Norihito MuramatsuRCNP, Osaka University

Nuclear Physics at J-PARC, 2 June 2007 @ Tokai

Search for the Θ+

with a Low Momentum K+ Beam

~~~ Contents ~~~- Summary of current situation- Objectives of Θ+ search at J-PARC- Considerations on experimental setup

Θ+ Photoproduction from deuteron

MMd(γ,pK－) GeV/c2

Θ+

1.6 GeV bump

mass ~1.53 GeV/c2, s/sqrt[s+b] = 4-5σ, Details are being shown at INPC.

LD2 data (preliminary)1.50< M(pK－)<1.54 GeV/c2

γ

pn

Θ＋

K－

p

Λ(1520)

No indications at CLAS, but acceptance coverage is different from LEPS. (No sensitivity of Λ* detection in extremely forward region)

0

20

40

60

80

100

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 10 20 30 40 50 60 70 80 90

Titov’s cross sectioncalculation at Eγ = 2.0 GeVwith spin-parity = 3/2‐

Isospin Asymmetry in quasi-free Θ+ PhotoproductionNam, Hosaka, and Kim, Phys. Rev. C74, 025204 (2006)

γN→KΘ+ : contact term (3/2) or no K* exchange (1/2)

neutron traget > proton target (CLAS-p)γN→KΛ* : neutron < proton

Indication is seen in LEPS data.

M(K-p) GeV/c2

M(K-p) GeV/c2

proton target

deuteron target

θCM(K-p) < 60°

cosθCM(K0) > 0.5

cosθCM(K0) > 0.5

Λ(1520)

g11@CLAS

Cou

nts

Cou

nts

M(nK+) (GeV)M(nK0) (GeV)

σ γ p → Θ+ K0 < 1.25 nb @ 1.54 GeV/c2

Θ+(1540) ?

Null results in high energy experiments. (BES, BaBar, Belle, LEP, HERA-B, SPHINX, HyperCP, CDF, FOCUS, PHENIX)

⇒ σ(Θ+)/σ(Λ*) < 2-3 % [Quark Fragmentation]

Energy Dependence?

Pentaquark less suppressed ?

Baryon fragmentation

s

e

dd

uud

s

Needs fewerquark pairs from the vacuum

Θ+

Quark fragmentation

e

q

uud

Pentaquark stronglysuppressed ?

Θ+

q

Objectives of Θ+ search at J-PARC (and LEPS2)• Θ+ is not established yet.• Affected by reaction mechanism ?

- Isospin asymmetry in γN→KΘ+

- Angle dependence in γd→Λ*Θ+

- Energy dependence in σ(Θ+)/σ(Λ*)• Width/spin/parity is not determined.⇒ Systematic studies of Θ+ photoproduction at LEPS2

- Understand reaction mechanisms with high intensityphoton beam (~107/sec) and large volume detector

⇒ Θ+ formation experiment by K+n resonance at J-PARC- Direct confirmation of Θ+ existence- Independent from reaction mechanism- Width can be measured from cross section

K+n Scattering Experiments

mpKs (GeV/c2)

N /

2 M

eV/c

2

Nch

DIANA

BelleK+ is ‘reconstructed’ from thereaction D*-→D0π-→(K+π-)π-

DIANAOld bubble chamber experiment

K+Xe → KS0pX

Need a modern experiment with high intensity K+ beam

Basic Concepts

C

RSSC

+

Stack

Barrel

Veto

DegraderI

TargetB4

Drift Chamber

Veto

EndcapT

Range

K Beam

π＋

π－

protonK+ beamK0.8 ?

ΔP/P=1.4%

417 MeV/c• Resonance formation reaction: K＋n→Θ+→KS

0p→π＋π－p‐P(K＋)=417 (442) MeV/c for M=1.53 (1.54) GeV/c2

‐neutron in scintillation fiber target• π＋π－ detection at Drift Chamber and proton detection at Sci. Tgt.

M(π＋π－)=M(KS0) ⇒ M(KS

0p)=M(Θ＋)• Λ* formation for calibrations and checks of data quality and analysis

procedure with the same beamline and detectors: K-p→Λ(1520)→Λπ＋π－

(It is worth to do even if K- intensity is a bit lower.)

Originally considered at BNL-E949- sophisticated for K+ beam experiment- large 4π volume with good resolutions

Similar but optimized experiment ispossible at J-PARC.

K0.8 (Sharing w/ stopped K+ exp.)

0

20

40

60

80

100

120

140

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

target

BeO degrader ~40 cm(high dens. and low A)

K+: ~420 MeV/c (β~0.648→nth~1.54)π+: ~600 MeV/c (β~0.974→nth~1.03)

K

π

K0.8

Fitch-typeCherenkov

Beam WireChamber

AerogelCherenkov

E787 Year 95 96 97-98K+ momentum (MeV/c) 790 730 710

Stopping Fraction 20% 25% 28%

800 → 420 MeV/c~ 40 cm

800 → 600 MeV/c

BeO: R vs. P

Kπ

Active Target

0

5

10

15

20

25

30

35

40

45

50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

proton: θ vs. P in Lab. sys.

protonπ

R vs. P in CH- K+ travels inside a target until momentumbecomes appropriate to produce Θ+.

- Proton is emitted in forward directions, and tends to stop inside the target.

- Kinetic energy and polar angle measure-ments of proton.

- Mometum correction for pions.⇒ Active target w/ fine segmentation

Spectrometer Considerations

Pions are detected at spectrometer& proton stops inside the target

cos(polar angle of KS0) of

generated Θ+ eventspion: θ vs. P in Lab. sys.

- Pions are emitted in side directions. ⇒ Cylindrical drift chamber inside a solenoid.

- In case that a 1 m-long drift chamber is placedat -40 cm to 60 cm of the target, geometrical acceptance is an order of 40%.

- PID by TOF would be enough.(See right figure: green R=50 cm, red R=90 cmin case of charged particles are emitted at 90°.Δt=50 psec is assumed.)

Mass Resolution

Mass resolution studies were done assuming BNL-E949 detector resolutions.Invariant mass of ππp : 7.6 MeV/c2 (assuming ΔP/P=1.4% at P=200-300 MeV/c,

ΔE/E=8.3% at EKIN=100 MeV, proton angle mes. error = 6 degree)Kinematic fit (using correlation with KS

0 mass) : 6.2 MeV/c2

(Note that initial neutron mass is also correlated with the reconstructedmass deviation, but it depends on K+ momentum resolution.)

M(π+π-)=M(KS0) GeV/c2

M(π+π-p)=M(Θ+) GeV/c2re

cons

truc

ted

mas

s –

true

mas

s (G

eV/c

2 )KS

0 mass (GeV/c2)

Dependence on momentum resolution

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0 0.01 0.02 0.03 0.04 0.05 0.06

B-field = 1 TeslaL=1.9 m, R=66 cmpos. res.=150 um

w/o multiple scatt.

w/ multiple scatt.

Although mass resolution w/ kinematic fit is not sensitive to momentum resolution,it is better to construct a spectrometer with ~1 % resolution. Tracking chambers with such resolutions are under considerations at LEPS2.

w/o kinematic fit

w/ kinematic fit

Other possibilities of detector setup• Inactive target + proton detection at spectrometer

Need studies of proton momentum resolution.• Only π＋π－ detection at Side Tracking Chamber

M(π＋π－)=M(KS0) & MM(K＋,π＋π－)=M(p)

⇒ M(K＋n) with Fermi-correction

Mass res. ~15.4 MeV with K+ beam mom. res. 8.4 MeV/c (LESB3)Select backward production of KS

0: 6.7 MeV (135°-180° in CMS)Need to measure K+ beam momentum with TOF. (L~4 m)

[ ] [ ] [ ] [ ]{ }2222 ),()()(0

p

KK

KC MKMMPP

PnKMnKM

S

−×−

−= −++++

+

+

ππ

forward chambers

protonπ

π

side tracker

targetK+ Momentum resolution at small polar angles has to be checked.

Effect of energy loss at target has to be checked.

Expected Yield (BNL case) and Backgrounds• LESB3: 1012 proton/pulse

⇒ 3⋅105 K+/pulse @710 MeV/c⇒ 3⋅104 K+/pulse @475 MeV/c w/o degrader

• Y=ρ ⋅ l ⋅ σ ⋅ NA ⋅ FK ⋅ fn = 1.032 g/cm3 ⋅ 25 cm ⋅ 10-27 cm2 ⋅6.022⋅1023 ⋅ 3⋅104 /pulse ⋅ (6/13) = 200 /mb/pulse

• σBW(E) = π/(4k2) ⋅ Γ2/[(E-M)2+Γ2/4] for spin1/2⇒ 26.4 ⋅ Γ mb/MeV

• Λ*: Γ=15.6 MeV ⇒ order of 100 mb• CEX BG: 7 mb [PRD15(1977)1846] & forward peaked• π+n→π+π-p: Pion contamination in beam can be

removed by Cherenkov detectors, KS0 reconstruction,

and 4-momentum conservation.

K0.8 as K1.1BR at Day-1 ?K0.8 w/ double separated beam at Day-2 ?

Summary• Existence of Θ+ can be confirmed in K+n resonance reaction

at J-PARC. P(K+) ~ 420 MeV/c• Θ+ is identified by invariant mass of two pions and proton.

Mass resolution ~8 MeV w/ ΔP/P=1.4% (Better resolution is expected with kinematic fit and/orlarge volume tracking chamber.)

• Θ+ signal should be distinguishable from chargeexchange background and contaminated pion reaction.

• Width can be measured from cross section. (26xΓ mb/MeV)• Λ(1520) formation with K- beam is useful as a reference

reaction.• Beamline and detector system can be shared with rare kaon

decay experiments.