Reconstruction of neutral mesons via conversion in Au+Au at 1.23 - PowerPoint PPT Presentation

Reconstruction of neutral mesons via conversion in Au+Au at 1.23 GeV/u with HADES e + material α 1 e - π 0 , η θ γγ e + α 2 Claudia Behnke for the HADES collaboration e -

Motivation Heavy-ion collision at 1-2 GeV/u First-chance NN Hot and dense stage Freeze-out ~ collisions ( 10 fm/c) stage Baryonic sources: Long-lived mesons: In-medium  NN Bremsstrahlung  π 0 ,η, ω spectral functions  ∆ ,N* → Ne + e - τ π = 7.7 × 10 7 fm/c τ η = 1.5 × 10 5 fm/c τ ω = 23 fm/c C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 2/19 26-30.01.15

The freeze-out “Cocktail” π 0 , η → γ e + e - ✔ Mult π0 is essential for spectrum normalization ✔ “no Mult η , no Mult Excess !” Phys.Rev. C84 (2011) How to reconstruct π 0 and η ? C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 3/19 26-30.01.15

The freeze-out “Cocktail” Phys. Rev. C,67:024903, 2003. 1.23 Uncertainties experiment simulation η Multiplicity of π 0 and η mesons, as a Mult AuAu ( π 0 ) = 6.4 ± 15% function of beam energy and system size. Au+Au points from Mult AuAu ( η ) = 0.09 ± 50% extrapolation to the measured data. No measurements at 1.23 GeV/u Phys.Rev. C84 (2011) 014902 C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 4/19 26-30.01.15

THE HADES AT GSI, DARMSTADT, GERMANY  Beams provided by SIS18: proton, nuclei (E Beam 1~2 GeV/u )  Secondary beam: π  High Acceptance (Fixed target)  Full azimuthal coverage  18° to 85° in polar angle  Hadron and lepton identification:  Tracking with 4x6 Multiwire Drift Chambers and superconducting magnet  Time of flight measurement with ToF and RPC Walls  Specific energy loss in MDC and ToF 27/01/15 5  RICH and shower detectors to identify leptons

How to measure π 0 and η with HADES? conv π 0 ,η → γγ ⇒ e + e - e + e - conv π 0 ,η → γ e + e - ⇒ e + e - e + e - RICH radiator gas (C4F10) Target holder (C) Beam pipe (C) How to reconstruct Identifying 4 Target(Au) γ without dedicated leptons using photon detector? photon conversion! Material Conv [%] ( π 0 ) Conv [%] ( η ) Target 0.05 0.09 0.05 0.08 δ -shield Beam pipe 0.04 0.06 Radiator gas 0.11 0.18 Mirror 0.08 0.14 Sum* 0.36 0.58 STAR: PhysRevC.70.044902 *all values are obtained from Geant3 simulations! CDF: PhysRevD.70.074008 HADES: - p+Nb 3.5 GeV: ALICE: Phys. Lett. B 717,162 PhysRevC.88.024904 C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 6/19 26-30.01.15

The analysis strategy - part 1 Lepton identification: e - e +  Momentum < 1000 MeV/c π - π + p  Momentum dependent K + He 3 d/He 4 velocity cuts  No RICH information t is used  Sharing of inner MDC segments is allowed p · sign |q| [MeV/c] C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 7/19 26-30.01.15

The analysis strategy - part 2 Combine 4 fully reconstructed lepton candidates  Background rejection cuts: π 0 → γγ η → γ ∗ γ Topological cuts on angles η → γγ between e + e - and γ : γγ e + e - e + e -  α 1 <2.5º, α 2 <20º  π 0 :10º< Θ γγ < 40º e +  η :40º< Θ γγ < 140º α 1 e - π 0 , η material θ γγ e + α 2 UrQMD simulation e - C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 8/19 26-30.01.15

π 0 statistics from 4.3 * 10 9 high multiplicity Au+Au events e + material α 1 P e - R E π 0 L I M θ γγ I N A R Y e + α 2 e -  Background rejection cuts: Topological cuts on angles between e + e - and γ :  α 1 <2.5º, α 2 <20º  π 0 : 10º< Θ γγ < 40° 27/01/15 Claudia Behnke 9 Background estimated using event mixing technique (red line)

Motivation for event-mixing technique Uncorrelated γγ pairs are the π 0 → γγ main background. π 0 → γ ∗ γ γγ π 0 γ ∗ γ e + eeeX e - e + e - π 0 UrQMD simulation analysed with same method C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 10/19 26-30.01.15

π 0 per Event  Spectrum after background subtraction is corrected with ε tot and normalized to the number of analysed events (4.3 •10 9 ) P R E  Integration of the spectra in L I M the 2σ range gives I N A Mult( π 0 ): 9.1 ± 3 R Y  Comparable with result of charged π from FOPI Mult( π 0 ): 10.4 and TAPS Mult( π 0 ): 6.4 (Min bias!) C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 11/19 26-30.01.15

Phase space coverage Rapidity coverage: Transverse momentum coverage:  π 0 with low momentum are not covered due to the acceptance of the decay products in HADES Coverage: -0.25 <Y cm <1 Coverage: 300 MeV/c <p ⊥ <650 MeV/c C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 12/19 26-30.01.15

Phase space coverage Invariant mass for different phase space bins, together with event-mixing P R E L I M I N A R Y C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 13/19 26-30.01.15

Yields of π 0 as function of m ⊥ - m π 0 PRELIMINARY  The efficiency corrected differential yields, together with Boltzmann fits  Single slope fit can describe the data  Points with large corrections are excluded from fit C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 14/19 26-30.01.15

Yields of π 0 as function of m ⊥ - m π 0  Filled circles: PRELIMINARY measured points  Open circles: reflected around mid-rapidiy  Resulting slopes can be fitted with a 1/cosh(y) distribution  Inverse slope parameter at mid-rapidity: T B (Y cm =0): 76 ± 15 MeV C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 15/19 26-30.01.15

η statistics from 4.3 * 10 9 high multiplicity Au+Au events e + material α 1 e - Y R η A N θ γγ I M I L E R P e + α 2 e -  Background rejection cuts: Topological cuts on angles between e + e - and γ :  α 1 <2.5º, α 2 <15º  η : 40º< Θ γγ < 140° 27/01/15 Claudia Behnke 16 Background estimated using event mixing technique (red line)

η background - subtracted  Integration of the spectra in the 2σ range 207 gives raw η counts: #( η ): 551 ± 207 PRELIMINARY  Corrections not yet done C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 17/19 26-30.01.15

Summary  Reconstruction of π 0 and η via γ conversion PRELIMINARY HADES  Number of π 0 per event:  Mult( π 0 ): 9.1 ± 3  Phase space coverage of π 0 signal:  - 0.25 <Y cm <1.0  300 <p ⊥ <650 MeV/c  Reconstructed inverse slope:  T B (Y cm = 0): 76 ± 15 MeV  Number of η in full beam time  #( η ): 551 ± 207 C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 18/19 26-30.01.15

Outlook  A part dependent analysis of π 0 and η  Phase space dependent analysis of η  Systematic error estimations  Compare the resulting cross sections with the results from dilepton analysis  EM Calorimeter will be added for SIS100, FAIR C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 19/19 26-30.01.15

Thank you for your attention The HADES Collaboration 27/01/15 Claudia Behnke 20

Backup Slides e + material α 1 e - π 0 , η θ γγ e + α 2 27/01/15 Claudia Behnke 21 e -

Efficiency and acceptance estimation  Simulation:  10 11 π 0 with Pluto Event Generator [PoS ACAT2007 076] T 1 = 39 MeV, T 2 = 79 MeV  Propagation through detector geometry using Geant3 package  ε Acc  Tracking and event selection like in measured data  ε 4e reco  Conversion and Branching Ratio: 2 + BR e+e- γ · conv γ  BR γγ · conv γ  Identification of leptons, γ and π 0  ε PID  ε Tot = ε Acc · ε 4e 2 + BR e+e- γ · conv γ ) · ε PID reco · (BR γγ · conv γ = 1.7% · 3.5% · (3.2 · 10 -4 ) · 44% = 8.99 · 10 -8 C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 22/19 26-30.01.15

Au+Au Run @ 1.23 AGeV, April 2012 HADES Au+Au beam time ✔ 24 days Au beam ✔ 8 kHz trigger rate ✔ 7.3 billion events ✔ Trigger on high multiplicity events (40% of most central collisions) Simulations ✔ UrQMD transport model ✔ Transport through the detector system using Geant3 and realistic digitizers Analysis of experimental and simulated data is identical C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 23/19 26-30.01.15

Challenges of (di)lepton reconstruction ✔ Low momenta ✔ Small opening angle – in 93% cases α is <3◦ - One lepton can be bend – RICH ring finder will often out by the magnetic field identify only 1 ring beam behind the inner MDCs - Reconstruction efficiency is between 15 - 55% α <3◦ 11% of e + /e - p< 50 MeV/c: ε = 15% 44% of e + /e - p< 100 MeV/c: ε = 55% Pluto p · sign |q| [MeV/c] C. Behnke - 53rd International Winter Meeting on Nuclear Physics (Bormio, Italy) 24/19 26-30.01.15