Whenever I read about quant strategies it leads me to stratergies which involve simultaneous buying and selling of two or more assets. Pairs trading, arbitrage, market neurtal or headging all these approaches involves a simultaneous buying and selling of some assets. I want to know if there exists a strategy which doesn't follow this pattern. For example a quant strategy which explains mathematically when to buy an asset and sell that same asset in future (independently of other assets). If those exist then what are those and can you give a brief explanation of them?
Broadly you're asking about directional versus relative value strategies. There are lots of directional approaches, but I've yet to see many discussed publicly in non-generic ways (I mean, if they work, why would anyone talk about them?).
As others have noted, trend following is a notable example. I'd consider a lot of equity factor approaches as a combination of directional and relative value (eg, buying low vol stocks typically involves first identifying low vol and high vol stocks and then taking some position that skews toward the low vol basket, either by going long-short, weighting toward low vol, etc.).
See also this article on Bloomberg: https://www.bloomberg.com/news/articles/2018-10-02/your-guide-to-the-many-flavors-of-quant-investing-quicktake