Open charm and beauty measurements from small to large systems

. Heavy-ﬂavor quarks (i.e. charm and beauty) are essential probes to investigate the properties of the quark-gluon plasma (QGP) created in ultra-relativistic heavy-ion collisions. The measurement of the nuclear modiﬁcation factor ( R AA ) gives insight into the in-medium parton energy loss in heavy-ion collisions. Furthermore, the measurements of heavy-ﬂavor elliptic ﬂow ( v 2 ) provide crucial information about the degree of thermalization of heavy quarks in the QGP, the path-length dependence of heavy-quark in-medium energy loss, and possible recombination e ﬀ ects. The higher ﬂow harmonics, such as the triangular ﬂow ( v 3 ), provide further constraints on the e ﬀ ect of ﬂuctuations in the initial state of the system. The measurements in proton-proton (pp) collisions allow for testing perturbative quantum chromodynamics (pQCD) calculations and are needed as a baseline for investigating the medium e ﬀ ects in heavy-ion collisions. In this contribution, recent results of open charm and beauty production measured with the ALICE detector are discussed.


Introduction
The ALICE apparatus was designed to study the quark-gluon plasma (QGP), a state of matter in which colored-partons are deconfined. Due to their large mass, heavy quarks are mainly produced in initial hard-scattering processes, which occur before the QGP formation [1]. They interact with the medium during the full evolution of the system. Therefore, they are effective probes to investigate the properties of the QGP.
While traversing the medium, heavy quarks interact with the medium constituents and lose their energy via collisional [2] and radiative processes [3]. The medium effects in heavy-ion collisions are studied measuring the nuclear modification factor (R AA ). The R AA is defined as the ratio between the p T -differential yield in nucleus-nucleus collisions (d 2 N AA /dηdp T ) and the p T -differential production cross section in pp collisions (d 2 σ pp /dηdp T ) scaled by the average nuclear thickness function T AA , which is a quantity proportional to the average number of binary nucleon-nucleon collisions. The measurement of heavy quark production in proton-proton (pp) collisions allows for testing pQCD calculations and provides the reference for measuring the R AA .
Open charm and beauty hadron production is measured with the ALICE detector via the reconstruction of hadronic and semi-leptonic decay channels at midrapidity. The D mesons are measured via the D 0 → π + K − , D + → π + π + K − , D * + → D 0 π + , D + s → π + φ → π + K + K − decay channels, and their charge conjugate. The D meson yields are extracted from an invariant mass analysis after applying topological and particle identification selections to enhance the signal-to-background ratio. Non-prompt D mesons are used to measure open-beauty hadron production. Exploiting machine-learning techniques based on Boosted Decision Trees, nonprompt D mesons are separated from the prompt D mesons [4]. Open heavy-flavor production is also measured by identifying the electrons from semi-electronic decays of charm and beauty hadrons. The production yield is extracted by subtracting the contamination of hadrons and of non-heavy flavor decay electrons from the inclusive sample. The production of beauty-hadron decay electrons is measured by determining statistically the fraction of beauty-decay electrons in the inclusive electron sample with an impact-parameter template fit method [5,6].

Results
The production of electrons from both inclusive heavy-flavor hadron decays [7] and beauty hadron decays was measured in pp collisions at √ s = 5.02 TeV at midrapidity. The fraction of the beauty hadron decay electrons to the one from inclusive heavy-flavor hadron decays in the transverse momentum (p T ) region 2 < p T < 8 GeV/c is shown in Fig. 1 (left). The result is described by the FONLL pQCD calculation [8][9][10] within the uncertainties. The measured values and the model prediction vary with p T , indicating that the charm contribution is dominant at low p T and the beauty contribution is increasing with p T . The prompt and non-prompt D 0 , D + and D + s meson cross sections were measured in pp collisions at √ s = 5.02 TeV [4]. From the results of non-prompt D mesons, the total bb production cross section was computed. Figure 1 (right) shows dσ bb /dy |y|=0 as a function of center-of-mass energy. The measurements are found to be compatible with FONLL [8][9][10] and NNLO [11] calculations. This result provides an important test for pQCD calculations in the beauty sector at the LHC.  The production of electrons from inclusive heavy-flavor hadron decays [7] and beauty hadron decays is also measured in Pb-Pb collisions at √ s NN = 5.02 TeV, as well as the production of prompt and non-prompt D 0 mesons. Figure 2 (left) shows the R AA comparison between inclusive heavy-flavor hadron decays and beauty hadron decays measured in 0-10% central Pb-Pb collisions. Both R AA values are smaller than unity, suggesting that the heavy-flavor production is suppressed in Pb-Pb collisions with respect to pp collisions in the intermediate p T region. The R AA of beauty decay electrons tends to be higher than that of inclusive heavy-flavor hadron decay electrons at low p T , though they are compatible within uncertainties. At high p T , they have very similar values. The observed trend is consistent with the expectation that the suppression of beauty hadrons is smaller than charm. Figure 2 (right) shows the R AA ratio of non-prompt D 0 over prompt D 0 in the 0-10% centrality Pb-Pb collisions at √ s NN = 5.02 TeV compared to several model predictions (MC@sHQ+EPOS2 [12], LGR [13,14], TAMU [15], and CUJET3.1 [16]). The measured ratio, above unity, suggests that non-prompt D 0 mesons are less suppressed than prompt D 0 ones. This is a hint of the mass dependence of the in-medium parton energy loss. At very low p T , the ratio decreases up to 3 GeV/c and increases from 3 GeV/c to 7 GeV/c. This behavior is mainly due to the formation of prompt D mesons via charm quark coalescence. Above 7 GeV/c, the ratio does not vary significantly with p T within sum of statistical and systematic uncertainties.  The azimuthal anisotropy of particles provides further insight into the interaction of heavy quarks in the QGP. It can be quantified by the Fourier coefficients of the particle azimuthal distribution defined as v n = cos(n(ϕ − Ψ n )) , where ϕ is particle azimuthal angle and Ψ n is n th -order asymmetry plane formed by impact parameter vector and the beam axis using V0 detector. The elliptic flow v 2 , which is the dominant coefficient in semi-central collisions, is sensitive to the degree of thermalization of the heavy quarks at low p T , while the path-length dependence of the in-medium parton energy loss becomes the main origin of a positive v 2 at high p T . The triangular flow v 3 originates from event-by-event fluctuations in initial collision geometry. Figure 3 shows the v 2 (left) and v 3 (right) for several particle species. For v 2 , the mass ordering hierarchy is seen for p T < 3 GeV/c while the v 2 of prompt D and π ± mesons are similar for 3 < p T < 6 GeV/c. This supports the hypothesis that charm quarks hadronize via coalescence with light quarks from the medium. For p T > 6 GeV/c, all v 2 values converge to a similar value, except for Υ(1S). This suggests a similar path-length dependence of energy loss for different partons. A positive v 3 for non-strange D mesons is observed, as shown in Fig. 3 (right) indicating that the charm quarks are sensitive to the initial state event-by-event fluctuation of the created medium.

Summary and Outlook
In this contribution, recent results on open heavy-flavor production in pp and Pb-Pb collisions from ALICE Collaboration were presented. All the measurements are conducted utilizing Run 2 data. The results show that the pQCD calculations reasonably describe the open heavy-flavor data in pp collisions. The R AA in central Pb-Pb collisions is lower than unity over a wide p T interval, indicating that open heavy-flavor production is suppressed. The comparison between the R AA of prompt and non-prompt D mesons as well as that between beauty-hadron decay electrons and the inclusive heavy-flavor hadron decay electrons suggest that the in-medium parton energy loss depends on the parton mass. The positive anisotropic flow indicates the participation of heavy quarks to the collective motion of the medium.