1. Study of hadronic resonances in the ALICE experiment
Alberto Pulvirenti
WISH'2010
Catania, 8-10 September 2010

2. Outline Motivations Analysis of LHC pp collisions @ 900 GeV
First results from analysis of LHC pp 7 TeV
Conclusions & outlook
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3. Motivations pp collision perspective Heavy-ions collisions perspective
test for QCD-inspired models of particle production
Heavy-ions collisions perspective
hadronic decays  study the evolution of fireball
interplay of rescattering and regeneration due to daughters interaction with surrounding medium
Strangeness:
pp collisions provide the comparison baseline
φ  strangeness enhancement vs. canonical suppression
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4. ALICE Overview
Strip
Drift
Pixel
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5. Data samples 900 GeV: 7 TeV Data Simulation Pulvirenti - WISH 2010
MB data from LHC pp 900 GeV
first LHC runs (november-december 2009)
Simulation
production of LHC pp 900 GeV with PYTHIA (tune D6T)
realistic simulation of ALICE detector geometry through GEANT3
7 TeV
selection of MB runs taken in march-april 2010
production of LHC pp 7 TeV with PYTHIA (tune D6T)
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6. Vertex reconstruction precision
Event selection
Good physics events:
have the correct event type (physics)
have the interaction trigger, i.e. trigger on bunch crossings
have at least 1 fired SPD chip or a beam-beam flag in one of the two V0 detectors
is not flagged as beam-gas by either V0A or V0C
Primary vertex quality
from tracks or SPD only
|vZ | ≤ 10 cm
Sample 900 GeV
data: ~250k events
sim: ~1.8M events
Sample 7 TeV
data: ~ 8M events
sim: ~ 1.6M events (being processed on larger sample)
Vertex reconstruction precision
ALICE performance
April 2010
ALICE Preliminary
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7. Track selection Quality assurance Primary tracks
check and cut on all track quality parameters, in order to select those with best resolution
number of track clusters, chi2
Primary tracks
resonances' daughters are indistinguishable from primaries, then they can be subject to the same cuts
cut on primary vertex reconstruction quality (event level)
cut on track impact parameter (track level)
Particle identification
to reduce the combinatorial background coming from misidentified “true” primaries or daughters from other resonances or from secondary vertices close to primary
comparison between track measured signal and expectation for a Kaon
 reduce background without killing too much signal
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8. Particle IDentification
Main contributors to present analysis: ITS, TPC and TOF
PID information currently implemented as a cut on compatibility between detector signal and expected response
TOF
ITS
TPC
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9. Resonances in ALICE pp 17 M min. bias events Decay channel Σ*(1385)
Fundamental baseline for heavy-ion studies aimed at investigating the fireball evolution
Σ*(1385)
76 M of MB events
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10. φ analysis: operative strategy
φ studied through the K+K- channel (BR = 49.2%)
Fixed rapidity range: | y | ≤ 0.6
select the region where efficiency is almost constant
Major detector contributions:
ITS  primary vertex, PID, track reconstruction
TPC  PID, track reconstruction
TOF  PID (when possible)
ITS PID used only in 7 TeV sample, for tracks reconstructed with ITS stand-alone after the first global tracking step.
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11. 900 GeV: peak extraction 1.0 ≤ pT ≤ 1.5 GeV/c Peak range = mPDG ±4σPDG
Build invariant mass distribution
Fit invariant mass distribution with a background function + Gaussian
Number of φ from unbinned counting:
Total entries in the peak – BG function integral inside the peak
1.0 ≤ pT ≤ 1.5 GeV/c
Peak range = mPDG ±4σPDG
Where σPDG = ΓPDG /(8ln2)0.5
Fit range  smallest 2 / NDF
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12. Corrected spectrum: fit
Errors = stat + sys
Small sample size (250k evts)  analysis feasible in 4 pT bins only
Statistical error dominates over systematics j
pp INEL √s = 900 GeV
Spectrum fitted with a Levy function
dN/dy  sum of measured histogram integral + fit function extrapolation (49%)
<pT>  fit function in whole range
dN/dy ± ± 0.002
<pT> ± ± 0.15 GeV/c
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13. Comparison with generatorsφ
K0s Λ
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14. 7 TeV: peak extraction Background: 3rd order polynomial outside peak
Subtract background
Fit background-subtracted spectrum with a Breit-Wigner function
Raw counts from full integral of fitted Breit-Wigner
Example: 1.8 ≤ pT ≤ 2.0 GeV/c
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15. 7 TeV: pT binning strategy (in GeV/c)
It is possible to extract the peak in very small pT bins
0.6 – 0.8
0.4 – 0.6
ALICE Performance - 30/08/2010
0.8 – 1.0
1.0 – 1.2
1.2 – 1.4
1.4 – 1.6
1.6 – 1.8
1.8 – 2.0
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16. φ resonance measured in LHC pp collisions @ 900 GeV
exploited status-of-the-art PID detectors capabilities
low statistics collected in 2009 allowed the study in few bins, but allowed to provide the first measurement of the φ at this energy
improvement foreseen with the large sample collected in 2010
measured yield at mid-rapidity and average pT:
dN/dy = ± ± 0.002
<pT> = ± 0.14 ± 0.15 GeV/c
promising preliminary results from 7 TeV analysis
possibility to extract the peak in very small pT bins
spectrum can extends down to 0.4 GeV and up to 10 TeV
improvements foreseen when exploiting all the available statistics
Work in progress for other resonances, in view of heavy-ion collisions coming at the end of the year
peaks extraction, spectra, yields
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17. Backup
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18. Mass & width Pulvirenti - WISH 2010 28/11/2018
At small pT mass is underestimated due to acceptance and momentum resolution
Width is overestimated due to invariant mass resolution
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19. 900 GeV: counts and efficiency
Small sample size (250k evts)  analysis feasible in 4 pT bins only
Statistical error dominates over systematics
Efficiency vs. pT,
computed with PYTHIA
Raw counts in the 4 chosen bins
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20. Track selection - TPC TPC PID Pulvirenti - WISH 2010 28/11/2018
pTPC < 350 MeV/c  ±5σ (to compensate some shift w.r. to computed BB)
pTPC ≥ 350 MeV/c  ±3σ
σ = 6 %
Checked quantity: relative difference between TPC signal and Bethe-Bloch with Kaon mass hypothesis:
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21. Track selection - TOF TOF PID Pulvirenti - WISH 2010 28/11/2018
measured value: tTOF = TOF signal – collision “time zero” (estimated offline)
reference value: tK = integrated time with K mass hypothesis
compatibility =
Accepted band defined by two hyperbolas
TOF PID is used only if the TOF has given a reliable PID signal,
otherwise: track identified by TPC only
Don’t affect the efficiency
Reduce the background
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22. Main specific systematic error sources
min – avg – max
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23. Influence of TOF check
pT (GeV/c)
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24. Track selection - II DCA (r) DCA (z) cm cm nTPC 2
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25. φ spectra: past results
AFS Collab., Nucl. Phys. B203 (1982) 27
STAR Collaboration, PRC 79 (2009), 64903
RHIC results 200 GeV, NSD:
- dN/dy = ± 0.01 ± 0.003
- <pT> 812 ± 30 ± 41 MeV/c
ISR 63 GeV
- dN/dy = ± ± 0.03
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