Admission criteria for top PhD programs in theoretical particle physics can vary among universities, and while research achievements during undergraduate years can be advantageous, they are not necessarily a strict requirement for admission.
Admission committees for prestigious PhD programs typically consider a combination of factors, including academic performance, letters of recommendation, statement of purpose, and performance in standardized tests (such as the GRE Physics Subject Test). They also take into account the potential and aptitude of applicants for research in theoretical physics.
While research experience as an undergraduate can certainly bolster an applicant's profile, it is not always expected or accessible to every student. Recognizing this, admission committees also consider other indicators of a student's potential, such as coursework, advanced courses taken, the rigor of the undergraduate institution, and the applicant's ability to demonstrate a deep understanding of physics through exams and interviews.
It's important to remember that each applicant is evaluated holistically, and research achievements alone do not guarantee admission. Many students are admitted to top PhD programs based on their exceptional academic performance, strong letters of recommendation, and potential demonstrated through coursework and exams, even if they have limited or no research experience.
Ultimately, while research achievements can enhance an application, they are not an absolute requirement, and admission decisions are based on a combination of factors to identify promising candidates with the potential to excel in theoretical particle physics research.