1. Search for Invisible Decay of Y(1S)
Osamu Tajima (KEK)
for Belle Collaboration
Oct. 17, QWG 2007 (DESY)

2. “Dark Matter” is 1/4 of universe B-factory may have strong sensitivity !?
Main issue of B-factory is
study for anti-matter which
is 0% in the universe !!
0% antimatter

3. Status of Dark Matter Search
LEP c c N’
90% CL upper limits
(Excluded region) N
Energy
deposition ?
LHC
Direct measurements have no sensitivity for ~GeV mass region
LHC will reach ~TeV mass region
Who will search ~GeV mass region ?

4. Status of Dark Matter Search (cont.)
LEP
Invisible width of Z
Limit for coupling with Z
single photon counting (e+e-  g invisible)
Limit for coupling with e+e-
Limit for coupling with qq ?
Not covered well,
Can be applied LEP limit in simple MSSM
However …
We can construct qq favored model as we like
PRD 72, (2005), B.McElrath, “Invisible quarkonium decays as a sensitive probe of dark matter”
hep-ph/ “Probing MeV Dark Matter at Low-Energy e+e- Colliders”
hep-ph/ “Light neutralino dark matter in the next-to-minimal”
hep-ph/ “Dark matter pair-production in bs transitions”
Charge of the Experiment : test all possibilities
We are very lucky if we find DM with Dark Horses e+ e- Z DM
LEP e+ e- g DM M _ q DM M _ _

5. Br( Y(1S)cc ) ~ 6x10-3 (mc<4.73GeV/c2 ~ mb)
How many events ?
Relic density is denoted as follows
W : relic density
h : Hubble constant
v : 1/20 ~ 1/25
0.1 pb ・ c
<s(ccSM) v >
Wh2 =  WMAP c q _ c q _
s(ccSM) ~ 18 pb
see PDG
s(ccSM), G(U(1S)cc) = fU2MUs(bbcc)
Br( Y(1S)cc ) ~ 6x10-3 (mc<4.73GeV/c2 ~ mb)
Assuming Time reversal …
PRD 72, (2005) , B.McElrath, “Invisible quarkonium decays as a sensitive probe of dark matter”
Past Best limit < 23x10-3 (90% CL) by ARGUS (1986)

6. How do we search such a “invisible” decay ?

7. KEKB accelerator & Belle detector
The world highest luminosity
collider, KEKB, can provide
~1 million U per day
Belle
Multi purpose detector,
Belle, catch the invisible
decay signals
KEKB
Linac

8. What is the most efficient way ?p+
Usual operation
on this resonance p-
Y(3S) Y*
Y(1S)
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
Invisible
No Signal

9. Reconstruction of “Invisible”
No direct detection of Y(1S) decay,
Detectable information
momentum of p+, p-
energy of initial state (EY(3S))
Missing particle is resonance
 recoil mass peak p+ p-
Y(3S) Y*
Y(1S)
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
Invisible

10. Reconstruction of “Invisible”
Demonstration with
Y(1S)  m+m- decay p+
4902 events p- Y*
Y(3S)
Y(1S) m+
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
U(1S)m+m- m-

11. Trigger logic is important
To improve the detection efficiency
Trigger logic is important
For trigger issue, we need two charged tracks
Reach to outer most layer of CDC (pt ~250 MeV/c)
Reach to middle layer of CDC (pt ~120 MeV/c)
“Opening angle cut” is necessary to distinguish tracks p+ p- x y

12. Special Trigger for Y(3S)p+p-Y(1S)invisible
Control sample
U(3S)  p+p-U(1S)
U(1S)  m+m-
data MC
Too low efficiency with usual condition (>135o)
Higher efficiency with looser condition
Special trigger condition was implemented
　　 (~850 Hz, twice rate as usual)
Single track trigger was implemented, too
with 1/500 pre-scale rate (pt>250 MeV/c)
2-track trigger & 1-track trigger
1-track trigger
for efficiency monitoring ?

13. Special Trigger for Y(3S)p+p-Y(1S)invisible
Control sample
U(3S)  p+p-U(1S)
U(1S)  m+m-
data MC
Too low efficiency with usual condition (>135o)
Higher efficiency with looser condition
Special trigger condition was implemented
　　 (~850 Hz, twice as usual condition)
Single track trigger was implemented, too
with 1/500 pre-scale rate (pt>250 MeV/c)
2-track trigger & 1-track trigger
1-track trigger
for efficiency monitoring
Trigger eff. = 89.8%
qrf > 30o
ptfull > 0.30 GeV/c
ptshort > 0.17 GeV/c
other cuts (following slides)
244 events predicted
Br(Y(1S)invisible)=6x10-3

14. What is background source ?

15. Y(3S)p+p-Y(1S)invisible Background
Two-photon
2 prong pp, ee, mm …
pt is balanced
Boosted (q distribution)
p+p-p0 ...
p0 veto, g energy cut
U(1S)
S/N:
1/301/8
recoil mass of p+p-
signal BG

16. Y(3S)p+p-Y(1S)invisible Background
Two-photon BG
recoil mass of p+p-
Y(1S) m+m-, e+e- … (outside of acceptance) p m
244 events predicted
Br(Y(1S)invisible)=6x10-3

17. Results

18. Results Br(Y(1S)invisible) < 2.5x10-3 (90%C.L.)
Nsignal = 38 ± 39  0 consistent
Br(Y(1S)invisible) < 2.5x10-3 (90%C.L.)
data
Fit BG
Prediction
Br(Y(1S)invisble)=0.6%

19. Originally, it was introduced to explain Solar neutrino anomaly
Another Impact
Originally, it was introduced to
explain Solar neutrino anomaly

20. Another DM Search : B  h χχ
Dark Matter
Dark Matter
Dark Matter
Dark Matter
Belle 535M BBbar, to appear in PRL

21. Summary Invisible decay of Y may indicate the
existence of light dark matter (mass < mb)
Direct search experiments have no sensitivity
Max. prediction : Br( Y(1S)invisible ) ~ 6x10-3
We took 2.9fb-1 data on Y(3S) with special trigger
 Search for invisible decay of Y(1S)
No indication whereas we reach the prediction
Br( Y(1S)invisible ) < 2.5x10-3 (90%CL)
Phys. Rev. Lett. 98, (2007)

23. Prospects

24. Prospects e- e+ 90 %C.L Super-forward m-detector
and Super-forward cal.
Can reach 2x10-4
~500 fb-1
SM : Y(1S)nnbar

25. Recent results from others
Submitted to PRD(RC) with one month decay
Belle still has the best limit
UL 3.9x10-3 (90%CL)  2.5x10-3 (Belle)
Based on 1.2 fb-1 on Y(2S) resonance
Y(2S)  p+p-Y(1S)invisible
with pre-scaled trigger 1/20

26. Other light Dark Matter physics by B-factory

27. FWD/BWD muon detectors
U(1S)m+m- : ~1/50 of now
+/-5o
+/-5o
We can cover < +/-1.5o region in principle
Because we have 100m straight section in tunnel
Belle
>99 % of acceptance is covered for muon

28. ~99 % of acceptance is covered for e+e-
EFC
Pb shield
 active detector !?

29. Other Impacts (2) our results gives lower limit for
describes U
our results gives lower limit for
gravitino mass m3/2 > 1.5x10-7 eV
previous limit: m3/2 > 0.3x10-7 eV

30. eff. = 9.2% (reconstruction) x 89.8%(trigger) = 8.2%