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

Search for Invisible Decay
of Y(1S)
Osamu Tajima (KEK)
for Belle Collaboration
Oct. 17, QWG 2007 (DESY)
“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
Status of Dark Matter Search
LEP
90% CL upper limits
(Excluded region)
c
c
N’
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 ?
LEP
Status of Dark Matter Search (cont.)
e+
Invisible width of Z
Limit for coupling with Z
eLEP
single photon counting (e+e-  g invisible)
+
e
+
Limit for coupling with e e
_
e
Limit for coupling with qq ?
Not covered well,
q
_
Can be applied LEP limit in simple MSSM q
However …
_
We can construct qq favored model as we like
Z
DM
DM
g
DM
M
DM
DM
M
DM
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 e+e- 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
How many events ?
Relic density is denoted as follows
Wh2
@
0.1 pb ・ c
<s(ccSM) v >
Wh2 = 0.113  WMAP
s(ccSM) ~ 18 pb
see PDG
s(SMcc) @ s(ccSM),
W : relic density
h : Hubble constant
v : 1/20 ~ 1/25
qc
_
qc
q
c
_
q
c
G(U(1S)cc) = fU2MUs(bbcc)
Assuming Time reversal …
Br( Y(1S)cc ) ~ 6x10-3 (mc<4.73GeV/c2 ~ mb)
PRD 72, 103508 (2005) , B.McElrath, “Invisible quarkonium decays as a sensitive probe of dark matter”
Past Best limit < 23x10-3 (90% CL) by ARGUS (1986)
How do we search
such a “invisible” decay ?
KEKB accelerator & Belle detector
The world highest luminosity
collider, KEKB, can provide
~1 million U per day
Belle
KEKB
Linac
Multi purpose detector,
Belle, catch the invisible
decay signals
What is the most efficient way ?
p+
Usual operation
on this resonance
pYY*
(3S)
Y(1S)
Invisible
No Signal
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
Reconstruction of “Invisible”
No direct detection of Y(1S) decay,
Detectable information
momentum of p+, penergy of initial state (EY(3S))
Missing particle is resonance
 recoil mass peak
p+
pYY*
(3S)
Y(1S)
Invisible
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
Reconstruction of “Invisible”
Demonstration with
Y(1S)  m+m- decay
p+
4902 events
pYY*
(3S)
Y(1S)
m+
U(1S)m+m-
m-
Y(3S) runs : 2.9 fb-1
(Feb, 2006 : 4days)
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+
py
x
Special Trigger for Y(3S)p+p-Y(1S)invisible
data
MC
Control sample
U(3S)  p+p-U(1S)
U(1S)  m+mToo 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
Special Trigger for Y(3S)p+p-Y(1S)invisible
data
MC
Control sample
U(3S)  p+p-U(1S)
U(1S)  m+mToo low efficiency with usual condition (>135o)
 Higher efficiency with looser condition
 Special trigger condition was implemented
(~850 Hz, twice as usual condition)
Trigger eff. = 89.8%
qrf > 30o
ptfull > 0.30 GeV/c
ptshort > 0.17 GeV/c
other cuts (following slides)
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
244 events predicted
Br(Y(1S)invisible)=6x10-3
What is background source ?
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
signal
BG
p
p
p
U(1S)
p
S/N:
1/301/8
recoil mass of p+p-
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)
m
p
p
m
244 events predicted
Br(Y(1S)invisible)=6x10-3
Results
Results
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%
Another Impact
Originally, it was introduced to
explain Solar neutrino anomaly
χχ
Another DM Search : B  h 
Dark Matter
Dark Matter
Belle 535M BBbar, to appear in PRL
Dark Matter
Dark Matter
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, 132001 (2007)
Prospects
Prospects
e90 %C.L
Super-forward m-detector
SM : Y(1S)bar
and Super-forward cal.
Can reach 2x10-4
~500 fb-1
e+
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

Other light Dark Matter physics
by B-factory
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
~99 % of acceptance is covered for e+e-
EFC
Pb shield
 active detector !?
Other Impacts (2)
describes
U
our results gives lower limit for
gravitino mass m3/2 > 1.5x10-7 eV
previous limit: m3/2 > 0.3x10-7 eV
eff. = 9.2% (reconstruction) x 89.8%(trigger) = 8.2%