Publication Authorship Credits Many physics papers now have dozens of authors per paper. Experimental physics may have multi-organizational and multi-country contributing staffs, but I'd guess that most of the names don't contribute a word or equation to a paper, yet they get individual authorship credit.
My question is who determines the author list, does everybody listed have editing privilages, and perhaps most importantly, who decides on their listed order?   
 A: OK, this is for experimental high energy physics as I worked in the field for over 40 years.
There are groups in institutions, universities and research ones. There are many such in each country, and there are many countries. The group leaders in the group decide who signs a paper, mainly by the man hours put in the construction and running of the experiment and also considering  contributions in analysis of present and other papers. The order is alphabetical per author within a group, per name of institution. There have been long discussions on changing the credit attribution, but I see that the same holds for LHC papers.
Why are there so many names? In my carreer I worked on one large ( previous had about 50 people) experiment( 350 people) from inception of the idea to taking data and analysis. It took 10 years to build the detector by hundreds of people, years that yielded very few publications from the full work put in, certainly over 8 hours a day. Credit was accumulating from analysis published/worked-on  previous experiment papers. Then another 15 years of data analysis where there are also large numbers of working groups, made up from people from all groups, and many people working on the same subject with their own analysis. The final paper is decided by the working group, a joining of all individual analysis. The names are still the ones the individual institute group leader gives to the working group. The working group proposes a preprint to the collaboration editing board, and if the board approves of the paper it goes to the full collaboration meeting, passed by consensus. Every person who signs can comment and ask for changes. They usually do not, as there is trust that the working groups are doing their job well.
The system is completely open. Any group member can join in the analysis and comment.
A: It is typical for large collaborations in nuclear and particle physics to have considerable internal bureaucracy, including a Memorandum of Understanding or similar document that lays out the rules for membership in the collaboration, authorship on papers published as a collaboration, vetting rules for separate publications of  papers on work done on the experiment, and so on.
As anna says the rules often put the choice into the hands of a institutional leader, but they also might require things like


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*Every author must sit at least three days of shift per year (which can result in some grad student having to baby a very senior, but somewhat thumb handed theorist through the shift process (though I learned a lot when I had that experience))

*Every author must attend at least one collaboration meeting per year in person

*Some monetary figure that must be contributed toward the collaboration budget (either put into a common fund or spent of capital equipment) for each "author" (this money, of course, generally comes from grants made to the institution, so it goes back to anna's point)


and similar "make sure you're still interested" type qualifications.
The paper is usually drafted by committee (the members of which are selected at collaboration meetings, and they generaly want more not fewer volunteers), and are made available to the whole collaboration for comments a few times before they are submitted (and usually again when the reviewer comments come back).
