Study on Λ-Hypernuclei with the Charge-Exchange Reactions

Atsushi Sakaguchi (Osaka Univ.)

Collaboration:Osaka U., KEK, Osaka E. U., RIKEN, Seoul Natl. U., JAEA, U. Torino, INFN

and INAF-IFSI

Proposal for J-PARC 50GeV PS

2

AbstractStudies on Λ-hypernuclei

with Charge-Exchange (CX) reactions

Proposal of experiments (at Day-1)Exp.1: Production of neutron-rich hypernuclei

Production of exotic hypernuclei⇒ Λ−N interaction in neutron-rich environment⇒ EoS and structure of neutron stars

Exp.2: Non-mesonic weak decay(NMWD) of 4ΛHeSpin-isospin structure of Λ-Nucleon weak int.⇒ “ΔI=1/2 rule” in NMWD of hypernuclei

3

Atomic NucleiStable carriers of baryon numbers

Attractive NN int.⇒ Guarantees formation of “Matter”

Repulsive NN int.⇒ Avoids collapse to black hole

Interplay of NN interactions⇒ Drives evolution of stars and the universe

Knowledge on NN interactionNuclear structureNN scattering data

4

Hypernuclei

S

“Stable” carriersof baryon numbers

“Stable” against strong int.Quasi-stable objects

Stable in neutron starsYN and YY interaction

Structure of neutron stars

Source of info. on YN and YY interaction

Structure of hypernucleiProduction of hypernucleiLevel energy measurement

5

Structure study on hypernucleiSurvey only limited region of isotopesLack of spectroscopic tools

+K+π

C12ΛC12Ordinary

production reaction

6

Charge-Exchange ReactionsNew category of production reaction

New hypernuclear speciesMirror and neutron-rich hypernuclei

+K−π

Be12ΛC12Charge-Exchange

reaction

7

Exotic ObjectsHeaviest stable hydrogen ?

TritonHeaviest stable hydrogen

Only with nucleonsSuper-heavy hydrogen1.7 MeV unbound

Addition of Λ hyperonHyper-heavy hydrogen~ 4 MeV boundHeaviest stable hydrogenDouble of 3H

H5

H6Λ

H3

8

Our StrategyDevelop charge-exchange reactions

Single charge-exchange (SCX) reactions

Suitable to produce mirror-hypernuclei

Double charge-exchange (DCX) reactions

Production of neutron-rich hypernuclei

( )0,K−π

He4

H

He

4

4

Λ

Λ

c

NCX

SCX

( ) ( )+−+− ππ ,, KK

Systematic study of hypernuclei

9

Practical problemsSmall detection efficiency for SCX: 10-1~10-2

Small reaction cross section for DCX: 10-3

High-intensity beam line for CX reactionPion beam intensity ~ 109/spill: 102

Override small yieldHigh-resolution achievable at the same time

A long-range planConstruction of new beam line and spectrometer

Experiments at Day-1Start study with available infrastructure

( )0, K−π

10

Experiments at Day-1 (1/2)Production of n-rich hypernuclei with DCX

Structure of neutron-rich hypernucleiWhat we can learn ?ΛN strong interaction in N≫Z environmentClose connection to EoS in neutron stars

Non-mesonic weak decay of 4ΛHeMeasurement of Λn→nn weak processComplementary with Λp→pn in 4ΛHWhat we can learn ?Spin-isospin structure of ΛN weak interactionTest of “ΔI=1/2 rule”

11

Neutron-Rich HypernucleiKEK-PS-E521

First successful experimentn-rich hypernucleiDCX reaction

Reaction studied

Cross section~10 nb/sr

small cross section~10-3 of ordinary reaction

ExperimentalNo problem except for the small cross sectionWe need good beam line and good spectrometer

( ) LiKB 1010 , Λ+−π

12

Experimental setup

K1.8 Beam lineπ beam intensity ~107 / spill

SKS Spectrometermoved from KEK-PSsolid angle ~ 100 msr

13

Proposed experimentReaction mechanism of DCX reactionProduction of typical n-rich hypernuclei

Another optionStudy on structure of n-rich hypernucleiProduction of exotic hypernuclei

( ) ( ) Be,CLi,B 12121010Λ

+−Λ

+− KK ππ

HHeLi 6910ΛΛΛ

Similar reactionand structure

Cross section and structure may differ

14

Expected yieldProduction of 9ΛHe

Assume similar cross section as 10ΛLi

Requested beamtime for this study2 weeks for 9ΛHe and 2 weeks for 6ΛH (4 weeks in total)+ Beam tuning and energy/efficiency calibration

Parameters Values

π− beam momentumπ− beam intensity

PS acceleration cycle9Be target thickness

Reaction cross sectionSpectrometer solid angleSpectrometer efficiency

1.2 GeV/c107 /spill3.4 sec

3.5 g/cm2

10 nb/sr0.1 sr0.5

NBeam

TCycle

NTarget

dσ/dΩΩSP

εSP

CycleSPSPA

TargetBeam T

TimeddN

NNYield ××Ω×

Ω×××= εσ

9~400 events/2weeks

15

Experiments at Day-1 (2/2)Production of n-rich hypernuclei with DCX

Structure of n-rich hypernucleiWhat we can learn ?ΛN strong interaction in N≫Z environmentClose connection to EoS in neutron stars

Non-mesonic weak decay of 4ΛHeMeasurement of Λn→nn weak processComplementary with Λp→pn in 4ΛHWhat we can learn ?Spin-isospin structure of ΛN weak interactionTest of “ΔI=1/2 rule”

Need SCX

16

Non-Mesonic Weak DecaySpin-isospin structure of NMWD

Branching ratio tellDecay asymmetry tellPossible answer: 1S0 amplitude (a and/or b)Observable sensitive to 1S0

othersfd >>≈fdorfd <<>>

( )( )npp

nnn→ΛΓ→ΛΓ

( )pθασ cos10 +

( ) ( )H;He; 44ΛΛ →ΛΓ→ΛΓ nppnnn

17

Beam line and spectrometerK1.8 beam line: 1.1GeV/c π+ beam ~107/spillSKS spectrometer: ~100 msr

Expected yield of 4ΛHe(0+)

Yield ~ 38k 4ΛHe(0+)/day

Parameters Values

π+ beam momentumπ+ beam intensity

PS acceleration cycleLiquid 4He target thickness

Reaction cross sectionSpectrometer solid angleSpectrometer efficiency

1.1 GeV/c107 /spill3.4 sec2 g/cm2

10 μb/sr0.1 sr0.5

NBeam

TCycle

NTarget

ds/dΩΩSP

εSP

CycleSPSPA

TargetBeam T

TimeddN

NNYield ××Ω×

Ω×××= εσ

4

18

Decay particle detector

proton

neutron

−π

Direct measurementDetect nn and np pairs

Estimated yieldsΛn→nn: ~120 events/2weeksΛp→np: ~3200 events/2weeks

( )( )pYieldnYield ( )

( )npYieldnnYield

Veto for negative pion BGRejection with CDCMC study tells S/N≫10

Back-to-back kinematicsSelection of ΛN→NN processRemove ΛNN→NNN process

19

SummaryWe propose studies on Λ-hypernuclei

with Charge-Exchange (CX) reactionsProposal of experiments (at Day-1)Exp.1: Production of neutron-rich hypernuclei

Exp.2: Detailed study on NMWD of 4ΛHe

Outputs expectedΛN strong int. in neutron-rich environmentSpin-isospin structure of ΛN weak interaction

HHe 69ΛΛ

( )Henn;n 4Λ→ΛΓ pin down 1S0 contribution

Production of exotic hypernuclei