Jet Splitting Scales Of Reclustered Large-radius Jets In High-energy Nuclear Collisions

The quark-gluon plasma(QGP),the new kind of nuclear matter with deconfined quarks and gluons,is expected to be formed in high-energy nuclear collisions.Fast partons produced from the initial hard-scattering may lose their energies through interactions with other partons in the plasma while propagating through the QGP,which is denoted as jet quenching phenomenon.In the last twenty years,jet quenching has become one of the most important hard probes to study the formation and the properties of the QGP.Many studies of jet quenching have been made so far,such as the suppression of leading hadron yield,attenuation of inclusive jet spectra,dijet and?/W/Z+jet correlations,as well as the medium modification of the internal structures of jets.Recently,a new approach of reconstructing jets is adopted by ATLAS Collabora-tion,which utilizes reconstructed jets with relatively small cone sizes to construct the full jets with large radii(referred to as "large-radius jets" in the following)in order to remove the soft components of jets,which is not significantly sensitive to nonpertur-bative processes of jet hadronization.In this paper,we make a detailed study of jet quenching on the hard splitting of large-radius jets proposed by ATLAS.We simulate production of the initial partons in the proton-proton collisions with Pythia8,and take advantage of the 3+1D viscous relativistic hydrodynamic model to describe the time-space evolution of the hot and dense medium,and utilize the linear Boltzmann model for fast parton propagating in the hot and dense medium.In our calculations,anti-kT algorithm is applied to reconstruct the jet,while kT algorithm is used to obtain jet splitting scales in a large-radius jet.In this thesis,we present the first theoretical calculations on the the medium modifications of large-radius jets production and jet splitting scales in high-energy nuclear collisions.The jet splitting scale((?)),whose theoretical calculations can provide a fairly good description of the experimental data,is an important observable to characterize the hardest splitting in a jet.Our calculations show that the nuclear modification factor of large-radius jets as a function of jet transverse momentum is different from that of inclusive jets due to its complex structure.The jet splitting scale spectrum indicates that jet suppression depends on jet structures.A further detailed analysis shows that the yields of large-radius jets with a.single subjet,(de-noted as one single subjet)are less suppressed than that of large-radius jets with complex structures(denoted as multiple subjets)due to jet production fraction and the evolution of the large-radius jets with multiple subjets,while jet quenching effect on jet splitting angle and jet splitting function results in the nuclear modification factor evaluated as a function of the splitting scale sharply decreases with increasing(?)for small values of the splitting scale followed by flattening for larger(?)for large-radius jets with complex structures.A particular attention will be paid to ex-ploring the underlying reasons of the yield and stru cture of large-radius jets modified by the medium,where the yields of those jets with small splitting angle values and small splitting values should be less suppressed in nucleus-nucleus collisions relative to that in proton-proton collisions.By investigating in-medium modification on dif-ferent processes,we found that due to nuclear medium effect,one single sub jet can evolve into two subjets(1?2),in which the yields of multiple subjets increase.And two subjets can become one single subjet(2? 1),and subsequently the yields of multiple subjets reduce.Furthermore,the number of subjets of the large-radius jets with two subjets remains unchanged(2?2),but the splitting angle of those two sub jets will change.As a result,at the small splitting angle region,the total contribution of the 1? 2 processes and the 2? 2 processes is larger than that of the 2?1 processes,while 2? 1 processes dominate at the larger splitting angle region which leads to less suppression for yields of the large-radius jets in small split-ting angle region compared to those in larger splitting angle region due to medium modification.