If a group of the smartest people were transported to the stone age, how long would it take to rebuild to our level of modern technology, e.x., the large hallidron collider?

They wouldn't be able to accomplish it in their lifetimes, or their childrens' lifetimes, and after that, things could break down very quickly. If there were enough smart people to get a self-sustaining society going (and I think this would be a big number—say, 50,000) with a diverse set of skills useful for reconstructing modern society, I could imagine that they'd get up to a Roman level of technology during their lifetimes, or their childrens'. But of course, after Rome came the dark ages. There's no reason to think that this group would go through exactly the same reversal, but there's no reason to think that somewhere along the way they wouldn't lose much of what they had built due to disease, outside invasion, some kind of internal religious fervor, whatever. The continuity of knowledge and the cultural drive to keep expanding knowledge would be the keys. Once human society got out of the dark ages, things started expanding very rapidly with the renaissance and the industrial revolution. If there were a continuity of knowledge, this group could skip the whole "let's invent the scientific method" step and jump straight into the industrial revolution, which has probably been proceeding as fast as possible, if not faster. So, say, 400-500 years if there were no reversals, but that's a big if.

Most of the answers here fail to take inte account the energy wasted in maintaining a population density. Depending on the size of the group agriculture might not be needed at all. Most hunter-gatherers worked somewhere between 12 to18 hours per week, leaving plenty of time to engage in other activites such as mining, refining, smithing, masonry etc. Efficient toolmaking with iron tools might even decrease the time spent foraging and hunting even more. So if the group is small enough to be supported by a given area (food has to be transported towards the main population without spoiling) there would be less of a die off. And when I say small enough this is still a sufficient population density to have a small city since the transition to city life happened BEFORE agriculture. (Not really surprising, you need a permanent residence, as well as a notion of private ownership before starting to plant stuff into the ground for you to harvest a season later). This if the land transported to is abundant enough of course as for instance the Levant during the paleothic. But my guess is that most subtropical climates would do fine. Temperate regions with harsh winters would be much tougher since its hard to store food for months. So instead of trying to resolve the question into how many would die before agriculture we could start by how many would die before aquiring sufficient survival skills to life comfortably as hunter-gatherers. Once that skillset is aquired and the population lives in an Eden of abundance, with breath-taking beauty, easy work hours and communal life providing plenty of social stimulus why on earth rush for the office cubicle?

It depends on what they had with them.  If there were no repositories of knowledge (e.g. books) and no tools at all it would take an extremely long time if ever. To put much of our current knowledge into practice demands a sophisticated web of precision production and logistics so a lot of knowledge alone would be fairly useless other than as a general guide. Ironically, stone age skills such as the ability to find edible roots would be more immediately valuable.  Better hope the survivors aren't just intelligent - special forces training would be helpful.