Effect of TCTVB collimator on ZDC A fraction of the spectator neutrons will hit the jaws Two physic parameters/measurements are affected –The energy mean value and resolution centrality determination –The phi azimuthal angle distribution determination of the reaction plane in nuclear collisions (measurement of the directed flow) The effect depends on the crossing angle value, the beam divergence and the Fermi momentum spread

Calculation of the effects of TCTVB GEANT description of the beam line Fermi momentum and the beam divergence applied to the spectator neutrons Energy released in the calorimeter determined by a fast simulation Transverse momentum due to Fermi  = 142 MeV/c corresponding to:  = 51  rad

Spectator neutron spot on TCTVB Two effects must be considered 1.Beam divergence (depend on the   at the collision) (assumed to be 30 microrad) 2.p T due to Fermi momentum Adding in quadrature we get giving at TCTVB a spot with  of ~4.4 mm  Gap between jaws should be +/ mm in order not to lose neutrons when the crossing angle is ZERO The crossing angle moves the spot vertically

jaw Spectator neutron Spot (~3 sigma) Crossing angle 0  rad Spectator neutron Spot (~3 sigma) Crossing angle 100  rad Neutron spot on the TCTVB 26 mm

Results We have calculated the percentage of spectator neutrons hitting the TCTVB jaws the azimuthal distribution of the spectator neutrons the total energy measured by the neutron calorimeter in selected impact parameter window

Effect of TCTVB - crossing 0 Percentage of spectator neutrons hitting the jaws = 0.8% Slight non uniformity already seen

Effect of TCTVB - crossing 40  rad Percentage of spectaror neutrons hitting the jaws = 1.2% Non uniformity in azimuth

Effect of TCTVB - crossing 100  rad Percentage of spectator neutrons hitting the jaws = 4.9% Strong non uniformity

General comments the measurement of the reaction plane and the determination of the flow of the spectator neutrons requires a flat distribution in the azimuthal angle –the determination of the reaction plane is affected we plan to equalize the different towers requiring the same response to a perfectly flat azimuthal distribution –calibration between towers ‘in situ’ at risk non uniformity depends on the crossing angle value

Effect on the energy measurement zero crossing angle 3 centrality class: 1)b<3 fm 2)3<b<6 fm 3)6<b<9 fm Mean value of the energy decreases of ~1 – 2 %

Effect on the energy measurement 100 microrad crossing angle Mean value of the energy decreases of ~ 5 – 9 %