Scientific program for the Forward Physics Facility

Abstract

The recent direct detection of neutrinos at the LHC has opened a new window on high-energy particle physics and highlighted the potential of forward physics for groundbreaking discoveries. In the last year, the physics case for forward physics has continued to grow, and there has been extensive work on defining the Forward Physics Facility and its experiments to realize this physics potential in a timely and cost-effective manner. Following a 2-page Executive Summary, we first present the status of the FPF, beginning with the FPF's unique potential to shed light on dark matter, new particles, neutrino physics, QCD, and astroparticle physics. We then summarize the current designs for the Facility and its experiments, FASER2, FASER $\nu$ 2, FORMOSA, and FLArE.

Fig. 1

The rich physics program at the FPF spans many topics and frontiers

Fig. 2

The FPF is located 627–702 m west of the ATLAS IP along the line of sight. The FPF cavern is 75 m long and 12 m wide and will house a diverse set of experiments to fully explore the forward region

Fig. 3

New particle searches and neutrino measurements at the FPF. Representative examples of DM and other new particles that can be discovered and studied at the FPF (top) and of some of the many topics that will be illuminated by TeV-energy neutrino measurements at the FPF (bottom)

Fig. 4

Inelastic dark matter searches at the FPF. The discovery potential of FASER2 and other experiments for two different realizations of inelastic DM. The left panel considers a the case of heavy inelastic DM interacting via a dark photon portal, as introduced in  [15], where the high energy of the LHC allows FASER2 to probe masses up to tens of GeV. The right panel considers the case of light inelastic DM with very small mass splittings that is mediated by a dipole portal as introduced in Ref. [16], where the large LHC energy boosts the signal to observable energies. In both scenarios, the reach of FASER2 extends beyond all other experiments, including direct and indirect DM searches, LHC experiments, and beam dump experiments, such as SHiP. It also covers the thermal DM relic target (solid black lines), that is the cosmologically-favored parameter space where the model predicts the observed dark matter relic abundance as produced through thermal freeze-out. For comparison, we have shown the leading constraints provided by BaBaR [17] and LEP [18, 19], as well as projections from a number of other proposed searches, including those for displaced muon jets (DMJ) and delayed particles (timing) at the main LHC experiments [20, 21] as well as displaced particle searches at LHCb [–24], SHiP [25], Belle 2 [26], and SeaQuest [27]

Fig. 5

New particle searches at the FPF. Left: The discovery reach of FORMOSA and FLArE for millicharged particles [14, 31]. Right: The discovery reach of FASER and FASER2 for color-neutral quirks [32]. In both panels, we also show existing bounds (gray shaded regions) and projected sensitivities of other experiments (dashed contours), including BEBC [33], SLAC [34], LEP [35, 36], CMS [37, 38], LSND [39], ArgoNeuT [40], Proto-milliQan [41], milliQan [42], FerMINI [43], SUBMET [44], monojet searches [–47], quirk searches at D0 [48], heavy stable charged particle searches (HSCP) [45, 49, 50], co-planar hits searches [51], and out-of-time searches [52, 53]

Fig. 6

Neutrino yields and cross sections at the FPF. The expected precision of FLArE measurements of neutrino interaction cross sections (top, statistical errors only) and the combined spectrum of neutrinos interacting the FPF experiments (bottom) as a function of energy for electron (left), muon (middle), and tau (right) neutrinos. In the case of muon and tau neutrinos, separate measurements of the neutrino and anti-neutrino measurement can be performed using muons passing through the FASER2 spectrometer, where a 17% branching fraction of taus into muons was considered. Existing data from accelerator experiments [63], IceCube [64], and the recent FASER$\nu$ result [65] are also shown, together with the prospects for SHiP

Fig. 7

Precision tau neutrino studies at the FPF. The projected sensitivity of FASER$\nu$2 to neutrino NSI parameters that violate lepton flavor universality [75] Past NOMAD bounds [76, 77] are presented based on Ref. [78]. Constraints from the measured ratio of pion decay widths to the electron and muon [79] are obtained based on Ref. [80]

Fig. 8

QCD physics at the FPF. Representative QCD targets at the FPF, classified into production at the ATLAS IP and scattering at the FPF neutrino detectors

Fig. 9

Impact of FPF neutrino measurements on cross-section measurements and traditional BSM Searches at the HL-LHC. Left: Reduction of the PDF uncertainties on Higgs- and weak gauge-boson cross sections at the HL-LHC, enabled by neutrino DIS measurements at the FPF [95]. Right: Signatures for a new heavy $W^{\prime }$ boson with $m_{W^{\prime }}=13.8$ TeV, namely a distortion of the high-mass charged-current Drell–Yan cross-sections, would be reabsorbed in a PDF fit with HL-LHC data ($f_{\text{BSM,noFPF}}$), unless the PDFs are constrained by the “low energy” FPF neutrino data ($f_{\text{BSM,FPF}}$) [102]

Fig. 10

Small-x QCD at the FPF. Impact of FPF data on the small-x gluon PDF, compared with non-linear QCD (saturation) models. The y axis displays xg evaluated at a scale of $Q=2$ GeV. The baseline prediction is NNPDF 3.1 [107]

Fig. 11

Astroparticle physics at the FPF. The central part of the figure shows the expected energy spectrum of interacting electron neutrinos in the FLArE detector at the FPF (solid gray curve) obtained using SIBYLL 2.3d [124] and POWHEG + Pythia [73] with the NNPDF 3.1 as well as expected statistical uncertainties (black error bars). The colored contours illustrate two examples of physics that can change the expected flux and be probed at the FPF: enhanced kaon production that solves the muon puzzle (blue dashed line) and small-x PDFs that lead to improved prompt atmospheric neutrino flux predictions (orange band). Left: Dimensionless muon shower content $R_{\mu }$ as predicted by piKswap model through simulations with SIBYLL 2.3 + AIRES [135] and compared with data from the Pierre Auger Observatory [118]; for details, see [133]. Right: Reduction of PDF uncertainties on the prompt neutrino flux $\mathrm{\Phi }$ enabled by FPF data as a function of $E_{\nu }$; see Fig. 10 for the corresponding PDF

Fig. 12

Left: Plan view showing the FPF location. Right: 3D view of the Facility. All distances are given in meters

Fig. 13

The baseline layout of the FPF facility, showing the four proposed experiments and the large infrastructure

Fig. 14

The muon fluence rate for ${\mu }^{-}$ (left) and ${\mu }^{+}$ (right) in the transverse plane in the FPF cavern for the HL-LHC baseline luminosity of $5\times {10}^{34}$ cm$_{}^{-2}$ s$_{}^{-1}$. The coordinate system is defined such that (0, 0) is the LOS, and -ve x is towards the center of the LHC ring

Fig. 15

Visualisation of the full FASER2 detector, showing the veto system, uninstrumented 10 m decay volume, tracker, magnet, electromagnetic calorimeter, hadronic calorimeter, iron absorber and muon detector

Fig. 16

Left: Design of the FASER$\nu$2 detector. Right: FASER$\nu$2 prototype module on the SPS-H8 beamline

Fig. 17

Left: Tau decay topology in the emulsion detector. Right: Charge measurement for a muon from a tau decay

Fig. 18

Left: An engineering drawing of the FORMOSA detector. Right: The FORMOSA demonstrator taking data in the forward region of the ATLAS interaction point

Fig. 19

Layout of the FLArE baseline design. The detector is shown with the $3\times 7$ modular segmentation. Three TPC modules are also shown withdrawn horizontally from the cryostat

Fig. 20

Left: TPC assembly with three TPC modules hanging from the cold door via cantilevered beams. Right: Inner view showing a conceptual high-voltage connection scheme to the cathode planes

Fig. 21

Preliminary design of the magnetized hadron/muon calorimeter downstream of the FLArE cryostat in the FPF cavern. The implementation is based on the Baby MIND concept [172]