Ph.D. defense of Peter Christiansen, 27. May 2003 Information Please switch off mobile phones....

Ph.D. defense of Peter Christiansen, 27. May 2003

Information

Please switch off mobile phones.

Corrections :

Light systems are Ni+Ni (FOPI), Si+Al(E802), S+S(NA35)

Acceptance plot (2.8 < y < 2.95) is wrong

Stopping in central GeV Au+Au collisions

at RHIC200NNs

Peter Harald Lindenov Christiansen

Niels Bohr Institute

Faculty of Science, University of Copenhagen

Outline of talk

• Introduction to heavy ion physics

• Stopping

• The BRAHMS experiment

• Analysis

• Results

• Conclusions

Quantum Chromo Dynamics (QCD)3 color charges (red, green, blue)

Hadrons have to be colorless

Baryons have all 3 colors

Mesons has a color and an anti-color

A single quark cannot be observed because it has color!

The quarks are confined inside the hadrons!

This talk will be about proton and anti-protons

Hadrons

Baryons

Mesons

QCD potential

Gluons carries color Gluons can interact with gluons

Quark Gluon Plasma

Confinemt

Deconfinemt

Lattice QCD

calculations

Experimental heavy ion physics

Simulations1 2 3 4

Relativistic Heavy Ion ColliderFirst heavy ion collider in the world.

L=2*1026cm-2s-1

R=1200Hz

Data presented here is from the first Au+Au run at GeV200NNs

PHOBOS

PHENIX

A Real Collision

What is stopping ?

Energy conservation. Kinetic energy of initial baryons is used to create a hot and dense zone. Baryon (qqq) number conservation.Before: 2*197 baryons After:2*197 net-baryons (baryons-anti-baryons)

Stopping is the study of the energy loss suffered by the baryons in the collision. The energy loss happens in 3 ways :• Initial interactions• Rescattering of partons and hadrons• Decays

How to measure stoppingUse rapidity variable Distributions are boost invariant

pEy log

Full stopping Full transparency

AFTER COLLISION2 extreme final states

BEFORE COLLISION

“Velocity” space

Physical space

Two physics pictures

Transparency – excited color field

Stopping – excited nucleons

p+p collisions

Because of the target and projectile symmetry the rapidity loss is symmetric around mid-rapidity.

target projectile

p+p collisions exhibits a large degree of transparency.

A+A Collisions Geometry

Geometric Glauber model calculations can be used to calculate the collision geometry.

participants spectators

b is the impact parameter.

Au+Au collisions at AGS• A+A collisions is more than a sum of p+p collisions

• p+p picture is recovered in peripheral collisions

• In central collisions the rapidity distibution peaks at mid-rapidity

• Can be described by two Gaussians.

Energy dependence

?NA49E866/E877

Au+Au Pb+Pb

Energy

How to quantify stoppingUse rapidity loss : finalinitial yyy

midbnet

y midfinal ydydy

ydNyyy

initial

For symmetric collisions the last term is calculated as :

Relative rapidity loss independent of beam energy!

What happens at RHIC ?

ettbeam yyYYy arg where/

What happens at RHIC ?Will there be stopping ? Or transparency ?

BRAHMS can tell!

The BRAHMS detector

A BRAHMS eventD2T2

MRS 90 deg

FFS 6 deg

Event reconstruction - Global

1. Interaction Point

2. Centrality

Event reconstruction - Tracks

1. Local tracking

2. Matching (momentum)

3. Particle identification

Proton PID using TOF

m2 momentum dependence parameterized by :2222

224224

)()1(4

TOFppm pmp

multiangle

Proton PID in the FS

The ring radius in the RICH depends on the velocity.

The RICH is used to identify protons directly and as a VETO counter for pions and kaons. Important to correct for contamination.

Ring ImagingCHerenkov

Proton and anti-proton acceptance

A single spectrometer setting covers a small fraction of phase space, but by combining different settings pT-spectra can be obtained at many different rapidities.MRS(0<y<1), FFS(1<y<2), FS(2.0<y<3.5?)

Constructing pT-spectra

DATA : Measured protons and anti-protons

ACC : Geometrical acceptance

CORRections

• Tracking efficiency

• PID efficiency (slat efficiency)

• Multiple scattering and nuclear absorption correction

DATACORR

NORMNp2

bineventsT

Ndyddpp

Invariant yield :

Data selection

Global cuts :

• Interaction point (BB & ZDC agrees, and close to nominal IP)

• Centrality : 0-5 % shown here

Track cuts :

• Pointing (Track points back to the IP)

• Magnet fiducial cut (No intersection)

PID cuts :

• TOF (TOFW, H1, H2) and RICH

Acceptance correction

Simulation with pions. Pions are stopped when they hit the magnet and all physical processes except energy loss have been turned off.

Acceptance correction

The acceptance correction should correct for the limited geometrical coverage of the spectrometers.

The correction is calculated by simulation.

THROWN

ACCEPTEDACC

A pT-spectrum

Extracting dN/dy

Fit pT spectra and use the fit to extrapolate into regions where we don’t measure to get dN/dy.

The difference between the fits depends on the fit-range. In the following mT-exponentials are used at all rapidities.

Rapidity Coverage

Examples of pT-spectra

0-5% central collisions

Rapidity densities dN/dy

Plots has statistical errors only.Typical systematic errors are :~1.0 (0<y<1) ~2.6 (y~2) ~1.6 (y~3)Net-proton distrubution is far from full stopping.

Full stopping Full transparency

Reflected

Net-proton energy dependence

The shape of the net-proton distribution measured at RHIC is different from what is observed at lower energies.

At RHIC the mid-rapidity region is almost net-proton free. Pair production dominates at RHIC.

Comparison to Models INet-protons measured includes protons from hyperon decays e.g. Λp+-.

To compare with models the protons from hyperon decays have to be removed. BRAHMS does not measure Λ, instead we use models and simulations to correct :

HIJING : s = 0.9/0.4

C~0.75 at all rapidities

Comparison to Models II

Hijing (Strings, no rescattering)

UrQMD (Transport calculation, resonance excitations, rescattering)

Hijing describes the data best, BUT Hijing does not reproduce Λ/p (y=0) or p-bar/p (0<y<3)

Rapidity Loss Estimates

All net-protons at y = 3.5Maximal rel. rap. loss = 0.24

All net-protons at y = 5.0Minimal rel. rap. loss = 0.16

29 net-protons measured (0 < y <3)

Estimate total :

350 participants 140 initial protons

Assume 140 total 70 (y>0)

41 outside acceptance (y>3)

Beam rapidity

Example of processes :

p+pn+p+π+ (pn)

n+nn+p+π - (n p)

p+N +K++N (p )

p+ π - ( p)

Rapidity Loss (MCM fit)

beamxY yyxexYexNxf where))(()( )(

Fit the data with the MCM inspired function :

Rapidity Loss Results

BLUE is DATARED is MODELS

Constant relative rapidity loss is broken at RHIC.

Net-proton energy dependence

Conclusions

• The observed net-proton yield increases from 7.3±0.5(stat.) ±1.0(syst.) at y = 0 to at 12.9±0.4(stat.) ±1.6(syst.) at y=3.

• The collisions exhibits a large degree of transparency. This has not been observed in collisions at lower energies.

• Hijing reproduce the observed net-proton yields while UrQMD over predicts the stopping power. This suggests that the same string physics as p+p can describe the results.

•Scaling of rapidity loss is broken at RHIC. The relative rapidity loss is lower than what was observed in collisions at SIS, AGS, and SPS energies.

Phase diagram of hadronic matter

Model predictions• Geometric Glauber model calculations can be used to

calculate the collision geometry.

• Most interactions are soft so pQCD can not be used.

• The physics learned from p+p collisions can be used as a starting point, but there are important differences :

Formation times, Off-shell cross sections, Rescattering

The models chosen are :

• MCM (Simple)

• Hijing (Strings)

• UrQMD (Transport)

Multi Chain Model)()()()()(

1/ yQmPWryYQnPWry

dN BA N

nAAnABBB

B is the projectile(y=Y), A is the target(y=0)

r is the ratio of protons to nucleons

W is the number of participants

P(n) is the fraction of nucleons that has n binary collisions

Q are the fragmentation functions that contains the physics

SIS AGS

SPS RHIC

HijingEnergy lost in hard scatterings is resolved first.

All the soft scatterings results in string excitations.

The strings decays after all collisions have been resolved according to Lund string model (JETSET).

The strings can be (de)excited by more scatterings after they are created with a modified probability.

Figure is taken from Phys. Lett. B 443, p 45

UrQMDTransport theory. Only 1234 scatterings. All particle production from decays. Propagate as free particle between scatterings.

Reduced cross section of strings and decay time of strings is important. Strings decay time .

σ=1GeV/fm σ=3GeV/fm

Tracking 1

Tracking 2

Tracking 3

Tracking 4

Y=3 discrepancy 1

4 deg HIGH value

3 deg LOW value

Y=3 discrepancy 2

4 deg HIGH value

3 deg LOW value

Net-protons vs Net-baryons 1

The effect of lambdas.

HIJING SIMULATION

Associated productionp+K+

Net-protons vs Net-baryons 2

The effect of neutrons :

ybeam = 3.7

19GeVp+Be,Al,Cu,Pb (min. bias) RHIC simulations

Rich efficiency 1

NO CONFIRMATION

Focus on veto method (essentially all yield) :1) Particle absorption or decay after T5 and decay

product is not identified in the RICH. p=10GeV/c, length=1m, P(pi)=0.2%, P(K)=1.3%2) Algorithm inefficiency.

Rich efficiency 2Use H2 to estimate contamination. 1/beta-1/beta(proton).

Shape of pion and kaon dist from those identified by the RICH.

Shape of protons from directly identified at higher momentum.

Fit H2 distribution of vetoed protons with sum of pi,K, p.

Fixed pi+K contamination(thesis) Different contamination of pi,K

Centrality dependence 1

Centrality dependence 2

. Checks

Ph.D. defense of Peter Christiansen, 27. May 2003 Information Please switch off mobile phones....

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