Diapositiva 1

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Event characterization
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• Many important aspects of heavy-ion dynamics
  require the characterization and selection of
  events.
• Two main examples
• Determination of event centrality
• Determination of reaction plane

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Determination of the event centrality
The classification of the events according to
their centrality is one of the main tasks in the
study of heavy ion reactions, from intermediate
energy to ultrarelativistic collisions Most of
the results obtained in HI collisions are studied
as a function of the event centrality Usually
some global property of the event is used to
determine the centrality (global
variables) However, there are several possible
choices and many critical points related to
detector limitations and theoretical models
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Several observables may be related to the impact
parameter in a nucleus-nucleus collision Particl
e multiplicity Due to the violence of the
collision, it is expected that at small impact
parameters, the probability for the system to
break up in more fragments is higher. Subsets of
the multiplicity, such as the forward or backward
multiplicity are sometimes considered. Total
detected charge While a perfect detector gives a
value of the total detected charge very close to
the charge of the colliding nuclei, for a real
detector some correlation exists between the
detected charge and the impact parameter. Also in
this case subsets (for instance the mid-rapidity
charge) exist.
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Total perpendicular momentum This is defined by
the sum of perpendicular momenta of all detected
particles. Average (or transverse) parallel
velocity It is defined by the weighted average
(over the masses) of the parallel (transverse)
velocities. For a perfect detector this would
give the C.M. velocity. Diagonal components of
the quadrupole momentum tensor The shape of the
momentum distribution of each event may be
analyzed by the sphericity tensor. The
eigenvalues give the axes of the ellipsoid
momentum distribution, allowing to define 2
values the eccentricity and the angle between
the symmetry axis and the beam direction. The
distribution of the events in this plane permits
to sort out them into central, intermediate and
peripheral collisions
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Method Generate events according to some
realistic model Filter generated events through
detector geometry, limitations and
response Select well measured events, i.e. events
which carry enough information to build the
global variables of interest Study the
correlation between simulated impact parameter
and the value of the global variable Extract from
filtered data the value of the reconstructed
impact parameter and compare to the original value
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An example from the MEDEA set-up in the study of
pion production (F.Riggi et al., Zeit. Physik
A344(1993)455)
Solid angle93 of 4 p
Detector structure 180 BaF2 modules arranged
into 7 rings (24 detectors each 1 ring with 12
detectors) 120 Phoswich detectors arranged into 5
rings 16 NE102 scintillators, arranged into 2
rings
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Detector limitations/1
Polar angle resolution 1.25 Ring 1
Hodoscope 2.5 Ring 2 Hodoscope 2.0 Rings
1-5 Phoswich 7.5 Rings 1-8 BaF2 Azimuthal
angle resolution 22.5 Hodoscope 15 BaF2
and Phoswich
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Detector limitations/2
Particle identification BaF2 charge and mass for
Z1 for Zgt1 Z2 and A4 assigned Phoswich
charge and mass for Z1 for Z2 A4 assigned
for Zgt2, Z3 and A6 assigned Hodoscope charge
identification up to Z6 mass assigned as the
most abundant isotope for Zgt6, Z7 and A14
assigned Velocity threshold From 3.5 and 5.0
cm/ns, depending on detectors Finite angular
resolution and multihit events considered
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It is important to compare the response of a
perfect detector to that of the real detector
Example the charged particle multiplicity
distribution, as measured In a 4 p detector In
the real MEDEA detector (with or w/o Hodoscope)
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To perform a centrality analysis, a preliminary
selection of the events is made (well measured
events), in order to ensure they carry enough
information to extract global variables. For
instance, in a perfect particle detector, the
total parallel momentum in each event should be
constant and equal to the projectile
momentum. Any real detector gives however a broad
distribution of the total parallel momentum, due
to undetected or misidentified particles.
Example MEDEA detector Hodoscope With MEDEA
only a large fraction of momentum is lost and
almost all events are badly measured
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One can also look at the correlation between two
global variables. Examples
In this case a good correlation is observed
between multiplicity and total detected charge.
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By means of one global variable (choose the
best!) values of the true and reconstructed
impact parameters may be compared.
MEDEA Hod
MEDEA only
Example of a realistic correlation between true
and reconstructed impact parameter, according to
the average parallel velocity
3 centrality regions (central, intermediate and
peripheral) may be extracted
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As before, for the total multiplicity as a global
variable
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An example of multiplicity analysis at SPS
energies the NA57 experiment Ref. The NA57
Collaboration, J.Phys. G31(2005)321
Multiplicity is important not only for
classification of events according to centrality,
but also beacuse it is related to the entropy of
the system and to the initial energy density. In
NA57 the production of strange and multistrange
hyperons is studied as a function of the
centrality
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The experimental set-up includes two planes of
microstrip detectors for centrality evaluation
Each plane has 3 arms with 200 strips/arm
(pitches from 100 to 400 µm) At 158 AGeV/c
(central rapidity ycm 2.9) the two planes
cover 1.9 lt ? lt 3.0 3.0 lt ? lt 4.0
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Reconstruction of multiplicity The detector
provides analogue signals roughly proportional to
the energy lost by particles in the strips.
Strip signals are equalized during calibration
runs and noisy strips removed from the
analysis From contiguous strips, the number of
clusters is determined The hit multiplicity is
then evaluated considering also the energy
deposited, to account for clusters produced by
the passage of more than one particle.
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Hit multiplicity distribution _at_160 AGeV/c
The contribution of the empty target (interaction
of the beam with air or other materials along the
beam line) is measured in a special run and
subtracted. This amount to about 6 , mostly at
low multiplicity.
Decrease at low multiplicity is due to the
trigger efficiency, which is mainly optimized for
central collisions.
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Hit multiplicity distribution _at_40 AGeV/c
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Correlation between the number of hits and the
charged particle multiplicity, according to
theoretical models filtered by the detector
response.
To obtain the total multiplicity over the full
azimuthal range, a comparison is made between the
data and a simulation of the collision according
to some model (here VENUS and RQMD), filtered by
the GEANT response of the detector.
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Comparison between the charged particle
multiplicity experimental and theoretical model
distributions.
The distribution of charged particle
multiplicity, evaluated as described, is used to
determine the centrality of the collision and to
evaluate the number of wounded nucleons Nwound
(nucleons which suffered at least one inelastic
collision in the process). Nwound is determined
by a geometrical (Glauber) model and Nch const
x (Nwound)a
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For such study, 9 centrality classes were
identified, according to the fraction of the
total inelastic cross section (evaluated by the
Glauber model for this case as 7.26 barn).
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For further studies of the centrality dependence
of the strangeness production however, more
stringent selection criteria have been used, and
only 5 centrality classes have been used
5 most central collisions
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Since each experiment may use a different
centrality definition, it is sometimes important
to compare reasults from different experiments
Comparison between the number of wounded nucleons
for each of the (same) five centrality classes in
experiments NA57 and NA50
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For the value of dN/d? a disagreement is observed
especially in central collisions between the
different SPS experiments.
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Determination of the reaction plane

• Several methods exist to determine the reaction
  plane
• Sphericity tensor method (Gyulassy et al.,
  Phys.Lett.110B(82)185)
• Transverse momentum analysis (Danielewicz et
  al., Phys.Lett.157B(85)146)
• Azimuthal correlation method (Wilson et al.,
  Phys.Rev. C45(92)738)

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• The applicability of any method to determine the
  reaction plane relies on two points
• The events are well measured, i.e. they are
  complete events with the momenta of all particles
  well determined
• The problem has a solution, i.e. the impact
  parameter differs from zero

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An example determination of the reaction plane
to study pion shadowing effects in heavy ion
collisions at 100 MeV/A (F.Riggi et al., Phys.
Rev. C55(1997)2506)
Geometrical picture of the pion production process
Dynamical picture of the process
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Selection of well measured events
Only about 1 of the inclusive events were
selected!
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Chosen method transverse momentum analysis
The reaction plane is determined by the vector Q
in the XY-plane This is calculated
event-by-event, by a weighted vectorial sum over
all the detected fragments
M total projtarget mass ? multiplicity w
appropriate weight of the event
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A widely used technique to estimate the
resolution of the method is to split the event
into two subevents and calculate the reaction
planes for the two subevents. Comparing the two
gives the resolution
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Result pions are emitted mainly perpendicular to
the reaction plane
whereas for photons the distribution is flat
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Microscopic BNV calculations predict this
squeeze-out effect when pion reabsorption is
included