Light Quark Fragmentation Research Articles

Time-Like heavy-flavour thresholds for fragmentation functions: the light-quark matching condition at NNLO

Matching conditions are universal ingredients that describe how fragmentation functions change when heavy-flavour thresholds are crossed during the factorisation scale evolution. They are the last missing piece for a consistent description of observables with identified final-state hadrons at next-to-next-to leading order accuracy in quantum chromodynamics. We present an analytical form of the matching condition for light-flavour to hadron fragmentation function at next-to-next-to leading order. The derivation is performed by extending the formalism employed in the extraction of the next-to leading order matching conditions to the subsequent order, making use of e+e-\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e^+e^-$$\\end{document} annihilation cross sections. We obtain the first non-trivial heavy-quark effect in the light-quark fragmentation functions and provide results in Mellin space.

Polarized J/ψ production in semi-inclusive DIS at large Q2 : Comparing quark fragmentation and photon-gluon fusion

We compare the relative importance of different mechanisms for polarized J/ψ production in semi-inclusive deep inelastic scattering processes at large Q2. The transverse momentum dependent (TMD) factorization framework and nonrelativistic quantum chromodynamics are used to study the leading contributions from light quark fragmentation to polarized J/ψ, and compared to direct production via photon-gluon fusion, which can proceed through color-singlet as well as color-octet mechanisms. We identify kinematic regimes where light quark fragmentation dominates, allowing for the extraction of the S31[8] matrix element, as well as regimes where photon gluon fusion dominates, suggesting that the gluon TMD parton distribution function can be probed. Published by the American Physical Society 2024

Polarized TMD fragmentation functions for J/ψ production

We calculate the matching, at leading order, of the transverse momentum-dependent fragmentation functions (TMDFFs) for light quarks and gluons fragmenting to a J/ψ onto polarized nonrelativistic QCD (NRQCD) TMDFFs. The NRQCD TMDFFs have an operator product expansion in terms of nonperturbative NRQCD production matrix elements. Using the results we obtain, we make predictions for the light quark fragmentation contribution to the production of polarized J/ψ in semi-inclusive deep inelastic scattering (SIDIS) both for unpolarized and longitudinally polarized beams. These results are an important contribution to polarized J/ψ production in SIDIS and thus are needed for comparison with experiments at the future Electron-Ion Collider. Published by the American Physical Society 2024

The Strong Force meets the Dark Sector: a robust estimate of QCD uncertainties for anti-matter dark matter searches

In dark-matter annihilation channels to hadronic final states, stable particles — such as positrons, photons, antiprotons, and antineutrinos — are produced via complex sequences of phenomena including QED/QCD radiation, hadronisation, and hadron decays. These processes are normally modelled by Monte Carlo (MC) event generators whose limited accuracy imply intrinsic QCD uncertainties on the predictions for indirect-detection experiments like Fermi-LAT, Pamela, IceCube or Ams–02. In this article, we perform a comprehensive analysis of QCD uncertainties, meaning both perturbative and nonperturbative sources of uncertainty are included — estimated via variations of MC renormalization-scale and fragmentation-function parameters, respectively — in antimatter spectra from dark-matter annihilation, based on parametric variations of the Pythia 8 event generator. After performing several retunings of light-quark fragmentation functions, we define a set of variations that span a conservative estimate of the QCD uncertainties. We estimate the effects on antimatter spectra for various annihilation channels and final-state particle species, and discuss their impact on fitted values for the dark-matter mass and thermally-averaged annihilation cross section. We find dramatic impacts which can go up to O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(40) GeV for uncertainties on the dark-matter mass and up to O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(10%) for the annihilation cross section. We provide the spectra in tabulated form including QCD uncertainties and code snippets to perform fast dark-matter fits, in this github repository.

Dead cone effect in charm and bottom quark jets

The evolution of a heavy quark initiated jet is mainly ruled by gluon bremsstrahlung. As a consequence of the dead-cone effect, this radiation is suppressed in the forward direction at angles smaller than that proportional to the heavy quark mass MQ, i.e. Θ0=MQ/EQ at energy EQ of the primary quark. In this paper, we unveil this effect in charm and bottom quark jets using DELPHI and OPAL data from Z0 boson decays in e+e− annihilation at center of mass energy 91.2 GeV. The analysis of the reconstructed heavy quark fragmentation function in momentum space shows the strong suppression of hadrons at high momenta in such events compared to light quark fragmentation by a factor ≲1/10. The amount of this suppression is well reproduced by perturbative QCD (pQCD) within the Modified Leading Logarithmic Aproximation and the compact scheme of Local Parton Hadron Duality (MLLA-LPHD). As a new result, we obtain an almost perfect agreement between the light quark fragmentation functions expected at W0∝MQ from DELPHI and OPAL data with Pythia8 and shed light on the reasons for the existence of the ultra-soft gluon excess at small momentum fraction in comparison with pQCD predictions.

Observation of the dead cone effect in charm and bottom quark jets and its QCD explanation

The production of a heavy quark is accompanied by gluon bremsstrahlung which is suppressed at small angles $\Theta\lesssim M_Q/E$ for mass $M_Q$ and high energy $E$ according to perturbative Quantum Chromo Dynamics (QCD) (``dead cone effect''). As particles at small angles typically have large momenta, the heavy quark mass also causes a suppression of high momentum particles. In this paper, we studied this effect in c- and b-quark events using data from Z boson decays in $e^+e^-$ annihilation. The heavy quark fragmentation function for charged particles is reconstructed in the momentum fraction variable $x$ or $\xi=\ln(1/x)$ by removing the decays of the heavy quark hadrons. Indeed, we find an increasing suppression of particles with rising $x$ down to a fraction of $\lesssim 1/10$ for particles with $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparison to light quark fragmentation. The sensitivity to the dead cone effect in the present momentum analysis is considerably increased in comparison to the recently presented angular analysis. This amount of suppression and the differences between c- and b-quark fragmentation are in good quantitative agreement with the expectations based on perturbative QCD within the Modified Leading Logarithmic Approximation (MLLA) in the central kinematic region. The data also support a two parameter description in the MLLA of these phenomena (``Limiting Spectrum''). The sensitivity of these measurements to the heavy quark mass is investigated.

Quarkonium TMD fragmentation functions in NRQCD

We study the transverse-momentum spectrum of quarkonium production from single light-parton fragmentation mechanism. In the case of semi-inclusive deep inelastic scattering, we observe that there are two possible initiating processes, namely photon-gluon fusion and light-quark fragmentation. For the second case we derive the factorization theorem, which involves a new hadronic quantity: the quarkonium transverse-momentum-dependent fragmentation functions in NRQCD. We calculate their matching onto the non-perturbative long distance matrix elements at the lowest order in the strong-coupling constant, mathcal{O}left({alpha}_s^2right) . Focusing on the case of the electron-ion collider, we make a comparative phenomenological study of the two production mechanisms and find the regions of the phase space where one is dominant over the other.

Proton fragmentation functions considering finite-mass corrections

We present new sets of proton fragmentation functions describing the production of protons from the gluon and each of the quarks, obtained by a global fit to all relevant data sets extracted from the single-inclusive electron-positron annihilation. Our analysis is in good agreement with $e^+e^-$ annihilation data. We also impose the finite-mass effects of proton in our calculations, a topic with a very little attention paid to in the literatures. Proton mass effects turn out to be appreciable for the gluon and light quark fragmentation functions, so the inclusion of finite-mass effects tends to improve the overall description of the data by reducing the minimized $\chi^2$ values, significantly. As an application, we also apply the extracted fragmentation functions to make our predictions for the scaled-energy distribution of protons inclusively produced in top quark decays at next-to-leading order, relying on the universality and scaling violations of fragmentation functions.

Vector meson fragmentation using a model with broken SU(3) at the next-to-leading order

A detailed study of fragmentation of vector mesons at the next-to-leading order (NLO) in QCD is given for e+e- scattering. A model with broken SU(3) symmetry using three input fragmentation functions α(x, Q2), β(x, Q2) and γ(x, Q2) and a strangeness suppression parameter λ describes all the light quark fragmentation functions for the entire vector meson octet. At a starting low energy scale of [Formula: see text] for three light quarks (u, d, s) along with initial parametrization, the fragmentation functions are evolved through DGLAP evolution equations at NLO and the cross-section is calculated. The heavy quarks contribution are added in appropriate thresholds during evolution. The results obtained are fitted at the momentum scale of [Formula: see text] for LEP and SLD data. Good-quality fits are obtained for ρ, K*, ω and ϕ mesons, implying the consistency and efficiency of this model. Strangeness suppression in this model is understood both in terms of ratios of quark fragmentation functions alone as well as in terms of observables; the latter yield a suppression through the K*/ρ multiplicity ratio of about 0.23 while the x dependence of this suppression is also parametrized through the cross-section ratios.

Fragmentation with a cut on thrust: Predictions forBfactories

When high-energy single-hadron production takes place inside an identified jet, there are important correlations between the fragmentation and phase-space cuts. For example, when one-hadron yields are measured in on-resonance B-factory data, a cut on the thrust event shape T is required to remove the large b-quark contribution. This leads to a dijet final state restriction for the light-quark fragmentation process. Here we complete our analysis of unpolarized fragmentation of (light) quarks and gluons to a light hadron h with energy fraction z in e+ e- -> dijet + h at the center-of-mass energy Q=10.58 GeV. In addition to the next-to-next-to-leading order resummation of logarithms of 1-T, we include the next-to-leading order (NLO) nonsingular O(1-T) contribution to the cross section, the resummation of threshold logarithms of 1-z, and the leading nonperturbative contribution to the soft function. Our results for the correlations between fragmentation and the thrust cut are presented in a way that can be directly tested against B-factory data. These correlations are also observed in Pythia, but are surprisingly smaller at NLO.

STUDY OF VECTOR MESON FRAGMENTATION USING A BROKEN SU(3) MODEL

Inclusive hadro-production in e+e- annihilation processes is examined to study the fragmentation process. A broken SU(3) model is used to determine the quark and gluon fragmentation functions of octet vector mesons, ρ and K*, in a simple way with an SU(3) breaking parameter λ. These are expressed in terms of just two light quark fragmentation functions, V(x, Q2) and γ(x, Q2) and the gluon fragmentation function Dg(x, Q2). These functions are parametrized at the low input scale of [Formula: see text], evolved through LO DGLAP evolution including charm and bottom flavor at appropriate thresholds, and fitted by comparison with data at the Z-pole. The model is extended with the introduction of a few additional parameters to include a study of singlet–octet mixing and hence ω and ϕ fragmentation. The model gives good fits to the available data for x ≳0.01, where x is the scaled energy of the hadron. The model is then applied successfully to ω, ϕ production in pp collisions at the relativistic heavy ion collider, RHIC, these data form an important baseline for the study of Quark Gluon Plasma in heavy nucleus collisions at RHIC, and also in future at the LHC.

RECENT RESULTS FROM RHIC SPIN AND BELLE FRAGMENTATION FUNCTION MEASUREMENTS

This contribution summarizes recent results from the RHIC spin program and the measurement of light quark fragmentation functions at Belle.

The influence of direct D-meson production on the determination of the nucleon strangeness asymmetry via dimuon events in neutrino experiments

Experimentally, the production of oppositely charged dimuon events by neutrino and antineutrino deep inelastic scattering (DIS) is used to determine the strangeness asymmetry inside a nucleon. Here we point out that the direct production of a D-meson in DIS may have a substantial influence on the measurement of nucleon strangeness distributions. Direct D-meson production is via heavy quark recombination (HQR) and via light-quark fragmentation from perturbative QCD (LQF-P). To see the influence precisely, we compute the direct D-meson productions via HQR and LQF-P quantitatively and estimate their corrections to the analysis of the strangeness asymmetry. The results show that HQR has a stronger effect than LQF-P does, and the former may influence the experimental determination of the nucleon strangeness asymmetry.

Determination of fragmentation functions and their uncertainties

Fragmentation functions and their uncertainties are determined for pion, kaon, and proton by a global ${\ensuremath{\chi}}^{2}$ analysis of charged-hadron production data in electron-positron annihilation and by the Hessian method for error estimation. It is especially important that the uncertainties of the fragmentation functions are estimated in this analysis. The results indicate that the fragmentation functions, especially gluon and light-quark fragmentation functions, have large uncertainties at small ${Q}^{2}$. There are large differences between widely used functions by Kniehl, Kramer, and P\otter (KKP) and Kretzer; however, they are compatible with each other and also with our functions if the uncertainties are taken into account. We find that determination of the fragmentation functions is improved in next-to-leading order (NLO) analyses for the pion and kaon in comparison with leading-order ones. Such a NLO improvement is not obvious in the proton. Since the uncertainties are large at small ${Q}^{2}$, the uncertainty estimation is very important for analyzing hadron-production data at small ${Q}^{2}$ or ${p}_{T}$ (${Q}^{2}$, ${p}_{T}^{2}\ensuremath{\ll}{M}_{Z}^{2}$) in lepton scattering and hadron-hadron collisions. A code is available for general users for calculating obtained fragmentation functions.

The leading particle effect from light quark fragmentation in charm hadroproduction

The asymmetry of D- and D+ meson production in π-N scattering observed by the E791 experiment is a phenomenon typical for what is known as the leading particle effect in charm hadroproduction. We show that the phenomenon can be explained by the effect of light quark fragmentation into charmed hadrons (LQF). Meanwhile, the size of the LQF effect is estimated from the data of the E791 experiment. A comparison is made with the estimate of the LQF effect from the prompt like-sign dimuon rate in neutrino experiments. The influence of the LQF effect on the measurement of the nucleon strange distribution asymmetry from charged current charm production processes is briefly discussed.

Fragmentation functions for [formula omitted] and Λ with complete quark flavour separation

We present new sets of next-to-leading order fragmentation functions for the production of KS0 and Λ particles from the gluon and from each of the quarks, obtained by fitting to all relevant data sets from e+e− annihilation. The individual light quark flavour fragmentation functions are constrained phenomenologically for the first time by including in the data the light quark tagging probabilities measured by the OPAL Collaboration.

Fragmentation functions for light charged hadrons with complete quark flavour separation

We present new sets of next-to-leading order fragmentation functions describing the production of charged pions, kaons and protons from the gluon and from each of the quarks, obtained by fitting to all relevant data sets from e + e − annihilation. The individual light quark flavour fragmentation functions are obtained phenomenologically for the first time by including in the data the light quark tagging probabilities obtained by the OPAL Collaboration.

Probing nucleon strange asymmetry from charm production in neutrino deep inelastic scattering

We propose a means to detect the nucleon strange quark-antiquark asymmetry, which is predicted as a non-perturbative effect, but still unchecked directly by available experiments. The difference for the $D(c\bar{q})$ and $\bar{D}(\bar{c}q)$ meson production cross sections in neutrino and antineutrino induced charged current deep inelastic scattering is illustrated to be sensitive to the nucleon strange asymmetry. Prospect is given and the effect due to the light quark fragmentation is also discussed for the extraction of the strange asymmetry in future experiments.

Multiplicities and particle production at LEP

Recent results on hadron multiplicities in heavy and light quark fragmentation above the Z peak (OPAL), and multiplicity distribution analysis (L3) and inclusive f_1 production (DELPHI) in hadronic Z decays are presented.

Multiplicities, fluctuations and correlations

The recent results on hadron multiplicities in heavy and light quark fragmentation, multiplicity local fluctuations and multiparticle correlations submitted to the Conference are reviewed.