II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Status of the J-PARC E16 experiment in 2020
S. Yokkaichi∗1 for the J-PARC E16 Collaboration
We proposed the experiment E161) to measure thevector meson decays in nuclei in order to investigate
the chiral symmetry restoration in dense nuclear matter.
The experiment started at the J-PARC Hadron Experimental Facility.
This experiment aims to systematically study thespectral modification of vector mesons in nuclei, particularly the ϕ meson, using the e+ e− decay channel withstatistics that are two orders larger in magnitude thanthose of the precedent E3252) experiment performed at
KEK-PS. In other words, it aims to accumulate 1 × 105
to 2 × 105 events for each nuclear target (H, C, Cu, andPb) and deduce the dependence of the spectral modification on the size of matter and meson momentum.
A scientific (“stage-1”) approval was granted to theexperiment E16 by PAC in March 2007. For the full(“stage-2”) approval, a technical design report was submitted to PAC in May 2014 and reviewed for experimental and budgetary feasibility. In the PAC meeting
held in July 2017, the stage-2 approval for 320 hoursof a commissioning run was granted. In this run, background measurement at the new beamline is required.
The construction of High-momentum beamline, where
the experiment will be conducted, has been completed
by KEK in June 2020.
Our proposed spectrometer has 26 modules. As shownin Fig. 1, a module consists of Lead-glass calorimeter(LG) and Hadron-blind detector (HBD) for electron
identification, as well as three-layers of GEM Trackers(GTR) and a single layer of silicon strip detecor (SSD) as
the tracking devices. Owing to budget limitations, thecommissioning run was started with a limited number ofmodules.
The first half of the commissioning run, called Run0a, was successfully performed in June 4–20, 2020, witha configuration of 6(SSD)-6(GTR)-4(HBD)-6(LG) modules.3) The beam was delayed from February in the original plan, owing to a bureaucratic issue and possibly because of the COVID-19 pandemic, in the Nuclear Regulatory Agency, to issue the permission for the new beamline. As a user beamtime, 159 hours were executed, including 10-hours of downtime in total; each downtime
was less than 30 min. (downtimes over 30 min were compensated). Here, “downtime” is due to problems causedby the accelerator or the beamline operation and not byusers.
As the design, a primary proton beam with an intensity of 1×1010 protons per 2 sec duration (5.2 sec cycle)
was delivered to our spectrometer. The interaction rate
at the targets (C and Cu) is 10 MHz because the totalinteraction length of the targets is 0.2%. One of ourconcerns, GEM breakdown under a high particle rate,∗1

RIKEN Nishina CenterFig. 1. Plan view of the proposed E16 spectrometer in theeight-module configuration for Run-1. The red lines show
the parts that have been constructed as of December
2021, for the commissioning run planned for February
2021 (Run-0b). The SSD located in the innermost layer
is not shown.

was not observed in HBD. In GTR, the breakdown of 13strips was observed in 300-GTR, which caused approximately 11% of dead region in total, but operation wascontinued. The electron ID performance of HBD andLG were roughly confirmed.
A background study was performed, and the LG single
rate was found to be two times as high as the expectation, which is a scaled value of the E325 experiment.
However, a factor of two is within the designed margin.
The origin of the background will be studied to improve
the situation.
As of January 2021, the second half of the commissioning run, Run-0b, is scheduled for February 2021.
GTR and HBD were uninstalled from the spectrometer after the Run-0a, and refurbished in J-PARC andRIKEN, respectively. Additionally, two GTR and twoHBD modules were newly constructed, and installed inNovember–December 2020 along with the refurbishedones. Consequently, a 6(SSD)-8(GTR)-6(HBD)-6(LG)were installed, as shown by the red lines in Fig. 1.
After Run-0b, Run-1 (physics run) is planned for autumn 2022. Its approval should be obtained at the JPARC PAC. We have to update the technical design
report for Run-1 based on the result of Run-0 and submit by December 2021, following which discussion will
be conducted at the PAC held in January 2022.
References1) S. Yokkaichi et al., J-PARC proposal No. 16 (http://j-parc.jp/researcher/Hadron/en/pac_0606/pdf/p16Yokkaichi_2.pdf); Y. Komatsu et al., JPS Conf. Proc.
12, 020005 (2017); S. Ashikaga et al., JPS Conf. Proc.
26, 024005 (2019).
2) R. Muto et al., Phys. Rev. Lett. 98, 042501 (2007).
3) S. Yokkaichi, RIKEN Accel. Prog. Rep. 53, 79 (2020).

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