Search for Invisible Decay of Y1S

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Search for Invisible Decayof Y(1S)

• Osamu Tajima (KEK)
• for Belle Collaboration
• Oct. 17, QWG 2007 (DESY)

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Dark Matter is 1/4 of universeB-factory may
have strong sensitivity !?
Main issue of B-factory is study for anti-matter
which is 0 in the universe !!
0 antimatter
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Status of Dark Matter Search
LEP
c
90 CL upper limits (Excluded region)
N
Direct measurements have no sensitivity for GeV
mass region LHC will reach TeV mass region Who
will search GeV mass region ?
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Status of Dark Matter Search (cont.)
LEP

• Invisible width of Z
• Limit for coupling with Z
• single photon counting (ee- ? g invisible)
• Limit for coupling with ee-
• 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, 103508 (2005), B.McElrath, Invisible
  quarkonium decays as a sensitive probe of dark
  matter
• hep-ph/0510147 Probing MeV Dark
  Matter at Low-Energy ee- Colliders
• hep-ph/0509024 Light neutralino dark
  matter in the next-to-minimal
• hep-ph/0601090 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

LEP
_
_
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How many events ?
Relic density is denoted as follows
W relic density h Hubble constant v 1/20
1/25
0.1 pb c
Wh2 _at_
lts(cc?SM) v gt
Wh2 0.113 ? WMAP
s(cc?SM) 18 pb
see PDG
s(SM?cc) _at_ s(cc?SM), G(U(1S)?cc) fU2MUs(bb?cc)
Assuming Time reversal
Br( Y(1S)?cc ) 6x10-3 (mclt4.73GeV/c2 mb)
PRD 72, 103508 (2005) , B.McElrath, Invisible
quarkonium decays as a sensitive probe of dark
matter
Past Best limit lt 23x10-3 (90 CL) by ARGUS (1986)
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How do we search such a invisible decay ?
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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
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What is the most efficient way ?
p
Usual operation on this resonance
p-
Y
Y(3S)
Y(1S)
Y(3S) runs 2.9 fb-1 (Feb, 2006 4days)
Invisible No Signal
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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
Y(3S)
Y(1S)
Y(3S) runs 2.9 fb-1 (Feb, 2006 4days)
Invisible
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Reconstruction of Invisible
Demonstration with Y(1S) ? mm- decay
p
4902 events
p-
Y
Y(3S)
Y(1S)
m
Y(3S) runs 2.9 fb-1 (Feb, 2006 4days)
U(1S)?mm-
m-
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Trigger logic is important
To improve the detection efficiency

• 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

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Special Trigger for Y(3S)?pp-Y(1S)invisible
Control sample U(3S) ? pp-U(1S)
U(1S) ? mm-

• Too low efficiency with usual condition (gt135o)
• Higher efficiency with looser condition
• Special trigger condition was implemented
• (850 Hz, twice rate as usual)

?
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Special Trigger for Y(3S)?pp-Y(1S)invisible
Control sample U(3S) ? pp-U(1S)
U(1S) ? mm-

• Too low efficiency with usual condition (gt135o)
• Higher efficiency with looser condition
• Special trigger condition was implemented
• (850 Hz, twice as usual condition)

Trigger eff. 89.8 qrf gt 30o ptfull
gt 0.30 GeV/c ptshort gt 0.17 GeV/c
other cuts (following slides)
244 events predicted Br(Y(1S)?invisible)6x10-3
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What is background source ?
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Y(3S)?pp-Y(1S)invisible Background

• Two-photon
• 2 prong pp, ee, mm
• pt is balanced
• Boosted (q distribution)
• pp-p0 ...
• p0 veto, g energy cut

S/N 1/30?1/8
recoil mass of pp-
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Y(3S)?pp-Y(1S)invisible Background
Two-photon BG
recoil mass of pp-
Y(1S)? mm-, ee- (outside of acceptance)
244 events predicted Br(Y(1S)?invisible)6x10-3
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Results
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Results
Nsignal 38 39 ? 0 consistent
Br(Y(1S)?invisible) lt 2.5x10-3 (90C.L.)
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Another Impact
Originally, it was introduced to explain Solar
neutrino anomaly
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Another DM Search B ? h ??
??
Dark Matter
Dark Matter
Dark Matter
Dark Matter
Belle 535M BBbar, to appear in PRL
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Summary

• Invisible decay of Y may indicate the
• existence of light dark matter (mass lt 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 ) lt 2.5x10-3 (90CL)
• Phys. Rev. Lett. 98, 132001 (2007)

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Prospects
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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
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Recent results from others

• Submitted to PRD(RC) with one month decay
• Belle still has the best limit
• UL 3.9x10-3 (90CL) ? 2.5x10-3 (Belle)
• Based on 1.2 fb-1 on Y(2S) resonance
• Y(2S) ? pp-Y(1S)invisible
• with pre-scaled trigger 1/20

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Other light Dark Matter physicsby B-factory
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FWD/BWD muon detectors
U(1S)?mm- 1/50 of now
/-5o
/-5o
We can cover lt /-1.5o region in
principle Because we have 100m straight section
in tunnel
Belle
gt99 of acceptance is covered for muon
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99 of acceptance is covered for ee-
EFC
Pb shield ? active detector !?
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Other Impacts (2)
describes
our results gives lower limit for
gravitino mass m3/2 gt 1.5x10-7 eV previous
limit m3/2 gt 0.3x10-7 eV
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eff. 9.2 (reconstruction) x 89.8(trigger)
8.2