Fermilab hosted a workshop dedicated to muon collider physics and detectors on Dec. 14-16, 2022, in which approximately 80 scientists from around the world participated in-person and many more participated remotely due to occupancy limitations. The workshop was organized jointly by the Snowmass Muon Collider Forum and the Fermilab Future Colliders Group. The primary goal was to engage the broader U.S. particle physics community in discussions and development of new ideas, in particular, in the areas of physics and detector development.
The recent resurgence of interest in muon colliders in the high-energy physics community is due to the fact that a multi-TeV muon collider could enable experiments to reach higher energy scales beyond those accessible at the High-Luminosity LHC, in a compact footprint and with high-energy efficiency. Muons, being much more massive than electrons, lose far less energy to synchrotron radiation when traveling in a ring, and unlike protons, muons are elementary particles and their entire energy goes into the energy of the collisions.
It became evident during the recent Snowmass studies in the U.S., that the physics program of a high-energy muon collider is extremely rich. A 8-10 TeV muon collider, which could fit within Fermilab site boundaries, would provide unprecedented sensitivities to new particles and a reach similar to a 60-100 TeV proton-proton collider for new physics processes. It could also help discover or unambiguously exclude dark matter particles under the so-called minimal-DM scenarios all the way to the thermal targets, something that cannot be achieved at current or planned direct detection or collider experiments.
At the same time, a muon collider can also be a precision machine, allowing for detailed exploration of the electroweak sector of the Standard Model and for an order-of-magnitude more precise studies of the Higgs boson than possible at the HL-LHC.
During the workshop, recent developments were presented and discussed by both theorists and experimentalists. Some of them were refined versions of studies that have been done in the past, but many are new ideas, including how to look for heavy neutrinos or to shed light on the g-2 anomaly from experiments at the muon collider.
Realization of a muon collider would require us to overcome many technological challenges. Following the previous European Strategy Update, CERN established an International Muon Collider Collaboration to study the feasibility of a high-energy muon collider. The Snowmass process and deliberations within the IMCC have allowed us to assess significant progress made in the development of high-power targets and of high-field magnets; in the demonstration of operation of radio-frequency cavities in magnetic fields; and of the self-consistent accelerator lattice designs of the various subsystems. Targeted R&D is necessary in order to make further engineering and design progress.
A major challenge for designing detectors for experiments at a muon collider and extracting physics is how to deal with beam-induced backgrounds or BIB. As muons are unstable particles, an unprecedented amount of secondary and tertiary decay products will enter the detector volume of any experiment. These decay products create challenges not only in operations of the detector, but also in distinguishing particles produced in the collision from the cloud of background noise. The detector design, choice of technology and reconstruction algorithms are therefore heavily influenced by the beam-induced background.
Fortunately, there has been major progress recently in the development of collider detector technologies, driven largely by the needs of the LHC high-luminosity upgrades. These new detector technologies allow for highly granular detectors, capable of providing not only spatial information about incident particles but also their precise timing of arrival information, which provides tremendous benefit in background suppression. Also event reconstruction algorithms have advanced significantly and feature advanced machine-learning techniques and much more.
The simulation work carried out at the workshop showed that high efficiency and quality reconstruction of muon collision events is possible. Many students and postdocs participated in the hands-on simulation tutorial offered on the first day of the workshop. Learning simulation tools will enable them to engage in exploring new ideas and directions in detector design.
Despite the workshop’s focus being on physics and detectors, many accelerator physicists also participated. In the end, it is only with close collaboration between accelerator physicists, experimentalists and theorists, that such an ambitious project can be realized. Recent developments in accelerator technology were presented, and an R&D program necessary to pursue for realization of a muon collider was discussed.
Beyond collider physics, another great appeal of the muon collider is that it offers strong synergies with neutrino physics, lepton flavor violation experiments, and searches for low-mass particles via beam dump experiments. The workshop helped the community to begin thinking about how all these ideas can be implemented in a single facility that can potentially be hosted at Fermilab and would offer an unprecedentedly rich and multifaceted physics program. The idea of building a muon collider is particularly popular among the HEP early career community, and a large number of them participated in this workshop.
Since the Fermilab workshop, a two-week workshop dedicated to muon colliders has been held at the Kavli Institute of Theoretical Physics at the University of California, Santa Barbara campus. The KITP workshop brought together theorists, experimentalists and accelerator physicists with the goal of charting a collaborative, international path toward a muon collider. Focal points of the program included coordinating efforts among international groups; developing the physics case for a demonstrator facility; assessing the central needs and opportunities for theory, experiment, and accelerator physics; and building bridges to the broader particle physics community. Several Fermilab scientists actively participated in the workshop, both in-person and remotely.
Mark your calendars for two upcoming workshops:
1) Muon4Future Workshop in Venice, Italy
2) International Muon Collider Collaboration annual meeting in Orsay, France
Sergo Jindariani and Pushpa Bhat lead the Fermilab Future Colliders Group.
Original source can be found here.