Tetsuro Itabashi 　 (Osaka University)

Measurement of proton induced pion paMeasurement of proton induced pion pair production in nucleiir production in nuclei

IntroductionIntroduction

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∝ ｆ π, ｆ π

2mπ2 ～ m<qq>

• Generate pion as Nambu-Goldstone boson– Light pion mass

IntroductionIntroduction

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

T. Hatsuda, T. Kunihiro, H. Shimizu, PRL 82. 2840. (1999)

Φ=1-Cρ/ρ ０

ω(GeV)

•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 2mπ 　　 threshold region

–Mππ 　 distribution

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

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

Our ExperimentOur Experiment

p+A→π+ + π － +X reaction• Ring Cyclotron of RCNP @ Osaka university.

– Tp ～ 420MeV

• Proton beamElementary process may be simple

• Subthreshold beam energyTthre(pN) = 600MeV for elementary process– influence from secondary process is small

• Mass number dependence of Mπ+π-

CERSIUSPRL 88 (2002) 192301

Subthreshold 2π productionSubthreshold 2π production

with beam energy Tp=416MeV • invariant mass region ：　 2mπ to 350 MeV/c

2

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

Experimental set upExperimental set up

solenoidal magnet (3kG)

opening angleθ : 15º-105ºφ : 2π

Cylindrical Drift Chamber (CDC) ⇒ Momentum

Inner HodoscopesOuter Hodoscopes ⇒ Trigger,TOF

acceptance for 2π event ~ 0.6

side view

photophoto

You can see

solenoidal magnet and CDC

• σππ 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 systemTrigger system

~10 -3

π- trigger --π- trigger --DesignDesign

≧3

≦２front 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

photophoto

Efficiency of π- trigger

• Determined by geometry

• optimized for π- from subthres

hold 2π production

Efficiency of 3-coin.

pT distribution of π

Test experimentTest experiment

• Trigger performance test• Search for π+π- events

– Proton beam energy : 392MeV– Beam current : ～ 0.2nA– Target : Cu 50μm

ππ‐‐ trigger performance trigger performance

π- triggerinnerOR×outer 3 coin.

p

Inclusive triggerinnerOR×outerOR

p

π-

π-

~2.1%

Trigger type inclusive π-

trigger rate(counts/nA)

1900k 41k

preliminary resultpreliminary 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+ detectionAdditional new counters – To catch some π+ which escapes t

hrough gaps of counters– Separate p and π+ well in PID

geometry is simple for determining flight length, dE/dx etc.

Test experiment IITest 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 Particle ID by flat counter

Protons are dominant but π+ locus clearly seenWe can distinguish π+ events from proton well

π+

pppp

π+

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)

OutlookOutlook

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

SummarySummary

• 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 syste

m, we will perform 4-day experiment soon.

1.34nb

0.38nb

4.10nb