Diacetyl is caused by higher fermentation temps, not lower, among other causes. You raise the temp to encourage the yeast to keep cleaning up unwanted compounds. Good article in an old issue of Brew Your Own magazine
Brew Your Own: The How-To Homebrew Beer Magazine - Story Index - Brewing Science - Diacetyl: Homebrew Science
This is from the article
Higher temperatures lead to more diacetyl formation. For example, a lager beer that is fermented at 57° F can produce up to three times as much diacetyl as a similar beer fermented at 40° F.
Diacetyl removal is accelerated at higher temperatures. Some lager brewers raise the temperature of the beer to around 57–61° F for a day or so after primary fermentation and prior to cold conditioning. This is called a diacetyl rest. Ale brewers can also perform a diacetyl rest by waiting for a couple of days after primary fermentation is over before chilling the beer.