1. The ALICE MasterClass exercise Looking for strange particles in ALICE
physics motivation
Strangeness enhancement : (first) signature for quark gluon plasma
the signature
V0 (and cascade) decays of strange hadrons (K0s, Λ, Ξ-)
the tools
Simplified ALICE event display based on ROOT
Combine visual analysis of small event sample with large statistics analysis
The data
First LHC data (900 GeV proton proton) : develop / run masterclasses 2011
7 TeV proton proton data for 2012
Pb-Pb collision data : in a next version
IPPOG CERN
ALICE exercise - D. Hatzifotiadou

2. ALICE exercise - D. Hatzifotiadou
Visual identification of V0 decays and cascades
Ξ-→π-Λ→ π- p π-
K0s→ p+p-
Bachelor (single track) and two opposite tracks coming from a secondary vertex
L ®p-p
anti Λ→ p-π+
Two opposite tracks from a secondary vertex
Monte Carlo Pb Pb event
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ALICE exercise - D. Hatzifotiadou

3. ALICE exercise - D. Hatzifotiadou
The analysis tool : simplified ALICE event display based on ROOT
3 views of ALICE – 3D, rφ, rz
V0 finder and cascade finder
Highlights V0 (cascade)
Recognise from decay pattern
Calculate invariant mass
Classify according to mass
Fill tables
Fill histograms
Runs on LINUX, Ubuntu, MacOSX
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ALICE exercise - D. Hatzifotiadou

4. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou
From Pawel Debski

5. ALICE exercise - D. Hatzifotiadou
From Pawel Debski
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ALICE exercise - D. Hatzifotiadou

6. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou

7. ALICE exercise - D. Hatzifotiadou
Many comments on how we deal with background
Questions from pupils : is this how you analyse the data?
2nd part of exercise (GSI version)
Analyse big event sample (2000 events)
Invariant mass histograms for Ks, L, anti-L
Fit 2nd degree polynomial to background
Fit gaussian to peak
Get number of Ks, L, anti-L after background subtraction
+ option to see combinatorial background
(without common secondary vertex requirement)
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ALICE exercise - D. Hatzifotiadou

8. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou

9. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou

10. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou

11. ALICE exercise - D. Hatzifotiadou
Goals
Flavour of data analysis
Concept of particle decays
Concept of some conservation laws and invariant mass
Results
Count numbers of particle species (K0s, Λ, anti-Λ and Ξ-)
Calculate yields of particles
Compare with ALICE published results
Note that
To see strangeness enhancement you need to make ratios, for example
yield of Λ in pp / yield of Λ in Pb-Pb
Show published strangeness enhancement plots from ALICE (+ other HI experiments)
Explain on the plots what they would need to measure, in addition, to reproduce them
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ALICE exercise - D. Hatzifotiadou

12. ALICE exercise - D. Hatzifotiadou
Strangeness enhancement
defined as: the particle yield normalised by the number of participating nucleons in the collision, and divided by the observed yield in proton-beryllium (or proton-proton) collisions
Enhancement increases with number of strange quarks in the hadron ( Ω has 3, Ξ has 2, Λ has 1)
Enhancement decreases with collision energy (going from SPS to RHIC)..and at the LHC?
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ALICE exercise - D. Hatzifotiadou

13. ALICE exercise - D. Hatzifotiadou
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ALICE exercise - D. Hatzifotiadou

14. ALICE exercise - D. Hatzifotiadou
The problem with lead ion collision data
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ALICE exercise - D. Hatzifotiadou

15. ALICE exercise - D. Hatzifotiadou
L ®p-p
K0s→ p+p-
Include event displays from lead lead
Ξ-→π-Λ→ π- p π-
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ALICE exercise - D. Hatzifotiadou

16. ALICE exercise - D. Hatzifotiadou
Outline of exercise
Students (each group): Visual analysis of 30 events – tables + invariant mass histograms
Institute (all groups): Add up numbers / merge histograms
Students (each group): Analysis of 2000 events - invariant mass histograms
Students (each group): Try to fit background / peak / subtract -> find number of particles
Institute (all groups): Merge histograms
Institute (all groups): Fit background / peak / subtract -> find number of particles
Calculate yields of each particle type
(assumptions on initial number of events from which the sample has been extracted
correction factors for efficiency)
Discussion on strangeness enhancement plots
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ALICE exercise - D. Hatzifotiadou

17. ALICE exercise - D. Hatzifotiadou
Status
30 event samples 7 TeV proton-proton for visual analysis (6) ok
2000 event samples 7 TeV proton-proton ok
Analysis tools ok
Accompanying Text almost ready (to be checked)
To be provided for the masterclasses
Proposed discussion for institutes
Proposed discussion for moderators
Excel sheets for results
IPPOG CERN
ALICE exercise - D. Hatzifotiadou