CREMA Collaboration

CREMA Collaboration
Name	CREMA Collaboration
Formation	2009
Type	Research collaboration
Headquarters	Paul Scherrer Institute
Fields	Precision spectroscopy, particle physics, atomic physics
Members	International institutions

CREMA Collaboration

The CREMA Collaboration is an international research consortium focused on precision measurements of muonic atoms to probe fundamental constants and test quantum electrodynamics. Operating at major accelerator and laboratory sites, CREMA brings together researchers from universities and institutes to measure atomic energy levels with muon probes, confronting predictions from Quantum Electrodynamics, Proton radius puzzle, CODATA adjustments and other high-precision determinations. Its work has influenced discussions in the communities of Particle Physics, Atomic Physics, Metrology, and Nuclear Physics.

History

CREMA formed in 2009 as a response to experimental advances at facilities such as the Paul Scherrer Institute, Rutherford Appleton Laboratory, J-PARC, and TRIUMF. Early membership included groups from University of Zurich, Massachusetts Institute of Technology, University of Bonn, University of Bern, and MIT Lincoln Laboratory, among others. The collaboration leveraged techniques developed in muon science at the Muon g−2 program and built on theoretical input from researchers associated with Stanford University, Max Planck Institute for Nuclear Physics, and Institut Laue–Langevin. Key milestones included the first measurement campaigns on muonic hydrogen and subsequent campaigns on muonic deuterium and muonic helium ions, prompting responses from communities at National Institute of Standards and Technology, Harvard University, and European Organization for Nuclear Research.

Research Goals and Methods

CREMA aims to extract nuclear charge radii and test bound-state predictions from Quantum Electrodynamics by measuring energy splittings in muonic atoms. The collaboration targets transitions such as the Lamb shift in muonic hydrogen, deuterium, and helium to determine the Proton charge radius, Deuteron radius, and Helium nucleus radius with unprecedented precision. Methodologically, CREMA combines pulsed muon beams from facilities like the Paul Scherrer Institute and TRIUMF with cryogenic target systems, fast laser spectroscopy similar to techniques used at Max Planck Institute for Quantum Optics and timing systems developed for Fermi National Accelerator Laboratory. Data analysis integrates theoretical corrections from groups at MIT, University of Warsaw, Los Alamos National Laboratory, and KIT to account for nuclear structure, two-photon exchange, and radiative recoil effects.

Key Experiments and Results

CREMA's most prominent result came from measurements on muonic hydrogen that yielded a proton radius notably smaller than previous electron-based determinations, a discrepancy that became known within communities studying the Proton radius puzzle. Subsequent muonic deuterium and muonic helium ion experiments produced consistent radii determinations that stimulated re-evaluations by committees contributing to CODATA and prompted comparisons with results from electron scattering experiments at Jefferson Lab, MIT-Bates Laboratory, and Mainz Microtron (MAMI). CREMA employed pulsed-laser spectroscopy and X-ray detectors akin to technologies used at European X-Ray Free-Electron Laser projects and collaborated with theorists who had ties to Perimeter Institute and Institute for Nuclear Theory to refine two-photon-exchange corrections. The experiments also influenced precision measurements related to the Rydberg constant and spurred cross-checks using atomic hydrogen spectroscopy groups at National Physical Laboratory and LKB (Laboratoire Kastler Brossel).

Collaboration Structure and Membership

The collaboration is organized with spokespeople, working groups, and institutional representatives from universities, national laboratories, and research institutes including Paul Scherrer Institute, University of Zurich, University of Bonn, Harvard University, Massachusetts Institute of Technology, TRIUMF, Rutherford Appleton Laboratory, Helmholtz Association, and Max Planck Society affiliated groups. Working groups focus on experiment operations, laser systems, detector development, data analysis, and theory integration; these interact with external theory collaborators at MPI for Physics, Perimeter Institute, Institute for Advanced Study, and university groups at University of Cambridge and Princeton University. Publications and review papers have included coauthors from institutions such as University of Warsaw, Los Alamos National Laboratory, CERN, University of Manchester, and KVI-CART (Groningen).

Impact and Significance

CREMA's findings reshaped conversations in precision measurement communities including those at CODATA and influenced experimental programs at Jefferson Lab, MAMI, and TRIUMF. Its proton radius results triggered theoretical reexaminations in groups at Perimeter Institute, Brookhaven National Laboratory, Los Alamos National Laboratory, and stimulated new atomic spectroscopy campaigns at National Physical Laboratory, LKB, Harvard–Smithsonian Center for Astrophysics, and University of Oxford. The collaboration's work strengthened links between muon physics and precision atomic metrology, prompting follow-up experiments in muonic atoms, proposals at J-PARC, and cross-disciplinary studies involving researchers from Max Planck Institute for Quantum Optics and Stanford University.

Funding and Facilities

CREMA's operations have been supported by national agencies and international funding bodies including the Swiss National Science Foundation, U.S. National Science Foundation, Deutsche Forschungsgemeinschaft, Natural Sciences and Engineering Research Council of Canada, and European research funding instruments. Major experimental campaigns relied on beam time and infrastructure at the Paul Scherrer Institute muon facilities, with complementary technical contributions from TRIUMF, Rutherford Appleton Laboratory, and accelerator laboratories such as J-PARC and CERN. Detector development and laser systems benefited from collaborations with instrument groups at Max Planck Institute, MIT Lincoln Laboratory, and national metrology institutes like National Institute of Standards and Technology and National Physical Laboratory.

Category:Physics collaborations