0000000000178310
AUTHOR
V. Kozhuharov
Precision measurement of the ratio BR(KS→π+π−e+e−)/BR(KL→π+π−πD0)
Abstract The K S → π + π − e + e − decay mode was investigated using the data collected in 2002 by the NA48/1 Collaboration. With about 23 k K S → π + π − e + e − events and 59 k K L → π + π − π D 0 normalization decays, the K S → π + π − e + e − branching ratio relative to the K L → π + π − π D 0 one was determined to be BR ( K S → π + π − e + e − ) / BR ( K L → π + π − π D 0 ) = ( 3.28 ± 0.06 stat ± 0.04 syst ) × 10 − 2 . This result was used to set the upper limit | g E1 / g BR | 3.0 at 90% CL on the presence, in the decay amplitude, of an E1 direct emission ( g E1 ) term relative to the dominant inner bremsstrahlung ( g BR ) term. The CP-violating asymmetry A ϕ in the sin ϕ cos ϕ distri…
Searches for lepton number violation and resonances in K± → πμμ decays
The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003–2004. A new upper limit on the rate of the lepton number violating decay K±→π∓μ±μ± is reported: B(K±→π∓μ±μ±)<8.6×10−11 at 90% CL. Searches for two-body resonances X in K±→πμμ decays (such as heavy neutral leptons N4 and inflatons χ ) are also presented. In the absence of signals, upper limits are set on the products of branching fractions B(K±→μ±N4)B(N4→πμ) and B(K±→π±X)B(X→μ+μ−) for ranges of assumed resonance masses and lifetimes. The limits are in the (10−11,10−9) range for resonance lifetimes below 100 ps.
Search for heavy neutrinos at the NA48/2 and NA62 experiments at CERN
© The Authors, published by EDP Sciences. The NA48/2 experiment at CERN has collected large samples of charged kaons decaying into a pion and two muons for the search of heavy nuetrinos. In addition, its successor NA62 has set new limits on the rate of charged kaon decay into a heavy neutral lepton (HNL) and a lepton, with = e, µ, using the data collected in 2007 and 2015. New limits on heavy neutrinos from kaon decays into pions, muons and positrons are presented in this report.
Empirical parameterization of the K±→π±π0π0 decay Dalitz plot
As first observed by the NA48/2 experiment at the CERN SPS, the π0π0 invariant mass (M00) distribution from K±→π±π0π0 decay shows a cusp-like anomaly at M00=2m+, where m+ is the charged pion mass. An analysis to extract the ππ scattering lengths in the isospin I=0 and I=2 states, a0 and a2, respectively, has been recently reported. In the present work the Dalitz plot of this decay is fitted to a new empirical parameterization suitable for practical purposes, such as Monte Carlo simulations of K±→π±π0π0 decays.
A new measurement of the K±→π±γγ decay at the NA48/2 experiment
Abstract The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay K ± → π ± γ γ from these data sets based on 149 decay candidates with an estimated background of 15.5 ± 0.7 events is reported. The model-independent branching ratio in the kinematic range z = ( m γ γ / m K ) 2 > 0.2 is measured to be B MI ( z > 0.2 ) = ( 0.877 ± 0.089 ) × 10 − 6 , and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be B ( K π γ γ ) = ( 0.910 ± 0.075 ) × 10 − 6 .
Searches for lepton number violating $K^+$ decays
The NA62 experiment at CERN reports a search for the lepton number violating decays K+ -> pi(-)e(+)e(+) and K+ -> pi(-)mu(+)mu(+) using a data sample collected in 2017. No signals are observed, and upper limits on the branching fractions of these decays of 2.2 x 10(-10) and 4.2 x 10(-11) are obtained, respectively, at 90% confidence level. These upper limits improve on previously reported measurements by factors of 3 and 2, respectively.
Search for heavy neutrinos in K + → μ + ν μ decays
The NA62 experiment recorded a large sample of K+→μ+νμ decays in 2007. A peak search has been performed in the reconstructed missing mass spectrum. In the absence of a signal, limits in the range 2×10−6 to 10−5 have been set on the squared mixing matrix element |Uμ4|2 between muon and heavy neutrino states, for heavy neutrino masses in the range 300–375 MeV/ c2 . The result extends the range of masses for which upper limits have been set on the value of |Uμ4|2 in previous production search experiments.
Search for direct CP violation in the decays K-+/- -> 3 pi(+/-)
Abstract We report a measurement of the direct CP violation asymmetry parameter A g in charged kaon decays K ± → π ± π + π − by the NA48/2 experiment at the CERN SPS. The experiment has been designed not to be limited by systematics in the asymmetry measurement. Using 1.67 × 10 9 such decays collected during the 2003 run, the charge asymmetry in the Dalitz plot linear slope parameter g has been measured to be A g = ( 1.7 ± 2.9 ) × 10 −4 . The precision of the result is limited by the statistics used.
Search for heavy neutral lepton production in K+ decays to positrons
A search for heavy neutral lepton ($N$) production in $K^+\to e^+N$ decays using the data sample collected by the NA62 experiment at CERN in 2017--2018 is reported. Upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the level of $10^{-9}$ over most of the accessible heavy neutral lepton mass range 144--462 MeV/$c^2$, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The $|U_{e4}|^2$ range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/$c^2$.
Measurement of the branching ratio of the decay Ξ0→Σ+μ−ν¯μ
Abstract From the 2002 data taking with a neutral kaon beam extracted from the CERN-SPS, the NA48/1 experiment observed 97 Ξ 0 → Σ + μ − ν ¯ μ candidates with a background contamination of 30.8 ± 4.2 events. From this sample, the BR ( Ξ 0 → Σ + μ − ν ¯ μ ) is measured to be ( 2.17 ± 0.32 stat ± 0.17 syst ) × 10 − 6 .
First observation and measurement of the decay K±→π±e+e−γ
Abstract Using the full data set of the NA48/2 experiment, the decay K ± → π ± e + e − γ is observed for the first time, selecting 120 candidates with 7.3 ± 1.7 estimated background events. With K ± → π ± π D 0 as normalisation channel, the branching ratio is determined in a model-independent way to be Br ( K ± → π ± e + e − γ , m e e γ > 260 MeV / c 2 ) = ( 1.19 ± 0.12 stat ± 0.04 syst ) × 10 −8 . This measured value and the spectrum of the e + e − γ invariant mass allow a comparison with predictions of chiral perturbation theory.
First observation and study of the K± → π±π0e+e− decay
The NA48/2 experiment at CERN reports the first observation of the K-+/- -> pi(+/-)pi(0)e(+)e(-) decay from an exposure of 1.7 x 10(11) charged kaon decays recorded in 2003-2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, BR(K-+/- -> pi(+/-)pi(0)e(+)e(-)) = (4.24 +/- 0.14) x 10(-6). The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated.
Search for K+→ π+νν¯ at NA62
Flavour physics is one of the most powerful fields for the search of new physics beyond the Standard Model. The kaon sector with the rare decay K+ → π+νν̅ provides one of the cleanest and most promising channels. NA62, a fixed target experiment at the CERN SPS, aims to measure BR (K+ → π+νν̅) with 10% precision to test the Standard Model validity up to an energy scale of hundreds of TeV. NA62 had dedicated data taking for the K+ → π+νν̅ measurement in 2016 and 2017 and will continue in 2018. Here preliminary results on a fraction of 2016 dataset are presented. The analysis of the complete 2016 data sample is expected to achieve the SM sensitivity.
NA48/62 latest results
The NA62 experiment at the CERN SPS recorded in 2007 a large sample of K+ ? µ+?µ decays. A peak search in the missing mass spectrum of this decay is performed. In the absence of observed signal, the limits obtained on B(K+ ? µ+?h) and on the mixing matrix element |Uµ 4| are reported. The upgraded NA62 experiment started data taking in 2015. About 5×1011K+ decays have been recorded so far to measure the branching ratio of the K+ ? ?+?? decay. Preliminary results from the K+ ? ?+?? analysis based on about 5% of the 2016 statistics are reported.
Search forBs0→μ+μ−andB0→μ+μ−Decays with CDF II
A search has been performed for B{sub s}{sup 0} {yields} {mu}{sup +}{mu}{sup -} and B{sup 0} {yields} {mu}{sup +}{mu}{sup -} decays using 7 fb{sup -1} of integrated luminosity collected by the CDF II detector at the Fermilab Tevatron collider. The observed number of B{sup 0} candidates is consistent with background-only expectations and yields an upper limit on the branching fraction of {Beta}(B{sup 0} {yields} {mu}{sup +}{mu}{sup -}) < 6.0 x 10{sup -9} at 95% confidence level. We observe an excess of B{sub s}{sup 0} candidates. The probability that the background processes alone could produce such an excess or larger is 0.27%. The probability that the combination of background and the expe…
Search for the dark photon in π0 decays
A sample of $1.69\times 10^7$ fully reconstructed $\pi^0\to\gamma e^+e^-$ decay candidates collected by the NA48/2 experiment at CERN in 2003--2004 is analysed to search for the dark photon ($A'$) production in the $\pi^0\to\gamma A'$ decay followed by the prompt $A'\to e^+e^-$ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass $m_{A'}$ and mixing parameter $\varepsilon^2$ is established. The obtained upper limits on $\varepsilon^2$ are more stringent than the previous limits in the mass range $9~{\rm MeV}/c^2<m_{A'}<70~{\rm MeV}/c^2$. The NA48/2 sensitivity to the dark photon production in the $K^\pm\to\pi^\pm A'$ decay is also evaluated.
New measurement of the K±→π±μ+μ− decay
Abstract A sample of 3120 K ± → π ± μ + μ − decay candidates with ( 3.3 ± 0.7 ) % background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a detailed study of the decay properties. The branching ratio was measured to be BR = ( 9.62 ± 0.25 ) × 10 − 8 . The form factor W ( z ) , where z = ( M μ μ / M K ) 2 , was parameterized according to several models. In particular, the slope of the linear form factor W ( z ) = W 0 ( 1 + δ z ) was measured to be δ = 3.11 ± 0.57 . Upper limits of 2.9 × 10 − 2 and 2.3 × 10 − 2 on possible charge asymmetry and forward–backward asymmetry were established at 90% CL. An upper limit BR ( K ± → π ∓ μ ± μ ± ) 1.1 × 10 − 9 was e…
Measurement of thett¯production cross section inpp¯collisions ats=1.96 TeVusing soft electronb-tagging
The authors present a measurement of the t{bar t} production cross section using events with one charged lepton and jets from p{bar p} collisions at a center-of-mass energy of 1.96 TeV. A b-tagging algorithm based on the probability of displaced tracks coming from the event interaction vertex is applied to identify b quarks from top decay. Using 318 pb{sup -1} of data collected with the CDF II detector, they measure the t{bar t} production cross section in events with at least one restrictive (tight) b-tagged jet and obtain 8.9{sub -1.0}{sup +1.0}(stat.){sub -1.0}{sup +1.1}(syst.) pb. The cross section value assumes a top quark mass of m{sub t} is presented in the paper. This result is cons…
Search for heavy neutral lepton production in K+ decays
A search for heavy neutral lepton production in $K^+$ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the $10^{-7}$ to $10^{-6}$ level are established on the elements of the extended neutrino mixing matrix $|U_{\ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{\rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.
Search for Lepton Number and Flavor Violation in K+ and π0 Decays
Searches for the lepton number violating $K^{+} \rightarrow \pi^{-} \mu^{+} e^{+}$ decay and the lepton flavour violating $K^{+} \rightarrow \pi^{+} \mu^{-} e^{+}$ and $\pi^{0} \rightarrow \mu^{-} e^{+}$ decays are reported using data collected by the NA62 experiment at CERN in $2017$-$2018$. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times 10^{-11}$, $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$ and $\mathcal{B}(\pi^{0}\rightarrow\mu^{-}e^{+})<3.2\times 10^{-10}$. These results improve by one order of magnitude over previous results for thes…
Study of the K±→π±γγ decay by the NA62 experiment
Abstract A study of the dynamics of the rare decay K ± → π ± γ γ has been performed on a sample of 232 decay candidates, with an estimated background of 17.4 ± 1.1 events, collected by the NA62 experiment at CERN in 2007. The results are combined with those from a measurement conducted by the NA48/2 Collaboration at CERN. The combined model-independent branching ratio in the kinematic range z = ( m γ γ / m K ) 2 > 0.2 is B MI ( z > 0.2 ) = ( 0.965 ± 0.063 ) × 10 − 6 , and the combined branching ratio in the full kinematic range assuming a Chiral Perturbation Theory description is B ( K π γ γ ) = ( 1.003 ± 0.056 ) × 10 − 6 . A detailed comparison of the results with the previous measurements…
Test of lepton flavour universality in K+→ℓ+ν decays
Abstract A precision test of lepton flavour universality has been performed by measuring the ratio R K of kaon leptonic decay rates K + → e + ν and K + → μ + ν in a sample of 59 813 reconstructed K + → e + ν candidates with ( 8.71 ± 0.24 ) % background contamination. The result R K = ( 2.487 ± 0.013 ) × 10 − 5 is in agreement with the Standard Model expectation.
Precise measurement of the K±→π±e+e− decay
Abstract A sample of 7253 K ± → π ± e + e − ( γ ) decay candidates with 1.0% background contamination has been collected by the NA48/2 experiment at the CERN SPS, which allowed a precise measurement of the decay properties. The branching ratio in the full kinematic range was measured to be BR = ( 3.11 ± 0.12 ) × 10 − 7 , where the uncertainty includes also the model dependence. The shape of the form factor W ( z ) , where z = ( M e e / M K ) 2 , was parameterized according to several models, and, in particular, the slope δ of the linear form factor W ( z ) = W 0 ( 1 + δ z ) was determined to be δ = 2.32 ± 0.18 . A possible CP violating asymmetry of K + and K − decay widths was investigated,…
Precision measurement of the ratio of the charged kaon leptonic decay rates
A precision measurement of the ratio RK of the rates of kaon leptonic decays K+- --> e nu and K+- --> mu nu with the full data sample collected by the NA62 experiment at CERN in 2007-2008 is reported. The result, obtained by analysing ~150000 reconstructed K+- --> e nu candidates with 11% background contamination, is RK = (2.488+-0.010)*10^{-5}, in agreement with the Standard Model expectation.
Measurement of the Dalitz plot slope parameters of the K±→π±π+π− decay
Abstract The distribution of the K ± → π ± π + π − decays in the Dalitz plot has been measured by the NA48/2 experiment at the CERN SPS with a sample of 4.71 × 10 8 fully reconstructed events. With the standard Particle Data Group parameterization the following values of the slope parameters were obtained: g = ( − 21.134 ± 0.017 ) % , h = ( 1.848 ± 0.040 ) % , k = ( − 0.463 ± 0.014 ) % . The quality and statistical accuracy of the data have allowed an improvement in precision by more than an order of magnitude, and are such as to warrant a more elaborate theoretical treatment, including pion–pion rescattering, which is in preparation.