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%% subsection 3.2 Slepton NLSP [slac-pub-7236-0-0-3-2 in slac-pub-7236-0-0-3: slac-pub-7236-0-0-3-3]
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\subsection{\usemenu{slac-pub-7236::context::slac-pub-7236-0-0-3-2}{Slepton NLSP}}\label{subsection::slac-pub-7236-0-0-3-2}
With low scale supersymmetry breaking it is equally possible that
the lightest standard model superpartner is a charged slepton.
For example, a gauge-mediated
messenger sector with two generations of ${\bf 5} + \bar{\bf 5}$
generally gives a right handed slepton as the NLSP.
In this case it decays by $\tilde{l}_R \to l + G$.
These decays also lead to very distinctive signatures.
At an $e^+e^-$ collider the signature for slepton pair production
with prompt decay to Goldstinos is $e^+e^- \to l^+l^- + \Emiss$.
The standard model backgrounds are easily manageable for this signal.
The decay leads to a flat lepton spectrum in the lab frame,
with the end points giving an important test that the missing
energy is carried by essentially massless particles.
At a hadron collider pair production of NLSP sleptons
with prompt decay to Goldstinos also gives final states
$ l^+l^- + \EmissT$.
This suffers from fairly large irreducible backgrounds,
and does not represent a clean signature.
However, production of heavier states which cascade to
$\tilde{l}_R$ can give clean signatures with multiple leptons.
For example, pair production of $\lL \lL$
followed by the cascade decays $\lL^{\pm} \to \tilde{l}_R^+ l^- l^{\pm}$
(through on- or off-shell $\chi_i^0$)
gives final states $6l + \EmissT$.
Such signatures do not suffer contamination by standard model backgrounds.
If the decay $\tilde{l} \to l + G$ takes place on the scale of
a detector very distinctive charged particle tracks can arise.
This is because heavy non-relativistic sleptons are more highly ionizing
than ultra-relativistic charged particles.
Decay over a finite distance can then be seen as a greater than minimum
ionizing track with a kink to minimum ionizing track.
Such kinks should be measurable down to the 100 $\mu$m level.
If the slepton decay length is long enough,
timing information can also be applied to isolate such events.
Measurement of the decay length distribution would give a direct
measure of the supersymmetry breaking scale.
Because of the larger Yukawa coupling, the $\tilde{\tau}_R$ can be
lighter than $\tilde{\mu}_R$ and $\tilde{e}_R$ from renormalization group
evolution.
If $m_{\tilde{\tau}_R} + m_{\tau} + m_{\mu} < m_{\tilde{\mu}}$
the electro-weak decay
$\tilde{\mu}_R^{\pm} \to \tilde{\tau}_R^+ \tau^- \mu^{\pm}$,
%and
% $\tilde{\tau}_R^{\pm} \to \tilde{e}^+ e^- \tau^{\pm}$
through the $B$-ino component of off-shell $\chi_i^0$,
can compete with the decay $\tilde{\mu} \to \mu + G$,
and likewise for $\tilde{e}_R$.
It is possible then that
nearly all cascades lead to $\tilde{\tau}_R$ and that all
the slepton signatures discussed above
occur with $\tau$'s in the final state.
Alternately,
if $m_{\tilde{\tau}_R} + m_{\tau} + m_{\mu,e} > m_{\tilde{\mu},\tilde{e}}$,
all the right handed sleptons
%$\tilde{\tau}_R, \tilde{\mu}_R$, and $\tilde{e}_R$
are stable against three body electro-weak decays
at lowest order, and the decay $\tilde{l}_R \to l + G$
can dominate.
In this case the above signatures occur with equal rates for all three
generations.
If the supersymmetry breaking scale is well above a few 1000 TeV,
the decay $\tilde{l} \to l + G$ takes place well outside the
detector.
At both $e^+e^-$ and hadron colliders the signature for supersymmetry
would then be $\lR^+ \lR^- X$, i.e. heavy charged particle pair production
without missing energy!
This very non-standard signature should not be overlooked in
the search for low scale supersymmetry breaking.