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Scientific Report 2007-2009
Particle physics
P26. Search for neutrino oscillations by the OPERA detector at Gran
Sasso
Increasing experimental evidence of neutrino oscillations
has been collected in the last decades by several
experiments, by exploiting both the natural neutrino
sources, like the sun and the atmosphere, and the available
reactor and accelerator facilities. However, a direct
observation of neutrino flavor appearance, complementary
to the widely reported flavor disappearance, yet remains
among the missing tiles of the picture.
The OPERA neutrino detector was designed to perform
the first detection of neutrino oscillations in the appearance
mode trough the study of the ν µ to ν τ channel.
The OPERA apparatus is installed in the underground
Gran Sasso Laboratory (LNGS) in the high energy longbaseline
CERN to LNGS neutrino beam (CNGS), 730
Km away from the neutrino source. The CNGS is an
almost ’pure’ ν µ beam, so that the observation of ν τ -
induced events in the apparatus would be an unambiguous
signal of in-flight oscillations.
Figure 1: The OPERA neutrino detector.
OPERA is a hybrid detector made of two identical Super
Modules, each consisting of a target section of about
625 tons made of emulsion/lead modules, of a scintillator
tracker detector and a muon spectrometer. Details
about the apparatus (fig. 1) can be found in [1].
The CNGS beam started to delivery neutrinos with
a technical run in 2006. The physics program was initiated
in 2007 with a very limited integrated intensity
of 8.24 × 10 17 protons on target (pot). Full-scale data
taking took place in the next two years, with 1.78 × 10 19
pot in 2008 and 3.52 × 10 19 pot in 2009. Overall, ≃ 5400
beam-induced events were reconstructed till now in the
OPERA target, by the pattern recognition of hits in the
target tracker and spectrometer sections of the detector.
As for the hybrid technique deployed by OPERA, next
steps after trigger are the extraction of selected target
units candidate to contain the events[2], the development
of photographic emulsion films therein, their fast
automated scanning by computer-assisted optical microscopes
[3], and, finally, the selection and study of peculiar
decay topologies in order to unveal the ν τ appearance
tagged as charged-current (CC) interactions producing
the short-lived massive τ lepton.
The location of beam-induced events in the emulsion
films was successful since the beginning of data-taking,
as reported in [4]. At the time of writing this note,
≃ 1500 neutrino events were located and studied, mostly
from the 2008 run, while the 2009 run data will require
some more months to be digested. The procedures for
the selection and extensive study of events featuring decay
topologies are under fine tuning. The production and
decay of charmed particles was observed in ν µ -induced
CC interactions, at a rate compatible with the known
production rate and the expected detection efficiency.
A few events with a prompt electron at a primary vertex
were also observed, due to the known ν e contamination
of CNGS. The excellent space resolution of nuclear
emulsions (≃ 1 µm), particle identification and momentum
measurements by multiple coulomb scattering were
shown to allow full topological and kinematical study of
interesting events.
In summary, OPERA is ready to detect the ν τ appearance,
and the analysis is in progress. The experimental
program is expected to continue with data taking at
higher intensity in 2010-2012 and consequent scanning
and analysis of the emulsion target data.
According to the computed sensitivity, for an integrated
intensity exceeding 2. × 10 20 pot the experiment
is expected to observe over 10 oscillation events against
less than 1 background.
As a member of the OPERA Collaboration spanning
several countries in Europe and Asia, the Rome group,
rooted in a long standing local experience with nuclear
emulsions dating back to the early ’50s, contributed to
the design and construction of vital infrastructure for
the emulsion handling at LNGS, as well as to the design,
setting-up, test and exploitation of automated microscopes.
The group is now part of the ’European scanning
team’, having its partner in Japan. Contribution are
also expected in the data handling (data-base) and in
the physics analysis of selected events.
References
1. R. Acquafredda, et al., J. Inst. 4, P04018 (2009).
2. A. Anokhina, et al., J. Inst. 3, P07005 (2008).
3. L. Arrabito, et al., J. Inst. 2, P05004 (2007).
4. N. Agafonova, et al., J. Inst. 4, P06020(2009).
Authors
G. Rosa
http://operaweb.lngs.infn.it/
Sapienza Università di Roma 133 Dipartimento di Fisica