The Alternate View
Cern Seeks Magnetic Monopoles
John G. Cramer
The magnetic monopole represents a kind of “unicorn” for the world of particle physics. It is a potentially stable and important could-be particle that, if it existed, would have an isolated magnetic charge, either a “north” source of magnetic lines of force or a “south” sink of force lines. It is the magnetic equivalent of the negative and positive electric charges carried by charged particles like electrons and protons. It fits neatly into several blank spots in an expanded version of the four Maxwell equations that describe electromagnetism.
Lodestone magnets were known in the ancient world. In the 12th century AD the magnetic compass came into use in navigation. It wasn’t until 1831 that Michael Faraday discovered magnetic induction, revealing a connection between electricity and magnetism. All permanent magnets have both a north and a south pole, and there was speculation on whether an isolated magnetic monopole, one magnet pole without the other, might be possible.
In 1931 Paul Dirac applied quantum mechanics to magnetic monopoles and concluded that if monopoles exist, then electric charge must be quantized (as it is!) and magnetic monopoles should have a quantized magnetic charge with minimum value gD, which is the electric charge e times half the inverse of the fine structure constant (gD =~137e/2 = 68.5e).
Many front-line physical theories like string theory, grand unified theories, and inflationary cosmology predict that magnetic monopoles should exist, although all of them are dismayingly vague about just what mass a monopole should have. Theoretically there might be magnetic monopoles in two mass classes, cosmic and fundamental. Cosmology tells us that in the Big Bang’s early stages a small black hole born in a tangle of magnetic flux might emerge with a net magnetic charge, which would render it stable and block its evaporation in a burst of Hawking radiation. Such a composite monopole, a Big Bang leftover, might have a diameter of about a Planck length (1.6 x 10-35 meters) and weigh in at around a Planck mass (2.18 x 10-8 kg or 1.22 x 1016 TeV). In contrast, a Dirac magnetic monopole should be much lighter. It would be a stable pointlike fundamental particle with a mass, as estimated from Standard Model extensions, that might be as low as a few TeV.
Extensions of the Standard Model predict that sufficiently energetic proton-proton collisions might produce Dirac monopoles in pairs (north + south) by two processes. One of these is Drell-Yan pair production, in which a quark and anti-quark annihilate to form an energetic virtual photon, which then turns into a monopole pair. The other process is photon fusion, in which two energetic photons exchange a virtual magnetic monopole, producing a monopole pair. Both processes are technically difficult to calculate, so that the present state of particle theory can provide only very rough estimates of the monopole production probability and mass.
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If Dirac monopoles do exist, they should be stable against decay into lighter particles because they are “stuck” with a net magnetic charge. Once produced, the only way of getting rid of a magnetic monopole would be to annihilate it with another monopole with the opposite magnetic charge (south + north), much as positive electrically charged positrons are annihilated by negative electrically charged electrons. Thus, once created monopoles cannot easily disappear, and so it makes sense for enterprising researchers to search for them in the natural world.
A magnetic monopole moving through space at a high velocity would induce loops of very strong electric field in its wake. A high velocity monopole traveling through matter would look like a highly charged ionizing particle. Thus, tracking detectors, e.g. stacks of special plastics and photographic emulsions that are used to study cosmic rays, can also be used to search for very highly ionizing monopole tracks.
There has been a long history of experimental searches for magnetic monopoles (see “Again Monopoles” and “When Proton Meets Monopole,” my first AV column). In 1962, three MIT physicists hiked to a magnetic iron ore outcropping in the Adirondack Mountains of New York and attempted (without positive results) to suck south monopoles out of the rock formation there, using a pulsed “magnetic vacuum cleaner” they had constructed. In 1973, a group led by P. Buford Price of UC Berkeley reported that a balloon-borne Lexan emulsion stack had recorded what appeared to be a magnetic monopole track. However, this result was later withdrawn when as an error was discovered in the assumed stack thickness.
In 1982, Blas Cabrera of Stanford reported a single event in which a current transient indicated that an object with a single Dirac magnetic charge had passed through the five-turn 20 cm2 superconducting loop of his detector system. In 1985, physicists at Imperial College London, using a similar but improved setup with two superconducting loops, reported another single event that could be attributed to the passage of a single monopole with one Dirac magnetic charge. While there have been many other monopole searches with superconducting loops, these are the only two with seemingly positive results. Physicists have also searched for monopoles trapped in magnetic iron ore, in accelerator beam stops, and in irradiated detector parts. They have also searched for monopole tracks produced by cosmic rays or high-energy reactions at accelerators. So far, despite false alarms, the two superconducting-loop events represent the only could-be detections of magnetic monopoles.
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Several accelerator-based searches for collision-produced magnetic monopoles, combined with extended standard model predictions, have all shown negative results and indicated that either the theoretical expectations are wrong or the magnetic monopole particle is too massive to be produced with the available particle-particle collision energies. Thus, the ongoing operation and energy upgrades of the CERN LHC facility, giving the highest collision energies available in a laboratory setting, provide new opportunities for monopole searches. At this writing, the LHC has has three runs: Run 1 (2009 to 2013, 1.8 to 4.0 TeV per beam at somewhat below design beam intensity), Run 2 (2015 to 2018, 6.5 TeV per beam at up to twice design intensity), and Run 3 (2023 to 2026[planned], 6.8 TeV per beam at up to four times design intensity). Monopole searches at the LHC have been reported by the MoEDAL Collaboration and the ATLAS Collaboration.
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The prototype detector used by the MoEDAL Collaboration to search for monopoles produced by LHC collisions in Run 2 is their Nuclear Track Detector array and Trapping Detector array. The system is basically a set of passive non-triggered stacks of plastic plates and aluminum panels, placed at strategic locations near the LHCb collision vertex point of the LHC, at which the p-p collisions might create magnetic monopoles that reach the detector system.
A collision-created monopole would leave a heavy ionization track in the plastic and might possibly be trapped in the aluminum. At the conclusion of one of the LHC runs, the plates were extracted, processed, and scanned for tracks. The plastic was placed in an alkaline solution that selectively etched away plastic damaged by ionization. The aluminum panels were passed through superconducting-loop SQUID detectors to search for isolated magnetic charges. No evidence of monopole tracks or trapping was found, but the negative result was used to establish upper limits on monopole production probability and lower limits on monopole mass. In particular, 13 TeV p+p data from Run 2 indicates mass greater than 1.8 TeV for monopoles having spin 1 and one gD unit of magnetic charge.
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The ATLAS detector, one of the two large general purpose LHC detector systems, uses an outer toroidal superconducting magnet and an inner solenoidal superconducting magnet to detect and track charged particles produced in LHC collisions. The problem with monopole detection with the ATLAS tracking detector is that while normal charged particles make tracks in the plane perpendicular to the magnetic field, magnetic monopoles follow the field lines, a direction in which the inner particle tracking system has no sensitivity. However, ATLAS is a versatile device, and while its principal tracker is blind to monopoles, its electromagnetic calorimeter and transition radiation tracker can identify particles with high ionization and provide monopole sensitivity.
No evidence of monopole tracks has been found by the ATLAS analysis of data from Runs 1-3, and, as in the MoEDAL case, this negative result is used to set limits on monopole mass and production probability. In particular, ATLAS 13 TeV p+p data from Run 2 was analyzed to set a lower mass limit of about 1.85 TeV for singly charged monopoles with spin 0 and 2.4 TeV for singly charged monopoles with spin ½. These are the most stringent limits on monopole mass so far.
The LHC’s Run 3 is still in progress and is scheduled to continue for two more years. It is expected that when it concludes and data is analyzed (assuming no monopoless are found), the MoEDAL monopole mass limits will increase by about a factor of 2 and the ATLAS monopole mass limits will increase by about a factor of 3.
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This is a science fiction magazine, so let’s consider what monopoles might make possible. If a trapped stable magnetic monopole was ever found, the experimental physicists who could play with it would have a field day. Once particle physicists understood the trick of producing magnetic monopoles, accelerator physicists could construct “monopole factories” that might cheaply mass-produce enough monopoles to allow their use in device construction.
Brushless DC motors and generators would be an immediate possible application. A long magnetic solenoid, i.e., a cylindrical coil of wire carrying a strong electric current, would becomes a powerful accelerator, along which injected monopoles could reach high energies. This might permit particle physicists to study monopole-matter and monopole-monopole collisions in search of new physics. It also suggests a “monopole ray gun” that might be used to blast victims with a beam of highly ionizing monopoles.
Conversely, a spinning tube containing an ensemble of trapped magnetic monopoles would constitute an “electric solenoid,” which would create a sizable electric field along its interior to accelerate ions. This might simplify the construction of ion drives for space applications. Bringing stored north and south monopoles together to annihilate them would release many TeV of energy with each combination. This might provide a convenient way of storing and releasing energy as needed. Or perhaps a “monopole bomb” . . .
And so on. Readers and authors are invited to invent other far-out uses of captured magnetic monopoles not listed above.
References:
- Goto, H. H. Kolm, and K. W. Ford, “Search for Ferromagnetically Trapped Magnetic Monopoles of Cosmic-Ray Origin, Physical Review 132, 387-396, (1963).
- Aad, et al, (The ATLAS Collaboration), “Search for magnetic monopoles and stable particles with high electric charges in √s = 13 TeV pp collisions with the ATLAS detector,” arXiv: 2308.04835 [hep-ex] (2023).
- Acharya, et al, (The MoEDAL Collaboration), “Search for Highly-Ionizing Particles in pp Collisions at the LHC’s Run-1 Using the Prototype MoEDAL Detector,” arXiv:2112.05806v2 [hep-ex] (2022).
John G. Cramer’s 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake—Entanglement, Nonlocality, and Transactions, (Springer, January 2016) is available online as a hardcover or eBook at: https://link.springer.com/book/10.1007/978-3-319-24642-0 or https://www.amazon.com/dp/3319246402. Editions of John’s hard SF novels Twistor and Einstein’s Bridge are available online at: https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/%C2%ADEinsteins-Bridge-John-Cramer/dp/0380788314. Electronic reprints of 207 or more “The Alternate View” columns written by John G. Cramer and previously published in Analog are currently available online at: https://www.npl.washington.edu/av.
Copyright © 2024 John G. Cramer