Difference between Roman Pots and VELO 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 100  m of steel are acceptable because of the large momentum. Rad.length of Fe = 5  of Al 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 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. beam

Main Problem with Material Multiple scattering The 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 100  m to 250  m, for reason of RF shielding, the efficiency of the Vertex Trigger decreased by about 20% = 20% loss of good events.   IP = r 1 2  s 2 / p t f 2   r f = distance from origin to last measured point / distance from first to last measured point r 1 = radius of first measured point s = multiple scattering factor

Other Problem Radiation length in 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.