  A study of forward energy flow in leptonically decaying W bosons using data
of an integrated luminosity corresponding to 36 pb-1 of pp collisions at a
center-of-mass energy of 7 TeV is presented. This data was recorded with the
CMS detector during the 2010 running of the LHC. In this sample of W events,
about 300 events with no significant energy deposits in one of the forward
calorimeters are observed. This corresponds to a large pseudorapidity gap of at
least 1.9 units. The majority of the charged leptons from these W decays are
found in the hemisphere opposite to the gap. This gives a strong indication of
a diffractive component in the W production, which can be explained in terms of
diffractive parton distribution functions (PDF) which have, on average, a
smaller x than the conventional parton PDFs.
