Measurement of prompt D-meson production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The $p_{\rm T}$-differential production cross sections of the prompt charmed mesons $D^0$, $D^+$, $D^{*+}$ and $D_{\rm s}^{+}$ and their charge conjugate in the rapidity interval $-0.96<y_{\rm cms}<0.04$ were measured in p-Pb collisions at a centre-of-mass energy $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. The nuclear modification factor $R_{\rm pPb}$, quantifying the D-meson yield in p-Pb collisions relative to the yield in pp collisions scaled by the number of binary nucleon-nucleon collisions, is compatible within the 15-20% uncertainties with unity in the transverse momentum interval $1<p_{\rm T}<24$ GeV/$c$. No significant difference among the $R_{\rm pPb}$ of the four D-meson species is observed. The results are described within uncertainties by theoretical calculations that include initial-state effects. The measurement adds experimental evidence that the modification of the momentum spectrum of D mesons observed in Pb-Pb collisions with respect to pp collisions is due to strong final-state effects induced by hot partonic matter.

D-meson production in p-Pb collisions at √ s NN = 5.02 TeV ALICE Collaboration Charged particles were reconstructed and identified with the central barrel detectors located within a 0.5 T solenoid magnet. Tracks were reconstructed with the Inner Tracking System (ITS) and the Time Projection Chamber (TPC). Particle IDentification (PID) was based on the specific energy loss dE/dx in the TPC gas and on the time of flight from the interaction point to the Time Of Flight (TOF) detector. The analysis was performed by using p-Pb data collected in 2013 with a minimum-bias trigger that required the arrival of bunches from both directions and coincident signals in both scintillator arrays of the V0 detector, covering the regions 2.8 < η < 5.1 and −3.7 < η < −1.7. Events were selected off-line by using the timing information from the V0 and the Zero Degree Calorimeters to remove background due to beam-gas interactions. Only events with a primary vertex reconstructed within ±10 cm from the centre of the detector along the beam line were considered. About 10 8 events, corresponding to an integrated luminosity of (48.6 ± 1.6) µb −1 , passed the selection criteria.
D-meson selection was based on the reconstruction of decay vertices displaced from the interaction vertex, exploiting the separation of a few hundred micrometers typical of the D-meson weak decays, as described in [6,[39][40][41]. D 0 , D + and D + s candidates were defined using pairs or triplets of tracks with the proper charge sign combination. Tracks were required to have |η| < 0.8, p T > 0.4 GeV/c, at least 70 out of 159 associated space points in the TPC and at least 2 out of 6 hits in the ITS, out of which at least one in the two innermost layers. D * + candidates were formed combining D 0 candidates with tracks with |η| < 0.8, p T > 0.1 GeV/c and at least three associated hits in the ITS. The selection strategy was based on the displacement of the tracks from the interaction vertex and the pointing of the reconstructed D-meson momentum to the primary vertex. At low-p T , further background rejection was obtained by identifying charged kaons with the TPC and TOF by applying cuts in units of resolution (±3σ ) around the expected mean values of dE/dx and time of flight. For D + s candidate selection, the invariant mass of at least one of the two opposite-charge track pairs was required to be compatible with the mass of the φ meson (±2σ ).
The total cross section for hard processes σ hard pA in proton-nucleus collisions scales as σ hard pA = A σ hard NN [42], where σ hard NN is the equivalent cross section in pp collisions. Therefore, the R pPb for prompt D mesons is given by (1) The production cross sections of prompt D mesons (not coming from beauty meson decays) were obtained as (e.g. for D + ) N D ± raw is the raw yield extracted in a given p T interval (of width ∆p T ) by means of a fit to the invariant mass distribution of the D-meson candidates. f prompt is the prompt fraction of the raw yield. (Acc × ε) prompt is the geometrical acceptance multiplied by the reconstruction and selection efficiency of prompt D mesons. The factor α y = y fid /0.5 normalizes the yields, measured in |y lab | < y fid , to one unit of rapidity |y lab | < 0.5. y fid is the p T -dependent fiducial acceptance cut (y fid increases from 0.5 at p T = 0 to 0.8 at p T = 5 GeV/c and becomes constant at 0.8 for p T > 5 GeV/c). The cross sections are given for particles; thus, a factor 1/2 was added to take into account that both particles and anti-particles are counted in the raw yield. The integrated luminosity, L int , was computed as N pPb,MB /σ pPb,MB where N pPb,MB is the number of p-Pb collisions passing the minimum-bias trigger condition and σ pPb,MB is the cross section of the V0 trigger, which was measured to be 2.09 b± 3.5% (syst) with the p-Pb van der Meer scan [43]. The minimum-bias trigger is 100% efficient for D mesons with p T > 1 GeV/c and |y lab | < 0.5. D-meson production in p-Pb collisions at √ s NN = 5.02 TeV ALICE Collaboration The acceptance-times-efficiency (Acc × ε) corrections were determined by using a Monte Carlo simulation. Proton-lead collisions were produced using the HIJING v1.36 [44] event generator. A cc or bb pair was added in each event using the PYTHIA v6.4.21 [45] generator with Perugia-0 tuning [46]. The generated particles were transported through the ALICE detector using GEANT3 [47]. The efficiency for D-meson reconstruction and selection varies from 0.5-1% for p T < 2 GeV/c to 20-30% for p T > 12 GeV/c because of the larger displacement of the decay vertex of high-p T candidates due to the Lorentz boost. Hence the generated D-meson spectrum used to calculate the efficiencies was tuned to reproduce the shape given by Fixed-Order Next-To-Leading-Log resummation (FONLL) [2] calculations at √ s = 5.02 TeV in each p T interval. The efficiency depends also on the multiplicity of charged particles produced in the collision since the primary vertex resolution, and consequently the resolution of the topological selection variables, improves with increasing multiplicity. This dependence is different for each meson species and p T interval: e.g. the D 0 efficiency in 5 < p T < 8 GeV/c increases by a factor 1.5 for low multiplicity events until it becomes constant at about 20 reconstructed primary particles. Therefore, the efficiency was calculated by weighting the simulated events according to their charged particle multiplicity in order to reproduce the multiplicity distribution observed in data. The fraction of prompt D mesons, f prompt , was estimated as in Ref. [6] by using the beauty production cross section from FONLL calculations [2], the B → D + X decay kinematics from the EvtGen package [48] and the reconstruction and selection efficiency for D mesons from B hadron decays. The R pPb of prompt and feed-down D mesons were assumed to be equal and were varied in the range 0.9 < R feed−down pPb /R prompt pPb < 1.3 to evaluate the systematic uncertainties. This range was chosen considering the predictions from calculations including initial state effects based on the Eskola-Paukkunen-Salgado 2009 (EPS09) [13] parameterizations of the nuclear modification of the PDF and CGC [16].
The reference pp cross sections at √ s = 5.02 TeV were obtained by a perturbative-QCD-based energy scaling of the p T -differential cross sections measured at √ s = 7 TeV [40]. The scaling factor for each D-meson species was determined as the ratio of the cross sections from the FONLL calculations at 5.02 and 7 TeV. The uncertainty on the scaling factor was evaluated by varying the calculation parameters as described in Ref. [49] and it ranges from +17.5%

−4%
at p T = 1 GeV/c to about ±3% for p T > 8 GeV/c. In addition, the pp reference is affected by the uncertainty coming from the 7 TeV measurement (∼17%) [40]. Since the D 0 cross section in pp collisions in the 1 < p T < 2 GeV/c interval was measured at both 7 and 2.76 TeV, both results were scaled to 5.02 TeV and averaged considering their relative statistical and systematic uncertainties as weights. Since the current measurement of the ALICE D 0 pp cross section at √ s = 7 TeV is limited to p T = 16 GeV/c, the cross section was extrapolated to higher p T using the spectrum predicted by FONLL [2] scaled to match pp data in 5 < p T < 16 GeV/c. Then the D 0 cross section at 7 TeV in 16 < p T < 24 GeV/c was scaled to 5.02 TeV.
The systematic uncertainties on the D-meson cross sections include contributions from yield extraction (from 2% to 17% depending on p T and D-meson species), an imperfect description of the cut variables in the simulation (from 5% to 8% for D 0 , D + and D * + , ∼20% for D + s ), tracking efficiency (3% for each track), simulated p T shapes (from 2% to 3% depending on p T and D-meson species), and the subtraction of feed-down D mesons from B decays (from 4% and 40% depending on p T and D-meson species). For the D 0 meson, the yield extraction systematic uncertainty also includes the contribution to the raw yield of signal candidates reconstructed assigning the wrong mass to the final-state hadrons. This contribution, which is strongly reduced by the PID selection, was estimated to be 3%(4%) at low(high) p T based on the invariant mass distribution of these candidates in the simulation. Details of the procedure for the systematic uncertainty estimation are reported in Refs. [6,[39][40][41]]. The measured cross sections have a global systematic uncertainty due to the determination of the integrated luminosity (3.7% [43]) and to the branching ratio [38]. For the R pPb , the pp and p-Pb uncertainties were added in quadrature except for the branching ratio uncertainty, which cancels out in the ratio, and the feed-down contribution, which partially cancels out.  The p T -differential production cross sections of prompt D 0 , D + , D * + and D + s mesons are shown in Fig. 1. The relative abundances of D mesons in p-Pb collisions are compatible within uncertainties with those measured in pp, ep and e + e − collisions at different energies [41]. The R pPb of the four D-meson species, shown in Fig. 2, are consistent, and they are compatible with unity within the uncertainties in the measured p T range. D-meson production in p-Pb collisions is consistent within statistical and systematic uncertainties with the binary collision scaling of the production in pp collisions. Moreover, within the uncertainties, the D + s nuclear modification factor is compatible with that of non-strange D mesons. The average of the R pPb of D 0 , D + and D * + in the p T range 1 < p T < 24 GeV/c was calculated D-meson production in p-Pb collisions at √ s NN = 5.02 TeV ALICE Collaboration  using the relative statistical uncertainties as weights. The systematic error on the average was calculated by propagating the uncertainties through the weighted average, where the contributions from tracking efficiency, B feed-down correction, and scaling of the pp reference were taken as fully correlated among the three species. Figure 3 shows the average R pPb compared to theoretical calculations. Predictions based either on next-to-leading order (NLO) pQCD calculations (MNR [50]) of D-meson production, including the EPS09 [13] nuclear modification of the CTEQ6M PDF [51], or on calculations based on the Color Glass Condensate [16] can describe the measurement by considering only initial-state effects. Data are also well described by calculations which include cold-nuclear-matter energy loss, nuclear D-meson production in p-Pb collisions at √ s NN = 5.02 TeV ALICE Collaboration shadowing and k T -broadening [9]. The possible effects due to the formation of a hydrodynamically expanding medium as calculated in Ref. [35] are expected to be small in minimum-bias collisions at LHC energies. The present uncertainties of the measurement do not allow any sensitivity to this effect. In Fig. 4, the average R AA of prompt D mesons in central (0-20%) and in semi-peripheral (40%-80%) Pb-Pb collisions at √ s NN = 2.76 TeV [6] is reported along with the average R pPb of prompt D mesons in p-Pb collisions at √ s NN = 5.02 TeV, showing that cold-nuclear-matter effects are smaller than the uncertainties for p T 3 GeV/c. In addition, as reported in Ref. [6], the same EPS09 nuclear PDF parametrization that describes the D-meson R pPb results predicts small initial state effects (less than 10% for p T > 5 GeV/c) for Pb-Pb collisions. As a consequence, the suppression observed in central Pb-Pb collisions for p T 2 GeV/c is predominantly induced by final-state effects, e.g. the charm energy loss in the medium [7][8][9][10].
In summary, we reported the measurement of the D-meson cross section and nuclear modification factor in p-Pb collisions at √ s NN = 5.02 TeV. The latter is consistent within uncertainties of about 15%-20% with unity and is compatible with theoretical calculations including gluon saturation. Thus, the suppression of D mesons with p T 2 GeV/c observed in Pb-Pb collisions cannot be explained in terms of initial-state effects but is due to strong final-state effects induced by hot partonic matter.