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Study of hadronic resonances in the ALICE experiment
Alberto Pulvirenti WISH'2010 Catania, 8-10 September 2010
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Outline Motivations Analysis of LHC pp collisions @ 900 GeV
First results from analysis of LHC pp 7 TeV Conclusions & outlook Pulvirenti - WISH 2010 28/11/2018 2
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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 Pulvirenti - WISH 2010 28/11/2018
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ALICE Overview Strip Drift Pixel Pulvirenti - WISH 2010 28/11/2018 4
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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) Pulvirenti - WISH 2010 28/11/2018 5
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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 Pulvirenti - WISH 2010 28/11/2018 6
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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 Pulvirenti - WISH 2010 28/11/2018 7
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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 Pulvirenti - WISH 2010 28/11/2018 8
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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 Pulvirenti - WISH 2010 28/11/2018 15 9
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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. Pulvirenti - WISH 2010 28/11/2018 10
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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 Pulvirenti - WISH 2010 28/11/2018 11
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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 Pulvirenti - WISH 2010 28/11/2018 12
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Comparison with generators
φ K0s Λ Pulvirenti - WISH 2010 28/11/2018 13
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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 Pulvirenti - WISH 2010 28/11/2018 14
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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 Pulvirenti - WISH 2010 28/11/2018 15
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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 Pulvirenti - WISH 2010 28/11/2018
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Backup Pulvirenti - WISH 2010 28/11/2018 17
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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 Pulvirenti - WISH 2010 28/11/2018 18
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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 Pulvirenti - WISH 2010 28/11/2018 19
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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: Pulvirenti - WISH 2010 28/11/2018 20
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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 Pulvirenti - WISH 2010 28/11/2018 21
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Main specific systematic error sources
min – avg – max Pulvirenti - WISH 2010 28/11/2018 22
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Influence of TOF check pT (GeV/c) Pulvirenti - WISH 2010 28/11/2018 23
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Track selection - II DCA (r) DCA (z) cm cm nTPC 2
Pulvirenti - WISH 2010 28/11/2018 24
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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 Pulvirenti - WISH 2010 28/11/2018 25
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