中性子過剰ラムダハイパー核の生成：実験的研究の背景 ... -...

中性子過剰ラムダハイパー核の生成：実験的研究の背景・経緯・現状

大阪大学・理学研究科阪口篤志

Properties of Λ Hypernuclei • Λ in nucleus: single particle picture OK

• Beautiful Λ shell structure • Deeply bound and highly excited Λ states

• “Glue-like role” of Λ hyperon • Attractive Λ-N interaction stabilizes nuclei • Doorway to strange matter

• Spin dependent interaction • Spin-orbit force is weak

• What else? • Strong ΛN-ΣN mixing • 3-body, many-body force?

第７回 J-PARC ハドロンサロン, 1 March 2013 2

p n

n

n

n

p Λ n

n n n

IA≠0 IA≠0 Λ

I=0 Σ

I=1

glue-like role

Nuclear Chart with Strangeness


“glue-like” role of Λ hyperon

Expand hypernuclear chart by using new spectroscopic tools

Strangeness Nuclear Physics

Production Reactions of Hypernuclei • Emulsion based experiment

• K− beams stopped in target (stopped K−) • Produce “hyperfragment” • Information of BE of ground state

• Missing-mass spectroscopy • “identified” hypernuclei • (K−,π−) or (π+,K+) reaction • Obtain level structure

• high statistics (>1000) • Mainly Non Charge-eXchange (NCX)

• produced only Z~N hypernuclei


~100 events

~10 events

Production Reactions of Hypernuclei (2) • Missing-mass spectroscopy: 2nd generation

• Charge exchange reactions • Single CX reactions (SCX)

• (e,e’K+) @JLAB • (K−,π0) or (π−,KS) reaction

• Double CX reactions (DCX) • (π−,K+) or (K−,π+) reaction • access neutron-rich hypernuclei • possible with existing setup • cross section is very small !

• roughly 1/1000 of NCX

第７回 J-PARC ハドロンサロン, 1 March 2013 5 J-PARC E10

Production of Exotic Λ hypernuclei • Example of “Hydrogen”


5ΛH No evidence

6ΛH Stable?

7ΛH Stable?

2ΛH Not bound

3ΛH Stable

4ΛH Stable

Λ

1H Stable 2H Stable 3H Stable

4H No evidence 5H Resonance 6H No evidence

n

p Super Heavy Hydrogen

Hyper Heavy Hydrogen

glue like role of Λ

We can produce at J-PARC

Super Heavy Hydrogen 5H • Resonance state in 5H system

• Missing-mass spectroscopy at Dubna • p(6He,ppt)nn (tnn=5H) PRL 87 (2001) 092501 • 3H(t,p)5H PL B566 (2003) 70 • 2H(6He,3He)5H NP A719 (2003) 229c


Resonant state of ~1.7MeV unbound p(6He,pp)X

Super Heavy Hydrogen 5H (2) • Other studies at GSI and LAMPF

• p(6He,2He)5H NP A723 (2003) 13 • 9Be(π−,pt)5H & 9Be(π−,dd)5H EPJ A24 (2005) 231

• 6ΛH structure ↔ 5H structure • Feedback to structure of normal neutron-rich nuclei


GSI LAMPF

ΛN-ΣN Mixing in Λ Hypernuclei • Strong mixing of ΛN and ΣN pairs

• B.F. Gibson et al. PR C6 (1972) 741

第７回 J-PARC ハドロンサロン, 1 March 2013 9 NN

∆N

ΛN

ΣN ΛΛ

ΞN ΣΛ

ΣΣ

∆m ~ 80 MeV

S=-1 S=-2

smaller ∆m enhances

ΛN-ΣN mixing

ordinary nuclei

hypernuclei

BB spectra

larger mixing expected in nuclei with larger IA

KNN Σ*N

overall conservation of isospin is required

IA≠0 IA≠0

How large is mixing in neutron-rich hypernuclei?

Λ I=0

Σ I=1

∆m ~ 300 MeV

Coherent ΛN-ΣN Coupling • Additional attraction for 4ΛH and 4ΛHe (I=1/2)

• Y. Akaishi et al. PRL 84 (2000) 3539 • Important to discuss structure of s-sell Λ-hypernuclei • a kind of ΛNN 3-body force (beyond 2-body force)


Mixing in Neutron-rich Hypernuclei

11

Normal ΛN interaction BΛ ~ 4.4 MeV

Coherent ΛN-ΣN mixing effect BΛ ~ 4.4 + 1.4 MeV

Precise measurement of B.E. → Estimation of mixing effect

Coherent ΛN-ΣN mixing originally introduced to explain A=3-5 hypernuclei

第７回 J-PARC ハドロンサロン, 1 March 2013 Y. Akaishi et al. Frascati Phys. Ser. XVI (1999) 59

Mixing and EoS in Neutron Stars • Degree of ΛN-Σ0N mixing and EoS

12

Λ-m

atte

r In

tera

ctio

n (M

eV)

large mixing

Hard EoS

Soft EoS

no mixing

with mixing

p-fraction

p-fraction

第７回 J-PARC ハドロンサロン, 1 March 2013 S. Shinmura et al. J.Phys G28 (2002) L1

Mixing and EoS in Neutron Stars • Appearance of strangeness looks robust

• What kinds of strangeness appear ? • Controlled by mass, charge and interaction.

13

Σ+Λ=Λ βα""

large n/p asymmetry (isospin»1) → strong ΛN-ΣN mixing expected

study on neutron-rich hypernuclei may provide information

第７回 J-PARC ハドロンサロン, 1 March 2013 S. Balberg and A. Gal NP A625 (1997) 435

No Σ−

Is 6ΛH Bound or Unbound ? • Systematics of BEΛ of known light hypernuclei


-4.6

MeV

from

sys

tem

atic

s Λ QF threshold

n bound threshold

Studies of Neutron-rich Hypernuclei • KEK-PS (SKY)

• K. Kubota et al. NP A602 (1996) 327 • (stopped K−,π+) reaction • Production upper limit


9ΛHe: < 2.4x10-4 /(stopped K-)

12ΛBe: < 6.1x10-5 /(stopped K-)

16ΛC: < 6.2x10-5 /(stopped K-)

12ΛC g.s.: 9.8±1.2 x10-4 /(stopped K-)

H. Tamura et al. PTPS 117 (1994) 1

DCX

NCX

limited by background from in-flight Σ+ decay

1/20 of NCX

Studies of Neutron-rich Hypernuclei (2) • DAΦNE (FINUDA)

• M. Agnello et al. PL B640 (2006) 145 • (stopped K−,π+) reaction

• (K− abs.)-π+ vertex cut • reject in-flight Σ+ decay


6ΛH: < 2.5x10-5 /(stopped K-)

7ΛH: < 4.5x10-5 /(stopped K-)

K−

π− π+

K−

π− π+

Σ+

Signal Background

production decay

Acce

ptan

ce C

ut?

1/40 of NCX

Li target Li target

Studies of Neutron-rich Hypernuclei (3) • M. Agnello et al. PRL 108 (2012) 042501 • Reanalysis of 6Li(stopped K−,π+) reaction

• used also information of π− from weak decay • Energy conservation: T(π+)+T(π−)

• 3 events of 6ΛH candidates


HeH

HLiK66

66

+→

+→+−

Λ

Λ+−

π

π

2.9±2.0x10-6 /(stopped K-)

Dalitz

Akaishi

1/300 of NCX 1 event = 1/1000

?*6HΛ

BΛ=0

g.s.

DCX (π−,K+) Reaction • KEK-PS (E521)

• P.K. Saha et al. PRL 94 (2005) 052501 • 10B(π−,K+)10

ΛLi reaction • Clean reaction


dσ/dΩ~10nb/sr (1/1000 of NCX)

47 events in bound region

J-PARC E10 is based on KEK-E521 result

DCX (π−,K+) Reaction (2) • Information from KEK-E521 data

• 2 sets of data: pBeam 1.05GeV/c and 1.2GeV/c • dσ/dΩ(1.05GeV/c) < dσ/dΩ(1.2GeV/c)


( ) LiKB 1010 , Λ+−π

pbeam = 1.2 GeV/c pbeam = 1.05 GeV/c

(π-,K+) Reaction Mechanism • Optimum π beam momentum ~ 1.2GeV/c

• E521 experiment tells

• Puzzle of reaction mechanism of DCX • Naïve two-step reaction


pion beam momentum 1.05 GeV/c 1.2 GeV/c 10B(π−,K+)10

ΛLi cross section 5.8 nb/sr 11.3 nb/sr

( )( )GeV/c2.1GeV/c05.1

σσ

>Λ+→++→+

+→+Λ+→++−

+−

KpnpnKpKKp

00

00

,,

πππ

π

pion beam momentum 1.05 GeV/c 1.2 GeV/c

Theory Lanskoy Harada et al.

38 nb/sr ~16 nb/sr

22 nb/sr ~12 nb/sr

(π-,K+) Reaction Mechanism (2) • Momentum dependence of elementary process


Λ+→++→+ +− Kpnp 00 , πππ

nKpKKp +→+Λ+→+ +− 00 ,π

One Step DCX Mechanism (2) • Possibility of other mechanisms

• One-step reaction with ΛN-ΣN mixing

• Elementary cross section


)()(, npKp Λ→ΣΣ+→+ −−+−π ( )( )GeV/c2.1GeV/c05.1

σσ

<

Σ channel opens at 1.045GeV/c

pion beam momentum 1.05 GeV/c 1.2 GeV/c Theory Harada et al. 2.4 nb/sr 5.4 nb/sr

ΛN-ΣN mixing

One Step DCX Mechanism (2) • ΛΝ-ΣΝ coupled channel calculation by Harada et al. • Spectrum is sensitive to mixing interaction VΛΣ and

Σ-N interaction WΣ

• Bound region → VΛΣ

• Continuum region → WΣ 23

VΛΣ WΣ

More yield is necessary to make detailed discussions

第７回 J-PARC ハドロンサロン, 1 March 2013

Measurement over Wide Mass-range • One step mechanism and mass spectra

• If one step mechanism is dominant:

• Affect to Λ(QF)/Σ−(QF) ratio, shape of Λ(QF), … • Measurement and understanding of full spectrum are

quite important 第７回 J-PARC ハドロンサロン, 1 March 2013 24

Λ region

Σ region

Missing Mass

(π−,K+)

ΛN-ΣN Mixing

interference?

J-PARC E10 Proposal • Subject

• Production of neutron-rich hypernuclei • higher statistics than previous experiments

• Hypernuclei close to neutron drip-line • 6

ΛH: core is 5H, beyond neutron drip-line • 9

ΛHe: core is 8He, typical neutron skin nucleus • Effect of ΛN-ΣN mixing in neutron-rich hypernuclei

• precise measurement of binding energies

• Method • Missing-mass spectroscopy by (π−,K+) reaction

• method has been established by KEK-PS E521


Experimental Setup • K1.8 beam line (about 40m long)

• 1.2 GeV/c high-intensity π− beams (107 π/spill) • Good momentum resolution: ∆p/p~3x10-4


30GeV proton

D1,D2

D3 D4

ESS1

ESS2

K1.8 beam line

SKS

Experimental Setup (2) • SKS spectrometer

• Good momentum resolution: ∆p/p~10-3

• Large angular acceptance: Ω~100msr


January 2010 before earthquake

by K. Tanaka

K+ momentum from (π−,K+) reaction ~ 850 MeV/c

↓ Best resolution ~ 1 MeV/c

Actual resolution determined by target thickness

Thicker target is necessary to obtain larger yields

↓ E10 Resolution ~ 2.5 MeV/c

Experimental Setup (3) • SKS was moved from KEK-PS K6 to J-PARC K1.8

• momentum acceptance enlarged: good for E10 • Simultaneous Λ and Σ measurement


SKS @KEK-PS K6

SDC3, 4

TOF

LC

Large AC

SKS @J-PARC K1.8

Λ hypernuclei production Σ− production

Yield Estimation for 6ΛH Production • Assumed cross section ~10nb/sr (~1/1000 of NCX)

• Major difficulty in this experiment

• About 300 events in 3 weeks of beamtime • About 3T (3x1012) pions on target


Parameters Values

π- beam momentum π- beam intensity PS acceleration cycle 6Li target thickness Reaction cross section Spectrometer solid angle Spectrometer efficiency Analysis efficiency

1.2 GeV/c 107 /spill 6 s/spill

3.5 g/cm2

10 nb/sr 0.1 sr

0.5 0.5

High intensity beams

Large acceptance

Thick target

Prospects on B.E. Measurement • Measurement of B.E. of 6ΛH


Λ bound

6ΛH

simulated spectrum

Assumptions overall energy resolution 6

ΛH yield 6

ΛH/QF ratio (Ex<23MeV)

MeV(FWHM)5.2≈

events300≈

Clear separation of g.s. and QF Statistical error of B.E. < 0.1MeV

10/1≈ estimated from 4

ΛHe and 10ΛLi

E10 Beamtime in Dec. and Jan. • Beamtime allocated for E10

• 15-27 Dec 2012 (run 45) and 8-16 Jan 2013 (run 46)


E10 Beamtime in Dec. and Jan. (2) • 295 hours in total, 250 hours (10.4 days) for 6ΛH

production = about 1/2 of requested (3 weeks)


0

20

40

60

80

100

120

140

160

180

14 16 18 20 22 24 26 28

Inte

grat

ed b

eam

time

(hou

r)

Date in December 2012

Total Allocated w/ Beam User Trouble Down Time

0

20

40

60

80

100

120

140

160

180

7 9 11 13 15 17

Inte

grat

ed b

eam

time

(hou

r)

Date in January 2013

Total Allocated w/ Beam Down Time

144 hors 151 hors

beam unstable

beam stable

E10 Beamtime in Dec. and Jan. (2) • Number of pion beams on target

• about 1.65 T pions ⇔ 3 T pions (in E10 proposal)


0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

20 25 30 35 40 45

Num

ber o

f pio

ns o

n ta

rget

(101

2 )

Date (start from Dec. 2012)

1.65 T pions

RUN45

RUN46

beam power 10 kW 10M/spill pion beams

15kW 12M/spill

Calibration of Missing-mass → BEΛ • No established calibration peak in (π−,K+)

• Direct calibration is not possible

• Calibration in 3 or 4 steps • Calibration with Σ+ production (beam: π+, SKS: K+) • Calibration with Σ− production (beam: π−, SKS: K+)

• Peak of p(π,K+)Σ reactions: (CH2)n target • 80MeV/c2 away from mass range of Λ hypernuclei

• Calibration with 12ΛC production (beam: π+, SKS: K+)

• charge of beam is opposite • Monitoring with Σ− production (beam: π−, SKS: K+)

• test of stability of missing-mass in production runs


12ΛC used

for NCX

special runs

Calibration of Missing-mass → BEΛ (2) • Results of quick analyses of calibration runs

• Σ− and Σ+ production (calibrate energy scale)


p(π−,K+)Σ−

Missing Mass (GeV/c2)

∆MX: 2.5MeV (FWHM)

5 hours 1 hour

p(π+,K+)Σ+

Missing Mass (GeV/c2)

∆MX: 3.0MeV (FWHM)

no Eloss correction

Calibration of Missing-mass → BEΛ (3) • 12

ΛC production (energy scale and resolution check) • First production of hypernuclei at J-PARC


12C(π+,K+)X

-BE(Λ) (GeV)

∆BΛ: 3.0MeV (FWHM)

g.s.

Ex=11MeV

13 hours (4M/spill, 3.5g/cm2)

⇔ 2.5MeV (expected)

Λ-QF

Σ+-QF?

Analysis Plan and Prospects • Procedures to get final results

• Background reduction: We need clean (π−,K+) reaction spectrum but we still have (π−,π+) and (π−,p) backgrounds.

• Energy resolution: We wish to improve energy resolution of missing-mass, say 3.0MeV(FWHM) → 2.5MeV(FWHM).

• Analysis efficiency: We wish to improve analysis efficiency to get more yields.

• Schedule in planning • results of quick analysis at JPS-meeting in Hiroshima • semi-final results in INPC2013 in Firenze


Summary • Λ hypernuclei are similar with normal nuclei and

also have new features • We (E10) are interested in “glue-like role” of Λ

hyperon and “ΛN-ΣN mixing” in hypernuclei • We wish to produce Λ hypernuclei close to neutron

drip-line, 6ΛH and 9ΛHe, because these phenomena become critical

• 1st phase of E10 done in Dec. 2012 - Jan. 2013 • Total beamtime for 6ΛH production ~250 hours, total

number of pions on target ~1.65T (1.65x1012), about ½ of proposed due to less beamtime allocation

• Analyses are now in progress 第７回 J-PARC ハドロンサロン, 1 March 2013 40

中性子過剰ラムダハイパー核の生成：実験的研究の背景 ... -...

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中性子過剰ラムダハイパー核の生成： 実験的研究の背景 ... -...

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Transcript of 中性子過剰ラムダハイパー核の生成： 実験的研究の背景 ... -...

中性子過剰ラムダハイパー核の生成：実験的研究の背景 ... -...

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