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

Collaboration T. Itabashi, S. Ajimura, T. Hayakawa, T. Kishimoto, K. Matsuoka, S. Minami, A. Sakaguchi, Y. Shimizu, K. Sugita and K. Terai Osaka University T. Ogaito and K. Tamura Fukui Medical University H. Noumi and M. Sekimoto KEK T. Fukuda, W. Imoto and P.K. Saha Osaka Electro-communication University

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

Introduction How to confirm it experimentally Phase transition at high energy :  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

T. Hatsuda, T. Kunihiro, H. Shimizu, PRL (1999) Φ=1-Cρ/ρ ０ ω(GeV) σmeson ( f 0 (600) or σ : I,J p =0,0 + ) –Chiral partner of pion –Change of spectral function at finite nuclear density –Enhancement at just above 2m π threshold region –M ππ distribution

CHAOS π + A→π + π － A ’ reaction heavier nuclear target, remarkable enhancement only in sigma channel Crystal ball –π - +A  π 0 +π 0 +X –No sharp peak

TAPS γA→π 0 π 0 A ’ reaction R Pb/C has mass shift to lower Invariant mass region Elementary process seems to be simple Still not conclusive  new type of experiments are desirable

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

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

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

photo You can see solenoidal magnet and CDC

σ ππ is 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- Trigger system ~10 -3

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

Hodoscopes before installing CDC photo

pulse height cut at proton peak For all 3 adjacent counters hit π-π- p upper threshold lower threshold ADC of Outer hodoscope accept π ‐ trigger --Pulse Height cut π ‐ trigger --Pulse Height cut

Efficiency of π - trigger Determined by geometry optimized for π - from subthreshold 2 π production Efficiency of 3-coin. p T distribution of π

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

innerOR×outer3coin.OR π - trigger (innerOR×outer3coin.OR ×pulse height cut) p Inclusive innerOR×outerOR p p π-π- π-π- π-π- Trigger typeinclusive3-coin. 3-coin w/ p.h. cut trigger rate (counts/nA) 1900k41k7.4k 2.2%0.34% π ‐ trigger performance π ‐ trigger performance

π- trigger innerOR×outer 3 coin. p Inclusive trigger innerOR×outerOR p π-π- π-π- ~2.1% Trigger typeinclusiveπ- trigger rate (counts/nA) 1900k41k

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

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.

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

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

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)Outlook

4 days experiment with one target is approved –Establish     measurement Present yield is too few –Discuss     production more clearly M , p  and   distributions –First step to measure A-dependence At least 2 more targets

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.

1.34nb 0.38nb 4.10nb