References - Bogoliubov Laboratory of Theoretical Physics - JINR

ing should be appropriate. We also analyze possibilities to avoid the betatron resonance
νx =1(νx is the horizontal tune).
The system of units � = c =1isused.
Let us consider spin dynamics in a usual storage ring with a noncontinuous magnetic
field and magnetic focusing. The general equation for the angular velocity of spin
precession in the cylindrical coordinates is given by (see Ref. [5])
ω (a) = − e
m
�
aB − aγ
�
1
β(β · B)+
γ +1 γ2 �
− a (β × E)
− 1
+ 1
�
B� −
γ
1
β2 (β × E) �
�
�
. (1)
Eq. (1) is useful for analytical calculations of spin dynamics with allowance for field
misalignments and beam oscillations. This equation does not contain terms depending
on an electric dipole moment and other small terms. The sign � denotes a horizontal
projection for any vector. The vertical magnetic field, Bz, and the radial electric one, Eρ,
are the main fields in the muon g − 2 experiment. The spin precession caused by these
fields is defined by
ω (a)
z = − e
� �
1
aBz −
m γ2 � �
− a βφEρ . (2)
− 1
Let Ω (a) denotes the average value of ω (a) . The spin coherence is kept when
B0
dΩ (a)
z
dp
=0. (3)
For a storage ring with noncontinuous fields, the quantities Bz and Eρ should be averaged.
Condition (3) can be satisfied for ordinary storage rings with usual magnets (Fig. 1).
If the field created by the magnets is given by Bz(ρ) =const · ρ−n , the field index and
betatron tunes into bending sections are equal to
n = − R0
� �
∂Bz
, νx =
∂ρ
√ 1 − n, νz = √ n,
ρ=R0
where B0 ≡ Bz(R0), x = ρ − R0. Average angular velocity of spin precession is given by
Ω (a) = ω(a) z πρ
πρ + L
πeaρBz(ρ)
= − , (4)
m(πρ + L)
where L is a half of the total length of the straight sections (Fig. 1). The muon anomaly
is given by
aμ = gp − 2 mμ
2 mp
Ω (a)
μ
Ω (a)
p
, (5)
where the fundamental constants gp and mμ/mp are measured with a high precision. The
magnetic field is the same for muons and protons when they move on the same trajectory.
In this case, their momenta coincide.
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