reconstruction tools for the study of short-lived

Reconstruction tools for the study of short-lived resonances in ALICE pp collisions at the LHC startup

1. The ALICE experiment @LHC2. Short-lived resonances in ALICE3. pp collisions: preparing the tools for LHC startup4. Results from local and distributed computing analysis

Outline

F.RiggiDept. of Physics and Astronomy and INFN, Catania

ACAT 2007, Amsterdam, 23-27 April, 2007

Z

The Large Hadron Collider (LHC) @ CERN

4 major experiments waiting for the first beam…

ALICE Detector @ Point2

Point 2

ALICE: A Large Ion Collider Experiment

ITS

TPC

TOF

PT > 1 GeV/c

ALICE will track and identify products from pp and nucleus-nucleus collisions in a large multiplicity environment

Main tracking detectors: ITS and TPC

The silicon Inner Tracking System (ITS)

Drift (SDD) 133,120 channels

Strips (SSD) 2,608,128 channels

Pixel (SPD) 9,830,400 channels

Time Projection Chamber (TPC)

Channels 557 568

Gas Ne/CO2 90/10 or + 5%N2

Volume 88 m3

Drift length 2.5 m

Drift field 400 V/cm

Drift velocity

2.84 cm/ms

Max drift time

88 ms

Arrival of the TPC in the ALICE cavern

Possible ALICE physics at the LHC startup (pp @0.9 TeV?)

Physics aspects Compare pp and pp data at 900 GeVCompare pp and pp data at 900 GeV Improve the precision in comparison with existing pp data at 900 GeVImprove the precision in comparison with existing pp data at 900 GeV Prepare the detector (calibration, alignment)Prepare the detector (calibration, alignment) Provide a reference for data at different cm energies (2.4 TeV, 5.5 TeV?, 14 TeV). Provide a reference for data at different cm energies (2.4 TeV, 5.5 TeV?, 14 TeV). Use first pp events to study global event properties:Use first pp events to study global event properties: Charged Multiplicity, Pseudorapidity distributions, pCharged Multiplicity, Pseudorapidity distributions, pT spectra spectra

First strange particle studiesFirst strange particle studies Short-lived resonances ?Short-lived resonances ?

Ks0 P P

Yield perevent

0.1 0.01 210–4 10–5 0.4 0.4

Statisticsneeded

104 104 104 104 104 104

PP eventsneeded

105 106 108 109 104 104

Resonance Life-time [fm/c] 1.3++ 1.7 f0(980) 2.6 K*(892) 4.0 (1520) 13 ω(783) 23(1020) 45

Short-lived resonances have typical lifetimes comparable to that of the hot and dense matter created in AA collisions

Time

ch

em

ical fr

eeze-

ou

t

signal lost

kin

eti

c f

reeze

-ou

t

signal measured late decay

signal measured

rescattering

regeneration

e+

e-

signal measuredearly decay

+

-signal measured late decay

e+

e-signal measuredlate decay

Observation of short-lived resonances is affected by two competing effects: rescattering vs regeneration

Resonance K*(892) Φ(1020) *(1520) Δ(1232)Decay channel (B.R.) K (~100%) K+K- (49%) N K (45%) Nπ (100%)

Width [MeV/c2] 50.8 4.5 15.6 100 Life time [fm/c] 3.9 44 13 1.7

Study of short-life resonances in ALICE

Main observables:

● Extraction of the signal/yields● Mass and widths of resonances● Transverse momentum and transverse mass spectra● Particle ratios● Elliptic flow● Nuclear modification factors: RCP and RAA

pp collisions @900 GeV and @14 TeV

● LHC running scenario at startup: 1-2 days for physics collisions@900 GeV ?

● Expected no. of events: 0 to 106

● To what extent is the study of short-lived resonances feasible?

Results from:

● Local analysis via LSF (@900 GeV): 2 x 105 events

● Distributed GRID analysis (@14 TeV): 1.5 x 106 events

Tuning of reconstruction tools: ● Study of the combinatorial background ● Perfect and realistic Particle IDentification

Detailed examples for K*(892), other resonances treated in the same way

Reconstruction of short-lived resonances requires optimal performance on:

primary and secondary vertex reconstruction

Efficiency

An example of D0-> K-π+

decay in a large multiplicity environment

σ ~ 40 μm for pp

σ ~ 5 μm for PbPb

3D resolution


primary vertex reconstructionprimary vertex reconstruction

tracking efficiency

Kalman filter strategy allows a good tracking performance down to very low momenta

Methods based on neural networks also implemented for stand-alone tracking in the ITS: 10% improvement

ITS + TPC



tracking efficiencytracking efficiency

momentum resolution

pT resolution

The inclusion of ITS, TPC and TRD results in pT resolution as good as 3 % up to 100 GeV/c




momentum resolutionmomentum resolution

track impact parameter

Information on impact parameter mainly provided by the ITS

Transverse impact parameter resolution




momentum resolutionmomentum resolution

track impact parametertrack impact parameter

particle identification

with ITS and TPC at low momenta…

and TOF at high momenta…

w(i | s)=r(s | i) Ci

Σk r(s | k) Ck

Bayesian PID for each detector

Inside same event, correlations between

K+ and π- candidates

K- and π+ candidates

Evaluate invariant mass spectrum

Combinatorial background

Signal extraction (unlike-sign)

Mixed-event technique

Like-sign technique

Example: K*(892) Kπ (~100%)

The AliROOT framework

STEERBase classes, overall control

AliReconstruction ESDReconstruction

Event Summary Data

Particle generation

DPMJET

HIJING

HBTP

PYTHIA PDF

ISAJET

Transport of particles

GEANT3 GEANT4 FLUKA

Analysis

HBT JET PWG0-4

Detectors

ZDCPHOS

EMCALHMPID

TOF

TRD

TPC

ITS

T0 V0PMDMUON

Response

Geometry Calibration

Alignment

Selection of primary and identified tracks

Size reduction by 200

AOD:

Analysis Object Data

Analysis Cuts,histograms,…

From generation and reconstruction

ESD: Event Summary Data

Kinematics

…

Local analysis of 200,000 pp min bias events via

LSF (Load Sharing Facility, A general purpose distributed computing system)

For such application:

● Cluster of 60 multiprocessors CPU (up to 30 simultaneous jobs running)

● Generation and full reconstruction of events (1 job= 100 events)

● CPU time/event: 110 s @ 900 GeV ( 230 s @14 TeV)

● Total CPU time: approx. 200 days (about 10 effective days)

No event selection

Only events with Δm<5 and Δzv < 3 cm mixed

Effect of event selection (multiplicity and vertex location) on mixing procedure

Selection in multiplicity is more effective than in primary vertex location

(True Background)

(Mixed events background)

True background = (Signal) – (True pairs)

Like-sign technique

Signal

Background

Effect of particle identification on the results

PID information from TPC and TOF used in the present analysis, with Bayesian procedure

Different scenarios

● Perfect PID● Realistic PID with no threshold on max_probability● Realistic PID with improvement both in K and pion identification● Realistic PID with improvement in K identification /no PID on pions

K*(892) invariant mass with perfect PID

Improving kaon identification in the Bayesian algorithm by a threshold on max.probability

From max_prob > 0 to max_prob > 0.7

No thresh on maxprob

Maxprob > 0.7 (K)

No PID (π)

Maxprob > 0.7 (K)

Maxprob > 0.7 (π)

Perfect PID Realistic PID

Summary of PID influence on K*(892) reconstruction True

Found

S/B = 0.11S/√B = 20.28

S/B = 0.111S/√B = 20.01

Φ(1020)

Λ*(1520)

Similar analyses also done on other short-lived resonances

Yields and particle ratios uncertainties expected at 900 GeV with 200K pp events

Particle ratios Statistical

K*/K- 1.6 %

Λ*/Λ 9 %

Φ/K* 8 %

Φ/Λ* 12 %

K*/Λ* 9 %

REAL DATA (MB) MonteCarlo (MB)

Raw ESD AOD Ev. Catalog Raw ESD

pp / event 1 0.04 0-004 0.01 0.4 0.04

pp / year (x 109) 1.1 0.18 0.4 0.1

HI / event 12.5 2.5 0.25 0.01 300 2.5

HI / year (x 109) 1.4 1.0 3.0 0.9

More detailed analyses require a large number of events

Distributed analysis of 1.5 Mevents pp min bias events @ 14 TeV

from PDC06 on the GRID via AliEn (ALICE Grid Environment)

Submitting and monitoring jobs on the GRID via AliEn

Distributed computing on the GRID:

~1.5 Mevents (pp @14 TeV) processed on the grid via AliEn middleware

Monitoring the jobs on the GRID in ALICE

Typical results at 14 TeV from distributed analysis on the GRID

pT-analysis with realistic PID

pT= 0 - 0.5 pT= 1.5 - 2

pT= 3.5 - 4

Correction matrix

pT=0-4 GeV/c (8 bins)

Y = -1.5 – 1.5 (4 bins)

Summary

● Study of short-lived resonances in pp and PbPb collisions @ LHC energies could be studied in ALICE from the very beginning

● With a small sample of events [O(105)] and realistic PID:

● Extraction of yields at least for K*(892), Φ(1020), Λ*(1520)

● Rough pT - distribution for K*(892) up to 1.5-2 GeV/c

● Particle ratios Φ/K*, Λ*/K*, Φ/Λ* measurable

● Analysis of O(106) pp events at 14 TeV fully reconstructed on the GRID

● Resonance yields with large statistics

● pT-analysis

● Correction matrix (y,pT)

● Extension to other resonances in progress

reconstruction tools for the study of short-lived

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