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The ALICE Measurement Looking for strange particles in ALICE Find channel possibl e to observe visually Requirement : physics measurement ALICE strong on tracking and PID – not enough calorimeter coverage Proposal to look for V0 and cascade decays (of strange hadrons)

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The ALICE Measurement

  • Looking for strange particles in ALICE
  • Find channel possible to observe visually
  • Requirement : physics measurement
  • ALICE strong on tracking and PID – not enough calorimeter coverage
  • Proposal to look for V0 and cascade decays (of strange hadrons)
  • Strangeness enhancement : first signature for quark gluon plasma
  • Seen by previous heavy ion experiments
  • Need to compare lead lead collision data with proton proton data
  • Develop and test with firstLHC data (900 GeV proton proton)

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 PbPb event

ALICE exercise - D. Hatzifotiadou


Looking for strange particles in ALICE (K0s, Λ, Ξ-)

recognisefrom decay pattern


invariant mass

classify according

to mass

fill tables


  • Simplified ALICE event display based on ROOT (LINUX, Ubuntu, MacOSX)
  • V0 finder and cascade finder
  • P.Debski, P.Foka, DH, B.Hippolyte, A.Maire, M.Tadel

ALICE exercise - D. Hatzifotiadou


From PawelDebski

ALICE exercise - D. Hatzifotiadou



  • Flavour of data analysis
  • Concept of particle decays
  • Concept of some conservation laws and invariant mass
  • 2.76 TeV lead collision data (end 2010) not available for the exercise
  • Count numbers of particle species (K0s, Λ, anti-Λ and Ξ-)*
  • Calculate yields of particles
  • Compare with Monte Carlo predictions
  • Compare with ALICE published results
  • Some discussion of errors
  • First (experimental) phase
  • No background subtraction, efficiency etc
  • No Windows implementation
  • *in a sample of 100 events

ALICE exercise - D. Hatzifotiadou


Some numbers

  • FrascatiMasterClass : 41 pupils from all over Italy, 2 pupils /school
  • part of a four-day programme in Frascati “Stage InvernaleResidenziale”, 7-10 February (outside “International Masterclasses” schedule)
  • Graz: problems with ROOT installation; did LEP exercise
  • Bergen : 70 pupils
  • Oslo : 35 pupils (?)
  • GSI Darmstadt/Frankfurt 30 pupils
  • Torino : 80 pupils in the morning, 56 staying for the exercise
  • Lodz : 60 pupils + 3 teachers
  • Nantes : 35 pupils + 3 adults (1 class)
  • Heidelberg : 36 pupils + 2 teachers (2 classes)
  • CERN : 27 pupils + 3 teachers (classe première scientifiqueLycée International de Ferney)
  • Debriefing meeting + EVO 18.3.2011
  • Debriefing EVO Meeting 1.4.2011

ALICE exercise - D. Hatzifotiadou

slide10 Nantes 25.3.2011

ALICE exercise - D. Hatzifotiadou



  • Generalappreciation varies
  • Some found difficult to understand - others liked the exercise
  • Somewere enthusiastic – others found it boring
  • Event samples generated a lot of discussion
  • In 100 events include K0s, Λ, anti-Λ(percentage as from data analysis)
  • and Ξ–(10 x percentage as from data analysis)
  • Too complicated (selected V0s, often V0 finder finds more V0s in the same event)
  • Event without V0s (“empty”) : should they exist or not?
  • Repetition of “same V0s” in many samples
  • Background
  • Not considered – another version developed to take it into account (see next)
  • V0 finder – cascade finder : mechanical (just click a button)
  • But can one find V0s by eye?
  • Some technical problems
  • Computer crashes (version of root? Graphics?) – no crash on MacOSX

ALICE exercise - D. Hatzifotiadou


Various remarks and suggestions

  • Should implement counter for events analysed
  • Display event number / sample number separately
  • Histograms not optimised
  • Change histogram binning
  • Need to update automatically
  • Add histogram facility : ok for single files, not functioning for multiple files
  • Book-keeping of results
  • Clicking of buttons required to save results in tables ad histograms
  • Often children forget – information lost
  • Have to modify

ALICE exercise - D. Hatzifotiadou


GSI version

  • Developed another version of the exercise
  • Analyse “visually” a small sample of events (20 instead of 100)
  • Run analysis program in the background analysing some thousands of events
  • Invariant mass plots
  • Fit background and substract
  • This certainly answers the question asked by many
  • Is this how you do the analysis in reality (scanning each event by eye)?

ALICE exercise - D. Hatzifotiadou


CERN MasterClass - Feedback

  • Teacher made an evaluation sheet
  • 20 replies from the pupils
  • Answers cover the whole spectrum
  • General impression positive
  • Enjoyed the day, feel that they learnt a lot
  • Proposed improvements
  • Same teacher had participated last year in CERN masterclass with OPAL exercise
  • He liked the new one (ALICE) better
  • One of the instructors: it was fun and it was something different
  • encouraging

ALICE exercise - D. Hatzifotiadou



  • Discussion on QGP and strangeness enhancement
  • too much at the end of a long day?
  • Proposed questions to ask (baryon number conservation) – too difficult – dropped
  • Opted for having the results reported and filling excel sheets “online”
  • No uploading of documents
  • Some of the pupils said that they liked the quiz
  • Some of the organisers question whether the quiz should be done locally
  • Some expressed the opinion that we should have an ALICE special quiz
  • The time of the video-conference makes the whole event end too late
  • Can everything be brought forward?
  • For CERN masterclass – less important (they have spent the day at CERN)
  • Some report technical problems, echo, etc.

ALICE exercise - D. Hatzifotiadou



  • “Looking for strange particles with ALICE”
  • starting point
  • developed with limited resources
  • Overall experience positive
  • Brought new institutes in the game
  • Generated interest, dialogue and new ideas
  • People get more involved when they present their experiment
  • Some technical problems (computer crashes)
  • Criticism to be taken into account for the future developments

ALICE exercise - D. Hatzifotiadou



  • Revisit the exercise and its aims
  • Original idea of looking for strangeness enhancement:
  • Compare data from Pb-Pb collisions and pp collisions at higher energies (Can the event display easily deal with thousands of tracks?)
  • Smaller data samples
  • Viewer, histogram facility, calculator : need modifications
  • Idea of small scale visual analysis combined with larger scale “real” analysis and background fitting and subtraction : interesting, to be investigated
  • ( time needed ; too complex ?)
  • In search of manpower

ALICE exercise - D. Hatzifotiadou