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

1. Hypothetical new scaler particle X for Q+ and its search by the (K+, X+) reaction T. Kishimoto Osaka University

2. 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)

3. First evidence of Q+ from SPring8 gn  Q+K-  K+K-n M = 1.540.01 GeV G < 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 K+K- production off the neutron/nucleus • … is nearly identical to non-resonant K+K- production off the proton Q+ Phys.Rev.Lett. 91 (2003) 012002 hep-ex/0301020

4. Positive results (Table: courtesy of A. Airapetian) Mass~1540 MeV Width <10 MeV

5. Q+ width from the KN database G=20 MeV G=1 MeV PK(GeV) A. Sibirtsev et al., hep-ph/0405099 GQ < 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.

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

7. +: Another explanation • Consistent theoretical (quark) model • yet to be worked out • KN bound state • arXiv:hep-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

8. 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)

9. 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 

10. How to Decay N p p K p N p s s K R(int) R() Decay (R(int)/R())6 N K p

11. Radius of + (and X) Asymptotic wave function () EB=30 MeV 15 MeV r~ 2 fm r~ 3 fm Extended object Seen only in Low momentum transfer reaction

12. 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

13. 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

14. X (Why missed) • Lowest order decay modeX ! K • No strong decay MX< MK + Mp • No X→K  decay • (0+ → 0- + 1- L non cons. ) • No X→K e+e- • (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.

15. Experiment to search for X • extended object→Low q2 reaction • r(X)~4fm: q2~ 50 MeV/c • Lowest q2 reaction • p(K-+ , X-+)p • @PK~1 GeV/c • q ~ 50 MeV/c • X ! K

16. 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

17. Cross section PK~1.05 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

18. K+ momentum (MeV/c) K+p → X+p K+ scattering angle (cos q) K+ momentum (MeV/c) K+p → K+p0p K+ scattering angle (cos q) K+ momentum (PK=1 GeV/c) Max 950 MeV/c Signal Background Max 830 MeV/c

19. Experimental condition(E548) • K+ beam from K2 beam line at KEK-PS • PK=1.2 GeV/c • 50k for 3x1012ppp • Target: 20cm thick H2O and 20 cm thick CH2 • Trigger • (K+, K+) x NaI(>5 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)

20. Spectrometer E548 Study of kaonic nuclei by (K-, p) reaction Search for X particle by (K+, K+ gg) reaction

21. Current setup of E548 NaI 5x5 KURAMA NaI 3x4 FAC γ 20cm VZH BAC 2 LC BS BAC 1 K+ Target(water) BQC K+Beam 1.2GeV/c 10.5cm γ CV

22. K+ momentum Tagged by 2γ PK=1.2 GeV/c K+ momentum X particle production 2 γ backward K+ momentum π0 production

23. Typical cross section • 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) K+ X+ p0 p p

24. Expected yield (Extension requested) • NK：50 k/spill • NT　：20cm （water target） • X+ cross section ~10 -3 of p0 Beam Time ‥ 18shift ⇒Xparticle ~1600 <r>~10fm, BE~1MeV

25. 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 σ) • It is not only a solution of + but opens new world. • Wait for our results