Descripcion de los diferentes algoritmos que se usan para la reconstruccion de las particulas de los jets.

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a) Definition of JET
b) JETs characteristics
c) Fragmentation
d) Pseudorapidity, Azimuthal and Pt
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a) Hard Scattering and Beam Remnant
b) Multiple Interactions, Underlying Events Pile-Up
c) Jet Measurement
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a) Cone Algorithm
b) Kt Algorithm
c) Energy Flow
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to the lifetimes and braching ratios for each unstable particle.
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Jets appear as showers of electromagnetic
and hadronic matter. They are observed as
cluster if energy located in adjacent detector
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Each jet is characterized by :
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The of an hadron collision consist of a set
of particles with four momento, pi, resulting from the X
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one only measured energy deposition
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The definition of a jet is not unique and the energy
and direction and
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characteristic is influenced by many factors: parton
fragmentation, FSR, underlying events, detector response and by the jet algorithm
itself
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The components
consists of the outcoming two ’jets’
which come from a hard 2-to-2 parton
scattering which interact at short distance
with large momentum transfer.
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and consist of:
- the beam-beam renmants
- ISR and FSR
- Secondary semi-hard interaction
Underlying events contribution must be removed from the jets before comparing to
QCD predictions. Precise jet measurements will require good modeling of the
underlying events.
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In additionto the hard 2-to-2 parton
scattering with transverse momentum
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PT(hard), sometimes there are
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:a second, a third...softer
2-to-2 scattering that contribute particles
to the ‘underlying events’
Due to the proton has 3 quarks, in each incoming
beam particle may leave behind a beam remnant,
which does not take part in the ISR or hard
scattering process. If nothing else, the remnant
need to be reconstructed and connected to the rest
of event.
Finally,in high luminosity, it is possible to have severalcollision between beam
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particles in one and the same beam crossing, ie, events.
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Jet algorithms are employed to map the observed (hadronic) final states
in the data, onto jets. The motivating idea is that these jets are surrogates
for the underlying energetic parton.
They make cluster (Clustering) based on - hadrons, Calorimeter
Cells,Towers etc., for nearness
Nearness in angle = Cone Algorithm.
Nearness in relative transverse
momentum = Kt algorithm.
Recombination Scheme - The momentum
addition rule of particles in a jet. KT jet Cone jet
Historically hadron collider use cone algorithms : easier calibration
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- Order all cells according to their !
- Use the cell with maximum ET in - a d
space as a seed for a jet candidate
- Make a cone of
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seed center (usually it takes 0.4 or 0.7)
- In order decreasing of ET , include in the ’jet’ all cells
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- Recalculate the centroid of the jet after each new cell
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- Cone iterated, until a ET weigthed cone is
achieved the centroid of the energy depositions
within the cone is aligned with the geometric axis.
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– According with the granularity of
the calorimeter it defines the
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a d Tower Tower
– A list of towers above a fixed ET
is create, to be used as seeds
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are formed from an unbroken chain of PreClus
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contiguous seed towers. They are grown into
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: defining a centroid cone in Et
weighted of the precluster and
all towers with ET threshold E0 Cluster
are incorporated into cluster Cluster
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– A new cluster center is calculated from the set
of towers into the clustering
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A single particle may belong to two or more cones. A procedure must be included in
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the cone algorithm to .
Cones whose shared ET is larger than a fixed !A¢DC
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fraction (f= 50-75%) of the energy cone are
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Otherwise, the shared particles are
between the cones the shared particles are
assigned to the cone that is closer in space.
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Two jets Merged jet Lost jet
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the presence of soft radiation between
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energy is split among several towers.
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The KT algorithm combines particles aproximated by the ’towers’ of the
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calorimeter, and pairs the
– Compute for each pair (i,j) and for each
particle (i) the quantities:
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– Find the minimum of {d_ij ,d_j} = dmin
– If dmin=d_i then it is called a jet and is
removed from the list
– If dmin=d_ij removed precluster i and j form the list and replace them with a new
merged particle ij to give a single jet by E-scheme: Eij=Ei+Ej and Pij=Pi+Pj
– Iterate until all particles are in jets
Every particle is assigned to a unique jet
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requires looping many times over the towers
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(advantages disandvantages)
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– easier calibration – theorically well behaved by design
–
overlapping jets need a – No overlapping
No merging or
split/merged step spliting needed
– eficient in CPU time: average – more time consuming: Reco time for
recon time for 1 GeV jet 0.7 ¡ 1000 GeV jet with Et cut 1 sec, ¡
sec without Et cut 1min ¡requiers a
method to terminate clustering
– neither infrared safe nor
collinear safe – infrared and collinear safe by design
– difficult to simulate in perturbat – can be applied in perturb calculations
calculations
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was proposed as the theorically preferred algorithm for hadrons studies,
although early studies would suggest that the presence of underlying event and
multiple interaction would make Kt not suitable for pp collisions .Alternatively,
improved cone-based jet algoritm have been developed
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This algorithm highly reduce the sensitivity to infrared and collinear sensitivity
emissions. This was achieved by adding seed locations for trial cones between pairs
of stable seed-based cones. It works as follow:
– iterate cones starting at each seed tower
– put seed in
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of stable cones whose centers pi and pj
separated by less than 2R
– iterate a cone starting at the midpoint pi+pj
– split or merge overlapping cones.
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However jet algorithm makes no use of tracking information
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Basic Idea: The well measured particle momentum substitutes random fluctuation
of energy in the calorimeter
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Use tracker for charged and
Calorimeter for neutrals.
Must locate and remove
charged calor. energy
Simple idea but challenge to realize:
build particle ID asociated with the track.Difficulties: high track multiplicity and
coarse calorimeter granularity needs especially good segmentation to separate
clusters and match extrapolated track correctly to some of them.
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in the cell of the calorimeter. We need to know more about this
effect and its influence in the analysis
Typical multi-jet event :
– 64% charged energy
– 25% photons
– 11% neutral hadron
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