1. Open Charm with the CBM Experiment
Volker Friese
GSI Helmholtzzentrum für Schwerionenforschung H4F-Workshop “Heavy Flavour Physics with CBM” FIAS, Frankfurt, 26 May 2014

2. The CBM Physics Program
comprehensive program to explore the phase diagram of strongly interacting matter at highest net baryon densities and moderate temperatures:
heavy-ion collisions from 2 – 45 GeV/nucleon at FAIR
SIS100
2 to 14 GeV/nucleon for nuclei
up to 29 GeV for protons
SIS300:
up to 45 GeV/nucleon for nuclei
up to 90 GeV for protons
Broad spectrum of observables: strangeness, flow, fluctuations, hypernuclei, low-mass dileptons, open charm, charmonia
Heavy Flavour Physics with CBM
V. Friese

3. Experiment Set-up Electron + Hadron setup Muon setup STS+MVD RICH TRD
TOF
ECAL
PSD
absorber + detectors
STS+MVD
Heavy Flavour Physics with CBM
V. Friese

4. O. Linnyk et al., Nucl. Phys. A 786 (2007) 183
Why Open Charm with CBM?
O. Linnyk et al., Nucl. Phys. A 786 (2007) 183
CBM p+A
elementary cross section experimentally unknown
no data in A+A below RHIC energies
open / hidden charm seems promising observable to distinguish phases
in-medium modification of D masses?
Heavy Flavour Physics with CBM
V. Friese

5. Experimental challenges: Rare probes
particle multiplicity  branching ratio
min. bias Au+Au collisions at 25 AGeV
(from HSD and thermal model)
SPS Pb+Pb 30 A GeV
STAR Au+Au sNN=7.7 GeV
Punchline: Open access to rare probes by unprecedented interaction rates
Heavy Flavour Physics with CBM
V. Friese

6. Experimental Task D Kππ Central Au+Au collision at 25AGeV
UrQMD + GEANT + CbmRoot
D Kππ
Heavy Flavour Physics with CBM
V. Friese

7. The Workhorse: Silicon Tracking System
CBM’s main tracking detector
large aperture
from ~ center-of mass to beam rapidity
polar angles: 2.5 deg <  < 25 (35) deg
redundant track point measurement
8 tracking stations
space point resolution ~ 25 µm
low material budget
double-sided silicon microstrip sensors
r/o electronics outside physics aperture;
0.3-1% X0 per tracking station
radiation tolerant silicon sensors
1 m
Heavy Flavour Physics with CBM
V. Friese

8. Detector performance simulations
detailed, realistic detector model based on tested prototype components
CbmRoot simulation framework
using Cellular Automaton / Kalman Filter algorithms
track reconstruction efficiency
momentum resolution
Heavy Flavour Physics with CBM
V. Friese

9. Detection of Open Charm
The detection principle is decay topology (displaced vertex)
The relevant variables are
impact parameter of daughters at primary (event) vertex
z-coordinate of pair vertex
decay angle in pair rest frame
impact parameter of pair momentum at primary vertex
Note: flight path until decay only several 100 mum. D0 + K-
p + pK b bK
Lab
CMpair vz *
Heavy Flavour Physics with CBM
V. Friese

10. Detection of Open Charm: Impact Parameter
Primary tracks will point back to the event vertex; signal daughters will have non-vanishing impact parameter.
Pairs from signal decays will predominantly be in different hemispheres in the target z-plane; hemisphere for primary tracks is random. Use signed product of impact parameters.
schematically.
only primary tracks
Heavy Flavour Physics with CBM
V. Friese

11. Detection of Open Charm: Vertex Position
Construct the pair vertex (point of closest approach)
Random background pairs from primary particles will be centered around primary vertex.
Signal pair vertex will be distributed according to the decay law, folded with the D momentum distribution.
Heavy Flavour Physics with CBM
V. Friese

12. Background Primary particles: reducible by precision vertexing
identification by TOF (in particular K)
Particles from abundant weak decays (Λ). Reducible by:
remove from combinatorics after positive identification as Λ daughter (inv. mass, Armenteros)
The vertexing precision of the detector depends crucially on:
resolution in coordinate space (needs pixel detector),
material budget (minimise mutiple scattering)
Heavy Flavour Physics with CBM
V. Friese

13. Particularities for CBM
Ratio of particles with open charm to bulk particles much smaller than at RHIC/LHC: background must be suppressed by many orders of magnitude. Needs very high precision and high statistics.
Fixed target: Lorentz boost increases momentum and thus lifetime in lab frame. Easier decoupling from background. Detection down to pt = 0 possible.
Heavy Flavour Physics with CBM
V. Friese

14. What Detector is Needed?
Requirements:
pixel detector with high resolution
very low material budget
fast (high rates)
radiation hard (high rates, close to target)
Our choice from the start: MAPS (Monolithic Active Pixel Sensors)
integrated electronics: very thin
small pixel size
not fast
not particularly radiation hard, but huge progress in last years
now almost the standard choice (STAR, ALICE upgrades)
Heavy Flavour Physics with CBM
V. Friese

15. The CBM Micro-Vertex Detector (MVD)
STS
2.5°
25°
Beam
Heavy Flavour Physics with CBM
V. Friese

16. Why We Invest in a MVD STS MVD resolution 20 / 120 mum 3 mum
mat. budget
1% X0 / layer
0.3% X0 / layer
distance from target
30 –100 cm
5 – 20 cm
rad. hardness + ~
r/o speed ++ -
Primary vertex resolution;
resolution for secondary vertices analog
Heavy Flavour Physics with CBM
V. Friese

17. CBM Data Taking
The complexity of e.g., the open charm signature forbids conventional triggers. Self-triggered data will be reconstructed and analysed online in a computer farm (FLES).
Needs precise and performant reconstruction software.
Heavy Flavour Physics with CBM
V. Friese

18. Example: Particle finder
Heavy Flavour Physics with CBM
V. Friese

19. Secondary Vertex Resolution
Au+Au, 25A GeV
Heavy Flavour Physics with CBM
V. Friese

20. Performance of open charm measurement
p+C collisions, 30 GeV (SIS100)
1012 centr.
D0 Kπππ
D Kππ
Au+Au collisions, 25 AGeV (SIS300)
D0 Kπππ
D Kππ
D0 Kπ
Heavy Flavour Physics with CBM
V. Friese

21. Performance of open charm measurement
D0+D0
D++D-
Ds+
c+
decay channel
K-+
K-+ +
K-K+ +
p K-+
MHSD
1.5·10-4
4.2·10-5
5.4·10-6
MSM
8.2·10-4
8.4·10-5
1.4·10-4
4.9·10-4
BR(%)
3.8
9.5
5.3
5.0
geo. acc.(%)
29.2
40.1
32.8 71
z-resolution (m) 52 56 60 69
total eff. (%)
3.95
4.75
1.0
0.05
m (MeV/c2)
~11
S/B2
0.16/0.5
1.24/2.5
0.6
Yield/1012mb HSD
14k+41k
47k+89k
0.7k
Yield/1012mb SHM
78k+225k
95k+179k
19k
3.2 k
Heavy Flavour Physics with CBM
V. Friese

22. CBM – Day One Modul 0 SIS100 Modul 1 CBM, APPA Modul 2 Super-FRS
Antiproton-target, CR, p-Linac, HESR
Heavy Flavour Physics with CBM
V. Friese

23. Rates and Prospects
The MVD developed for day-1 CBM (SIS-100) will limit the Au+Au interaction rate to ~100 kHz.
Sub-threshold open charm measurements at SIS-100 will be extremely challenging.
The open charm programme of the first years of CBM will focus on p+A measurements (up to 30 GeV).
The full open charm physics potential in A+A will open with SIS-300.
Since the development of CMOS sensors is very rapid, a likely scenario is that for CBM at SIS-300, a new MVD based on improved detector technology will be deployed.
SIS-300
SIS-100
D thresh.
Heavy Flavour Physics with CBM
V. Friese