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Scientific Report 2007-2009
Particle physics
P17. NA62 experiment with a high-intensity charged kaon beam at
CERN: search of Standard Model violations in the K → πν¯ν decays
The Branching Ratio (BR) K + → π + ν¯ν can be related
to the CKM element V td (the less well known one).
The precise theoretical estimation in the Standard Model
(SM) and in the SUper SYmmetry (SUSY) will allow us
to have a probe of the flavour sector or the evidence of
physics beyond the Standard Model, if deviation from
the predicted SM value will be observed. The SM prediction
is (8.22 ± 0.84) × 10 −11 . Until now only seven
events have been collected and the experimental value is
1.47 −0.89 +1.30 × 10−10 . The NA62 experiment has been proposed
and approved to detect ≈100 events with a signal
to background of at least 10. The NA62 experiment will
be housed in the CERN North Area, using the same SPS
extraction line and target of the NA48 experiment. With
a new high-acceptance beam-line, a secondary positive
hadron beam 50 times more intense will be available.
The intense 400 GeV/c proton beam, extracted from
the SPS, produces a secondary charged beam by impinging
on a Be target. A 100 m long beam line selects a 75
GeV/c momentum beam with 1% RMS momentum bite
and an average rate of about 800 MHz integrated over an
area of 14 cm 2 . The beam is positron free and is composed
by 6% of K + . A system of subdetectors placed
about 100 m downstream to the beginning of the decay
region provides the detection of the K + decay products:
the decay rate in the 120 m long fiducial volume will be
≈11 MHz.
The success of the experiment depends crucially in obtaining
the required level of background rejection. Key
points of NA62 are: accurate kinematic reconstruction;
precise timing to associate the π + with its K + parent;
a system of efficient vetoes to reject events with γ and
µ; a particle identification system to identify the kaons
in the charged beam and to distinguish π + from µ + and
e + in the final state. Indeed the main backgrounds to be
rejected are: three-body K + decays, K + → π + π 0 , K µ2
and K e4 .
Due to the finite resolution of the reconstructed kinematic
thresholds, low mass and high precision detectors
placed in vacuum are required for the tracking. The very
high rate in the beam detector (800 MHz) requires to associate
the incoming kaon to the downstream pion track
by means of tight spatial and time coincidences. Any
mismatch between them, in fact, causes a loss of kinematic
rejection power. A Cerenkov Threshold Counter
(CEDAR) placed on the beam line, the beam tracker
itself and a RICH, provide the timing of the experiment.
The beam tracker must reconstruct the beam tracks
with at least 200 ps time resolution. The designed beam
spectrometer (Gigatracker) consists of three Si pixels stations
60×27mm 2 , made up by 300×300 µm 2 pixels each
of them composed by a 200µm thick Si sensor. A readout
chip 100µm thick constructed with a 0.13µm technology
and bump-bonded on the sensor guarantees the required
time resolution. The total material budget amounts to
less than 0.5% radiation length per station. Dedicated
radiation damage tests on prototypes proved the usage
of this detector at an average rate of 60 MHz/cm 2 .
The RICH is made of a 17 m long vessel placed after
the pion spectrometer and filled with Ne gas at atmospheric
pressure. A mosaic of mirrors at the end, having
17 m focal length, reflects the Cerenkov light towards
two arrays of about 1000 phototubes each, placed on
both the sides of the vessel at the entrance window.
Straw Tube is the building technology for the pion
spectrometer. Four chambers placed in the same vacuum
of the decay region form the detector. Each chamber
includes four view-planes rotated by 45 degree one to
another.
The main detector for the rejection of photons will be
the quasi-homogenous liquid-Krypton calorimeter from
the NA48 experiment, covering angles from 2 to 8.5
mrad. The decay fiducial volume is surrounded by 12
ring shaped detectors, in order to veto photons in the
angular range from 8.5 to 50 mrad (“large angle” vetoes,
LAV). In addiction, the LAV system must have
a good energy and time resolution in order to define a
precise energy threshold and to use the system in the
trigger logic. For this purpose, each single detector station
will be realized using the lead glass crystals formerly
used for the OPAL barrel calorimeter, re-arranged in five
staggered layers, radially arranged in order to ensure the
required photon-detection efficiency.
In the three years 2006-2008, different test beams
allowed the testing of the innovative detectors. In addition,
150 000 K + → e + ν decay candidates have been
collected, in order to determine the ratio of K e2 /K µ2
branching ratios to better than 0.5%, which is an
interesting test of new physics, since it can be predicted
with high accuracy within the Standard Model. The
construction of the apparatus will start in 2010 and
the beginning of the data taking is foreseen in 2012-2013.
References
1. F. Ambrosino et al., Nucl. Instrum. Meth. A581 389
(2007).
2. S. Venditti et al., Nuovo Cim. 123B 844 (2008).
3. A. Antonelli et al., J. Phys. Conf. Ser. 160 012020 (2009).
4. G. Saracino et al., Nucl. Phys. Proc. Suppl. 187 78 (2009).
Authors
N. Cabibbo, G. D’Agostini, E. Leonardi 1 , M. Serra 1 , P.
Valente 1
http://na62.web.cern.ch/NA62/
Sapienza Università di Roma 124 Dipartimento di Fisica