The spectrum of the Hamiltonian is therefore also discrete and has a mass gap. From
the commutator of two supercharges one may deduce a BPS equation. Since spectrum is
discrete and we have supersymmetric pairing of non-BPS states, the Witten index picks
up contributions only from those BPS states.
If we consider round S
5
, then in our R-gauge field background (2.2) we find that our
Hamiltonian can be expressed in the form
H = H
0
+
1
2r
(R
12
+ R
34
) + m (R
12
− R
34
)
where H
0
is the nonsupersymmetric Hamiltonian we compute with R gauge field turned
off (and which would correspond to ∆ in radial quantization). Here by R
12
and R
34
we
denote the two Cartan generators of SO(5) which is the R-symmetry of the M5 brane
(and which we denoted as R
1
and R
2
above). However, a generic mass parameter m
breaks this symmetry down to SU(2)
R
× SU(2)
F
where SU(2)
R
is the R symmetry and
SU(2)
F
is another global symmetry. The 8 real supercharges (N = (1, 0) supersymmetry)
that we preserve do not commute with H
0
. The purpose with turning on some R gauge
field background is to preserve some supersymmetry. Thus supercharges that we preserve
commute with H, and this is true for any value of the mass parameter m, so in particular
they commute with R
12
−R
34
. The points at ±m =
1
2
have enhanced supersymmetry with
16 real supercharges. On a squashed S
5
where the squashing is along the Hopf fiber, it is
plausible that this critical value gets changed to the value ±m =
1
2
+
a
2
where we saw the
simplification of the index was happening above. But to show this we would need to derive
the Killing spinor solution on this squashed S
5
.
We can consider a general squashing on S
5
by turning on three chemical potentials
a
i
= (a, b, c) for the three Cartan generators of the isometry group SO(6), without imposing
any restrictions on these chemical potentials other than they shall be real-valued. If we
use a parametrization
z
i
= r
i
e
iφ
i
of S
5
with r
i
lying on S
2
, say r
i
= r (sin θ sin φ, sin θ cos φ, cos θ), we get the induced metric
ds
2
= dr
2
1
+ r
2
1
dφ
2
1
+ dr
2
2
+ r
2
2
dφ
2
2
+ dr
2
3
+ r
2
3
dφ
2
3
on S
5
. In Lorentzian signature squashing by turning on these chemical potentials amounts
to changing the metric on R × S
5
into [18]
ds
2
= −dt
2
+ dr
2
1
+ r
2
1
(dφ
1
+ adt)
2
+ dr
2
2
+ r
2
2
(dφ
2
+ bdt)
2
+ dr
2
3
+ r
2
3
(dφ
3
+ cdt)
2
Wick rotation of t alone would lead to a complex metric, so we propose that we should
Wick rotate the a
i
’s as well so that the Euclidean metric becomes
ds
2
= dt
2
+ dr
2
1
+ r
2
1
(dφ
1
+ adt)
2
+ dr
2
2
+ r
2
2
(dφ
2
+ bdt)
2
+ dr
2
3
+ r
2
3
(dφ
3
+ cdt)
2
We have not been able to rigorously compute the Witten index with generic squashing
parameters a
i
since the representation theory of SO(6) is complicated. Instead we will
compute the Witten index in Lorentzian signature with a = b = c by compensating this
– 6 –