neutrino energy $-L/E_{\nu}$.

  • fig_7_left_number_ift_clusters

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    10.17182/hepdata.156838.v1/t28

    Distribution of the number of clusters in the IFT for neutrino-like data, muon-like data, and simulation.

  • fig_7_center_theta

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    10.17182/hepdata.156838.v1/t29

    Distribution of the muon angle for neutrino-like data, muon-like data, and simulation.

  • fig_7_right_qop

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    10.17182/hepdata.156838.v1/t30

    Distribution of the muon charge over momentum for neutrino-like data, muon-like data, and simulation.

  • table_2_efficiency

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    10.17182/hepdata.156838.v1/t31

    Efficiency in bins of the neutrino energy $-L/E_{\nu}$.

  • covariance_matrix

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    10.17182/hepdata.156838.v1/t32

    Covariance matrix from the likelihood of the number of neutrino interactions in bins of the neutrino energy $-L/E_{\nu}$.

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