The Path to an Energy Frontier Muon Collider

Minutes

On May 30, 2025, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,516th meeting of the Society to order at 8:01 p.m. ET. He began by welcoming attendees, thanking sponsors for their support, announcing new members, and inviting guests to join the society. Scott Mathews then read the minutes of the previous meeting which included the lecture by Carey Lisse, titled “Real-Time Observations of Early Solar System Evolution”. The minutes were approved as read.

President Millstein then introduced the speaker for the evening, Mark Palmer, of the Brookhaven National Laboratory. His lecture was titled “The Path to an Energy Frontier Muon Collider”.

The speaker began by saying that particle physics is the exploration of the subatomic particles that make up our universe and the laws that govern their interactions. He briefly described the groups or families of subatomic particles: quarks, leptons, and bosons (or force carriers). Palmer describe the types of high energy physics particle colliders currently in operation, giving examples of each type. These included: proton-proton or proton-antiproton colliders and electron-positron colliders, in both linear and circular configurations. He presented the so called “Livingston Plot”, showing the increase in collision energy as a function of time from the 1970’s to the present day, as well as projections of possible future colliders.

The speaker provided an overview of multi-TeV machine concepts. These included: TeV-class conventional linear colliders, proton-proton energy frontier machines, lepton-ion machines, and lepton-lepton energy frontier machines. He showed schematic diagrams and gave details about some of the potential designs of a proton-proton collider. These included:

• The Future Circular Collider or FCC, with a circumference of 100 km, a CoM (center of mass) collision energy of 100 TeV, and 16 Tesla dipoles
• The SppC, a Chinese version of the FCC, also with a circumference of 100 km, a CoM collision energy of 125 TeV, and 20 Tesla dipoles
• And the “Collider-in-Sea”, which would be constructed underwater in the Gulf of Mexico (or perhaps the Gulf of America), with a circumference of 1,900 km, a CoM collision energy of 500 TeV, and 3.5 Tesla dipoles

Palmer then discussed energy frontier lepton colliders, including muon colliders and wake field accelerators. He discussed the technical maturity of each of these approaches, indicating that “almost none of these machines are ready for construction today.”

The speaker then discussed the potential advantages of a muon collider. These included:

• intense, cold beams, which enable measurement of lepton flavor violation, anomalous magnetic moments, and act as a precision source of neutrinos
• new opportunities in high energy physics, by providing strong coupling to the Higgs boson, reduced synchrotron radiation, and beams with a very narrow energy distribution
• and, collider synergies, in the areas of long-baseline neutrino experiments and materials science

Palmer discussed some of the challenges associated with muon colliders. These included the short lifetime of the muons, the ionizing radiation from neutrinos, the need for a liquid target, and the decay particles which must be distinguished from the collision products. He gave an equation for the luminosity of a muon beam, indicating that for a 10 TeV accelerator, there would be 2 x 1012 particles/bunch, occurring at 5 bunches/sec, in a 1 um diameter beam, resulting in a luminosity of approximately 1035 per cm2 per second.

The speaker discussed the history of muon colliders, saying that the idea was first proposed in 1969, and further developed at the 1996 Snowmass Meeting. During the 2000’s, the concept of combining a muon collider and a neutrino factory was discussed, and in 2010 the Muon Accelerator Program (or MAP) was proposed in the US. In 2014, the MAP effort was ramped down, largely due to interest in the neutrino and the Higgs boson. In 2018, renewed interest in muon colliders was expressed in the European Particle Physics Strategy Update. In 2023 the US P5 report emphasized the need to explore a 10 TeV collider and the so-called “Muon Shot”.

Palmer showed a schematic diagram of a proposed muon collider. It consisted of a proton driver, a “front end” with the target and buncher, an ionization cooling stage, an accelerator stage, and a collider ring. He claimed that this type of collider could provide an energy efficient route to the 10 TeV energy frontier.

The speaker ended his talk by discussing several of the technological developments that would be required to ensure the feasibility of a muon collider. He said that the community is aiming for a reference design by the end of the decade.

The lecture was followed by a Question and Answer session.

A member asked about the ionizing radiation produced by neutrinos. Palmer responded that while neutrinos rarely interact with matter, the intensity and energy of the neutrinos from a muon collider would be sufficient to cause ionizing radiation and could therefore constitute a health risk.

A member on the live stream asked how many Higgs particles have been generated and observed, and how do those numbers compare to a “Higgs factory”. Palmer responded that he did not know the exact numbers, but that the idea of a Higgs factory was to dramatically increase the number of particles observed, and not necessarily to increase the number being produced.

A member asked about the benefits a muon collider could provide to the materials science community. Palmer responded that muon spin resonance studies are of particular interest to materials scientists, and that there are currently very few machines capable of making such measurements.

After the question and answer period, President Millstein thanked the speaker and presented him with a PSW rosette, a signed copy of the announcement of his talk, and a signed copy of Volume 17 of the PSW Bulletin. He then announced speakers of up-coming lectures and made a number of housekeeping announcements. He adjourned the 2,516th meeting of the society at 10:01 pm ET.

Temperature in Washington, DC: 17.8° Celsius
Weather: Partly cloudy
Audience in the Powell auditorium: 33
Viewers on the live stream: 38
For a total of 71 viewers
Views of the video in the first two weeks: 336

Respectfully submitted, Scott Mathews: Recording Secretary