Decay KL Research Articles

Scrutinizing CKM unitarity with a new measurement of the Kμ3/Kμ2 branching fraction

Precision tests of first-row unitarity of the Cabibbo-Kobayashi-Maskawa matrix currently display two intriguing tensions, both at the $3\sigma$ level. First, combining determinations of $V_{ud}$ from superallowed $\beta$ decays with $V_{us}$ from kaon decays suggests a deficit in the unitarity relation. At the same time, a tension of similar significance has emerged between $K_{\ell 2}$ and $K_{\ell 3}$ decays. In this Letter, we point out that a measurement of the $K_{\mu3}/K_{\mu 2}$ branching fraction at the level of $0.2\%$ would have considerable impact on clarifying the experimental situation in the kaon sector, especially in view of tensions in the global fit to kaon data as well as the fact that the $K_{\mu2}$ channel is currently dominated by a single experiment. Such a measurement, as possible for example at NA62, would further provide important constraints on physics beyond the Standard Model, most notably on the role of right-handed vector currents.

KOTO II at J-PARC : toward measurement of the branching ratio of

The KOTO II experiment at J-PARC is a next-generation experiment that is under design aiming at the measurement of the branching ratio of the decay. The KOTO II experiment is planned in the extended Hadron Experimental Facility at J-PARC using the second production target. With the second production target, the KL extraction angle of 5 degrees is chosen instead of 16 degrees in order to obtain a larger KL flux and harder KL momentum spectrum while keeping the same ratio of neutron and KL fluxes. To realize the extraction angle, the KOTO II detector is behind the primary beam dump. The KOTO II detector has a longer decay volume and a larger calorimeter to achieve a larger decay probability and higher acceptance of the KL decay. With the designed beam line and the detector, 40 signal events are expected for the branching ratio of 3×10−11 with 60 background events in the running period of 3 × 107 s, which corresponds to approximately 5-σ discovery of the decay.

First thought on a high-intensity KS experiment

The K → µµ decays have recently been identified as another golden kaon physics mode along-side the rare processes. These golden modes provide precision tests of the Standard Model with very high sensitivity to New Physics. The presented study is exploring the possibility to address the KL − KS → μ + μ − interference experimentally and outlines the challenges associated with such an ambitious project for the far future. A next-generation experiment at the intensity frontier is required that should be capable of collecting a large sample of \U0001d4aa(1014 − 1015 ) KL and KS decays. Challenges related to the beamline design and detector technology need to be overcome if we want to address this mode experimentally. A significant background suppression of KS → π + π − and radiative KL → μ + μ − γ decays is imperative for a few % measurement, which would require excellent kinematic resolution and efficient photon detection. The first attempt at a possible experimental setup to measure this effect is presented. Last but not least, a huge number of neutral particles produced offers the possibility to study a plethora of other rare KL , KS decays as well as hyperon decays enhancing the physics motivation for such an initiative.

New experimental limits on double-beta decay of osmium

Double-beta processes in 184Os and 192Os were searched for over 15 851 h at the Gran Sasso National Laboratory (LNGS) of the I.N.F.N. by using a 118 g ultra-pure osmium sample installed on the endcap of a 112 cm3 ultra-low-background broad-energy germanium detector. New limits on double-electron capture and electron capture with positron emission in 184Os were set at the level of lim T 1/2 ∼ 1016–1017 years. In particular the 2ν2K and 2ν KL decays of 184Os to the ground state of 184W are restricted as T 1/2 ⩾ 3.0 × 1016 years and T 1/2 ⩾ 2.0 × 1016 years, respectively. A lower limit on the half-life for the double-beta decay of 192Os to the first excited level of 192Pt was set as lim T 1/2 = 2.0 × 1020 years at 90% confidence level

Four-dimensional calorimeter to discriminate gammas from neutrons for the KOTO experiment

We study the rare decay KL→π0νν̄ at the J-PARC KOTO experiment. This decay mode is highly suppressed in the Standard Model (SM). The signature of the decay is two gammas from a π0 decay and no other detectable particles. In the KOTO experiment, the two gammas are measured with a calorimeter which is composed of undoped cesium iodide (CsI) crystals. Each crystal is read out with a photomultiplier tube (PMT) attached on its rear side. One of the main backgrounds to KL→π0νν̄ is caused by neutrons in the beam halo. To reach the sensitivity predicted in the SM, we need to suppress the background further by a factor of ten. We developed a method to attach Multi Pixel Photon Counters (MPPCs) on the front end of the crystals. The depth of the interaction, measured with the timing difference between the MPPC signal from upstream and the PMT signal from downstream, is used to discriminate gammas from neutrons. We obtain four-dimensional information on the showers (timing and three-dimensional position) as well as their energies from the calorimeter. We installed MPPCs in the autumn of 2018 and took data during the beam time in 2019. Using the four-dimensional calorimeter, the background is suppressed by a factor of 45.

A light scalar explanation of (g − 2)μ and the KOTO anomaly

The KOTO experiment has recently performed a search for neutral Kaons decaying into neutral pions and a pair of neutrinos. Three events were observed in the KOTO signal region, with an expected background of about 0.05. Since no clear signal of systematic errors have been found, the excess of events in the decay KL→ π0νv¯ is quite intriguing. One possibility to explain this anomaly would be the presence of a scalar \U0001d719 with mass of the order of the pion mass and inducing decays KL→ π0\U0001d719 which mimic the observed signal. A scalar with mass of the order of the pion mass and a coupling to muons of the order of the Standard Model Higgs coupling could also explain the muon anomalous magnetic moment anomaly (g − 2)μ. We built on these facts to show that a light singlet scalar with couplings to the leptons and quarks as the ones induced by mixing with Higgs states in two Higgs doublet models may lead to an explanation of both anomalies. More specifically, we show that this is the case in the so-called type-X models in which leptons and quarks couple to two different Higgs doublets, and for scalar masses that are in the range between 40 and 70 MeV. Due to the relatively large coupling to leptons required to fit (g − 2)μ, the scalar lifetime accidentally falls into the sub-nanosecond range which is essential to evade the severe proton beam dump experiments and astrophysical constraints, though it becomes sensitive to constraints from electron beam dump experiments. The additional phenomenological properties of this model are discussed.

A new theory framework for the electroweak radiative corrections in Kl3 decays

We propose a new theory framework to study the electroweak radiative corrections in Kl3 decays by combining the classic current algebra approach with the modern effective field theory. Under this framework, the most important mathcal{O} (GFα) radiative corrections are described by a single tensor Tμν involving the time-ordered product between the charged weak current and the electromagnetic current, and all remaining pieces are calculable order-by-order in Chiral Perturbation Theory. We further point out a special advantage in the {K}_{l3}^0 channel that it suffers the least impact from the poorly-constrained low-energy constants. This finding may serve as a basis for a more precise extraction of the matrix element Vus in the future.

Study of Rare Mesonic Decays Involving Di-Neutrinos in Their Final State

We have studied phenomenological implication of R-parity violating (Rp) Minimal Supersymmetric Model (MSSM) via analyses of pure leptonic (M→νν¯) and semileptonic decays of pseudoscalar mesons (M→Xνν¯). These analyses involve comparison between theoretical predictions made by Rp MSSM and the Standard Model (SM) with the experimental results like branching fractions (Br) of the said process. We have found, in general, that Rp contribution dominates over the SM contribution, i.e., by a factor of 10 for the pure leptonic decays of KL,S and by 102 and 104 in case of Bs and Bd, respectively. Furthermore, the limits obtained on Rp Yukawa couplings (λαkβ′λαkγ′⁎) by using Br (M→Xνν¯) are used to calculate Br (M→νν¯). This demonstrates the role of Rp MSSM as a viable model for the study of new physics contribution in rare decays at places like Super B factories, KOTO (J-PARC) and NA62 at CERN.

Recent results from NA62 and NA48

This paper reviews recent results form the NA62 and NA48 experiments at CERN. NA62 has searched for the K → πνν¯ with an in-flight decay technique. The analysis of 1.2 · 1011 decays has found one event compatible with the Standard Model prediction, giving an upper limit for the branching ratio of 1.4 · 10-9 (95% CL) in the background assumption. Prospects for improvements in the measurement are given. Results on the Kl3 decay form factors from NA48/2 are presented as well as the first observation of the decay K± → π ± π0e+e- and the measurement of its branching ratio from the same experiment.

James Watson Cronin

James Watson Cronin

Evaluating the Charged Background Rejection Requirement in an Experiment to Measure BR(KL → π0νν-) at the CERN SPS

Measuring the rate at which the long-lived, neutral kaon decays into a neutral pion, neutrino, and anti-neutrino allows physicists an opportunity to test precise predictions made by the Standard Model. Differences between theoretical predictions and experimental measurements may point to new physics. Not only does the Standard Model predict a very low probability at approximately 3 KL→ π0νν-) decay in 100 billion KL decays, but many of the common decays leave false signals in the detector that look the same as the true signal. Charged decays have been studied to determine the required detection efficiency necessary to eliminate them. The conclusion of these studies is that a reduction by a factor of 1/(3 × 109) will be required to achieve the 10:1 signal to charged background ratio necessary for the experiment.

Val Logsdon Fitch

Val Logsdon Fitch

Performance study of a prototype pure CsI calorimeter for the KOTO experiment

Abstract The performance of the CsI electromagnetic calorimeter to be used for the J-PARC E14 KOTO experiment for a rare decay K L → π 0 ν ν ¯ was studied by using a positron beam from an accelerator. The energy, timing, and position resolutions for energies up to 800 MeV were measured. We also developed a new method to reconstruct the incident angle of photons based on the shower shape recorded with the calorimeter, and evaluated the method to suppress possible backgrounds for K L → π 0 ν ν ¯ decay signals.

The Kπ form factors from Analyticity and Unitarity

Analyticity and unitarity techniques are employed to obtain bounds on the shape parameters of the scalar and vector form factors of semileptonic Kl3 decays. For this purpose we use vector and scalar correlators evaluated in pQCD, a low energy theorem for scalar form factor, lattice results for the ratio of kaon and pion decay constants, chiral perturbation theory calculations for the scalar form factor at the Callan-Treiman point and experimental information on the phase and modulus of Kπ form factors up to an energy tin = 1GeV2. We further derive regions on the real axis and in the complex-energy plane where the form factors cannot have zeros.

Search for the Rare DecaysKL→π0π0μ+μ−andKL→π0π0X0→π0π0μ+μ−

The KTeV E799 experiment has conducted a search for the rare decays KL->pi0pi0mu+mu- and KL->pi0pi0X0->pi0pi0mu+mu-, where the X0 is a possible new neutral boson that was reported by the HyperCP experiment with a mass of (214.3 pm 0.5) MeV/c^{2}. We find no evidence for either decay. We obtain upper limits of Br(KL->pi0pi0X0->pi0pi0mu+mu-) < 1.0 x 10^{-10} and Br(KL->pi0pi0mu+mu-) < 9.2 x 10^{-11} at the 90% confidence level. This result rules out the pseudoscalar X0 as an explanation of the HyperCP result under the scenario that the \bar{d}sX0 coupling is completely real.

Designing Klod detector prototypes and results of their experimental tests

The projected KLOD facility is intended for finding and investigating the ultrarare decay KL0 → π0ν\( \bar \nu \); its branching ratio predicted by the Standard Model is Br = (3.0 ± 0.6) × 10−11. Designing and testing the prototype of the decay-volume veto system and beam veto calorimeter, which are the most important detectors of the facility, are considered. It is shown that the proposed beam veto calorimeter is able to detect γ rays with a high efficiency at the neutron flux of 300 MHz.

Yangian description for decays and possible explanation of X in the decay [formula omitted

In this letter, hadronic decay channels of light pseudoscalar mesons are realized in Yangian algebra. In the framework of Yangian, we find that these decay channels can be formulated by acting transition operators, composed of the generators of Yangian, on the corresponding pseudoscalar mesons. This new description of decays allows us to present a possible interpretation of the new unknown particle $X$ in the decay $K^0_L\to \pi^0 \pi^0 X$: it is an entangled state of $\pi^0$ and $\eta$.

Search for the K L 0 → π0ν % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB % PrgifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0x % c9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8fr % Fve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeaaeaaakeaaieGace % WF2bGbaGaaaaa!38B3! $$ \tilde v $$ decay at the INEP U-70 accelerator: The KLOD project

The rare decay KL0 → π0ν\( \tilde v \) branching ratio measurement is one of the clearest Standard Model test. Calculations based on the SM predict Br(KL0 → π0ν\( \tilde v \)) ≈ 2.8 × 10−11, but the most accurate experimental value Br(KL0 → π0ν\( \tilde v \)) < 6.7 × 10−8 (90% C.L.). We present design of a new experimental setup KLOD (U-70 accelerator, IHEP, Protvino) for KL0 → π0ν\( \tilde v \) branching ratio measurement. Sensitivity of the KLOD experiment will be enough for registration of 2.4 events KL0 → π0ν\( \tilde v \) for every 10 days of the data taking (according to SM predictions).

K→πlνsemileptonic form factors from two-flavor lattice QCD

We present new lattice results of the K -> pion semileptonic form factors obtained from simulations with two flavors of dynamical twisted-mass fermions, using pion masses as light as 260 MeV. Our main result is f+(0) = 0.9560 (84), which, combined with the latest experimental data for Kl3 decays, leads to |V_{us}| = 0.2267 (5)_exp (20)_f+(0). Using the PDG(2008) determinations of |Vud| and |Vub| our result implies for the unitarity relation |Vud|**2 + |Vus|**2 + |Vub|**2 = 1.0004 (15). For the O(p**6) term of the chiral expansion of f+(0) we get Df = f+(0) - 1 - f2 = -0.0214 (84).

Electromagnetic effects inKℓ3decays

We study the radiative corrections to all Kl3 decay modes to leading non-trivial order in the chiral effective field theory, working with a fully inclusive prescription on real photon emission. We present new results for Kμ3 modes and update previous results on Ke3 modes. Our analysis provides important theoretical input for the extraction of the CKM element Vus from Kl3 decays.