NA64

NA64 is a fixed-target, beam-dump experiment operating at the CERN Super Proton Synchrotron facility near Geneva. It searches for weakly coupled, light hidden-sector particles produced in high-intensity electron and muon beams by looking for missing energy and visible decay signatures. The experiment builds on techniques developed at the SLAC National Accelerator Laboratory, DESY, and Fermilab and contributes to the worldwide program of intensity-frontier searches that includes efforts at J-PARC, KEK, and the Large Hadron Collider collaborations.

Overview

NA64 uses a primary high-energy lepton beam directed onto a passive target to produce hypothetical mediators connecting the Standard Model to hidden sectors, such as dark photons, light scalar bosons, axion-like particles, and heavy neutral leptons. The concept is related to beam-dump and missing-energy strategies previously employed by experiments like E137, E141, LSND, and MiniBooNE. NA64 complements collider-based searches from ATLAS, CMS, and precision experiments like BaBar, Belle II, and KLOE by exploiting extreme beam intensity and hermetic vetoing. The collaboration gathers expertise from institutions including Moscow State University, JINR, Czech Technical University in Prague, University of Geneva, and national laboratories across Europe and Asia.

Experimental Setup

The experiment is installed in the H4 beamline of the Super Proton Synchrotron and uses a high-energy secondary electron or muon beam delivered by the CERN accelerator complex. Incoming particles are tracked with magnetic spectrometers derived from technologies used at CERN experiments such as NA62 and COMPASS. The target region (active dump) is instrumented with electromagnetic calorimetry adapted from projects at SLAC and DESY, while downstream detectors include hadronic calorimeters, scintillator veto systems, and muon detectors with heritage from LHCb and ALEPH. The trigger and data acquisition system integrates electronics and firmware developed in collaboration with groups experienced on ALICE and CMS upgrades. Shielding and beam transport optics are coordinated with the Super Proton Synchrotron operations team and beam instrumentation groups at CERN.

Physics Goals and Methodology

NA64 aims to probe parameter space for light mediators that couple feebly to charged leptons or to photons, targeting models motivated by anomalies such as the muon anomalous magnetic moment tension reported by the Muon g−2 program and astrophysical observations tied to dark matter phenomenology studied by groups at Princeton University and Caltech. Primary search channels include invisible decays of mediators (missing energy/missing momentum) and visible displaced decays into e+e− or μ+μ− pairs. The methodology combines precision beam monitoring from CERN beam instrumentation, event-by-event kinematic reconstruction inspired by analyses at BaBar and Belle II, and background suppression strategies used in rare process searches like PIENU and NA62. Statistical interpretation employs hypothesis testing frameworks and limit-setting techniques shared with collaborations such as ATLAS and CMS, while theoretical inputs and cross-section calculations are benchmarked against predictions from groups working with the Institute for Theoretical and Experimental Physics and the Perimeter Institute.

Key Results and Publications

NA64 has produced exclusion limits that constrain dark photon models in the sub-GeV mass range, surpassing previous bounds set by experiments like E137 and complementing results from BaBar and KLOE. Publications from the collaboration report null results in targeted parameter regions, improving upper limits on coupling constants for vector and scalar mediators and placing bounds on heavy neutral lepton mixing angles comparable to constraints from T2K and PS191. Results have been presented at conferences including the Rencontres de Moriond, EPS Conference on High Energy Physics, and workshops hosted by CERN and IPPP. The collaboration maintains a steady output of technical notes on detector performance and beamline characterization that reference design principles from COMPASS and NA61/SHINE.

Collaborations and Funding

The NA64 collaboration is multinational, composed of research groups from universities and institutes across Europe, Russia, and Asia, with institutional partners such as CERN, JINR, and national funding agencies including agencies analogous to the European Research Council and national science foundations. Project funding and in-kind contributions come from laboratories experienced in accelerator-based experiments, such as IHEP, PNPI, and university groups affiliated with Oxford University and Moscow State University. Collaborative management follows models used in medium-scale experiments like NA62 and COMPASS, coordinating physics, detector, software, and commissioning tasks through working groups that interact with the CERN accelerator and safety offices.

Category:Particle physics experiments