TWiki> FASER Web>WebHome (revision 29)EditAttachPDF

Welcome to the FASER Webpage


FASER (ForwArd Search ExpeRiment at the LHC) is a proposed experiment to be situated 480m along the line-of-sight of the proton collisions in front of the ATLAS interaction point at the LHC. Preliminary studies show that a small experiment at this location has significant prospects for discovering a variety of light, weakly-coupled new particles, such as dark photons, dark Higgs bosons, heavy neutral leptons (sterile neutrinos), and axion-like particles, which could be produced in the decay of particles produced in the LHC collisions, or in the interaction of these particles with material. The FASER experiment is part of the CERN Physics Beyond Colliders study group.
For the internal FASER site (protected) please follow this link: InternalFaserSite.

FASER in a Nutshell


For decades, the leading examples of physics beyond the standard model were particles with TeV-scale masses and O(1) couplings to the standard model (SM). More recently, however, there is a growing and complementary interest in new particles that are much lighter and more weakly-coupled. Among their many motivations, such particles may yield dark matter with the correct thermal relic density and resolve outstanding discrepancies between theory and low-energy experiments. Perhaps most importantly, new particles that are light and weakly-coupled can be discovered by relatively inexpensive, small, and fast experiments with potentially revolutionary implications for particle physics and cosmology.

If new particles are light and very weakly coupled, the focus at the LHC on searches for new particles at high pT may be completely misguided. In contrast to TeV-scale particles, which are produced more or less isotropically, light particles with masses in the MeV- GeV range are dominantly produced at low pT. In addition, because the new particles are extremely weakly coupled, very large standard model event rates are required to discover the rare new physics events. These rates are not available at high pT, but they are available at low pT: at the 13 TeV LHC, the total inelastic pp scattering cross section is σinel(13 TeV) ≈ 75 mb, with most of it in the very forward direction. This implies

inelastic pp scattering events for an integrated luminosity of 300 fb-1 at the LHC (3 ab-1 at the HL-LHC). Even extremely weakly-coupled new particles may therefore be produced in sufficient numbers in the very forward region. Due to their weak coupling to the SM, such particles are typically long-lived and travel a macroscopic distance before decaying back into SM particles. Moreover, such particles may be highly collimated. For example, new particles that are produced in pion or B-meson decays are typically produced within angles of θ ~ ΛQCD / E or mB / E of the beam collision axis, where E is the energy of the particle. For E ~ TeV, this implies that even ~500m downstream, such particles have only spread out ~ 10 cm - 1 m in the transverse plane. A small and inexpensive detector placed in the very forward region may therefore be capable of extremely sensitive searches, provided a suitable location can be found and the signal can be differentiated from the SM background.

FASER, the ForwArd Search ExpeRiment, is an experiment designed to take advantage of this opportunity. It is a small detector, with volume ~1 m3, that will be placed along the beam collision axis, several hundreds of meters downstream from the ATLAS or CMS interaction point (IP).


Infrastructure Photo.png
Figure 1: Location for FASER. Top panel: A schematic drawing of the LHC and the very forward infrastructure downstream from the ATLAS and CMS interaction points; FASER is to be located 480 m from the IP, after the LHC ring starts to curve. Bottom panels: a map of the tunnel including the beam collision axis (left), a photo of this location (center), and a diagram showing the intersection of the beam collision axis with tunnel TI18 (right). We thank Francesco Cerutti, Paolo Fessia, Mike Lamont, and their groups for providing the photo and diagrams.

As shown in Fig. 1, FASER will be placed along the beam collision axis, several hundreds of meters downstream from the ATLAS or CMS IP after the LHC tunnel starts to curve. A particularly promising location is a few meters outside the main LHC tunnel, 480 m downstream from the ATLAS IP, in service tunnel TI18, as shown in the bottom panels of Fig. 1. (A symmetric location on the other side of ATLAS in tunnel TI12 is also possible.) This tunnel was formerly used to connect the SPS to the LEP tunnel, but is currently empty and unused. As shown on the tunnel map in the lower left panel of Fig. 1, the beam collision axis passes through TI18 close to where it merges with the main LHC tunnel. A more detailed study of the intersection between the beam collision axis and TI18 verifies that there exists space for FASER in the tunnel, as shown in the lower right panel of Fig. 1.

In this location, FASER harnesses the enormous, previously "wasted,'' cross section for very forward physics (σ ~ 100 mb), which implies that even very weakly-coupled new particles can be produced in large numbers at the LHC. In addition, the production of long-lived particles (LLPs) at high center-of-mass energy results in long propagation distances d ~ O(100) m and decays that are far beyond the main LHC infrastructure in regions where the backgrounds are expected to be negligible.

Signals, Detector Design and Backgrounds

FASER will search for LLPs that are produced at or close to the IP, move along the beam collision axis, and decay within the volume of FASER into visible decay products

LLPs produced in the very forward region of the beam collision axis typically have very high energies E ~ TeV. Although the identity of the LLP decay products depends on the mass of the LLP and the concrete new physics model, a characteristic signature is expected of two or more stable charged particles, such as electrons, muons or pions. This leads to a striking signature at FASER: two oppositely charged tracks with very high energy that emanate from a vertex inside the detector and which have a combined momentum that points back to the IP. A measurement of individual tracks with sufficient resolution and an identification of their charges is therefore imperative if the apparatus is to make use of kinematic features to distinguish signal from background. A tracking-based technology, supplemented by a magnet and possibly a calorimeter to allow for an energy measurement, will be the key components of FASER.

The FASER signals are two extremely energetic (~ TeV) coincident tracks or photons that start at a common vertex and point back to the ATLAS IP. Muons and neutrinos are the only known particles that can transport such energies through 90 m of rock between the IP and FASER. Preliminary estimates show that muon-associated radiative processes and neutrino-induced backgrounds may be reduced to negligible levels.

Recently a FLUKA study from the CERN Sources, Targets and Interactions group has been carried out to assess possible backgrounds and the radiation level in the FASER location. The study shows that no high energy (>100 GeV) particles are expected to enter FASER from proton showers in the dispersion suppressor or from beam-gas interactions. In addition, the radiation level expected at the FASER location is very low due to the dispersion function in the LHC cell closest to FASER.

An emulsion detector and a battery-operated radiation monitor were installed at the FASER site in June 2018. The results from these first in situ measurements will complement and validate the background estimates and inform future work, which includes refining background estimates, evaluating signal efficiencies, and optimizing the detector.

Discovery Prospects for FASER

Figure 2: Projected FASER exclusion reach for dark photons (left), dark Higgs bosons (center), and HNLs (right) parameter space in the corresponding coupling vs. mass plane. The gray shaded regions are excluded by current experimental bounds, and the colored contours represent projected future sensitivities of other proposed experiments that search for long-lived particles.

In previous work, we studied FASER's potential to detect dark photons, dark Higgs bosons, heavy neutral leptons (HNLs). Combined, these studies established the significant potential for FASER to extend the LHC's discovery reach in models with renormalizable portals, where the new physics couples to the SM through dimension-4 interactions:

The physics reach at FASER for these models is shown in Fig. 2. Here we assume that backgrounds can be reduced to negligible levels. The gray-shaded regions of parameter space have already been excluded by previous experiments. For comparison we also show the projected reaches of other proposed experiments that search for long-lived particles.

Dark photons (left) are mainly produced in the decay of light mesons or via dark bremsstrahlung and are therefore very collimated around the beam collision axis. Already a very small detector with radius R = 20 cm and length ∆ = 3 m is able to probe large and unconstrained regions of parameter space, making dark photons an ideal short term goal for FASER. In contrast, dark Higgs bosons and HNLs define a good long term physics goal. They are both mainly produced in heavy meson decays, leading to a larger spread around the beam collision axis. A larger, but still relatively small, detector with R ~ 1 m is then required to exploit the full potential of FASER.

Note that FASER's physics potential is not restricted to the models mentioned above. It has been shown that FASER can probe flavor-specific scalar mediators, neutralinos and U(1)B-L-gauge bosons. Additionally, studies for FASER's potential to discover axion-like particles and inelastic dark matter and strongly interacting massive particles are underway.

Timeline and Detector Benchmarks

The proposed timeline is for FASER to be installed in TI18 during Long Shutdown 2 (LS2), in time to collect data during Run 3 of the 14 TeV LHC from 2021-23. FASER's cylindrical active decay volume has a radius R = 10 cm and depth D = 1.5 m, and the detector's total length is under 5 m. FASER will run concurrently with the LHC and require no beam modifications. Its interactions with existing experiments are limited only to requiring bunch crossing timing and luminosity information from ATLAS.

If FASER is successful, a larger version, FASER 2, with an active decay volume with R = 1 m and D = 5 m, could be installed during LS3 and take data in the 14 TeV HL-LHC era. FASER 2 would require extending TI18 or widening the adjacent staging area UJ18.

If you are interested in working on FASER

If you are interested in studying FASER's physics potential, please consider the following two benchmark geometries for FASER's decay volume:

Please also contact a FASER theorist - we are happy to assist and and answer your questions.

Additional Resources

Papers about FASER

Presentations about FASER

Meetings, Conferences, Workshops

Photos and Pictures

  • Photos
IMG_1575.jpg IMG_1557.jpg
  • Line of Sight Maps
Forward_Infrastructure.png Infrastructure_Full.png Infrastructure_Photo.png
  • Schematics of the Detector


FASER e-group is: Active members:
  • Akitaka Ariga (Bern, experimentalist)
  • Tomoko Ariga (Kyushu/Bern, experimentalist)
  • Jamie Boyd (CERN, experimentalist) (contact with PBC accelerator group)
  • Dave Casper (UC Irvine, experimentalist)
  • Jonathan Feng (UC Irvine, theorist) (contact with PBC BSM group)
  • Iftah Galon (Rutgers, theorist)
  • Shih-Chieh Hsu (Washington, experimentalist)
  • Peppe Iacobucci (Geneva, experimentalist)
  • Felix Kling (UC Irvine, theorist)
  • Hidetoshi Otono (Kyushu, experimentalist)
  • Brian Petersen (CERN, experimentalist)
  • Osamu Sato (Nagoya, experimentalist)
  • Anna Sfyrla (Geneva, experimentalist)
  • Jordan Smolinsky (UC Irvine, experimentalist)
  • Aaron Soffa (UC Irvine, experimentalist)
  • Yosuke Takubo (KEK, experimentalist)
  • Sebastian Trojanowski (Warsaw, theorist)

FASER Web Utilities

Topic attachments
I Attachment History Action Size Date Who Comment
JPEGjpeg FaserLogo.jpeg r1 manage 37.1 K 2018-07-17 - 00:54 FelixKlingExternal1  
PDFpdf FaserLogo.pdf r1 manage 6.5 K 2018-07-17 - 00:54 FelixKlingExternal1  
PNGpng FaserLogo.png r1 manage 16.9 K 2018-07-17 - 00:54 FelixKlingExternal1  
PNGpng Forward_Infrastructure.png r1 manage 121.8 K 2018-05-10 - 23:53 FelixKlingExternal1 Forward Infrastructure
JPEGjpg IMG_1557.jpg r1 manage 6671.1 K 2018-04-26 - 23:12 JlfExternal  
JPEGjpg IMG_1575.jpg r1 manage 6697.8 K 2018-04-26 - 23:05 JlfExternal FASER Proposed Site
PNGpng Infrastructure_Full.png r1 manage 158.8 K 2018-05-11 - 00:02 FelixKlingExternal1 LHC Infrastructure and Location of FASER
PNGpng Infrastructure_Photo.png r1 manage 268.6 K 2018-05-10 - 23:59 FelixKlingExternal1 Location of FASER with Photos and Maps
PDFpdf LHCLayout2.pdf r1 manage 203.9 K 2018-04-26 - 23:22 JlfExternal  
JPEGjpg Reach.jpg r1 manage 202.4 K 2018-06-14 - 04:01 FelixKlingExternal1  
Edit | Attach | Watch | Print version | History: r103 | r31 < r30 < r29 < r28 | Backlinks | Raw View | Raw edit | More topic actions...
Topic revision: r29 - 2018-09-18 - JamieBoyd
    • Cern Search Icon Cern Search
    • TWiki Search Icon TWiki Search
    • Google Search Icon Google Search

    FASER All webs login

This site is powered by the TWiki collaboration platform Powered by PerlCopyright & 2008-2020 by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback