Measurements of the dilepton continuum in ALICE. Christoph Baumann Resonance Workshop, Austin

Transcription

1 Measurements of the dilepton continuum in ALICE Christoph Baumann Resonance Workshop, Austin

2 ALICE Central Detectors: Inner Tracking System Time Projection Chamber Time-of-Flight Transition Radiation Detector Spectrometers: High Momentum PID (RICH) Photon Multiplicity Forward Multiplicity Muon Spectrometer Calorimeters: EM Calorimeter Photon Spectrometer (PHOS) Zero Degree Calorimeter Trigger: Trigger Detectors pp High-Level-Trigger Acceptance of central detectors: 0 <φ < 2π, η < 0.9 2

3 ITS SSD SDD SPD Louter=97.6 cm Router=43.6 cm 6 different layers 2xSPD: inner radius 3.9 cm, 9.8 M channels, 0.2 m2 2xSDD: 133k channels, 1.3 m2 2xSSD: 2.6M channels, 4.75 m2 3

4 TPC central electrode (100 kv) 85 m3 NeCO2N2 (85.7/9.5/4.8) 5m 557,568 readout channels 94 μs maximum drift time 10 bit ADC at 10 MHz 4

5 TOF used as PID detector rejection of non-electron tracks 5

6 TRD TRD 13 TRD modules currently installed (7 for 2010, 10 for 2011) Due to limited acceptance currently not included in PID for dielectrons Use as trigger and for enhancement of PID being evaluated 6

7 Triggers SPD2 V0A SPD1 SDD1 SDD2 pp online trigger: coincidence of beam counters and SPD V0A V0C efficiency: ~95% of σinel SSD1 V0C SSD2 PbPb MB trigger: coincidence of V0s & ZDCs Centrality triggers in Pb+Pb defined via total charge in V0 7

8 Dielectron - Motivation Measurement of Dielectrons in Pb-Pb Electromagnetic probe, no strong final state interaction Contains information from all stages of the HI-collision Continuum also allows measurement of heavy flavor particles Dielectrons in p+p Necessary as baseline for Pb+Pb Check consistency with vacuum decay of hadrons 8

9 Recent RHIC Results PHENIX STAR Preliminary Jie Zhao, QM11 Phys.Rev.C81:034911,2010 Enhancement at masses ~0.5 GeV/c2 seen by PHENIX and STAR (although with different magnitude) 9

10 Virtual Photon Measurement PHENIX reported an enhancement in the dielectron production at small invariant masses from pp to Au+Au collisons can be interpreted as signal from virtual photons 0.1 < mee < 0.3 GeV/c2 PRL104,132301(2010), arxiv:

11 ALICE Data - Pb+Pb, pp ALICE data used for analyses: pp: 350M MB events available for analysis (2010 data) Pb-Pb 2010: ~12M MB Events Pb-Pb 2011: 8M MB Events 27M Central Events 32M Semicentral Events 11

12 Analysis Strategy PID: Electron-ID in TPC & TOF yields high purity with good efficiency Effective kinematic cut-off at p ~ 0.4 GeV/c for lower momenta: TPC, exclusion of p, K additional background reduction: rejection of π0-dalitz candidates, using ITS information Outlook: Evaluate potential of TRD-PID 12

13 Invariant Mass Spectrum in pp unlike-sign distribution for pair-pt > 0.8 GeV/c combinatorial background from like-sign pairs: CB N +- =2 N + + N -- 13

14 Dielectron Signal background subtracted dielectron signal resonances up to J/Psi clearly visible 14

15 S/B Ratio S/B-ratio typically 1/10 where no resonance 15

16 S/B Ratio S/B-ratio typically 1/10 where no resonance comparable to result from muons Dielectron channel Dimuon channel 16

17 pp Results in the Dimuon Channel Invariant mass spectrum for pair pt > 1.0 GeV/c Shape reproduced by hadronic cocktail simulation arxiv: v1 [nucl-ex] 17

18 Extraction of Resonance Spectra φ yield in good agreement with measurement in Kaonchannel φ arxiv: v1 [nucl-ex] LHCb: Phys.Lett. B703 (2011)

19 Extraction of Resonance Spectra φ yield in good agreement with measurement in Kaonchannel φ/(ρ+ω) underestimated by most generators, reasonable agreement with ATLAS-CSC & PYTHIA D6T ω yield described by Perugia-0 tune φ ω arxiv: v1 [nucl-ex] 19

20 Outlook: Resonances in e+e Current developments: Optimization of PID at pt < 0.4 GeV/c Background reduction via identification of π0-dalitz candidates Further improvement of signal in mass region for ω, φ extraction of pt-spectra 20

21 pp: Virtual photon region extraction of dielectron signal low mass region comparison to cocktail simulation for meson decays Further studies needed, e.g., inclusion of open charm sources 21

22 Dielectrons in Pb+Pb Analysis strategy mostly identical to pp Identification of electrons with information from TPC and TOF Currently no background rejection strategies applied Available data 2010: MB only, ~12M events 2011: Trigger on central & semi-central event 8M MB Events 27M Central Events 32M Semicentral Events increase in events over previous year by ~ factor 10 22

23 Dielectron pairs in Pb+Pb semi-central events combinatorial background from Like-sign mix mix distribution corrected by R=N UL / N LS 23

24 Dielectron pairs in Pb+Pb central events combinatorial background from Like-sign mix mix distribution corrected by R=N UL / N LS 24

25 S/B-ratios central semi-central comparable to results from RHIC experiments 25

26 Pb+Pb: Dielectron Yield in Low Mass Region Extraction of ptdependent dielectron yield Next steps: Efficiency corrections Comparison to cocktail simulation of hadronic decays 26

27 Summary Resonance Spectra extracted in dimuon channel Analysis of Low Mass Dielectrons in pp and Pb+Pb in ALICE ongoing First results on full mass range from pp optimization of background description, Hadronic cocktail, PID Preparations for search for virtual photon signal dielectron signal extraction in pp and Pb+Pb 27

28 Backup 28

29 29

30 Recent RHIC Results PHENIX STAR Phys.Rev.C81:034911,2010 Jie Zhao, QM11 Typical S/B values at mee = 0.5 GeV/c2: pp: 10-1 to 1 AA: