/SLAC

Halyo, Valerie

Kim, Peter C.

Lee, Eric R.

Lee, Irwin T.

Loomba, Dinesh

Perl, Martin L.

U., /New Mexico

U., /Princeton

UNT Digital Library

Work supported in part by US Department of Energy contract DE-AC02-76SF00515Search for Fractional-Charge Particles in Meteoritic MaterialPeter C. Kim, Eric R. Lee, Irwin T. Lee, and Martin L. Perl*Stanford Linear Accelerator Center, Stanford University, 2575 Sand Hill Road, Menlo Park, California 94025, USAValerie HalyoDepartment of Physics, Princeton University, Princeton, New Jersey 08544, USADinesh LoombaDepartment of Physics, University of New Mexico, Albuquerque, New Mexico 87131, USA*We have used an automated Millikan oil drop method to search for free fractional-charge particles in asample containing in total 3.9 mg of pulverized Allende meteorite suspended in 259 mg of mineral oil.The average diameter of the drops was 26:5 m with the charge on about 42 500 000 drops beingmeasured. This search was motivated by the speculation that isolatable, fractional-charge particlesproduced in the early Universe and present in our Solar System are more likely to be accumulated inasteroids than on Earth‘s surface. No evidence for fractional-charge particles was found. With 95%confidence, the concentration of particles with fractional-charge more than 0.25 e (e being the magnitudeof the electron charge) from the nearest integer charge is less than 1:3 1021 particles per nucleon in themeteoritic material and less than 1:9 1023 particles per nucleon in the mineral oil.Since the electron‘s identification and the measurementof its charge [1] e about 100 years ago, there has been thequestion as to whether there are other isolatable elementaryparticles with fractional charge such as, for example, 0:5e or  e. No known physics law requires the unit ofelementary particle charge to be unique; hence, the exis-tence of fractional-charge, isolatable, elementary particlesis an experimental question. Of course the quarks are takento have fractional charge 2=3 or 1=3, but they are notisolatable.Since about 1950, there have been many searches forfractional-charge, isolatable, elementary particles using avariety of search methods: detection in cosmic rays, pro-duction in accelerators, production in colliding beamsmachines, searches in bulk matter [2–14]. All but twosearches reported null results. McCusker claimed detectionof free quarks in early cosmic rays searches [12], but thishas not been confirmed in much more extensive modernsearches. Using a superconducting levitometer method,LaRue et al. [11] reported the existence of fractional-charged particles in niobium, but Smith et al. [13] foundno such particles using a ferromagnetic levitometer on a5 times larger niobium sample.Two searches for fractional-charge particles in meteor-itic material have been carried out: the 1989 ferromagneticlevitometer search by Jones et al. [14] and the searchreported in this Letter. Jones et al. searched through threesamples of meteoritic material—Hoba iron-nickel, ForsythCounty iron-nickel, and Murchison stoney—with a totalmass of 2.8 mg. No fractional-charge particles were found.If isolatable, fractional-charge particles exist in nature,then there should be some relic abundance left over fromthe early Universe. Where they would be located on Earthdepends strongly on their chemical properties [15,16]which require assumptions about their charge and mass.Geochemical and geophysical processes occurring duringEarth’s early history generally produced differentiation bymass, with heavier atoms generally tending to sink deepinside Earth. Null results from accelerator and collidersearches for fractional-charge particles indicate that theyare likely to be heavy; hence, they may have sunk deep intothe Earth during its early formation history. However, thereare important exceptions to differentiation by density; forexample, minerals rich in uranium, plutonium, and leadhave been pushed back to the surface of Earth by geo-chemical processes at the crust-mantle boundary. We havewanted our search to be as model independent as possible,and therefore these considerations have motivated us totarget our search to the carbonaceous chondritic meteor-ites, which are some of the most primordial and unpro-cessed sources of materials inside the solar system. At over4.5 Gyrs old, these meteorites are some of the oldest datedobjects in the solar system and, indeed, are used to date thesolar system. It is also believed that they have undergonevery little processing since formation; therefore, little dif-ferentiation within the meteoritic material has occurred.Other than the loss of volatile elements, the initial relativeelemental abundances have been maintained in these ob-jects. Thus, any fractional-charge particles initially boundin these meteorites should be there at roughly the sameabundances today. Finally, their elemental abundances arebelieved to be representative of the material from whichthe solar system collapsed. That is, data from these mete-orites together with that from the Sun’s photosphere areSLAC-PUB-12940October 2007Submitted to Physical Review Lettersused to specify the initial elemental abundances of the solarsystem. If fractional charge particles exist in nature, theseobjects are likely to contain them.In our search, we used a commercially purchased samplefrom the Allende carbonaceous chondrite meteorite [17].The meteorite was first ground by hand and then furtherpulverized using a jet pulverizer, giving a range of powdersizes from 0.04 to 6:0 m. It is an art, somewhat hit andmiss, to make a colloidal suspension using a very diversemineral powder. After much experimentation, using theEdison method [18], we found we could suspend 1.5%by weight powder in a mixture of 5% by volume ofCastrol brand 10W30 weight motor oil and 95% by volumeof Ultraol 50 NF [19]. We were not able to significantlyincrease the proportion of meteorite in the suspension.Our development of an automated Millikan oil dropmethod to search for free fractional-charge particles beganin 1994 [20] with some of the inspiration for a modernMillikan oil drop apparatus coming form the earlier workof Bland and his colleagues at San Francisco StateUniversity [21,22].The principle of our measurement of drop charge is thata drop of radius r, density , and charge Q falls in air (zdirection) through a horizontal (x direction) electric field ofstrength E. From Stokes‘ law, the horizontal terminalvelocity, vx, is vx  QE6r (1)where  is the viscosity of air. Hence, measuring vx givesQ providing r is known. The drop radius is determinedfrom the vx of integer charge particles. The measurementof Q does not depend on the density of the drop and is alsoindependent of the gravitational force on the drop. Theelectric field alternates in the x and x direction so thatthe drop is moved back and forth along the x axis. If thedrop were falling in still air, the vertical terminal velocitywould be given by vz;term  2r2g9(2)where g is the acceleration of gravity. However, we use anupward flow of air of velocity vair in the z direction.Hence, the net downward velocity of the drop is vz  2r2g9 vair: (3)We want vz to be small; hence, we set vair to be close tovz;term but slightly smaller.The apparatus for this search, shown schematically inFig. 1, consisted of the Lee et al. [23] apparatus we usedpreviously, augmented (a) by a device, called a neutralizer,[24,25] that brings the average drop charge to zero and(b) by increased automatic computer control of the droptrajectories. The piezoelectrically driven, drop-on-demanddrop generator [26] produced 0.833 drops per second withan average drop diameter of 26:5 m and a short term (onehour) diameter spread of 0:016 m (0.06%). The datawere analyzed in 1 h segments, so a gradual long-termdrift in the average drop diameter, which did in fact occur,was acceptable so long as the drop diameter was stableover durations of the order of hours.The drops emerging from the generator had chargesspread over  several thousand e [24]. However, theprecision measurement of drop charge required a verymuch smaller charge spread centered at zero charge. Theneutralizer we built [25] consisted of a still air verticalspace through which the drops fell, the air being continu-ously ionized by a 200 C 90Sr source. A horizontalelectric field is produced across the vertical space by metalplates, the total voltage, V, can be varied from 3 to 3volts. By adjusting V, we obtained a Gaussian drop chargedistribution centered close to zero with an rms of about15e. The drops leaving the neutralizer fall directly into thecharge measurement chamber.As a drop falls though the measurement chamber, thereare nimages independent measurements of its charge,nimagesave  8. The motion of the drops in the measurementchamber is recorded using a CCD through an f=11,135 mm focal length lens. The image of the drops isproduced by a 10 Hz, stroboscopic light source made of20 red LEDs. The CCD pixel images of the drops iscomputer processed and stored on a hard disk as describedin Lee et al. [23].Since there is a slow drift in the diameter of the ejecteddrops between drop ejector servicing due to debris build upFIG. 1. Search apparatus.in the ejection aperture, drop charge measurements weregrouped in intervals of 1 h each for purposes of analysisand initial statistical cuts. Within each 1 h analysis interval,drop diameter consistency, trajectory, mean charge andcharge spread must be within preset intervals, or the hourrun is rejected. Within each good 1 h interval, each indi-vidual fluid drop must pass cut criteria based on consis-tency of multiple charge measurements of the same drop,mean drop charge, and physical location of the drop withinthe imaging field.In addition to storing the data for offline processing, thecomputer system measures in real time the drop chargedistribution and keeps the distribution centered by varyingthe voltage in the neutralizer. The computer system alsodetermines in real time the average drop trajectory in themeasurement chamber and corrects the trajectory, if nec-essary, by moving the drop generator horizontally.The search began on January 15, 2004, and ended onDecember 31, 2006, with 42:5 106 drops producedwhich passed all measurement cuts. In a search for rareparticles, the experimenter must avoid the bias that occurswhen the full data set is examined, and then the data se-lection criteria are changed so that a statistical fluctuationbecomes significant. We avoided this bias by substantiallysetting the drop selection criteria using early data and thenlooking at newly accumulated data every several months tosee that the apparatus was running properly. The percent-age of cut events is ill defined since a malfunctioning dropgenerator can skip or eject multiple off axis drops withrandomly determined detectability. Using the criteria of100% up time at an ejection rate of 0.833 drops per second,the experimental apparatus could hypothetically havebeen able to measure the charges on 77:8 106 drops inthe absence of any measurement cuts. Combining bothhardware downtime and measurement cuts, the chargesof 42:5 106 drops were measured in the actualexperiment.After the application of these criteria, we had a final datasample of 42 537 104 drops of average diameter 26:5 m.Figure 2 shows the charge distribution in units of e. We seesharp peaks at integer numbers of charges and no dropsfurther than 0.25 e from the nearest integer. We emphasizethat there is no background subtraction here; this is all thedata after the application of the criteria previouslydiscussed.To show the shape of the peaks at integer values of q, wesuperimpose them in Fig. 3 using the charge distribution,qc, defined by qc  q Nc where Nc is the signed integerclosest to q. The peaks have a near Gaussian distributionwith a standard deviation of 0.042 e.In Fig. 4, we superimpose the valleys between the peaksusing the residual charge distribution, qr, defined by qr jqj  Nl where Nl is the largest integer less than jqj. Wedid not find any drops with residual charge between 0.25 eand 0.75 e.In conclusion, with 95% confidence, the concentrationof particles with fractional-charge more than 0.25 e fromthe nearest integer charge is less than 1:3 1021 particlesper nucleon in the meteoritic material and less than 1:91023 particles per nucleon in the mineral oil. The totalmass of our meteoritic material is 3.9 mg. Thus, our nullresults agree with that of Jones et al. [14], and the twosearches have covered a variety of meteoritic materialswith a total mass of 5.7 mg. The 259 mg of mineral oil isthe largest, fractional-charge search sample used in theMillikan oil drop method or the magnetic levitometermethod.We do not plan to continue the search for fractional-charge particles using our drop method because the nextsearch should use samples a hundred times larger, but wehave not found a practical method to accomplish thisdream. We have considered the expensive possibility ofbuilding and simultaneously operating a hundred copies ofour present apparatus, but we have not solved the problemof insuring that all the machines are operating correctly.We think the next step in meteorite searches should be aFIG. 2. The q charge distribution in units of e.FIG. 3. The qc charge distribution in units of e.ferromagnetic levitometer using full computer system au-tomatic data acquisition and control. At present, we do nothave the time or resources to carry out this proposal.We are very grateful to our collaborators in previousfractional-charge searches: Charles Hendricks, KlausLackner, Nancy Mar, Howard Rogers, and Gordon Shaw;and to student programmers Adie Jain and Jason Seidman.This work was supported by Contract No. DE-AC02-76SF00515 with the U. S. Department of Energy.*martin@slac.stanford.edu[1] R. A. Millikan, Phys. Rev. 32, 349 (1911).[2] L. W. Jones, Rev. Mod. Phys. 49, 717 (1977).[3] M. Marinelli and G. Morpurgo, Phys. Rep. 85, 161 (1982).[4] L. Lyons, Phys. Rep. 129, 225 (1985).[5] P. F. Smith, Annu. Rev. Nucl. Part. Sci. 39, 73 (1989).[6] M. L. Perl and E. R. Lee, Am. J. Phys. 65, 698 (1997).[7] M. L. Perl et al., Int. J. Mod. Phys. A 16, 2137 (2001).[8] M. L. Perl et al., Int. J. Mod. Phys. A 19, 2595 (2004).[9] M. Marinelli and G. Morpurgo, Phys. Lett. B 137, 439(1984).[10] G. Morpugo et al., Nucl. Instrum. Methods 79, 95 (1970).[11] G. S. LaRue et al., Phys. Rev. Lett. 46, 967 (1981).[12] B. McCusker, The Quest for Quarks (CambridgeUniversity Press, Cambridge, 1983).[13] P. F. Smith et al., Phys. Lett. B 171, 129 (1986); P. F. Smithet al., Phys. Lett. B 181, 407 (1986); P. F. Smith et al.,Phys. Lett. B 153, 188 (1985).[14] W. G. Jones et al., Z. Phys. C 43, 349 (1989).[15] K. S. Lackner and G. Zweig, Phys. Rev. D 28, 1671(1983).[16] K. S. Lackner and G. Zweig, Novel Results in ParticlePhysics (Vanderbilt, 1982), edited by R. S. Panvini,S. Alam, and S. E. Csorna (AIP, New York, 1982), p. 1.[17] The Allende meteorite, weighing at least two tons, fell inMexico in 1969, leaving fragments over a 60 square milearea.[18] We tried a large variety of organic and inorganic liquids,mixed with all sorts of surfactants. We were led to the useof Castrol motor oil by the reasoning that gasoline enginelubricating oil contains additives whose purpose is to keepmineral and metal particles in suspension.[19] Ultraol 50 NF is a mineral oil used in pharmaceutical andcosmetic preparations manufactured by Ultra Chemical,Inc.[20] C. D. Hendricks et al., Meas. Sci. Technol. 5, 337 (1994).[21] D. C. Joyce et al., Phys. Rev. Lett. 51, 731 (1983).[22] M. L. Savage et al., Phys. Lett. B 167, 481 (1986).[23] I. T. Lee et al., Phys. Rev. D 66, 012002 (2002).[24] H.-C. Yeh in Aerosol Measurements, edited by K. Willekeand P. A. Baron (Wiley, New York, 1993), pp. 410– 417.[25] S. Fan et al., Rev. Sci. Instrum. 74, 4305 (2003).[26] E. R. Lee et al., Metrologia 41, S147 (2004).FIG. 4. The qc residual charge distribution in units of e.

Search for Fractional-Charge Particles in Meteoritic Material

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Search for Fractional-Charge Particles in Meteoritic Material

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