New dark matter experiment at CERN moves forward with construction of prototype detector

The 5600-tonne LHCb experiment sits 100 meters underground at CERN, close to the Geneva airport. Over 1700 physicists from 23 countries at 98 institutes participate in probing the deepest mysteries, such as understanding the nature of dark matter.  Image source: CERN.

Another 27% percent comprises a mysterious dark matter that feels only feebly (if at all) any of the three fundamental forces known to physicists – weak, strong and electromagnetism. The only way we “see” dark matter is via its gravitational effects in cosmological observations. For example, our galaxies are rotating at such spectacular speeds that they should be ripping themselves apart, only to be held together by gravity from dark matter.

The fact that dark matter interacts feebly with the visible (so-called Standard Model) sector means that they are incredibly hard to detect.

The LHCb detector at CERN is one of the leading experiments in the world looking for dark matter signatures in the aftermath of very high energy proton-proton collisions.

The rich content of the (left) visible Standard Model particles – quarks, leptons, force carriers and the Higgs – hints at a hidden dark sector (right) with new particles and interactions. The visible and dark sectors became decoupled in the early hot Universe, but can still talk to each other feebly via so-called “portal interactions”

More generally, physicists suspect that there could be a hidden or dark sector with as rich a particle content as the visible sector described by the Standard Model. For example, accompanying the normal massless photon (carrier of the electromagnetic force), there could be massive dark photons. Similarly, like the Standard Model Higgs boson, there can be additional dark Higgs bosons. Strong theoretical motivation for such new feebly interacting long-lived particles (LLPs) come from almost any viable extension of the Standard Model such as theories of supersymmetry and dark matter. 

The experimental signature of LLPs produced at the LHC are flashes of particles seemingly coming out of nowhere and occurring far away from the proton-proton collision point.

That is, the LLPs are produced close to the collision point, but travel several tens of meters before decaying into a shower of visible particles. The problem is that the existing main LHC detectors (LHCb, ATLAS and CMS) are optimized for discovering new heavy particles decaying close to the collision point. LLP searches therefore require unconventional reconstruction algorithms, triggers and simulation models. While huge efforts are ongoing on this front at the main LHC detectors, dedicated detectors are needed.

The full CODEX-b detector is proposed to be a 10x10x10m3 box in the access region of the LHCb cavern, behind a thick concrete wall, to shield against backgrounds from Standard Model processes. Several locations in the access region (“UX85A”) are being investigated currently.

CODEX-b (COmpact Detector for EXotics at LHCb) will be a novel LLP detector aligned with the existing main LHCb detector, to harness the extraordinary luminosities in the upcoming High-Luminosity LHC era. CODEX-b will be stationed behind a 3.2m thick concrete wall, thereby benefiting from the existing shielding against Standard Model backgrounds, a major limitation for the existing LHC detectors. The online data acquisition can proceed in the same data stream as the main LHCb detector, thereby enabling joint CODEX-b/LHCb triggers. The location of CODEX-b at high perpendicularity to the beamline and the LHCb geometry will give access to LLPs produced via heavy intermediate particles, such as the Higgs boson. Given that the High-Lumi LHC will be a Higgs factory, CODEX-b will critically be able to probe exotic decays of the Higgs boson. The CODEX-b box will be instrumented with multiple tracking layers with good spatial and timing resolution. A cost-effective solution for the detector technology is usage of Resistive Plate Chambers (RPCs) already used extensively at CERN. The target cost of the full detector is of the order of ~$10 million, modest compared to typical particle physics experiments.

CODEX-b features in the 2023 US particle physics prioritization (P5) report as one of the potential “auxiliary experiments”, an important step towards eventual funding, both in the US and the EU.

The CODEX-b collaboration has published an Expression of Interest in 2020. 

As the next important step, a small 2x2x2m3 pilot detector (1/125th of the full volume), CODEX-β, is being constructed to collect data during the ongoing Run3 data-taking at LHCb. CODEX-β is designed to mimic the full detector as closely as possible, including 14 full-scale RPCs, similar to those expected in CODEX-b. The development of online readout and joint LHCb/CODEX-b triggers will be important milestones, as will be gaining experience with handling RPC detectors, new to LHCb. CODEX-β has been reviewed and approved as an R&D project by the LHCb collaboration. The Technical Design Report has just been submitted to the arXiv for publication.

ELTE student Andras Burucs (left-most) with colleagues from the CODEX-b collaboration, at the CODEX-beta construction site at CERN.

The ELTE LHCb/CODEX-b group led by assistant professor Biplab Dey, has been playing a strong role in the CODEX-b effort.

In 2018, Dey led the first background measurements in the shielded region of the LHCb cavern, a critical step to understand the Standard Model backgrounds to be expected for CODEX-b. Following up on this work, ELTE undergraduate student Andras Burucs has just submitted his thesis on developing the software framework for CODEX-b(β). Andras also spent around 2.5 months at CERN earlier this year, to participate in the RPC construction work. His primary task was to set up and test the data acquisition for the chambers under construction. The ELTE contribution is supported by the "Astro- and Particle Physics Thematic Excellence Program" funded by NKFIH.

The LHCb approval of CODEX-β, subsequent journal submission of the TDR, finding mention in the US P5 report – all represent significant milestones for the CODEX-b project. The chamber construction for CODEX-β is on schedule to start data-taking with LHCb Run3 in early 2025. Successful operation of the pilot project will be of paramount importance for eventual funding and construction of the full project. In parallel, the CODEX-b collaboration is further solidifying the physics case for approval of the full CODEX-b detector. The ELTE LHCb/CODEX-b group will continue to play significant roles in this endeavor to harness the tremendous capabilities of the High-Luminosity LHC in the coming years to probe the dark sector.