FASER experiment

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FASER ForwArd Search ExpeRiment
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FASER (ForwArd Search ExpeRiment) is one of the nine particle physics experiments in 2022 at the Large Hadron Collider at CERN. It is designed to both search for new light and weakly coupled elementary particles, and to detect and study the interactions of high-energy collider neutrinos. [1] In 2023, FASER and SND@LHC reported the first observation of collider neutrinos. [2]

Contents

The experiment is installed in the service tunnel TI12, which is 480 m downstream from the interaction point used by the ATLAS experiment. [3] This tunnel was formerly used to inject the beam from the SPS into the LEP accelerator. In this location, the FASER experiment is placed into an intense and highly collimated beam of both neutrinos as well as possible new particles. Additionally, it is shielded from ATLAS by about 100 meters of rock and concrete, providing a low background environment. The FASER experiment was approved in 2019. [4] [5] The detector was built in the following two years and installed in 2021. [6] The experiment started taking data at the beginning of Run 3 of the LHC in summer 2022. [7] [8] [9]

New physics searches

The primary goal of the FASER experiment is to search for new light and weakly interacting particles, that have not been discovered yet, such as dark photons, axion-like particles and sterile neutrinos. [10] [11] If these particles are sufficiently light, they can be produced in rare decays of hadrons. Such particles will therefore be dominantly produced in the forward direction along the collision axis, forming a highly collimated beam, and can inherit a large fraction of the LHC proton beam energy. Additionally, due to their small couplings to the standard model particles and large boosts, these particles are long-lived and can easily travel hundreds of meters without interacting before they decay to standard model particles. These decays lead to a spectacular signal, the appearance of highly energetic particles, which FASER aims to detect.

In March 2023, the FASER Collaboration reported their first results on the search for dark photons. No signal consistent with a dark photon was seen in their 2022 data and limits on previously unconstrained parameter space were set.

Neutrino physics

Position of FASER experiment in side tunnel TI12 in the LHC at CERN FASER in LHC.jpg
Position of FASER experiment in side tunnel TI12 in the LHC at CERN

The LHC is the highest energy particle collider built so far, and therefore also the source of the most energetic neutrinos created in a controlled laboratory environment. Indeed, collisions at the LHC lead to a large flux of high-energy neutrinos of all flavours, which are highly collimated around the beam collision axis and stream through the FASER location.

In 2021, the FASER Collaboration announced the first detection of collider neutrino candidates. [12] [13] [14] [15] [16] The data used for this discovery was collected by a small emulsion pilot detector with a target mass of 11 kg. The detector was placed in the service tunnel TI18, and the data was collected for only four weeks during LHC Run 2 in 2018. While this outcome falls short of being a discovery of collider neutrinos, it highlights the potential and feasibility of conducting dedicated neutrino experiments at the LHC.

In 2023, the FASER Collaboration announced [17] [18] [19] and published [2] the first observation of collider neutrinos. For this, they searched for events in which a high momentum track emerges from the central part of the FASERv detector volume and no activity in the most upstream veto layers, as expected from a muon neutrino interaction. This search was performed using only the electronic detector components.

FASER experiment setup in the LHC at CERN FASER.jpg
FASER experiment setup in the LHC at CERN

To study these neutrino interactions in greater detail, FASER also contains the dedicated FASERv sub-detector (which is pronounced FASERnu). [20] [21] During its nominal run time of a few years, about 10000 neutrinos are expected to be recorded by FASERν. [22] These neutrinos typically have TeV scale energies, allowing FASERv to study their interactions in a regime where they are currently unconstrained.

FASERnu will be capable of exploring the following physics domains:

  1. FASERv will measure neutrino-nucleus interaction cross sections for all three neutrino flavours at the TeV energy scale. With the ability to identify the neutrino flavor, it will allow to test lepton flavour universality in neutrino scattering.
  2. FASERv will be able to see the highest number of tau neutrino interactions, allowing to study this elusive particle in greater detail.
  3. FASERv will carry out very precise measurements of muon neutrino interactions at an energy scale never explored before. These measurements will allow to probe the proton's structure and constrain parton distribution functions.
  4. Neutrinos at FASERv are primarily produced in the decay of pions, kaons and charmed hadrons. The measurement of the neutrino fluxes therefore allows to constrain the production of these particles in kinematic regime that is inaccessible for the other LHC experiments. This will provide new key inputs for astro-particle physics experiments.

Detector

Layout of the FASER detector FASER Detector Layout.png
Layout of the FASER detector

Located at the front end of FASER is the FASERν neutrino detector. It consists of many layers of emulsion films interleaved with tungsten plates as target material for neutrino interactions. Behind FASERν and at the entrance to the main detector is a charged particle veto consisting of plastic scintillators. [23] [24] This is followed by a 1.5 meter long empty decay volume and a 2 meter long spectrometer, which are placed in a 0.55 T magnetic field. The spectrometer consists of three tracking stations, composed of layers of precision silicon strip detectors, to detect charged particles produced in the decay of long-lived particles. Located at the end is an electromagnetic calorimeter.

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References

  1. ↑ "FASER detector at the Large Hadron Collider to seek clues about hidden matter in the universe". UW News. 2019-03-05. Retrieved 11 April 2021.
  2. 1 2 Worcester, Elizabeth (July 19, 2023). "The Dawn of Collider Neutrino Physics". Physics. 16: 113. Retrieved July 23, 2023.
  3. ↑ "LS2 Report: FASER is born". CERN. Retrieved 2021-03-25.
  4. ↑ "FASER: CERN approves new experiment to look for long-lived, exotic particles". CERN. Retrieved 2019-12-19.
  5. ↑ "FASER's new detector expected to catch first collider neutrino". CERN. Retrieved 2019-12-19.
  6. ↑ "FASER Detector Installation". EP News. Retrieved 2023-03-17.
  7. ↑ Garisto, Daniel. "Large Hadron Collider Seeks New Particles after Major Upgrade". Scientific American. Retrieved 2023-03-17.
  8. ↑ Liverpool, University of. "Large Hadron Collider takes first data in record-breaking run". phys.org. Retrieved 2023-03-17.
  9. ↑ "LHC Run 3: physics at record energy starts tomorrow". ATLAS. Retrieved 2023-03-17.
  10. ↑ Feng, Jonathan L.; Galon, Iftah; Kling, Felix; Trojanowski, Sebastian (2018-02-05). "FASER: ForwArd Search ExpeRiment at the LHC". Physical Review D. 97 (3): 035001. arXiv: 1708.09389 . doi:10.1103/PhysRevD.97.035001. ISSN   2470-0010. S2CID   119101090.
  11. ↑ Ariga et al. (FASER Collaboration) (2019-05-15). "FASER's Physics Reach for Long-Lived Particles". Physical Review D. 99 (9): 095011. arXiv: 1811.12522 . Bibcode:2019PhRvD..99i5011A. doi:10.1103/PhysRevD.99.095011. ISSN   2470-0010. S2CID   119103743.
  12. ↑ FASER Collaboration; Abreu, Henso; Afik, Yoav; Antel, Claire; Arakawa, Jason; Ariga, Akitaka; Ariga, Tomoko; Bernlochner, Florian; Boeckh, Tobias; Boyd, Jamie; Brenner, Lydia; Cadoux, Franck; Casper, David W.; Cavanagh, Charlotte; Cerutti, Francesco (2021-11-24). "First neutrino interaction candidates at the LHC". Physical Review D. 104 (9): L091101. arXiv: 2105.06197 . doi: 10.1103/PhysRevD.104.L091101 .
  13. ↑ "Neutrinos were caught interacting at the Large Hadron Collider | Science News". 2021-05-26. Retrieved 2023-03-17.
  14. ↑ "UCI-led team of physicists detects signs of neutrinos at Large Hadron Collider". UCI News. 2021-11-24. Retrieved 2023-03-17.
  15. ↑ "Neutrinos detected in particle collider for first time at CERN". The Jerusalem Post | JPost.com. Retrieved 2023-03-17.
  16. ↑ Dorminey, Bruce. "Suitcase-Sized Neutrino Detector Hits Pay Dirt At Large Hadron Collider". Forbes. Retrieved 2021-11-26.
  17. ↑ "UC Irvine-led team is first to detect neutrinos made by a particle collider". UCI News. 2023-03-20. Retrieved 2023-03-20.
  18. ↑ "First detection of neutrinos made at a particle collider". ScienceDaily. Retrieved 2023-03-20.
  19. ↑ "DESY News: Res eaname rch team detects first neutrinos made by a particle collider". www.desy.de. Retrieved 2023-03-21.
  20. ↑ Abreu et al. (FASER collaboration) (2020). "Detecting and Studying High-Energy Collider Neutrinos with FASER at the LHC". The European Physical Journal C. 80 (1): 61. arXiv: 1908.02310 . Bibcode:2020EPJC...80...61A. doi:10.1140/epjc/s10052-020-7631-5. S2CID   199472668.
  21. ↑ Krishna, Chetna. "Catching neutrinos at the LHC". symmetry magazine. Retrieved 2023-03-17.
  22. ↑ Kling, Felix; Nevay, Laurence J. (2021-12-29). "Forward neutrino fluxes at the LHC". Physical Review D. 104 (11): 113008. arXiv: 2105.08270 . doi:10.1103/PhysRevD.104.113008.
  23. ↑ Ariga et al. (FASER Collaboration) (2018-11-26). "Letter of Intent for FASER: ForwArd Search ExpeRiment at the LHC". arXiv: 1811.10243 [physics.ins-det].
  24. ↑ Ariga et al. (FASER Collaboration) (2018-12-21). "Technical Proposal for FASER: ForwArd Search ExpeRiment at the LHC". arXiv: 1812.09139 [physics.ins-det].

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