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Measurement of proton induced pion pair production in nuclei

Measurement of proton induced pion pair production in nuclei. Tetsuro Itabashi (Osaka University). Introduction. QCD and spontaneous breaking of Chiral Symmetry quarks & qluons ⇒ hadrons at low energy Quark and anti-quark pair condensation <qq>=0 ⇒ <qq>≠0 hadrons have effective mass

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Measurement of proton induced pion pair production in nuclei

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  1. Measurement of proton induced pion pair production in nuclei Tetsuro Itabashi(Osaka University)

  2. Introduction QCD and spontaneous breaking of Chiral Symmetry • quarks & qluons ⇒ hadrons at low energy • Quark and anti-quark pair condensation • <qq>=0 ⇒ <qq>≠0 • hadrons have effective mass • M∝ｆπ, ｆπ2mπ2～m<qq> • Generate pion as Nambu-Goldstone boson • Light pion mass

  3. Introduction How to confirm it experimentally • Phase transition at high energy : <qq>0 • New phase of matter – QGP • Heavy ion collider RHIC, LHC • Partial restoration in nuclear medium • Meson mass shift in nuclear medium ρ/ω/φ(KEK E325) σ(CHAOS,TAPS) quark condensate

  4. σmeson ( f0(600) or σ : I,Jp=0,0+ ) • Chiral partner of pion • Change of spectral function at finite nuclear density • Enhancement at just above 2mπthreshold region • Mππdistribution Φ=1-Cρ/ρ０ ω(GeV) T. Hatsuda, T. Kunihiro, H. Shimizu, PRL 82. 2840. (1999)

  5. CHAOS • π+A→π+π－A’ reaction • heavier nuclear target, remarkable enhancement only in sigma channel Crystal ball • π-+A  π0+π0+X • No sharp peak

  6. TAPS • γA→π0π0A’ reaction • RPb/C has mass shift to lower Invariant mass region Elementary process seems to be simple Still not conclusive  new type of experiments are desirable

  7. Our Experiment p+A→π+ +π－ +X reaction • Ring Cyclotron of RCNP @ Osaka university. • Tp～420MeV • Proton beam Elementary process may be simple • Subthreshold beam energy Tthre(pN) = 600MeV for elementary process • influence from secondary process is small • Mass number dependence of Mπ+π- CERSIUS PRL 88 (2002) 192301

  8. Subthreshold 2π production with beam energy Tp=416MeV • invariant mass region ：　2mπ to 350 MeV/c2 Enough to see around threshold region ( observed in Mπ+π- <300MeV/c2 ) • cross section ～1nb/nucleon simple calculation using • elementary pp→π+π－pp cross section data • Fermi motion distribution of nuclei • nucleon binding in nuclei

  9. solenoidal magnet (3kG) • opening angle • θ : 15º-105º • φ : 2π • Cylindrical Drift Chamber (CDC) • ⇒ Momentum • Inner Hodoscopes • Outer Hodoscopes • ⇒ Trigger,TOF • acceptance for 2π event ~ 0.6 Experimental set up side view

  10. photo You can see solenoidal magnet and CDC

  11. Trigger system • σππis very small compared to proton scattering • efficient trigger system is needed proton scattering : σ～ a few 10mb pn→ppπ- : σ～ 50μb pN→pNπ+π- : σ～ 1nb negative charge trigger is quite useful for rare π+π- ~10 -3

  12. π- trigger --Design ≧3 Tilted counters with magnetic field distinguish charge by # of hit negative particle : ≧ 3 hits positive particle : ≦ 2 hits Requiring 3 adjacent hits ⇒ π- trigger ≦２ front view

  13. photo Hodoscopes before installing CDC

  14. Efficiency of 3-coin. Efficiency of π- trigger • Determined by geometry • optimized for π- from subthreshold 2π production pT distribution of π

  15. Test experiment • Trigger performance test • Search for π+π- events • Proton beam energy : 392MeV • Beam current : ～0.2nA • Target : Cu 50μm

  16. π‐ trigger performance Inclusive trigger innerOR×outerOR π- trigger innerOR×outer 3 coin. p p π- π- ~2.1%

  17. preliminary result Search for π+ π - like pairs One π- and another h+ 8 hours run(2.7h live time) Sensitivity down to ～0.1nb/nucleon Low statistics

  18. Improvement for π+ detection Additional new counters • To catch some π+ which escapes through gaps of counters • Separate p and π+ well in PID geometry is simple for determining flight length, dE/dx etc.

  19. Test experiment II To check performance of new counters, a part of counters were placed flatly • proton beam energy : 392MeV • Beam current : ～ 0.1 nA • Target : Cu 50μm

  20. Particle ID by flat counter preliminary Protons are dominant but π+locus clearly seen We can distinguish π+events from proton well π+ p p π+

  21. Outlook Further trigger improvement for high statistics • Combination of outer & inner hodoscopes • Combination of outer & CDC wire hit require 2-cluster of them 2-track event/all～1/10 Trigger logic w/ ULM (FPGA base)

  22. 4 days experiment with one target is approved • Establish p+p- measurement • Present yield is too few • Discuss p+p- production more clearly • Mpp, ppp and qpp distributions • First step to measure A-dependence • At least 2 more targets

  23. Summary • Medium modification of π+π- pair in sigma channel is still not conclusive • We are preparing another experiment, proton induced subthreshold pion pair production in nuclei • large acceptance detector and effective π- trigger • π+π- like events are seen, but low statistics • With additional detectors and improved trigger system, we will perform 4-day experiment soon.

  24. 4.10nb 1.34nb 0.38nb

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