Difference between Roman Pots and VELO
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Difference between Difference between Roman Pots and VELO 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 100m 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 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. beam beam
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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. - PowerPoint PPT Presentation
Transcript of Difference between Roman Pots and VELO
Difference between Difference between Roman Pots and VELORoman 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 100m 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 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.
beambeam
Main Problem with MaterialMain 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 100m to 250m, for reason of RF shielding, the efficiency of the Vertex Trigger decreased by about 20% = 20% loss of good events.
IP = r1
2s2 / pt2 + 2 f2
r
f = distance from origin to last measured point / distance from first to last measured pointr1
= radius of first measured point s = multiple scattering factor
Other ProblemOther 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.
