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Leticia Cunqueiro INFN Frascati

Tecniche Monte Carlo per la fisica dei jet. Leticia Cunqueiro INFN Frascati. 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:

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Leticia Cunqueiro INFN Frascati

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  1. Tecniche Monte Carlo per la fisica dei jet Leticia Cunqueiro INFN Frascati Roma QUARK MATTER ITALIA

  2. A jetis 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. Jets in pp i A B Roma QUARK MATTER ITALIA

  3. 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. Roma QUARK MATTER ITALIA

  4. 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? Roma QUARK MATTER ITALIA

  5. “quasi” Jets @RHIC Slide from S.Salur, QM09 Roma QUARK MATTER ITALIA

  6. 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. Roma QUARK MATTER ITALIA

  7. 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. Roma QUARK MATTER ITALIA

  8. Fragmentation Function: how does matter affect evolution? The gluon radiation will be modified when the parton traverses medium: Medium induced gluon radiation: Roma QUARK MATTER ITALIA

  9. 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. Roma QUARK MATTER ITALIA

  10. Medium-induced gluon radiation Roma QUARK MATTER ITALIA

  11. Medium-modified Fragmentation Function: old Roma QUARK MATTER ITALIA

  12. New: modeling of the inmedium splittings Roma QUARK MATTER ITALIA

  13. Medium-modified splittings Roma QUARK MATTER ITALIA Slide from C.Salgado at QM09

  14. 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. Roma QUARK MATTER ITALIA

  15. 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 Roma QUARK MATTER ITALIA

  16. 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. Roma QUARK MATTER ITALIA

  17. 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. D’Enterria 2008 Roma QUARK MATTER ITALIA

  18. 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. Roma QUARK MATTER ITALIA

  19. 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… Roma QUARK MATTER ITALIA

  20. QPYTHIA: comercial break • QPYTHIA [Armesto-Cunqueiro-Salgado, arXiv:08094433] • is a Monte Carlo for the Jet Quenching based on the ideas in [Armesto-Cunqueiro-Salgado-Xiang, JHEP 0802:048,2008] • It is a tuning of the final state routine PYSHOW in PYTHIA. • It can be downloaded from: http://igfae.usc.es/qatmc/ Roma QUARK MATTER ITALIA

  21. QPYTHIA: implementation Roma QUARK MATTER ITALIA

  22. Qpythia: formation time effects Roma QUARK MATTER ITALIA

  23. 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]: Roma QUARK MATTER ITALIA

  24. List of ingredients • Energy-momentum conserved at each splitting. • All procesess, not only QCD, included. • At qhat=0, 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. Roma QUARK MATTER ITALIA

  25. 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. Roma QUARK MATTER ITALIA

  26. Jet shape distribution Roma QUARK MATTER ITALIA

  27. Hardest pt distribution Roma QUARK MATTER ITALIA

  28. Subjet structure Subjet analysis: Take the hardest jet in the event clustered with maximum resolution Rmax. Recluster its constituents with decreasing R. When R=Rmax P(1subjet)=1 When R0, P(+6subjets)1. The 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. Roma QUARK MATTER ITALIA

  29. 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? ? Roma QUARK MATTER ITALIA

  30. 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. Roma QUARK MATTER ITALIA

  31. 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. Roma QUARK MATTER ITALIA

  32. 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 Roma QUARK MATTER ITALIA

  33. 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). Roma QUARK MATTER ITALIA

  34. 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. Roma QUARK MATTER ITALIA

  35. EXTRAS Roma QUARK MATTER ITALIA

  36. Jet definition Slide by G.Salam 2008 Roma QUARK MATTER ITALIA

  37. Other MC:YaJEM Roma QUARK MATTER ITALIA

  38. Other MC: JEWEL Roma QUARK MATTER ITALIA

  39. Other MC:MARTINI Roma QUARK MATTER ITALIA

  40. Roma QUARK MATTER ITALIA Slide stolen to T.Renk (QM 2009)

  41. Extras Roma QUARK MATTER ITALIA

  42. Schematic representation of previous/new approaches Slide from T.Renk at QM09 Roma QUARK MATTER ITALIA

  43. Extras Roma QUARK MATTER ITALIA

  44. kt jets are bigger than antikts Roma QUARK MATTER ITALIA

  45. Kt jets are bigger than antikts Roma QUARK MATTER ITALIA

  46. Jet areas Roma QUARK MATTER ITALIA

  47. 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: Roma QUARK MATTER ITALIA

  48. Jet reconstructionat LHC Roma QUARK MATTER ITALIA

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