We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood .
High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars , and nearby Hipparcos counterparts were also observed .
We determined their distances , velocities , and metallicities to measure the gravitational force law perpendicular to the Galactic plane .
As in most previous studies , we applied one-parameter models of the vertical gravitational potential .
We obtained a disk surface mass density within 1.1 kpc of the Galactic plane , \Sigma _ { 1.1 kpc } = 64 \pm 5 \mathrm { M } _ { \sun } \mathrm { pc } ^ { -2 } , with an excellent formal accuracy , however we found that such one-parameter models can underestimate the real uncertainties .
Applying two-parameter models , we derived more realistic estimates of the total surface mass density within 800 pc from the Galactic plane , \Sigma _ { 0.8 kpc } = 57-66 \mathrm { M } _ { \sun } \mathrm { pc } ^ { -2 } , and within 1.1 kpc , \Sigma _ { 1.1 kpc } = 57-79 \mathrm { M } _ { \sun } \mathrm { pc } ^ { -2 } .
This can be compared to literature estimates of \sim 40 \mathrm { M } _ { \sun } \mathrm { pc } ^ { -2 } in stars and to 13 \mathrm { M } _ { \sun } \mathrm { pc } ^ { -2 } in the less accurately measured ISM contribution .
We conclude that there is no evidence of large amounts of dark matter in the disk and , furthermore , that the dark matter halo is round or not vey much flattened .

A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane , 42 \pm 2 Myr , which can not be related to the possible period of large terrestrial impact craters \sim 33-37 Myr .