An experiment was performed to capture fractionally charged particles produced in heavy-ion collisions and to concentrate them in samples suitable for analysis by various techniques. Two of the samples so produced have been searched, with use of an automated version of Millikan\u27s oil-drop apparatus. The beam was 56Fe at 1.9 GeV/nucleon, incident on a lead target. Less than one fractional charge per 1.0× 104 Fe-Pb collisions was found to be produced, and, with further assumptions, less than one per 2.0× 106 collisions

Abrams, Peter C.

Bland, Roger W.

Hahn, Alan A.

Hodges, Christopher L.

Johnson, Robert T.

Joyce, David C.

Knoop, Tanya D.

Lackner, Klaus S.

Lindgren, Michael A.

Pugh, Howel G.

Savage, Maureen H.

Scholz, Marion H.

Shaw, Gordon L.

Slansky, Richard

Young, Betty A.

English

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Santa Clara University Scholar Commons Physics College of Arts & Sciences 10-31-1983 Search for Fractional Charges Produced in Heavy-Ion Collisions at 1.9 GeV/nucleon Michael A. Lindgren David C. Joyce Peter C. Abrams Roger W. Bland Robert T. Johnson See next page for additional authors Follow this and additional works at: https://scholarcommons.scu.edu/physics  Part of the Physics Commons Recommended Citation Lindgren, M. A., Joyce, D. C., Abrams, P. C., Bland, R. W., Johnson, R. T., Knoop, T. D., Savage, M. H., Scholz, M. H., Young, B. A., Hodges, C. L., Hahn, A. A., Shaw, G. L., Lackner, K. S., Pugh, H. G., & Slansky, R. (1983). Search for Fractional Charges Produced in Heavy-Ion Collisions at 1.9 GeV/nucleon. Physical Review Letters, 51(18), 1621–1624. https://doi.org/10.1103/PhysRevLett.51.1621 Copyright © 1983 American Physical Society. Reprinted with permission. This Article is brought to you for free and open access by the College of Arts & Sciences at Scholar Commons. It has been accepted for inclusion in Physics by an authorized administrator of Scholar Commons. For more information, please contact rscroggin@scu.edu. Authors Michael A. Lindgren, David C. Joyce, Peter C. Abrams, Roger W. Bland, Robert T. Johnson, Tanya D. Knoop, Maureen H. Savage, Marion H. Scholz, Betty A. Young, Christopher L. Hodges, Alan A. Hahn, Gordon L. Shaw, Klaus S. Lackner, Howel G. Pugh, and Richard Slansky This article is available at Scholar Commons: https://scholarcommons.scu.edu/physics/41 VOLUME 51, NUMBER 18 PHYSICAL REVIEW LETTERS 31 OcToBER 1983Search for Fractional Charges Produced in Heavy-Ion Collisions at 1.9 GeVlnucleonMichael. A. Lindgren, David C. Joyce, Peter C. Abrams, Roger W. Bland, +' Robert T. Johnson,Tanya D. Knoop, Maureen H. Savage, Marion H. Scholz, and Betty A. YoungPhysics and Astronomy Department, San Francisco State University, San Francisco, California 94132andChristopher L. HodgesPhysics and Astronomy Department, San Francisco State University, San Francisco, California 941', andPhysics Department, University of California, Riverside, California 92521Alan A. Hahn and Gordon L. ShawPhysics Department, University of California, Irvine, California 9271 7Kl,aus S. LacknerStanford Jinear Accelerator Center, Stanford, California 94805andHowel. G. PughLawrence Berkeley Laboratory, University of California, Berkeley, California 94790Richard Sl.ansky7.'heowetica/ Division, I.os A/amos Na, iona/ Iaboxatoxy, Ios A/amos, +em Mexico 87545{Received 22 August 1983)An experiment was performed to capture fractionally charged particles produced inheavy-ion collisions and to concentrate them in samples suitable for analysis by varioustechniques. Two of the samples so produced have been searched, with use of an auto-mated version of Millikan's oil-drop apparatus. The beam was ' Fe at 1.9 GeV/nucleon,incident on a lead target. Less than one fractional charge per 1.0&&10 Fe-Pb collisionswas found to be produced, and, with further assumptions, less than one per 2.0& 10 col-lisions.PACS numbers: 14.80.nq, 25.70.NpSearches for fractionally charged particleshave been made in many kinds of elementary-particle interactions, including searches at e+eand hadron storage rings. ' These experimentstypically look for tracks with ionization belowthe minimum for singly charged particles. Theyhave seen no evidence for fractional charges.%ith the high-energy beams of heavy nucleiavailable at the Beval.ac, it is possible to searchfor fractional charge produced in interactionsinvolving huge numbers of quarks and large had-ronic volumes. ' Processes that involve heavynuclei might yield fractional. charge by mecha-nisms not possible in elementary-particl. e inter-actions. ' That is, where it is difficult to sepa-rate a particle with fractional charge from theremaining colored fragment in the vacuum, theseparation process may be greatly enhanced inthe environment of a quark-gl. uon sea created ina heavy-ion collision.The experiment is done in two steps. The firststage is a scheme to collect and concentrate frac-tional charges produced in heavy-ion collisionsat the Bevalac. In the second stage, the targetand concentrated materials from the Beval.acexposure are brought to one of the existing ex-periments designed to look for free fractionalcharge in bulk material. . These include magneticlevitometers, ' Van de Graaff charge spectrom-eters, ' and droplet experiments. " In this paperwe report the details of the initial Beval.ac ex-posure, and then describe the analysis of thisexposure with the San Francisco State Universityautomated Mil. likan apparatus. Other analysesof the Bevalac materials will be reported else-where.1983 The American Physical Society 1621VOLUME 51, NUMBER 18 PHYSICAL REVIEW LETTERS 31 OcToBER 1983A total of 3& 10"Fe ions of 1.9 GeV/nucleonkinetic energy were incident on a target config-uration consisting of a number of heavy material. s(mainly Pb). Within a 30' cone behind the targetwere 26 five-gal. ion tanks of carbon tetrachl. oride.The layout of the tanks and the composition ofthe target are described more compl. etely inFig. 1. Each tank had a thin central wire ma, in-tained at a potential of + 90 V with respect to itsinside wall. Fractionall. y charged particles thatstop in the tank form fractionall. y charged atomsor molecules which drift to the wire' and aretrapped there by the image charge, since no sub-sequent reaction can make them natural. .The choice of fully ionized Fe was made tooptimize the energy per nucleon, atomic number,and intensity of the beam. The Pb target wasselected to optimize the probability of a centralcollision. Since following the nucl. ear collision,fractional charge may be part of a nuclear frag-ment, the total. collection configuration was de-signed to stop both low- and high-charge frag-ments. Most high-charge fragments (Z & 6) areexpected to stop in the target, while most low-charge fragments pass through the target pl.ates.0Fe BEAM60TARGETCOLLECT IONTANKSFIG. 1. Schematic top view of the experimental setupat the Bevalac showing the target configuration and thestopping tanks of CC14. Some of the five-gallon tanksare arranged in a double layer as denoted by the 2.The 26 centra1 collection wires included Au, In, Cu,Nb, and W wires, each maintained at + 90 V or —90 Vwith respect to the inside wall of its tank. The targetconfiguration consisted of 22 Pb and 9 In (interspersed)wafers &8 in. thick and 3 cm in diameter. Near themiddle of the target there were some Cu plates, total-ing &8 in. in thickness, 10 g of Hg, and several hundredsteel balls, each with a mass of about 0.1 m. The Febeam was 1.5 cm in diameter and was accurately main-tained in the center of the target.into the tanks. The stopping efficiency of thetanks was estimated with use of Monte Carlotechniques and the Bethe-Hooch formula, . Wefind, for a large range of mass and reasonablemomentum distributions for the produced parti-cles, that about 1% of all particles with chargee/3 and about 8% of those with charge 4e/3 arestopped in each of the forward tanks. The col-lection efficiencies of the other tanks decreasewith production angle and distance from the tar-get.We now turn to the analysis of some of thesesamples by the automated Millikan technique. 'One sample was prepared from the 36-p, m-diamcentral gold wires from two of the forward tanks,one of each polarity. Each wire was passedbventy times through a small drop of mercuryin a capillary tube in order to remove the sur-face layer of the wire. These two drops werecombined to form sampleA. In this process thefractional charge is transferred to the mercurywith an unknown efficiency. As this efficiencycould be small, a second sampl. e B was preparedby dissolving the gold wires in some mercuryfrom the target assembly. Thus sample B com-bines collection from the target and the tanks.The measuring apparatus is a modification ofthe Millikan oil-drop experiment in which smalldrops are introduced one at a, time into a meas-uring chamber by a piezoel. ectric drop ejector.The drop's drift vel.ocity in a switched electricfiel. d is measured by timing the passage of animage of the ill.uminated drop over a series ofslits. The measured velocity of a typical dropas a function of time is shown in Fig. 2. Thefield switches from positive to negative at slit39. The change in velocity seen at this pointcorresponds to a charge on the drop of 14e. Thefield returns to its original polarity at sl.it 74,and the velocity returns to nearly its original.value. A change in charge during the measure-ment can be detected by comparing the final ve-locity with the original one. Drops which changecharge during the measurement are identified inthis way and rejected. The occurrence of suchcharge changes is less than one per 1000 drops.Drops with substantial. residual. charge are fur-ther analyzed to ensure that their charge meas-urements are not erroneous. Limits are pl.acedon the initial and final transverse positions, thecharge change, the X' of the ve l.ocity fit, andthe drop radius such that 5')D of all drops wouldfail any one cut. These l.imits remove twentydrops which compose a fairly fl.at distribution1622VOLUME 51, NUMBER 18 PH YSICAL REVIEW LETTERS 31 OcToBER 198320— 'a'~41 +'~15—E0010—0'CLO1C)104O10~-O1o 100 I I I I40Position {slit number)80I I I Il.5Residual charge {units of e)FIQ. 2. Drift velocity as a function of position for atypical mercury drop. Reversals of the electric fieldoccur at slits 39 and 74.of residual. charges between 0.2e and 0.8e. Theradius limit is particularly significant since mostdisturbances that affect the charge determinationalso affect the radius determination. Under goodrunning conditions the radius is constant to O. l/0.The radius test rejects drops which have radiiabout 0.7% too big or too small.To ensure objectivity in the analysis, + e/3 isadded at random intervals to a drop's measuredcharge. There are 236 of these test events, ofwhich 197 remain in the sample after all. limitshave been applied. The test events are identifiedas such and removed from the sample only atthe end of the analysis.Drops can be measured at a rate of 1 sec ',and 5 p, g of mercury can be measured in an hourof good running. The charge on a 6- p. m-diammercury drop is measured to an accuracy ofabout 3.5% of e. To keep the charge on the dropsnear zero, a bias wire is inserted into a col.umnof water which is in contact with the mercurydrop in the drop ejector's tip. The potential ofthis wire (a few volts) is adjusted by the com-puter. Drops with charge between -168 and +16eare accepted.A total of 500 p, g of mercury was measured,200 p. g from samplers and 300 LILg from sampleB. A histogram of residual charge for both sam-ples, a total of 260000 drops, is shown in Fig. 3.The residual charge is the charge in excess ofan integral value. The charge measurement iscalibrated directly from the integral-chargepeaks by assuming that the peak to peak separa-FIQ. 3. Combined distribution of residual chargefor samples A and B. The arrows indicate the posi-tions where measurements of drops containing 1/3-in-teger charges would fall. The curve shown is a Gauss-ian with standard deviation of 0.035e.tion is e. There are no measurements near frac-tional-charge values of e/3 or 2e/3.A limit on the fractional-charge productionrate can be set for sample A by noting that 200p. g is 2%%up of the mercury bead used to rinse thegold wire and by using the calcul. ated l%%uo captureefficiency of the tanks for particles of chargea e/3. If the efficiency in transferring fractionalcharges from the gold wires to the mercury insampl. e A is 100%, then we obtain a productionlimit of less than 5&&10 ' fra.ctionally chargedparticles per coll. i sion with 95% confidence. Ifthe fractional charges were not removed by themercury rinse and remained on the gold wire,then the fractional charges from the two tanksare concentrated instead in sampl. e B. We meas-ured approximately 0.02'%%uo of sample B. Thus,assuming that the mercury rinse did not removethe fractional charges, the 95% confidence levelon the production rate is less than 5&10 ' frac-tionally charged particles per collision. (Morerestrictive limits apply if the produced particl. eshave total charge greater than e/3, for then thestopping efficiency is greater. )Sample B is al.so used to set the l.imit on theproduction of high-Z fractionall. y charged nucl. earfragments, all. of which woul. d be stopped in theportion of the target covered by the beam. Themercury measured from sample 8 correspondsto 1&&10 ' of the target, whi, ch gives a productionlimit of l.ess than 1&&10 ~ fractional charges per1623VOLUME 51, NUMBER 18 PHYSICAL REVIEW LETTERS 31 OcToBER 1983collision with 95'%%up confidence.In summary, we have carried out a first-gen-eration experiment to detect fractionally chargedparticl. es produced in Fe-Pb nuclear collisions at1.9 GeV/nucleon incident beam energy. We detectno &-integra1. l,y charged particles. We can ruleout production of fractional charges in the targetat the rate of one per 1&104 collisions, or undercertain assumptions, to one per 2.0&&10' colli-sions,We thank J. Carroll and collaborators, of Be-valac experiment 6718, for permitting a test ofparts of this experiment, and the staff of theBeva1.ac, especial. ly Fred Lothrop and Bob Mi11.er9for their enthusiastic help. We al.so thank RogerMcWilliams and George Basile for assistance inthe construction of the collection tanks and Cher-ril. l. Spencer for useful discussions. This workwas supported in part by the U.S. Department ofEnergy and by the National Science Foundation.Current address: Laboratoire de Physique Nucle-aire des Hautes Energies, Ecole Polytechnique, Routede Saclay, F-01128 Palaiseau, France.'J. M. Weiss etal. , Phys. Lett. 101B, 439 (1981);W. Bartel et al. , Z. Phys. C 6, 295 (1980); J. Napoli-tano et al. , Phys. Rev. D 25, 2837 (1982); C. W. Fabianet al. , Null. Phys. B101, 349 (1975); M. Basile etal. ,Nuovo Cimento 40, 41 (1977).An experiment performed at the Bevalac by P. B.Price etal. , Phys. Rev. Lett. 50, 566 (1983), set alimit that less than 1 in 300 of the fragments producedin 1.85-6eV/nucleon argon collisions in a plastic trackdetector have fractional charge.3B. Slansky, T. Goldman, and G. L. Shaw, Phys.Rev. Lett. 47, 887 (1981).4G. L. Shaw and R. Slansky, Phys. Rev. Lett. 50,1967 (1983).G. S. LaRue, J. D. Phillips, and W. M. Fairbank,Phys. Rev. Lett. 46, 967 (1981); M. Marinelli andG. Morpurgo, Phys. Rep. 85, 161 (1982); D. Liebo-witz, M. Binder, and K. O. H. Ziock, Phys. Rev. Lett.50, 1640 (1983).6K. H. Chang et al. , to be published; D. Elmore,private communication; R. McKeown, private com-munication.7C. L. Hodges et al. , Phys. Rev. Lett. 47, 1651(1981); P. C. Abrams et al. , in Novel Results in Parti-cle Physics, edited by B. S. Panvini et a/. , AlP Con-ference Proceedings No. 93 (American Institute ofPhysics, New York, 1982), p. 15; D. Joyce et al. ,Phys. Rev. Lett. 51, 731 (1983).G. Hirsch, R. Hagstrom, and C. D. Hendricks,Lawrence Berkeley Laboratory Report No. LBL-13636, 1981 (unpublished) .~A. O. Allen, M. P. de Haas, and A. Hummel, J.Chem. Phys. 64, 2587 (1976).1624

Search for Fractional Charges Produced in Heavy-Ion Collisions at 1.9 GeV/nucleon

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Search for Fractional Charges Produced in Heavy-Ion Collisions at 1.9 GeV/nucleon

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