FASER2

Introduction Contact information and useful links Meetings Physics Priorities and Milestones Activity areas Person power Open studies Baseline Geometry Tools Detector technologies Background estimates Links

Introduction

Welcome to the FASER2 webpage. This page documents information related to the FASER2 detector for the Forwards Physics Facility (FPF). FASER2 is primarily focused on searching for exotic and long-lived beyond the Standard Model particles, which, along with other motivations, are possible candidates for Dark Matter.
FASER2 is a detector proposed to be housed in the FPF. It's physics goals are similar to that of the existing FASER (ForwArd Search ExpeRiment), however although FASER is already set to probe new parameter space in the search for BSM physics the overall size of FASER is been heavily constrained by the available space underground. This restricts the size of the possible decay volume and therefore sensitivity to various new physics models, which for many representative BSM models is directly related to the length and radius of the decay volume. This strongly motivates the case for an enlarged detector, FASER2, which would be housed in the dedicated cavern of the FPF where significantly more space is available.

Contact information and useful links

Contact:

• FASER2 contacts: Jamie Boyd, Josh McFayden
• Mailing list: PCB-acc-FPF-faser2@cernNOSPAMPLEASE.ch
• Slack channel: https://join.slack.com/t/forwardphysic-hrq8935/shared_invite/zt-1i43qe7rk-SozssB0Vez7OGTjR8nwmmA

Publications:

• The Forward Physics Facility: Sites, experiments, and physics potential”, arXiv:2109.10905, Physics Reports Volume 968, 19 July 2022, Pages 1-50.
• FPF Snowmass Whitepaper arXiv:2203.05090

Meetings

Regular meetings are held every other Thursday at 1pm CERN time.

Meetings can be found in the FASER2 indico category.

Recent meetings

• 29/06/2022 - FASER2 mini workshop at FASER Collaboration meeting: https://indico.cern.ch/event/1146599/#b-465642-faser2-discussion-fas
• 24/05/2022 - FASER2 Brainstorming: https://indico.cern.ch/event/1160661/

Physics Priorities and Milestones

FASER2 is expected to have sensitivity to a number of physics models not accessible to FASER, as summarised in this table from the FASER Physics Reach Paper:

The larger volume of FASER2 and the richer phenomenology means that there are several important design considerations:

• Larger aperture imposes strong restrictions on possible magnet technology.
• Larger radius means being on-axis is less important.
• More LLP decay channel means particle ID will be important.
• A larger detector means a larger background rate, may required different detector technology.
• More sophisticated signal reconstruction strongly desirable (mass and pointing).
• Link to FASERν2 - need to be able to measure muon charge (and momentum) from (τ and μ) neutrino interactions.

Milestones:

• FPF5: 15-16th Nov 2022
• P5 committee?: Q3 2022?
• CDR: Q1 2023
• LHCC discussion? Q2 2023

HL-LHC timeline:

Activity areas

Person power

Open studies

Baseline Geometry

A possible FASER2 geometry very similar to FASER but with larger dimensions was already explored in the LoI, Technical Proposal and Physics Reach publications from FASER.

This geometry had a decay volume of dimensions: Length=5m, Diameter=2m. This results in 4 orders of magnitude improvement in sensitivity reach and the angular acceptance of of all neutral pions goes from 0.6% in FASER to 10% in FASER2. It also improves sensitivities to LLPs produced in decays of heavy mesons and in general improves sensitivity to larger LLP masses.

However, further investigations of the available technology have indicated that such long and wide-aperture superconducting magnets are infeasible for the expected cost envelope for FASER2. Therefore, several other proposals have been considered for slightly reduced size and different shape magnet apertures have been studied which look promising. As a result, an updated baseline FASER2 geometry has been defined, as shown in the figure below:

This new baseline uses a single magnet, it does not immerse the decay volume in a magnetic field and in general has a significantly reduced length of magnetic field. However, the charged particle separations required still appear to be feasible due to the longer detector length. The baseline has a rectangular aperture of 1m x 3m in the transverse plane and the decay volume is 10m long. The effect of different scenarios on the Dark Higgs reach is shown in the following figure, the new baseline scenario corresponds to the magenta dashed line ("R1 L=10m X=3m Y=1m"), where it can be seen that the expected sensitivity is similar to that of the old default FASER2 geometry shown in dark red ("F2 L=5m D=2m"):

Tools

FORESEE

Developed by Felix Kling and Sebastian Trojanowski, FORESEE (FORward Experiment SEnsitivity Estimator) is a package for simulating the expected sensitivity reach of experiments placed in the far-forward direction from the proton-proton interaction point to new physics models. It can also be used to create HepMC files of these models for input into e.g. Geant4 simulations.

• FORESEE paper: https://arxiv.org/abs/2105.07077
• FORESEE github: https://github.com/KlingFelix/FORESEE/

Simple FORESEE+Geant4 simulation

To test different magnet and detector geometries the FORESEE package was slightly adapted to produce HepMC output files and a platform for creating simple Geant4 geometries has been developed for FASER2 R&D studies:

• Josh's fork of FORESEE (for input to G4): https://github.com/joshmcfayden/FORESEE
• Josh's G4 setup: https://github.com/joshmcfayden/FASER2_G4

Detector technologies

Magnet

SciFi

Dual-readout calorimeter

Background estimates

TODO: Add FLUKA estimates of the muon flux as a function of position in the FPF. Can be used for background and trigger studies.

Links

• Physic Beyond Colliders: https://pbc.web.cern.ch/
• FPF resources: https://pbc.web.cern.ch/fpf-resources
• FASER webpage: https://faser.web.cern.ch/about-the-experiment
• FASER Public Results page: https://twiki.cern.ch/twiki/bin/view/FASER/FaserPapers

-- JoshMcFayden - 2022-10-11