...
Collisions
...
at
...
the
...
LHC
...
will
...
contain
...
events
...
with
...
unprecedented
...
numbers
...
of
...
simultaneous
...
proton-proton
...
interactions,
...
or
...
"pile-up''.
...
Pile-up
...
refers
...
to
...
the
...
presence
...
of
...
a
...
signal
...
in
...
the
...
detector
...
which
...
does
...
not
...
originate
...
from
...
the
...
high
...
pT
...
interaction.
...
It
...
consists
...
of
...
two
...
components:
...
in-time
...
pile-up,
...
due
...
to
...
multiple
...
proton-proton
...
interactions
...
occurring
...
simultaneously
...
within
...
a
...
single
...
event,
...
and
...
out-of-time
...
pile-up,
...
resulting
...
from
...
contributions
...
in
...
previous
...
bunch-crossings
...
due
...
to
...
the
...
large
...
calorimeter
...
integration
...
time.
...
Pile-up
...
poses
...
special
...
challenges
...
to
...
jet
...
and
...
missing
...
ET
...
reconstruction
...
and
...
calibration.
...
At
...
the
...
LHC
...
design
...
luminosity,
...
each
...
bunch
...
crossing
...
will
...
contain,
...
on
...
average,
...
20
...
additional
...
minimum
...
bias
...
p-p
...
interactions,
...
contaminating
...
jet
...
signals
...
originating
...
from
...
the
...
high
...
pT
...
interaction
...
and
...
giving
...
rise
...
to
...
extra
...
jets,
...
uncorrelated
...
to
...
those
...
from
...
the
...
hard
...
scatter.
...
Even
...
for
...
the
...
initial
...
physics
...
searches,
...
pile-up
...
is
...
a
...
difficult
...
issue.
...
And,
...
obviously,
...
the
...
strategy
...
for
...
dealing
...
with
...
pile-up
...
will
...
be
...
a
...
major
...
consideration
...
in
...
designs
...
for
...
the
...
LHC
...
luminosity
...
upgrades,
...
with
...
up-to
...
400
...
additional
...
minimum
...
bias
...
interactions.
...
The
...
next
...
figures
...
show
...
the
...
effect
...
of
...
pile-up
...
on
...
calorimeter
...
signals
...
on
...
the
...
same
...
ATLAS
...
di-jet
...
event
...
with
...
2
...
and
...
10
...
minimum
...
bias
...
events
...
overlaid.
The SLAC group has initiated a broad effort in ATLAS to understand the global effects of pile-up on calorimeter signals, jets (reconstruction, energy scale, energy resolution, sub-structure, and properties) and missing ET, and to develop strategies and tools to account for such effects.
The present project consist of the study of calorimeter signal and jet reconstruction performance at the designed instantaneous luminosity of 10^34cm-2s-1, for which the present techniques to identify and calibrate hadronic jets have never been tested.