Using the full statistics of the DELPHI experiment at $\sqrt{s}=91 GeV$ 3-jet
events are selected and gluon respectively quark jet enriched subsamples are
defined. The leading systems of the two kinds of jets are determined using
rapidity gaps. The sum of charges of the leading systems is studied. It is
found that for gluon-jets there is a significant excess of leading systems with
total charge zero when compared to Monte Carlo simulations with JETSET. The
corresponding leading systems of quark-jets do not exhibit such an excess. The
mass spectra of the leading systems with total charge zero are studied.Comment: 8 pages, 6 figures (in eps) talk given at XXXI International
  Symposium on Multiparticle Dynamics, Sep. 1-7, 2001, Datong China URL
  http://ismd31.ccnu.edu.cn

Buschbeck, B.

Mandl, F.

English

arXiv

Using the full statistics of the DELPHI experiment at $\sqrt{s}=91 GeV$ 3-jet events are selected and gluon respectively quark jet enriched subsamples are defined. The leading systems of the two kinds of jets are determined using rapidity gaps. The sum of charges of the leading systems is studied. It is found that for gluon-jets there is a significant excess of leading systems with total charge zero when compared to Monte Carlo simulations with JETSET. The corresponding leading systems of quark-jets do not exhibit such an excess. The mass spectra of the leading systems with total charge zero are studied.Using the full statistics of the DELPHI experiment at $\sqrt{s}=91 GeV$ 3-jet events are selected and gluon respectively quark jet enriched subsamples are defined. The leading systems of the two kinds of jets are determined using rapidity gaps. The sum of charges of the leading systems is studied. It is found that for gluon-jets there is a significant excess of leading systems with total charge zero when compared to Monte Carlo simulations with JETSET. The corresponding leading systems of quark-jets do not exhibit such an excess. The mass spectra of the leading systems with total charge zero are studied

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STUDY OF GLUON FRAGMENTATION AND COLOUROCTET NEUTRALIZATION IN DELPHIB.BUSCHBECK AND F.MANDLInstitute for High Energy Physics of the OAW, Nikolsdorferstrasze 18,A-1050 Wien, AustriaE-mail:mandl@hephy.oeaw.ac.atUsing the full statistics of the DELPHI experiment at√s = 91 GeV 3-jet eventsare selected and gluon respectively quark jet enriched subsamples are defined. Theleading systems of the two kinds of jets are determined using rapidity gaps. Thesum of charges of the leading systems is studied. It is found that for gluon-jets thereis a significant excess of leading systems with total charge zero when compared toMonte Carlo simulations with JETSET. The corresponding leading systems ofquark-jets do not exhibit such an excess. The mass spectra of the leading systemswith total charge zero are studied.1 IntroductionThe study of the leading hadrons in gluon jets may give interesting insightinto the mechanism of colour neutralization. In principle there could exist adirect neutralization of the colour octet eld of the fast gluon by the creationof gluon pairs. This mechanism is called ‘colour octet neutralization’ 1. As aconsequence bound states with valence gluons (glueballs and gluonic mesons)might be eventually produced and observed. The existence of gluonic statesis predicted by non-perturbative QCD and has been searched for since morethan 20 years in dierent experiments 2. The very existence of such states,however, is not yet established.The present study is following a suggestion of Minkowski and Ochs 1,3 tosearch for colour octet neutralization (and for glueballs and gluonic mesons)in gluon jets, produced in 3-jet events in e+e− reactions, by dening a leadingsystem followed by a rapidity gap y empty of hadrons. The leading systemsof both gluon- and quark-jets are then compared to the predictions of theMonte Carlo model JETSET which is not including the mechanism of octetneutralization. The price to pay for a selection of jets with a rapidity gap ishowever a strong reduction of the number of events because of the Sudakovform factor 4. The sum of charges (SQ) in the leading particle system ofgluon jets is compared with the resp. Monte Carlo prediction to search for asurplus of events with SQ=0. Furthermore the same investigation for quarkjets should not result in a surplus of any charge.In ref. 5 it has been observed that there were no signicant deviations fromMonte Carlo predictions for resonance production - in particular that of the in quark- and gluon-jets. Therefore one can expect that octet neutralizationis a relatively rare process - if it exists at all.2 Data sample and 3-jet event selectionThe data sample used has been taken by the DELPHI experiment at the LEPcollider at√s = 91 GeV in the years 1992-1995. About 230000 3-jet-eventhave been selected, which have been obtained by using the appropriate cutsfor track quality and for the hadronic event type 6 as well as applying a ktcluster algorithm (Durham) 7 with ycut = 0:015. For the jet determinationall topologies have been used with 2; 3 = 135o ± 35o, where the jets hadbeen numbered with respect to the calculated energy i.e. E3 ≤ E2 ≤ E1and with e.g. 2 being the angle between the rst and third jet (so called"asymmetric events") 8,9,10. The jet with the highest energy E1 (jet-1) is inmost cases a quark-jet, that with the smallest energy E3 (jet-3) the gluon jet.Monte Carlo simulations show for the above mentioned conditions for jet-1a quark-jet contribution of ≥ 90% and for jet-3 a gluon-jet contribution ofabout 70%. This is e.g. in agreement with the numbers quoted by L3 withsimilar jet energies and selections 11 Heavy quark (b- and c- quarks) eventsare classied using an impact parameter technique 12,13. In the present studyevents are only accepted if they do not exhibit a b-quark signal. The intentionis to compare gluon jets only to ’light-quark’ jets. A corresponding sample ofMonte Carlo simulations (JETSET) of about twice the event- statistics hasbeen created for comparisons. All comparisons are done at the detector level.3 Preliminary Results for 3-Jet Events3.1 Sum of Charges in the Leading System with Rapidity GapAfter the selection of 3-jet events and the determination of enriched quarkand gluon jet samples the leading hadronic system of the jet is singled outby requiring a rapidity interval (y ≥ 2) between the particles (charged andneutral) belonging to this system and the rest of hadrons produced in eitherkind of jet. For the charged particles the momenta are required to be largerthan 0.2 GeV, for the neutrals this requirement is 0.5 GeV. The requirementof the rapidity interval y ≥ 2 below the leading system to be empty ofhadrons reduces the number of events to only 3%.The sum of charges of the particles belonging to the leading system de-ned as above is given in Fig.1 for gluon-jets and in Fig.2 for quark-jets (fullcircles) and compared to JETSET Monte Carlo simulations (open circles).Fig.1 Sum of charges, leadingsystem, gluon-jetsFig.2 Sum of charges, leadingsystem, quark-jetsThe numbers P(SQ) in the upper plots are dened as the number of events(or Monte Carlo events) with a certain SQ divided by the total number ofselected y ≥ 2 events - (or the corresponding Monte Carlo events). Theyare therefore an estimate for the probability of an event to have a certain SQ.The SQ distributions of the leading system for the gluon-jet (Fig.1) shows,for SQ=0, a striking difference between data and simulation. There is a sig-nicant enhancement of the SQ of the data at SQ=0 as expected (see Section1, ref. 1,3) when the process of colour octet neutralization is present (it is, asmentioned above, absent in the simulation). On the other hand, there is nosignificant difference of the SQ of the leading system, for SQ=0, between dataand JETSET simulation in the case of quark-jets (Fig.2). The lower parts ofFigs. 2 and 3 show the dierence of the P(SQ) between the data and theJETSET Monte Carlo simulation. This dierence amounts, for the gluon-jetFig.3 Eective mass distribution of the leading system for bothgluon-jets (a) and quark-jets (c) as well as the respective ratiosata/JETSET Monte Carlo simulations (b resp. c)(Fig.1) to about 9%, that is more than 4 standard deviations from zero, forthe quark-jet (Fig.2) this dierence is compatible with zero !3.2 Mass SpectraFig.3 shows, for both the gluon-jet and the quark-jet, the eective mass dis-tribution M of the leading system, no neutrals included, with rapidity gaprequired for the charged particles y ≥ 2 and the total charge of the systembeing zero. The number of charged particles in the leading system has tobe 2,4,6 etc. Several peaks can be observed for the gluon-jet (Fig.3a). Onepeak around M ∼ 0:8 GeV might be attributed to the  resonance, anotherat M ≤ 0:5 GeV to a reflection of , ‘ and !. Other peaks (M ≤ 1 GeV ,M ∼ 1:4 GeV , ...) are not understood yet and their statistical signicance isweak. The ratio of the M distributions of the leading system data/simulationis given in Fig.3b. The  peak and the enhancement at M ≤ 0:5 GeV seem tobe understood by the simulation. Besides the region at very low M ≤ 0:3 GeV ,the 2 enhancements in the M distributions of the leading system of the gluon-jet at M ≤ 1 GeV and M ∼ 1:4 GeV are not reproduced by the JETSETMonte Carlo simulation.Fig.3c shows the corresponding mass distribution of quark-jets and Fig.3dshows the ratio of the M distributions of the leading system data/simulationfor the quark-jet. Compared to the gluon-jet there is no enhancement atM ≤ 1 GeV in Fig.3c and in Fig.3d no excess compared to the simulation atM ∼ 1:4 GeV .Since the eect of the neutrals are not well understood yet (γ’s, 0’s, andneutral decay modes of ’s, ‘’s, !’s etc.) and because of the limited statisticsno conclusions can be drawn yet from the mass distributions.4 Summary and ConclusionsIn the present study rst eorts have been undertaken to search for the ex-istence of octet neutralization in the fragmentation of gluon-jets. The fullstatistics of 1992-1995 at√s = 91:1 GeV obtained by the DELPHI collabora-tion is used to select 3-jet events and to single out thereof quark-jets (purity≥ 90%) and gluon-jets (purity ≥ 70%). A leading system of a jet is denedwhich is separated from the rest of the low energy particles by a rapidity gapof width y ≥ 2 being empty of hadrons. The sum of charges of this lead-ing system is studied. For the gluon-jets an enhancement of neutral leadingsystems over the Monte Carlo prediction of about 9% is seen (more than 4standart deviations above zero), on the other hand, no such enhancement isseen in the quark-jet ! An even more signicant deviation is revealed (Fig.not shown) for the sum of charges of the 2 fastest tracks without demandinga rapidity cut (3:5% eect but with very high signicance).In order to assess the existence of colour octet neutralization furtherchecks have to be done - determination of the quantum numbers of the leadingsystem, better separation of the gluon-jet, better insight into the role of theneutrals etc. It can, however, be argued that there is an intrinsic shortcomingfor the JETSET Monte Carlo simulation describing the sum of charges in theleading system of the gluon-jet !The eective mass of the leading system with sum of charges SQ = 0 isstudied. An enhancement for the gluon-jet at M ≤ 1 GeV , which is not seenin the simulation nor for the quarkjet and another at M ∼ 1:4 GeV is yet ofonly weak statistical signicance.AcknowledgementsWe thank W.Ochs for encouraging us to start the above study and for discus-sions in the course of it, O.Klapp for technical support for the jet selectionand M.Siebel for valuable comments.References1. P.Minkowski and W.Ochs, Phys.Lett. B485 (2000) 139; H.Spiesberger andP.M.Zerwas, Phys.Lett.B B481 (2000) 236.2. Gluonia, Scalar and Exotic Hadrons, Session 7, Proc. of the QCD 99 Eurocon-ference, Montpellier, France, 7-13 July 1999, ed. S.Narison, Nucl.Phys B (procSuppl) 86 (2000) June 2000, p 341 ff.3. P.Minkowski and W.Ochs, Proceedings 30th Int. Symp. on Multiparticle Dy-namics, eds. R. Csorgo et al (WSPC, Singapore, 2001).4. V.V.Sudakov, Sov.Phys. JETP3 (1956) 65; W.Ochs and T.Shimade, Proc. ofXXIV Int. Symp. QCD and Multiparticle Production 1999.5. P.Abreu eyt al., DELPHI Collab., Eu.Phys.J. C17 (2000) 207, C.Peterson andT.F.Walsh, Phys.Lett.B B91 (1980) 455; L3 COLLAB., CERN-PPE/92-83.6. P.Abreu et al. (DELPHI-Collab.), Phys.Lett. B355 (1995) 415.7. S.Catani et al., Nucl.Phys. B269 (1991) 432; S.Bethke et al., Nucl. Phys.B370 (1992) 310.8. P.Abreu et al. (DELPHI-Collab.) Eur.Phys.J. C4 (1998) 1.9. K.Hamacher et al, ICHEP’98, paper 147, DELPHI 98-86 CONF154; ICHEP’99,DELPHI 99-127 CONF 314.10. P.Abreu et al. (DELPHI-Collab.), Z.Phys. C70 (1996) 179.11. L3-Collab., Phys.Lett. B407 (1997) 38.12. DELPHI Collab., Nucl. Instr. and Meth. A378 (1996) 57;G.Borisov and C.Mariotti, Nucl. Instr. and Meth. A372 (1996) 181.13. DELPHI-Collab, P.Abreu et al, Phys.Lett. B462 (1999) 425; O.Klapp, Thesis,WUB-DIS 99-16.

Study of gluon fragmentation and colour octet neutralization in DELPHI

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