The Excitation Function of the Omega Meson in Medium

Transcription

1 Omega Meson in CB Meeting Edinburgh II. Physikalisches Institut Justus-Liebig-Universtiy Gießen 14th of September 009

2 Outline E γ < 900 MeV

3 Deviation from excitation function on free nucleon expected, depending on scenario [Mühlich and Mosel, Nucl. Phys. A773 (006)]. Different approach to in-medium modifications than direct signal shape analysis.

4 Final state: γγγ + p/n. Two ways of analysis possible: Incl. (n γ = 3 or n γ = 4): Much better statistics, more complicated background. Excl. (n γ = 3, n p = 1): Proton ID, clean signal. Incl. preferred for excitation function (low statistics below threshold of 18 MeV).

5 Protons in the Exclusive Branch CB_DeltaE17_v_Echarged17 TACB008 Entries Mean x Mean y RMS x RMS y Proton ID with the PID E [MeV] de [MeV] Neutral and charged hits in TAPS neutral charged Veto dead time effects Test7_TAPS_neutral_times Entries e+07 Mean RMS 18.7 [ns] P time time [ns] TAPS_proton_TOF_dEcut Proton ID via TOF in TAPS TAPS_proton_TOF_dEcut Entries Mean x Mean y RMS x 0.6 RMS y P E [MeV] Neutral hits in TAPS Misidentified protons in the neutral TOF spectrum Test8_TAPS_neutral_TOF Entries Mean x 85.4 Mean y RMS x RMS y Time [ns] E [MeV] Proton ID by the PID or by TOF and/or de/e in TAPS. Veto dead time for high rates leads to misidentification. Warning: consider dead time for high rates or high Z targets!

6 incl_cbonly_mass_prompt_e7t8 incl_cbtapsmixed_mass_prompt_e7t8 π γ 4 3 mass for CB photons only, prompt, inclusive all γ s in CB Prompt+background Normalized background Entries 6718 Mean 65.3 RMS π γ Counts / 1 MeV 3 mass (CB+TAPS), prompt, incl 1 Prompt+background Normalized L1 SB Normalized L SB Normalized L3 SB Events with TAPS contrib. Entries 7949 Mean 54.8 RMS E [MeV] Blue BG: η prompt CB +γ TAPS 700 MeV < E γ < 800 MeV Separation of events with and without TAPS contribution. Detailed treatment of background. Main contribution from random tagger hits. Background subtraction strongly suppresses ω signal for E γ < 900 MeV!

7 π Sideband Subtraction γ mass vs. 3 γ mass, CB only, prompt, incl Pion_Sideband_d_prompt_CBonly_incl_total_raw 00 mγ γ [MeV] π η, one γ from each Entries e+07 Mean x 71.9 Mean y RMS x RMS y Entries Mean RMS twogamma_proj_cbonly twogamma_proj_cbonly mγ γ γ [MeV] Signal and Sidebands, CB only threegamma_proj_cbonly_raw threegamma_proj_cbonly_cleaned threegamma_proj_cbonly_cleaned Entries Mean 54.8 RMS Entries.17634e+07 5 Mean 69. RMS MeV < mπ < 160 MeV 1 75 MeV < mπ < 0 MeV MeV < mπ < 195 MeV Massγ γ /MeV π SBs subtracted Massπ γ /MeV Massπ γ /MeV Large background contribution from πη 4γ with one γ not detected. I Background is reduced by π sideband subtraction. I Method includes timing background subtraction. I

8 A final ω spectrum Counts/1MeV Omega Spectrum for 600 MeV < E < 1400 MeV, incl, whole Apr08 run γ 3 m ω σ ω = 781 MeV = 7 MeV ω s: clone1rb Entries Mean 711 RMS χ / ndf.486e+06 / 494 p ± 4.8 p ± 0.9 p 6.95 ± 0.70 p ± p ± 0.01 p6.687e-05 ± 1.733e / MeV m π 0 γ Calibration stable up to high energies by second order corrections ( mω m ω < 1%, fit dependent) ω mesons (all E γ ) in the April Nb run.

9 clone1rb Entries Mean RMS χ / ndf.77e+05 / 494 p ±.04 p1 784 ±.0 p 5.83 ± 1.54 p ± p4 0.7 ± 0.0 p e-05 ± 5.3e-08 0 clone1rb Entries Mean RMS χ / ndf.483e+05 / 494 p ± 1.19 p ±.9 p 5.83 ±.88 p ± p ± 0.0 p e-05 ± 5.961e-08 0 E γ < 900 MeV Counts/1MeV Omega_final_et11_incl 3 00 MeV < E γ < 10 MeV m π γ / MeV Omega_final_e8t9_incl MeV < E γ < 900 MeV > 00 MeV below threshold! Omega_final_e8t9_incl Entries 9539 Mean 51 RMS Mass /MeV Counts/1MeV Omega_final_e9t_incl MeV < E γ < 00 MeV m π γ / MeV E thresh = 18 MeV ω signal observed at incident photon energies below threshold (18 MeV). ω signal coming closer to the edge and getting weaker. Signal very sensitive to BG subtraction. BG has to be correctly subtracted. Using GiBUU for BG simulations.

10 TAKensLadder nice part of the beam time Ladder_ScalerAcc Entries Mean RMS TAKensLadder 11 9 Ladder_ScalerAcc Entries Mean RMS another problematic of 7 part the beam time TAKensLadder 11 Ladder_ScalerAcc Entries Mean RMS problematic part of the beam time TAKensLadder the whole beam time Ladder_ScalerAcc Entries e+08 Mean 41.6 RMS Total tagger scalers needed to get the photon flux for the cross section determination. Clean spectrum distorted by low channel blocks. Observed in all runs (April, June, August 08). Config file or readout problem?

11 First Try of an Deviation from excitation function on the free nucleon established. Photon flux yet only normalized to 1 E γ. No acceptance correction. Background fitted.

12 Detailed study of background from random coincidences was performed and false events were subtracted. Pion sideband subtraction technique enables to clean the spectra from πη background. Got a first glimpse of the excitation function. New source for inefficiency found (Veto Wall dead times). Photon flux still needed (does someone have an idea?). 4 γ branch will be included (contribution of % expected). GiBUU simulations will help to describe background and test different in-medium scenarios. Excitation function will be analyzed in finer E γ binning and also for the Carbon run.

13 Backup Slides

14 Veto Dead Time for High Currents and/or high Ztarget Left: Nb run (1 na). Dead time effects decrease with TAPS ring number. Right: C run (6 na). Dead time effects not that pronounced. (Time still running from the right to the left.)

15 ω fit for 800 MeV to 900 MeV E γ

16 Branches Excl.: Nice proton ID, clear signal. Incl.: Much better statistics, more complicated background. Excl. for shape analysis, incl. for excitation function (low statistics below threshold).

17 nd Order Corrections For The Energy Calibration (TAPS) For a calibrated π the η mass is deviating, increasing with ring number. without nd order correction factor mη m η A ringwise correction factor ( nd order) helps here. with nd order correction factor mη m η

18 de vs. E (TAPS) de vs. E with TAPS