Tau polarization
in SUSY cascade decays
Kentarou Mawatari
(馬渡 健太郎)
 hep-ph/0612237 (appear in PLB)
for LHC
“Tau polarization in SUSY cascade decay”
with S.Y.Choi (Chonbuk U.), K.Hagiwara (KEK),
Y.G.Kim (Sejong U.), P.M.Zerwas (DESY)
 hep-ph/0612301
for ILC
“Spin analysis of supersymmetric particles”
with S.Y.Choi (Chonbuk U.), K.Hagiwara (KEK),
U. Martyn (DESY), P.M.Zerwas (DESY)
March 1, 2007
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Contents
1. Introduction
 SUSY cascades
 Tau polarization
2. Invariant mass distributions
 Tau-tau vs pi-pi invariant mass distribution
3. Numerical studies
 SUSY vs UED’
 Experimental effects
 Determination of the stau mixing
4. Summary
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1. Introduction: SUSY cascades
 A rich source of information on SUSY particles and
the structure of the underlying theory can be
provided at LHC by cascade decays.
 After squarks and gluinos are produced, they
cascade down, generally in several steps, to the LSP.
 The SPS1a cascade
30%
10% for e,μ 100%
90% for τ
 Analyzing the visible qℓℓ final state, the cascade
provides the information on the masses and spins of
the SUSY particles involved.
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1. Introduction: Tau polarization
 So far, the cascade have primarily been studied
involving 1st and 2nd generation leptons/sleptons.
 We explore how the tau polarization can be
exploited to study L/R chirality and mixing effects in
both the stau and neutralino sectors.
 As polarization analyzer, we use single pion decays of
the tau’s.
z = Eπ/Eτ
(hard fragment)
(soft fragment)
 For notational convenience we characterize the tau
states by chirality (not helicity).
 The techniques can readily be applied to other decay
modes, e.g., ρ and a1.
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2. Invariant mass distributions
 Tau-tau vs pi-pi invariant mass
distribution
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The distribution of the visible final
state particles in the squark cascade

 The distribution depends only on the near tau angle.

The scalar character of the intermediate stau erases all
angular correlations.
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Invariant mass distribution
 The angles in the cascade are related to the
invariant mass
 In the following analyses, we use the
invariant masses in units of their maximum
values.
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Pion invariant mass distribution
 Pion distributions
chirality of
the tau leptons
 τL/R→π fragmentation func.
 The distributions do not
require the experimental
reconstruction of tau energy.
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The tau-tau vs pi-pi invariant mass
in the X2 decays
chirality of the
near/far tau
 The distributions do
not depend on the X2
polarization.
(∵ stau intermediate)
 The expectation value
of the invariant mass
The tau polarization measurements has the great
potential for determining spin and mixing phenomena.
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3. Numerical studies
 SUSY vs UED’
 Experimental effects
 Determination of the stau mixing
 SPS1a point: M(squark)=570 GeV, M(neutralino2)=175 GeV,
M(stau1)=135 GeV, M(neutralino1)=100 GeV,
neutralino2～wino, stau1～stauR
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SUSY vs UED’
(mass spectrum: SUSY SPS1a)
 The UED’ (universal
extra dimension)
cascade
LKP
 SUSY:
R-dominated
(∵ stau1～stauR)
 UED’:
L-dominated
(∵ Z1=W13)
 The difference comes
mainly from the
chirality difference
rather than the spin’s.
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Transverse momentum cut
for experimental analyses
 In fact, experimental analyses of tau leptons are a
difficult task at LHC.
 Transverse momentum cut
 increase the
efficiencies
 reduce the
primary event
number, and
L/R sensitivity
 The SUSY R-dominated
distribution is mildly
affected.
 The UED’ L-dominated
distribution is shifted
strongly.
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PT[π]>15GeV
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Determination of the stau mixing
 The stau mixing
 The large sensitivity can be
traced back to the fact that
X2 is nearly wino-like.
SPS1a
X2: pure wino-like
R
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L
R
 For |θ|=π/2, stau1=stauR
couples to X2 only through its
small higgsino and U(1)
gaugino components.
 For |θ|<π/2, the near tau
coupling is L-dominated.
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4. Summary
 The SUSY cascade decay
for the tau/stau case.
 The analysis of tau polarization in cascade
decays provides valuable information on
chirality-type and mixing of SUSY particles.
 The large size of the polarization effects was
predicted and exemplified quantitatively by
the pion channel.
 The effects can be exploited experimentally
in practice.
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