OOpen charm measurements in NA61/SHINE atCERN SPS
Pawel Staszel for the NA61/SHINE CollaborationJagiellonian University Institute of Physics,ul. Lojasiewicza 11, 30-348 Krakow, PLOctober 2, 2018
Abstract
The measurements of open charm production was proposed as an im-portant tool to investigate the properties of hot and dense matter formedin nucleus-nucleus collisions as well as to provide the means for modelindependent interpretation of the existing data on J /ψ suppression. Re-cently, the experimental setup of the NA61/SHINE experiment was sup-plemented with a Vertex Detector which was motivated by the importanceand the possibility of the first direct measurements of open charm mesonproduction in heavy ion collisions at SPS energies. First test data takenin December 2016 on Pb+Pb collisions at 150 A GeV/ c allowed to validatethe general concept of D meson detection via it’s D → π + + 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 ofopen charm measurements in the NA61/SHINE experiment after LS2 arepresented. The SPS Heavy Ion and Neutrino Experiment (NA61/SHINE) [1] is a fixed-target experiment located at the CERN Super Proton Synchrotron (SPS). TheNA61/SHINE detector is optimized to study hadron production in hadron-proton, hadron-nucleus and nucleus-nucleus collisions. The strong interactionresearch program of NA61/SHINE is dedicated to the study of the propertiesof the onset of deconfinement and the search for the critical point of stronglyinteracting matter. These goals are being pursued by investigating p+p, p+Aand A+A collisions at different beam momenta from 13A to 150A GeV/c. In2016 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 thepossibility of the first direct measurements of open charm meson production in1 a r X i v : . [ nu c l - e x ] O c t eavy ion collisions at SPS energies. Precise measurements of charm hadronproduction by NA61/SHINE are expected to be performed in 2022–2024. Therelated preparations have started already. One of the important aspects of relativistic heavy-ion collisions is the mecha-nism of charm production. Several models were developed to describe charmproduction. Some of them are based on dynamical and others on statistical ap-proaches. The estimates from these models for the average number of producedc and c pairs ( (cid:104) cc (cid:105) ) in central Pb+Pb collisions at 158 A GeV /c differ by up toa factor of 50 [2, 3] ss illustrated in Fig. 1 ( left ). Therefore, obtaining preciseFigure 1: ( Left: ) Mean multiplicity of charm quark pairs produced in the fullphase space in central Pb+Pb collisions at 158 A GeV /c calculated with dynami-cal models (blue bars): HSD [4, 5], pQCD–inspired [6, 7], and Dynamical QuarkCoalescence [8], as well as statistical models (green bars): HRG [9], StatisticalQuark Coalescence [9], and SMES [10]. ( Right: )The ratio of σ J /ψ /σ DY as afunction of transverse energy (a measure of collision violence or centrality) inPb+Pb collisions at 158 A GeV measured by NA50. The curve represents theJ /ψ suppression due to ordinary nuclear absorption [11].data on (cid:104) cc (cid:105) will allow to distinguish between theoretical predictions and learnabout the charm quark and hadron production mechanism. A good estimate of (cid:104) cc (cid:105) can be obtained by measuring the yields of D , D + and their antiparticlesbecause these mesons carry about 85% of the total produced charm [12].Charm mesons are of special interest in the context of the phase transitionbetween confined hadronic matter and the quark gluon plasma (QGP). Thecc pairs produced in the collisions are converted into open charm mesons andcharmonia (J /ψ mesons and the excited states). The production of charm isexpected to be different in confined and deconfined matter. This is caused bydifferent properties of charm carriers in these phases. In confined matter thelightest charm carriers are D mesons, whereas in deconfined matter the lightestcarriers are charm quarks. Production of a DD pair (2m D = 3 . c = 2 . (cid:104) cc (cid:105) may be a signal of the onset of deconfinement.Figure 1 ( right ) shows results on (cid:104) J /ψ (cid:105) production normalized to the meanmultiplicity of Drell-Yan pairs in Pb+Pb collisions at the top SPS energy ob-tained by the NA50 collaboration. The solid line shows a model predictionfor normal nuclear absorption of J /ψ in the medium. NA50 observed thatJ /ψ production is consistent with normal nuclear matter absorption for pe-ripheral collisions and is suppressed for more central collisions. This so calledanomalous suppression was attributed to the J /ψ dissociation effect in the de-confined medium. However, the above result is based on the assumption that (cid:104) cc (cid:105) ∼ (cid:104) DY (cid:105) which may be incorrect due to several effects, such as shadowingor parton energy loss [15]. Thus the effect of the medium on cc binding canonly be quantitatively determined by comparing the ratio of (cid:104) J /ψ (cid:105) to (cid:104) cc (cid:105) innucleus-nucleus to that in proton-proton reactions. In Pb+Pb collisions theonset of color screening should already be seen in the centrality dependence ofthe (cid:104) J /ψ (cid:105) to (cid:104) cc (cid:105) ratio. This clearly shows the need for large statistic data on (cid:104) cc (cid:105) . The SAVD was built using sixteen CMOS MIMOSA-26 sensors [16]. The ba-sic sensor properties are: 18 . × . µ m pixels, 115 µ s time resolution, 10 ×
20 mm surface, 0.66 MPixel, 50 µ m thick. The estimated material budgetper layer, including the mechanical support, is 0.3% of a radiation length. Thesensors were glued to eight ALICE ITS ladders [17], which were mounted ontwo horizontally movable arms and spaced by 5 cm along the z (beam) direc-tion. The detector box was filled with He (to reduce beam-gas interactions) andcontained an integrated target holder to avoid unwanted material and multipleCoulomb scattering between target and detector. More details related to theSAVD project can be found in [18].The first test of the device was performed in December 2016 during a Pb+Pbtest run. The test allowed to demonstrate: tracking in a large track multiplic-ity environment, precise primary vertex reconstruction, TPC and SAVD trackmatching. Furthermore, it allowed to make a first search for the D and D signals. The obtained primary vertex resolution along the beam direction of30 µ m was sufficient to perform the search for the D and D signals. Figure 2( right ) shows the first indication of a D and D peak obtained using the datacollected during the Pb+Pb run in 2016.Successful performance of the SAVD in 2016 led to the decision to also useit during the Xe+La data taking in 2017. About 5 · events of central Xe+Lacollisions at 150 A GeV /c were collected in October and November 2017. During3igure 2: Left:
The SAVD used by NA61/SHINE during the data taking in 2016and 2017.
Right:
The invariant mass distribution of D and D candidates incentral Pb+Pb collisions at 150 A GeV /c after the background suppression cuts.The particle identification capability of NA61/SHINE was not used at this stageof the analysis [2].these measurements the thresholds of the MIMOSA-26 sensors were tuned toobtain high hit detection efficiency which led to significant improvement in theprimary vertex reconstruction precision, namely the spatial resolution of theprimary vertices obtained for Xe+La data is on the level of 1 µm and 15 µm in the transverse and longitudinal coordinates, respectively. The distributionof 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 andare expected to lead to physics results in the coming months.The SAVD will also be used during three weeks of Pb+Pb data taking in2018. About 1 · central collisions should be recorded and 2500 D and D decays can be expected to be reconstructed in this data set. During the Long Shutdown 2 at CERN (2019-2020), a significant modification ofthe NA61/SHINE spectrometer is planned. The upgrade is primarily motivatedby the charm program which requires a tenfold increase of the data taking rateto about 1 kHz and an increase of the phase-space coverage of the Vertex De-tector by a factor of about 2. This, in particular, requires construction of a newVertex Detector (VD), replacement of the TPC read-out electronics, implemen-tation of new trigger and data acquisition systems and upgrade of the ProjectileSpectator Detector. Finally, new ToF detectors are planned to be constructedfor particle identification at mid-rapidity. This is mainly motivated by possiblefuture measurements related to the onset of fireball formation. The detectorupgrades are discussed in detail in Ref. [2]. The data taking plan related to theopen charm measurements forsees measurements of 500M inelastic Pb+Pb colli-sions at 150 A GeV /c in 2022 and 2023. This data will provide the mean numberof cc pairs in central Pb+Pb collisions needed to investigate the mechanism ofcharm production in this reaction. Moreover, the data will allow to establish4igure 3: ( Left: ) Distribution of longitudinal coordinate of the primary vertex z prim for interactions in the La target, which was composed of three 1 mm plates. Right:
Rapidity ( left ) and transverse momentum ( right ) distributions of D +D mesons produced in about 500M inelastic Pb+Pb collisions at 150 A GeV /c .Dots indicate all generated mesons, triangles mesons within the VD acceptanceand squares mesons within the VD acceptance and passing background suppres-sion cuts.the centrality dependence of (cid:104) cc (cid:105) in Pb+Pb collisions at 150 A GeV /c and thusaddress the question of how the formation of QGP impacts J /ψ production. Ta-ble 1 lists the expected number of charm mesons in centrality selected Pb+Pbcollisions at 150 A GeV /c assuming the above mentioned statistics of minimumbias collisions. The estimate was performed assuming that the mean multiplic-ity of charm hadrons is proportional to the number of collisions and used yieldscalculated for central Pb+Pb collisions within the HSD model [4, 5]. Central (0-30%) Pb+Pb collisions at 40 A GeV /c are planned to be recorded in 2024. Thisdata together with the result for central Pb+Pb collisions at 150 A GeV /c willstart a long-term effort to establish the collision energy dependence of (cid:104) cc (cid:105) andaddress the question of how the onset of deconfinement impacts charm produc-tion. The expected high statistics of reconstructed D and D decays is due toTable 1: Expected number of charm mesons in centrality selected Pb+Pbcollisions at 150 A GeV /c assuming 500M minimum bias events recorded in 2022and 2023, see text for detail. The mean number of wounded nucleons (cid:104) W (cid:105) calculated within the Wounded Nucleon Model is also given.0–10% 10–20% 20–30% 30–60% 60–90% 0–90% + D ) 31k 20k 11k 13k 1.3k 76k + + D − ) 19k 12k 7k 8k 0.8k 46k (cid:104) W (cid:105)
327 226 156 70 11 105the high event rate and the relatively large efficiencies of open charm detectionin the VD. The efficiency will be about 13% (3 times better than for the SAVD)for the D → π + + K − decay channel and about 9% for D + decaying into π + + π + + K − . The quoted efficiencies include the geometrical acceptance for D → π + + K − (D + → π + + π + + K − ) decays and the efficiency of the analysis quality cuts used to reduce thecombinatorial background. right ) shows distributions of D + D mesons in rapidity andtransverse momentum for all generated particles (black symbols) and for parti-cles that passed the acceptance and background reduction cuts (blue symbols).The presented plots refer to 500M inelastic Pb+Pb collisions at 150 A GeV /c . To-tal uncertainty of (cid:104) D (cid:105) and (cid:104) D (cid:105) is expected to be about 10% and is dominatedby systematic uncertainty.In summary it is emphasized that only NA61/SHINE is able to measure opencharm production in heavy ion collisions in full phase space and at the beginningof the next decade. The corresponding potential measurements at higher (LHC,RHIC) and lower (FAIR, J-PARC) energies are necessary to complement theNA61/SHINE results and establish the collision energy dependence of charmproduction. Acknowledgments: this work was supported by the Polish National Centerfor Science grants2014/15/B/ST2/02537 and 2015/18/M/ST2/00125.
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