Experimental properties of gluon and quark jets from a point source

Abstract

Abstract. Gluon jets are identified in hadronic Z0 decays as all the particles in a hemisphere opposite to a hemisphere containing two tagged quark jets. Gluon jets defined in this manner are equivalent to gluon jets produced from a color singlet point source and thus correspond to the definition employed for most theoretical calculations. In a separate stage of the analysis, we select quark jets in a manner to correspond to calculations, as the particles in hemispheres of flavor tagged light quark (uds) events. We present the distributions of rapidity, scaled energy, the logarithm of the momentum, and transverse momentum with respect to the jet axes, for charged particles in these gluon and quark jets. We also examine the charged particle multiplicity distributions of the jets in restricted intervals of rapidity. For soft particles at large pT, we observe the charged particle multiplicity ratio of gluon to quark jets to be 2.29±0.09 (stat.)±0.15 (syst.), in agreement with the prediction that this ratio should approximately equal the ratio of QCD color factors, CA/CF = 2.25. The intervals used to define soft particles and large pT for this result, p < 4 GeV/c and 0.8 < pT < 3.0 GeV/c, are motivated by the predictions of the Herwig Monte Carlo multihadronic event generator. Additionally, our gluon jet data allow a sensitive test of the phenomenon of non-leading QCD terms known as color reconnection. We test the model of color reconnection implemented in the Ariadne Monte Carlo multihadronic event generator and find it to be disfavored by our data. 218The OPAL Collaboration: Experimental properties of gluon and quark jets from a point source

Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb+Pb Collisions in ATLAS

“…The γ-tagged jet FF in pp collisions is observed to be harder than the FF for inclusive jets at the same collision energy with p jet T in the range 80-110 GeV, coinciding with the peak of the γ-tagged p jet T distribution [47]. This is consistent with the two samples having different quark jet fractions, and with expectations from, for example, data from the Large Electron-Positron collider [51][52][53], where harder FFs for quark jets were observed compared with those for gluon jets. The pp data are also compared with generator distributions, which are typically compatible with the data at low to moderate values of z or p T within uncertainties.…”

Theoretical aspects of inclusive light-hadron production

“…These results are in perfect agreement with what the Particle Data Group currently quotes as the world average, α (5) s (M Z ) = 0.1181 ± 0.002 [17]. Our strategy was to only include in our fits LEP1 and SLC data with both flavour separation and hadron identification [8][9][10], gluon-tagged three-jet samples with a fixed gluon-jet energy [11,12] and the π ± , K ± and p/ p datasets from the pre-LEP1/SLC era with the highest statistics and the finest binning in x [15]. Other data served us for cross checks.…”

“…The overall goodness of the NLO (LO) fit is given by χ 2 DF = 0.98 (0.97). The goodness of our fit may also be judged from figures 1(a) and (b), where our LO and NLO fit results are compared with the ALEPH [8,11], DELPHI [9], OPAL [12] and SLD [10] data. In figure 1(a), we study the differential cross section (1/σ tot ) dσ h /dx for π ± , K ± , p/ p and unidentified charged hadrons at √ s = 91.2 GeV, normalized to the total hadronic cross section σ tot , as a function of the scaled momentum x = 2p h / √ s. As in [9, 10], we assume that the sum of the π ± , K ± and p/ p data exhausts the full-charged-hadron data.…”

“…During the last five years, the experiments at LEP1 and SLAC SLC have provided us with a wealth of high-precision information on how partons fragment into low-mass charged hadrons, so as to cure this problem. The data partly come as light-, c-and b-quark-enriched samples without [5][6][7] or with identified final-state hadrons (π ± , K ± and p/ p) [8][9][10] or as gluon-tagged three-jet samples without hadron identification [11][12][13]. Motivated by this new situation, the author, together with Kramer and Pötter, recently updated, refined and extended the BKK [4] analysis by generating new LO and NLO sets of π ± , K ± and p/ p FFs [14] 1 .…”

Measurement of jet fragmentation in Pb+Pb and pp collisions at sNN=5.02 TeV with the ATLAS detector

“…In this case, the jets fragment as in vacuum, but R D(z) differs from unity due to an increase in the fraction of quark jets in Pb+Pb collisions relative to pp collisions at a fixed p jet T . Since quark jets are more likely to produce high-z particles than gluon jets [53,54] this causes R D(z) > 1 at high z in the model predictions. The EQ model does not have a description of the soft processes from soft gluon radiation or the response of the hot QCD matter to the jet passing through it, so the comparison with data is only appropriate at z > 0.1.…”