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
2/25
Time Scales at LHC• QGP life time
10 fm/c ≈ 310-23 s
• thermalization time0.2 fm/c ≈ 710-25 s
• formation time(e.g. charm quark):1/2mc = 0.08 fm/c
≈ 310-25 s
• collision time2R/ = 0.005 fm/c≈ 210-26 s
Plot: courtesy of R. Stock.
3/25
• 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 , W±, Z at LHC)
RAA(pT) = 1
Charm at high energiesNA60, SPS 0907.5004 STAR, RHIC 1404.6185 ALICE, LHC 1405.2001
4/25
X. Zhu, M. Bleicher, S.L. Huang, ks, H. Stöcker,N. Xu, and P. Zhuang, PLB 647 (2007) 366.
mc,b » QCD : new scale
mc,b ≈ const., mu,d,s ≠ const.
initial conditions:
test pQCDprobe gluon distribution
early partonic stage: diffusion (), drag ()flow, jets, correlationsprobe thermalization
hadronization:chiral symmetry restorationconfinementstatistical coalescenceJ/ enhancement / suppression
Heavy - flavor: a unique probe
Q2
time
5/25
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)
6/25
Where does all the charm go ?hadro-production in vacuo
• 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
plot: courtesy of J. Mercado.
7/25
Heavy-quark detection
• e.g., D0 K- + , c = 123 m
• displaced decay vertex is signature of heavy-quark decay
Open-charm reco. in ALICE
D0 KD+ KD* D0Ds KK
Under study:c pKc c K0
Sp
plot: courtesy of D. Tlusty.
8/25
9
Size: 16 x 16 x 25 meters
Resolution: 600M pixels (750 Mbytes)
Readout: 17.5 terabytes/s, ~4 Gbytes/s to tape
TPC
TRD
ITS
Central barrel (ITS, TPC, TRD, TOF) || < 0.9
Muon spectrometer: -4.0 < y < -2.5
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
11/25
• At low multiplicity, primary
vertex reconstruction
significantly deteriorates
dominant contribution to
spatial resolution
• Use beam spot size in
transverse direction
(~40-60 m)
• z position [-5 cm, 5 cm]
reconstructed from tracks
• (?) Multiple vertices at
CBM
Single-hit resolution• Design: 10 m
• After alignment (here
cosmics): 12 m
• Due to residual mis-
alignment
• Ever-decreasing pixel
size has limits
• Impact on open-
charm program
intensively checked
before data taking12/25
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 (c)
• High-speed readout:
enables beauty
measurements13/25
ALICE, CERN-LHCC-2013-024
secondary vertexing at hadron colliders
experiment pixel size (m2)
resolutionr x z (m2)
radius 1st layer (mm)
pointing resolution d0 (m) @ 1GeV
STAR HFT 30 x 30 12 x 12 25 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
14/25
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 important15/25
Open-charm spectra from pp @ 7 TeV
• 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 town16/25
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).
Open-charm cross section
• 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(?)17/25
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).
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
18/25
Spectrum covered by experiment
• At LHC at mid-rapidity, ~50% of spectrum below 2 GeV/c
• Fraction gets larger at lower energies
19/25
Giving up secondary vertexing
• New analysis method
• Signal observed down
to pT = 0
• Next step: apply
Bayesian PID,
i.e. instead of ncuts,
take into account
relative abundances of
particles (priors)
20/25
pp D0 analysis: C. Moehler, Master thesis, Univ. HD, in preparation.
c 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 c production yet published at hadron colliders
21/25
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
22/25
Anisotropy Parameter v2
y
x
py
px
coordinate-space-anisotropy momentum-space-anisotropy
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
24/25
R. Grajcarek, doctoral thesis, Univ. HD (2013).
• 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
Model comparison: RAA and v2
25/25
Summary
26/25
• 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