We use the Baryon Oscillation Spectroscopic Survey ( BOSS ) Data Release 9 ( DR9 ) to detect and measure the position of the Baryonic Acoustic Oscillation ( BAO ) feature in the three-dimensional correlation function in the Lyman- \alpha forest flux fluctuations at a redshift z _ { eff } = 2.4 .
The feature is clearly detected at significance between 3 and 5 sigma ( depending on the broadband model and method of error covariance matrix estimation ) and is consistent with predictions of the standard \Lambda CDM model .
We assess the biases in our method , stability of the error covariance matrix and possible systematic effects .
We fit the resulting correlation function with several models that decouple the broadband and acoustic scale information .
For an isotropic dilation factor , we measure 100 \times ( \alpha _ { iso } -1 ) = -1.6 ^ { +2.0 +4.3 +7.4 } _ { -2.0 -4.1 -6.8 } ( stat . ) \pm 1.0 ( syst . )
( multiple statistical errors denote 1,2 and 3 sigma confidence limits ) with respect to the acoustic scale in the fiducial cosmological model ( flat \Lambda CDM with \Omega _ { m } = 0.27 , h = 0.7 ) .
When fitting separately for the radial and transversal dilation factors we find marginalised constraints 100 \times ( \alpha _ { \parallel } -1 ) = -1.3 ^ { +3.5 +7.6 +12.3 } _ { -3.3 -6.7 -10.2 } ( stat . ) \pm 2.0 ( syst . )
and 100 \times ( \alpha _ { \perp } -1 ) = -2.2 ^ { +7.4 +17 } _ { -7.1 -15 } ( stat . ) \pm 3.0 ( syst . ) .
The dilation factor measurements are significantly correlated with cross-correlation coefficient of \sim - 0.55 .
Errors become significantly non-Gaussian for deviations over 3 standard deviations from best fit value .
Because of the data cuts and analysis method , these measurements give tighter constraints than a previous BAO analysis of the BOSS DR9 Lyman- \alpha forest sample , providing an important consistency test of the standard cosmological model in a new redshift regime .