Jet modification in absence of QGP-medium: the role of multiparton interactions and color reconnection

Recent studies of high-multiplicity events in small collision systems (proton-proton and proton-lead) have drawn research interest towards the possibility of the formation of partonic medium in such systems. One of the important consequences of the formation of dense partonic medium is quenching of high-momentum final-state particles resulting in several experimental observations such as suppression in nuclear modification factor $R_{\rm AA}$, modification of jet shape observable $\rho(r)$, etc. In this work, we study $\rho(r)$ for inclusive charged-particle jets in proton-proton (pp) collisions at $\sqrt{s}$ = 13 TeV using PYTHIA 8 Monash 2013 Monte Carlo simulation and show that the color reconnection (CR) and multiparton interaction (MPI) mechanisms in PYTHIA 8 can also lead to a significant amount of modification of $\rho(r)$ in high-multiplicity events compared to minimum bias events for 10 $<p_{\rm T,\,jet}^{\rm ch}<$ 20 GeV/$c$. The enhanced gluonic contribution in high-multiplicity events is also found to play a role in the observed modification of $\rho(r)$. We notice a direct connection between the number of MPIs and the amount of modification in $\rho(r)$ - the larger the number of MPIs, the larger the amount of modification of $\rho(r)$.Comment: 14 pages, 7 figures, 1 tabl

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Forward-central two-particle correlations in p-Pb collisions at root s(NN)=5.02 TeV

Two-particle angular correlations between trigger particles in the forward pseudorapidity range (2.5 2GeV/c. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B. V.Peer reviewe

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions at root(NN)-N-S=2.76 TeV

Peer reviewe

Elliptic flow of muons from heavy-flavour hadron decays at forward rapidity in Pb-Pb collisions at root s(NN)=2.76TeV

The elliptic flow, v(2), of muons from heavy-flavour hadron decays at forward rapidity (2.5 <y <4) is measured in Pb-Pb collisions at root s(NN)= 2.76TeVwith the ALICE detector at the LHC. The scalar product, two- and four-particle Q cumulants and Lee-Yang zeros methods are used. The dependence of the v(2) of muons from heavy-flavour hadron decays on the collision centrality, in the range 0-40%, and on transverse momentum, p(T), is studied in the interval 3 <p(T)<10 GeV/c. A positive v(2) is observed with the scalar product and two-particle Q cumulants in semi-central collisions (10-20% and 20-40% centrality classes) for the p(T) interval from 3 to about 5GeV/c with a significance larger than 3 sigma, based on the combination of statistical and systematic uncertainties. The v(2) magnitude tends to decrease towards more central collisions and with increasing pT. It becomes compatible with zero in the interval 6 <p(T)<10 GeV/c. The results are compared to models describing the interaction of heavy quarks and open heavy-flavour hadrons with the high-density medium formed in high-energy heavy-ion collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Study of jet cross-sections and properties in hadronic and heavy-ion collisions with ALICE at the Large Hadron Collider

In ultra-relativistic heavy-ion (AA) collisions, a high density decon ned state of quarks and gluons is produced which is known as the Quark Gluon Plasma (QGP). This state of matter is supposed to have existed microseconds after the Big Bang and resemble the state of the early universe. Attempts have been made to produce this system over a wide range of energy and/or energy-density using facilities such as the Relativistic Heavy Ion Collider (RHIC), the Large Hadron Collider (LHC). This produced system is very short lived and the main challenge of this eld of research is to characterise this short lived state of matter. There are few proposed signals to probe the QGP and a family of such probes are termed as the hard probes which are produced at the early stage of the collision and are considered to be the novel probes for the QGP medium. The massive available collision energy at the LHC makes it possible to produce hard probes with increased cross-sections. Jets belong to the family of such hard probes. A jet is the concentrated collection of hadrons, produced from the fragmentation of a hard scattered parton. In heavy-ion collisions, a decon ned dense medium of free partons is created and the hard initial partons lose their energy while traversing through the medium by the interaction with the medium's partons. This phenomena is termed "jet quenching" and is a smoking gun signature of the QGP production. Though the study of heavy-ion collisions is at the heart of this eld, there is also the need for baseline references (in vacuum) to re ect the e ects of the medium properly. Study of proton- proton (pp) collisions serves this purpose. The pp collision is also signi cant in it's own term for providing the testing ground for pQCD processes. The jet study in pp collisions, provides detailed insight on parton fragmentation. In this thesis, a jet properties study done with ALICE pp data at 2.76 TeV has been presented. The data measurements have also been compared with those obtained from Monte Carlo models to show the agreement with theory predictions. Jet properties study in heavy-ion collisions has also been conducted using the Pb-Pb events generated in the framework of a dynamical jet quenching model, JEWEL. Jet medium interactions have been investigated and the performance of the model against available experimental measurements have been presented in this thesis

燒津鰹漁業に於ける船仲組織(上) - 本邦漁業に特異なる勞働組織の一例 -

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at √sNN = 2.76 TeV as a function of collision centrality and the average transverse momentum of the pair kT. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged onedimensional radii are extracted. The radii decrease with kT, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with _dNch/dη_1/3. This behavior is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller √sNN, a decrease in the ratio Rout/Rside is seen, which is in qualitative agreement with a specific prediction from hydrodynamic models: a change from inside-out to outside-in freeze-out configuration. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the CERN Large Hadron Collider