Kai Schweda GSI Darmstadt / University of Heidelberg Open Charm reconstruction at ALICE, reconstruction techniques and lessons learnt for CBM Kai Schweda GSI Darmstadt / University of Heidelberg

Outline Charm physics in NN collisions – some remarks Topological selection – spatial resolution Ultra-low pT – particle identification Correlations – elliptic flow Summary

Time Scales at LHC QGP life time 10 fm/c ≈ 310-23 s thermalization time 0.2 fm/c ≈ 710-25 s formation time (e.g. charm quark): 1/2mc = 0.08 fm/c ≈ 310-25 s collision time 2R/g = 0.005 fm/c ≈ 210-26 s Plot: courtesy of R. Stock.

Charm at high energies NA60, SPS 0907.5004 STAR, RHIC 1404.6185 ALICE, LHC 1405.2001 charm: collective flow + energy loss at RHIC and LHC energies inconclusive at top SPS energies when charm does not interact with the medium, it becomes a standard candle at SIS 300 energies (like g, W±, Z at LHC)  RAA(pT) = 1

Heavy - flavor: a unique probe mc,b » QCD : new scale mc,b ≈ const., mu,d,s ≠ const. initial conditions: test pQCD probe gluon distribution early partonic stage: diffusion (), drag () flow, jets, correlations probe thermalization hadronization: chiral symmetry restoration confinement statistical coalescence J/ enhancement / suppression Q2 time X. Zhu, M. Bleicher, S.L. Huang, ks, H. Stöcker, N. Xu, and P. Zhuang, PLB 647 (2007) 366.

Heavy  quark production at LHC Charm is conserved quantum number in strong interactions Charm and anti-charm quarks created in pairs LO and NLO matrix elements enter state-of-the-art calculations in pQCD (FONLL, GM-VFNS, etc.) Flavor creation (LO) dominant for CBM Gluon splitting (NLO) Flavor excitation (NLO)

Where does all the charm go ? hadro-production in vacuo plot: courtesy of J. Mercado. Total charm cross section: open-charmed hadrons, e.g. D0, D+, D+s , D*+, c, … and c,b  e() + X Maybe significantly altered at SIS 300 energies

Heavy-quark detection Open-charm reco. in ALICE D0  K D+  K D*  D0 Ds  KK Under study: c  pK c   c  K0Sp plot: courtesy of D. Tlusty. e.g., D0  K- +  , c = 123 m displaced decay vertex is signature of heavy-quark decay

Resolution: 600M pixels (750 Mbytes) Size: 16 x 16 x 25 meters Resolution: 600M pixels (750 Mbytes) Readout: 17.5 terabytes/s, ~4 Gbytes/s to tape TRD ITS TPC Central barrel (ITS, TPC, TRD, TOF) || < 0.9 Muon spectrometer: -4.0 < y < -2.5 8

A single Pb-Pb collision in ALICE ALICE is designed for Highest multiplicities dN/d up to 6000 Excellent tracking & particle identification down to lowest momentum ~ 100 MeV/c

Primary vertex reconstruction At low multiplicity, primary vertex reconstruction significantly deteriorates  dominant contribution to spatial resolution Use beam spot size in transverse direction (~40-60 mm) z position [-5 cm, 5 cm] reconstructed from tracks (?) Multiple vertices at CBM

Single-hit resolution Design: 10 mm After alignment (here cosmics): 12 mm Due to residual mis-alignment Ever-decreasing pixel size has limits Impact on open-charm program intensively checked before data taking ALICE, JINST 5 , P03003 (2010).

Pointing resolution in ALICE ALICE ITS upgrade (2018) Improvement of factor of 3 Improves D detection + makes new measurements possible (Lc) High-speed readout: enables beauty measurements ALICE, CERN-LHCC-2013-024

secondary vertexing at hadron colliders experiment pixel size (mm2) resolution rf x z (mm2) radius 1st layer (mm) pointing resolution d0 (mm) @ 1GeV STAR HFT 30 x 30 12 x 12 25 ALICE ITS 50 x 425 12 x 100 39 60 ALICE ITS upgrade O(30 x 30) 5 x 5 22 20 CDF II pitch:60-150 12 (axial) 24 50 Get as close to the interaction point as possible for a competitive open-charm physics program

Particle identification is key kaon over pion ratio ~ 1/10, kaon identification becomes key to enhance signal / background ratio dE/dx + ToF effective up to ~5 GeV/c for D mesons, then combinatorial background less important

Open-charm spectra from pp @ 7 TeV ALICE, JHEP 1201 (2012) 128; arXiv:1111.1553 [hep-ex]; D*+ analysis: Y. Wang, doctoral thesis, Univ. HD (2014); F. Schaefer, bachelor thesis, Univ. HD (2012). pp covers spectrum from 1 up to 24 GeV/c Pb-Pb spectrum goes up to 36 GeV/c (!) pp reference becomes limit in high-pT studies access ultra-low pT  give up topological selection  PID becomes only game in town

Open-charm cross section ALICE, : JHEP 1207, 191 (2012), arXiv:1205.4007 [hep-ex]; J. Wilkinson, bachelor thesis, Univ. HD (2011); S. Stiefelmaier, bachelor thesis, Univ. HD (2012); H. Cakir, bachelor thesis, Univ. HD (2013). LHC: First collider measurements at TeV scale ATLAS & LHCb agree with ALICE experimental data sits in the upper theory band hints at lower charm mass, mc < 1.5 GeV(?)

Extrapolation of charm production Limited phase space: ALICE covers ~10% of total charm production Increase pT range down to 0 D meson does not move away from the collision vertex anymore Give up toplogical selection Sophisticated methods of PID (Bayesian approach) + controlling the background shape (track rotation, event mixing) necessary

Spectrum covered by experiment At LHC at mid-rapidity, ~50% of spectrum below 2 GeV/c Fraction gets larger at lower energies

Giving up secondary vertexing New analysis method Signal observed down to pT = 0 Next step: apply Bayesian PID, i.e. instead of ns cuts, take into account relative abundances of particles (priors) pp D0 analysis: C. Moehler, Master thesis, Univ. HD, in preparation.

Lc from ALICE No topological selection, pure combinatorics using Bayesian PID After ALICE-ITS upgrade: spectrum starts at pT > 2 GeV/c STAR-HFT: spectrum starts at pT > 2 GeV/c No prompt Lc production yet published at hadron colliders

Charmed resonance: D*(2010)+ D meson width typically 10 - 20 MeV D*+ < 1 MeV At kinematic boundary Substantially enhances S/B D0 and D*+ have comparable performance in ALICE

Anisotropy Parameter v2 coordinate-space-anisotropy  momentum-space-anisotropy y py x px low-pT: initial/final conditions, EoS, degrees of freedom high-pT: path-length dependence of energy loss

Determining the event plane Small in-homogeneities in azimuthal acceptance (sector gaps, dead modules) lead to substantial deviations in event plane reco. Sophisticated algorithms exist to recover Precision necessary depends on strength of flow signal R. Grajcarek, doctoral thesis, Univ. HD (2013).

Model comparison: RAA and v2 Simultaneous description of RAA and v2poses serious challenge to all models Charm conserved, decelerated charm quark should re-appear at bump around 2 GeV Models have limited power to predict production yields

Summary At low multiplicities, primary vertex reconstruction becomes an issue – pp reference Ever-decreasing pixel size might be countered by residual mis-alignment – get as close to the interaction vertex as possible ! Bulk production of charm below 2-3 GeV/c At sufficiently low pT, topological selection fails, sophisticated methods of PID essential Correlations studies necessitate homogenous (azimuthal) detector acceptance