Jet Fragmentation Properties Research Articles

Measurement of inclusive and leading subjet fragmentation in pp and Pb–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

This article presents new measurements of the fragmentation properties of jets in both proton–proton (pp) and heavy-ion collisions with the ALICE experiment at the Large Hadron Collider (LHC). We report distributions of the fraction zr of transverse momentum carried by subjets of radius r within jets of radius R. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with jet radius R = 0.4, and subjets are reconstructed by reclustering the jet constituents using the anti-kT algorithm with radii r = 0.1 and r = 0.2. In proton–proton collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calculations at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the zr distribution. In heavy-ion collisions, we measure the leading subjet distributions, which allow access to a region of harder jet frag- mentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The zr distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark–gluon plasma (QGP). We find no significant modification of zr distributions in Pb–Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for zr< 0.95, as predicted by several jet quenching models. As zr → 1 our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to disentangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.

Jet fragmentation properties with CMS open-data

The CMS collaboration has made some of their data public, making it possible for external users to perform physics analysis using real data. In this work, we report the measurement of jet properties such as fragmentation function, and the average multiplicity distribution of leptons and charged hadrons within the jets, all of them produced in pp collisions at = 7 TeV. We show that by implementing high energy cuts, the uncorrected experimental data approaches the behavior of corrected data, facilitating the use of open-data for simple analysis. The analysis is restricted in pseudorapidity and energy of the jets to: ∣η Jet∣ < 2.1 and E Jet > 120 GeV respectively. For the study we used the anti − K T algorithm applied to four samples of 2011 CMS open-data, to explore the possible differences obtained by using different trigger conditions, and the results are compared with predictions from the Pythia 8 event generator.

Measurements of jet substructure and jet fragmentation using the ATLAS detector

Measurements of the internal properties of jets at the LHC interaction scale allow QCD to be studied in a new energy regime. In this talk, we discuss recent measurements of jet substructure and jet fragmentation, which were performed using data collected by the ATLAS experiment at a centre-of-mass energy of \sqrt{s}=13s=13 TeV. The fragmentation properties of jets, such as the jet charge and summed fragmentation function, are measured using charged particles. A comprehensive suite of jet substructure observables is also measured for jets reconstructed with the soft-drop algorithm applied. In addition, a measurement of the Lund plane is performed using charged particles. All of the measurements are corrected for detector effects and are compared to the predictions of state-of-the-art Monte Carlo event generators.

Measurement of b-quark fragmentation properties in jets using the decay B\xb1\u2192 J/\u03c8K\xb1 in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

The fragmentation properties of jets containing b-hadrons are studied using charged B mesons in 139 fb−1 of pp collisions at sqrt{s} = 13 TeV, recorded with the ATLAS detector at the LHC during the period from 2015 to 2018. The B mesons are reconstructed using the decay of B± into J/ψK±, with the J/ψ decaying into a pair of muons. Jets are reconstructed using the anti-kt algorithm with radius parameter R = 0.4. The measurement determines the longitudinal and transverse momentum profiles of the reconstructed B hadrons with respect to the axes of the jets to which they are geometrically associated. These distributions are measured in intervals of the jet transverse momentum, ranging from 50 GeV to above 100 GeV. The results are corrected for detector effects and compared with several Monte Carlo predictions using different parton shower and hadronisation models. The results for the longitudinal and transverse profiles provide useful inputs to improve the description of heavy-flavour fragmentation in jets.

Soft modifications to jet fragmentation in high energy proton–proton collisions

The discovery of collectivity in proton–proton collisions, is one of the most puzzling outcomes from the first two runs at LHC, as it points to the possibility of creation of a Quark–Gluon Plasma, earlier believed to only be created in heavy ion collisions. One key observation from heavy ion collisions is still not observed in proton–proton, namely jet-quenching. In this letter it is shown how a model capable of describing soft collective features of proton–proton collisions, also predicts modifications to jet fragmentation properties. With this starting point, several new observables suited for the present and future hunt for jet quenching in small collision systems are proposed.

Fragmentation patterns of jets in pPb collisions in CMS

The nuclear parton distribution function and flavor composition of hard scattering processes can be accurately studied using the jet fragmentation functions. Recent measurements of the pPb nuclear modification factor (RpPb), with diverging values for inclusive jets and charged hadrons, have raised question on jet fragmentation properties in pPb collisions. These spectra measurements are performed with pp reference at 5.02 TeV constructed by interpolation or extrapolation from different s, and on steeply falling power-law spectra. As the jet fragmentation function is only evolving logarithmically with s, this further underscores the importance of a direct measurement. Together with the CMS results in pPb inclusive jets and charged hadron RpPb, we introduce the new CMS measurement of fragmentation function in pPb collisions, where within our uncertainties, jets in pPb is found to have identical fragmentation property vs. pp jets. We will further discuss the consistency and tension among the results, and their possible origins. This measurement also gives important reference for the interpretation of the fragmentation function in PbPb collisions.

Photon tagged correlations in heavy ion collisions

A detailed study of various two-particle correlation functions involving photons and neutral pions is presented in proton-proton and lead-lead collisions at the LHC energy. The aim is to use these correlation functions to quantify the effect of the medium (in lead-lead collisions) on the jet decay properties. The calculations are carried out at the leading order in QCD but the next-to-leading order corrections are also discussed. The competition between different production mechanisms makes the connection between the jet energy loss spectrum and the gamma-pi correlations somewhat indirect while the gamma-gamma correlations have a clearer relation to the jet fragmentation properties.

Light meson production at the Fermilab Tevatron to next-to-leading order.

Inclusive production of light mesons (${\mathrm{\ensuremath{\pi}}}^{0}$,\ensuremath{\eta},${\mathrm{\ensuremath{\pi}}}^{\ifmmode\pm\else\textpm\fi{}}$,${\mathit{K}}^{\ifmmode\pm\else\textpm\fi{}}$) at the Fermilab Tevatron is considered in QCD to next-to-leading order in the formalism of fragmentation functions. We present various distributions of phenomenological interest, along with a new set of K meson fragmentation functions. The fragmentation properties of jets in charged and neutral pions are also considered.

Some comments on jet fragmentation models andα s determinations

A number of interrelated topics on jet properties in e+e- annihilation is discussed. The need for different αs values in different fragmentation models is explained, with particular emphasis on the sensitivity to the choice of momentum conservation scheme in independent fragmentation models. Also other factors leading to a broad range of experimental αs values are discussed. Old and new methods to distinguish different fragmentation models are presented, with particular emphasis on gluon jet fragmentation properties.