\section{Introduction}
\label{sec:intro}

In this note we discuss the possibility to perform time-dependent measurements at \DDbar threshold. 
This study has been triggered by the opportunity of having \emph{relatively large} center-of-mass boost at \DDbar threshold, 
possibly up to  $\beta\gamma \simeq 0.9$, and we will try to address the issue if it is worth it or not for having such a large boost.
We will base our advice on sensitivity estimates for \CP violation (\CPV) searches in the \DzDzbar system and make a comparison with
previous estimates reported in ``SuperB Progress Reports -- Physics'' (2010) \cite{O'Leary:2010af}, where this possibility for 
 time-dependent \CPV (\TDCPV) measurements at \DDbar threshold was not considered.
In the following, we assume an integrated luminosity of 500\invfb accumulated at the $\Psi(3770)$ (or \DDbar threshold) 
with BR($\Psi(3770) \to \DzDzbar$)=$(52\pm5)\times 10^{-2}$. 
\\
\par
First of all, we should note an important difference between the neutral $B$ and $D$ meson systems related to the 
flavor tagging. The flavor tagging consist in determining the flavor of a neutral meson, $\M$, at time $t=0$ and 
 it represents a crucial information for time-dependent analyses. 
At $B$ factories, the flavor of the \Dz is tagged at production ($t=0$) through the decay $\Dstarp \to \Dz \pip$, allowing for 
 time-dependent measurements of \Dz originated in $\epem \to \ccbar$ reactions ( i.e. incoherent production).
For the case of the neutral $B$ mesons an analog method does not exist. The usage of a coherent production of neutral $B$ mesons 
 in the $\Upsilon(4S) \to \Bz\Bzbar$ decay allows for time-dependent measurement of the other $B$ of the event 
 with respect to the $B$ for which we tag the flavor. 
In this case, a center-of-mass boost is needed since the $B$ mesons are produced almost at
 rest in the $\Upsilon(4S)$ rest frame. On the other hand, for the \DzDzbar system, time-dependent measurements are possible at the 
  $\Upsilon(4S)$ and the \Dz coherent production in the $\Psi(3770) \to \DzDzbar$ decay in principle is not necessary.
\\
\par
For this reason, in the following we will focus our attention on the peculiarities of \DDbar threshold environment.
In particular, the possibility to perform almost zero background measurements and the usage of \CP-tagged \Dz events 
 for time-dependent analyses, with the intent to identify golden modes for \TDCPV sensitivity studies. 

The capability of \CP tagging is a unique feature of \DDbar threshold: by reconstructing one $D$ of the event in a \CP eigenstate
 the other $D$ is determined to be in the opposite \CP eigenstate. This technique has successfully exploited by 
 CLEOc~\cite{Briere:2009aa}, which  provided crucial information for a model-independent measurement of the CKM angle $\gamma$ by 
 Belle~\cite{gammaMI_Belle}.


