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Scientific Report 2007-2009
Particle physics
P27. Neutrino oscillation in long baseline experiments
The discovery of neutrino flavour oscillation allows to
study neutrino mixing and masses. The story of neutrino
oscillation starts with the detection of neutrinos
from the Sun by the pioneering Homestake mine experiment
led by Davis in the early 1970s. Bruno Pontecorvo
was the first to interpret the deficit of the solar neutrino
flux as a possible hint of neutrino oscillation. In the
80s and 90s a leading role in confirming the solar neutrino
oscillation was played by the large water Cherenkov
detectors Kamiokande (3 Kton mass) and its successor
SuperKamiokande (50 Kton mass).
2007 to August 2008 both in neutrino and anti-neutrino
mode. Several analysis are in progress and preliminary
results have been presented at conferences. The first
publication reports the search for coherent pion production
in neutrino charged current interactions [3].
The Rome group is also participating to T2K, the first
accelerator experiment searching for the subdominant ν µ
to ν e oscillation, which has not been observed up to now.
This process is related to a non zero θ 13 neutrino mixing
angle. The other two angles describing the neutrino
mixing are known to be large from the oscillation of solar
neutrinos and from the dominant ν µ to ν τ oscillation in
atmospheric neutrinos. On the contrary we only know
an upper limit on the angle θ 13 and a measurement is
needed in order to complete our understanding of neutrino
oscillation. The observation of a non zero value
may foster the measurement of leptonic CP symmetry
violation in neutrino oscillation, since the CP violation
effects are proportional to θ 13 and leptonic CP violation
can only exists if this angle is different from zero. T2K
will also provide measurement at a few percent precision
of the ∆m 2 23 and θ 23 parameters.
Figure 1: The SuperKamiokande detector.
In 1998 SuperKamiokande announced the discovery
of oscillation of atmospheric neutrinos, i.e. the neutrinos
produced by cosmic rays in the earth’s atmosphere. This
phenomenon can also be studied with man-made neutrinos
produced by accelerators with a detector of suitable
mass, located several hundreds kilometers away from the
neutrino source. K2K is the first of these “long baseline”
experiments. It uses the SuperKamiokande detector and
a muon neutrino beam produced 250 Km away at KEK.
K2K was the first to observe oscillation at an accelerator
in 2005, thus confirming the discovery of atmospheric
neutrino oscillation and improving the ∆m 2 measurement.
The Rome group has started its participation to
K2K in 2002 proposing, assembling and operating the
electromagnetic calorimeter used in the near detector.
To study neutrino oscillation the near detector plays
a crucial role by measuring the flux before neutrino oscillate
and by providing precision measurements of neutrino
interactions properties and cross-sections [1,2].
Few experimental data exist for neutrino crosssections
at 1 GeV energy and some processes have never
been measured. The present and next generation of neutrino
oscillation experiments at accelerators require better
experimental data. To this goal part of the K2K near
detector has been used to assemble a new experiment,
SciBooNE, at the Fermilab Booster neutrino beam. The
Rome group has been responsible for the installation and
operation of the electromagnetic calorimeter. The collaboration
is now analysing the data taken from June
Figure 2: The layout of the T2K experiment.
As a successor of K2K, the T2K experiment uses
again the SuperKamiokande detector and a new near
detector. The neutrino beam is extracted from proton
accelerated by the very high power (0.75 MW) accelerator
complex now under commissioning at J-PARC
in Japan. The Rome group proposed the adoption
of a magnetised design for the near detector and the
refurbishement of the large aperture dipole magnet built
at CERN for the UA1 collaboration. The discovery
of a non zero θ 13 within a factor 20 with respect to
the present upper limit is within reach of T2K after
five years data taking. First results are expected in 2011.
References
1. S. Mine et al., Phys. Rev. D 77, 032003 (2008).
2. A. Rodriguez et al., Phys. Rev. D 78, 032003 (2008).
3. K. Hiraide et al., Phys. Rev. D 78, 112004 (2008).
Authors
U. Dore, P.F. Loverre, L.Ludovici 1 , C.Mariani
http://www.phys.uniroma1.it/gr/T2K/index.html
Sapienza Università di Roma 134 Dipartimento di Fisica