The NA48/2 experiment at CERN reports the first observation of the <math><msup><mrow><mi>K</mi></mrow><mrow><mo>±</mo></mrow></msup><mo>→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math> decay from an exposure of <math><mn>1.7</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mn>11</mn></mrow></msup></math> 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, <math><mi>B</mi><mi>R</mi><mo>(</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>±</mo></mrow></msup><mo>→</mo><msup><mrow><mi>π</mi></mrow><mrow><mo>±</mo></mrow></msup><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mo>)</mo><mo>=</mo><mo>(</mo><mn>4.24</mn><mo>±</mo><mn>0.14</mn><mo>)</mo><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup></math>. 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.

United Kingdom

J.R. Batley

G. Kalmus

C. Lazzeroni

D.J. Munday

M.W. Slater

S.A. Wotton

CERN

R. Arcidiacono

G. Bocquet

N. Cabibbo

A. Ceccucci

D. Cundy

V. Falaleev

M. Fidecaro

L. Gatignon

A. Gonidec

W. Kubischta

A. Maier

A. Norton

M. Patel

A. Peters

JINR

S. Balev

P.L. Frabetti

E. Gersabeck

E. Goudzovski

P. Hristov

V. Kekelidze

V. Kozhuharov

L. Litov

D. Madigozhin

M. Misheva

N. Molokanova

I. Polenkevich

Yu. Potrebenikov

S. Stoynev

A. Zinchenko

United States

E. Monnier

E. Swallow

R. Winston

P. Rubin

A. Walker

Italy

P. Dalpiaz

C. Damiani

M. Fiorini

M. Martini

F. Petrucci

M. Savrié

M. Scarpa

H. Wahl

W. Baldini

A. Cotta Ramusino

A. Gianoli

M. Calvetti

E. Celeghini

E. Iacopini

M. Lenti

G. Ruggiero

A. Bizzeti

M. Veltri

Germany

M. Behler

K. Eppard

M. Hita-Hochgesand

K. Kleinknecht

P. Marouelli

L. Masetti

U. Moosbrugger

C. Morales Morales

B. Renk

M. Wache

R. Wanke

A. Winhart

D. Coward

A. Dabrowski

T. Fonseca Martin

M. Shieh

M. Szleper

M. Velasco

M.D. Wood

G. Anzivino

E. Imbergamo

A. Nappi

M. Piccini

M. Raggi

M. Valdata-Nappi

P. Cenci

M. Pepe

M.C. Petrucci

F. Costantini

N. Doble

L. Fiorini

S. Giudici

G. Pierazzini

M. Sozzi

S. Venditti

G. Collazuol

L. DiLella

G. Lamanna

I. Mannelli

A. Michetti

C. Cerri

R. Fantechi

First observation and study of the → Decay

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±)&lt;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.

K. Massri

France

B. Bloch-Devaux

Searches for lepton number violation and resonances in K ± → πμμ decays

A sample of 1.69Ã107 fully reconstructed Ï0âÎ³e+eâ decay candidates collected by the NA48/2 experiment at CERN in 2003â2004 is analyzed to search for the dark photon ( Aâ² ) production in the Ï0âÎ³Aâ² decay followed by the prompt Aâ²âe+eâ decay. No signal is observed, and an exclusion region in the plane of the dark photon mass mAâ² and mixing parameter Îµ2 is established. The obtained upper limits on Îµ2 are more stringent than the previous limits in the mass range 9Â MeV/c2&lt;mAâ²&lt;70Â MeV/c2 . The NA48/2 sensitivity to the dark photon production in the KÂ±âÏÂ±Aâ² decay is also evaluated.

C. Cheshkov

Search for the dark photon in π0 decays

A sample of 65210 K ± → π 0 π 0 e ± ν (K e4 00 ) decay candidates with 1% background contamination has been collected in 2003-2004 by the NA48/2 collaboration at the CERN SPS. A study of the differential rate provides the first measurement of the hadronic form factor variation in the plane ( M ππ 2 ,  M eν 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around 4 m π + 2 $$ 4{m}_{\pi^{+}}^2 $$ . Exploiting a model independent description of this form factor, the branching ratio, inclusive of radiative decays, is obtained using the K ± → π 0 π 0 π ± decay mode as normalization. It is measured to be BR(K e4 00 ) = (2.552 ± 0.010 stat ± 0.010 syst ± 0.032 ext ) × 10 −5 , which improves the current world average precision by an order of magnitude while the 1.4% relative precision is dominated by the external uncertainty from the normalization mode. A comparison with the properties of the corresponding mode involving a π + π − pair (K e4 + − ) is also presented.

M. Gersabeck

Detailed study of the K ± → π 0 π 0 e ± ν (K e4 00 ) decay properties

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±γγ 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=(mγγ/mK)2&gt;0.2 is measured to be BMI(z&gt;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(Kπγγ)=(0.910±0.075)×10−6 .

A new measurement of the K±→π±γγ decay at the NA48/2 experiment