Ph.D. defense of Peter Christiansen, 27. May 2003 Information Please switch off mobile phones....
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Transcript of 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 :
Page 23 : Light systems are Ni+Ni (FOPI), Si+Al(E802), S+S(NA35)
Page 97 : Acceptance plot (2.8 < y < 2.95) is wrong
Ph.D. defense of Peter Christiansen, 27. May 2003
Stopping in central GeV Au+Au collisions
at RHIC200NNs
Peter Harald Lindenov Christiansen
Niels Bohr Institute
Faculty of Science, University of Copenhagen
Ph.D. defense of Peter Christiansen, 27. May 2003
Outline of talk
• Introduction to heavy ion physics
• Stopping
• The BRAHMS experiment
• Analysis
• Results
• Conclusions
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
QCD potential
Gluons carries color Gluons can interact with gluons
Ph.D. defense of Peter Christiansen, 27. May 2003
Quark Gluon Plasma
Confinemt
Deconfinemt
?
Lattice QCD
calculations
Experimental heavy ion physics
Simulations1 2 3 4
Ph.D. defense of Peter Christiansen, 27. May 2003
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
STAR
PHOBOS
PHENIX
A Real Collision
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
How to measure stoppingUse rapidity variable Distributions are boost invariant
z
z
pE
pEy log
2
1
Full stopping Full transparency
AFTER COLLISION2 extreme final states
BEFORE COLLISION
“Velocity” space
Physical space
Ph.D. defense of Peter Christiansen, 27. May 2003
Two physics pictures
Transparency – excited color field
Stopping – excited nucleons
Ph.D. defense of Peter Christiansen, 27. May 2003
p+p collisions
beam
laby
yy *
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.
Ph.D. defense of Peter Christiansen, 27. May 2003
A+A Collisions Geometry
Geometric Glauber model calculations can be used to calculate the collision geometry.
participants spectators
?
b is the impact parameter.
Ph.D. defense of Peter Christiansen, 27. May 2003
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.
E917
Ph.D. defense of Peter Christiansen, 27. May 2003
Energy dependence
?NA49E866/E877
Au+Au Pb+Pb
Energy
Ph.D. defense of Peter Christiansen, 27. May 2003
How to quantify stoppingUse rapidity loss : finalinitial yyy
midbnet
y
y midfinal ydydy
ydNyyy
initial
mid
)(
)(
For symmetric collisions the last term is calculated as :
MAX
MIN
Relative rapidity loss independent of beam energy!
What happens at RHIC ?
ettbeam yyYYy arg where/
Ph.D. defense of Peter Christiansen, 27. May 2003
What happens at RHIC ?Will there be stopping ? Or transparency ?
BRAHMS can tell!
The BRAHMS detector
A BRAHMS eventD2T2
T1
TPM2
BEAM
TPM1
D5
D1
MRS 90 deg
FFS 6 deg
Ph.D. defense of Peter Christiansen, 27. May 2003
Event reconstruction - Global
1. Interaction Point
2. Centrality
Ph.D. defense of Peter Christiansen, 27. May 2003
Event reconstruction - Tracks
1. Local tracking
2. Matching (momentum)
3. Particle identification
1
12
22
pm
TOF
L
Ph.D. defense of Peter Christiansen, 27. May 2003
Proton PID using TOF
m2 momentum dependence parameterized by :2222
2
224224
2
)()1(4
2
TOFppm pmp
p
mmpm
multiangle
K
p
2cuts
1
12
22
pm
Ph.D. defense of Peter Christiansen, 27. May 2003
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
K
p
Ph.D. defense of Peter Christiansen, 27. May 2003
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?)
Ph.D. defense of Peter Christiansen, 27. May 2003
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
ACC
DATACORR
NORMNp2
1
d
p2
1
bineventsT
2
T
Tdydp
Ndyddpp
Nd
dp
NdE
TT
3
3
3
Invariant yield :
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
Acceptance correction
Simulation with pions. Pions are stopped when they hit the magnet and all physical processes except energy loss have been turned off.
~0.5%
Ph.D. defense of Peter Christiansen, 27. May 2003
Acceptance correction
The acceptance correction should correct for the limited geometrical coverage of the spectrometers.
The correction is calculated by simulation.
THROWN
ACCEPTEDACC
Ph.D. defense of Peter Christiansen, 27. May 2003
A pT-spectrum
Ph.D. defense of Peter Christiansen, 27. May 2003
Extracting dN/dy
Fit pT spectra and use the fit to extrapolate into regions where we don’t measure to get dN/dy.
TmTT
TmT
TpT
TT
T
T
T
emNpf
eNpf
eNpf
pmm
/
/
/
22
)(
)(
)(
The difference between the fits depends on the fit-range. In the following mT-exponentials are used at all rapidities.
Ph.D. defense of Peter Christiansen, 27. May 2003
Rapidity Coverage
Ph.D. defense of Peter Christiansen, 27. May 2003
Examples of pT-spectra
0-5% central collisions
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
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.
Ph.D. defense of Peter Christiansen, 27. May 2003
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 :
NsNsN
NC
p
p
HIJING : s = 0.9/0.4
C~0.75 at all rapidities
Ph.D. defense of Peter Christiansen, 27. May 2003
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)
Ph.D. defense of Peter Christiansen, 27. May 2003
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)
Ph.D. defense of Peter Christiansen, 27. May 2003
Rapidity Loss (MCM fit)
beamxY yyxexYexNxf where))(()( )(
Fit the data with the MCM inspired function :
Ph.D. defense of Peter Christiansen, 27. May 2003
Rapidity Loss Results
BLUE is DATARED is MODELS
Constant relative rapidity loss is broken at RHIC.
Ph.D. defense of Peter Christiansen, 27. May 2003
Net-proton energy dependence
Ph.D. defense of Peter Christiansen, 27. May 2003
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.
Ph.D. defense of Peter Christiansen, 27. May 2003
Phase diagram of hadronic matter
Ph.D. defense of Peter Christiansen, 27. May 2003
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)
Ph.D. defense of Peter Christiansen, 27. May 2003
Multi Chain Model)()()()()(
1/
1/ yQmPWryYQnPWry
dy
dN BA N
mmBA
N
nAAnABBB
pXBA
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
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
Tracking 1
Ph.D. defense of Peter Christiansen, 27. May 2003
Tracking 2
Ph.D. defense of Peter Christiansen, 27. May 2003
Tracking 3
Ph.D. defense of Peter Christiansen, 27. May 2003
Tracking 4
Ph.D. defense of Peter Christiansen, 27. May 2003
Y=3 discrepancy 1
4 deg HIGH value
3 deg LOW value
Nffs
Nfs
Ph.D. defense of Peter Christiansen, 27. May 2003
Y=3 discrepancy 2
4 deg HIGH value
3 deg LOW value
Ph.D. defense of Peter Christiansen, 27. May 2003
Net-protons vs Net-baryons 1
The effect of lambdas.
HIJING SIMULATION
Associated productionp+K+
p+π-
Ph.D. defense of Peter Christiansen, 27. May 2003
Net-protons vs Net-baryons 2
The effect of neutrons :
E941
ybeam = 3.7
19GeVp+Be,Al,Cu,Pb (min. bias) RHIC simulations
Ph.D. defense of Peter Christiansen, 27. May 2003
Rich efficiency 1
T5 H2
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.
70 cm
Ph.D. defense of Peter Christiansen, 27. May 2003
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
Ph.D. defense of Peter Christiansen, 27. May 2003
Centrality dependence 1
Ph.D. defense of Peter Christiansen, 27. May 2003
Centrality dependence 2
Ph.D. defense of Peter Christiansen, 27. May 2003
. Checks