Leticia CunqueiroINFN Frascati

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Tecniche Monte Carlo per la fisica dei jet

• Leticia Cunqueiro INFN Frascati

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Jets in pp

• A jet is produced in a hard elementary
  interaction with high virtuality Q2 (
  )
• Its perturbative cross section can be factorized
  into short/long distance terms
• A proton-proton jet is a well defined
• perturbative object and can be used
• as a reference in HIC.

i
A
B
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Jets in HIC
How does the QGP modify jets? Very
schematically -High virtuality interaction
means early in time time 1/Q -So
jets are produced at the early stages of the HIC,
long before soft interactions leading to QGP
formation.
It is the parton evolution encoded in the
Fragmentation Function that will be affected by
QGP.
The PDFs and the elementary cross section are
not affected by the QGP.
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Jets in HIC
How does the QGP modify jets? Very
schematically -High virtuality interaction
means early in time time prop 1/Q -So jets
are produced at the early stages of the HIC,
before soft interactions leading to QGP
formation.
Jets probe the medium medium properties can be
studied from the modification in the cross
section compared to the pp case. Complications
Nuclear initial conditions?
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quasi Jets _at_RHIC
Slide from S.Salur, QM09
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Jets in experiment
Complications measuring jets in a HIC is an
experimental challenge.
In a HIC a jet is no longer what you can see
-Need to have a jet definition ?well defined
algos so measured quantities can be
compared to theory. -Need of powerful
background subtraction techniques.
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Fragmentation Function how does matter affect
evolution?
A parton with virtuality t traveling in
vacuum will radiate gluons to become onshell.
The gluon radiation will be modified when the
parton traverses medium Medium induced gluon
radiation.
Note that for sufficiently boosted jets the
medium modifies the perturbative evolution and
hadronization happens in vacuum.
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Fragmentation Function how does matter affect
evolution?
The gluon radiation will be modified when the
parton traverses medium Medium induced gluon
radiation
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Medium-induced gluon radiation
No analytical solution for the path integral
expresion above approximations have to be used
-BDMPS-Z or multiple soft scattering
approximation (brownian motion)?

Assume the elem. interaction to be of short range
This defines the transport coefficient
It encodes all the information of the elementary
interaction and it represents the average
transverse momentum given by the medium to the
parton per unit path length.
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Medium-induced gluon radiation
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Medium-modified Fragmentation Function old
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New modeling of the inmedium splittings
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Medium-modified splittings
Slide from C.Salgado at QM09
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Medium-modified sudakov factor
Medium enters here
The probability of radiation is enhanced in
medium. The Sudakov form factor is the main tool
for our MC implementation.
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Medium-modified fragmentation function new
Our new method -energy-momentum conserved at
each splitting. -medium and vacuum treated in the
same footing.
Medium enters here
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The need for a Monte Carlo

• 1 Previous DGLAP procedure allows just to
  compute single inclusive high pt observables like
  RAA (we are just following the evolution of the
  leading parton). However RHIC has taught us that
  RAA is biased and not so good to discriminate
  between models.

Matter is opaque at RHIC and measured high pt
particles are produced in the corona low
sensitivity to transport coefficient.
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The need for a Monte Carlo

• 2 At LHC a full unbiased jet reconstruction
  will be possible.
• This will give acess to new observables such
  as
• fragmentation functions, intrajet
  correlations, jet shapes,jet multiplicities

The rate of high energy jets will be high. Signal
will be disentangled from background.
DEnterria 2008
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The need for a Monte Carlo

• 3 Jet Quenching will be well constrained, not
  only a single high pt observable!!
• ?But to go beyond single
  inclusive and access exclusive
• and differential observables a
  Monte Carlo is needed.
• A Monte Carlo for the Jet Quenching is thus a
  needful tool.
• However any probabilistic interpretation of
  medium induced gluon radiation
• relies on assumptions (i.e. time ordered
  induced gluon emissions)
• ?are there assumptions that cannot be
  tested experimentally??
• . MCs will hopefully allow to discriminate
  between energy loss models and to
• understand the mechanisms of energy loss.

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Recent MC developments

• JEWEL K.Zapp et al induced gluon radiation
  modelled by a multiplicative increase of the soft
  splitting function.
• YAYEM T.Renk enlargement of the QCD evolution
  by giving aditional virtuality to the partons.
• MARTINI McGill group HT corrections to the
  splittings.
• PYQUEN I.Lokhtin et al radiation collisional
  effects supreimposed.
• QPYTHIA LC,Santiago group modelling of the in
  medium splittings.
• QHERWIG LC,Santiago group,G.Corcella same
  phylosophy as QPYTHIA but in a different shower
  angular instead of virtuality ordering,
  different radiation phase space, cutoffs and
  hadronization mechanisms

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QPYTHIA comercial break

• QPYTHIA Armesto-Cunqueiro-Salgado,
  arXiv08094433
• is a Monte Carlo for the Jet Quenching based on
  the ideas in
• Armesto-Cunqueiro-Salgado-Xiang, JHEP
  0802048,2008
• It is a tuning of the final state routine PYSHOW
  in PYTHIA.
• It can be downloaded from
• http//igfae.usc.es/qatmc/

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QPYTHIA implementation
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Qpythia formation time effects
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QPYTHIA geometry

• Generate centrality, impact parameter b.
• Build transverse overlapping almond for that b.
• Generate production point for the hard scattering
  (x0,y0) acording to Glauber.
• The transverse path length to scape the medium as
  well as the integrated qhat along that pathlength
  are computed like in PQM A.Dainese et al

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List of ingredients

• Energy-momentum conserved at each splitting.
• All procesess, not only QCD, included.
• At qhat0, default pythia.
• Space-time evolution of the shower, mapped.
• Energy loss transverse broadening dynamically
  related through the transport coefficient.
• New Realistic Glauber like collision geometry is
  defined. Given the position and direction of each
  parton in the reaction plane, qhat and L are
  computed locally like in PQM.
• Eikonal approach?elastic corrections not included
  yet.
• No change on the color flow.

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Intrajet distributions for the hardest jet in
the pp event
- Increase of the total multiplicity.
-Suppression/enhancement of high/small z
particles. -Suppression of high pt particles
(pt broadening screened by energy
conservation). -Broadening of the distribution
with respect to jet axis.
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Jet shape distribution
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Hardest pt distribution
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Subjet structure
Subjet analysis Take the hardest jet in the
event clustered with maximum resolution Rmax. Rec
luster its constituents with decreasing R. When
RRmax ?P(1subjet)1 When R?0, P(6subjets)?1. Th
e jet structure is changed by the quenching at
the same R the jet algorithms split the jet into
more (and smaller) subjets than in vacuum.
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Quenching and the jet area
antikt is not affected by soft particles on the
border of the jet the area remains unchanged
by quenching.
kt is soft adaptable and the area increases up to
a 10 with quenching.
Key point under study can this be observed with
background?
If so (very speculative) why not take area
increase as a legitimate signature for the
quenching? How to relate Area and qhat?
?
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Open/ongoing issues

• Color structure of the shower. Role of
  hadronization.
• Finite-energy corrections to our eikonal BDMPS
  approach
• ?include elastic
  scattering effects.
• Interplay between virtuality and length
• -Space-time picture of the shower.
• -Ordering variable in the medium?
• any probabilistic description
  of gluon radiation in medium needs assumptions.
  i.e.virtuality being the ordering parameter in
  medium is not theoretically proved.
• Energy flow from/to the medium
• Study all previous effects in a realistic
  detector environment.
• 
  

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Role of hadronization
-The medium modifies the perturbative evolution
of the jet. Hadronization happens in vacuum for
sufficiently boosted partons. -However medium
modifications in the evolution affect further
hadronization - The interaction of
the gluon with the medium is a color exchange
that modifies the color flow and affects
further string formation and
hadronization.
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Space-time evolution of the shower

• -Interplay between the evolution of the jet
  the evolution of the medium.
• -Bjorken/Hydrodynamical evolution of the
  energy density.
• -but Background models should be
  accordingly developed!

Work by Konrad Tywoniuk
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Space-time evolution of the shower
-To include formation time effects in the shower
allows us to have a Chronography of the jet
evolution. -Looking to external/internal coronas
of the jet is selecting old/recent particles of
the shower (poster at QM by I.Dominguez,E.Cuautle,
LC,G.Paic,A.Morsch).
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Space-time evolution of the shower
I.Dominquez et al
Particles at external coronas are created first
in time, soon after the hard scattering they
face the same medium as the leading particles but
they have lower energies ?the quenching is
expected to be strong.
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EXTRAS
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Jet definition
Slide by G.Salam 2008
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Other MCYaJEM
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Other MC JEWEL
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Other MCMARTINI
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Slide stolen to T.Renk (QM 2009)
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Extras
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Schematic representation of previous/new
approaches
Slide from T.Renk at QM09
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Extras
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kt jets are bigger than antikts
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Kt jets are bigger than antikts
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Jet areas
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Jet reconstruction
I merge my pp jets in a PbPb 0-10 central
backround generated with HIJING (with its
quenched minijets). Area based subtraction
method -Cluster the whole event into
jets -Compute an average pt density -Your
correction is then
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Jet reconstructionat LHC