Hamacher, K

Klapp, O

Langefeld, P

Siebel, M

CERN Document Server

OPEN-99-38815/06/1999HEP'99 # 1 145 DELPHI 99-115 CONF 302Submitted to Pa 1, 3 15 June 1999Pl 1, 3A Test of QCD Coherence andLPHD using Symmetric 3-Jet EventsPreliminaryDELPHI CollaborationPaper submitted to the HEP'99 ConferenceTampere, Finland, July 15-21DELPHI Collaboration DELPHI 99-115 CONF 302June 21, 1999, 1999 DRAFT 0.98A Test of QCD Coherence and LPHDusing Symmetric 3-Jet EventsK. Hamacher, O. Klapp, P. Langefeld, M. SiebelFachbereich Physik, University of Wuppertal, Postfach 100 127, DE-42097 Wuppertal, GermanyAbstractData collected at the Z resonance using the Delphi detector at LEP are usedto determine the charged hadron multiplicity in a cone perpendicular to the eventplane of symmetric three jet events. The dependence of the multiplicity on the eventopening angle between the jets is predicted by QCD. The measurement constitutesa test of the colour coherence property of QCD and of LPHD.Paper submitted to the HEP'99 ConferenceTampere, Finland, July 15-211 IntroductionThe experimental verication of coherence eects has been important to show the validityof the analytical perturbative approach to jet physics (for a report see [1]). Beyond theearly, more qualitative tests in the recent years, quantitative tests in a direct comparisonof data and theoretical predictions became feasible. The so called string eect has beenquantitatively tested [1, 2] and the energy evolution of the maximum of the p= lnpmaxphadrondistribution is found to precisely follow the perturbative prediction [3] (for a summary ofother results see [1]). Further evidence comes from the universal behaviour of the hadronmomentum distributions for very low momenta which is well described by a perturbativecalculation in the energy range 3 GeV - 189 GeV [4, 3]In this brief note we present evidence for colour coherence from the evolution of thehadron multiplicity perpendicular to the event plane in mirror symmetric three jet events(the so called Y events) as a function of the opening angle between the jets.2 Theory and AnalysisSoft radiation is sensitive to the total colour ow in the underlying hard partonic events.Consider for example a qqg event. If the gluon is collinear to one of the quarks, the colourow is identical to the qq (two jet) case whereas if the gluon exactly recoils w.r.t. the twoquarks, the colour ow corresponds to that of a gg (two jet) event. Soft radiation at largeangle in this case is expected to be increased by the colour factor ratio CA=CFcomparedto the qq case. In [4] the evolution between those extreme cases has been calculated as afunction of the opening angles between the jets (gq; gq; qq) in three jet qqg events.θ2θ3θ1Jet 1Jet 3Jet 2Y eventsFigure 1: Mirror symmetric eventsWe apply this prediction to strictly mirror symmetric events (see Fig. 1) and measurethe multiplicity in a cone of 30opening angle perpendicular to the event plane (see Fig. 2).Symmetric events were selected by demanding that 2should be equal to 3within 2.Here  is half the angular bin width of 1taken to be 2:5. The event selection, analysistechniques, and corrections are identical to those given in [5] with the exception of anadditional cut ycut= 0:02 applied using the Durham algorithm [6] in the selection of the130jet 2jet 1event planeojet3Figure 2: Denition of the cone perpendicu-lar to the event planeevents. This cut has been applied since the prediction [4] refers to three jet events [7].The result of this cut is equivalent to a lower angle cut of about 30. Furthermore thiscut deselects four and more jet events. As described in the second part of [5], no gluonjet identication is needed for mirror symmetric events.Dierent from the proposal made in [4] we do not compare the radiation in two andsymmetric three jet events but study instead the evolution of the multiplicitywith openingangle 1of the three jet event. This study avoids an additional dependence on a ycutusedfor the denition of the two jet events (as discussed in [8]).Up to a normalization constant, a, the dependence of the multiplicity on the angles isdetermined in [4] to beNqqg?/ [2  cos 1+  cos 1  1N2C(1  cos + )] (1)with 1+= qg=    1=2;1 = qg 1;+ = qq= 2   1+  1 ;NC= 3 is the number of colours. Assuming rstly that the gluon is always less energeticthan the quarks, for mirror symmetric events this transforms to:Nqqg?= a  [2 + cos12  cos 1 1N2C(1 + cos12)] (2)A correction has been applied for the cases where the gluon is highly energetic analogouslyto [5]. Thus the comparison of Equ. 2 to the data directly tests the prediction [4] andthus the coherence of soft gluon emission from the hard underlying partons as well as theLPHD assumption for these large angle, mainly low energy tracks.3 ResultFig. 3 shows the measured multiplicity as a function of the angle 1. No data is availablebelow 1= 30due to the presence of a ycut.Omitting this ycutleads to a stronger decreasing multiplicity to smaller angles (i.e.close to the two jet region) as the ycutimplicitly imposed by the angle cut also reduces200.10.20.30.40.50.60.70.80 20 40 60 80 100 120<Nch,cone> for symmetric three jet eventsDataFit with free normalisationθ1[o]<Nch,cone> DELPHIFit rangeFigure 3: Cone multiplicitythe multiplicity in two jet like events [8]. The multiplicity at large angle decreases as(possibly aplanar) four and more jet events are removed from the sample.The prediction Equ. 2 was tted to the data. The normalization is determined to bea = 0:28  0:01 2=n:d:f: = 0:6. The theoretical expectation Equ. 2 is found to describethe behaviour of the data very well (see Fig. 3). This good agreement provides evidencefor colour coherence from a direct comparison of a perturbative calculation [4] to data,i.e. independent of fragmentation models. Also the LPHD assumption is tested for the(low energetic) tracks emitted perpendicular to the event plane.References[1] V.A. Khoze, W. Ochs, Int. J. of Modern Phys. A12 No.17 (1997), 2949.[2] DELPHI Coll., Z. Phys. C70 (1996), 179[3] DELPHI Coll., CERN-EP/99-57, Contributed Paper 1 225 to this Conference.[4] V.A. Khoze, S. Lupia, W. Ochs, Phys. Let. B394 (1997), 179.[5] DELPHI Coll., Phys. Lett. B449 (1999) 383.[6] S. Catani, Yu.L. Dokshitzer, M. Olsson, G. Turnrock, B.R. Webber,Phys. Lett. B269 (1991) 432;S. Bethke, Z. Kunszt, D.E. Soper, W.J. Stirling, Nuclear Physics B370 (1992) 310.[7] Private communication W. Ochs.[8] P. Eden, G. Gustafson, JHEP09, (1998) 15.3

A test of QCD coherence and LPHD using symmetric 3-Jet events

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