Even at such a crucial juncture of your career, you want to go back to school. US would be really good for you.

Lot of research is happening in Particle physics, I dont need to tell you that.

I think you should not waste your time going after Pak unis and convincing them to admit you. They do not care for someone who is really willing to research, and frankly will not care after you complete your education and want to contribute to the field in your country.

But US unis will. And thats why I am sure they you have a good chance of acquiring RA/TA type assistantship.

I myself am pursuing studies in US in engineering. I would suggest that you sit down for a week and do thorough browsing US/Canada unis for particle physics programs.

Stanford is a great choice, but its highly completive and expensive. Make a list of about 6 unis that you like. Send emails to their program directors/professors and tell them your background and what you want to research in. Also mention that you positively need funding either in form of RA or TA.

Once you get their reply (it can take weeks, sometimes no reply), pick out 4-5 unis.

While you are waiting for reply, start preparing for TOEFL. By reading your question, I think it shouldn’t take you more than 2 weeks.

After you get some response (could be positive or negative), register for TOEFL test at one of the 3 test centers in Pakistan, wherever you might be. Information is available online at www.toefl.org. You can register and pick a test date yourself, all online if you have a credit card.

The complete list of required docs for admission to unis is different for each institute, and will be available online on uni site. University’s website is your most accurate and complete information resource. Use it.

You will need good BS GPA, very good GRE scores, interesting research proposal among other things to increase your chances of admission. This is not UK. You will be rejected by some universities.

I must also give you a heads-up. The technology you intend to pursue, I believe, is classified among sensitive fields in US. You may NOT be able to get admission in it if you are not a US citizen or permanent resident. You may want to think of other places like EU.