50:50 sugar/fat mixture
1. What was the original research paper?

This one appears close: Hyperphagia in rats produced by a mixture of fat and sugar (PubMed, 1990)


  Experimental groups were fed sugar (sucrose), fat (corn oil), or a
  sugar-fat mixture as an option to chow; options were in the form of
  water solutions or emulsions. The control group was fed only chow. The
  sugar-fat group displayed a robust hyperphagia (greater than 36%),
  relative to the control group; the hyperphagic response was greater
  than that observed in the fat group but not in the sugar group. The
  sugar-fat group selected more calories from the option than the other
  two experimental groups. Body weight gains were also greater in the
  sugar-fat group than in the fat and sugar groups. Addition of
  saccharin to the fat emulsion increased fat and total intakes to
  levels close to those of the sugar-fat mixture. In a second
  experiment, the relative palatability of the plain and sweet fat
  emulsions was assessed with two-bottle preference tests. The sugar-fat
  mixture was preferred to the saccharin-fat mixture, which in turn was
  preferred to the plain-fat emulsion. These results suggest that the
  sweetness of the sugar-fat mixture contributed to the pronounced hyperphagia and obesity obtained with this diet option.


2. Have there been any related new results with other mixtures like sugar/salt fat/salt?

A free-choice high-fat high-sugar diet induces changes in arcuate neuropeptide expression that support hyperphagia (PubMed, 2010) (a study in rats)


  Our data suggest that the specific combination of saturated fat and
  a 30% sugar solution results in hyperphagia-induced obesity.


Snack food intake in ad libitum fed rats is triggered by the combination of fat and carbohydrates (PubMed Central, 2014)


  Thus, it can be concluded that the combination of fat and
  carbohydrates is a major molecular determinant of potato chips
  triggering hedonic hyperphagia.


Fat/carbohydrate ratio but not energy density determines snack food intake and activates brain reward areas (PubMed Central, 2015)


  We conclude from our behavioural data that the ratio of fat and
  carbohydrates, but not the absolute energy density, is the major
  determinant of the palatability and intake of snack food during
  short-term two-choice preference tests in rats.


Which Foods May Be Addictive? The Roles of Processing, Fat Content, and Glycemic Load (PubMed Central, 2015) (a human study)


  In summary, the current study found that highly processed foods,
  with added amounts of fat and/or refined carbohydrates (e.g., sugar,
  white flour), were most likely to be associated with behavioral
  indicators of addictive-like eating. Additionally, foods with high
  GL [glycemic load)] were especially related to addictive-like eating
  problems for individuals endorsing elevated symptoms of “food
  addiction.”



In conclusion, it can be either a combination of fats and sweetness (due to sugar or non-sugar sweeteners) or a combination of fats and high glycemic carbs (sugar, white flour, potatoes) that can induce overeating.