I believe you are referring to the Nobel laureate's Abdus Salam's PhD student who himself became a particle physicist like his advisor. Riazuddin, physicist. A quick search of Google Scholar shows his early papers in particle physics as M. Riazuddin, which is mostly likely an abbreviated version of Mohammad Riazuddin. It is very common in South Asia to skip the first name and call by the middle name if the first name starts with Mohammad (out of respect).
You are right, in Urdu, Riaz is an independent name which is plural of "garden" , but "Uddin" cannot exist independently.
So to parse his name M. Riazuddin, it could be Mohammad Riaz Uddin but his joint spelling Riazuddin is also common.
On the other hand, it seems that associating "M." with Riazuddin is a mistake of CERN / Google Scholar.
Edit: If we check Cambridge University library for his PhD thesis, his only name appears as Riazuddin as the author. In CERN scanned papers, the databases list him as M. Riazuddin but actually only Riazuddin appears in the actual pdf of the paper. See for example: https://cds.cern.ch/record/143116
My (pure) speculation is that both last names of the twin brothers rhyme, so if they ever had first names, they just dropped them for convenience. It is very likely that he never had or used M. before his name. He just had a single name which just means "gardens of faith".