Decay-Pion Spectroscopy of Λ-Hypernuclei at MAMI

Transcription

1 Decay-Pion Spectroscopy of Λ-Hypernuclei at MAMI for the A1 Collaboration at MAMI

2 Outline Experimental setup Pioneering run 2011 Experimental run 2012 Kaon identification Preliminary results

3 This project is performed in close collaboration with Tohoku Univ. (Sendai, Japan) The analysis is done in major parts for Mainz: Anselm Esser, for Tohoku: Kyo Tsukada, Sho Nagao Mainz Microtron Continuous wave electron beam Maximum beam energy: 1.6 GeV Maximum beam current: 100 μa Spectrometer facility 3 high resolution spectrometers A/B/C, used in precisions experiments for almost 2 decades A short orbit spectrometer Kaos

4 Decay-pion spectroscopy of Λ-hypernuclei Measuring ground state binding energy of different Λ-hypernuclei by two body decay Spectrometer A and C high resolution spectrometers δp / p = 10-4 B C A Kaos as kaon tagger marking strangeness production Target Beam energy Beam current Spectrometer Kaos angle Kaos momentum : 9 Be, 22 mg/cm 2 : GeV : up to 20 μa : Kaos with (A or C) : 0 : MeV/c Kaos

5 Decay-pion spectroscopy of Λ-hypernuclei γ * e' e - K + B C A 9 Be * 9 Λ Li* e - K + Kaos * T. Motoba et al.: Photoproduction of Polarized Hypernuclei, Prog. Theor. Phys. Suppl. No. 117, 1994

6 Decay-pion spectroscopy of Λ-hypernuclei γ * e' e - K + p n γ One possible fragmentation process B π - C A π - 9 Be 9 Λ Li* 7 He Λ π - Decay at rest 7 Li e - K + Kaos

7 Decay-pion spectroscopy Accessible Hypernuclei 12 C target 9 Be target 6,7 Li target Makes a wide range of light hypernuclei accessible with one experimental method Ground state masses can be measured with uncertainties < 40 kev/c 2 Isotopes in red: Inaccessible by missing mass measurements with stable targets Allowing to test predictions for the charge symmetry breaking of the ΛN interaction in A = 7 hypernuclei (made e.g. by few-body calculations)

8 Pioneering Run 2011 Kaos multi-purpose setup Observables for PID Kaos Specific energy loss in G & H Time-of-Flight between G & H Detected Cherenkov light from β > 0.95 particles MWPCs Wall G Aerogel Cherenkov particle track Wall H SpekA / SpekC Gas threshold Cherenkov for electron suppression Combined Coincidence timing Kaos with high resolution spectrometer

9 Pioneering Run 2011 Proton cut (time-of-flight, energy loss) p π - Huge positron background from bremsstrahlung (1 MHz/μA trigger rate) p µ - p e -

10 Conclusion of 2011 The Kaos spectrometer can be operated at 0 scattering angle with all detector systems for up to 5 μa beam current Identifying different particle types and coincidences is possible, but data quality suffers from high multiplicity A clear identification of K + π - coincidences is not possible in this setup A physical suppression of positrons is necessary to reach for high luminosity

11 Simulation of an Energy Degrader Geant4 simulation of a 10 cm thick lead wall for particles at 900 MeV/c (corresponding central momentum) Positrons: stopped by electro-magnetic shower production Pions / Kaons: lost by large angle scattering out of acceptance or hadronic processes e+ π+ K+ p Protons: completely stopped for momenta below ~800 MeV/c All particles: momentum reduction by energy loss, deflection through scattering

12 Simulation of an Energy Degrader Result Segmented lead wall of 10/12/14 cm thickness Positron suppression > radiation lengths 10-3 Kaon loss of % Small angle scattering limits momentum resolution Separation power increases for slower particles absorber thickness [cm]

13 Kaos dedicated kaon tagger setup Physics Run 2012 Setup changes Momentum resolution not crucial for kaon tagging: MWPCs removed Physical suppression of positrons: cm lead energy degrader added Lead absorber Improved pion suppression: additional Cherenkov detector Wall G Wall I AC2 AC1 Wall H Maintaining tracking: one TOF wall split up in two walls

14 Drop in Kaos trigger rate from 1 MHz/μA to 1.5 khz/μa Stable data taking at 20 μa instead of 2 μa, two weeks of data aquired Still drastically reduced background Physics Run 2012 K + π - at 0 Cut: π + Cut: p Coincidence time peaks of protons and pions are easily identified, but are still strongly overlapping with possible kaon peaks

15 Data Analysis Kaon cut χ 2 Kaon < threshold Kaon Definition for PID Cut Particle specific parameters in red

16 Time-of-Flight

17 Time-of-Flight Expectation parameter adjustment for a kaon sample Cut used: and ± 1.5 ns coincidence time window < 1

18 Time-of-Flight Width parameter adjustment for a kaon sample Cut used: and ± 1.5 ns coincidence time window < 1 K + π -

19 Kaos SpekA Kaon Identification Kaos: π + Kaos: π + Kaos SpekC Kaos: K + Kaos: K + Kaos: p Kaos: p

20 Kaon Identification Kaos SpekA K + π - Kaos: K + Kaos: K + Kaos SpekC K + π - K + µ - K + e - K + µ - K + e - K + π - : K + µ - : coincident π - from weak decay coincident µ - from in-flight π - decay at target area K + e - : coincident e - from K + electroproduction (large Q 2 ) K + µ - / K + π ± ± 0.03 K + e - / K + π ± ± 0.03

21 Kaon Identification Kaos SpekA Kaos: K + / p / π + Kaos: K + / p / π + Kaos SpekC Peak ratios for K + are not reproducible by mixing those of p and π +

22 Expected Decay-Pions Signal Mesonic two body decays of hyperfragments at rest Expected ⁷ Λ He yield per week and µa: (4-8 in total) Physical background In-flight decay of hyperfragments Decay of free Λ (direct or fragmentation) Decay pion momentum [MeV/c] Decay of free Σ - Three body decays of hyperfragments

23 Decay-Pion spectra Kaon cuts used: Wide Cut (significant pion leakage) Medium Cut (low pion leakage) Narrow Cut (not negligible kaon loss) Additionally coincidence time cuts of ±0.5 ns, ±1.0 ns, and ±1.5 ns around the indicated peak center are used

24 Kaos SpekA Decay-Pion spectra 100 kev/c / bin ± 0.5 ns ± 0.5 ns ± 0.5 ns ± 1.0 ns ± 1.0 ns ± 1.0 ns ± 1.5 ns ± 1.5 ns ± 1.5 ns

25 Kaos SpekC Decay-Pion spectra 100 kev/c / bin ± 0.5 ns ± 0.5 ns ± 0.5 ns ± 1.0 ns ± 1.0 ns ± 1.0 ns ± 1.5 ns ± 1.5 ns ± 1.5 ns

26 π - Background Reaction vertex along beam direction K + π - : centre at 5.9 mm K + random coincidence: centre at 2.7 mm Vertex displacement suggests decay of free unstopped Σ -

27 Outlook Short term goal Finalize analysis of 2012 data Mid term goal Current analysis suggests statistics and Σ - decays as limiting issues Continuation of the program with luminosity increased by one order of magnitude; scheduled for spring 2014

28 Improvements & Preparations Target: stack of three cooled 6 Li strips high momentum resolution reduced random π - background high two body decay rate of directly produced 6 He Λ Li cooling tested up to 30 μa add absorber for unstopped particles 91 Beam Kaos 50 mm spectrometer acceptance 126 High resolution spectrometers: new absolute momentum calibration with new target geometry direct connection to scattering chamber (no kapton windows) implement coincidence trigger of two scintillator layers with high pion detection efficiency to remove neutron events (~80%) Kaon spectrometer: improved shielding of Kaos (inaccessible during beam time), reduced single trigger rate by ~30% change chicane dipole position to adjust central momentum (~20% lower) and absorber thickness

29 Thank you for your attention Collaboration list Institut für Kernphysik, Johannes Gutenberg-Universität, Mainz, Germany: Patrick Achenbach, Carlos Ayerbe, Ralph Böhm, Michael O. Distler, Anselm Esser, Mar Gomez, Alicia Sanchez-Lorente, Harald Merkel, Ulrich Müller, Josef Pochodzalla, Takehiko Saito, Björn Sören Schlimme, Matthias Schoth,, Concettina Sfienti, Adrian Weber University of Ljubljana and Institut "Josef Stefan", Ljubljana, Slovenia: Luka Debenjak, Simon Sirca Department of Physics, University of Zagreb, Croatia: Damir Bosnar, Ivica Friscic Department of Physics, Hampton University, VA, USA: Liguang Tang Department of Physics, Florida International University, Miami, FL, USA: Joerg Reinhold Yerevan Physics Institute, Yerevan, Armenia: Amur Margaryan Department of Physics, Tohoku University, Sendai, Japan: Osamu Hashimoto, Satoshi N. Nakamura, Kyo Tsukada, Toshiyuki Gogami, Sho Nagao GSI, Darmstadt, Germany: Olga Borodina, Vakkas Bozkurt, Eunhee Kim, Christophe Rappold