  The exponential growth in the number of scientific papers makes it
increasingly difficult for researchers to keep track of all the publications
relevant to their work. Consequently, the attention that can be devoted to
individual papers, measured by their citation counts, is bound to decay
rapidly. In this work we make a thorough study of the life-cycle of papers in
different disciplines. Typically, the citation rate of a paper increases up to
a few years after its publication, reaches a peak and then decreases rapidly.
This decay can be described by an exponential or a power law behavior, as in
ultradiffusive processes, with exponential fitting better than power law for
the majority of cases. The decay is also becoming faster over the years,
signaling that nowadays papers are forgotten more quickly. However, when time
is counted in terms of the number of published papers, the rate of decay of
citations is fairly independent of the period considered. This indicates that
the attention of scholars depends on the number of published items, and not on
real time.
