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  • Open Access

FASER’s physics reach for long-lived particles

Akitaka Ariga et al. (FASER Collaboration)
Phys. Rev. D 99, 095011 – Published 15 May 2019
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  • INTRODUCTION
  • THE FASER DETECTOR
  • PRODUCTION OF LLPs
  • FASER REACH FOR DARK VECTORS
  • FASER REACH FOR DARK SCALARS
  • FASER REACH FOR HEAVY NEUTRAL LEPTONS
  • FASER REACH FOR AXIONLIKE PARTICLES
  • FASER REACH FOR DARK PSEUDOSCALARS
  • DEPENDENCE ON BEAM OFFSET, MONTE CARLO…
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The ForwArd Search ExpeRiment (FASER) is an approved experiment dedicated to searching for light, extremely weakly interacting particles at the LHC. Such particles may be produced in the LHC’s high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons; light B−L and Li−Lj gauge bosons; axionlike particles that are coupled dominantly to photons, fermions, and gluons through nonrenormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

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    • Received 8 February 2019

    DOI:https://doi.org/10.1103/PhysRevD.99.095011

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    AxionsBranching fractionDiffractive productionExtensions of Higgs sectorExtensions of fermion sectorExtensions of gauge sectorHadronic decaysLeptonic, semileptonic & radiative decaysRare decays
    1. Physical Systems
    Hypothetical fermionsHypothetical gauge bosonsHypothetical particlesHypothetical scalarsSterile neutrinos
    1. Techniques
    Gaseous detectorsHadron collidersMonte Carlo methodsParticle detectors
    Particles & Fields

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    Issue

    Vol. 99, Iss. 9 — 1 May 2019

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    Images

    • Figure 1
      Figure 1

      Left panel: The arrow points to FASER’s location in service tunnel TI12, roughly 480 m east of the ATLAS IP. Credit: CERN Geographical Information System. Right panel: View of FASER in tunnel TI12. The trench lowers the floor by 45 cm at the front of FASER to allow FASER to be centered on the beam collision axis. Credit: CERN Site Management and Buildings Department.

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    • Figure 2
      Figure 2

      Schematic view of the far-forward region downstream of ATLAS and various particle trajectories. Upper panel: FASER is located 480 m downstream of ATLAS along the beam collision axis (dotted line) after the main LHC tunnel curves away. Lower left panel: High-energy particles produced at the IP in the far-forward direction. Charged particles are deflected by LHC magnets, and neutral hadrons are absorbed by either the TAS or TAN, but LLPs pass through the LHC infrastructure without interacting. Note the extreme difference in horizontal and vertical scales. Lower right panel: LLPs may then travel ∼480  m further downstream and decay within FASER in TI12.

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    • Figure 3
      Figure 3

      Layout of the FASER detector. LLPs enter from the left and the entire length of the detector is roughly 5 m. The detector components include scintillators (gray), dipole magnets (red), tracking stations (blue), a calorimeter (dark purple), and support structures (green).

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    • Figure 4
      Figure 4

      Representative Feynman diagrams for the LLP production processes outlined in this section: dark photon production from pion decay (left), dark photon production via dark bremsstrahlung (center left), dark photon production in hard scattering (center right), and ALP production via the Primakoff process from photons scattering in the TAN (right).

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    • Figure 5
      Figure 5

      Differential meson production rate in each hemisphere in the (θ,p) plane, where θ and p are the meson’s angle with respect to the beam axis and momentum, respectively. The bin thickness is 1/10 of a decade along each axis. We show the π0 spectrum (left), obtained via EPOS-LHC [42], and the B meson spectrum (right), obtained using FONLL with CTEQ6.6 [49]. The diagonal black dashed lines highlight the characteristic transverse momentum scale pT∼ΛQCD∼250  MeV for pions and pT∼mB for B mesons. The angular acceptances for FASER and FASER 2 are indicated by the vertical gray dashed lines.

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    • Figure 6
      Figure 6

      Benchmark model V1. The dark photon decay length (top left panel), its branching fractions into hadronic and leptonic final states (bottom left panel) and FASER’s reach (right panel). In the right panel, the gray-shaded regions are excluded by current bounds, and the projected future sensitivities of other experiments are shown as colored contours. See the text for details.

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    • Figure 7
      Figure 7

      Benchmark model V2. As in Fig. 6, but for the B−L gauge boson. In the right panel, projected future sensitivities of other experiments are shown following Ref. [30].

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    • Figure 8
      Figure 8

      Benchmark model V3. As in Fig. 6, but for the Lμ−Le gauge boson. In the right panel, the gray-shaded regions excluded by current bounds and projected future sensitivities of other experiments are adapted from Ref. [30].

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    • Figure 9
      Figure 9

      Benchmark model V3. As in Fig. 6, but for the Le−Lτ gauge boson. In the right panel, the gray-shaded regions excluded by current bounds and projected future sensitivities of other experiments are adapted from Ref. [30].

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    • Figure 10
      Figure 10

      Benchmark model S1. The decay length (top left panel), decay branching fractions (bottom left panel), and FASER’s reach (right panel) for the dark Higgs boson with negligible trilinear coupling to the SM Higgs. The gray shaded regions are excluded, and the colored contours are the projected sensitivities of other proposed experiments; see text for details.

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    • Figure 11
      Figure 11

      Benchmark model S2. As in Fig. 10, but the reach shown in the right panel is for dark Higgs bosons pair produced through B→Xsϕϕ with trilinear couplings λ=0.0046, 0.0015 corresponding to B(h→ϕϕ)≈4700λ2=10%, 1%, as indicated. The region probed by B→Xsϕ is also shown by the dashed black line. The projected sensitivities of MATHUSLA and Codex-b to the trilinear couplings through the SM Higgs decay h→ϕϕ are also shown for λ=0.0046. Note that the projected sensitivities of other experiments for vanishing trilinear coupling, λ=0, also apply; they are not shown in this figure, but can be found in Fig. 10.

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    • Figure 12
      Figure 12

      Benchmark model F1. The decay length (top left panel), decay branching fractions (bottom left panel), and FASER’s reach (right panel) for the HNL that mixes only with the electron neutrino νe. The gray shaded regions are excluded by current limits, and the colored contours are the projected sensitivities for other proposed experiments. See the text for details.

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    • Figure 13
      Figure 13

      Benchmark model F2. As in Fig. 12, but for an HNL that only mixes with νμ.

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    • Figure 14
      Figure 14

      Benchmark model F3. As in Fig. 12, but for an HNL that only mixes with ντ.

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    • Figure 15
      Figure 15

      Benchmark model A1. The decay length (top left panel), decay branching fractions (bottom left panel) and FASER’s reach (right panel) for ALPs with dominantly diphoton couplinga. The gray-shaded regions are excluded by current limits, and the colored contours give the projected sensitivities of several other proposed experiments. See the text for details.

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    • Figure 16
      Figure 16

      Benchmark model A2. As in Fig. 15, but for ALPs with dominantly fermion couplings.

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    • Figure 17
      Figure 17

      Benchmark model A3. As in Fig. 15, but for ALP with dominantly gluon couplings.

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    • Figure 18
      Figure 18

      Benchmark model P1. The decay length (top left panel), decay branching fractions (bottom left panel) and FASERs reach (right panel) for a CP-odd scalar with Yukawa-like couplings. The gray-shaded regions are excluded by current limits, and the colored contours give the projected sensitivities of several other proposed experiments. See the text for details.

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    • Figure 19
      Figure 19

      FASER reach for dark photons (left) and ALPs with dominantly fermion couplings (right) for different offsets d between the beam collision axis and the center of FASER.

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    • Figure 20
      Figure 20

      FASER reach for dark photons (left) and ALPs with dominant couplings to fermions (right). For the dark photon, we vary the forward Monte Carlo generators used to produce the light meson spectrum as well as the validity on the transverse momentum of the dark photon used in the bremsstahlung approximation. For the ALPs, we change the PDF used to estimate the forward B-meson spectra in FONLL.

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    • Figure 21
      Figure 21

      FASER reach for dark photons (left) and ALPs with dominant couplings to fermions (right) for different LLP energy threshold cuts.

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