In the $m_q \rightarrow 0$ limit the QCD lagrangian has the symmetry $U(N)_V \times U(N)_A$. Including just the two lightest quarks, $N=2$, and looking at the $U(2)_A=SU(2)_A \times U(1)_A$ part, we have upon SSB 8 PNGB's from the $SU(2)_A$. However, no goldstone boson is observed that matches the $U(1)_A$-part ("The $U(1)_A$ problem"). The way I see this explained is that $U(1)_A$ was never a real symmetry (the associated axial current is anomalous), and thus there is no SSB or GB either.
QUESTION: InIn the Peccei-Quinn solution to the strong CP problem one adds a new anomalous U(1) symmetry to cancel the CP-violating theta-term. How come we can have axions as a PNGB of this anomalous symmetry, when in the case of the $U(1)_A$ problem anomalousness was used to explain why there should not be a PNGB?
RelatedThis question has been answered here: Spontaneous symmetry breaking of anomalous global abelian symmetries and $U(1)$ of QCD
The answer (I think) seems to say that for $U(1)_{PQ}$ the symmetry is exact in the $m_a \ll \lambda_{QCD}$ limit (the anomaly is small) and thus we get PNGBs from SSB, while for $U(1)_{A}$ the anomaly is not small so we get no goldstone bosons.
- Are anomalies then merely the reason for explicit symmetry breaking, and if this breaking is small enough (so that we can have SSB) we will have (pseudo-) goldstone bosons?
- What happens to the goldstone bosons between this limit where the anomaly is small and a limit where it isn't (maybe for example with time dependent axion mass around when the axion mass is "turned on" at $T=T_{QCD}$)?
Brendan Darrer
- 1.3k
- 1
- 9
- 20