<s>
In	O
particle	O
physics	O
,	O
a	O
hermetic	B-Algorithm
detector	I-Algorithm
(	O
also	O
called	O
a	O
4π	B-Algorithm
detector	I-Algorithm
)	O
is	O
a	O
particle	B-Algorithm
detector	I-Algorithm
designed	O
to	O
observe	O
all	O
possible	O
decay	O
products	O
of	O
an	O
interaction	O
between	O
subatomic	O
particles	O
in	O
a	O
collider	O
by	O
covering	O
as	O
large	O
an	O
area	O
around	O
the	O
interaction	O
point	O
as	O
possible	O
and	O
incorporating	O
multiple	O
types	O
of	O
sub-detectors	O
.	O
</s>
<s>
Such	O
detectors	O
are	O
called	O
"	O
hermetic	O
"	O
because	O
they	O
are	O
constructed	O
so	O
as	O
the	O
motion	O
of	O
particles	O
are	O
ceased	O
at	O
the	O
boundaries	O
of	O
the	O
chamber	O
without	O
any	O
moving	O
beyond	O
due	O
to	O
the	O
seals	O
;	O
the	O
name	O
"	O
4π	B-Algorithm
detector	I-Algorithm
"	O
comes	O
from	O
the	O
fact	O
that	O
such	O
detectors	O
are	O
designed	O
to	O
cover	O
nearly	O
all	O
of	O
the	O
4π	O
steradians	O
of	O
solid	O
angle	O
around	O
the	O
interaction	O
point	O
;	O
in	O
terms	O
of	O
the	O
standard	O
coordinate	O
system	O
used	O
in	O
collider	O
physics	O
,	O
this	O
is	O
equivalent	O
to	O
coverage	O
of	O
the	O
entire	O
range	O
of	O
azimuthal	O
angle	O
(	O
)	O
and	O
pseudorapidity	O
(	O
)	O
.	O
</s>
<s>
the	O
range	O
of	O
phase	O
space	O
which	O
it	O
is	O
able	O
to	O
observe	O
)	O
;	O
broadly	O
speaking	O
,	O
the	O
main	O
design	O
objective	O
of	O
a	O
hermetic	B-Algorithm
detector	I-Algorithm
is	O
to	O
maximise	O
acceptance	O
,	O
i.e.	O
</s>
<s>
Prior	O
to	O
the	O
building	O
of	O
the	O
Mark	O
I	O
,	O
it	O
was	O
thought	O
that	O
most	O
particle	O
decay	O
products	O
would	O
have	O
relatively	O
low	O
transverse	O
momentum	B-Algorithm
(	O
i.e.	O
</s>
<s>
momentum	B-Algorithm
perpendicular	O
to	O
the	O
beamline	O
)	O
,	O
so	O
that	O
detectors	O
could	O
cover	O
this	O
area	O
only	O
.	O
</s>
<s>
More	O
recent	O
hermetic	B-Algorithm
detectors	I-Algorithm
include	O
the	O
CDF	O
and	O
DØ	O
detectors	O
at	O
Fermilab	O
's	O
Tevatron	O
accelerator	O
,	O
as	O
well	O
as	O
the	O
ATLAS	O
and	O
CMS	O
detectors	O
at	O
CERN	O
's	O
LHC	O
.	O
</s>
<s>
There	O
are	O
three	O
main	O
components	O
of	O
a	O
hermetic	B-Algorithm
detector	I-Algorithm
.	O
</s>
<s>
From	O
the	O
inside	O
out	O
,	O
the	O
first	O
is	O
a	O
tracker	O
,	O
which	O
measures	O
the	O
momentum	B-Algorithm
of	O
charged	O
particles	O
as	O
they	O
curve	O
in	O
a	O
magnetic	O
field	O
.	O
</s>
<s>
The	O
tracking	O
system	O
plots	O
the	O
helix	B-Application
traced	O
by	O
such	O
a	O
charged	O
particle	O
as	O
it	O
travels	O
through	O
a	O
magnetic	O
field	O
by	O
localizing	O
it	O
in	O
space	O
in	O
finely-segmented	O
layers	O
of	O
detecting	O
material	O
,	O
usually	O
silicon	O
.	O
</s>
<s>
The	O
particle	O
's	O
radius	O
of	O
curvature	O
is	O
proportional	O
to	O
its	O
momentum	B-Algorithm
perpendicular	O
to	O
the	O
beam	O
(	O
i.e.	O
</s>
<s>
transverse	O
momentum	B-Algorithm
or	O
)	O
according	O
to	O
the	O
formula	O
(	O
where	O
is	O
the	O
particle	O
's	O
charge	O
and	O
is	O
the	O
magnetic	O
induction	O
)	O
,	O
while	O
the	O
degree	O
to	O
which	O
it	O
drifts	O
in	O
the	O
direction	O
of	O
the	O
beam	O
axis	O
gives	O
its	O
momentum	B-Algorithm
in	O
that	O
direction	O
.	O
</s>
<s>
By	O
contrast	O
,	O
a	O
photon	B-Application
is	O
neutral	O
and	O
interacts	O
electromagnetically	O
,	O
so	O
it	O
deposits	O
its	O
energy	O
in	O
the	O
calorimeter	O
without	O
leaving	O
a	O
track	O
.	O
</s>
