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Tau polarization in SUSY cascade decays
Kentarou Mawatari (馬渡 健太郎) hep-ph/ (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/ 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 K. KEKPH07
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Contents Introduction Invariant mass distributions Numerical studies
SUSY cascades Tau polarization Invariant mass distributions Tau-tau vs pi-pi invariant mass distribution Numerical studies SUSY vs UED’ Experimental effects Determination of the stau mixing Summary March 1, 2007 K. KEKPH07
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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 Analyzing the visible qℓℓ final state, the cascade provides the information on the masses and spins of the SUSY particles involved. 30% % for e,μ 100% 90% for τ March 1, 2007 K. KEKPH07
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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 (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. z = Eπ/Eτ March 1, 2007 K. KEKPH07
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2. Invariant mass distributions
Tau-tau vs pi-pi invariant mass distribution March 1, 2007 K. KEKPH07
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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. March 1, 2007 K. KEKPH07
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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. March 1, 2007 K. KEKPH07
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Pion invariant mass distribution
Pion distributions τL/R→π fragmentation func. The distributions do not require the experimental reconstruction of tau energy. chirality of the tau leptons March 1, 2007 K. KEKPH07
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The tau-tau vs pi-pi invariant mass in the X2 decays
The distributions do not depend on the X2 polarization. (∵ stau intermediate) The expectation value of the invariant mass chirality of the near/far tau The tau polarization measurements has the great potential for determining spin and mixing phenomena. March 1, 2007 K. KEKPH07
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SUSY vs UED’ Experimental effects Determination of the stau mixing
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 March 1, 2007 K. KEKPH07
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SUSY vs UED’ (mass spectrum: SUSY SPS1a)
The UED’ (universal extra dimension) cascade SUSY: R-dominated (∵ stau1~stauR) UED’: L-dominated (∵ Z1=W13) The difference comes mainly from the chirality difference rather than the spin’s. LKP March 1, 2007 K. KEKPH07
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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. PT[π]>15GeV March 1, 2007 K. KEKPH07
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Determination of the stau mixing
The large sensitivity can be traced back to the fact that X2 is nearly wino-like. 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. SPS1a X2: pure wino-like R L R March 1, 2007 K. KEKPH07
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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. March 1, 2007 K. KEKPH07
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