0. 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

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

2. 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 = fm/c ≈ 210-26 s
Plot: courtesy of R. Stock.

3. Charm at high energies
NA60, SPS STAR, RHIC ALICE, LHC
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

4. 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.

5. 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)

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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).

12. 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

13. secondary vertexing at hadron colliders
experiment
pixel size (mm2)
resolution rf x z (mm2)
radius 1st layer (mm)
pointing resolution d0 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

14. 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

15. Open-charm spectra from pp @ 7 TeV
ALICE, JHEP 1201 (2012) 128; arXiv: [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

16. Open-charm cross section
ALICE, : JHEP 1207, 191 (2012), arXiv: [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(?)

17. 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

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

19. 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.

20. 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

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

22. 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

23. 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).

24. 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

25. 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