Open charm measurements in NA61/SHINE at CERN SPS

The measurements of open charm production was proposed as an important tool to investigate the properties of hot and dense matter formed in nucleus-nucleus collisions as well as to provide the means for model independent interpretation of the existing data on $\text{J}/\psi$ suppression. Recently, the experimental setup of the NA61/SHINE experiment was supplemented with a Vertex Detector which was motivated by the importance and the possibility of the first direct measurements of open charm meson production in heavy ion collisions at SPS energies. First test data taken in December 2016 on Pb+Pb collisions at 150$A$ GeV/$c$ allowed to validate the general concept of D$^0$ meson detection via its D$^0 \to \pi^+ + K^-$ decay channel and delivered a first indication of open charm production. The physics motivation of open charm measurements at SPS energies, pilot results on open charm production, and finally, the future plans of open charm measurements in the NA61/SHINE experiment after LS2 are presented.


Introduction
The SPS Heavy Ion and Neutrino Experiment (NA61/SHINE) [1] is a fixedtarget experiment located at the CERN Super Proton Synchrotron (SPS). The NA61/SHINE detector is optimized to study hadron production in hadronproton, hadron-nucleus and nucleus-nucleus collisions. The strong interaction research program of NA61/SHINE is dedicated to the study of the properties of the onset of deconfinement and the search for the critical point of strongly interacting matter. These goals are being pursued by investigating p+p, p+A and A+A collisions at different beam momenta from 13A to 150A GeV/c. In 2016 NA61/SHINE was upgraded with the Small Acceptance Vertex Detector (SAVD) based on MIMOSA-26AHR sensors developed in IPHC Strasbourg. Construction of this device was mostly motivated by the importance and the possibility of the first direct measurements of open charm meson production in heavy ion collisions at SPS energies. Precise measurements of charm hadron production by NA61/SHINE are expected to be performed in 2022-2024. The related preparations have started already.

Physics motivation for open charm measurements
One of the important aspects of relativistic heavy-ion collisions is the mechanism of charm production. Several models were developed to describe charm production. Some of them are based on dynamical and others on statistical approaches. The estimates from these models for the average number of produced c and c pairs ( cc ) in central Pb+Pb collisions at 158A GeV/c differ by up to a factor of 50 [2,3] ss illustrated in Fig. 1 (left). Therefore, obtaining precise  [4,5], pQCD-inspired [6,7], and Dynamical Quark Coalescence [8], as well as statistical models (green bars): HRG [9], Statistical Quark Coalescence [9], and SMES [10]. (Right:)The ratio of σ J/ψ /σ DY as a function of transverse energy (a measure of collision violence or centrality) in Pb+Pb collisions at 158A GeV measured by NA50. The curve represents the J/ψ suppression due to ordinary nuclear absorption [11].
data on cc will allow to distinguish between theoretical predictions and learn about the charm quark and hadron production mechanism. A good estimate of cc can be obtained by measuring the yields of D 0 , D + and their antiparticles because these mesons carry about 85% of the total produced charm [12]. Charm mesons are of special interest in the context of the phase transition between confined hadronic matter and the quark gluon plasma (QGP). The cc pairs produced in the collisions are converted into open charm mesons and charmonia (J/ψ mesons and the excited states). The production of charm is expected to be different in confined and deconfined matter. This is caused by different properties of charm carriers in these phases. In confined matter the lightest charm carriers are D mesons, whereas in deconfined matter the lightest carriers are charm quarks. Production of a DD pair (2m D = 3.7 GeV) requires an energy about 1 GeV higher than production of a cc pair (2m c = 2.6 GeV). The effective number of degrees of freedom of charm hadrons and charm quarks is similar [13]. Thus, in the statistical approach more abundant charm production is expected in deconfined than in confined matter. Consequently, in analogy to strangeness production [3,14], a change of collision energy dependence of cc may be a signal of the onset of deconfinement. Figure 1 (right) shows results on J/ψ production normalized to the mean multiplicity of Drell-Yan pairs in Pb+Pb collisions at the top SPS energy obtained by the NA50 collaboration. The solid line shows a model prediction for normal nuclear absorption of J/ψ in the medium. NA50 observed that J/ψ production is consistent with normal nuclear matter absorption for peripheral collisions and is suppressed for more central collisions. This so called anomalous suppression was attributed to the J/ψ dissociation effect in the deconfined medium. However, the above result is based on the assumption that cc ∼ DY which may be incorrect due to several effects, such as shadowing or parton energy loss [15]. Thus the effect of the medium on cc binding can only be quantitatively determined by comparing the ratio of J/ψ to cc in nucleus-nucleus to that in proton-proton reactions. In Pb+Pb collisions the onset of color screening should already be seen in the centrality dependence of the J/ψ to cc ratio. This clearly shows the need for large statistic data on cc .

Performance of SAVD
The SAVD was built using sixteen CMOS MIMOSA-26 sensors [16]. The basic sensor properties are: 18.4 × 18.4 µm 2 pixels, 115 µs time resolution, 10× 20 mm 2 surface, 0.66 MPixel, 50 µm thick. The estimated material budget per layer, including the mechanical support, is 0.3% of a radiation length. The sensors were glued to eight ALICE ITS ladders [17], which were mounted on two horizontally movable arms and spaced by 5 cm along the z (beam) direction. The detector box was filled with He (to reduce beam-gas interactions) and contained an integrated target holder to avoid unwanted material and multiple Coulomb scattering between target and detector. More details related to the SAVD project can be found in [18].
The first test of the device was performed in December 2016 during a Pb+Pb test run. The test allowed to demonstrate: tracking in a large track multiplicity environment, precise primary vertex reconstruction, TPC and SAVD track matching. Furthermore, it allowed to make a first search for the D 0 and D 0 signals. The obtained primary vertex resolution along the beam direction of 30 µm was sufficient to perform the search for the D 0 and D 0 signals.  these measurements the thresholds of the MIMOSA-26 sensors were tuned to obtain high hit detection efficiency which led to significant improvement in the primary vertex reconstruction precision, namely the spatial resolution of the primary vertices obtained for Xe+La data is on the level of 1 µm and 15 µm in the transverse and longitudinal coordinates, respectively. The distribution of the longitudinal coordinate (z prim ) of the primary vertex is shown in Fig. 3 (left) (see Ref. [2] for details) The Xe+La data are currently under analysis and are expected to lead to physics results in the coming months.
The SAVD will also be used during three weeks of Pb+Pb data taking in 2018. About 1 · 10 7 central collisions should be recorded and 2500 D 0 and D 0 decays can be expected to be reconstructed in this data set.

Proposed measurements after LS2
During the Long Shutdown 2 at CERN (2019-2020), a significant modification of the NA61/SHINE spectrometer is planned. The upgrade is primarily motivated by the charm program which requires a tenfold increase of the data taking rate to about 1 kHz and an increase of the phase-space coverage of the Vertex Detector by a factor of about 2. This, in particular, requires construction of a new Vertex Detector (VD), replacement of the TPC read-out electronics, implementation of new trigger and data acquisition systems and upgrade of the Projectile Spectator Detector. Finally, new ToF detectors are planned to be constructed for particle identification at mid-rapidity. This is mainly motivated by possible future measurements related to the onset of fireball formation. The detector upgrades are discussed in detail in Ref. [2]. The data taking plan related to the open charm measurements forsees measurements of 500M inelastic Pb+Pb collisions at 150A GeV/c in 2022 and 2023. This data will provide the mean number of cc pairs in central Pb+Pb collisions needed to investigate the mechanism of charm production in this reaction. Moreover, the data will allow to establish the centrality dependence of cc in Pb+Pb collisions at 150A GeV/c and thus address the question of how the formation of QGP impacts J/ψ production. Table 1 lists the expected number of charm mesons in centrality selected Pb+Pb collisions at 150A GeV/c assuming the above mentioned statistics of minimum bias collisions. The estimate was performed assuming that the mean multiplicity of charm hadrons is proportional to the number of collisions and used yields calculated for central Pb+Pb collisions within the HSD model [4,5]. Central (0-30%) Pb+Pb collisions at 40A GeV/c are planned to be recorded in 2024. This data together with the result for central Pb+Pb collisions at 150A GeV/c will start a long-term effort to establish the collision energy dependence of cc and address the question of how the onset of deconfinement impacts charm production. The expected high statistics of reconstructed D 0 and D 0 decays is due to the high event rate and the relatively large efficiencies of open charm detection in the VD. The efficiency will be about 13% (3 times better than for the SAVD) for the D 0 → π + + K − decay channel and about 9% 1 for D + decaying into π + + π + + K − . Figure 3 (right) shows distributions of D 0 + D 0 mesons in rapidity and transverse momentum for all generated particles (black symbols) and for particles that passed the acceptance and background reduction cuts (blue symbols). The presented plots refer to 500M inelastic Pb+Pb collisions at 150A GeV/c. Total uncertainty of D 0 and D 0 is expected to be about 10% and is dominated by systematic uncertainty.
In summary it is emphasized that only NA61/SHINE is able to measure open charm production in heavy ion collisions in full phase space and at the beginning of the next decade. The corresponding potential measurements at higher (LHC, RHIC) and lower (FAIR, J-PARC) energies are necessary to complement the NA61/SHINE results and establish the collision energy dependence of charm production.