- #1
CAF123
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Homework Statement
Construct the decompositions ##\mathbf 2 \otimes \mathbf 2 = \mathbf 3 \oplus \mathbf 1##, where ##\mathbf N## is the representation of su(2) with ##\mathbf N## states and thus spin j=1/2 (N-1).
Homework Equations
Substates within a state labelled by j can take on values -j to j in integer steps
3. The Attempt at a Solution
I think I get the idea but was hoping someone could just make sure I understand things correctly.
So we consider some states in the ##\mathbf 2## representation of SU(2), labelled as ##|j_1, m_1 \rangle## and take the tensor product of this with another state ##|j_2, m_2 \rangle##. If N=2, then j=1/2. So states are |1/2, 1/2> and |1/2,-1/2>, So out of these two states can form four possible tensor products. Take for example, $$|1/2, 1/2 \rangle \otimes |1/2, 1/2 \rangle$$ Then by Clebsch Gordan, possible states are ##|J.M\rangle## where ##|j_1 - j_2| < J < j_1 + j_2## and ##-J < M < J##? So the r.h.s is ##|0,0\rangle + |1,0\rangle + |1,-1 \rangle + |1,1\rangle## which is exactly those states in ##\mathbf 3 \oplus \mathbf 1##?
I am just wondering how the |0,0> state is part of ##\mathbf 3 \oplus \mathbf 1## on the r.hs? ##\mathbf 1## contains |0,0> but ##\mathbf 3## is always of the form ##|1,-1>, |1,0> ## or ##|1,1>##.
Thanks!