The Little Bang! Results from RHIC
Barbara V. Jaak
StonyBrook University,SUNY,StonyBrook,NY11794,USA
Reeivedon30September,2002
I report rst results on Au + Au ollisions at p
s = 130 GeV/ per nuleon pair from all four
experimentsat theRelativistiHeavyIonCollider. Theenergydensityahievediswellabovethe
thresholdpreditedbyLattieQCDforquarkdeonnement. Largepressureisdevelopedearlyin
theollision,leadingtoolletiveasymmetriowofthepartiletransversetothebeamenergyand
explosiveexpansionofthehotsystem. Thehadronyieldsindiatepreseneofanequilibratedhadron
gasinwhihthehemistryisxedatatemperaturenear170MeV,whihisalsothetemperatureat
whihthequarkgluonplasmatohadronphasetransition isexpeted. Adeitofhightransverse
momentumpartiles is observed inentral Au+Auollisions, ompared to expetedyields from
independent nuleon-nuleonollisions orexpetationsfromperipheralAu+Auollisions. Thisis
likely the rstindiationofjetquenhingbyenergyloss ofhardsatteredpartons traversing the
densemediumreatedintheollision.
I Introdution
ThegoalofexperimentsatRHICistoollidethe
heavi-estpossibleionsatthehighestpossibleenergy,toreate
matter at maximum temperature and density. These
ollisionsshould reproduethe onditionsthatexisted
in therstmiroseondsafterbigbang,wherethe
dis-tanesamonghadronsweremuhsmallerthanthesize
ofthehadronsthemselves. Suhonditionsarethought
tostillourintheurrentuniverse,attheoreof
neu-tronstars. Quantum hromodynamis postulatesthat
under suh extreme onditionsthe quarks and gluons
arenolongeronnedintohadrons,butratherexistas
aplasma,withonstituentsfreetoroamovertheentire
volume ofthe hotmatter. It isof extreme interestto
reproduetheseonditionsinthelaboratoryandstudy
the properties of suh matter. I will desribewhat is
knownaboutthis kindof matterfrom the rstrun of
theRelativistiHeavyIonColliderat Brookhaven
Na-tionalLaboratory. In2000,goldionswereollidedwith
eah other ata enter ofmass energyof 130GeVper
nuleonpair. The2001runofRHIC reahed200GeV
pernuleonpair.
Quantum Chromodynamis (QCD), the theory of
the strong interation, yields a potential whih
in-reases linearly in strength with the distane between
quarks. This strongattrativefore is responsible for
theonnementofquarksintobaryons(3-quarkbound
states) andmesons(boundstatesof aquarkand
anti-quark). Inmatterwhihisverydenseorhot,orboth,
theolorhargesofthequarksbeomesreenedandthe
potential dereases. At suÆiently high temperature
altogether,leadingtothedeonnementofquarks.
Inanalogyto phasediagrams foreveryday
materi-als,onemayskethaphasediagramofhadronimatter.
At lowtemperatures, below 8MeV, and near normal
nulearmatterdensity,onendshadronsonsisting
pri-marilyofnuleonsbound intonulei. Asthe
tempera-tureordensity inreases,thenuleonsmelt into agas
ofhadrons -the initial nuleonsalongwith pions, the
main arriers of the inter-nuleonattrative fore. If
thetemperatureexeeds 150-200 MeV,or thedensity
exeeds5timesnormalnulearmatterdensity,thenthe
onnement ofquarksandgluons intohadronsshould
vanish, and aphase transition to quark-gluon plasma
take plae. QCD predits an additional phase
transi-tion,atlowtemperaturebut veryhighdensity. Inthe
super-dense phase, thequarksand gluonsbehaveasa
olorsuperondutor. This newphaseisunderintense
theoretial investigation, but diÆult to reah in the
laboratory. It may, however, exist deep in theore of
quarkstars.
Inorder to identify the onditionsrequired to
re-ateandstudyquarkgluonplasma,wemustreplyupon
theory. Unfortunately, in thestrongly oupledregime
ofaquark-gluonplasma,QCDannotbesolved
pertur-batively, and alulationsmustbearried outby
sim-ulationon alattie. Reentprogressin omputational
tehnology has allowed large-sale lattie simulations
of QCD at high temperature. Karsh, Laermann and
Peikert showed that the energy density of a 3 avor
systemshowsa veryrapid rise when the temperature
reahes17010MeV.Exatlysuhariseis
indi-of1-3GeV/fm 3
toenterthequarkgluonplasmaregime.
II Heavy ion ollisions at RHIC
Theexperiments I will desribehavebeenarried out
at the Relativisti Heavy Ion Collider at Brookhaven
NationalLaboratoryinUpton,NY.RHICisatuallya
suiteofaelerators,withbeamsstartingin aTandem
VandeGraafaelerator. Whenthebeamreahesafew
MeV pernuleonitissenttotheBooster,andthento
the Alternating Gradient Synhrotron, with stripping
of eletronsfrom the beam atomsbetweeneah
ael-erator. Aubeamsreahmorethan11GeVpernuleon
in theAGS,while lighterbeamsan beaeleratedto
somewhathigherenergies. TheAGSthen sendsbeam
pulsesintothetworingsofRHIC;uponenteringRHIC
the beam is fully stripped. After lling, RHIC
ael-erates eah beam to 100 GeV per nuleon, and then
brings thebeamsinto ollision in upto 6intersetion
regions around the ring. The design of RHIC is for
luminositiesof210 26
m 2
se 1
.
Currently, four intersetion regions are
instru-mented with a suite of omplementary experiments
built by international ollaborations. There are two
large and two smaller experiments. BRAHMS, one
of the small experiments, onsists of a pair of
mov-able small-aeptane spetrometers with good
parti-le identiationapability. BRAHMSis optimizedto
sample the partile distributions overawide range of
longitudinal veloity. The other small experiment is
PHOBOS, whih is a \table-top" experiment (if you
havealargetable). PHOBOSemployshighlygranular
siliondetetorstoountallhargedpartilesand
mea-surethepartilespetraatlowmomentumnearthe
ra-pidity oftheenter ofmassofthe ollision. PHOBOS
also has hadron identiation by time-of-ight. It is
optimizedto searhfor largedistane phenomenaand
utuationsin partileprodution. PHENIXisalarge
experimentoptimizedto measureleptonsand photons
toprobetheearlytimeoftheollisionvia
eletromag-netially interating probes. PHENIX has high rate
apability and seletive triggers to measure rare
pro-esses, alsoin thehadroni setor. STAR is theother
largeexperimentandonsistsprimarilyofalargetime
projetionhamber. STAR is optimizedto have large
aeptaneforhadronstostudypartileprodutionand
event-by-eventutuations.
Whentwonulei ollide at RHIC, they are highly
Lorentz ontratedandtheinitialnuleon-nuleon
ol-lisions all take plae in less than 1 fm/. Beause of
theontration,itisnotpossibletoordertheollisions
in time, however, many of the ollisions our among
nuleons already disturbed by an enounter with
an-other nuleon. This makes theoretial desription of
the low momentum transfer proesses very
halleng-theaidofmodels,ratherthandiretlyalulatedusing
QCD.At p
sof130or200GeV/nuleonpair,the
olli-sionsanprobepartondistributionsnearx=10 2
. At
suh short distanes, the interations that take plae
are neessarily at the partoni, rather than hadroni
level. Large Q 2
proesses are in the weakly oupled
regime and may be alulated by perturbative QCD,
withtheollisionprobabilitygivenbynulearstruture
funtions. The large number of individual ollisions
givesrisetoopiousseondarypartileprodutionand
a parton asade results. Around 10 4
gluons, quarks
andantiquarksare produed,and itisthese produed
partons whih are expeted to thermalize and form a
quarkgluon plasma. Partonasades anbemodeled
with theaid ofautoseparating\hard" from \soft"
proesses, handlingthe former perturbatively and the
latterviastringmodelsofpartileprodution(suhas
the Lund model). Parton asadealulations predit
thermalization onthe timesale of 1fm/. Of ourse,
thethermalizedsystemthenexpandsintothevauum,
oolingandpassingthroughthephasetransitionbak
into hadrons.
III Initial onditions and
dy-namis of the ollisions
Experimentally, thehallenge atRHIC is todealwith
manythousandsofpartilesinthenalstateand
iden-tify and measureobservableswhih illuminatethe
un-derlying physis. I separate the observables into two
lasses. One lass of observables allows study of the
ollision dynamis and addresses to what extent the
partiles equilibrate. Measures of olletive behavior
andthepressuregeneratedintheollisionfallintothis
lass. Hadronyields and spetratell aboutthe
evolu-tionof thesystemand propertieslater in theollision
andthusalsoshedlightupontheollisiondynamis.
Theotherlassof observablesonsistsofprobesof
the early, hotphase of the ollision. Suh probesare
partiles whih are reated early in the ollision and
either donot interat with the hot, dense medium at
all (suh as thermalradiation) orwhih interat with
themediumdierentlythanwithnormalnulear
mat-ter. Thermal radiation an be measured via diret
photon emission orvirtualphotons deayingto
oppo-sitelyharged leptonpairs; bothofthese interat only
eletromagnetially so are unaeted by strong
inter-ationswiththedensemediumformedintheollision.
Of ourse, there an be thermalradiation from a hot
hadrongasaswell,andthismustbeontrolledifoneis
to detetthequarkgluonplasmaradiation. Probesof
thedensemedium itselfinlude fastquarkswhihlose
energy depending on the density of sattering enters
inaoloredmedium,harmquarksandantiquarks
quarkprodutionalsotellsabouttheearlymedium,as
theywhihanbeformedmoreeasilyinahotmedium
ifthetemperatureisnearthestrangequarkmass.
Beforestudyingmediumeetsontheprobes,
how-ever,itisimportanttoasertainthattheinitial
ondi-tionsatRHICareinfatsuÆientforreationofquark
gluon plasma. This is generally addressedby
measur-ing the number of produed partiles and the energy
ow perpendiular to the diretion of the beams. At
RHIC, over5000harged partilesareformed in
olli-sionsoftwogoldnuleiatsmallimpatparameter,i.e.
in entral ollisions. Compared top-pollisionsat the
same p
s,thenumberofhargedpartilesproduedper
interatingnuleonpairisonsiderablyhigher. Allfour
experimentsndmorethan3hargedpartilesper
nu-leon pairin entralAu + Au,ratherthan the2seen
in p-p. The inrease an beasribed to multiple
su-essiveollisionssueredbythenuleonsintheoverlap
region betweenthetwonulei. Asallof theprodued
partiles omeintoexistenein avolumeroughlythat
of twonulei, thedensityreahedis muh largerthan
normalnulearmatterdensity.
Measurement of the energy ow transverse to the
beamallowsestimation oftheenergy density attained
in the early phase of the ollision, prior to expansion
andooling. PHENIXmeasuredthetransverseenergy,
E
T =
X
i E
i sin
i
where the sum runs over produed partiles at
mid-rapidity. For entral ollisions at p
s = 130
GeV/nuleon pair, E
T
per unit rapidity = 503 2
GeV. Thisanbeused toestimate theenergydensity
assuming longitudinalexpansionat the speedof light,
usingBjorken'sformula
Bj =
1
R 2
1
2
0 (2
dE
T
dy )
yielding 4:6GeV =fm 3
. This is 50% higher than
previously observed and is well above the threshold
predited by lattieQCD. The valuehas asigniant
unertaintybeausethevalueof theparton formation
time
0
isnotwellknown. Thevaluegivenhereuses1
fm/, whihis almostertainlyanoverestimate.
FirstPHENIX,thenPHOBOSstudiedtheollision
entralitydependene ofthenumber ofharged
parti-les produed. Charged partile prodution inreases,
ofourse,astheimpatparameterdereasesandmore
nuleons are involved in the ollision. However, the
absolute number of partiles is larger than that
pre-dited by HIJING, an event generator inorporating
parton asading and energy loss of partons as they
traversethedensemedium. Theentralitydependene
is strongerthanexpetedfrom amodel of gluon
\sat-uration",orreombinationwhenthegluondensity
be-to ontrol model parameters; the disagreement is not
largeenoughtoruleoutthebasiphysisassumptions
theyinorporate. Itisquiteommontodeomposethe
partileyieldsinto aomponentwhih saleswiththe
numberofnuleonspartiipatingintheollisionanda
omponentsalingwiththenumberofbinary
nuleon-nuleonollisions.Thoughitistemptingtoidentifythe
ollisionsalingpartoftheyieldwithhard,orlargeQ 2
proesses,suhatwo-omponentmodelisnot
quantita-tively orret. Nevertheless, high momentum transfer
proessesareindeed importantatRHIC energy.
As many partiles are produed in a rather small
volume,onemayexpetthatsigniantpressureis
de-velopedin theearlystageof theollision. Toquantify
this experimentally, weneed a \barometer"for heavy
ionollisions. Suhabarometerisavailableby
measur-ingolletive\elliptiow"ofpartilesineahollision.
Theoriginoftheellipti owis thespatial anisotropy
of the overlap region of two nulei (this overlap
re-giononlyapproahesisotropyforthemostentral
ol-lisions). Extensive resatteringof thepartiles in the
evolvingsystemantranslatethespatialanisotropyto
a momentum spae anisotropy as it is easier to emit
partilesinthethinnerdiretionofthealmondshaped
overlapregion. Theanisotropyisexperimentally
aes-sibleby Fourieranalysis of theazimuthaldistribution
ofpartiles. Oneanidentifyin eaheventapreferred
diretion,whih isaligned with thereationplane, or
diretionoftheimpatparameterbetweenthetwo
nu-lei. The seond harmoni Fourier oeÆient of the
azimuthal distribution of partileswith respet to the
reationplane,knownasv
2
isusedtoquantifythe
ol-letive elliptiow. STAR showedthat v
2
reahes6%
insemi-peripheralollisionsofAu+Au,andthe
magni-tudeofv
2
isquitewellreproduedbyhydrodynamial
models of the ollision. As hydrodynamis, by
deni-tion,treatsthemediumasfullyequilibrated,its
appli-abilitytotheearlystageoftheollisionwhilethe
spa-tialanisotropyis still large impliesearly equilibration
ofthematterat RHIC. Itshould benotedthatRHIC
produesthehighestenergyheavyionollisionssofar,
andthis isthersttime thathydrodynamis provides
anauratedesriptionofexperimentalobservables.
IV Thermodynami properties
The thermal history of ollisions at RHIC an be
studiedbytwokindsofmeasurements. Boththequark
gluon plasma phase and the hot hadron gas formed
after ooling bak through the phase transition an
emit real and virtual photons. In the rst ase, the
photonsare emitted by quark-antiquarkannihilations
andquark-gluonComptonsattering. Inahadrongas,
thermal radiation arises from hadroni ollisions and
thephotons anbemeasured, andinterpretedaslong
asthe hadronyields and momentum distributions are
alsomeasuredtoallowhighpreisionsubtrationofthe
hadronibakgrounds.
Later in the ollision, as the hadron gas further
expands and ools, its thermodynami properties an
be measuredby studying the distributions of emitted
hadrons in the nal state. These reet the
temper-ature of the system when the hadrons ease to
inter-at: therelativeyieldsarexedwhentheinelasti
ol-lisions stop (hemialfreeze-out), and the spetraare
determined later, whenelasti ollisions ease (kineti
freeze-out). The hydrodynami ow - known to
ex-ist from measurementof ellipti ow -aets the
sys-tem during its entire thermal history. Thus
measure-ment of hadron spetra yields dynamial information
as well as a snapshot of the system at kineti
freeze-out. Hadronsareidentiedbytime-of-ight(PHENIX,
PHOBOSand BRAHMS) orenergylossin gas
dete-tors(STARandBRAHMS).
To evaluate the hemial freeze-out temperature,
hadron yields from all four experiments are used.
Braun-Munzinger, Magestro, Redlih and Stahel
as-sumedemissionofhadronsfromahemiallyand
ther-mally equilibrated gas. They t the ratios of
dif-ferent hadron yields aording to a Grand Canonial
ensemble and extrated the temperature and
baryo-hemial potential best mathing the observed data
in entral Au+Au ollisions at RHIC. They found a
baryo-hemialpotentialof 51 MeV,orresponding to
anearly, but notquite, net baryon-freegas at entral
rapidity. The hemial freeze-out temperature of 175
MeVfromtheirtissurprisinglynearthatexpetedfor
the hadronization phasetransition. This implies that
the hadrons are reated in hemial equilibrium from
ahemiallyequilibrated plasma,and thatthe
expan-sion is so explosive that the hadrons deouple
imme-diately and undergono inelasti ollisions. This is in
qualitativeagreementwithpredition of
hydrodynam-is, though the density at hadronization is predited
to be very large and one would naively expet some
hadroni interations to take plae one the hadrons
areformed.
Atual baryon yields have been reported by
PHENIX and BRAHMS, and though the net baryon
densityisquitelow,theatualnumberofprotonsand
antiprotonsatmidrapidityissubstantial. Therapidity
densityofprotonsis28inentralAu+Auat p
s=130
GeV/nuleonpair, while it is20 for antiprotons. The
numberofnetbaryons(p p)perpartiipantnuleonis
approximately0.05,whereasitwas0.18inlowerenergy
ollisionsattheSPS,wheretheinitialbaryonsfromthe
interatingnuleiweremoresuessfullytransportedto
theenterofmassrapidity. Ofourse,thisrequiredless
hangeinthelongitudinalveloityforthelowerenergy
ollisions. All experiments nd that the antibaryon
the ase forantiasades to asades, asmeasured by
STAR.
Strangenessprodutionwaspreditedtobea
possi-blesignatureofquarkgluonplasmaprodution,though
the interpretation is ompliated by the possibility of
produing strange and anti-strange hadrons in a hot
hadron gas. PHENIX has measured the K = ratio
in Au+Auollisions, andnds that both K +
= +
and
K = inreasewithollisionentralityfroma
periph-eral ollision value of 10% as seen also by UA5 in
proton-antiproton ollisionsat similar p
s. Both
posi-tiveandnegativeratiosinreasetogetherwith
entral-ity to 15%, whereas they dier in entral Pb+Pb
ollisionsat p
s=17GeV/A.Atthelowerenergy,only
K +
= +
inreases with ollision entrality; the
dier-ene between positive and negative kaons an be
un-derstood from the larger net baryon density in heavy
ionollisionsat lowerenergy.
Thehydrodynamimodels whih reproduethe
el-liptiow,v
2
,indiateaveryexplosiveexpansionofthe
ollisionsystem. Suhanexplosionshouldgiverisetoa
olletiveradialexpansionin addition tothe observed
ellipti ow. The radial expansion an be measured
bythespetrumofhadronmomenta transverseto the
beam. Inorderto usehadronsof dierentmasses,the
spetra are plotted as a funtion of transverse mass,
m 2
T = p
2
T +m
2
0
. PHENIX and STAR, followed by
BRHAMS,haveshownthattheprotonspetrumis
at-ter in m
T
than the lighter mesons. Suh a attening
wouldbeexpetedifallpartilesreeiveaommon
ol-letiveveloityboost, resultingin alargermomentum
boost for the heavier hadrons. Fitting the observed
pion, kaon, proton and antiproton spetra
simultane-ously,PHENIXfoundthatthedataanbedesribedby
emission from agasat 140-150 MeVtemperature,
ex-pandingradiallyaboutthebeamdiretionwithamean
veloityofapproximatelyhalfthespeedoflight. In
pe-ripheral ollisions, the kineti freeze-out temperature
doesnothange muh,but theradialowis
onsider-ablyless.
Thelargeradialowveloityausestheprotonyield
tonearlyequalthatofthepionsattransversemomenta
larger than approximately 2 GeV/. Suh a
\ross-ing"ofthespetrahasneverbeenobservedbefore,and
impatsthe interpretationof highmomentum partile
prodution. It isnoteworthythat thehydrodynamial
alulationssuessfullyreproduethehadronspetra,
inluding therossing ofthe baryonspetraoverthat
ofthemesons.
V Probes of the hottest, densest
phase
We have already seen that there is opious
pres-served radial and elliptiowsand indiate that high
densities are ahieved. There exists, however, amore
diretprobeoftheearlydensityanditseetsongluon
transportbythemedium. Theprobeisahigh
momen-tum quark or gluon, arising from hard sattering of
partons intheinitialnuleonollisions. Thesepartons
are sattered early, at a rate alulable by
perturba-tiveQCD,andtraversethehot,densemediumontheir
way out of the ollision region. As they traverse the
olored medium, these sattered partons radiate
glu-ons, with a radiation rate sensitive to the density of
the medium. There is aharateristi formation time
for the bremsstrahlung gluons, whih depends on the
transverse momentum of theradiated gluon. Thus, if
the medium is suÆiently dense, the mean free path
anbelessthanthedistanetheparton travelsbefore
the radiation is omplete. In this ase, the radiation
beomes oherent, and theamountof energy radiated
an inrease substantially. Large energy radiation by
partoni probes has observable eets as it dereases
theprodutionofhighp
T
partileswhenthesattered
parton fragments into ahadroni jet. The proess is
referredtoas\jetquenhing"andanbemeasured
ex-perimentallyviathespetrumofleadinghighp
T
parti-lesfromjetfragmentationorbyazimuthalorrelations
of theleadingpartilefrom eah ofthetwohard
sat-tering partons.
Both PHENIX and STAR measured the hadron
spetrum to large transverse momentum (p
T 5
GeV/). PHENIXalonemadethreeseparate
measure-mentsofthespetrumofhargedhadronsandof
iden-tied 0
's. Inall hannelsthe observedspetrain
pe-ripheralollisionsofAu+Aunuleiagreewellwiththe
spetrumpreditedbyfoldingindividual p-pollisions
bythenumberofbinarynuleon-nuleonollisions
or-responding to the seletedentralityrange. However,
this is notthe ase in entral ollisions. Thespetral
shapeismoreexponentialthatthesaledp-pspetrum,
and the observedyield is well below that expeted by
saling individual p-p ollisions by the approximately
900binarynuleon-nuleonollisionsorrespondingto
themostentral10%ofAu+Auollisions.Theyieldof
high momentum partiles is suppressed bya fatorof
3-4 for 0
andafatorofapproximately2forharged
hadrons.
Comparingthe measurementsat RHICto thoseat
lower energy at the SPS makes the observation even
morestriking. AttheSPS,theyieldin entralPb+Pb
wasnotonlynotsuppressed,itwasatuallyenhaned
due to multiple sattering of the inoming partons in
thenulearmediumpriortothehardsatteringwhih
sends them to large p
T
. This initial state sattering,
referred to as the \Cronineet" is well known from
p+nuleusollisionsandshouldouratRHICaswell.
Ifso,theatualsuppressionofhighp
T
hadronsiseven
larger than that inferred by omparing to saled p-p
other known nulear eet upon the parton
distribu-tions,namelynulearshadowing. Thisoursat small
momentumfration,x,inlargenulei,dueto
reombi-nationof nearby partons from theoverlappingparton
loudsin neighboring nuleons. Moderateto largeQ 2
proessesat RHICreahx210 2
. However,data
and alulations show that quark and gluon
shadow-inghaveless than10% eet at these modestly small
x values. Thus nulear shadowingannot explain the
suppression,supportingitsinterpretationasarst
ob-servationofjetquenhing.
It is importantto understand thedierene in the
observed suppression between harged hadrons and
identiedneutralpions. Thisturns outnotto be
sur-prising, ifwereallthe eet ofthe momentum boost
on protons from the substantial radial olletive ow
developedintheollision. Thenearparityofbaryonor
antibaryonyieldsandpionyieldsat2GeV/leadsone
to expet asmaller suppression fator for all hadrons
omparedto just pions. This isbeausetheboost
ap-plies to partiles produed by\soft" proesses, rather
thanthose arising from jet fragmentation. These soft
partilesshould notbesensitiveto thehard sattered
partonenergy loss. It would appear from theexisting
datathatthisisso. Animportantorollaryisthatthe
largehydrodynamiboostextendsthep
T
regionwhere
non-perturbative proessesontribute substantially to
thepartileyields. ThisseemstobetheaseinAu+Au
ollisionsatRHICforp
T
3GeV/.
The data suggest that there is substantial energy
loss by partons traversing the dense medium reated
in entral Au+Au ollisions. This an be tested and
quantied by omparison to model alulations. One
suhalulationhasbeenmadebyX.-N.Wangand
o-workers,takingintoaounttheexpetednulear
shad-owingandCronineets. Thedataarebest desribed
by inlusion of an energy loss of 0.25 GeV/fm. This
valueseemsratherlow,andisinfatverylosetothat
expeted in old nulear matter. However, this is an
averagevalueoverthelifetimeofthemedium,whihis
notat allstati. Takinginto aounttherapid
expan-sionofthe medium, thedata requireaninitial energy
lossloserto7GeV/fm. Thisislargeindeed.
Anotherprobeofthemediumisharmedquark
pro-dution. Theprimaryformationmehanismfor harm
and anti-harm quarkpairs is gluon fusion. Thus the
produtionrateshould besensitiveto thegluon
num-ber anddistribution and may be expeted to inrease
in thepreseneof hotplasmawithmanygluons.
Fur-thermore,spetrosopyoftheboundstatesoersan
exellentprobeoftheolorsreeningapabilityofthe
medium.
By observation of single eletron prodution in
Au+Au ollisions, PHENIX an measure the rate of
openharm(Dmeson)prodution. Unambiguous
PHENIX aomplishes this by use of a Ring
Imag-ingCherenkovCounterwithphotomultipliertube
read-out, in onert with a very granular eletromagneti
alorimeter. EletronsprodueCherenkovlightand
en-ergy whih is measuredin thealorimeter. Requiring
Cherenkovlightandthatthemeasuredenergymathes
themeasuredpartilemomentum yieldsalean
identi-ation of eletrons. Comparison to a alulation of
hadroni deay soures of eletrons along with
pho-ton onversions in the detetor material of PHENIX
provides a measure of the \exess" eletron
produ-tion. The bakground alulation is performed using
the measured hadron yields as input, and the
dier-eneyieldstheDmesonprodutionrosssetion. The
result shows that the harm yield mathes very well
thatexpetedfromrosssetionsmeasuredin p-pand
proton-antiprotonollisions. Thus,thereappearstobe
no enhanement ofharmprodution, norenergy loss
asobservedforthe lightquarks. It may, ofourse, be
thatMotherNatureisveryslyandthetwoeets
hap-pentoanelexatly. Butthis isnotextremelylikely.
VI Conlusions
FirstresultsfromRHIChaveshownthatthenalstate
inentralAu+Auollisionsonsistsofmorethan5000
hargedpartiles,andtheenergydensityahievedearly
in the ollision is well above the threshold predited
by Lattie QCD for deonnement. Large pressureis
developed earlyin theollision, leading to avery
sig-niant olletive elliptiow. The fat that the
sys-tem \remembers" the initial spatial asymmetry
indi-atesthatthermalizationmustourrapidly. Chemial
freeze-outofthehadronsfrom thissystemoursnear
170 MeV,whih is also the temperature at whih the
transitionbaktohadronsisexpeted.
Hard proesses with large momentum transfer at
RHIC are important. Theyboost thenumber of
pro-dued partiles and result in signiantprodution of
harmed quarksand satteredlightquarksand gluons
to probe the medium. A deit of high p
T
partiles
from the fragmentation of these sattered partons is
observed in entral ollisions, whih may be the rst
indiationofjetquenhing.
TheseondrunatRHIColletedsigniantlymore
data, allowingmeasurementsto highertransverse
mo-menta, orrelations among partile from jets,
multi-strangebaryonspetraandarstlookatharmonium
spetrosopy. The baseline physis in proton-proton
ollisionshasbeenmeasuredintheverysamedetetors.
SubsequentRHICrunswillprovideproton-nuleusdata
toquantifynuleareetsand willthenallowstudyof
thevolumeandenergydependeneof theollision
