--- UserCode/Friis/TancNote/note/introduction.tex	2010/03/17 20:01:26	1.1
+++ UserCode/Friis/TancNote/note/introduction.tex	2010/04/15 23:22:39	1.3
@@ -1,30 +1,47 @@
-Tau lepton identification ($\tau$-ID) is a challenging but important endeavor
-at hadron colliders.  Standard Model (SM) Higgs boson searches and many new
-physics scenarios Beyond the Standard Model (BSM) process have discovery
-channels involving taus.  In the Standard Model, the Higgs boson Yukawa
-couplings are proportional to mass, resulting in decays to taus ten percent of
-the time for scenearios where the higgs mass is below the diboson threshold.
-In the Minimal Supersymmetric Model (MSSM), the coupling of members of the
-Higgs sector doublet to the tau lepton is enhanced by a factor of $tan\beta$.
+High tau identification performance is important for the discovery potential of
+many possible new physics signals at the Compact Muon Solenoid (CMS).  Events
+with tau leptons are typically signal events; the Standard Model background
+rates with true tau leptons are typically the same order of magnitude as the
+expected signal rate in many searches.  The challenge of doing physics with
+taus is dominated by the rate at which objects are incorrectly tagged as taus.
+In paticular, quark and gluon jets have a significantly higher production
+cross-section and events where these objects are incorrectly identified as tau
+leptons can dominate the backgrounds of searches for new physics using taus.
+Efficient identification of hadronic tau decays and and low misidentification
+rate for quarks and gluons is thus essential to maximize the significance of
+searches for new physics at CMS.
 
-The large mass of the tau makes it unique among the leptons in that can hadron
-final states.  These hadronic decays account for approximately 65\% of all tau
-decays and have a signature of small number of collimated pions. The hadronic
-decays are dominated by small number of collimated pions.  This signature is
-very similar QCD jet production, which in general has cross sections many
-orders of magnitude larger than signal processes of interest.  An additional
-complication at hadron colliders is the presence of underlying event (UE), due
-to secondary interactions in the $pp$ collision.  These underlying event
-particles are dominated by large numbers of soft pions which can overlap true
-tau decays.
+New physics signals may be discovered through tau lepton hadronic decay channels
+in early CMS data.  The tau lepton plays a paticularly important role in
+searches for Higgs bosons.  In the Minimal Supersymmetric Model (MSSM), the
+production cross--section is enhanced by the parameter $\tan\beta$.  The
+coupling of the MSSM Higgs to the tau lepton is also enchaced. \fixme(finish
+this)
 
-The criterion for a successful $\tau$-ID is twofold: the algorithm must have
-high tau efficiency to facilitate searches for rare new physics while
-supressing the common backgrounds found at hadron colliders.   This paper will
-focus on novel algorithms designed to identify true hadronic tau decays and
-reject common backgrounds.
+%The tau plays a paticularly important role in the search for Higgs
+%boson particle.  In the Standard Model (SM), the Higgs boson couplings to fermions
+%are proportional to the fermion mass, which enhances the $H \rightarrow \tau^{+}
+%\tau^{-}$ branching ratio relative to other leptonic decay modes.  For SM Higgs
+%masses below the $W^{+}W^{-}$ and $ZZ$ production threshold, the SM Higgs decays
+%to tau lepton pairs approximately 10\% of the time.  The significance of the tau
+%is enhanced in the Minimal Supersymmetric Model (MSSM), where the MSSM Higgs
+%coupling to the tau is enhanced by a factor of $\tan\beta$.
 
-Tau identification in CMS is performed using objects from the ``Particle Flow''
-algorithm.  The particle flow algorithm provides a global and unique
-reconstruction of the event. Signals in various subdectors are linked together
-to reconstruct physics objects at particle granularity.  
+Tau leptons are unique in that they are the only type of leptons which are heavy
+enough to decay to hadrons.  The hadronic decays compose approximately 65\% of
+all tau decays, the remainder being split nearly evenly between $\tau^{-}
+\rightarrow \mu^{-} \bar \nu_\mu \nu_\tau$ and $\tau^{-} \rightarrow e^{-} \bar
+\nu_e \nu_\tau$.  The hadronic decays typically decay to one or three charged
+pions and zero to two neutral pions.  The neutral pions decay almost
+instantaneously to pairs of photons.
+
+In this note, we will describe a technique to identify hadronic tau decays.  Tau
+decays to electrons and muons are difficult to distinguish from electrons and
+muons produced in $pp$ collisions.  Analyses that use exclusively
+non-hadronically decaying taus typically require that the leptonic ($e,\mu$)
+decays be of opposite flavor.  The discrimination of hadronic tau decays from
+electrons and muons is described in~\ref{PFT08001}.  With the Tau Neural
+Classifier, we aim to improve the identification of true hadronic tau decays
+associated with a collimated jet containing either one or three tracks
+reconstructed in the pixel and silicon strip tracker, plus a low number of
+neutral electromagnetic showers reconstructed in the calorimeter.