March 4, 2019
High Energy Accelerator Research Organization (KEK)
Japan Proton Accelerator Research Complex (J-PARC) Center

KOTO Experiment at J-PARC Hadron Experimental Facility
Broke World’s Best Sensitivity on Rare Neutral Kaon Decay
by an Order of Magnitude- first step forward elucidation of “matter-antimatter asymmetry” -Notable Points○ In the J-PARC experiment, the collaboration achieved the improvement by anorder of magnitude in the sensitivity to search for the rare neutral-kaondecay.
○ The collaboration has begun to elucidate the matter-antimatter asymmetry(CP symmetry breaking) in the universe through kaon decays.

【Synopsis】
An international collaboration reported the first major results from theexperiment KOTO, which is being conducted at the Hadron Experimental FacilityofJapan

ProtonAccelerator

ResearchComplex(J-PARC).

The

KOTO

collaboration set the upper limit of once in three hundred million on therate of the neutral K-meson (kaon) decay into a neutral pi meson and a pairof neutrinos from the analysis of the data set collected in 2015, and improved
the world’s best sensitivity by an order of magnitude. The collaboration
has begun to elucidate the matter-antimatter asymmetry (CP symmetrybreaking) in the universe through kaon decays. As the prospects for future,
further improvements on the sensitivity with data collected since 2016 areanticipated. The collaboration upgraded the KOTO detector in the autumn of
2018, and started to take the new data with it in February 2019.
The results have been published in the 18-January-2019 issue of the journal
PHYSICAL REVIEW LETTERS.

【Background of this research】
The matter-antimatter asymmetry (CP symmetry breaking) in the universe isa big mystery in modern physics. This phenomenon was discovered in 1964 inmeasuring K-meson (kaon) #1 decays #2 ; United States physicists J.Cronin andV.Fitch, who performed the experiment, received the Nobel Prize in Physics
in 1980. At KEK, the Belle experiment of the KEK B Factory measured the CP
symmetry breaking in B-meson decays and verified the theory proposed byM.Kobayashi and T.Maskawa, who received the Nobel Prize in Physics in 2008.
The Kobayashi-Maskawa theory explained the properties of B-mesons well, andis embedded in the standard model (SM) of particle physics. However, theasymmetries measured so far by experiments are insufficient to account for
the reason why the antimatter has disappeared in the current universe. Inaddition to the Kobayashi-Maskawa theory, there should be new physics beyond
the SM and the new matter-antimatter asymmetry due to it. The Belle IIexperiment of the Super KEK B Factory, the T2K long-baseline neutrino
oscillation experiment of J-PARC, and many research projects in the world
are underway for the purpose to pursue the mystery of matter-antimatterasymmetry.
The decay of neutral kaon K L , where L of K L means the long-lived one of neutralkaons #3 , into a neutral pi meson (pion) π 0 , a neutrino ν and an antineutrino,
in enormous number of kaons, is searched for to elucidate the,matter-antimatter asymmetry through kaon decays. This decay,
occurs once in thirty billion decays in the prediction of the SM. New sourcesof matter-antimatter asymmetry due to new physics may be revealed most clearly
decay, particularly in the cases where the decay occurs inin theseveral times or several-score times higher rates than the predicted one.
Thus, efforts have been underway for many years to improve the sensitivity
decay. The world’s best limit so far, once into search for theforty million, was obtained by the E391a experiment performed in the 2000s
for six months at the KEK 12GeV proton synchrotron. To improve the sensitivity
further and aim for observation of the rare decay, a new KOTO experiment byusing the high-intensity proton beam of the Japan Proton Accelerator ResearchComplex (J-PARC), located at Tokai-mura in Ibaraki Prefecture, Japan, isbeing conducted.

【Description of the research and results】
At the Hadron Experimental Facility of J-PARC, an international
collaboration (Spokesperson: Prof. Taku Yamanaka, Department of Physics,Osaka University, Co-spokesperson: Prof. Yau W. Wah, Enrico Fermi Institute,
University of Chicago) is conducting the search for the rare neutral-kaon. The name of the experiment KOTO is short for “K0 at Tokai”;
decay
69 collaborators are from Osaka University, KEK, J-PARC Center, Kyoto
University, Yamagata University, National Defense Academy, Saga University
and Okayama University of Japan, University of Chicago, University of Michiganand Arizona State University of the United States, National Taiwan University

of Taiwan, Korea University, Chonbuk National University and Jeju National
University of Korea and JINR of Russia (Figure 1). The experiment is supportedby Ministry of Education, Culture, Sports, Science, and Technology (MEXT) ofJapan; the Japan Society for the Promotion of Science (JSPS); the U.S.
Department of Energy, Office of Science, Office of High Energy Physics; theMinistry of Education and the National Science Council/Ministry of Scienceand Technology in Taiwan; and the National Research Foundation of Korea. No
otherexperiment is present as of 2019.

Figure 1: Group photo of the KOTO collaborators taken in June 2018 at Tokai.
In the Hadron Experimental Facility, a proton beam with the kinetic energyof 30 GeV, extracted from the Main Ring accelerator to the experimental hall,strikes a gold target and produces secondary particles for particle andnuclear physics experiments. For the KOTO experiment, neutral kaons producedin the target are transported via the 20-meter-long neutral beam line to theexperimental area in the south floor of the hall. Detector components arelocated inside the cylindrical vacuum vessel with the diameter of 4 meters
and the length of 6 meters, and catch the particles from neutral kaon decays
decay arein vacuum (Figures 2 and 3). Neutrinos in theundetected; two gamma-rays from a neutral pion that decays immediately are
precisely measured with the electromagnetic calorimeter #4 located downstream.
If there were other detectable particles in a kaon decay the event should; for hermetic detection of extra particles to reject such
not bebackground events, gamma-ray and charged-particle veto counters covers thekaon decay volume.

Figure 2: Schematic view of the detector system of the KOTO experiment.
The neutral kaon beam enters from the left. The electromagnetic calorimeterin purple measures two gamma-rays from a neutral pion.

Figure 3: Photo of the detector system of the KOTO experiment in the Hadron
Experimental Facility of J-PARC. The tank painted in blue is the vacuum vessel.
The KOTO collaboration constructed the beam line in 2009 and theelectromagnetic calorimeter in 2010. After the East Japan Earthquake thecollaboration finished the detector construction in 2012 and collected thefirst data for a few days in 2013. From April 2015, when the Hadron
Experimental Facility resumed the user operation, KOTO started the genuine
data taking.
signal
It is very difficult to discriminate and identify the000
from the background events due to primary kaon decays: K L → π π π , K L →π 0 π 0 , K L → π 0 π + π - , etc. and due to neutrons contained in the neutral beam.
Several detector components have been upgraded even after the detector
construction was finished. A long and thoughtful data analysis has to be made.
The first major results reported this time is from the analysis of the data
set collected for 68 days in total during the user operation of the Hadron
Experimental Facility in 2015 from April to June and from October to December.

The rate of thedecay is less than once in three hundred million,
which is so far the most stringent upper limit on the decay（ Figure 4）.

Figure 4: Final plot representing the results of the KOTO experiment fromthe analysis of the 2015 data set. The decay vertex position (Z vtx ) and thetransverse momentum (P t ) of the neutral pion in the horizontal and verticalaxes, respectively, were calculated assuming that the vertex was on the beamaxis. The black italic (red regular) numbers indicate the numbers of observed(expected background) events for the regions inside the lines. Nocandidate events were observed in the signal region surrounded by red lines.

【 Significance of the results and future prospects】
The new results from the KOTO experiment improved the world’s best
decay by an order of magnitude.
sensitivity to search for theThe KOTO collaboration has begun to elucidate the matter-antimatterasymmetry in the universe through kaon decays. The collaboration upgraded
the gamma-ray detectors in 2016 and collected data from 2016 to 2018, whichcorresponds to 1.4 times larger than the data in 2015. The analysis iscurrently undertaken intensively, and further improvements onthesensitivity are anticipated. In the autumn of 2018 the collaboration
upgraded the electromagnetic calorimeter to significantly suppress thebackground events due to neutrons. A data taking with the new detector systemstarted in February 2019. By improving the sensitivity and approaching to
the predicted rate in the SM, the experiment makes strides toward exploringnew physics beyond the SM and the new matter-antimatter asymmetry due toit.

【 Publication】
J.K.Ahn et al. (KOTO Collaboration),“Search forandDecays at the J-PARC KOTO Experiment,”
Physical Review Letters 122, 021802 - Published 15 January 2019
DOI: https://doi.org/10.1103/PhysRevLett.122.021802 .

【 Contacts】< on physics contents >Osaka University
Department of Physics, Professor / KOTO Spokesperson
Taku YAMANAKA
E-mail： taku@champ.hep.sci.osaka-u.ac.jp

University of Chicago
Enrico Fermi Institute, Professor / KOTO Co-spokesperson
Yau W. Wah
E-mail: ywah@uchicago.edu
High Energy Accelerator Research Organization (KEK)
Institute of Particle and Nuclear Studies, Professor
Takeshi KOMATSUBARA
E-mail： takeshi.komatsubara@kek.jp< for Media >High Energy Accelerator Research Organization (KEK)
Public Relation Office, HeadHajime HIKINO
E-mail： press@kek.jp
High Energy Accelerator Research Organization (KEK)
Institute of Particle and Nuclear Studies, Public Relation Coordinator
Hiroko TADA
E-mail： htada@post.kek.jp
J-PARC Center
Public Relation Section, Leader
Minako ABE
E-mail： abe.minako@jaea.go.jp【Footnotes】#1 K-meson (kaon)
A K-meson is any of a group of particles which contains a strange quark (s),which belongs to the second generation of quarks in particle physics. Moreexplicitly speaking, a “charged K-meson（ K + ） ”is the bound state of a strange
antiquark and an up quark (u) and a “neutral K-meson（K 0 ） ”is the bound stateof a strange antiquark and a down quark (d); both the up and down quarks belong
to the first generation. There also be the antiparticles of a charged K-mesonand a neutral K-meson, K - and0K-mesons, K and, as shown in Figure 5. There are two neutral, which are a particle and an antiparticle.

Figure 5: Four K-mesons (kaons) containing a strange quark.
#2 decay
Heavy particles, produces by high-energy collisions of particles, have atransition to lighter particles over time. This occurs in a very short time,
say in one billionth of a second. This phenomenon, due to the interactionsof the quarks in the particles, is called “decay” in particle physics. The
decay of a particle proceeds via several paths or decay modes. By examining
the pattern of decays, quark interactions can be understood.
#3 long-lived neutral kaon
Two neutral kaons K 0 andare a particle and an antiparticle. Theirsuperposition makes them into a long-lived neutral kaon（ K L ） and ashort-lived one（K S ）. Due to the way of superposition, K L has a propertyof CP odd. The system of a neutral pion and a pair of neutrinos has a property
decay changes the CP property from odd toof CP even. Thus, theeven during the transition and breaks the CP symmetry.
#4 electromagnetic calorimeter
An electromagnetic calorimeter is a particle detector to measure theincident position, timing and energy of gamma-rays. The calorimeter of theKOTO experiment was built by stacking two types of 50-centimeter-longundoped Cesium Iodide (CsI) crystals, with 2.5-centimeter-squared and5-centimeter-squared in their cross sections, inside the cylinder of 2 meterin the diameter (Figure 6). The CsI crystals had originally been used ina kaon decay experiment named KTeV at the Fermi National Accelerator
Laboratory in the United States in 1990s, and were transferred to Japanthrough the Japan-U.S. Cooperative Research Program in High Energy Physics.
The energy of gamma-rays is transformed to light emission of CsI crystals,
the emission is converted to electronic signals and their waveform isrecorded by custom-made high speed electronic circuits made by Universityof Chicago and a data acquisition system built by University of Michigan.

Figure 6: Photo of the electromagnetic calorimeter taken in February 2011,
when the stacking of the CsI crystals was completed.

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