When people talk about CERN, they inevitably think of the large-scale experiments at the Large Hadron Collider (LHC). But the European Laboratory for Particle Physics has even more to offer: A small but powerful experiment called FASER is currently being assembled at CERN, involving research teams led by Geneva physics professors Anna Sfyrla and Giuseppe Iacobucci and by Bern physics professor Akitaka Ariga. FASER proves: Even comparatively inexpensive experiments have the potential for groundbreaking scientific findings.
Anyone who visits the four large experiments currently being conducted at the LHC will be amazed: deep underground are rock caverns that house enormous research equipment. One of these devices is the detector of the ATLAS experiment: a 46-metre-long cylinder, 25 metres in diameter, weighing 7000 tonnes. The colossus consists of several layers of different types of sensors. The sensors detect the particles that are created when two protons collide inside the detector and create new particles.
This detector is exactly the right size for the research goals of the ATLAS experiment. However, this does not mean that it is impossible to explore the smallest components of matter in simpler ways. This is proven by the FASER experiment, which is currently being set up only a few hundred metres away from the ATLAS detector, also 80 metres below the Earth's surface. The FASER detector looks like ATLAS' little brother: it consists of a five-metre-long tube with a diameter of 20 cm and weighing 3 tonnes. "We are a cosy collaboration where everyone knows everyone else and everyone overlooks the whole experiment," says Claire Antel, a trained particle physicist of South African origin. She completed her PhD at the University of Heidelberg and now works as a postdoc with particle physics professor Anna Sfyrla at the University of Geneva. She is one of 64 researchers from eight countries working on the FASER experiment.
Measuring what the ATLAS detector cannot detect
With FASER, the "cosy collaboration" wants to contribute to clarifying some of the big scientific questions of the 21st century: What particles does the dark matter consist of that fills the universe but has never been observed? How does the mass of the neutrino come about? Why is there much less antimatter in the universe than one would expect? To find answers to these questions, physicists are feverishly searching for previously unknown elementary particles with new experiments. FASER is one of the latest experiments: The idea for it was born in 2017, and since then it has been set up at record speed.
ATLAS measures properties of the particles that are created when two protons accelerated in the LHC collide. After years of collecting data it has become apparent that new particles are not in easy reach of the ATLAS detector, be it because they are too heavy or they interact too weakly. A paradigm shift is needed to look for something slightly different and ensure no obvious signal for new physics is being missed. This is what FASER aims to do a few hundred metres away from the ATLAS detector. Its goal is to detect previously unobserved elementary particles (such as the 'dark photon') that – according to some theoretical models – could be produced in proton-proton collisions but cannot be easily detected by the ATLAS detector as they are very light and tend to shoot out along the beam pipes - exactly where ATLAS has no detection sensitivity.
A 'dark photon', if it exists, could not be detected directly by FASER either. However, the experiment aims at capturing such a particle decaying within the FASER detector (e.g. into an electron and a positron), and so detect these visible decay products in the detector. “The FASER experiment introduces a novel approach to exploiting LHC collisions. It has the potential to either make a revolutionary discovery, or constrain new theories and models in regions where no current experiment has access to. At the same time, it will provide first measurements of collider neutrinos”, Anna Sfyrla explains.
A favourable location for novel observations
The FASER detector is designed precisely for this task. Its location is chosen such that as few other particles as possible interfere with the measurements. The strong magnets of the LHC help deflect most of the charged particles (in which FASER is not interested) so that they do not even reach the FASER detector. Four so-called scintillators at the front of the detector flag incoming muons, which can hence be removed when searching for tell-tale signs of new physics. If an unknown particle (e.g. a 'dark photon') were to enter the detector, it could decay in a certain sector of the detector ('decay volume'). The decay products would then be measured in the three-stage detection chamber ('tracker') (for details see legend to illustration 01). The FASER experiment is complemented by an emulsion detector: 1000 photographic layers, interleaved with tungsten plates, will capture neutrinos traversing its volume, at the highest human-made energies ever recorded. Some components of the detector were donated by the latge LHC experiments as they were spares or no longer needed. This allowed construction of the detector in a short time scale and reduced cost.
In order to detect new particles such as the dark photons, the FASER physicists have to measure particle tracks in the detection chamber for a long time and then evaluate the recorded 'events' using statistical methods. If successful, this would allow conclusions to be drawn about the existence of new types of particles. The readout system for the FASER data is Claire Antel's speciality. She has been working on this detector component since she joined the FASER experiment in 2019.
“One challenge is to make sure that when we start up the experiment all the detector components are reading out their part of data synchronised to within 25 nanoseconds so that when we stitch the data together later on, we correctly reconstruct the full picture.”, says Claire Antel.
Launch in February 2022
Despite CERN’s restrictive regulations related to the pandemic, precautions were put in place to allow fast progress with the installation of the FASER detector. Extensive tests were performed - including over Christmas - during which the researchers successfully tested various components connected to the data readout system by measuring cosmic rays. The detector was fully installed at the end of March 2021, but it will not go into operation until February 2022, when the LHC's current maintenance break ends. The first results from FASER are expected at the end of 2022.
Although the FASER detector has still not started taking data, the FASER collaboration is already thinking about a new detector to complement the current one after its three-year lifetime. The new FASER-2 is planned to be significantly larger and more powerful, allowing physicists to search for a broader spectrum of new phenomena.
Author: Benedikt Vogel