B-factory Programme Advisory Committee
Short Report for
Annual Review Meeting
5-7 February 2024, Tsukuba, Japan
A. Andreazza∗ (Milano), P. Collins∗ (CERN), G. Corti (CERN),
M. Demarteau (ORNL), R. Forty (CERN), B. Gavela (Madrid),
S. Gori (UCSC), W. Hulsbergen∗& (NIKHEF), M. Ishino (Tokyo),P. Mato∗ (CERN), F. Meijers∗ (CERN), N. Neufeld (CERN),
B. Ratcliff∗ (SLAC), A. A. Petrov∗ (Univ. South Carolina),
M. Sullivan& (SLAC), H. Tajima (Nagoya), M. Titov∗ (Saclay)
and chaired by T. Nakada (EPFL)&Remote participation∗Expert member.
N. Pastrone (Torino) participated remotely as a designated member

25 February 2024

Short Summary
The annual review meeting of the B-factory Programme Advisory Committee (BPAC)
took place at KEK from 5th to 7th of February, where the committee heard presentations
from the accelerator and Belle II groups on the status and upgrade plan. This shortreport gives the committee’s feedback for the six questions asked by the management of
the Institute of Particle and Nuclear Studies. A detailed report on the findings of thecommittee will be found in a separate document.
1. Reviewing the overall project, is there anything on which the collaboration should focus more to strengthen the project?
The committee finds that the overall project, with the accelerator complex andexperiment, is well focused on obtaining large statistics of high quality data, processing them rapidly and exploiting them for physics analyses.
2. Are the objectives of SuperKEKB and Belle II upgrades well definedand achievable?
The accelerator group and Belle II collaboration have been developing plans for upgrading the accelerator complex and Belle II detector components during the next
long shutdown, LS2, with the aim to reach a peak luminosity of 6 × 1035 cm−2 s−11

and to collect 50 ab−1 of data by around 2035. The Conceptual Design Report(CDR) for the LS2 upgrade plans, presented during this meeting, described various
ideas for how to increase the machine luminosities, enhance the detector performance and the tolerance against beam backgrounds and radiation damage. Someof them are well advanced, in particular the new vertex detector system (VTX),
which is envisioned to replace the current vertex detector, for which the committee
recommends that the group should advance towards the engineering level of studies,including the aspects of system integration and operation.
Some of the other subsystems appear to be still at the exploration stage and further
technical studies are required. Although those items are marked as “long term”,
this prevents drawing a coherent picture of the upgrade plan. The machine grouppresented an upgrade path with a new set of final focusing magnets. Since they
have just started machine operations after the Long Shutdown 1 with a lot ofchanges and improvements having been made, they need first to understand thebehaviour of the machine in order to identify the remaining causes of luminosity
limitation and to provide a definite scheme for the upgrade.
While the CDR provides an excellent initial framework to continue the development of the plans for the upgrade, the BPAC is not yet in a position to judge
the validity of the plan described in the CDR. While the committee recommends
further R&D work to continue, it also thinks that a new document providing a coherent and integrated description of the accelerator and detector upgrade plan witha quantitative demonstration of expected physics performance should be submitted
for making a decision on the project. Such a document should also demonstrate
the tolerance against background rates at the peak luminosity of 6 × 1035 cm−2 s−1
and integrated radiation damage up to 50 ab−1 .
As for the idea to collect data with a longitudinally polarised electron beam, theproposed test of production of polarised e− in the injector and measuring thepolarisation decay time in the SuperKEKB ring will provide essential information
for making further decisions. However, it should be noted that the highest priorityof the coming period must be to achieve stable machine operation with a luminosity
above 1035 cm−2 s−1 for data taking.
3. Can the updated scheme of the data and run dependent MC production
accommodate the current and coming high luminosity data?
The committee supports the new format for the data recording being adequate
for the forthcoming high luminosity run. Production of run-dependent simulation
data will be crucial for controlling systematics in the analysis. Presented progress
for the planning of large production of data is satisfactory. The committee took
note of the increased effort in event generators and encourages the software group
towards further collaboration with existing activities elsewhere. Fast calibration by
the subdetector groups remains critical for smooth data processing.
4. Have all the concerns about the detector operation been addressed?
The committee was pleased by the progress being made in the detector hardware,2

online and detector control, on and offline monitoring framework, software andcomputing for the start of Run 2 data taking.
5. Are physics analysis plans for the winter conferences and also toward
summer convincing? Are we missing any physics opportunities?
The BPAC considers the analysis activities presented for the winter conferences
and beyond very adequate and convincing. Published results demonstrate thatthe Belle II collaboration has been exploiting well the advantage of being able toreconstruct final states with neutral particles including neutrinos. Merging of theBelle and Belle II analyses has also been successful. The committee is looking
forward to hear the progress in the analysis of dark sector and tau physics, as wellas other analyses relevant to the understanding of the muon g − 2 anomaly. WithRun 2, the Belle II experiment should be able to start collecting data with statistics
competitive with those of the BaBar and Belle experiments, and the analysis effort
on the core B physics should be prepared.
6. Is the computing resource estimate adequate?
The committee took note of the presented computing resource accounting for 2023
and estimates for 2025 to 2028. A dedicated discussion and recommendation will
be given by the expert group consisting of G. Carlino, W. Hulsbergen, P. Mcbrideand P. Mato, and chaired by the BPAC chair, after receiving the written accounting
report and request.

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