Search for Disoriented Chiral Condensates in 158 AGeV Pb+Pb(5)

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

determinationofthecentralityofeachcollision,weusethetransverseenergy(ET)measuredintheMidrapidityCalorimeter(MIRAC[13]).Forremovalofbackgroundevents,wealsousetheZero-DegreeCalorimeter(ZDC)andthePlasticBalldetector[14].

2.1Chargedparticlemultiplicity

WecountchargedparticlesusingacircularSiliconPadMultiplicityDetector(SPMD)[15]located32.8cmfromthetargetcovering2.35<η<3.75,thecentralrapidityregionatSPSenergies(whereηCMS=2.9),andfullazimuth.Thedetectorconsistsoffouroverlappingquadrants,eachfabricatedfromasingle300µmthicksiliconwafer.Theactiveareaofeachquadrantisdividedinto1012padsforming46azimuthalwedgesand22radialbinswithapadsizeincreasingwithradiustoprovideuniformpseudorapiditycoverage.Thee ciencyofdetectingachargedparticleintheactiveareahasbeendeterminedinatestbeamtobebetterthan99%.Conversely,thedetectoristransparenttohighenergyphotons,sinceonlyabout0.2%areexpectedtointeractinthesilicon.Duringthedatataking,95%ofthepadsworkedproperlyandareusedinthisanalysis.

Inacentralion-ioncollision,theoccupancycanbeashighas20%,imply-ingthat≈20%ofthepadscontaintwoormorehits.Anunbiasedwaytoestimatethetotalnumberofchargedparticlesinagiveneventundersuchconditionsistousethesumoftheenergydepositedinpadsexceeding1/2ofthemostprobableenergylossdividedbythemeanenergylossperparticleasdeterminedinlow-multiplicityevents:

Nch= dE/dxi

i=hits

N.For

typicalcentraleventswithamultiplicityof≈600,thisgivesanuncertaintyofabout2%.Tochecktheoverallscale,wecomparetheresultswiththemul-tiplicityobtainedbyassumingthattheparticlesaredistributeduniformlysothemulti-hitprobabilityisgivenbyPoissonstatistics.Asimplecalculation

′givesNch= Npadslog(1 Nhits/Npads),whereNpadsisthetotalnumberofpads,ingthisasacheck,weestimatethesystematicerroronNch,duetouncertaintiesinthegainsandbackgrounds,tobeabout4%.