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