New results from PHENIXWhat’s happening at high pT?

A preview of PRL covers to come

Outline & summary

The high pT suppression is real!

Continues to higher pT

In both 0 and charged particles

High pT particles do come from jets!Must use caution to avoid confusion with v2

Hadronic composition at high pT

Is mysteriousChanges with centrality

Goals of RHIC

Collide Au + Au ions at high energy130 GeV/nucleon pair c.m. energy in 2000s = 200 GeV/nucleon pair in 2001

Create in the laboratory high temperature and density matteras existed ~1 sec after the Big Banginter-hadron distances comparable to that in neutron starsheavy ions to achieve maximum volume

Study the hot, dense systemthermal equilibrium?do the nuclei dissolve into a quark gluon plasma?characteristics of the phase transition?transport properties of plasma? equation of state?

QCD Phase Transition

transition affects evolution of early universelatent heat & surface tension matter inhomogeneity in evolving universe?

equation of state of nuclear matter compression in stellar explosions

we don’t understandhow process of quark confinement workshow symmetries are broken by nature massive particles from ~ massless quarks

did something new happen at RHIC?

Study collision dynamics (via final state)

Probe the early (hot) phase

Equilibrium?hadron spectra, yields

Collective behaviori.e. pressure and expansion?elliptic, radial flow

vacuum

QGP

Particles created early in predictable quantity interact differently in QGP vs. hadron matterfast quarks, J/fast quarks, J/, strange, strangequark content, quark content, thermal radiation

PHENIX at RHIC

2 Central spectrometers

2 Forward spectrometers

3 Global detectors

PHENIX philosophy: optimize for

signals / sample soft physics

fast partons as probe of the plasma

hadrons

q

q

hadronsleadingparticle

leading particle

schematic view of jet productionJets in heavy ion collisions: observed via fast leading particles or azimuthal correlations between the leading particles

But, before they create jets, the scattered quarks radiate energy in the colored medium

decreases their momentum fewer high pt particles“jet quenching” affect away side jet

Pion spectrum - at low pTs = 200 GeV per nucleon pair

Pion spectrum - 0 to high pT

Pion spectrum – charged at very high pT

Use RICH to taghigh pT pions

PHENIX 0 spectrum in p-p collisions

Measure reference spectrum in the SAME experiment

Remove extrapolation errors

Reach higher pT than UA1

Agrees with NLO calculation

Compare Au+Au to p+p

Use measured p+p to predict rate of plasma probe in Au+Au

Hard scattering probability scales with # of binary nucleon-nucleon collisions

Construct RAA = pp

centralbinarycentral

Yield

NYield /

RAA should be 1 if nothing happens to the probe

0 yield in AuAu vs. p-p collisions

70-80% PeripheralNcoll =12.3 ±4.0

30-40% Semi-centralNcoll =220±14

PHENIX Preliminary

pp

centralbinarycentral

Yield

NYield /

Suppression due to parton energy loss?

Data not consistent with predictions including no energy loss

GLV L/ =4 somewhat better agreement

Both predictions including energy loss consistent with data up to 5 or 6 GeV/c

but maybe not quite…

P.Levai, Nuclear Physics A698 (2002) 631.

X.N. Wang, Phys. Rev. C61, 064910 (2000).

Charged particle pT spectra from 200 GeV

pT <2 GeV/c, slope increase flow

pT >2 GeV/c, slope decrease suppression

h+ + h-

Centrality dependence of change

suppression stronger with centrality & increased pT

peripheralbinaryperipheral

centralbinarycentral

NYield

NYield

//

Suppression to 9 GeV/c!

Factor consistent for 3 independent measurements

Difference in charged hadron ratio and neutral pion ratio accounted for by particle composition

Comparing different channels

How do high pT yields scale?

vs. binary collisions:continuous decrease as

function of centralityfactor ~ 3.5 from

peripheral to central vs. participants:

first increase, then decrease as function of centrality

for Npart > 100 have 3 change (scaling or no?)

surface emission? re-interactions?accident?

18% scaling uncertainty from corrections

x of struck parton

• if pT(had) / pT(jet) ~ 1 then xT ~ x(parton) at y=0

• SPS and RHIC at different x!

RHIC:~1.6 x 10-2 at 200 GeVstill not very small…

xT =

Learn from xT

Hard scatterings most probably of gluons Shadowing not a large effect as x is not very small

Beyond leading twist not as clear… Natural to compare with gluon jets studied in e+e-, BUT

Our leading hadrons are very soft (<10 GeV/c)We mostly see just part of the fragmentation functionjet falls faster thanD(z), so we probe“kinematic limit”with large zhadron spectra maybe dominated by q jets

Parent x for 4 GeV/c hadron

From X.N. Wang

High pT hadrons do come from jets

Look at particle correlations for jet signature“trigger” on leading photon with pT > 2.5 GeV/calso look at charged-charged correlationsJets are observed in Au + Au

v2 (nominally collectively elliptic flow) at high pT is also sensitive to jets

Bias effect:Trigger requirement requires leading particle!Systematic study via trigger , hadron

use 2.5 GeV , perhaps NOT dominated by 0…?

Identifying Jets - Angular Correlations

Remove soft background by subtraction of mixed event distribution

Fit remainder:Jet correlation in ; shape taken from PYTHIAAdditional v2 component to correct flow effects

PHENIX Preliminaryraw differential yields

2-4 GeV

Verify PYTHIA using p+p collisions

(neutral E>2.5 GeV + 1-2 GeV/c charged partner)

||<.35 ||>.35

ake cuts in to enhance near or far-side correlationsBlue = PYTHIA

In Au+Au collisions

1-2 GeV partner

(neutral E>2.5 GeV + charged partner)

||<.35 ||>.35

1/N

trig d

N/d

1/N

trig d

N/d

Correlation after mixed event background subtraction

Clear jet signal in Au + AuDifferent away side effect than in p+p

Jet strengthSee non-zero jet strength as partner pT increases!

jets or flow correlations? fit pythia + 2v2vjcos(2)

partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c

1/N

trig d

N/d

v2

vj

1-2 GeV/c

min bias 200 GeV Au+ Au

v2 at high pT

v2 via reaction plane at =3-4 and via 2-particle correlations similarNo jet contamination of

reaction planeDiverge at pT> 4 GeV/c?

Low pT as expected from hydrodynamics

v2 > 0.15 at pT>3 GeV/c interpretation? 15% jets per STARflow vs. hard processes

contribution unclear

Au+Au at sNN=200GeV

v2

r.p. ||=3~4min. bias

v2 of identified hadrons

Negativespi-&K-,pbar

Positivespi+&K+,p

PHENIX Preliminary PHENIX Preliminary

pT (GeV/c) pT (GeV/c)

v2

p cross ,K

not expected from hydro

modifiedand p not??

Look at charged particle spectra

0 – 5 % 5 -10 %10- 15 %15 – 20 %20 – 30 %30 – 40 %40 – 50 %50 – 60 %60 – 70 %70 – 80 %

80 – 93 %

Au+Au at s = 200 GeV PHENIX preliminary

PHENIX Preliminary

hydrodynamic analysis of spectra

PHENIX Preliminary

T = 1224 MeVt = 0.72 0.012/dof = 30.0/40.0

Simultaneous fit tomT-m0 < 1 Gev/C

Au+Au at s = 130 GeV

200 GeV similar but T, a bit

Extrapolate soft component using hydrodynamics

Hydrodynamic flow modifies pt threshold where hard physics starts to dominate

physics has soft (thermal) contributions until pt 3 GeV/c

Calculate spectra usinghydro parametersh+ + h - = , K, p

Compare sum to measured Charged particle pT spectrum

J. Burward-Hoy

Particle composition?

Dynamics affect p/pion ratios

hydro boosts baryons to higher pt Jet quenching should reduce yield

(by ~3-5)baryons less depleted as less likely to be jet leading particles

Vitev & Gyulassy nucl-th/0104066

pbar/ pi-

Ratio of protons to pions ~1 at high pT for central collisions

Flattens. Turnover not seen.

Now extend to higher energy, pT

Centrality dependence of p/pi

+

-

•Ratios reach ~1 for central collisions

•Peripheral collisions lower, but still above gluon jet ratios at high pT

•Maybe not so surprising 1)“peripheral” means 60-91.4% of total

2) p/pi = 0.3 at ISR

How do protons scale with Ncoll/Npart?

Scale with Ncoll (unlike )?!

High pT baryons scale with Ncoll!

Low pT near Npart scaling

But baryons with pT > 2 GeV/cbehave very differently!From jets? Unsuppressed??

J. Velkovska

Use pi/h to look at higher pT

What’s this?protons??

How about electrons?

PHENIX looks for J/ e+e- and

There is the electron.

A needle in a haystack:find electron without mistaking a pion at the level of one in 10,000

Ring Imaging Cherenkovcounter to tag the electrons“RICH”

uses optical “boom” whenvpart. > cmedium

We do find the electrons

Energy/Momentum

Electron enriched sample (using RICH)

All tracks

And J/

Centrality dependence of charm

conclusions

The high pT suppression is real!

Continues to higher pT

In both 0 and charged particlesCharmed quarks do not show energy loss

High pT particles do come from jets!Must use caution to avoid confusion with v2

Hadronic composition at high pT

Is mysteriousChanges with centralityWhat’s going on with protons & antiprotons??

Need theoretical help!!

3 GeV/c region of spectrum is complicatedMix of soft & hard processes

via single particle extrapolationv2 large & not a measurement artefact

Large proton contribution to spectrumflow seems a reasonable explanation

BUT – why is pi/h so low out to 8 GeV/c????

Suppression of pions, but not leading baryons?PHENIX has and correlations to help figure it out

Charmed quarks do not indicate large energy loss

Backup slides

Charged hadron correlations - small

•Fit charged correlations with v2 + Gaussian (fixed pT)

•Jet signal visible via Width of near-side Gaussian decreases with pT

No significant centrality dependence on near-side

Cor

rela

tion

wid

th

jT

pT Correlation width jT/pT

Note pbar/p behavior

Centrality dependence only for pT > 3 GeV/c

Peripheral collisions have quite a few protons at mid-y

Considerable baryon stopping still! Caution for high pT physics interpretation!!

High pT -/+ ratio

ratio ~1 at high pT in Minimum Bias data

Slightly decreasing in large Npart region?

Hydrodynamics-inspired fit

emperatureFreezeOutTT

ocitySurfaceVelR

r

dT

pI

T

mKfmA

dmm

dN

FO

s

ns

FO

T

FO

TT

TT

)(tanh;

)sinh()cosh()(

1

01

After Schnedermann, et al. Phys. Rev. C48, 2462 (1993)

<pt> increases with centrality

Expect such a trend from radial flowbut also from partonic multiple scatteringand gluon saturation

don’t know whether final or initial state effect

Can also get v2 from correlations

PHENIX (and PHOBOS) measure correlation function in azimuthal angle

from same event

from mixed events

C() =ratio

dN/d() [1 + 21cos() + 22cos(2)] 2 v2Impose pT threshold & see jet correlations

At high pT

jet correlations weak or missing!

Reaction plane results a mystery...

Hydrodynamicsno longer dominates

Correlation method on HIJING picks out back-to-back particles from jets

For datacorrelation & reaction plane methods agree

J. Rak

hard/soft competition as probe

22/ 1 2v ( ) cos 2T TdN dydp d p

Above pt ~ 1.5 GeV/c, hydrodynamic flow in the reaction plane has competition from hard processes, which are not correlated with that plane

so look for disappearance of elliptic flowdepends on amount of energy loss!

Nuclear effects in initial stage

Structure functions are modified in nuclei Shadowing in small-x region

due to high parton density from superposition of all the nucleons

F2A(x)

---------AF2

N(x)

Accessible at RHIC

Measure radial expansion

Teff = Tfo + m <radial>2

Tfo = 140 - 150 MeV radial = 0.5 - 0.6 (higher for central collisions)

was 0.4 at lower energyless flow in peripheral collisions!

baryon yields

PHENIX preliminary

antiproton dN/dy = 20!

Was 0.18 at SPS

strangeness production

PHENIX preliminary

s=17 GeV Pb+PbPhys.Lett.B 471, 6 (1999)

Both K+/ and K-/increase with NpartPeripheral collisions near pp valueK+/ and K-/do not diverge as at SPS,AGS

K/ ats=200 p+p

Z.Phys.C41,179(1988)(UA5)

For RHIC and CERNThanks to Xin-Nian Wang!

For pt = 4 GeV/c hadron from jet fragmentation, what is x distribution of parent parton?

Does not include ktbroadening

From Xin-Nian’s calculation

For CERN energy<z> ~ 0.87distribution is fairly symmetric

for RHICpeak z ~ 1long tail to higher x parent parton

i.e. tail to smaller zestimate with z range 0.7-1.0

remake x plot

more realistic fragmentation function

z range indicated by horizontal bar:answer is not very different

to excludeknown softphysicsregime

PT = 3 GeV/c(is a safer boundary for hard processes)

Thanks to X.N. Wang