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