  Despite extensive theoretical \cite{GanterPRL1998,
ElliotPRL2001,SchirmacherPRL2007, TanakaNatureM2008, MonacoPNAS2009,
MarruzzoSCIRP2013} and experimental studies \cite{ChumakovPRL2011,
ChumakovPRL2014, KayaScience2010,KChenPRL2010, LXuPRL2012, BonnPreprint2014}, a
longstanding puzzle in condensed matter physics remains regarding the origin
and nature of "Boson peak" (BP), where the vibrational density of states (DOS)
in glasses possesses an excess of states compared with the crystalline
counterpart. Here we show that BP is successfully observed in 2D hexagonal
granular packing, where the disorder is due to the force network, i.e. the
spatial heterogeneity of elasticity \cite{MarruzzoSCIRP2013}. Using
photo-elastic techniques \cite{TrushNature2005}, the disordered particle
interaction can be precisely measured to resolve the origin and nature of BP
for the first time in a real scenario. We discover the structures of DOS of
disordered crystals reminiscent of the corresponding perfect crystals, which is
consistent with the recent studies in $SiO_2$\cite{ChumakovPRL2014} and in
disordered gels\cite{LorenzoGelPaper2011}, notwithstanding the drastically
different systems. Moreover, we propose a mechanism to clarify that BP is not
merely a broadened and shifted van Hove singularity \cite{ElliotPRL2001,
ChumakovPRL2011} but instead it is due to an interplay of the mesoscopic
screening effect and the microscopic elasticity disorder -- causing,
respectively, the broadening and the attenuation of the first and the second
van Hove singularity. This may lead to an in-depth understanding of BP in
structure glasses.
