1. Impact of Efficient e Veto on Stau SUSY Dark Matter Analyses at ILC
Introduction
BeamCal for e vetoing SM backgrounds
Desired other PID capability
Summary
Based on
P. Bambade, V. Drugakov, W. Lohmann, physics/
Z. Zhang, arXiv: v1 & earlier studies
(Sendai), 5/3/2008

2. Introduction
Search for DM and understanding its nature is a key subject
ILC is expected to play a unique role
However the precision achievable at ILC does not come without effort
(Sendai), 5/3/2008

3. Example Results on Relic DM Density
Method one: (L=500fb-1)
Scenario A C D G J
DM (GeV)
Ecm (GeV)
s (fb)
Efficiency (%) <1.0
dmstau (GeV) >1.0
dWh (%) >14*
Method two: (L= fb fb-1)
Scenario Modified SPS 1a D
DM (GeV)
Ecm (GeV)
Pol 0.8(e-)/0.6(e+) yes yes yes yes no yes
s (fb)
Efficiency (%)
dmstau (GeV)
dWh (%) * 4.1* 6.7*
microMegas
*: Wh2<0.094(WMAP lower limit)
H.U.Martyn
hep-ph/060822
Z. Z. arXiv: v1
[hep-ph]
(Sendai), 5/3/2008

4. Expected Signature at an ILC Detector
Stau production & decays
@ e+e- collider t+ e+ e-
t - x-
Difficulty no one:
Missing energy from both LSP
and neutrino(s) in tau decay final state
Difficulty no two:
Large SM background contributions
(Sendai), 5/3/2008

5. Cross Sections: Signal versus SM Backgrounds
Signal (Scenario D’):
Ecm (GeV)
Beam Pol.
s (fb)
442
Unpol.
0.456
500 10
0.8(e-)/0.6(e+) 25
600 20 50
Method one: Optimal Ecm
(hep-ph/ ) 
Method two: Large Ecm
(hep-ph/ )
SM Backgrounds:
g*g*  t+t-(Et>4.5GeV): s~4.3x105 fb
 m+m- (Et>2GeV): s~5.2x106 fb
 hadrons (direct*direct dominant)
ccbar s~8.2x105 fb
 WW
e+e-  m+m-, t+t-: s~1.0x103 fb
(Sendai), 5/3/2008

6. Example: Dominant gg Background
SM background production & e+e- collider e+ e+ g* t+
t - g* e- e-
Tau decay final states:
Measured in the main detector
Spectator e+ and e-
Mostly going into the BeamCal
(Sendai), 5/3/2008

7. Background Rejection Analysis cuts relying on the main detector
A big fraction of background can be rejected using these cuts
but not sufficient for a quasi-background free analysis
 Forward veto is needed
(Sendai), 5/3/2008

8. Forward (BeamCal) Veto
Identify energetic spectator e+ and/or e- from gg events
Complication from beamstrahlung
GeV
Very challenging to have a radiation hard yet a very efficient BeamCal
for e/g ID
(Sendai), 5/3/2008

9. Forward (BeamCal) Veto Efficiency
A study by P. Bambade, V. Drugakov, W. Lohmann, physics/ :
Fine granularity tungsten/diamond sample 370cm from IP
Design depends on beam configuration
370cm
e/g VETO efficiency
Identify spectator e+/e- out
of huge beamstrahlung e+e- pairs
Efficiency is energy and angle
dependent
(Sendai), 5/3/2008

10. Summary on Final Selection/Rejection
The angular distribution of spectator e±
SM background ggtt generated
at Ecm of 500GeV
Method 1
Method 1 2
ssignal[fb]*eeff
0.456*5.7%
10*6.4%
sbkg[fb]
(w/o VETO)
561
168
(+VETO)
0.08
0.26
S/B
~0.3
~2.5
VETO eff. is pretty good for method 2 but needs improvement for method 1
(Sendai), 5/3/2008

11. How to Improve?
Very limited efficiency (e.g. ~6% in method one for scenario D’)
one reason: mm & eX topologies excluded (>20% eff. lost)
To improve on this, one needs to improve/extend PID to low angles
Background free stau detection
needs this capability:
eeeemm, eeeett:
m+e or t+e visible in the detector  signal like
Another e in the beam-pipe,
another m or tm/p low angle e
m/t
For more details refer to my ILD contribution on Friday
(Sendai), 5/3/2008

12. Summary Excellent veto efficiency of the BeamCal is a must
m/p PID capability at low angles is also desirable
Depending on SUSY scenario, DM density ILC can compete with expected precision from e.g. Planck
(Sendai), 5/3/2008

13. mSUGRA SUSY DM Scenarios after WMAP
Benchmark points:
Battaglia-De Roeck
Ellis-Gianatti-Olive
-Pape,
hep-ph/
important
when
DM=mstau-mc
is small
Challenging
scenarios
The precision on SUSY DM prediction depends on DM & thus
dmc  Needs smuon (or selectron) analysis
dmstau  Needs stau analysis
(Sendai), 5/3/2008