Using the helicity method we derive complete formulas for the joint angular decay distributions occurring in semileptonic hyperon decays including lepton-mass and polarization effects. Compared to ...the traditional covariant calculation, the helicity method allows one to organize the calculation of the angular decay distributions in a very compact and efficient way. In the helicity method the angular analysis is of cascade type, i.e. each decay in the decay chain is analyzed in the respective rest system of that particle. Such an approach is ideally suited as input for a Monte Carlo event generation program. As a specific example we take the decay
(
l
−
=
e
−
,
μ
−
) followed by the nonleptonic decay
Σ
+
→
p
+
π
0
for which we show a few examples of decay distributions which are generated from a Monte Carlo program based on the formulas presented in this paper. All the results of this paper are also applicable to the semileptonic and nonleptonic decays of ground-state charm and bottom baryons, and to the decays of the top quark.
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.
We report results from a new measurement of the K
e4
decay K
±
→π
+
π
-
e
±
ν by the NA48/2 collaboration at the CERN SPS, based on a partial sample of more than 670 000 K
e4
decays in both charged ...modes collected in 2003. The form factors of the hadronic current (F,G,H) and ππ phase difference (δ=δ
s
-δ
p
) have been measured in ten independent bins of the ππ mass spectrum to investigate their variation. A sizeable acceptance at large ππ mass, a low background and a very good resolution contribute to an improved experimental accuracy, a factor two better than in the previous measurement, when extracting the ππ scattering lengths a
0
0
and a
0
2
. Under the assumption of isospin symmetry and using numerical solutions of the Roy equations, the following values are obtained in the plane (a
0
0
,a
0
2
): a
0
0
=0.233±0.016stat±0.007syst,a
0
2
=-0.0471±0.011stat±0.004syst. The presence of potentially large isospin effects is also considered and will allow comparison with precise predictions from Chiral Perturbation Theory.
The NA48/2 experiment at CERN reports the first observation of the K±→π±π0e+e− decay from an exposure of 1.7×1011 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±→π±π0e+e−)=(4.24±0.14)×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.
A sample of 1.69×107 fully reconstructed π0→γe+e− decay candidates collected by the NA48/2 experiment at CERN in 2003–2004 is analyzed to search for the dark photon (A′) production in the π0→γA′ ...decay followed by the prompt A′→e+e− decay. No signal is observed, and an exclusion region in the plane of the dark photon mass mA′ and mixing parameter ε2 is established. The obtained upper limits on ε2 are more stringent than the previous limits in the mass range 9 MeV/c2<mA′<70 MeV/c2. The NA48/2 sensitivity to the dark photon production in the K±→π±A′ decay is also evaluated.
The NA48/2 experiment at CERN reports the first observation of the $K^± → π^0π^0μ^±$ν decay based on a sample of 2437 candidates with 15% background contamination collected in 2003–2004. The decay ...branching ratio in the kinematic region of the squared dilepton mass above 0.03 GeV2/c4 is measured to be (0.65 ± 0.03) × 10–6. The extrapolation to the full kinematic space, using a specific model, is found to be (3.45 ± 0.16) × 10–6, in agreement with chiral perturbation theory predictions.
A
bstract
The NA48/2 experiment at CERN reports the first observation of the
K
±
→
π
0
π
0
μ
±
ν
decay based on a sample of 2437 candidates with 15% background contamination collected in 2003–2004. ...The decay branching ratio in the kinematic region of the squared dilepton mass above 0.03 GeV
2
/
c
4
is measured to be (0.65 ± 0.03) × 10
−
6
. The extrapolation to the full kinematic space, using a specific model, is found to be (3.45 ± 0.16) × 10
−
6
, in agreement with chiral perturbation theory predictions.
A
bstract
A measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 10
6
K
±
→
π
0
e
±
ν
e
(
K
e
3
±
) and 2.3 × 10
6
K
±
→
π
0
μ
±
ν
μ
(
K
μ
3
±
) decays ...collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the
K
e
3
±
and
K
μ
3
±
modes is also presented.