GRENDEL Detector Proposed for LHC Long-Lived

Physicists have proposed a new experiment called GRENDEL to search for long-lived particles at the Large Hadron Collider. The GalleRy ExperimeNt for Decays of Exotic LLPs would be installed in an existing drainage gallery at the CMS experiment to probe particles that current detectors miss.

Long-lived particles are predicted by many theories that extend the Standard Model of particle physics. According to the proposal, these exotic particles could help explain major puzzles like the hierarchy problem, the origin of neutrino masses, the universe's matter-antimatter asymmetry, and the nature of dark matter. Despite this strong theoretical motivation, neutral particles with masses around the GeV scale and long lifetimes have proven difficult to detect with the LHC's main detectors.

The Challenge of Long-Lived Particles

Trigger limitations and challenging backgrounds in the primary detectors curtail sensitivity across a wide range of particle lifetimes. The proposed GRENDEL detector aims to overcome these hurdles by using a different location and strategy.

It would be situated in the PX56 drainage gallery at CMS Point 5, approximately 22 meters above the LHC interaction point. The detector would run largely transverse to the beam and span around 110 meters, covering the central region with an absolute pseudorapidity of less than 0.6.

Detector Design and Location

The substantial rock and concrete between the gallery and the collision point would serve as passive shielding against background particles. GRENDEL would also employ active background rejection techniques. The detector concept makes use of extruded scintillator bars.

The authors state this design allows GRENDEL to achieve competitive sensitivity with other proposed detectors while requiring minimal new infrastructure. A key advantage is that it would not need new civil engineering work, as it uses an existing gallery.

Projected Physics Reach

The proposal presents projected sensitivities for several theoretical benchmarks using 3 inverse attobarns of data from the High-Luminosity LHC. The targets are particles in the 0.2 to 15 GeV mass range. The benchmarks span three theoretical "portals": the neutrino portal, the scalar portal, and the axion portal.

These exotic particles could be produced through several mechanisms. The source lists exotic Higgs boson decays, decays of B- and D-mesons, and decays of W or Z bosons.

The GRENDEL proposal, submitted to the arXiv preprint server, outlines the full physics case and detector concept. Its goal is to open a new window on phenomena that remain hidden from current LHC experiments.