The MACHO Project is a search for dark matter in the form of massive compact halo objects ( Machos ) .
Photometric monitoring of millions of stars in the Large Magellanic Cloud ( LMC ) , Small Magellanic Cloud ( SMC ) , and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects .
Analysis of the first 2.1 years of photometry of 8.5 million stars in the LMC reveals 8 candidate microlensing events .
This is substantially more than the number expected ( \sim 1.1 ) from lensing by known stellar populations .
The timescales ( { \widehat { t } } ) of the events range from 34 to 145 days .
We estimate the total microlensing optical depth towards the LMC from events with 2 < { \widehat { t } } < 200 days to be \tau _ { 2 } ^ { 200 } = 2.9 ^ { +1.4 } _ { -0.9 } \times 10 ^ { -7 } based upon our 8 event sample .
This exceeds the optical depth , \tau _ { backgnd } = 0.5 \times 10 ^ { -7 } , expected from known stars , and the difference is to be compared with the optical depth predicted for a “ standard ” halo composed entirely of Machos : \tau _ { halo } = 4.7 \times 10 ^ { -7 } .
To compare with Galactic halo models , we perform likelihood analyses on the full 8 event sample and a 6 event subsample ( which allows for 2 events to be caused by a non-halo “ background ” ) .
This gives a fairly model independent estimate of the halo mass in Machos within 50 kpc of 2.0 ^ { +1.2 } _ { -0.7 } \times 10 ^ { 11 } { M _ { \odot } } , which is about half of the “ standard halo ” value .
We also find a most probable Macho mass of 0.5 ^ { +0.3 } _ { -0.2 } { M _ { \odot } } , although this value is strongly model dependent .
Additionally , the absence of short duration events places stringent upper limits on the contribution of low-mass Machos : objects from 10 ^ { -4 } { M _ { \odot } } to 0.03 { M _ { \odot } } contribute \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 316 $ } } 20 \% of the “ standard ” dark halo .