Measurement of charges (1 ≤ Z ≤ 3) of the projectile fragments in ultra–relativistic heavy–ion collisions (16O–emulsion at 60A GeV/c) was carried out by the measurement of lacunarity (L) of the track structure in nuclear emulsion track detector. No fractionally charged projectile fragment has been found in the present investigation.Proučavani su ultrarelativistički sudari teških iona (60 GeV/c 16O) u nuklearnoj emulziji. Naboji čestica (1 ≤ Z ≤ 3) određeni su prema lacunarnosti (L) tragova čestica u emulziji. Nisu nađene čestice s razlomljenim nabojem

Das, Sunil K.

Ghosh, Anit

Ghosh, Dipak

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ISSN1330–0016CODENFIZBE7MEASUREMENT OF CHARGE OF PROJECTILE FRAGMENTS (1 ≤ Z ≤ 3)IN 60A GeV/c 16O–EMULSION INTERACTION – A SEARCH FORFRACTIONALLY CHARGED FRAGMENTDIPAK GHOSH, SUNIL K. DAS and ANIT GHOSHHigh Energy Physics Division, Department of Physics, Jadavpur University, Calcutta,IndiaReceived 11 April 1996UDC 539.1.074PACS 29.40.RgMeasurement of charges (1 ≤ Z ≤ 3) of the projectile fragments in ultra–relativisticheavy–ion collisions (16O–emulsion at 60A GeV/c) was carried out by the measure-ment of lacunarity (L) of the track structure in nuclear emulsion track detector. Nofractionally charged projectile fragment has been found in the present investigation.1. IntroductionSeveral groups of workers have performed quark search experiments at accel-erators and in cosmic rays using different experimental techniques [1]. Still, thereis no convincing evidence regarding the existence of a free quark. Recently, it hasbeen suggested that quark search experiments should be performed in relativis-tic heavy–ion interactions where quarks and gluons, which are confined withinindividual nucleons, may be free over the whole nuclear volume to form a quark–gluon–plasma [2]. Plastic detector experiment [3], Cerenkov–radiation experiment[4], and a search technique within stable matter [5], all gave negative results re-garding search for fractional charges. Bloomer, a pioneer researcher for fractionalcharges using nuclear–emulsion technique [6], also obtained negative result. Still,FIZIKA B 5 (1996) 2, 135–140 135ghosh et al.: measurement of charge of projectile . . .there is hope that signatures of fractionally charged projectile fragments (PF) maybe found. In this paper, we report an emulsion experiment with 16O beam at 60AGeV/c to search for PF having charges 4/3, 5/3, 7/3 and 8/3 times the electroniccharge. Here, we followed the method of Bloomer [6]. The measurement of charge isbased on the determination of lacunarity i.e., fractional transparency or fractionalopacity in the linear track structure of ionization as observed in developed emulsion.This method yields an estimate of charges for relativistic PF having 1 ≤ Z ≤ 3with the standard deviation (Sz) ≤ 0.03e in the measured charges of each track.2. Theory of measurementWhen a charged particle traverses a silver halide crystal of the nuclear emulsion,it deposits energy in the crystal by ionization. The excited crystals are convertedinto metallic silver by chemical reduction and the remaining AgBr crystals areremoved. The opaque Ag–grains form the structure of the track. The probabilitythat a given crystal will develop depends on (1) the emulsion sensitivity, (2) thetotal energy deposited in the crystal, (3) where the particle traversed the crystaland (4) the possible contribution of secondary ionization from δ–rays. The energyloss attributable to primary ionization is called the “restricted energy loss”. Fora given sensitivity of the emulsion, the primary grain density, g, is assumed to beproportional to the restricted energy loss rateg = ki, (1)where k is the sensitivity factor.The restricted energy loss rate can be calculated from the differential energy–transfer cross–section (dσ/dω)dω, asi = neω′∫ωmindσdωωdω, (2)where ne is the electron density, ω′ the upper limit of δ–ray energies and ω, respon-sible for the production of primary grains, ≈ 2− 5 keV. Under the approximationthat δ–rays of energy ω ≥ ω′ do not contribute to primary ionization, Eq. (2) istaken to be exact. In general, the restricted energy loss has the formi =Z2β2f(β), (3)where βc is the particle velocity and Ze the particle charge. A semi–empiricalcalculation yieldsf(β) = 2πr20mc2ne[ln(2mc2I2β2γ2ω)− 2β2 − 2C], (4)136 FIZIKA B 5 (1996) 2, 135–140ghosh et al.: measurement of charge of projectile . . .where I is the mean ionization of excitation potential, m the electron rest mass,r0 classical electron radius, C correction term, ne density of electrons in stoppingmaterial and γ = (1− β2)−1/2.In actual practice, the number of grains in a track segment is not amenable todirect measurement. O’Ceallaigh [7] made the key observation that the gap lengthdistribution is exponential in a trackH(≥ l) = B exp(−g′l), (5)where H(≥ l) represents the density of gaps greater than or equal to l, g′ is theslope of the exponential distribution, known as the ionization parameter, l is theminimum chosen gap length and B is the blob density (i.e., the total number ofresolvable gaps per unit length). Fowler and Perkins [8] confirmed the form of thedistribution for wide range of ionization parameters and for wide gap lengths. Theyalso determined that B, the blob density or equivalently the gap density, is identicalto H(≥ 0) and is governed byB = g′ exp(−g′α), (6)where α is the mean diameter of a developed grain. Barkas [9] proved that theionization parameter, g′ in the experimental distribution of gap is, in fact, theactual grain density, i.e., g = g′.Some important results follow as a consequence. If we define the lacunarity, L,to be the linear fraction of a track made up of gaps, thenL =α∫0dHdlldl = exp(gα). (7)From Eqs. (6) and (7), it follows that measurements of both B and L over a giventrack segment can be used to estimate the grain density,g =BL(8)and the mean grain diameterα = −LBlnL. (9)From the above relations, the dependence of L on α, β, K and Z is− lnL =αKf(β)β2Z2. (10)For a given development of the emulsion, α is a constant. If we deal withsufficiently high energy, β is also a constant. In that case, the charge of an ionizingparticle is simply related to L of its track byZ = K0√− lnL, (11)FIZIKA B 5 (1996) 2, 135–140 137ghosh et al.: measurement of charge of projectile . . .where K0 is a proportionality constant that is most conveniently determined em-pirically. For convinience, the quantity proportional to charge Z can be definedasρ =√− lnL =√− ln(1− φ), (12)where φ is defined as the opacity of the track structure.We have now ionization parameters, L and φ, which are operationally welldefined, easy to measure and related to charge by the simple relation Z = K0ρ.The precision, δZ, with which the charge Z of an ionizing particle can be de-termined from its ionization track, is obtained from Eq. (11) asδZ =K202ZδLL, (13)where δL/L = (σL/L)N−1/2, σL is the variance of L, and N is the number of cells,each of length S, over which L is measured. Barkas [10] has given a theoreticalupper limit to the quantity δL/L for a model based on completely random spacingsbetween individual grains in blobs. It isδLL=√2αΛ[−1− LL lnL− 1], (14)where α is the mean grain diameter in µm of the developed grains and Λ = NSthe total path length in µm over which L is measured.3. Experimental detailsWe have used a stack of Illford–K2 plates exposed to the 16O beam with theaverage momentum 60A GeV/c obtained from CERN, Geneva. A Leitz–metalloplanmicroscope with oil immersion objective, having a magnification 100× and ocularlens 10×, along with an image processor, is used for scanning. Data were taken withASM 68K semi–automatic measuring system. SUSY–system disk and a suitablydeveloped programme is used to measure lacunarity (L) and/or opacity (φ) of thetrack structure.From a sample of 226 events, with Nh > 8 and Ns > 20 where Nh and Nsrepresent the number of heavy tracks and shower tracks, respectively, final mea-surement of charge was taken on 477 projectile fragments. We measured opacity(φ) for Z = 1 tracks for a minimum length of 1000 µm of the linear track structure.Such a long–range of measurement yields a charge precision of less than 0.03e. La-cunarity values obtained directly or calculated from the opacity (φ) [L = 1 − φ]belong to three distinct groups. Each group thus obtained is separately normalisedto ρ, where ρ = (− lnL)1/2. From this value of ρ, the value of K0 is calculatedusing the relation Z = K0ρ.138 FIZIKA B 5 (1996) 2, 135–140ghosh et al.: measurement of charge of projectile . . .4. Experimental results and discussionFigure 1 shows the distribution of charges of the PF. It is transparent from thefigure that the charges are distributed sharply around the integer values of Z, whereZ = 1, 2 and 3. To determine if any candidate really possesses a fractional charge,one has to be sure about the width of the distribution of the integer charges that isexpected from statistical fluctuation for the charges. Thus we have to measure thedispersion Dz of the charge distribution around the integer charge Z (Z = 1, 2 and3) and check if there is any candidate for which the charge lies beyond Z ± 3Dz.Table 1 shows that this experiment does not find a single candidate of a PF whosecharge lies beyond the Z ± 3Dz value for any of the distributions around Z = 1, 2and 3.Fig. 1. Distribution of the charges (Z) of the projectile fragments emitted in 16O–emulsion interaction at 60 A GeV/c.The track saturation, i.e., conglomeration of Ag–grains into blobs may distortthe linear relationship between mean gap length (g) and charge (Z) of the ionizingtracks (i.e., g ∼ Z2) as Z increases, and the effect is dependent on the quality ofdevelopment of the emulsion–pellicle and that of the emulsion batch itself. Actuallythis is the cause that restricted us to measuring charges of the PFs having 1 ≤ Z ≤3.All possible sorts of check were performed to ascertain that these candidatesare really projectile fragments. Specifically, we carried an extensive check on theemission angle of the PF and ensured that they all lie within a 3◦ forward conewith the incident beam direction. That resulted in the rejection of twenty onecandidates. This angular cut ensures that these candidates have suffered a tooFIZIKA B 5 (1996) 2, 135–140 139ghosh et al.: measurement of charge of projectile . . .small longitudinal momentum transfer in their production for precise measurementof charge with this technique.TABLE 1.Results of charge measurements 1 ≤ Z ≤ 3 projectile fragments. K0 = Z/ρ, whereρ = (− lnL)1/2, L is the lacunarity or fractional transparency of the trackstructure.No. of PFsZ No. of K0 = Z/ρ Dispersion with chargetracks Dz beyond Z ± 3Dz1 100 2.973 0.068 02 307 2.981 0.075 03 70 2.974 0.073 05. ConclusionThe objective of the experiment was to find if there are any fractionally chargedPF in 16O–emulsion interaction at 60A GeV/c. The result is negative, i.e., we getno fractionally charged PF. All the PF lie within three times the dispersion (Dz)around the integer charges for the respective charge groups. Thus, this experimentshows, within experimental limit, only integrally charged PF.References1) M. A. Lindgren et al., Phys. Rev. Lett. 51 (1983) 1621;2) H. Heseroth, M. G. Albrow, I. Otterlund, H. Satz and L. van Hove, Talks president inworkshop on SPS fixed target physics in the years 1984–1989, Ed. I. Manelli, CERNpreprint 83–02;3) P. B. Price, M. L. Tincknell, G. Tarle, S. P. Ahlen, K. A. Frankel and S. Perlmutter,Phys. Rev. Lett. 50 (1983) 566;4) S. W. Barwick et al., Phys. Rev. D30 (1984) 691;5) M. A. Bloomer, E. M. Friedlander, H. H. Heckman and Y. J. Karant, Phys. Lett. 138B(1984) 373;6) M. A. Bloomer, H. H. Heckman and Y. J. Karant, Nucl. Inst. Meths. 215 (1983) 247;7) C. O’Ceallaigh, CERN BS–II, Geneva (1954) also Nuovo Cim. Suppl. 12 (1954) 412;8) P. H. Fowler and D. H. Perkins, Phil. Mag. 46 (1955) 587;9) W. H. Barkas, Phys. Rev. 124 (1961) 897;10) W. H. Barkas, Nuclear Research Emulsion, Vol. 1. (Academic, New York, 1963).MJERENJE NABOJA ČESTICE (1 ≤ Z ≤ 3) PROIZVEDENIH U SUDARIMA60 A GeV/c 16O U EMULZIJI I TRAŽENJE ČESTICA S RAZLOMLJENIMNABOJEMProučavani su ultrarelativistički sudari teških iona (60 GeV/c 16O) u nuklearnojemulziji. Naboji čestica (1 ≤ Z ≤ 3) odredeni su prema lacunarnosti (L) tragovačestica u emulziji. Nisu nadene čestice s razlomljenim nabojem.140 FIZIKA B 5 (1996) 2, 135–140

Mjerenje naboja čestice (1 ≤ Z ≤ 3) proizvedenih u sudarima 60 A GeV/c 16O u emulziji i traženje čestica s razlomljenim nabojem

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