Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THP059

DEVELOPMENT OF A POLARIZED BEAM FOR SuperKEKB
Z. J. Liptak∗, Hiroshima University, Higashihiroshima, Japan
Abstract
The SuperKEKB accelerator is currently in operationin Tsukuba, Japan, with a planned long shutdown in 2026.
Among the possible upgrades being considered during thisperiod is the change to a polarized electron beam in the HighEnergy Ring. Such a change would require modifications inthe source generation and transport, geometrical and lattice
variations to provide spin rotation, and polarimetry. A Polarized SuperKEKB Working Group has been formed frommembers of the Belle II experiment and the SuperKEKB
accelerator team to investigate the possibilities and challenges of these modifications. This talk lays out the goals of
the proposed upgrade, considers the necessary changes tothe existing accelerator and their feasibility and lays out thephysics motivation behind such an effort.

INTRODUCTION
The SuperKEKB [1, 2] accelerator began operation in2016 at KEK in Tsukuba, Ibaraki, Japan, and has been providing electron-positron collisions for the Belle II experiment’s full physics data taking operation since 2019. While
the planned lifetime of the experiment is approximately 10years, the KEK road map includes several long shutdowns
for planned upgrades and maintenance.
At the moment, neither of SuperKEKB’s beams are polarized; among the proposed upgrades during that time is
the inclusion of spin polarization of the high-energy electron ring. This would open up new areas of measurement
for Belle II with minimal changes necessary in the existing
beam line.
The major improvements needed to produce a polarized
beam are:of neutral currents are considered one of the highest-priority
avenues for discovery of physics beyond the SM. Any measurement finding a deviation of sin2 𝜃 𝑊 from SM prediction
would be a clear indication of new physics.
The introduction of an electron beam spin-polarized above
70% would open a new avenue for measurement of weakneutral currents in a manner complementary to existing experiments. In particular, a polarized beam would enable
Belle II to measure the weak neutral vector coupling constants of the 𝑏 and 𝑐 quarks and muon with a substantial
improvement in precision over past experiments [3, 4].
Figures 1 and 2 display the predicted precision of weakneutral vector current measurements with a data set of
20 ab−1 taken with a polarized beam. In addition to precision, it can also be seen that Belle II’s measurements would
be in a parameter space of order 10 GeV, complementary to
existing higher- and lower-energy range searches. Figure 3shows the predicted resolving power for the vector couplingsof the 𝑏 and 𝑐 quark compared to current SM predictions.
Furthermore, recent predictions also indicate that measurement of a chiral asymmetry in the process 𝑒 + 𝑒 − → 𝜏 + 𝜏 −at 1% precision would provide a measurement of the 𝜏 magnetic moment on the order of the current 4𝜎 tension between
the SM and experiment [5]. Such a measurement would bea new avenue at investigating anomalous lepton magnetic
moments without the need to build a dedicated accelerator
or experiment.

• Generation of a highly-polarized beam, and transportof polarized electrons from the electron source to themain storage ring
• Vertical spin preservation during storage in the main
ring, and rotation to a longitudinal spin for collisions,followed by restoration to the vertical• Precision polarimetry
A beam polarization working group, consisting of members of the SuperKEKB accelerator team and Belle II researchers, has been created to investigate possibilities for
upgrading to a polarized electron beam and to determine
and meet the challenges involved.

PHYSICS MOTIVATION
The weak mixing angle 𝜃 𝑊 is a fundamental parameterof the Standard Model (SM), and precision measurements

Figure 1: Scale dependence of sin2 𝜃 𝑊 (giving the weakmixing angle) defined in the 𝑀𝑆 renormalization scheme.
Adapted from Ref. [6] to include projected errors on proposed/upcoming experiments at particular energy scales,
including that at an upgraded SuperKEKB that includes polarized electron beams.

∗ zjl@hiroshima-u.ac.jp- 881 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THP059

Figure 2: Predicted Q2 -dependent shift in sin2 𝜃 𝑊 caused
by a 15 GeV mass dark Z, represented by the dark blue band.
Adapted from [7].

Figure 3: Predicted resolving ability of vector coupling constants of the 𝑏 and 𝑐 quarks with 20 ab−1 of data taken witha polarized beam.

PROPOSED UPGRADES
To facilitate the inclusion of a polarized electron beam inSuperKEKB, all upgrades under consideration are intended
to minimize disruption of the existing beamline.

Beam Source and Transport
To achieve the stated physics goals, a high beam polarization (≥ 70%) is desired. GaAs sources have in the past been
demonstrated to produce polarization >90% with a quantum
efficiency (QE) of 1.6% [8], but a wide band gap makes accelerating the generated electrons difficult. To alleviate thisissue, research is underway in the development of thin-film
Negative Electron Affinity (NEA) surfaces applied to theGaAs cathode, as shown in Fig. 4.
While this technique has been in use in the past, cathodelifetimes are notably short. To ameliorate this issue, a DC
gun can be used instead of an RF gun; such a backup system
exists already at KEK and can be utilized without significanttrouble.
A beam buncher can also be employed to produce bunchesof 4 nC, in line with SuperKEKB’s standard.
Simulations have shown that vertical beam polarization
can be achieved with the use of a Wien filter at the source

Figure 4: The NEA film mechanism. A thin surface film
applied to the GaAs base reduces the band gap and imparts
energy to generated electrons, enabling more efficient acceleration.

point, and that polarization at the source can be maintained
through the SuperKEKB linac to the Main Ring injector.

Spin Preservation in the Main Ring
In the 3 km Main Storage Ring (MR), the beam would
need to be circulated with a vertical polarization vector witha rotation to (from) longitudinal polarization before (after)
collisions at the IP. Spin-polarization vectors in this scheme
are shown graphically in Fig. 5.
Challenges inherent to this problem include the preservation of a vertical spin vector around the MR; precise rotationof polarization vectors to that needed for collision; minimal
disturbance of the existing beam lattice; and satisfying spinmatching conditions enough to avoid negatively affecting
the polarization lifetime.
KEK has prepared an optical model of the beam latticefor use in simulation software, and simulations have demonstrated that this can be achieved in a number of ways which
require minimal changes to the existing beam lattice. One
such option is the inclusion of superconducting combined
di/quadrupole magnets between 100 and 150 m before andafter the IP; alternately, rotator magnets could be installedin drift sections of the ring to achieve the same effect.

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THP059

Figure 5: Simulated spin vectors in the proposed rotation
scheme for the main storage ring. Electron spins are maintained vertically throughout the majority of the ring androtated to (from) the horizontal before (after) collisions.

Polarimetry
Polarimetry with high precision and short integration timeis needed to measure the beam polarization state and relate
it to physics measurements taken in Belle II.
A Compton polarimeter can be deployed for this purpose,and can be used continuously as it is non-destructive. In thisscheme, the beam electrons are incident on polarized laser
light, and a fraction of them undergo Compton scattering.
The asymmetry between electron and photon polarizationsaligned parallel and antiparallel is well known and can beused to measure the beam polarization; backscatter electrons can also be used for a largely independent polarization
measurement.
A Compton Polarimetry system for use in SuperKEKB,
shown in Fig. 6 is under development [9] based on that usedby the QWeak Collaboration, which carried out the mostprecise polarization measurement to date [10].

PROPOSED TIMELINE FOR UPGRADES
The current KEK Roadmap for the near future provides
two long shutdowns for Belle II, during which time planned
maintenance and upgrades may be carried out. The first ofthese is slated to occur in 2022, to be focused mainly on
replacement of sensitive physics detectors near the BelleII Interaction Point (IP). The next planned long shutdown
will occur in 2026. Because of the time involved in R&D
for the components necessary for a polarized beam and thenecessity to carefully plan changes to the beamline, including
the possibility of changing magnets, the 2026 shutdown isconsidered the more realistic possibility.

Figure 6: Compton polarimeter demonstrator under development at the University of Manitoba.

CONCLUSION
The inclusion of a polarized electron beam at SuperKEKB
provides an opportunity to make unique measurements ofweak neutral vector currents in a parameter space complementary to existing experiments and at unprecedented precision.
A working group has been assembled with members of theSuperKEKB accelerator team as well as Belle II researchers,
and has identified key challenges in upgrading to a spinpolarized beam. The working group is actively working
on surmounting the technical challenges involved in beam
generation and transport, spin preservation and rotation andpolarimetry measurement necessary for producing a usefulspin-polarized beam.
Work is currently ongoing in the development of robust
spin-polarized cathodes and magnet design, and first-ordersimulations have shown that electron spin can be preserved
through the injector linac, beam transport lines, and mainring, and that spin rotation to horizontal configuration ispossible with minimal changes to the existing SuperKEKB
lattice.

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[2] SuperKEKB Design Report. https://kds.kek.jp/indico/event/15914/- 883 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 THP059[3] A. Aleksejevs, S. Barkanova, C. Miller, J. M. Roney,and V. Zykunov, “NLO radiative corrections for ForwardBackward and Left-Right Asymmetries at a B-Factory”, Phys.
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