May2705

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Hypothetical new scaler particle X for Q and its
search by the (K, X) reaction

• T. Kishimoto
• Osaka University

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Contents

• Current status of ? search
• Another explanation of ?
• Could ? be made from K ? N?
• X as a K ? bound state
• Experiment to search for X (KEK-PS E548)

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First evidence of Q from SPring8
gn ? QK- ? KK- n
M 1.54?0.01 GeV G lt 25 MeV Gaussian
significance 4.6s

• Target neutron in Carbon nucleus
• Background level is estimated by a fit in a mass
  region above 1.59 GeV.
• Assumption
• Background is from non-resonant KK- production
  off the neutron/nucleus
• is nearly identical to non-resonant KK-
  production off the proton

Q
Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020
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Positive results
(Table courtesy of A. Airapetian)
Mass1540 MeV Width lt10 MeV
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Q width from the KN database
G20 MeV
G1 MeV
PK(GeV)
A. Sibirtsev et al., hep-ph/0405099
GQ lt 1 MeV

• The background is smooth (4 mb).
• The Q with a narrow width should appear as a
  bump.
• If not, a strong limit on the width can be put.

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Negative results
(Table courtesy of A. Airapetian)
Consistent explanation?
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? Another explanation

• Consistent theoretical (quark) model
• yet to be worked out
• K?N bound state
• arXivhep-ex/0312003 (T.K and T.Sato and others)
• Mass 1540 MeV
• M(K?N) 1570 MeV (30 MeV bound)
• 10 MeV/particle (usual)
• Spin parity ½ (original prediction)
• K ? N system (s-wave) ½
• This conjecture explains all Q properties
  particularly
• Narrow width
• Seen in Low Q, unseen in High Q

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Could ? be a K?N bound state

• Known two Body interaction
• ?N weakly attractive
• KN weakly repulsive
• K? weakly attractive
• K? int. may have ambiguity
• So strong to make K? bound state

No bound state
X (new scaler particle)
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Width of ? in K?N conjecture

• In order ? to decay
• K?N ! KN (? has to be absorbed in N)
• ?N (s-wave) ! ?N (p-wave)
• K-?N (s-wave) ! K-?N (p-wave)
• K?N have to interact simultaneously
• Width ?(strong) (R(int)/R(?))6
• R(int) Radius of Interaction
• R(?) Radius of ?

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How to Decay
N
p
p
K
p
N
p
s
s
K
R(int)
R(?)
Decay
(R(int)/R(?))6
N
K
p
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Radius of ? (and X)
Asymptotic wave function (?)
EB30 MeV 15 MeV r 2 fm
r 3 fm Extended object
Seen only in Low momentum transfer reaction
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Width of ? in K?N conjecture

• Width ?(strong) (R(int)/R(?))6
• ?(strong) a few 100 MeV
• R(int) 1 fm Radius of Interaction
• R(?) 3 fm Radius of ?
• ?(?) 300 x (1/3)6 0.3 MeV
• Consistent with observation
• Production of Q ? low q2 reaction

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New hypothetical particle X K? bound state

• K? interaction may be strong enough to make a
  bound state New particle X
• 0- 0- ? 0 scaler particle
• Binding energy less than 30 MeV
• If deeper than 30 MeV Q decays into N and X
• Probably a few MeV bound
• Very extended object

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X (Why missed)

• Lowest order decay mode X ! K??
• No strong decay MXlt MK Mp
• No X?K ? decay
• (0 ? 0- 1- L non cons. )
• No X?K ee-
• (vec. curr. vs axial charge (0? 0-)
• No such decay mode is listed in PDG
• No experimental searches ever made (Probably)
• If it exists.
• Hard to believe but possibility is there.

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Experiment to search for X

• extended object?Low q2 reaction
• r(X)4fm q2 50 MeV/c
• Lowest q2 reaction
• p(K- , X-)p
• _at_PK1 GeV/c
• q 50 MeV/c
• X ! K??

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How to measure (KEK-PS-E548)

• p(K- , X-)p, X! K ? ?
• Measure Kaon momentum in coincidence with ? rays
• BG process p(K- , K - ?0), ?0 ! ? ?
• X particle gives highest PK
• Measure invariant mass of K and two ?s
• Dedicated detector system
• Study of K? final state interaction

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Cross section
PK1.05 GeV/c Inelastic ? 5 mb (?
production) ?(X) ?(K?) x f f low relative
momentum phase space Reaction
mechanism ?b/sr order (1/1000) KEK-PS E548
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K momentum (PK1 GeV/c)
Max 950 MeV/c
Signal
Background
Max 830 MeV/c
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Experimental condition (E548)

• K beam from K2 beam line at KEK-PS
• PK1.2 GeV/c
• 50k for 3x1012ppp
• Target 20cm thick H2O and 20 cm thick CH2
• Trigger
• (K, K) x NaI(gt5 MeV)
• Rate 500 ev/spill
• NaI
• Target region 5x5 top and bottom
• 10.5 cm from the beam center
• Forward counter 3x4 top and bottom
• 13cm from the beam center
• Data taking went well (2 days in this April)

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Spectrometer
E548 Study of kaonic nuclei by (K-, p)
reaction Search for X particle by (K, K gg)
reaction
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Current setup of E548
NaI 5x5
KURAMA
NaI 3x4
FAC
?
20cm
VZH
BAC 2
LC
BS
BAC 1
K
Target(water)
BQC
KBeam 1.2GeV/c
10.5cm
?
CV
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Gammas
Signal
g energy (MeV)
g g invariant mass (MeV/c2)
Kg g invariant mass (MeV/c2)
Background
g energy (MeV)
g g invariant mass (MeV/c2)
Kg g invariant mass (MeV/c2)
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K momentum Tagged by 2?
PK1.2 GeV/c
K momentum
X particle production
2 ? backward
K momentum
p0 production
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Typical cross section
K
X

• p0 production cross section 5 mb
• 10-3 of p0
• 1 MeV bound X (r 10 fm)
• 10-2 of p0
• 5 MeV bound X (r 4 fm)

p0
p
p
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Expected yield (Extension requested)

• NK 50 k/spill
• NT 20cm (water target)
• X cross section
• 10 -3 of p0

Beam Time ? 18shift ?Xparticle
1600 ltrgt10fm, BE1MeV
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Summary

• K? bound system X is hard to believe
• Might have been missed experimentally
• Explains almost all ? properties
• If found, it means we found a new scaler particle
  (like k, strange partner of s)
• It is not only a solution of ? but opens new
  world.
• Wait for our results