Solution

ID: past-exam-of-the-groups-and-representations-course-of-the-university-of-oxford/2022/1/d/ii/solution

Now the doublet invariant tensor would satisfy . Its four entries acquire factors , , , and , respectively. None equals , so . Equivalently, has no trivial representation. The mixed doublet and third-family terms are again forbidden because their tensor product of group representations contain no trivial representation. Only the product of the two fields survives:

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