Forward neutrino fluxes at the LHC

With the upcoming Run 3 of the LHC, the $\mathrm{FASER}\nu$ and SND@LHC detectors will start a new era of neutrino physics using the far-forward high-energy neutrino beam produced in collisions at ATLAS. This emerging LHC neutrino physics program requires reliable estimates of the LHC’s forward neutrino fluxes and their uncertainties. In this paper, we provide a new fast-neutrino flux simulation, implemented as a RIVET module, to address this issue. We present the expected energy distributions going through the $\mathrm{FASER}\nu$ and SND@LHC detectors based on various commonly used event generators, analyze the origin of those neutrinos, and present the expected neutrino event rates.

Figure 1

Beam pipe geometry: the boundaries of the LHC’s beam pipe in the horizontal (top) and vertical (bottom) panels are shown as a black line. The shaded areas correspond to the quadrupole (light gray) and dipole (dark gray) magnets. The red dashed line shows the trajectory of the proton beam with initial half beam crossing angle of $150\mu \mathrm{rad}$ vertically upwards. The dotted blue and dotted-dashed green curve show the trajectories of two oppositely charged 2 TeV energy pions with the same initial orientations.

Figure 2

Validation: comparison of the neutrino flux obtained using the fast neutrino flux simulation implemented as a RIVET module (solid) and a full detector simulation using BDSIM (dashed) for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$. We show the energy spectrum of the neutrino passing through a $20\mathrm{cm}\times 20\mathrm{cm}$ (left) and $40\mathrm{cm}\times 40\mathrm{cm}$ (right) cross sectional area that is centered around the beam collision axis and located at $z=480\mathrm{m}$. Electron and muon neutrinos are shown in red and blue, respectively.

Figure 3

Neutrino energy distribution: panels show the neutrino energy spectrum for electron (top), muon (center), and tau (bottom) neutrinos passing through $\mathrm{FASER}\nu$ (left) and SND@LHC (right) for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$. The vertical axis shows the number of neutrinos per energy bin that go through the detector’s cross sectional area for an integrated luminosity of $150{\mathrm{fb}}^{-1}$. We separate the different production modes: pion decays (red), kaon decays (orange), hyperon decays (magenta), charm decays (blue), and secondary hadronic showers (green). The different line styles correspond to predictions obtained from sibyll 2.3d (solid), dpmjet iii.2017.1 (short dashed), eposlhc (long dashed), qgsjet ii-04 (dotted), and pythia 8.2 using the softqcd processes (dotted-dashed) and pythia 8.2 with the hardqcd process for charm production (double-dotted-dashed).

Figure 4

Neutrino asymmetry: panels show the ratio of the antineutrino and neutrino fluxes as function of the neutrino energy for electron (red), muon (blue), and tau (green) neutrinos passing through $\mathrm{FASER}\nu$ (left) and SND@LHC (right). The different line styles correspond to predictions obtained from sibyll 2.3d (solid), dpmjet iii.2017.1 (dotted-dashed), EPOSLHC for light hadrons/ pythia 8.2 using the hardqcd for charm production (dashed), and qgsjet ii-04 for light hadrons/ pythia 8.2 using the softqcd for charm production (dotted).

Figure 5

Neutrino angular distribution: neutrino pseudorapidity distribution for electron (left), muon (center), and tau (right) neutrinos, as a function of pseudorapidity $\eta$ or equivalently the radial displacement from the LoS at $z=480\mathrm{m}$ for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$. In the top panel, we show the number of neutrinos, in units of particles per bin, while the bottom panels show the neutrino flux, in units of particles per area per bin. The flux components from light hadron decays and charmed hadron decays are shown in red and blue, respectively. The line styles denote the different event generators. All energies $E_{\nu }>10\mathrm{GeV}$ are included. Shown at the bottom of each panel is the angular coverage of $\mathrm{FASER}\nu$ and SND@LHC.

Figure 6

Interacting neutrino energy distribution: panels show the number of charged current neutrino interactions with the $\mathrm{FASER}\nu$ (left) and SND@LHC (right) detectors during LHC Run 3 with $150{\mathrm{fb}}^{-1}$ integrated luminosity as function of the neutrino energy. Here, we assume a target mass of 1.2 tons for $\mathrm{FASER}\nu$ and 800 kg for SND@LHC; further details on the experimental setup are provided in Sec. 3a. The red solid, blue dashed, and green dotted lines correspond to electron, muon, and tau neutrinos, respectively. The thick line denotes the average prediction of different generators, while the shaded band corresponds to the range of predictions obtained with different generators.

Figure 7

Beam configuration: We show the neutrino flux going through $\mathrm{FASER}\nu$ (left) and SND@LHC (right) for different orientations of the beam crossing angle considered for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$. The simulations were performed using sibyll 2.3d. The muon neutrino and electron neutrino fluxes are shown as solid and dashed lines, respectively. The lower panels show the change of the neutrino flux relative to the nominal setup with an angle of $150\mu \mathrm{rad}$ vertically upwards as shown in black. For $\mathrm{FASER}\nu$, we also show results for a detector that was moved to be aligned with the center of the neutrino beam.

Figure 8

Predictions for the energy spectra of ${\pi }^{+}$ (top row), $K_L$ (center row), and $D^{+}$ (bottom row) in three pseudorapidity intervals using six commonly used MC generators for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$.

Figure 9

Comparison of the neutrino flux obtained using the far neutrino flux simulation implemented as RIVET module (solid) and the a full FLUKA simulation (dashed) using dpmjet iii generator for LHC Run 3 with an integrated luminosity of $150{\mathrm{fb}}^{-1}$. The FLUKA simulation was performed by CERN Sources, Targets and Interactions Group in the context of the XSEN proposal [7], and its results have further been presented in Refs. [8, 12, 13]. See text for discussion.