Azimuthal correlations of heavy-flavor hadron decay electrons with charged particles in pp and p–Pb collisions at $$\pmb {\sqrt{s_{\mathrm{{NN}}}}}$$ = 5.02 TeV

Eur. Phys. J. C (2023) 83:741 https://doi.org/10.1140/epjc/s10052-023-11835-x Regular Article - Experimental Physics Azimuthal correlations of heavy-flavor hadron decay electrons with √ charged particles in pp and p–Pb collisions at sNN = 5.02 TeV ALICE Collaboration CERN,1211 Geneva 23, Switzerland Received: 14 April 2023 / Accepted: 11 July 2023 © CERN for the benefit of the ALICE Collaboration 2023 Abstract The azimuthal (ϕ) correlation distributions between heavy-flavor decay electrons and associated charged √ particles are measured in pp and p–Pb collisions at sNN = 5.02 TeV. Results are reported for electrons with transverse momentum 4 < pT < 16 GeV/c and pseudorapidity |η| < 0.6. The associated charged particles are selected with transverse momentum 1 < pT < 7 GeV/c, and relative pseudorapidity separation with the leading electron |η| < 1. The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p– Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The ϕ distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators. 1 Introduction In high-energy hadronic collisions, heavy quarks (charm and beauty) are mainly produced in hard parton scattering processes. Due to the large momentum transfer characterizing these processes, their inclusive production cross sections can be calculated in the framework of perturbative quantum chromodynamics (pQCD) [1–5]. The production cross sections of several open heavy-flavor hadrons and of their decay leptons in pp collisions were measured at both mid- and forwardrapidity at the LHC [6–27], and are described by pQCD calculations [28–30] with large theoretical uncertainties. The charm-hadron production cross section calculations in the pQCD frameworks are based on the factorization of parton  e-mail: 0123456789().: V,-vol distribution functions (PDF), the partonic cross section, and the fragmentation function. Recent measurements of charmbaryon production at midrapidity in pp collisions [31–42] are not reproduced by pQCD calculations and event generators adopting a fragmentation model tuned on e+ e− data. A better description of these measurements can be obtained by models including hadronization mechanisms such as quark coalescence [43], additional color reconnections among parton fragments [44], or by including enhanced feed-down from higher-mass charm-baryon states within a statistical hadronization approach [45], where the higher-mass excited charm-baryon states are predicted by the Relativistic Quark Model [46] but not yet measured. More differential measurements are needed to better understand the fragmentation (parton showering) and hadronization of heavy quarks. Twoparticle angular correlations originating from heavy-flavor particles allow such processes to be characterized. The typical structure of a two-particle angular correlation distribution of high transverse-momentum ( pT ) trigger particles with associated charged particles features a “nearside” (NS) peak at (ϕ, η) = (0, 0) and an “away-side” (AS) peak at ϕ = π , extending over a wide pseudorapidity range. The NS peak is mainly induced by particles emerging from the fragmentation of the same parton that produced the trigger particle. The AS peak is related to the fragmentation of the other parton produced in the hard scattering. Here, η is the difference in pseudorapidity between the trigger and associated particles. The peaks lie on top of an approximately flat continuum extending over the full (ϕ, η) range [47]. At leading order (LO) accuracy in QCD, heavy quark–antiquark pairs are produced back-to-back in azimuth [48]. At next-toleading order (NLO), the correlation shapes can significantly differ from such a topology [48,49]. Gluon radiation of heavy quarks can smear the back-to-back topology and broaden the near- and away-side peaks. In the gluon splitting process, the two heavy quarks can be produced with a small opening angle, depending on the pT of the gluon and the mass of the produced quark, generating two sprays of hadrons that 123 741 Page 2 of 30 can partially overlap, leading to a broader near-side peak. In the flavor excitation process [49], the heavy-quark pairs can be significantly separated in rapidity, and the hadrons from the opposite quark with respect to the trigger particle induce a nearly flat contribution to the ϕ distribution. The correlation measurements provide insight into heavy-flavor jet properties at low transverse momentum. By varying the pT interval of the trigger and associated particles, the correlation measurements allow the details of jet fragmentation to be studied, such as the jet angular profile and the momentum distribution of the particles produced in the fragmentation of the hard parton. The azimuthal correlation distributions of prompt D mesons and charged particles were measured by the ALICE √ Collaboration in pp collisions at s = 5.02, 7, and 13 TeV for pTD of the D mesons up to 36 GeV/c and associated charged particles up to pTassoc = 3 GeV/c [47,50,51]. The measurements were compared with Monte Carlo (MC) simulations with different event generators, like PYTHIA [52– 54], HERWIG [55,56], EPOS [57,58], and POWHEG coupled with PYTHIA8 for the parton shower and hadronization (POWHEG+PYTHIA8) [59,60]. A substantial difference among the generators was observed, with PYTHIA8 and POWHEG+PYTHIA8 providing the best description of the measured observables. These differences can be ascribed to the specific implementation of features such as hard-parton scattering matrix elements, parton showering, hadronization algorithm, and underlying event generation, affecting the correlation functions of heavy-flavor hadrons and charged particles. Measuring the correlation distribution between heavyflavor decay electrons and charged particles grants a substantially larger sample of correlation pairs, compared to measurements of D mesons and charged particle azimuthal correlations [47,50]. This allows a significant extension of the pTassoc range of associated particles and can provide a more complete picture of the heavy quark fragmentation. In addition, electrons originating from beauty-hadron decays (b → (c →) e) dominate the heavy-flavor hadron decay electron spectrum (> 50%) at high pTe (> 5 GeV/c) [61]. Hence, probing large enough trigger electron transverse momenta enables the study of the correlation function of particles originating from beauty-hadron decays, and provides information on the different correlation structures for charm (...truncated)