Measurement of D-meson production in Pb–Pb collisions at the LHC with ALICE

Measurement of D-meson production in Pb–Pb collisions at the LHC with ALICE

Abstract

The measurement of D-meson production in heavy-ion collisions at LHC energy provides insights into the mechanisms of interaction of charm quarks in the hot and dense medium formed in these collisions. ALICE results on the D-meson nuclear modification factor and azimuthal anisotropy in Pb–Pb collisions at  TeV are presented.

1 Introduction

Heavy-flavour hadrons are suitable to probe the conditions of the high-energy-density Quark-Gluon Plasma (QGP) medium formed in ultra-relativistic heavy-ion collisions. Heavy quarks are mainly produced in hard scattering processes in the early stage of Pb–Pb collisions. The time-scale of their production () is, in general, shorter than the formation time of the QGP,  fm/. During their propagation through the medium, heavy quarks interact with its constituents and lose energy. QCD energy loss is expected to occur via both inelastic (radiative energy loss via medium-induced gluon radiation) [1] and elastic (collisional energy loss) processes [2]. The energy loss for quarks is expected to be smaller than for gluons, due to the smaller colour coupling factor of quarks with respect to gluons. In addition, the “dead-cone effect” should reduce small-angle gluon radiation for heavy quarks with moderate energy compared to their mass, thus further attenuating the effect of the medium [3].

The nuclear modification factor , where is the average nuclear overlap function for a given centrality class, is sensitive to the interaction of hard partons with the medium. At large , is expected to be mostly sensitive to the average energy loss of heavy-quarks in the hot and dense medium. The questions whether low-momentum heavy quarks can reach thermal equilibrium with the medium constituents and participate in the collective expansion of the system [4, 5], and whether heavy quarks can hadronise also via recombination with other quarks from the medium [5, 6] are still open. These questions are addressed by studying at low and intermediate and measuring the azimuthal anisotropy of heavy-flavour hadron production with respect to the reaction plane, defined by the beam axis and the impact parameter of the collision. The hadronisation mechanisms of the c quark are also investigated through the measurement of production in nucleus–nucleus collisions compared to that in pp collisions [7].

2 D-meson reconstruction

The decays , and , and their charge conjugates, were reconstructed at mid-rapidity () in minimum-bias Pb–Pb collisions using the ALICE central barrel detectors. The D-meson selection was based on the precise reconstruction of the primary and secondary (decay) vertices, which is provided by the Inner Tracking System (ITS). Charged pions and kaons were identified using the information provided by the Time Projection Chamber (TPC) and the Time Of Flight (TOF) detectors [8]. The reference proton–proton cross section at  TeV, needed to compute , was obtained by a pQCD-based energy scaling of the -differential cross section measured at  TeV [9].

3 D-meson nuclear modification factor

Figure 1: Average of prompt , and mesons [10] compared to the prompt D-meson in Pb–Pb collisions in the 0–20% and 40–80% centrality classes [11].
Figure 2: Average prompt D-meson in Pb–Pb collisions in the 0–7.5% centrality class compared to theoretical models including parton energy loss.

A large suppression of the D-meson (factor 3-5) was observed for  GeV/ in central Pb–Pb collisions at (Figure 2[11]. The comparison of with the D-meson nuclear modification factor measured in p–Pb collisions at  [10] shows that the expected cold nuclear matter effects are smaller than the uncertainties on for . Therefore, the suppression observed in central Pb–Pb collisions is predominantly induced by final-state effects due to the presence of a hot and dense partonic medium. Figure 2 shows the D-meson measured in Pb–Pb collisions in the centrality class 0–7.5%, compared to theoretical models including charm interactions with the medium constituents. The large suppression observed, e.g. of a factor 6 at , is described by the models that include radiative and collisional heavy-quark energy loss.

Figure 3: of D mesons [12] and charged pions [13] as a function of centrality compared to a pQCD model including mass dependent radiative and collisional energy loss [14].
Figure 4: [12] and non-prompt J/ meson [15] vs. centrality compared to a pQCD model including mass dependent radiative and collisional energy loss [14].

Figures 4 and 4 show the D-meson as a function of centrality (quantified in terms of the average number of participant nucleons in the Pb–Pb collision) [12] along with the of charged pions [13] and non-prompt J/ mesons measured by the CMS Collaboration [15], respectively. The focus here is on the study of the parton energy loss, thus, the results are presented for the high- interval for the pions and D mesons and for for J/ from B-meson decays. The of charged pions and D mesons are compatible within uncertainties in all centrality classes. The average difference of the values of D mesons and non-prompt J/ in the 0–10% and 10–20% centrality classes is larger than zero with a significance of 3.5 . A pQCD model including mass-dependent radiative and collisional energy loss [14] describes well the similarity of the D-meson and charged-pion . In this model, the colour-charge dependence of energy loss is compensated by the softer fragmentation and spectrum of gluons with respect to those of c quarks leading to a very similar for D mesons and pions. This calculation results in a larger suppression of D mesons with respect to non-prompt J/, in agreement with the data. The large difference in the two derives predominantly from the quark-mass dependence of parton energy loss, as demonstrated by the non-prompt J/ calculated considering the c-quark mass in the energy loss of b quarks (dotted lines in Figure 4[14, 16, 17].

4 D-meson azimuthal anisotropy

Figure 5: Model comparison for the average D-meson in the 30–50% centrality class [18, 19].
Figure 6: in the 30–50% centrality class in two -wide orthogonal azimuthal intervals [19].

is defined as the second coefficient of the Fourier expansion of the -dependent particle distribution in azimuthal angle relative to the reaction plane. It quantifies the momentum azimuthal anisotropy of final-state D mesons that might result from the spatial asymmetry of the initial parton distribution in non-central collisions, as a consequence of the collective expansion of the QGP. Figure 6 shows the D-meson in Pb–Pb collisions in the 30–50% centrality class [18, 19] compared to theoretical models including charm-quark interactions with the medium constituents. The value of is larger than zero in the interval  GeV/ with a significance of about 5  and it is best described by models including mechanisms that transfer to the charm quarks the elliptic flow of the medium during the system expansion (namely collisional processes and hadronisation by recombination with light quarks). A smaller suppression is observed in the in-plane direction (Figure 6[19]; the ordering is equivalent to the observation of and, thus, consistent with the expectations from collective flow.

5 Conclusions

The results obtained by ALICE using the data from the LHC Run-1 (2010–2013) indicate a strong suppression of the D-meson production in central Pb–Pb collisions for , which is mainly due the interactions of heavy quarks with the hot and dense medium. The smaller observed for D mesons with respect to non-prompt J/ confirms the mass-dependent nature of the energy-loss mechanisms. The non-zero of D mesons and the azimuthal dependence of the indicate that, during the collective expansion of the medium, the interactions between its constituents and the charm quarks transfer to the latter information on the azimuthal anisotropy of the system. During the LHC Run-2 we expect to collect a data sample larger by a factor 5-10 with respect to Run-1, depending on collision centrality. It will be, thus, possible to measure the D-meson and with a better precision and in an extended range.

References

References

  1. Baier R, Dokshitzer Y, Mueller A, Peigné S and Schiff D 1997 Nuclear Physics B 484 265–282 ISSN 0550-3213
  2. Braaten E and Thoma M H 1991 Phys. Rev. D 44(4) 1298–1310
  3. Armesto N, Salgado C A and Wiedemann U A 2004 Phys. Rev. D 69(11) 114003
  4. Batsouli S, Kelly S, Gyulassy M and Nagle J 2003 Physics Letters B 557 26–32 ISSN 0370-2693
  5. Greco V, Ko C and Rapp R 2004 Physics Letters B 595 202–208 ISSN 0370-2693
  6. Andronic A, Braun-Munzinger P, Redlich K and Stachel J 2003 Physics Letters B 571 36–44 ISSN 0370-2693
  7. Barbano A (for the ALICE Collaboration) 2015 These proceedings
  8. Abelev B B et al. (ALICE) 2014 Int. J. Mod. Phys. A29 1430044 (Preprint 1402.4476)
  9. Abelev B B et al. (ALICE) 2012 JHEP 01 128 (Preprint 1111.1553)
  10. Abelev B B et al. (ALICE) 2014 Phys. Rev. Lett. 113 232301 (Preprint 1405.3452)
  11. Abelev B B et al. (ALICE) 2012 JHEP 09 112 (Preprint 1203.2160)
  12. Adam J et al. (ALICE) 2015 (Preprint 1506.06604)
  13. Abelev B B et al. (ALICE) 2014 Physics Letters B 736 196–207 ISSN 0370-2693
  14. Djordjevic M, Djordjevic M and Blagojevic B 2014 Physics Letters B 737 298–302 ISSN 0370-2693
  15. CMS Collaboration (CMS) 2012 CMS-PAS-HIN-12-014
  16. Nahrgang M, Aichelin J, Gossiaux P B and Werner K 2014 Phys. Rev. C 89(1) 014905
  17. He M, Fries R J and Rapp R 2013 Nuclear Physics A 910–911 409–412 ISSN 0375-9474
  18. Abelev B B et al. (ALICE) 2013 Phys.Rev.Lett. 111 102301 (Preprint 1305.2707)
  19. Abelev B B et al. (ALICE) 2014 Phys. Rev. C90 034904 (Preprint 1405.2001)
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