First results of the OPERA experiment This paper is dedicated to the ...

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First results of the OPERA experiment This paper is dedicated to the ...

First results of the OPERA experiment

N. Di Marco

Department of Physics, L’Aquila University, Via Vetoio Localit Coppito

67100 L’Aquila, Italy

OPERA (Oscillation Project with Emulsion tRacking Apparatus) is a long baseline neutrino

experiment, designed to search for ν µ → ν τ oscillation through the direct observation of ν τ in

the almost pure CNGS ν µ beam produced at CERN and delivered at the large underground

Gran Sasso National Laboratory, 730 km away from CERN. The detector construction was

completed in summer 2008 and data taking started at the end of June 2008. In this paper,

we review the physics potential of the experiment, the performances of the detector and the

first results obtained in the analysis of the first physics run events.

This paper is dedicated to the memory of the 306 people killed by the earthquake that, on

April 6 2009, destroyed the city of L’Aquila.

1 Introduction

Increasing experimental evidence of neutrino oscillation has been collected in the last decade,

by exploiting both the natural neutrino sources and the available artificial sources like reactors

and dedicated accelerator lines. However, despite the fact that there are many experimental

indications supporting the ν µ → ν τ solution for the atmospheric neutrino oscillation channel, a

direct evidence of the ν τ appearance is still missing.

The OPERA 1 experiment aims at measuring the ν τ appearance in an almost pure ν µ beam

produced at CERN SPS, 730 km far from the detector. The ν τ appearance signal is detected

through the measurement of the decay daughter particles of the τ lepton produced in CC ν τ

interactions. Since the short-lived τ particle has, at the energy of the beam, an average decay

length of about ∼1 mm, a micrometric detection resolution is needed. For this purpose the

OPERA detector, placed in the hall ”C” of the Gran Sasso National Underground Laboratories,

makes use of nuclear emulsions, the tracking detector with highest spatial resolution, combined

with lead plates in a structure called ”brick”.

The CNGS 2 beam is designed to provide 4.5 · 10 19 proton-on-target/year (p.o.t./y) with

a running time of 200 days per year. The beam parameters have been designed in order to

optimize the number of ν τ charged current interactions in the OPERA detector. The average

neutrino energy is =17 GeV with a small contamination of ν µ (4%) and of ν e , ν e (less than

1%). The average L/E ratio is 43 km/GeV, far from the oscillation maximum, but dictated by

the high energy needed for τ appearance.

In 5 years of data taking, OPERA is able to observe 10 to 15 ν τ events after oscillation at

full mixing in the range 2.5 × 10 −3 < ∆m 2 < 3 × 10 −3 eV 2 , with a total background of 0.75

events.


Figure 1: Schematic view of the OPERA detector

2 Detector overview

The OPERA apparatus (Fig. 1) consists of 2 identical parts called super-modules (SMs). Each

super-module consists of a target section of about 625 tons made of emulsion/lead bricks, of

a scintillator tracker detector (TT) and of a muon spectrometer. TT planes serve as trigger

devices and allow selecting the brick containing a neutrino interaction. The muon spectrometer

at the downstream end of each SM, allows measuring charge and momentum of penetrating

tracks. A large size anti-coincidence detector (VETO), made of two glass RPC planes mounted

in front of the first target, allows to reject charged particles originating from outside the target

fiducial region coming from neutrino interactions in the surrounding rock material.

2.1 Emulsion Target

Each target section is a sequence of vertical steel containers (walls) hosting the bricks, interleaved

with double-layered plastic scintillator planes as Target Trackers (TT). Each brick wall contains

∼ 2900 bricks for a total of 150000 bricks in the whole apparatus.

A target brick consists of 56 lead plates of 1 mm thickness interleaved with 57 emulsion

films. The plate material is a lead alloy with a small calcium content to improve its mechanical

properties. The transverse dimensions of a brick are 12.8 × 10.2 cm 2 and the thickness along

the beam direction is 7.9 cm (about 10 radiation lengths). The weight is 8.3 kg.

The large amount of bricks used in the apparatus, translates in an overall emulsion area larger

than 100,000 m 2 . Therefore an industrial film production was set-up after an R&D program

carried out in collaboration with the Fuji Film company. An OPERA film has 2 emulsion layers

(each 44 µm thick) on both sides of a transparent triacetylcellulose base (205 µm thick). The

total thickness is 293±5 µm.

In order to reduce the number of tracks integrated by nuclear emulsion from their production

to their development, a new procedure called refreshing (realized in a facility located in the

Tono mine in Gifu, Japan) was set up. It consists in keeping the emulsion films at high relative

humidity (RH ∼ 98%) and high temperature (∼ 27 ◦ C). This process reduces the integrated

number of tracks from ∼3000 to less than 100 tracks/cm 2 without affecting the sensitivity to

tracks detected later on (34 grains/100 µm).

In order to have a better signal to noise ratio and to reduce the emulsion scanning load the

use of Changeable Sheets (CS) film interface 3 , successfully applied in past experiments, was

extended to OPERA. Tightly packed doublets of emulsion films are glued to the downstream

face of each brick and can be removed without opening the brick. The global layout of bricks,


Figure 2: Schematic view of two bricks with their Changeable Sheets and target tracker planes.

CS and TT is schematically shown in Figure 2.

The construction of 150000 bricks for the neutrino target was accomplished in 80 weeks by

an automatic machine, the Brick Assembly Machine (BAM), operating underground in order to

minimize the number of background tracks from cosmic-rays and environmental radiation. Two

Brick Manipulating Systems (BMS) on the lateral sides of the detector position the bricks in

the target walls and also extract those bricks containing neutrino interactions.

In order to cope with the analysis of the large number of emulsion sheets related to neutrino

interactions, two new generations of fast automatic optical microscopes was developed:

the European Scanning System (ESS) 4 and the Japanese S-UTS 5 (Figure 3). Although the

implementation differs both in the hardware and software architecture, both systems have comparable

performances ensuring a scanning speed two order of magnitude greater than that of

the systems used in past experiments, and a spatial and angular resolution of the order of ∼1

µm and 1 mrad respectively.

2.2 Target Tracker

The main role of the Target Tracker is to provide a trigger and identify the bricks where the event

vertex should be located. Each TT wall is composed of two planes of respectively horizontal

and vertical plastic scintillator strips (680 cm×2.6 cm × 1 cm). The strips are made of extruded

polystyrene with 2% p-terphenyl and 0.02% POPOP, coated with a thin diffusing white layer of

TiO 2 . Charged particle crossing the strips will create a blue scintillation light which is collected

by wavelength-shifting fibers which propagate light at both extremities of the strip. All fibers

are connected at both ends to multianode Hamamatsu PMTs.

2.3 Muon Spectrometers

Each muon spectrometer consists of a large iron magnet instrumented with plastic Resistive Plate

Chambers (RPC). The deflection of charged particles inside the magnetized iron is measured by

six stations of drift tubes (Precision Trackers, PT). Left-right ambiguities in the reconstruction

of particle trajectories inside the PT are removed by means of additional RPC (XPC) with

readout strips rotated by ±45 ◦ with respect to the horizontal and positioned near the first two

PT stations.


Figure 3: A photograph of the European (left) and Japanese (right) scanning system.

3 Event analysis chain

We describe in the following the different steps carried out to analyze neutrino interaction events

from the identification of the ”fired” brick up to the detailed kinematical analysis of the vertex

in the emulsion films.

Once a trigger in the electronic detectors is selected to be compatible with an interaction

inside a brick, electronic detector data are processed by a software reconstruction program

that selects the brick with the highest probability to contain the neutrino interaction vertex.

This brick is removed from the target wall by the BMS and exposed to X-rays for film-to-film

alignment. There are two independent X-ray exposures: the first one ensures a common reference

system to the CS film doublet and the most downstream film of the brick (frontal exposure);

the second one produces thick lateral marks on the brick edges, used for film to film alignment

inside the brick.

After the first X-ray exposure the CS doublet is detached from the brick and developed

underground, while the brick is kept in a box made of 5 cm thick iron shielding to reduce the

radioactivity background. The scanning of the CS is done in two laboratories hosting a farm of

microscopes (Scanning Stations), one at LNGS and the other in Nagoya. CS films are analyzed

with a procedure called ”general scanning” that looks for tracks in all the available angular

range (typically ±400 mrad around the perpendicular to the film) in an area of about 50 cm 2

around the TT prediction. The residuals with respect to the TT prediction, are of the order of

about 1 cm in position and 20 mrad in angle.

The agreement between the two reconstructed tracks (each of them called ”base-track”,

and composed by the 2 track segments recorded in the two emulsion layers and called ”microtrack”)

in the CS doublet is quite stringent, given the precise alignment provided by the X-ray

exposure. The intrinsic position accuracy is less than 2 µm while the residual is dominated

by the systematic uncertainty in the mark measurement which is of the order of 10 µm. All

candidate tracks are then validated by an eye inspection to reject the small remnant background

due to accidental coincidences. The tolerances between tracks reconstructed on the two CS films,

are set accordingly: 40 µm in position and 10 mrad in angle, the latter enlarged according to

the track slope. The tracking efficiency has been measured on the double-refreshed CS films by

using tracks produced in neutrino interactions and hence it is averaged on the relevant angular

spectrum. The result shows a reconstruction efficiency of 86% at the level of single tracks.

This implies a particle trajectory reconstruction efficiency of 75% at the level of a CS doublet.

Using Compton electrons from environmental radioactivity, the systematic uncertainties in the

alignment between the two CS films are reduced, bringing the position accuracy at the level

of 1 µm 6 . Given the larger background in the micro-track reconstruction, such an accuracy


allows for the application of tighter cuts and thus it makes the use of single micro-tracks in the

track formation possible. If no double base-track candidate is found, after the application of the

fine alignment procedure, candidates made of 3 micro-tracks are selected, thus increasing the

particle trajectory reconstruction efficiency to 90%.

If the CS scanning detects tracks compatible with those reconstructed in the electronic

detectors the second X-ray exposure (lateral marking) is performed and the brick is brought

to the surface laboratory. The brick is then exposed to cosmic-rays for about 24 hours in

a dedicated pit in order to select high-energy cosmic muons to provide straight tracks for a

refined (sub-micrometric) film-to-film alignment. The brick emulsion films are then developed

and dispatched to the various scanning laboratories in Europe and Japan.

All tracks measured in the CS are sought in the most downstream films of the brick and

followed back until they are not found in three consecutive films. The stopping point is considered

as the signature either for a primary or a secondary vertex. The vertex is then confirmed by

scanning a volume with a transverse size of 1 cm 2 for 11 films in total, upstream and downstream

of the stopping point. The data are processed by an off-line program to reconstruct all the tracks

originating inside the volume: these tracks are input for a vertex reconstruction algorithm which

is tuned to find also decay topologies. The tracks attached to the primary vertex are followed

downstream, with a procedure analogous to the scan-back but in the opposite direction, to check

whether they show any decay topology and to measure their momentum from the Coulomb

multiple scattering.

4 First physics results

After a short commissioning run in 2006 the CNGS operation started on September 2007 at

rather low intensity. Unfortunately, due to a fault of the CNGS facility, the physics run lasted

only a few days. During this run 8.2 × 10 17 protons on target (p.o.t.) were accumulated: this

corresponds to about ∼ 3.6 effective nominal days of running. With such an integrated intensity

32 neutrino interactions in the bricks and 3 in the scintillator material of the target tracker were

expected; we actually observed 38 events on time with the arrival of the beam at Gran Sasso.

A much longer run took place in 2008 when 1.782 × 10 19 protons were delivered on the

CNGS target. OPERA collected 10100 events on time and among them 1700 interactions in

the targets, which are presently under analysis. At the time of the conference 484 events were

located and their interaction topology was reconstructed: among them 412 were CC events while

72 were NC events.

Charm production and decay topology events have a great importance in OPERA for two

main reasons. On the one hand, since charm decays exhibit the same topology as τ decays,

measuring the charm-like event reconstruction efficiency provides an important cross-check of

the τ event reconstruction capability. On the other hand, charm events are a potential source

of background, in particular if the muon at the primary vertex is not identified. Therefore,

searching for charm-decays in events with the primary muon correctly identified provides a

direct measurement of this background. Charm decay topologies are searched for in the sample

of located neutrino interactions. As an example Figure 4 shows an event with a charm-like

topology: the primary vertex has high track multiplicity and one of the scan-back tracks shows

a kink topology. At the 90% C.L. the transverse momentum ranges between 600 MeV/c and

1150 MeV/c. We evaluated that in this case, the probability that a hadron interaction mimics

a charm-decay with transverse momentum larger than 600 MeV/c is only 4 × 10 −4 .


Figure 4: Online display of the OPERA electronic detector of a ν µ charged-current interaction with a charm-like

topology (top panel). The emulsion reconstruction is shown in the bottom panels where the charm-like topology

is seen as a track with a kink: top view (bottom left), side view (bottom center), frontal view (bottom right).

5 Conclusions and outlook

After the two pilot runs in 2006 and 2007, in the 2008 run OPERA detected about 1700 neutrinos

interacting in the targets, consistently with the value expected from the CNGS integrated

intensity. At the time of the Moriond conference 484 interactions were located in the bricks:

72 NC and 412 CC interactions. The Scanning Stations and the scanning laboratories of the

Collaboration are completing the analysis of the bricks, locating the events which will be finally

analysed for the search of the ν τ candidates.

A new physics run is foreseen to start in June 2009: about 2.4 × 10 13 protons per extraction

will be delivered, for a total integrated intensity of 3.5 × 10 19 protons on target in less than 170

days of run. With such integrated intensity we expect to collect ∼ 3500 events in the target and

eventually to observe the first τ event candidates.

References

1. OPERA Collaboration, R. Acquafredda et al., JINST 4 P04018 (2009).

2. CNGS project: http://proj-cngs.web.cern.ch/proj-cngs/;

G. Acquispace at al., CERN-98-02 (1998);

R. Bailey et al., CERN-SL/99-034 (1999);

A. E. Ball et al., CERN-SL/Note-2000-063 (2000).

3. OPERA collaboration, A. Anokhina et al., JINST 3 P07005 (2008).

4. L. Arrabito et al., Nucl. Instrum. Meth. A 568 (2006) 578;

L. Arrabito et al., JINST 2 P05004 (2007);

Armenise et al., Nucl. Instrum. Meth. A 551 (2005) 261.

5. T. Nakano, Automatic analysis of nuclear emulsion, Ph.D. Thesis, Nagoya University,

Japan (1997).

6. S. Miyamoto et al., Nucl. Instrum. Meth. A 575 (2007) 466.

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