Transcript Document

First-day physics with Pb
beam at ALICE
Francesco Prino
INFN – Sezione di Torino
Firenze – February 17th 2006
Questions that can be answered
with few days of data taking
• How many particles are produced ?
• How does the number of particles scale with centrality ?
 Charged multiplicity in different centrality bins
• Which is the relative contribution of hard ( Ncoll ) and soft ( Npart )
processes in particle production?
• Which energy and parton densities are attained ?
 Particle density at midrapidity in different centrality bins (dNch/dhmax)
• Which degree of stopping/transparency characterize the reaction ?
• What happens in the target and projectile fragmentation regions?
 Pseudorapidity (polar angle) distributions of unidentified particles (dNch/dh)
• Is the system strongly interacting and “early thermalized” ?
 Azimuthal distributions of unidentified charged particles (v2, and v4?)
Day 1 - statistics and analysis
• dN/dh and v2 of unidentified particles require:
 ~104 events
• Hadronic physics (pT spectra, particle ratios) require:
 ~105 events
 Particle identification
 Better understanding of the detector
 Calibrations, alignment …
• Data taking scenario and statistics:
 Also in a low luminosity scenario (ex. trigger rate <≈ 10 Hz)
104 events can be collected in few hours and 105 in few days
• Day 1 results will probably be more limited by analysis
(i.e. understanding the detector) than by statistics
Centrality determination
Event geometry
• The impact parameter (b) detemines the number of
nucleons that participate in the collision (Npart)
Small impact parameter
Many participant nucleons
Big System
Many produced particles
Large impact parameter
Few participant nucleons
Small System
Few produced particles
ZDC detectors
ZP
•
Two sets of calorimeters on both
sides of the interaction point
 made in Torino and Cagliari
•
Each set composed by:
 2 hadronic “spaghetti calorimeters”
 1 for spectator neutrons, 1 for spectator protons
 Placed at 116 m from the interaction point
 1 forward electromagnetic calorimeter (ZEM)
 Placed at 7 m from the interaction point
Proton ZDC
Neutron ZDC
Beam pipes
ZN
ZDC and centrality determination
• EZDC correlated with number of spectators BUT
two branches in the correlation
• Break-up of correlation due to production of
fragments (mainly in peripheral collisions)
% of sinelastic
• ZEM needed to solve the ambiguity
• Signal with relatively low
resolution, but whose amplitude
increases monotonically with
centrality
Multiplicity and pseudorapidity
distributions
Key measurements
• Scaling of particle multiplicity vs. energy
 Change the energy available for particle production
 Change the number of collision per participant
Handle for changing the balance between soft and hard processes
• Scaling of particle multiplicity vs. centrality of the collision
 Change the volume of particle production region ( Npart)
 Change the number of collision per participant
Second handle for changing the balance between soft and hard processes
dN/dh – basics
pT = pL
q = 45 (135) degrees
h = ±0.88
pL>>pT
pT>pL
• Midrapidity peak / plateau
 Sensitive to hadroproduction
details
 Related to energy density
 Bjorken formula (requires a
“central-plateau structure” in the y
distribution of produced particles)
pL>>pT
 BJ
mT  dN


Ac  f  dy


 y 0
• Width of the distribution
 Information on longitudinal
expansion, stopping vs.
trasparency
• Fragmentation regions
 Investigate effects connected
with target and projectile
fragmentation
Width of dN/dh distribution
• Information on longitudinal expansion and degree of
stopping/transparency
• In the case of full-stopping:
 Baryon-rich fireball
 Single thermal source emitting at rest (isotropically)
FWHM = 1.8
dN
 const
d
dN
 sin 
d
dN
1

dh cosh2 h
Width vs. centrality at SPS and RHIC
NA50 at 158 GeV/c (√s=17.2 GeV)
√s= 200 GeV
√s= 130 GeV
• Widths larger than expected from a single thermal source emitting at
rest
• Gaussian width (FWHM) decreases with increasing centrality
Observed also by NA35, WA80, Helios/Emulsion, E802
Stopping power effect
Decreasing contribution of protons from target and projectile fragmentation
Midrapidity peak / plateau
• The maximum of pseudorapidity distribution (dNch/dh |
max
) at hcm=0:
 Most frequently used variable to characterize the multiplicity of the
interaction
Independent of phase space acceptance  allows comparison between different experiments
 Increases with collision energy (√s) and centrality
central
central
peripheral
peripheral
dN/dhmax vs. centrality
PHOBOS PRC 2004, nucl-ex/0405027
• Introduce yield per participant pair = dN/dhmax /(Npart/2)
 A flat behaviour reflects the linear dependence of dN/dhmax on Npart
• Yield per participant pair increases by ≈ 25% from peripheral to
central Au-Au collisions
 Contribution of the hard component of particle production ?
 BUT:
The ratio 200 / 19.6 is independent of centrality
A two-compoment fit with dN/dh  [ (1-x) Npart /2 + x Ncoll ] gives compatible values of x (≈
0.13) at the two energies.
 Explained by models based on gluon saturation (Color Glass Condensate)
Warning
• Npart is not a direct experimental observable and affects the scale
of both axes of plots of yield per participant vs. Npart
• Different methods of evaluating Npart give significantly different
results!
NA50 at 158 A GeV/c
s = 130 GeV
Density at midrapidity vs. energy
• WARNING when comparing dN/dhmax between collider and fixed
target experiments: pseudo-rapidity h is not boost invariant
 Conversion from dN/dh|lab to dN/dy (Lorentz invariant) and then to
dN/dh|cm
dN ch
dN ch
m2
 1 2
dpT dh
mT cosh 2 y dpT dy
dN/dhmax in central heavy
ion collisions increases as
ln(s) from AGS to top
RHIC energies
Different √s
dependence in pp and
heavy ion collisions
Limiting fragmentation (I)
• Study particle production in the rest frame of one of the
two nuclei
 Introduce the variable y’ = y - ybeam (or h’ = h – ybeam )
• Limiting fragmentation
Benecke et al., Phys. Rev. 188 (1969) 2159.
 At high enough collision energy both
d2N/dpTdy and the particle mix reach a
limiting value in a region around y’ = 0
 Also dN/dh’ reach a limiting value and
become energy independent around h’=0
 Observed for p-p and p-A collisions
• In nucleus-nucleus collisions
 Particle production in the fragmentation
region independent of energy, but NOT
necessarily independent of centrality
Limiting fragmentation (II)
PHOBOS Phys. Rev. Lett. 91, 052303 (2003)
• Particle production
independent of
energy in
fragmentation
regions
 Extended limiting
fragmentation (4
units of h at 200
GeV)
 No evidence for
boost invariant
central plateau
Spectator emission ?
Total multiplicity vs. Npart
• Total multiplicity obtained
integrating dN/dh distributions
 Small extrapolation thanks to the
wide h coverage of PHOBOS
• Total charged-particle multiplicity
proportional to Npart
 dN/dhmax increases more than
linearly with Npart
 dN/dh width decreases with
increasing centrality
• Total yield per participant is the
same as in e+e- collisions at the
same energy
Integrated yield vs. energy
• Multiplicity in pp
collisions lower
than in e+e• Understood as
due to leading
particle effect
• Multiplicity in AA collisions
 Below pp and e+e- at AGS energies
 Cross through pp at SPS energies
 Joins e+e- data above top SPS energy
• No leading particle effect AA collisions
at RHIC energies
 Due to multiple collisions per participant ?
 The outgoing
proton takes away
a substantial
amount of energy
Conclusions from RHIC
• Charged particle multiplicities follow simple scaling behaviours
 Total yield at RHIC energies ≈ Npart  multiplicity in e+e- at the same energy
 Extended (up to 4 h units) fragmentation regions where particle production
is independent of energy (BUT not of centrality)
 No evidence for a boost invariant central plateau also at top RHIC energy
• From STAR White paper:
 “Most bulk properties measured appear to fall on quite SMOOTH CURVES
with similar results from lower energy collisions…Similarly the centrality
dependences observed at RHIC are generally smooth… These experimental
results contrast with theoretical speculations and predictions… which often
suggested strong energy dependences accompanying the hadron-to-QGP
phase transition”
 Energy density from Bjorken formula and measured dN/dy (dN/dh) at top
RHIC energy gives values of ~ 5 GeV/fm3 “well above the critical density (1
GeV/fm3) predicted by Lattice QCD for a transition to the QGP
 BJ 
mT  dN

Ac  0  dy

0.6 GeV / c
3


 

700


1
.
1


2
145
fm

c


2


 y 0
0
2
≈15 GeV/fm3 (0= 0.35 fm/c)
≈5 GeV/fm3 (0= 1 fm/c)
Perspectives for ALICE at the
LHC
Energy dependence and the LHC
Detectors planned for
dN/dh > 5000
Saturation model
Armesto, Salgado, Wiedemann hep-ph/0407018
Models prior to RHIC
dN/dη ~ 1800
dN/dη ~ 1100
Log extrapolation
Limiting fragmentation and the LHC
dN/dη ~
1800
Limiting fragmentation
dN/dη ~ 1100
W. Busza, Zakopane ’04
Experimental issues
• Acceptance:
 Large h coverage to measure particles at mid-rapidity and in fragmentation
regions
 Low pT cut-off if a magnetic field is present
• Analysis techniques
 Count fired channels (hits) on detectors
NA50, PHOBOS
In general 1 hit NOT EQUAL to 1 particle because of:
– PHYSICAL PROCESSES in the detector volume (multiple occupancy,
charge sharing…)
– INSTRUMENTAL PROBLEMS (electronic noise, cross-talk …)
What is done is to count CLUSTERS (i.e. groups of contiguous strips firing together) and
apply a correction to go from clusters to crossing particles
 Measure energy deposition in detector channels
NA57, BRAHMS, PHOBOS
Correction for Landau distribution of energy deposition required
 Match hits between 2 detectors (TRACKLETS)
PHOBOS, PHENIX
More precise alignment and knowledge of primary vertex required
Correction for tracking efficiency to be applied
 Full tracking
NA49, STAR
ALICE at the LHC
Forward Multiplicity Detector (FMD)
Inner Tracking System (ITS)
Time Projection Chamber (TPC)
ALICE pseudorapidity coverage
p-p collisions at LHC:
s = 14 TeV
ybeam = 9.6
• Different measurement techniques
 CLUSTERS on innermost ITS layers (Silicon Pixels)
 TRACKLETS with 2 innemost layers of ITS (Silicon Pixels)
 FULL TRACKING (ITS+TPC)
 ENERGY DEPOSITION in the pads of Forward Multiplicity Detector
dN/dh measurement with ITS
• Multiplicity from:
 2 innermost layers of Silicon Pixel Detectors:
 Wider h coverage
 No energy loss information
• Analysis techniques:
 Count “clusters” on the 2 layers
 Count “Tracklets” (associations between 2 layers)
 ALICE collab. - Pysics Performance Report - Vol II
Silicon Pixel Detectors (2D)
Silicon Drift Detectors (2D)
Silicon Strip Detectors (1D)
L= 97.6 cm
R= 43.6 cm
dN/dh at mid-rapidity with ITS
• dN/dh in |h|<0.5 for:
100 HIJING events
Standard noise level
No magnetic field
zVERTEX = 0
• Hits = number of primary
particles crossing a layer
• Number of clusters
Lower than generated
multiplicity in layer 1
 due cluster merging at high
multiplicity
Enhanced in layer 2
 due to secondary particles
produced in the inner layer
• Tracklets
Association efficiency
decreases with increasing
multiplicity
Systematic effects
Magnetic field effect
 Clusters in layer 1
insensitive to the field
 low pT tracks do not
reach layer 2
 Field = 0 best condition
to measure multiplicities
Noise level effect
(looser cut)
(tighter cut)
Standard noise level
 Tracklet method more
stable against noise level
 Noise effect almost
completely removed at
Generated mult.
the price of a decrease
of efficiency (larger
MonteCarlo correction
needed)
dN/dh reconstruction in ITS (I)
• dN/dh distribution for:
1 central HIJING event
(dN/dh = 6000)
Standard noise level
No magnetic field
zVERTEX = 0
• With zVERTEX smearing
an acceptance
correction has to be
included
dN/dh reconstruction in ITS (II)
• dN/dh distribution for:
300 semi-central HIJING events (dNch/dh ≈ 3000)
Standard noise level
No magnetic field
zVERTEX spread allows to increase
zVERTEX spread = ± 5 cm
• + acceptance correction
the h coverage
Anisotropic transverse flow
Flow in heavy-ion collisions
• Flow = collective motion of particles (due to high pressure arising from
compression and heating of nuclear matter) superimposed on top of the
thermal motion
 Flow is natural in hydrodynamic language, but flow as intended in heavy ion
collisions does not necessarily imply (ideal) hydrodynamic behaviour
• Isotropic expansion of the fireball:
 Radial transverse flow
y
 Only type of flow for b=0
 Relevant observables: pT (mT) spectra
x
• Anisotropic patterns in non-central collisions:
 Directed flow
 Generated very early when the nuclei penetrate each other
x
– Expected weaker with increasing collision energy
z
 Dominated by early non-equilibrium processes
 Elliptic flow (and hexadecupole…)
 Caused by initial geometrical anisotropy for b ≠ 0
– Larger pressure gradient along X than along Y
 Develops early in the collision ( first 5 fm/c )
y
x
Anisotropic transverse flow
• Correlation between azimuthal angle of outgoing particles and
the direction of the impact parameter
• Fourier expansion of particle azimuthal distributions relative to
the reaction plane:
dX X 0
1  2v1 cos(  RP )  2v2 cos2  RP ))  ....)

d 2
y
RP
x
View along beamline
REACTION PLANE = plane
defined by beam direction and
impact parameter
Directed flow
dX X 0
1  2v1 cos(  RP )  2v2 cos2  RP ))  ....)

d 2
Directed flow coefficient
v1  cos  RP )
Elliptic flow
dX X 0
1  2v1 cos(  RP )  2v2 cos2  RP ))  ....)

d 2
Elliptic flow coefficient
v2  cos2  RP ))
Higher order harmonics
dX X 0
1  2v1 cos(  RP )  2v2 cos2  RP ))  ....)

d 2
• Fourth order coefficient v4:
Restore the elliptically deformed
shape of particle distribution
Magnitude and sign sensitive to
initial conditions of hydro
 Kolb, PRC 68, 031902(R)
Ideal hydro: v4/v22 = 0.5
 Borghini, Ollitault, nuclth/0506045
Why elliptic flow ?
• At t=0:
 geometrical anisotropy (almond shape)
 momentum distribution isotropic
• Interaction among constituents generate a
pressure gradient which transforms the initial
spatial anisotropy into a momentum anisotropy
 Multiple interactions can lead to local thermal
equilibrium at an early stage  limiting behaviour
= ideal hydrodynamic flow
• The mechanism is self quenching
 The driving force dominate at early times
 Sensitive to Equation Of State at early times
Theoretical tools
Kolb, Sollfrank, Heinz,
Phys. Rev. C62 (2000) 054909
• Hydrodynamics:
Macroscopic approach
 Zhang, Gyulassy, Ko,
Phys. Lett. B455 (1999) 45
• Transport (cascade) models:
Valid if mean free path << system size
v2 depends on equation of state
sound velocity (  dp/d )
Phase transition (soft point)
 Microscopic approach
 partonic and/or hadronic
 v2 depends on:
 interaction cross-sections
 density of partons/hadrons
First-day @ RHIC: v2 vs. centrality
• Observed elliptic flow depends on:
 Eccentricity
 Amount of rescatterings
 decreases with increasing centrality
 increases with increasing centrality
• Measured v2 well described
by hydro from mid-central
to central collisions
Hydrodynamic limit
STAR
PHOBOS
 Incomplete thermalization
for peripheral collisions
 Hint for rapid and complete
thermalization for midcentral and central collisions
• Flow larger than expected
from hadronic cascade
models
 Evidence for a strongly
interacting (partonic) phase
s=130 GeV
RQMD
 STAR: Phys. Rev. Lett. 86 (2001) 402.
 PHOBOS: Phys. Rev. Lett. 89 (2002) 222301.
First-day @ RHIC: v2 vs. pT
 STAR: Phys.Rev.Lett. 86 (2001) 402.
 STAR: Phys.Rev.Lett. 90 (2003) 032301.
s=130 GeV
just 20k events
•
v2(pT) sensitive to the evolution and freeze-out conditions of the system
 At low pT follow hydrodynamics
 Deviation at high pT where:
 Hydro not applicable because high pT partons have not undergone sufficent re-scatterings to
come to thermal equilibrium
 Parton energy loss in the opaque medium is a source of anisotropy
•
More information from v2(pT) of pions and protons
 Pions are more senistive to freeze-out temperature and radial flow velocity
 Protons (and in general heavier particles) more sensitive to the EOS
v2 vs. beam energy
Low energy:
Squeeze-out
High energy:
In-plane
• Different physical mechanisms in different energy regimes
• Hydro limit reached at top RHIC energy
 Hint for early and complete thermalization ?
• BUT no hint for a saturation of v2 in the data
 In the low-density limit (mean free path ≈ system size) a monotonic
dependence of v2/ versus 1/S dN/dy is expected
Conclusions from RHIC
• Observed at top RHIC energy:
 Strong elliptic flow
 Hydrodynamics reproduces observed magnitude, pT and mass
dependence of elliptic-flow using an EOS incorporating a soft phasetransition from QGP to hadronic matter
 At intermediate pT, v2 of indentified mesons and baryons scales with
the number of constituents quarks
• Elliptic flow is one of the main pieces of evidence for:
 Attainment of local thermal equilibrium at an early stage
 Perfect liquid behaviour
 mean path << system size AND viscosity=0
 Partonic degrees of freedom
 Strongly interacting QGP (sQGP)
• But:
 Also hints for incomplete thermalization
Bhalerao et al., Phys. Lett. B 627 (2005) 49
 Theoretical uncertainties within hydrodynamics not completely treated
 Can a harder EOS coupled with late thermalization and significant viscosity describe
the data ?
Perspectives for ALICE at the
LHC
Experimental issues
• Analysis techniques to estimate vn
Event plane method (Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671.)
 Calculate an estimator of the reaction plane (EVENT PLANE) from the anisotropy of particle
azimuthal distributions
 Correlate azimuth of each particle with the event plane calculated with all the other particles
 WEAK POINT: assumes that the only azimuthal correlation between particles is due to their
correlation to the reaction plane (i.e. to flow)
 BUT other sources of correlation (NON-FLOW) are in due to momentum conservation,
resonance decays, jets + detector granularity  SYSTEMATIC UNCERTAINTY
Two particle correlations (S. Wang et al, Phys. Rev. C44 (1991) 1091.)
 No need for event plane determination
 Calculate two-particle correlations for all possible pairs of particles
 WEAK POINT: same bias from non-flow correlations as in event-plane method
“Cumulants” method (Borghini et al, Phys Rev C 63 (2001) 054906.)
 Extract vn from multi-particle azimuthal correlations
 Based on the fact that flow correlates ALL particles in the event while non-flow effects
typically induce FEW-particle correlations
 DRAWBACK: larger statistical error and more sensitivity to fluctuation effects
Lee-Yang zeroes method (Bhalerao et al, Nucl. Phys. A727 (2003) 373.)
 Extension of cumulants method to infinite order
Method comparison
Event-plane method
v2in=0.1
4-th order cumulant
back-to-back
track pairs
embedded
s  130 GeV
Predictions for the LHC
• Multiplicity larger than at RHIC
 by a factor 1.5-2
• v2/ expected larger than at RHIC
  = geometrical eccentricity
 Few predictions:
Teaney, Shuryak, Phys.Rev.Lett. 83 (1999) 4951.
Kolb, Sollfrank, Heinz, Phys.Rev. C62 (2000) 054909.
Bhalerao et al., Phys.Lett. B627 (2005) 49
– incomplete thermalization at RHIC
• Easier measurement (feasible on day 1)
• BUT larger non-flow contribution from jets
 Important to compare different methods
ALICE at the LHC
Forward Multiplicity Detector (FMD)
Inner Tracking System (ITS)
Zero
Degree
Calorimeters
(ZDC)
Time Projection Chamber (TPC)
Event plane from track anisotropy
• Main tracking detector: TPC
 Rin
90 cm
 Rext
250 cm
 Length (active volume)
500 cm
 Pseudorapidity coverage: -0.9 < h < 0.9
 Azimuthal coverage:
2
 Maximum drift time:
88 ms
 Gas mixture:
90% Ne 10%
CO2
• Provides:
 Tracking efficiency > 90%
 Particle identification by dE/dx in the
low-momentum region
• Event plane resolution depends on:
 Amount of anisotropy (v2)
 Number of used tracks
Event plane from spectator neutrons
• Neutron ZDC segmented in 4 “towers”
 Rough localization of neutron spot
 Estimation of the reaction plane
from spectator neutron bounce-off
V1=0 %
• Essential for directed flow
v1 = first harmonic
in dN/d Fourier
expansion = shift
of the particle
source in the
transverse plane
Dominated by beam parameters
rather than centroid resolution
V1=20%
v2 from central barrel detectors
dN/dφ distributions from SPD
for 1 HIJING event with
dN/dhmax = 3000, v2=0.1
v2 vs. pT for TPC tracks:
• 100 events
• 2000 tracks per event
Backup slides
“Glauber” calculations
• Optical approximation
 Czyz and Maximon, Annals Phys. 52 (1969) 59.
Nucleus thickness
functions
Nucleus-nucleus
thickness function
Nucleon-nucleon
collision probability
Npart distributions
Centrality selection
on generated Npart
Centrality selection
on EZDC-EZEM correl.
Density of participants
Out of plane
In Plane
dN/dh - basics (I)
Particle momenta decomposed
Longitudinal momentum (pL)
• Rapidity variable
1  E  pL 

y   ln 
2  E  p L 
 Lorentz invariant
• Pseudorapidity variable
   
1  p  pL 
h   ln


ln
tan 


2  p  pL 
  2 
 h≈y for large momenta
 h more easily accessed
experimentally
Transverse momentum (pT)
Width of dN/dh distribution
E917 at AGS
Incomplete stopping
already at AGS energies
PHOBOS at RHIC
At RHIC energies only 22% of the particles
emitted with pT>pL ( |h| < 0.88 )
Width vs. energy
NA50 most central Pb-Pb
E877
central
Au-Au
s h  0.58  0.32  ln s
• Available phase space in rapidity increases with √s
 Fit with the simple scaling law sh = a + b · ln s
 At SPS energies dN /dh (dN/dy) are twice as large as the one expected
from a thermal fireball (Senger and Strobele, nucl-ex/9810007)
dN/dhmax vs. centrality (SPS)
• Fit with the power law dN/dhmax  Nparta
Values of exponent a between 1.00 (NA50)
and 1.08 (WA98)
Depends on the model to calculate Npart (NA50 finds
a=1.00 with a Glauber estimation of Npart and 1.08
with a VENUS estimation)
• Two-component fit dN/dhmax  A·Npart
+B·Ncoll
NA50
Values of B compatible with 0
• Npart describes the centrality dependence
of particle production at midrapidity at SPS
energies
Negligible contribution from hard processes
• Introduce yield per participant pair
dN/dhmax /(Npart/2)
A flat behaviour reflects the linear
dependence of dN/dhmax on Npart
Scaling with centrality at the SPS
• Factor 1.7 between NA50 and
NA57 measurements
Quite different experimental
conditions and analysis techniques
for the different experiments
EZDC not available for NA50 at this
energy, so not all the cleaning cuts were
applied to this data sample
NA57 uses the multiplicity to define
centrality classes (autocorrelations?)
• Fit with the power law
dN
dh
 N apart
max
Values of exponent a between
1.02 (NA50) and 1.09 (NA57)
Npart GLAUBER vs. VENUS
Multiplicity vs. density at midrapidity
central
peripheral
• The shape of pseudorapidity distributions
is not independent of centrality (Npart)
 Height increases more than linearly with
Npart
 Width decreases with increasing centrality
 BUT Height  Width ≈ constant
√s= 200 GeV
Gold vs. copper
62.4 GeV
200 GeV
Cu+Cu
PHOBOS
PHOBOS
Preliminary
3-6%, Npart = 100
Cu+Cu
Preliminary
3-6%, Npart = 96
Au+Au
35-40%, Npart = 99
Au+Au
Preliminary
35-40%,Npart = 98
• Unscaled dN/dh very similar for Au-Au and Cu-Cu
collisions with the same Npart
 Compare central Cu-Cu with semi-peripheral Au-Au
 For the same system size (Npart) Au-Au and Cu-Cu are very similar
Limiting fragmentation (II)
• Different limiting curves
for central and peripheral
data
 Particle production in the
fragmentation region changes
significantly with centrality
 The hypothesis of limiting
fragmentation does not imply
that the limiting curve is
independent of centrality
• BUT both (central and
peripheral) energy
independent
Boost invariant central plateau?
• Pseudorapidity distorts the distributions for production angles
near 0° and 90°
• Rapidity distributions from BRAHMS at RHIC very similar to
data at lower energies and well represented by gaussian fits
• No evidence of a plateau at midrapidity
dNch/dh in p-p
LHC
C. Jorgensen
Predictions
Predictions before RHIC startup
Predictions before LHC startup
2000
4000
6000
dNch / dy
8000
10000
Pseudorapidity distribution ITS+FMD
• Central Hijing event (dNch/dhmax ≈ 6000)
In-plane vs. out-of-plane
dX X 0
1  2v1 cos(  RP )  2v2 cos2  RP ))  ....)

d 2
Isotropic
V2=10%
Elliptic flow coefficient:
v2>0 In plane elliptic flow
v2<0 Out of plane elliptic flow
V2= - 10%
Equation of state
Pions vs. protons
• Pions more sensitive to freeze-out temperature and
radial flow velocity
• Protons (and massive particles) more sensitive to EOS
Partonic flow?
• Constituent quark scaling observed at intermediate pT
 Predicted by hadronization model based on
recombination/coalescence
v2 scaled by
number of
constituent
quarks
(n=2 for mesons,
n=3 for baryons)
pT scaled by
number of
constituent
quarks
(n=2 for mesons,
n=3 for baryons)
The neutron ZDC (ZN)
Each ZN is made by 44 grooved W-alloy slabs, each of
them 1.6 mm thick, stacked to form a parallelepiped of
7.2 x 7.2 x 100 cm3.
The active part is made of 1936 quartz fibers,
embedded in the absorber with a pitch of 1.6 mm.
The fibers, hosted in the slab grooves, are placed at 00
with respect to the incident particle direction.
PMT 1
PMT 2
PMT 3
Fibers come out from the rear face of the calorimeter,
directly bringing the light to 5 photomultipliers (one
for each of the 4 towers + 1 for the total energy).
PMT 4
PMT c