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Scientific Report 2007-2009
Particle physics
P18. NA48/2 experiment at CERN with simultaneous K ± beams:
measurement of CP -violating asymmetries and ππ scattering lengths
The primary goal of the NA48/2 experiment at the and K ± → π ± π 0 π 0 decay. In particular, a new technique
ear slopes A g = (g + − g − )/(g + + g − ) in K ± → π ± π + π −
CERN SPS is the measurement of the slope charge asymmetries
of asymmetry measurement involving simultaneear
A g in both K ± → π ± π + π − and K ± → π ± π 0 π 0 ous K + and K − beams and the large data sample
processes with a sensitivity at least one order of magnitude
collected, allowed a result of an unprecedented preci-
better than previous experiments. The new level of sion. The charge asymmetries were measured to be
precision can explore effects, albeit larger than the SM A ± g = (1.5 ± 2.2) × 10 −4 with 3.11 × 10 9 decays for
predictions, induced by new physics, and is achieved by the charged mode, and A 00
g = (1.8 ± 1.8) × 10 −4 with
using a novel measurement technique based on simultaneous
9.13 × 10 7 decays for the neutral mode, the precision
K + and K − beams overlapping in space. being mainly limited by the statistics [1].
The simultaneous K + and K − beams are produced The distribution of the K ± → π ± π + π − decays in
by 400 GeV/c primary SPS protons impinging on a the Dalitz plot has been also measured with a sample
beryllium target. Charged particles with momentum
of 4.71 × 10 8 fully reconstructed events. With the
(60±3) GeV/c are selected by an achromatic system
which splits the two beams in the vertical plane and
then recombines them on a common axis.
The beams then enter the fiducial decay volume
housed in a 114 m long cylindrical vacuum tank. Both
beams follow the same path in the decay volume: their
axes coincide within 1 mm, while the transverse size of
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)% [2]. This allowed an improvement in
precision by more than an order of magnitude, allowing
a more elaborate theoretical treatment, including pionpion
rescattering.
the beams is about 1 cm. With 7×10 11 protons incident Using the full data-set, the K ± → π ± e + e − γ decay
on the target per SPS spill of 4.8 s duration, the positive
has been observed for the first time. 120 events
(negative) beam flux at the entrance of the decay volume
have been selected over an estimated background of
is 3.8 × 10 7 (2.6 × 10 7 ) particles per pulse, of which 7.3 ± 1.3 events. Using the K ± → π ± π
5.7% (4.9%) are K + (K − ). The K + /K − D 0 as normalisation
channel, the branching ratio has been measured
flux ratio is
1.79. The fraction of beam kaons decaying in the decay
volume at nominal momentum is 22%.
The decay volume is followed by a magnetic spectrometer
in a model-independent way [3]: BR(K ± → π ± e + e − γ,
m eeγ > 260 MeV/c 2 ) = (1.19±0.12 stat ±0.04 syst )×10 −8 ,
allowing comparison with ChPT predictions.
housed in a tank filled with helium at nearly atmo-
Another important result was a new measurement of
spheric pressure, separated from the vacuum tank by the K e4 decay, based on a sample of more than 670 000
a thin (0.31%X 0 ) Kevlar composite window. A thinwalled
aluminium beam pipe of 16 cm outer diameter
decays in both charged modes. 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
traversing the centre of the spectrometer (and all the
bins of the ππ mass spectrum to investigate their
following detectors) allows the undecayed beam particles
and the muon halo from decays of beam pions to ππ mass, low background and a very good resolu-
variation. Thanks to the sizeable acceptance at large
continue their path in vacuum.
tion, the ππ scattering lengths have been measured
The magnetic spectrometer, consisting of four drift with improved accuracy (under the assumption of
chambers (two upstream and two downstream of a dipole
magnet), allows to measure the momentum of charged
isospin symmetry): a 0 0 = 0.233 ± 0.016 stat ± 0.007 syst ,
a 2 0 = 0.0471 ± 0.011 stat ± 0.004 syst [4].
particles with a resolution of σ(p)/p=(1.02 ⊕ 0.044 · p)%
(p in GeV/c). It is followed by a plastic scintillator hodoscope
used to produce fast trigger signals and to pro-
References
1. J. R. Batley et al., Eur. Phys. J. C 52 875 (2007).
2. J. R. Batley et al., Phys. Lett. B 649 349 (2007).
vide precise time measurements of charged particles.
3. J. R. Batley et al., Phys. Lett. B 659 493 (2008).
A liquid krypton electromagnetic calorimeter is used 4. J. R. Batley et al., Eur. Phys. J. C 54 411 (2008).
for photon detection and particle identification. It is an
almost homogeneous ionization chamber with an active
volume of 7 m 3 of liquid krypton, segmented transversally
Authors
N. Cabibbo, G. D’Agostini
into 13248 projective cells, 27 X 0 deep, and with an
energy resolution σ(E)/E = 0.032/E ⊕ 0.09/E ⊕ 0.0042 http://www.cern.ch/na48/NA48-2/
(E in GeV). The LKr is followed by a hadronic calorimeter
and a muon detector.
Among the many interesting physics results obtained
in the last three years, the measurement of the direct
CP violating charge asymmetries of the Dalitz plot lin-
Sapienza Università di Roma 125 Dipartimento di Fisica