Physics Today Cabrera counts flux quanta to find a Dirac monopoleGloria B. Lubkin Citation: Physics Today 35(6), 17 (1982); doi: 10.1063/1.2915122 View online: http://dx.doi.org/10.1063/1.2915122 View Table of Contents: http://scitation.aip.org/content/aip/magazine/physicstoday/35/6?ver=pdfcov Published by the AIP Publishing

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Cabrera counts flux quanta to find a Dirac monopoleTake a superconducting loop, place it inan ultralow magnetic-field device, andmonitor the current with a Supercon-ducting Quantum Interference Devicefor many months. That was the experi-ment that Bias Cabrera of StanfordUniversity did in his search for a mov-ing magnetic monopole. On Valen-tine's Day he found a single eventconsistent with one Dirac unit of mag-netic charge. When rumors of the ob-servation spread, the organizers of theThird Workshop on Grand Unificationphoned Cabrera in Blacksburg, Virgin-ia (where he was giving a colloquium),and asked him to address the Work-shop the next morning (16 April).After a long, wee-hours-of-the-morningdrive to Chapel Hill, N. C, Cabreraelectrified the workshop participantswith his careful, low-key account of hispossible monopole discovery.

One noted theorist remarked, "Wecame to scoff and stayed to praise. Itwas a very impressive glitch. Oneshouldn't be convinced by one event.But it's about as impressive as oneevent can be."

By using a superconducting ring, theCabrera experiment can detect a mov-ing magnetic charge solely from thelong-range electromagnetic interac-tions between the magnetic charge andthe macroscopic quantum state of the

ring. Such a detector is insensitive tothe monopole velocity, mass, electriccharge or magnetic dipole moment.

A similar approach was used in 1970by Philippe Eberhard, Ronald Ross andLuis Alvarez (Berkeley) and RobertWatt (SLAC). They circulated moondust (collected by the Apollo 11 astro-nauts, Neal Armstrong, Edwin Aldrinand Michael Collins) through a super-conducting solenoid and looked for theemf generated. Cabrera's paper waspublished in the 17 May issue of Phys-ical Review Letters.

The existence of magnetic monopoleswas proposed by Paul Dirac in 1931 inan attempt to explain the existence ofthe smallest electric charge, e. Dirac'squantization condition said that thestrength of a single magnetic pole, g,would be restricted by the relation

eg = fe/2

In 1948 Dirac showed that if a magneticcharge exists, the integrity of quantummechanics, and in particular the quan-tization of angular momentum, is vio-lated unless there is a smallest electriccharge.

In remarks prepared for the Mono-pole Meeting held in Trieste last De-cember, Dirac said, "I am inclined nowto believe that monopoles do not exist.So many years have gone by without

any encouragement from the experi-mental side."

But practically all previous searchesfor monopoles—in cosmic rays, at parti-cle accelerators, and in matter—wouldnot have detected an extremely mas-sive monopole. Currently populargrand unified theories predict the exis-tence of monopoles, which are indeedextremely massive—10 ~8 grams or1016 GeV.

In 1975 P. Buford Price, EdwardShirk (Berkeley), Zack Osborne andLawrence Pinsky (University of Hous-ton) reported finding in cosmic rays amonopole with twice the Dirac strengthand a mass at least 200 proton masses(PHYSICS TODAY, October 1975, page 17).The Price group later withdrew itsclaim to have found a monopole.

Cabrera's way to detect a monopole isto use the quantized flux in a supercon-ductor. The flux quantum <pQ = hc/2egives a direct measurement of magnet-ic charge.

Cabrera's apparatus is a four-turn, 5-cm-diameter loop made of niobium, po-sitioned with its axis vertical; the loopis connected by twisted pair leads to thesuperconducting input coil of a SQUIDmagnetometer. If a single Dirac chargepasses through the loop, one wouldexpect an Bxj>0 change in the fluxthrough the superconducting circuit,

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Candidate monopole event found by Bias Cabrera of StanfordUniversity. If a single Dirac magnetic charge passes through the

detection loop, one would expect an 8<A0 change in the flux throughthe superconducting circuit.

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consisting of the detection loop and theSQUID input coil (a factor of 2 from4irg = 2<f>0 and of 4 from the number ofturns in the pick-up loop).

The SQUID and loop are mountedinside a 20-cm-diameter, 1-meter-longcylindrical superconducting shieldclosed at the bottom. Surrounding thesuperconducting shield is a single mu-metal cylinder that reduces the Earth'sfield to a few milligauss. The combinedshielding provides 180 db isolationfrom external magnetic-field changesand an ambient field of 5x 10 " 8 gauss.A flux quantum in this size cylinderwould produce a field of about 3x 10"9

gauss. So, Cabrera told us, the cylinderhas a few unpaired vortices. Cabrerahad used a similar ultralow field devicefor his PhD research (which he earnedin 1974 under William Fairbank's di-rection).

[Bias Cabrera is a third-generationphysicist. His father, Nicolas, is a sol-id-state physicist who spent 23 years atthe University of Virginia (where hisson earned his BS), and is now at theAutonomous University of Madrid.The father of Nicolas, also named Bias,specialized in magnetism and was anorganizer of the 1930 Solvay Congress,along with Niels Bohr, Marie Curie andAlbert Einstein. He is sometimes cred-ited with founding modern experimen-tal physics in Spain.]

To make the superconducting shield,Cabrera takes an expandable lead foil,60 microns thick, and cools it to liquid-helium temperature while it is accor-dion-pleated into a long, thin ribbon.When the foil container becomes super-conducting, it has trapped the ambientfield. Then Cabrera uses a mechanicalplunger to expand the foil into a cylin-der, still keeping it at liquid-heliumtemperature. During the expansion,the magnetic field that occupied thevolume of cylinder before expansion isexpelled from the volume by inducedsupercurrents on the surface, leaving alower magnetic field inside the foilcontainer. Although some trappedfield remains inside the shield becauseof the folded configuration, the insideof the cylinder has a field a factor of 10-100 lower than the field outside.

Then Cabrera takes a second foldedshield ribbon, placed inside a coolingtube, which can be thought of as avacuum jacket with a long, glass tube.The tube allows him to control thetemperature of the inner shield whilethe outer shield is kept superconduct-ing. He lets warm helium gas flowthrough the glass tube, then slowlyreduces the flow so that the innershield is cooled through its transitiontemperature. Because the inner shieldis in the lower magnetic field providedby the previously expanded shield, astill lower field is trapped. He contin-ues this bootstrap process, so that after

Monopole-search apparatus. The loop isconnected to the input coil of a SQUID magne-tometer. The combined shielding provides180 db isolation from external field changesand an ambient field of 5x10~ 8 gauss.

three or four expansions, the field in-side the apparatus is down to 5xlO~8

gauss. Then Cabrera lowers his appa-ratus into the ultralow field regionthrough an airlock. (Stanford has foursuch ultralow field devices operating;two of them are used for the gyrorelativity experiment.) The Dewar ismaintained continuously at liquid-heli-um temperature.

The four-turn loop is a flip coil thatcan be rotated 180° in the plane of thecoil (to allow calibration of the absolutemagnetic field). Cabrera had been us-ing the facility for an experiment tomeasure h/me. In his monopolesearch, he left the coil in a fixed posi-tion with its axis coincident to theshield axis.

By mid-May, Cabrera had been moni-toring the dc current in the supercon-ducting loop continuously for over 200days and had seen only one candidateevent—recorded at 1:53 PM on Sunday,14 February, when the lab was unoccu-pied. Cabrera found the event an hourand a half later, when he read his stripchart recorder.

In his Phys. Rev. Letter, Cabrerareports that the event is consistentwith the passage of a single Diraccharge within a combined uncertaintyof ± 5%. It is the largest event of anykind in the record. He found a total of27 events exceeding a threshold of 0.2<j>0, which remain after excludingknown disturbances such as transfersof liquid helium and nitrogen. He de-fines an event as a sharp offset with

well-defined stable levels for at leastone hour before and after. Only sixevents were recorded during the 70% ofthe running time when the lab wasunoccupied.

Sources of error. Discussing possiblesources of spurious detector response,Cabrera says line-voltage fluctuationscaused by two power outages and ac-companying transients did not causedetectable offsets. Radio-frequency in-terference from the motor brushes of aheat gun failed to produce any offsetswhen operated close to the detector.The critical current of the loop is notreached for currents 1000 timesgreater. No seismic disturbance oc-curred on 14 February.

Could an energetic cosmic ray hittingthe wire cause a piece of it to gonormal? Cabrera says that such rayscould deposit as much as 1 GeV/cm intraversing the wire, raising the localwire temperature by about 0.01 K. Buta 5-K change would be needed to reachthe critical temperature.

Cabrera has intentionally generatedmechanically induced offsets by hittingthe detector with a screwdriver handle,for example. Out of 25 attempts, twoproduced offsets up to 6</>0, but thesignal was not clean enough to mimic amonopole event, Cabrera told us.There was always an overshoot andsettling-in period lasting several hours.

The only reasonable occurrence thatwould mimic a monopole passage, Ca-brera feels, is a spontaneous internalstress-release mechanism. If the turnsin the loop shifted with respect to eachother, the inductance would changeand produce a current change. A 1%shift in inductance could mimic the jevent. I

Cabrera says his signal-to-noise levelapproaches 15:1 for the passage of asingle Dirac charge. The 1970 experi-ment of Alvarez and his collaboratorswas not sensitive to a single Diraccharge in a single pass. So they circu-lated their moon-rock samples severaltimes through the superconducting coilto increase the signal-to-noise ratio.Cabrera told us that the advantage ofthis type of experiment, compared toother monopole searches, is that a par-ticle with any velocity or mass carryinga magnetic charge affects the experi-ment in the same way. The coupling isonly through long-range electromag-netic fields. Although his present sys-tem, with its low bandwidth, is unableto discern the expected monopole veloc-ity (about 10 " 3 c), some SQUID systemsbuilt for research have been made withsufficient bandwidth.

Improved experiment. Cabrera, Mi-chael Taber, Susan Felch, RobertGardner and John Bourg are building anew detector with three mutually orth-ogonal loops wound on a spherical Py-rex bulb. Unlike the flip coil, the three-

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loop arrangement is highly stablemechanically. For hypothetical mono-poles passing through the loops, about70% of the trajectories will intersect atleast two of the loops, providing coinci-dence information. The total area isten times that of the flip coil. Anadditional factor of five in effectivedetecting area is gained because Ca-brera also will be able to detect near-miss trajectories that do not passthrough because they would cause afield change within the ultralow-fieldshield. Cabrera expects that the groupwill be taking data with the new equip-ment beginning this month.

Cabrera stresses that the event hehas reported is "not yet a discovery.It's an interesting event. We're work-ing hard with new apparatus to seedefinitively if there are such particles.It's tough to be very extravagant withonly one data point. But it's also diffi-cult to make it go away. We're caughton a knife edge."

Theory. In 1974 Gerard 't Hooft (Uni-versity of Utrecht) and A. M. Polyakov(Landau Institute for Theoretical Phy-sics, Moscow) independently showedthat if you have a non-Abelian gaugegroup that is semisimple and that spon-taneously undergoes a breakdown ofscale, such theories have solutions cor-responding to Dirac magnetic mono-poles. They further showed that themass of the monopole would be thecharacteristic scale of symmetry break-ing divided by the fine-structure con-stant. That same year Howard Georgi,Helen Quinn and Steven Weinberg (allthen at Harvard) showed that for alarge class of grand unified theories,the unification scale—the region wherestrong, weak and electromagnetic cou-plings become equal—is about 1014

GeV. All the grand unified theorypredictions for monopole mass are inthe ballpark of a hundred times theunification scale, that is 1016 GeV.

If any monopoles exist now, presuma-bly they would have been produced inthe very early Universe, about 10 ~36

sec after the Big Bang, at a time whenthe unified interactions break apartinto strong, weak and electromagnetic.

Cabrera cites an observational upperbound on the mass density of mono-poles to be given by the local "missingmass." This limit is in the range 0.03-0.05 solar masses/cubic parsec. Onecan assume that because the monopolesare so massive their velocities would beno greater than about 300 km/sec (asAlvarez says, just sauntering by anatom so that little or no ionizationtakes place). Then Cabrera estimatesthat the number of monopoles passingthrough the Earth's surface would be4xlO~10 cm^sec - ' s t e r - 1 . Such aflux would yield 1.5 events per yearthrough his loop.

In 1969 Eugene Parker (University of

Chicago) pointed out that if monopolesare distributed throughout the galaxy,they would leach energy out of thegalactic magnetic field and soon des-troy the field. Some argue that mono-poles are concentrated locally, but it'sdifficult to produce a mechanism forthis concentration. At the GUT work-shop in April, Glashow mentioned anidea developed by him, Savas Dimopou-los, Edward Purcell (Harvard) andFrank Wilczek (University of Califor-nia, Santa Barbara). They speculatethat the monopole flux arriving onEarth originates in the Sun. Alvarezhas called attention to measurementsof the Sun's magnetic field for the lastfive "quiet periods" that once led JohnWilcox (Stanford) to write a 1972 papercalled, "Why does the Sun sometimes

look like a magnetic monopole?" A fewmonths ago Alvarez calculated, assum-ing those measurements are correct,that the monopoles in the Sun musthave masses greater than 1012 GeV tokeep them together by gravitationalattraction in spite of their mutual re-pulsion.

Perhaps monopoles collect at the cen-ter of the Earth, too, Alvarez specu-lates. You couldn't hope to put a mono-pole on a table and experiment with it.As Paul Frampton (University of NorthCarolina) points out, "Because of itsenormous mass, the thing would beunimpressed by a table. Earth's gravi-tational pull could be greater than theelectromagnetic force between a mono-pole and a typical atom. So the mono-pole would go straight through."—GBL

Hunting neitron-antineutron oscillationWhen Murray Gell-Mann and Abra-ham Pais predicted in 1955 that the K°meson should exhibit an oscillatingprobability for metamorphosis into itsantiparticle, the K°, they cited the neu-tron as a counterexample. The conser-vation of baryon number, they pointedout, would prevent the neutron-anti-neutron analog of neutral-kaon oscilla-tion. But nowadays, with grand unifiedtheories of the elementary particlesvery much in favor, all bets based onbaryon-number conservation are off. Aprodigious experimental effort (PHYSICSTODAY, January 1980, page 17) attests tothe widespread expectation that protondecay will be seen with a lifetime ofabout 1031 years.

But the same SU(5) grand unifiedtheory that predicts this finite protonlifetime does not (in its simplest form)permit neutron oscillation. The SU(5)unification of quarks and leptons in asingle gauge-theoretic framework, pro-posed by Howard Georgi and SheldonGlashow (both at Harvard) in 1974,replaces baryon conservation by theconservation of B — L, the differencebetween baryon and lepton number,thus forbidding n<^n, the neutron-oscillation transition.

There are, however, rival unificationtheories. Robert Marshak (VirginiaPolytechnic Institute) and RabindraMohapatra (City College of New York)have recently put forward1 a "partial(as distinguished from grand) unifica-tion theory" based on a left-right sym-metric electroweak scheme which,when joined to the usual color-SU(3)group of quantum chromodynamics,yields a color-SU(4) group structuremathematically similar to an earlierproposal of Jogesh Pati (University ofMaryland) and Abdus Salam (Interna-tional Centre for Theoretical Physics,Trieste, and Imperial College, London).

In contrast to the SU(5) grand unifica-tion, the Marshak-Mohapatra theorypredicts neutron oscillation on a timescale that may well be accessible toexperiment. Proton decay, on the oth-er hand, is not permitted in the Mar-shak-Mohapatra partial unificationscheme.

One could complicate the spontane-ous-symmetry-breaking mechanism inthe SU(5) theory to get neutron oscilla-tions at an observable level. But suchdepartures from "minimal SU(5)"would deprive the theory of much of itssimple elegance, Georgi contends. "Iwould be greatly surprised if we findneutron oscillation," he told us. "But ifwe do, it will be most instructive."

Aesthetic prejudices notwithstand-ing, Lay-nam Chang (Virginia Poly-technic Institute) and Ngee-pongChang (City College of New York) haveproposed2 a modification of SU(5) thatwould permit both neutron oscillationand proton decay at observable levelsby extending the theory's minimalsymmetry-breaking mechanism to in-clude Higgs bosons with masses around104 or 10s GeV.

Because neutron oscillation playssuch a central role in choosing amongcompeting unification theories, a num-ber of experimental attempts to lookfor this exotic phenomenon are now invarious stages of planning, and onegroup has recently announced prelimi-nary results. At the International Con-ference on Baryon Nonconservation,held in Bombay in January, MillaBaldo-Ceolin (University of Padua) re-ported the first results obtained by aCERN, Laue-Langevin, Padua, Ruth-erford, Sussex collaboration using thecold-neutron facilities of the Laue-Lan-gevin Institute at Grenoble. Their firstnull finding—that the characteristicneutron oscillation time is greater than

PHYSICS TODAY / JUNE 1982 19

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