Charmed meson and baryon measurements in pp and p–Pb col- lisions with ALICE at the LHC

We present here recent open heavy-flavour results from the ALICE experiment, including measurements of D-meson, Λc baryon and Ξc baryon production in pp collisions at √ s = 7 TeV and p–Pb collisions at √ snn = 5.02 TeV.


Introduction
The measurement of charm production in pp collisions is an important test of perturbative QCD, and in p-Pb collisions the study of charm production can help disentangle cold nuclear matter effects from the effects modifying the p T spectrum of charmed hadrons in Pb-Pb collisions due to the high-temperature and high energy-density medium formed. The relative abundance of baryons and mesons can shed light on the process of fragmentation -a non-perturbative process -and deviations from measurements made at e + e − colliders may hint at specific processes occuring in the higher partonic density environment of pp and p-Pb collisions. In addition, these measurements also provide a reference for future measurements in nucleus-nucleus collisions, where the baryon-to-meson ratio is expected to be sensitive to modified hadronisation mechanisms such as coalescence [1]. Preliminary results from STAR [2] hint at an enhanced Λ + c /D 0 ratio in Au-Au collisions.

Heavy-flavour reconstruction at ALICE
Heavy-flavour decays are reconstructed at ALICE in the central barrel, which consists of the Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time-of-Flight detector (TOF), covering the entire azimuthal range and designed to track and identify charged particles over a wide momentum range. Charmed hadrons are reconstructed at mid-rapidity (|y| < 0.8) via their hadronic decays including D 0 → K − π + , D + → K − π + π + , D * + → D 0 π + , D + s → φπ + , Λ + c → pK − π + and Λ + c → pK 0 S . Selection is made on the hadron decay topology, the signal is extracted via an invariant mass analysis, and corrections are made for the efficiency, acceptance, and the fraction of non-prompt hadrons in the signal sample. The semileptonic decays of Λ + c and Ξ 0 c baryons are also reconstructed via the decay channels Λ + c → e + ν e Λ and (Ξ 0 b →)Ξ 0 c → e + ν e Ξ − . Here the analyses are not based on topological selections, and are instead based on subtracting the 'wrong-sign' e − Λ(e − Ξ − ) pair spectra from the 'right-sign' e + Λ(e + Ξ − ) spectra. Additional corrections include correcting for contributions to the wrong-sign spectra from Λ b (Ξ 0 b ), and unfolding the e − Λ(e − Ξ − ) p T spectra to obtain the Λ + c (Ξ 0 c ) p T spectrum. A correction for feed-down from Ξ +,0 c is also included for the Λ + c measurement. e-mail: jaime.norman@cern.ch

Results
The nuclear modification factor (R pPb ) of D 0 , D + , D * + and D + s mesons was measured in p-Pb collisions at √ s nn = 5.02 TeV [3]. Figure 1 (left) shows the p T -differential averaged D 0 , D + and D * + R pPb in comparison with models that include cold nuclear matter effects and models that assume a Quark-Gluon Plasma is formed and include hydrodynamical effects [3]. The statistical precision of the measurements has been improved by approximately a factor of 2 with respect to the previous measurement [4] due to an increased integrated luminosity. The models describe the data well, although a suppression larger than 15-20% for p T > 5 GeV/c, expected from the POWLANG(HTL) and Duke models, is slightly disfavoured by the data. D-meson production has also been measured as a function of centrality in p-Pb collisions at √ s nn = 5.02 TeV [3]. Figure 1 (right) shows the D 0 Q CP calculated as the ratio of the D 0 -meson nuclear modification factor in central (0-10%) and peripheral (60-100%) centrality intervals. The Q CP tends to increases in the interval 1 < p T < 4 GeV/c and reaches about 1.25, and then decreases in the interval 7 < p T < 24 GeV/c. The average value of the D 0 Q CP is larger than unity in the interval 3 < p T < 8 GeV/c by 1.7 standard deviations of the statistical and systematic uncertainty. The p T -differential cross section of the Λ + c baryon was measured in pp collisions at √ s = 7 TeV and in p-Pb collisions at √ s nn = 5.02 TeV, and is reported in figure 2. The cross section is compared to perturbative calculations at NLO using the GM-VFNS [5,6] scheme in pp collisions, and at NLO with powheg [7] matched with pythia 6.4.25 [8] in pp and p-Pb collisions. For the p-Pb powheg the EPS09 nuclear PDF was used [9]. GM-VFNS underpredicts the data by a factor of 2.5 on average, and powheg underpredicts the data by a factor of 18(4) at low(high) p T in pp collisions, and by a similar amount in p-Pb collisions. Figure 3 (left) shows the baryon-to-meson ratio Λ + c /D 0 measured using the Λ + c cross sections presented in this paper and the D 0 cross sections measured by ALICE [4,10]. In the same figure theoretical predictions in pp collisions are shown including pythia8 with and without a tune including enhanced colour reconnection [11], dipsy [12] with rope hadronisation and herwig7 [13] with hadronisation via clusters. The Λ + c /D 0 ratio in pp collisions is compatible with the same ratio in p-Pb collisions within uncertainties. While all models underpredict the data, pythia8 with enhanced colour reconnection brings the prediction closer to data. nuclear PDF [9] and a prediction for charmed hadrons which assumes a small-size QGP is formed [14]. The R pPb of Λ + c is consistent with unity and with the D-meson R pPb , and does not allow to distinguish between the models presented within the current experimental uncertainties.    Figure 4 shows the p T -differential cross section times branching ratio of the Ξ 0 c baryon (including prompt and non-prompt contributions), and the baryon-to-meson ratio Ξ 0 c → e + ν e Ξ − /D 0 in comparison to predictions from pythia with the aforementioned tunes. The shaded band for the models spans the range of theoretical predictions for the Ξ 0 c → e + ν e Ξ − branching ratio [15][16][17]. As for the Λ + c /D 0 ratio, all predictions significantly underestimate the data.

Summary
Recent charmed meson and baryon measurements by the ALICE collaboration have been presented.
The R pPb of D mesons is found to be consistent with unity. The Q CP of D 0 mesons shows a hint of D 0 enhancement at 3 < p T < 8 GeV/c in central p-Pb collisions. The cross section of the Λ + c baryon and the Λ + c /D 0 and Ξ 0 c → e + ν e Ξ − /D 0 ratios are found to be underpredicted by theoretical calculations. Finally the R pPb of Λ + c baryons is found to be consistent with unity, with the D-meson R pPb and with theoretical predictions within current uncertainties.