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D-meson production in p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV
and in pp collisions at $\sqrt{s}=7$ TeV
The production cross sections of the prompt charmed mesons D$^0$, D$^+$, D$^{*+}$ and D$_s$ were measured at mid-rapidity in p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D$^0\rightarrow{\rm K}^-\pi^+$, D$^+\rightarrow{\rm K}^-\pi^+\pi^+$, D$^{*+}\rightarrow D^0\pi^+$, D$_s^+\rightarrow\phi\pi^+\rightarrow{\rm K}^-{\rm K}^+\pi^+$, and their charge conjugates. The $p_{\rm T}$-differential production cross sections were measured at mid-rapidity in the interval $1<~p_{\rm T}<~24$ GeV/$c$ for D$^0$, D$^+$ and D$^{*+}$ mesons and in $2<~p_{\rm T}<~12$ GeV/$c$ for D$_s$ mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D$^0$, D$^+$ and D$^{*+}$ mesons were also measured in three $p_{\rm T}$ intervals as a function of the rapidity $y_{\rm cms}$ in the centre-of-mass system in $-1.26<~y_{\rm cms}<~0.34$. In addition, the prompt D$^0$ cross section was measured in pp collisions at $\sqrt{s}=7$ TeV and p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV down to $p_{\rm T}=0$ using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D$^0$ decay vertex. The nuclear modification factor $R_{\rm pPb}(p_{\rm T})$, defined as the ratio of the $p_{\rm T}$-differential D-meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.

Phys. Rev. C 94 (2016) 054908
HEP Data e-Print: arXiv:1605.07569 | PDF | inSPIRE
CERN-EP-2016-127

Figures
Distributions of the invariant mass for $\Dzero$ (top left), $\Dplus$ (top right), $\Ds$ (bottom left) candidates and their charge conjugates and of the mass difference for $\Dstar$ (bottom right) candidates (and charge conjugates) in the rapidity interval $|y_{\rm lab}|< y_{\mathrm{fid}}(\pt)$ in p--Pb collisions. The dashed lines represent the fit to the background while the solid lines represent the total fit function. One $\pt$ interval is shown for each species: $1< \pt< 2~\gevc$ for $\Dzero$, $5< \pt< 6~\gevc$ for $\Dplus$, $8< \pt< 12~\gevc$ for $\Ds$ and $16< \pt< 24~\gevc$ for $\Dstar$.

Distributions of the invariant mass for $\Dzero$ (left column), $\Dplus$ (middle column) candidates and their charge conjugates and of the mass difference for $\Dstar$ (right column) candidates (and charge conjugates) in p--Pb collisions in the rapidity intervals $|y_{\rm lab}| < 0.1 $ (top row) and $-0.8 < y_{\mathrm{lab}} < -0.4$ ($-0.7 < y_{\mathrm{lab}} < -0.4$ for $\pt< 5~\gev/c$) (bottom row). The dashed lines represent the fit to the background while the solid lines represent the total fit function. One $\pt$ interval is shown for each species: $2< \pt< 5~\gevc$ for $\Dzero$, $5< \pt< 8~\gevc$ for $\Dplus$ and $8< \pt< 16~\gevc$ for $\Dstar$.

Product of acceptance and efficiency for $\Dzero$ mesons as a function of $\pt$ (left) and as a function of $y_{\rm lab}$ (right). In the left panel, efficiencies are shown for prompt $\Dzero$ with (solid line) and without (dashed line) PID selection applied, and feed-down $\Dzero$ (dotted line). In the right panel, the ${\rm Acc}\times\epsilon$ values are shown for prompt $\Dzero$ mesons for the three $\pt$ intervals considered in the analysis as a function of rapidity.

Left: Examples of fits to $\Dzero$ (top), $\Dplus$ (middle) and $\Dstar$ (bottom) impact-parameter distributions in the $\pt$ intervals$3< \pt< 4~\gevc$, $5< \pt< 6~\gevc$ and $6< \pt< 8~\gevc$, respectively The curves show the fit functions describing the prompt, feed-down and background contributions, as well as their sum, as described in the text Right: fraction of prompt $\Dzero$ (top), $\Dplus$ (middle) and $\Dstar$ (bottom) raw yield as a function of $\pt$ compared to the FONLL-based approach The results from the data-driven method are shown as square markers with theerror bars (boxes) representing the statistical (systematic) uncertainty The arrow in the interval $1< \pt< 2~\gev/c$ represents the minimum value within a 95% confidence level The central values of $f_{\rm prompt}$ from the FONLL-based approach are shownby the dashed line and their uncertainty by the red boxes.

Invariant-mass distributions of $\DtoKpi$ candidates (andcharge conjugates) in pp collisions at $\sqrts=7~\TeV$ fortwo $\pt$ intervals: $0< \pt< 1~\gev/c$ (top panels) and $1< \pt< 2~\gev/c$(bottom panels). For both $\pt$ intervals, the left panels displaythe invariant-mass distribution of all ULS K$\pi$ pairs together with thebackground distributions estimated with the LS, event-mixing andtrack-rotation techniques. The middle and right panels show the invariant-massdistributions after subtraction of the background from the track-rotationand LS techniques. Fit functions are superimposed.

Invariant-mass distributions of $\DtoKpi$ candidates (andcharge conjugates) in p--Pb collisions at $\sqrtsNN=5.02~\TeV$ fortwo $\pt$ intervals: $0< \pt< 1~\gev/c$ (top panels) and $1< \pt< 2~\gev/c$(bottom panels). For both $\pt$ intervals, the left panels displaythe invariant-mass distribution of all ULS K$\pi$ pairs together with thebackground distributions estimated with the LS, event-mixing andtrack-rotation techniques. The middle and right panels show the invariant-massdistributions after subtraction of the background from the event-mixingand side-band fit techniques. Fit functions are superimposed.

Product of acceptance and efficiency in p--Pb collisions The ${\rm Acc}\times\epsilon$ values from the analysis with decay-vertexreconstruction were rescaled to account for the different fiducial acceptance,$y_{\rm fid}(\pt)$, selection on the $\Dzero$ rapidity.

$\pt$-differential production cross section of $\Dzero$ mesons with $|y|< 0.5$ in pp collisions at $\sqrt s =7~\tev$. Left: comparison of prompt and inclusive $\Dzero$ mesons (the latter including also $\Dzero$ mesons from beauty-hadron decays) from the analysis without decay-vertex reconstruction. Right: comparison between the prompt $\Dzero$ cross sections measured with and without decay-vertex reconstruction. Here and in all the following figures the symbols are plotted at the centre of the $\pt$ intervals (shown by the horizontal lines), the vertical lines represent the statistical uncertainties and the vertical size of the boxes corresponds to the systematic uncertainties.

Total inclusive charm production cross section in nucleon--nucleon collisionsas a function of $\sqrt{s}$. Data are from pA collisions for $\sqrt s< 100$ GeV and from pp collisions for $\sqrt s>100$ GeV. Data from pA collisionswere scaled by $1/A$ Results from NLO pQCD calculations (MNR) and theiruncertainties are shown as solid and dashed lines.

$\pt$-differential production cross section of $\Dzero$ mesons with $-0.96< y_{\rm cms}< 0.04$ in p-Pb collisions at $\sqrtsNN =5.02~\tev$ Left: comparison of prompt and inclusive $\Dzero$ mesons (the latter including also $\Dzero$ mesons from beauty-hadron decays) from the analysis without decay-vertex reconstruction. Right: comparison between the prompt $\Dzero$ cross sections measured with and without decay-vertex reconstruction.

Ratios of prompt D-meson production cross sections as a function of $\pt$ in pp collisions at $\sqrt s=7~\tev$ ($|y_{\rm cms}|< 0.5$) and p--Pb collisions at $\sqrtsNN=5.02~\tev$ ($-0.96< y_{\rm cms}< 0.04$).

Nuclear modification factor $R_{\rm pPb}$ of prompt D mesons in p--Pb collisions at $\sqrtsNN=5.02~\tev$ Top: $R_{\rm pPb}$ of $\Dzero$, $\Dplus$ and $\Dstar$ mesons. Bottom: average $R_{\rm pPb}$ of the three non-strange D-meson species and $R_{\rm pPb}$ of $\Ds$ mesons. All results are obtained from the analysis with decay-vertex reconstruction.

Comparison of the nuclear modification factors of prompt $\Dzero$ mesons as obtained in the analysis with decay-vertex reconstruction and in the analysis without decay-vertex reconstruction.

Nuclear modification factor $R_{\rm pPb}$ of prompt D mesons in p-Pb collisions at $\sqrtsNN=5.02~\tev$: average $R_{\rm pPb}$ of $\Dzero$, $\Dplus$ and $\Dstar$ mesons in the interval $1< \pt< 24~\gev/c$, shown together with the $\Dzero$ $R_{\rm pPb}$ in $0< \pt< 1~\GeV/c$. In the top panel, the data are compared with results of theoretical calculations including only CNM effects: CGC, NLO pQCD with EPS09 nPDFs, a LO pQCD calculation with CNM effects (Vitev et al.) and a calculation based on incoherent multiple scatterings (Kang et al.). In the bottom panel, the results of the Duke and POWLANG transport models are compared to the measured D-meson $R_{\rm pPb}$.