CERN Accelerating science

FASER joins the exploration for new physics

by Jamie Boyd (CERN)

On March 5th the CERN Research Board approved the ForwArd Search ExpeRiment (FASER), for installation at the LHC during Long Shutdown 2. FASER is an experiment designed to broaden the search for new physics at the LHC, by looking for light, weakly- interacting new particles that could be produced in the LHC collisions in the extreme forward direction.

The idea for such an experiment was proposed by J. L. Feng, I. Galon, F. Kling, and S. Trojanowski, theorists at the University of Irvine, California in a paper released in the summer of 2017 [1]. In 2018 they started to work with experimentalists, forming a collaboration to realise this idea in an experiment at the LHC. By the summer 2017 the project had attracted funding from the Heising-Simons foundation and the Simons foundation, two private foundations from the US, allowing the experiment to rapidly progress through the review by the LHCC, with a letter of intent [2] submitted in July 2018, followed by a Technical Proposal [2] submitted in November 2018. Following successful review, and further scrutiny from the LHC Machine Committee and the LS2 Committee the experiment was formally approved in March 2019.

The basic idea of FASER is based on the fact that a huge number of standard model particles, such as pions, are produced in the LHC collisions, predominantly very closely aligned around the colliding beam axis. New physics particles that can be very rarely produced in the decay of such pions, could then be detected in a very small detector with an active area of only 20cm across, when placed 500m from the collision point. In fact 2% of π0’s produced in the LHC collisions (with E>10 GeV) are produced in this angular region, which covers only 2-6% of the solid angle.

Fig 1: (Top) A schematic of the LHC in the region up to 500m from the ATLAS collision point (shown at the left hand side). The bottom right panel shows the location where FASER will be installed aligned with the beam collision axis in the TI12 tunnel.

A particular target is the ‘dark photon’, which can act as a mediator particle for light dark matter, and is discussed in more detail in Ref [4]. For a dark photon with a mass of 100  MeV and a coupling to SM particles of eps=10-5, these would only produced once in about 1010 π0 decays. However in Run-3 of the LHC an expected 10^15 pi0s will be produced in the angular region corresponding to the FASER detector, meaning ~105 dark photons could be produced pointing towards FASER. The large energy of these pi0s mean that the dark photon is likely to travel long distances before it decays, and a few 100s of these 100,000 dark photons could decay inside the FASER detector.

Amazingly, it turns out that there already exists an ideal location for the FASER detector to be placed, so that it can be aligned directly on the collision axis. This is an unused service tunnel that joins the LHC 480m from the ATLAS collision point, called TI12. In the past this tunnel was used to inject leptons from the SPS into the LEP collider, but it is no longer used.  Figure 1 shows a sketch of the LHC tunnel for 500m on one side of ATLAS, showing the location where FASER will be installed in TI12. With just a small amount of digging in this tunnel, a 5.5m-long detector can be situated on the collision axis, allowing good sensitivity to dark photons and other light weakly interacting new particles, in unexplored, and theoretically well motivated, regions of parameter space.

The extremely fast turn around for FASER, from an idea in a paper, to an approved experiment was made possible by support from the CERN Physics Beyond Colliders study group, which provided resources for running background simulations, measuring backgrounds, mapping out the collision axis in TI12, and studying the infrastructure work that needs be done for FASER to be installed and operated. In addition, the FASER detector will re-use spare modules from the ATLAS silicon micro-strip tracker (SCT), and from the LHCb electromagnetic calorimeter, allowing it to skip the design and construction phase for these new detectors, and saving both time and money.

Fig 2: A schematic of the FASER detector. Showing the scintillators (grey), magnets (red), tracking stations (blue), and electromagnetic calorimeter (purple). Particles from the ATLAS collisions enter from the left hand side. The first magnet represents the decay volume for the dark photons.

A sketch of the detector is shown in Figure 2. At the entrance there are a number of scintillator planes, which will be used to ensure charged particles are not entering the detector when searching for the signal topology. This is followed by a 1.5m-long decay volume, enclosed in a 0.6T dipole magnet. Following this there is a spectrometer to measure the trajectories of charged particles produced in dark photon decays inside the decay volume. The spectrometer is made up of two 1m-long dipole magnets (also with 0.6T), with tracking stations, positioned at the start, middle and end of the spectrometer. Each tracking station is made up of three layers of double-sided silicon strip SCT modules. At the end of the detector the electromagnetic calorimeter allows to measure the electromagnetic energy in the event, and to distinguish between decays to electrons or photons, compared to muons or hadrons. The expected physics sensitivity of FASER across dark photon parameter space is shown in Figure 3 and the reach has been estimated in many potential new physics models [4].

Fig 3: The expected exclusion limit in dark photon parameter space for 10/fb (expected in 2021) and 150/fb (expected during the full LHC Run-3) of data at the LHC. Projected limits from proposed future experiments are also shown.

There is an ambitious timeline to build, test and install the detector into TI12 before summer 2020, when the LHC machine in that sector will be cooled down. The schedule foresees to have the individual systems ready by the end of 2019, and then for combined commissioning on the surface during the first part of 2020. The magnets, which are being built by the CERN magnet group, are expected to become available in the first quarter of 2020. The Collaboration currently consists of 38 researches from 16 institutes in 8 countries, and includes CERN fellows, and staff, working part time on FASER. Given the tight timeline, and small collaboration, it represents a nice opportunity for young researchers to get involved in construction and commissioning in a short timescale compared to hardware projects on the big LHC experiments.

As well as searching for weakly interacting light new physics particles FASER may be able to make neutrino measurements, based on the huge flux of high energy neutrinos that are produced in the LHC collisions and traverse the detector location. Studies are ongoing to see what measurements may be possible.

Despite the fact that FASER is not up and running yet, there are already ideas for a potential upgrade, which would see a bigger detector, installed in the same location. Such a detector (with a transverse radius of 1m compared to 10cm) would have sensitivity to new physics particles produced in heavy meson decays (such as B-decays), which are produced more spread out around the beam axis.

However, given the tight timeline the team are currently focussing on getting FASER installed and working in LS2. They are looking forward to increasing the ability to search for new physics with the LHC, in a complementary way to the bigger LHC experiments.

 

Further Reading

[1] - J. L. Feng, I. Galon, F. Kling, and S. Trojanowski ,Phys. Rev. D 97, 035001 (2018) hep-ph:1708.09389

[2] – FASER Collaboration, “Letter of Intent for FASER: ForwArd Search ExpeRiment at the LHC”, arXiv:1811.10243

[3] - FASER Collaboration, “Technical Proposal for FASER: ForwArd Search ExpeRiment at the LHC”, arXiv:1812.09139

[4] - https://ep-news.web.cern.ch/content/lifetime-frontier

[5] - FASER Collaboration, “FASER's Physics Reach for Long-Lived Particles”,  arXiv:1811.12522