Difference between Roman Pots and VELO

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Difference between Roman Pots and VELO
The VELO is designed for a precise primary and
secondary vertex reconstruction and for
standalone tracking in an early trigger level.
The vacuum tank is around the interaction
region. MANY particles, amongst them photons and
electrons, are produced with LOW momentum.
Particles enter under a SHALLOW angle, average
100mrad, minimum 10mrad. The material seen by a
particle can be 10-100 times the
thickness. Because of the spread of the
interaction point, we need many detectors.
Because we have to reconstruct down to small
angles, the vacuum tank becomes quite long.
Very forward tracking is typically done using
detectors located in Roman pots. They are far
away from the interaction point. Particles,
typically ONE proton, carrying a LARGE fraction
of the beam momentum, enter through a thin steel
window almost PERPENDICULAR . Several 100mm of
steel are acceptable because of the large
momentum. Rad.length of Fe 5 ? of Al
beam
beam
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Main Problem with Material

• Multiple scatteringThe trigger decision is based
  on tracks which are displaced from the primary
  vertex. Because there is no momentum information
  at this early trigger stage, particles of low
  momentum which undergo multiple scattering can
  fake the signature of a displaced secondary
  vertex.

By increasing the thickness of the Al foil from
100mm to 250mm, for reason of RF shielding, the
efficiency of the Vertex Trigger decreased by
about 20 20 loss of good events.
f distance from origin to last measured point
/ distance from first to last measured
pointr1 radius of first measured point s
multiple scattering factor
s2IP r12?s2 / pt2 2 f2 s2r
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Other Problem

• Radiation lengthin the present design, there are
  regions where particles see more than 50 of a
  radiation length. Problem for downstream
  detectors !Moving from TP design to new design
  with increased Al thickness, the number of
  secondary particles produced in the VELO
  increased by a factor of 2.