measurement of proton induced pion pair production in nuclei
DESCRIPTION
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 =0 ⇒ ≠0 hadrons have effective mass - PowerPoint PPT PresentationTRANSCRIPT
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Tetsuro Itabashi (Osaka University)
Measurement of proton induced pion paMeasurement of proton induced pion pair production in nucleiir production in nuclei
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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
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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
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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
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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
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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
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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
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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
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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
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photophoto
You can see
solenoidal magnet and CDC
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• 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
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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
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Hodoscopes
before installing CDC
photophoto
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Efficiency of π- trigger
• Determined by geometry
• optimized for π- from subthres
hold 2π production
Efficiency of 3-coin.
pT distribution of π
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Test experimentTest experiment
• Trigger performance test• Search for π+π- events
– Proton beam energy : 392MeV– Beam current : ~ 0.2nA– Target : Cu 50μm
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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
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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
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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.
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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
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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
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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
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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
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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.
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1.34nb
0.38nb
4.10nb