Quenched jets Leticia Cunqueiro INFN Frascati 9252020 LNF

Quenched jets Leticia Cunqueiro INFN Frascati 9/25/2020 LNF

Introduction Many new avaliable tools for the simulation of quenching in jets: • YAYEM [T. Renk]: QCD evolution enlarged, the medium gives virtuality to the partons. • JEWEL [K. Zapp et al]: induced gluon radiation modelled by a multiplicative increase of the soft splitting function. • MARTINI [Mc. Gill group]: HT corrections to the splittings. • PYQUEN [I. Lokhtin et al]: radiation + collisional effects supreimposed. • QPYTHIA [Santiago group]: modelling of the in medium splittings. • QHERWIG [Santiago+G. Corcella]: same phylosophy as QPYTHIA but in a different shower (angular instead of virtuality ordering, different radiation phase space, cutoffs and hadronization mechanisms…) • Different modeling of the radiation process, different parameters involved, not so easy to compare and discriminate. 9/25/2020 LNF

Tool 1: Qpythia: 1 physical idea: modified splittings 9/25/2020 LNF Slide stolen to C. Salgado

Tool 1: Qpythia: 2 physical idea: formation time effects 9/25/2020 LNF

For details on the implementation see for example Salgado’s talk at QM 09. Here I list what is contained in the MC: • • 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. Ours is an eikonal approach elastic corrections not included yet. No change on the color flow. To start with I generated pp events with Qpythia @ 5. 5 Te. V, pthard>100 Ge. V. I set a 0. 5 Ge. V momentum cutoff for the particles and define the geometry for a 0 -10% central collision. I use fastjet to reconstruct my jets and to compute intrajet distributions and other observables. 9/25/2020 LNF

Tool 2: k. T/antik. T (from Fast. Jet) Recombination algorithms: undo QCD branchings k. T algo: To minimize dij is to maximize QCD branching probability ( ) -It starts clustering up soft particles among themselves (min(dij) is set by the softest). -Soft adaptable. antik. T algo: Not inversion of the QCD branching but with some advantages: -The minimum of dij is set by the hardest particle and then it is a pure angular (pure cone) problem. So it starts clustering up soft particles around the hardest. -Soft resilient. 9/25/2020 LNF

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. 9/25/2020 LNF

Jet shape distribution: true energy (Infrared safe equivalent) of the fraction of the jet energy contained in different subcones of radius Rsj. 20% of the energy out of cone with R=0. 3 for a dense medium. 9/25/2020 LNF

p. T distribution of the hardest jet 9/25/2020 LNF

p. T cut bias Cluster the event and take the hardest <p. T> for different p. Tcuts. The smaller R, the less affected by removing soft particles. With R=0. 3 one can safely go around p. Tcut=1 wo biasing the jet spectrum. 9/25/2020 LNF

kt jets are bigger than antikts 9/25/2020 LNF

kt jets are bigger than antikts Antikt (p=-1): min(dij) is set by the hardest particle and then it is a pure cone. A soft particle distant >R from the hardest will not be clustered up in the jet soft resilient. 9/25/2020 LNF Kt (p=1): min(dij) is set by the softest particles. Soft particles are clustered up first. A soft particle distant >R from the hardest can be clustered up in the jet soft adaptable.

Jet areas 9/25/2020 LNF

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 (speculative) why not take area increase as a legitimate signature for the quenching? How to relate Area and qhat? 9/25/2020 LNF theoretical work under development ?

Area-based subtraction method -pp (quenched/unquenched) merged in a HIJING Pb. Pb background. Area based subtraction method: -Cluster the whole event into “jets” -Compute an average pt density: -Your correction is then: 9/25/2020 LNF

How to control background? - Local control of -method assumption: real jet contribution to the background median is negligible. But this really depends on the acceptance. How to suppress background? a) Reduce the area of the jet A(Jet): reduce R you loose energy/information. 9/25/2020 LNF b) Reduce fluctuations: make p. T cut on towers and traks before jet finding. you also take away particles from the jet and your jet spectrum is biased.

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Event structure: kt/antikt The cluster structure of the event is different with kt/antikt. Antikt creates more clusters of low p. T. Differences disappear when very soft particles are removed from the event. 9/25/2020 LNF

Event structure: kt/antikt /Area Differences are softened when weighting by the area 9/25/2020 LNF

Example: resolution: kt/antikt kt: distribution is shifted to postive values backreaction. antikt: for small p. Tcut background is subestimated because of minijets: simple median not valid. 9/25/2020 LNF

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 (also background models should be developed accordingly) • Studies in a realistic detector environment. 9/25/2020 LNF

Role of hadronization -The medium affects the perturbative evolution of the jet. Hadronization happens in vacuum (this is true for sufficiently boosted jets). -However modifications in the perturbative shower can lead to changes in hadronization. -The interaction of the radiated gluon with the medium is a color exchange. That changes the color flow of the shower affects further string formation and hadroquemistry. 9/25/2020 LNF

Space time evolution of the shower. -Interplay between the extension of the jet and the extension of the medium. -Bjorken/Hydrodynamical evolution of the energy density. -Background models should be accordingly developed! Small initial transport coefficient 9/25/2020 LNF Work by Konrad Tywoniuk

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. (see poster at QM by I. Dominguez, E. Cuautle, LC, G. Paic, A. Morsch) 9/25/2020 LNF

Space time evolution of the shower I. Dominquez et al Particles at external coronas are created first in time , cl the face the same medium as the leading particles but they have lower energies the quenching is expected to be strong. 9/25/2020 LNF

Extras 9/25/2020 LNF

Extras 9/25/2020 LNF

Example of easy reconstruction Good reconstruction with an easy background. Pb generated with PSM model with no hard component (no minijets). Figure stolen to J. Rojo 9/25/2020 LNF

Kt jets are bigger than antikts 9/25/2020 LNF

Qpythia: Sudakov factor When medium-modified splitting functions are included the radiation probability is enhanced. 9/25/2020 LNF

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Jet reconstruction The presence of minijets compicates the subtraction respect to other softer models like HYDJET or PSM (see J. Rojo’s talk at HP 2008). 9/25/2020 LNF

Jet reconstruction VACUUM Quenched jets are less efficientely reconstructed. Ongoing work: study the effect of different background models. 9/25/2020 LNF