High p T Charged Hadron Production at RHIC Claus O. E. Jørgensen Ph.D. defense Friday the 17 th of...

High pT Charged Hadron Production at RHIC

Claus O. E. Jørgensen

Ph.D. defenseFriday the 17th of September, 2004

14:30 in Auditorium A

- a search for the quark gluon plasma -

Ph.D defence, Claus Jørgensen, Friday 17th of September, 2004

Thanks to…

…and many more!


Outline

QCD and the Quark Gluon Plasma

Heavy Ion Collisions

High pT Particle Production

The BRAHMS Experiment

The Results

- What is it that we want to understand?

- How do we compress/heat the matter?

- What can we use to probe the dense matter?

- How do we measure the high pT particles?

- What have we found out?


The Structure of Matter

quark

sle

pto

ns

“forc

e m

edia

tors

”(b

oso

ns)

photons:electromagneticgluons:strong force

Z and W bosons:weak force

Building blocks of The Standard Model:

0.1m 10-8m 10-10m 10-14m 10-15m

Can we get a better

understanding of the forces of

nature?

size:


Quantum Chromo Dynamics (QCD)

QCD is the theory of the strong interaction

The force mediators (the gluons) carry the color charge (red, green, blue) gluons can interact.

• Confinement: the quarks (and gluons) are confined in white objects (hadrons)

The electromagnetic force:

The strong force:

r

V(r)

qualitative differencebetween the q-q and the Coulomb potentials

If you try to separate two quarks, you will just get twonew ones…

If you compress the QCD matter…

… the q-q potentialis screened and thehadrons disolve quark gluon plasma


icewater

steam

Phase Transitions

nucleus quark gluon plasmahadron gas

The transition to the quark gluon plasma (QGP) is predicted by lattice QCD…

temperature

degre

es

of

freedom

The first phase transitionhas been observed.

tem

pera

ture

[M

eV

]

excitation energy [MeV]

Where can we find a

quark gluon plasma?

(H2O and QCD matter)


The History of the Universe

Big Bang (15 billion years ago)

10-32 sec.: Inflation

Quark Gluon Plasma (in danish: ursuppe)

1 million years: Atoms are formed

1 billion years: Galaxies are formed

3-15 min: Atomic nuclei are formed

10-6 sec: Hadronization

15 billion years: Claus defends his thesis


The QCD Phase Diagram

Nuclear Matter

early universe

B

T

TC~170 MeV

940 MeVbaryon chemical potential

tem

per

atu

re

SIS

AGS

SPSRHIC

crab nebulaneutron stars

hadron gas

Where can we find QGP?

•The early Universe•Neutron stars•Heavy Ion Collisions

quark gluon plasma


Outline





The Results








Collision zone • is small (volume 0.00000000001 cm3) • is hot (1000 billion C)• is dense (1 billion kg/cm3)• evolves over short time-scales (life-time 0.000000000000000000001 sec)

Only the “final state” particles are observable information on the earlierstages can only be extractedvia models.


Evolution and Observables

before collision

initial collisions

quark matter production

collective expansion

hadronization

collective expansion

thermalfreeze out

time

A small but importantpiece of the puzzle!

elliptic flow

high pT hadronsthermal photons, J/Ψ

relative hadronabundances (stat. model)

transverse spectra(blast wave model)


(Definition of Variables)

We need to characterize the collision…

lower impact parametermore particles producedlower centrality

b

multiplicity (N charged particles)

Centralitymeasured in %(0-10%, 10-20%, 20-40%…)

Energy available?

…and the particles emitted from it.

Center-of-mass energy sNN

measured in GeV (=1.610-10 J)Ex: sNN=200GeV (99.99995%c)

central Au+Au collisions

beam

Transverse momentum:

pT = p sinθPseudo-rapidity:

η = -ln(tan(θ/2))

Spectrum (example)


Outline





The Results







•nucleon-nucleon collisions: - high pT particles are created in jets - well described


Like using X-rays to measurethe (dense) bones.

(fracture on the 4th metacarpal)

q

q

hadronsleadingparticle

leading particle

hadrons

jet

q

q

hadronsleadingparticle

•nucleus-nucleus collisions: - high pT particles can be used to probe the medium (tomography)

We only measure the yields(pT spectra), so how can we compare the nucleon-nucleon collisions to the nucleus-nucleus collisions?


Nuclear Modification Factor

ddpNdN

ddpNdR

TNN

bin

TAB

AB /

/2

2

How do we compare the high pT particle productionin nucleus-nucleus collisions and nucleon-nucleon collisions?

The number of binary collisions

Naïve expectation, i.e. no nuclear effects (no funny stuff going on) binary scaling at high pT

(RAB = 1)

High pT particles are created in hard collision:large energy transfer short collisions time incoherent collisions

Yields in nucleus-nucleus collisons

Yields in nucleon-nucleon collisions

Mean number of binary collisions


•Explained by multiple scattering in the initial state.•Modeled by transverse momentum broadening.

Cronin Enhancement Nuclear effects:

ShadowingCronin enhancement

Enhancement of high pT particles first observed by Cronin in the 70’s

Nuclear Modifications

Jet quenching


Shadowing

Low x partons are depleted (x is the momentum fraction carried by the parton)

The structure of a nucleon changeswhen it’s put inside a nucleus.


Nuclear effects:


Jet quenching

1

RExplained by:• multiple coherent scattering • saturation (boosted nuclei)

low energyhigher x

high energysmaller x

gluon density can saturate.

Effect below the saturationscale QSA1/3ey

It’s the Color Glass Condensate!


Energy loss of jets by•medium induced radiative energy loss (gluon bremstrahlung)•hadronic rescattering (is not enough to quench jets)

Jet quenching

Note that the medium is expanding!We need to model the dynamics to extract information on the density.


Nuclear effects:


Jet quenching


How can we disentangle the effects?

p+p collisions: •we need this as reference

d+Au collisions:•Cronin enh. due to mult. scattering?•shadowing in the Au nucleus?•jet quenching must be a small effect!

Au+Au (central):•Cronin enh. due to mult. scattering?•shadowing in the Au nuclei?•jet quenching?


Outline





The Results







Relativistic Heavy Ion Collider

RHIC


RHIC pictures

Two concentric rings6 interaction regions3.8 km long1740 superconducting magnets

boosterinjector

tandem

RHIC blue and yellow rings

BRAHMS


95°

30°

30°15°

2.3°


Midrapidity Spectrometer

Front Forward Spectrometer

Back ForwardSpectrometer

GlobalDetectors

Zero DegreeCalorimeter

| 18 m

It’s the Broad RAnge Hadron Magnetic Spectrometers!


A BRAHMS Event

TPM2

TPM1D5

D1T1

D2T2

beam

beamcollision point(vertex)

MRS at 90 degrees

FFS at 6 degrees

Reconstructed tracks


Event Counting

We want to make spectra,

we need to count the numberof collisions (events) Nevents and the number of tracks Ntracks

…still we need the centrality from the multiplicity (MA)…

multiplicity [a.u.]

T

tracks

Tevents dpd

dN

pN 2

11

…and the collision point (vertex) from the BB, ZDC and INEL counters.

MA

INEL

ZDC ZDCBB BB

We measure (almost) all the collisions and counting is easy!


x

y

TPM1D5

TPM2

(side view of MRS)

TOFW

Not detected!

Track Counting

We make surethat the trackscome fromthe collision.

We want to make spectra,

we need to count the numberof collisions (events) Nevents and the number of tracks Ntracks

T

tracks

Tevents dpd

dN

pN 2

11

Acceptance corrections are done by Monte Carlo simulations(vertex bins of 5 cm)

+ corrections for resolution and bin size effects…

•Inefficiencies of the detectors•Decay of particles

•Limited acceptance• .• .

We miss some tracks due to:


Outline





The Results







Results – The Spectra

Mmmh - very nice Claus, but it’s not easy to conclude anything from this! Why don’t you show the nuclear modification factors RAB?

Au+Au at sNN=200GeV

d+Au and p+p at sNN=200GeV


40-60%

Nuclear Modification at η=020-40%10-20%0-10%•Approximate binary scaling

in 40-60% central•Strong supp. in central collisions•What causes the suppression shadowing or jet quenching?

Au+Au @ sNN=200GeV

•Cronin enhancement!!! shadowing in the gold nucleus is not important•Suppression in the central Au+Au must be due to jet quenching!

d+Au @ sNN=200GeV

We made it to the

cover of PRL!


Energy systematics (pT=4GeV/c)central heavy ion collisionssNN= 17 GeV: Large Cronin enhancement (RAB>1)sNN= 62.4 GeV (NEW!): Slight suppression in central collisions.sNN= 200 GeV: Large suppression in central collisions.

What does it tell us about the medium?

This is also what other experts say: “…jet tomography analysis at RHIC presents strong evidence for the creation of the deconfined state of QCD.”[Vitev,qm04]

HIJING, Vitev-Gyulassy and Hirano-Nara:medium induced radiative energy loss

Cassing et. al.: hadronic rescattering & pre-hadronic interactions.“…there should be some additional and early partonicinteraction in the dense and possible colored medium.”[Greiner, qm04]

It must be due to radiative energy loss in

a deconfined medium!

Could the strong suppression be explained by jet quenching in a hadronic medium?


q

qq

q

40-60%20-40%10-20%

Nuclear Modification at η=2.2

•Reaction dynamics are important! The medium extended in the longtudinal direction. [Hirano&Nara, PRC68, 064902(2003)]

Suppression is similar to η=0•Does it simply scale with the density?

dN

/dη

η

•How important is saturation? Not easy to say in a symmetric collision system… maybe d+Au can help?

Au+Au @ sNN=200GeV0-10%


Nuclear Modification in d+Aud+Au @ sNN=200GeV

η=0

Cronin enhancement disappearsat higher pseudo-rapidities.

η=1

Higher η means lower x of the Au nuclei.•Is the suppression at higher η an effect of low-x gluon saturation?•Does the enhancement/suppression cancel out in the Au+Au collisions?

Summary

High pT particles provide a unique window to the early stages of heavy ion collisions at RHIC.

It’s not due to shadowing -we have made the d+Au check!

We’ve discovered a new effect:High pT particles are suppressed in central Au+Au collisions at RHIC the jets are quenched in the dense medium!

Energy loss studies show that the medium must be deconfined – we’ve made “free” quarks and gluons!

So what do you think?Have we discovered

the Quark Gluon Plasma?

The medium is extended in the longitudinal

direction, but I think we need more data to

understand the saturation effect.

Yes, I think so…

Come and have a drink and a snack in the T-

villa…


Backup - Models

HIJING: pQCD (hard) + strings (soft),shadowing, very schematic jet quenching (1992)

Vitev-Gyulassy: pQCD (hard), no softCronin kT broadening, shadowing and (GLV) jet quenching

Cassing et al: pQCD (hard) + strings (soft)kT broadening, shadowing and energy loss (pre-hadronic and hadronic)

Hirano-Nara: pQCD (hard) + hydro (soft)Cronin kT broadening, shadowing and (GLV) jet quenching


Backup – the CGC

ln(1/x)


sNN=62.4GeV

At sNN=62.4GeV: Cronin enhancement in semi-peripheralcollisions. Canceled out by energy loss in central.

At sNN=200GeV: Strong suppression in central collisions.

40-60%20-40%10-20%At SPS (sNN=200GeV): RAB is “consistenly above1 due to strong Cronin effect via initial multiple scattering, leaving not much room for parton energy loss…” [Wang, nucl-th/00405029]

0-10%Au+Au @ sNN=62.4GeV

High p T Charged Hadron Production at RHIC Claus O. E. Jørgensen Ph.D. defense Friday the 17 th of...

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Transcript of High p T Charged Hadron Production at RHIC Claus O. E. Jørgensen Ph.D. defense Friday the 17 th of...