Search for Disoriented Chiral Condensates in 158 AGeV Pb+Pb(6)

The restoration of chiral symmetry and its subsequent breaking through a phase transition has been predicted to create regions of Disoriented Chiral Condensates (DCC). This phenomenon has been predicted to cause anomalous fluctuations in the relative produ

2.2Photonmultiplicity

WecountphotonsinthepreshowerPhotonMultiplicityDetector(PMD)sit-uated21.5mfromthetarget,coveringtheregion2.8<η<4.4.Thephotonsimpingingonthedetectorareconvertedin3.34X0thickleadandironandthesecondariesaredetectedin3mm-thicksquareplasticscintillatorpadsofvaryingsizes(15mm,20mmand23mm).Amatrixof50×38padsisplacedinonelight-tightboxmoduleandreadoutindividuallyviawavelengthshiftingoptical berscoupledtoanimageintensi erandCCDcamerasystemsimilartothatdescribedin[16].Themoduleswithsmallerpadsweremountedintheforwardangleregiontominimizeclusteroverlapatlargemultiplicitiesandtoprovidereasonablyuniformoccupancy.Outofatotalof28boxmodulesimplementedinthePMD,thedatapresentedherecorrespondto19boxmod-uleshaving35524pads.Theaverageoccupancyforthepartofthedetectorconsideredinthepresentcaseisaround15%forcentralevents.

Theprincipleofphotonidenti cationmakesuseofthefactthatphotonsaremorelikelytoshowerintheleadconverterandproducealargesignalinthescintillatorpads,whilenon-showeringhadronswillproduceasignalcorre-spondingtoasingleminimumionizingparticle(MIP).Signalsfromseveralneighbouringpadsarecombinedtoformclustersandthosewithenergydepo-sitionlargerthanthatcorrespondingto3MIPsareconsideredtobe”γ-like”clusters.Thisselectiongivesanaveragephotoncountinge ciencyofabout70%whichisalmostuniformovertherangeofcentralityandpseudorapidityconsidered.Italsocreatesane ectivelowerpTcuto of30MeV/c,atwhichpointthee ciencyfallsbelow35%.About15%oftheproducedhadronsim-pingingonthePMDinteractintheconverter,generatingsecondarieswhichalsodepositlargeenergyonthedetector.Thiscontaminationconstitutesabackgroundtophotoncounting.Inordertominimizee ectsduetovariationsintheangulardistributionsofchargedparticles,weonlyusedatawiththeGoliathmagnetturnedo .

Thephotoncountinge ciency,hadroncontaminationandtheassociateder-rorsarederivedusingtestbeamdataandGEANTsimulationusingamethodsimilartotheonesdescribedin[16,17].ThelevelofhadroncontaminationinthePMDwasveri edbycomparingtheazimuthaldistributionofhitsformagnet-onandmagnet-o data[18].Theazimuthaldistributionofchargedtracksbecomesverynon-uniforminthepresenceofthemagnetic eld,theamountofnon-uniformityindicatingthemagnitudeofthehadroncontamina-tion.

Itshouldbeemphasizedthatinthisanalysis,wedonotcorrectthedatausingtheseparameters.Insteadweaccountforallofthedetectore ectsbyfullysimulatingtheconversionofparticlesinthedetector,asdescribedbelow.