The observations of CP-symmetry breaking, first in neutral K decays and 
more recently in B mesons, are consistent with the standard model. Local 
Lorentz invariant quantum field theories imply CPT invariance, in 
accordance with all experimental evidence. Hence, it is expected that 
the CP-violating weak interaction also violates time-reversal invariance.

Experiments that could provide direct evidence supporting T 
noninvariance, without using an observation which also violates CP, 
involve either nonvanishing expectation values of T-odd observables 
(upper limits for electric dipole moments of the neutron and the 
electron), or the exchange of initial and final states, which are not CP 
conjugates to each other, in the time evolution for transition 
processes. The latter, requiring neutrinos or unstable particles, are 
particularly difficult to implement.

We report the direct observation of T violation in the B meson system, 
through the exchange
of initial and final states in transitions that can only be connected by 
a T-symmetry transformation. For this study we used 468 10^6 BBbar pairs 
produced in Y(4S) decays collected by the BABAR detector at the PEP-II 
asymmetric-energy e+e- colliderSLAC. Using the entanglement of the B 
mesons produced in the Y(4S) decay, the "flavor tagging" has been 
combined, for the first time, with the "CP tagging", for the 
construction of T-transformed processes.