measurement of proton induced pion pair production in nuclei

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