Measurement of D*+/- meson production in jets from pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This ATLAS study measures the production of D*± mesons within jets in proton-proton collisions at 7 TeV, defining the observable R(pT,z) as the ratio of D*±-containing jets to inclusive jets and mapping it across jet pT and the momentum fraction z. D*± mesons are fully reconstructed via the decay chain D*+ -> D0 pi+, D0 -> K- pi+. Unfolding to particle level with Bayesian methods reveals R = 0.025 ± 0.001 (stat) ± 0.004 (syst) for 25 < pT < 70 GeV and 0.3 < z < 1, while MC predictions consistently underpredict at low z and fail to capture the pT dependence of R. The data show z-distribution shapes and normalization that differ from PYTHIA, HERWIG, and POWHEG-based predictions, indicating that current heavy-flavor production and fragmentation models at 7 TeV require refinement. Overall, the results highlight deficiencies in QCD-based descriptions of high-pT D-meson production and fragmentation in this energy regime, motivating further theoretical development and more precise measurements.

Abstract

This paper reports a measurement of D*+/- meson production in jets from proton-proton collisions at a center-of-mass energy of sqrt(s) = 7 TeV at the CERN Large Hadron Collider. The measurement is based on a data sample recorded with the ATLAS detector with an integrated luminosity of 0.30 pb^-1 for jets with transverse momentum between 25 and 70 GeV in the pseudorapidity range |eta| < 2.5. D*+/- mesons found in jets are fully reconstructed in the decay chain: D*+ -> D0pi+, D0 -> K-pi+, and its charge conjugate. The production rate is found to be N(D*+/-)/N(jet) = 0.025 +/- 0.001(stat.) +/- 0.004(syst.) for D*+/- mesons that carry a fraction z of the jet momentum in the range 0.3 < z < 1. Monte Carlo predictions fail to describe the data at small values of z, and this is most marked at low jet transverse momentum.

Figures (5)

• Figure 1: Examples of the distributions of the mass difference of the $D^{*+}\to D^0\pi^+$ and its charge conjugate inside jets for different $p_{\rm T}$ and $z$ bins. The solid line is the fit result. The dotted line represents the background component. For comparison, the distribution of the difference between the invariant mass of the wrong sign candidates, $m(K^\pm\pi^\pm\pi^\mp)-m(K^\pm\pi^\pm)-m(\pi^\mp)$, is also shown with a dashed line in Figure (f), where identical event selection criteria as the signal reconstruction have been applied except that the two tracks from the $D^0$ ($\bar{D}^0$) candidates are required to have the same charge. No structure is observed and the distribution of wrong sign candidates can be also well described by the same PDF as the signal candidates with slightly different values of the PDF parameters.
• Figure 2: Comparison of the $p_{\rm T}$ and $z$ distributions of the $D^{*\pm}$ jets between data and MC. The $D^{*\pm}$ decay products are matched to jets at the particle level by using a geometrical matching with $\Delta R < 0.6$. The MC distributions are then reweighted and subsequently normalized to have the same number of events as the data.
• Figure 3: Relative systematic uncertainties of the measured $D^{*\pm}$ jet production rate ${\cal R}(p_{\rm T},z)$ in different jet $p_{\text{T}}$ and $z$ bins. The values corresponding to the integrated $p_{\rm T}$ range are shown in the bottom right Figure.
• Figure 4: Comparison of the $D^{*\pm}$ production rate ${\cal R}(p_{\rm T},z)/\Delta z$ in different jet $p_{\text{T}}$ and $z$ bins between the measurement and the MC predictions of PYTHIA, HERWIG, POWHEG+PYTHIA and POWHEG+HERWIG. The values corresponding to the integrated $p_{\rm T}$ range are shown in the bottom right Figure. The insets show the ratio of the measurement to the POWHEG+PYTHIA prediction.
• Figure 5: Comparison of the $D^{*\pm}$ production rate ${\cal R}(p_{\rm T},z)/\Delta z$ in different jet $p_{\text{T}}$ and $z$ bins between the measurement and the MC predictions of POWHEG+PYTHIA (left) and POWHEG+HERWIG (right). The $c$- and $b$ fractions predicted by MC are also shown. Only the central values of the MC predictions are shown here.