B→η(′) form factors in QCD

We calculate the semileptonic form factors f(+)(B ->eta)(q(2)) and f(+)(B ->eta ')(q(2)) from QCD sum rules on the light-cone (LCSRs), to NLO in QCD, and for small to moderate q(2), 0 <= q(2) <= 16 GeV2. We include in particular the so-called singlet contribution, i.e. weak annihilation of the B meson with the emission of two gluons which, thanks to the U(1)(A) anomaly, couple directly to eta((')). This effect is included to leading-twist accuracy. This contribution has been neglected in previous calculations of the form factors from LCSRS. We find that the singlet contribution to f(+)(B ->eta ') can be up to 20%, while that to f(+)(B ->eta) is, as expected, much smaller and below 3%. We also suggest to measure the ratio B(B ->eta ' e nu)B(B -> eta e nu) to better constrain the size of the singlet contribution.