Studies of the FASER Calorimeter and Searches for New Physics with LHC Run 3 Data

FASER is a new experiment at the LHC designed to detect light and weakly-interacting particles predicted by several extensions of the Standard Model of particle physics [1]. It aims to identify new particles belonging to a dark sector, called dark photons, which are possibly long-lived particles (LLPs) produced at the ATLAS interaction point [2]. These dark photons are thought to decay to e+e- (45%), (45%) or (10%). The decay products of such light, weakly-interacting particles are highly collimated and in the very forward region, requiring only a small detector. The FASER detector is located in TI12 - an unused tunnel previously used to connect the Super Proton Synchrotron (SPS) to LEP [2]. Installation will be completed during the current long shutdown ready for data acquisition during Run 3, which is scheduled for 2022. The main components of the FASER detector are the scintillators, the tracking stations, the dipole magnets, and the calorimeter [1].

The initial focus of this project, prior to data acquisition, is the study and simulation of FASER's sampling electromagnetic calorimeter (ECAL) [3]. This is building on previous work that used examples in Geant4 [4] to build a simple simulation setup and evaluated the impact of angular tilt and performing angular scans to investigate the loss of energy in the gap in the calorimeter. Work was also carried out which aimed to implement fast simulations using a more complex setup [5].

Simulation of the ECAL is required for the development of data analysis in the search for new physics. Current work is being performed towards the implementation of a full simulation of the detector, which requires careful evaluation of the impact of materials and geometry on the deposited energies in the ECAL. Additional work will subsequently be carried out to realise a fast simulation to reduce the computational times and make future analysis efficient. Once this is completed, the software needed to analyse the data will be developed and used for scientific results.

Initial studies confirm the results previously shown using simple Geant4 simulations in terms of the fraction of deposited energy and behaviour as a function of injected energy. Further simulation studies using FASER's software framework, Calypso [6], are on-going to evaluate the energy dependence more precisely for high momentum particles. The goal is to validate the electromagnetic calorimeter simulation with first data. The ultimate aim is to go on to analyse data taken at FASER in the search for new physics in the form of dark photons or other long-lived particles. In the event that no such discoveries are made, the goal moves towards setting world-best constraints over a range of parameter space that is currently unexplored.

References:
[1] FASER Collaboration (2019) FASER's Physics Reach for Long-Lived Particles, Phys. Rev. D 10.1103/PhysRevD.99.095011
[2] FASER Collaboration (2018) FASER: FORWARD SEARCH EXPERIMENT AT THE LHC, Technical Report arXiv:1812.09139v1
[3] Fabjan, C. W., Gianotti, F. (2003) Calorimetry for Particle Physics, CERN
[4] Geant4 Calorimetry Example: https://geant4-userdoc.web.cern.ch/geant4-userdoc/Doxygen/examples_doc/html/ExampleB4.html
[5] Fellers, D. (2020) FASER Fast Calorimeter Simulation, FASER Offline Meeting 25/05/2020 https://indico.cern.ch/event/918531/
[6] https://gitlab.cern.ch/dcasper/calypso/-/tree/master-ecal-2020Oct28