EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH
CERN–PPE/97–059
26 May 1997
THE NOMAD EXPERIMENT AT THE CERN SPS
The NOMAD Collaboration
J. Altegoer
6
M. Anfreville
20
C. Angelini
18
P. Astier
15
M. Authier
20
D. Autiero
9
A. Baldisseri
20
M. Baldo-Ceolin
14
G. Ballocchi
9
M. Banner
15
S. Basa
10
G. Bassompierre
2
K. Benslama
10
I. Bird
10
B. Blumenfeld
3
F. Bobisut
14
J. Bouchez
20
S. Boyd
22
A. Bueno
1,4
S. Bunyatov
7
L. Camilleri
9
A. Cardini
11
A. Castera
15
P.W. Cattaneo
16
V. Cavasinni
18
V. Cavestro†
14
O. Clou´e
20
G. Conforto
23
C. Conta
16
R. Cousins
11
A. De Santo
18
T. Del Prete
18
L. Di Lella
9
T. Dignan
4
E. do Couto e Silva
9
I.J. Donnelly
21,22
J. Dumarchez
15
L. Dumps
9
C. Engster
9
T. Fazio
2
G.J. Feldman
4
R. Ferrari
16
D. Ferr`ere
9
V. Flaminio
18
M. Fraternali
16
G. Fumagalli†
16
J.-M. Gaillard
2
P. Galumian
10
E. Gangler
15
A. Geiser
9
D. Geppert
6
D. Gibin
14
S. Gninenko
13
J.-J. Gomez-Cadenas
1,9
J. Gosset
20
C. G¨oßling
6
M. Gouan`ere
2
A. Grant
9
G. Graziani
8
A. Guglielmi
14
C. Hagner
20
J. Hernando
1
D. Hubbard
4
P. Hurst
4
W. Huta
9
N. Hyett
12
E. Iacopini
8
C. Joseph
10
D. Kekez
24
M. Kirsanov
8,13
B. Khomenko
9
O. Klimov
7
A. Kovzelev
13
V. Kuznetsov
7
A. Lanza
16
L. La Rotonda
5
M. Laveder
14
C. Lazzeroni
18
A. Letessier-Selvon
15
J.-M. Levy
15
L. Linssen
9
A. Ljubiˇci´c
24
J. Long
3
A. Lupi
8
E. Manola-Poggioli
2
A. Marchionni
8
F. Martelli
23
J.-P. Mendiburu
2
J.-P. Meyer
20
M. Mezzetto
14
S.R. Mishra
4
G.F. Moorhead
12
L. Mossuz
2
P. N´ed´elec
2,9
Yu. Nefedov
7
C. Nguyen-Mau
10
D. Orestano
16,19
J.-P. Pass´erieux
20
F. Pastore
16,19
L.S. Peak
22
E. Pennacchio
23
J.-P. Perroud
10
H. Pessard
2
P. Petitpas
2
R. Petti
16
A. Placci
9
H. Plothow-Besch
9
A. Pluquet
20
J. Poinsignon
20
G. Polesello
16
D. Pollmann
6
B.G. Pope
9
B. Popov
7,15
C. Poulsen
12
P. Rathouit
20
G. Renzoni
18
C. Roda
9
A. Rubbia
9
F. Salvatore
16
K. Schahmaneche
15
B. Schmidt
6
A. Sconza
14
M. Serrano
15
M.E. Sevior
12
D. Sillou
2
C. Sobczynski
9
F.J.P. Soler
22
G. Sozzi
10
D. Steele
3
M. Steininger
10
M. Stipˇcevi´c
24
T. Stolarczyk
20
G.N. Taylor
12
V. Tereshchenko
7
A. Toropin
13
A.-M. Touchard
15
S.N. Tovey
12
M.-T. Tran
10
E. Tsesmelis
9
J. Ulrichs
22
V. Uros
15
M. Valdata-Nappi
5,17
V. Valuev
7,2
F. Vannucci
15
K.E. Varvell
21,22
M. Veltri
23
V. Vercesi
16
D. Verkindt
2
J.-M. Vieira
10
M.-K. Vo
20
S. Volkov
13
F. Weber
9,4
T. Weisse
6
M. Werlen
10
P. Wicht
9
F.F. Wilson
9
L.J. Winton
12
B.D. Yabsley
22
and H. Zaccone
20
Submitted to Nucl. Instrum. Methods Phys. Res. A
1)
Univ. of Massachusetts, Amherst, MA, USA
2)
LAPP, Annecy, France
3)
Johns Hopkins Univ., Baltimore, MD, USA
4)
Harvard Univ., Cambridge, MA, USA
5)
Univ. of Calabria and INFN, Cosenza, Italy
6)
Dortmund Univ., Dortmund, Germany
7)
JINR, Dubna, Russia
8)
Univ. of Florence and INFN, Florence, Italy
9)
CERN, Geneva, Switzerland
10)
University of Lausanne, Lausanne, Switzerland
11)
UCLA, Los Angeles, CA, USA
12)
University of Melbourne, Melbourne, Australia
13)
Inst. Nucl. Research, INR Moscow, Russia
14)
Univ. of Padova and INFN, Padova, Italy
15)
LPNHE, Univ. of Paris, Paris VI and VII, France
16)
Univ. of Pavia and INFN, Pavia, Italy
17)
Now at Perugia Univ., Perugia, Italy
18)
Univ. of Pisa and INFN, Pisa, Italy
19)
Now at Roma-III Univ., Rome, Italy
20)
DAPNIA, CEA Saclay, France
21)
ANSTO Sydney, Menai, Australia
22)
University of Sydney, Sydney, Australia
23)
Univ. of Urbino, Urbino, and INFN Florence, Italy
24)
Rudjer Boˇskovi´c Institute, Zagreb, Croatia
†
)
Deceased
Abstract
The NOMAD experiment is a short base-line search for ν
µ
→ ν
τ
oscillations in the
CERN neutrino beam. The ν
τ
’s are searched for through their charged-current inter-
actions followed by the observation of the resulting τ
−
through its electronic, muonic
or hadronic decays. These decays are recognized using kinematical criteria necessitat-
ing the use of a light target which enables the reconstruction of individual particles
produced in the neutrino interactions. This paper describes the various components
of the NOMAD detector: the target and muon drift chambers, the electromagnetic
and hadronic calorimeters, the preshower and transition radiation detectors, and the
veto and trigger scintillation counters. The beam and data acquisition system are also
described. The quality of the reconstruction of individual particles is demonstrated
through the ability of NOMAD to observe K
0
s
’s, Λ
0
’s and π
0
’s. Finally, the observa-
tion of τ
−
through its electronic decay being one of the most promising channels in
the search, the identification of electrons in NOMAD is discussed.
1 INTRODUCTION
The main goal of the NOMAD (Neutrino Oscillation MAgnetic Detector) experi-
ment is to search for the appearance of tau neutrinos (ν
τ
) in the CERN SPS wideband
neutrino beam. This beam has a mean energy of 24 GeV and the predominant neutrino
type is ν
µ
. As a by-product the experiment can set limits on ν
µ
→ ν
e
oscillations.
The NOMAD detector measures and identifies most of the particles, charged and
neutral, produced in neutrino interactions within the detector. The active target is a
set of drift chambers with a fiducial mass of about 2.7 tons and a low average density
(98.6 kg/m
3
). The detector is located in a dipole magnetic field of 0.4 T which allows
the determination of the momenta of charged tracks via their curvature, with minimal
degradation due to multiple scattering. The active target is followed by a Transition
Radiation Detector (TRD) to identify electrons, an electromagnetic calorimeter including
a preshower detector, a hadronic calorimeter, and muon chambers. This paper describes
the detector in detail.
In three years’ running at the CERN SPS, NOMAD should collect data with more
than 1 million charged current (CC) ν
µ
events.
The ν
τ
will be searched for via its CC interactions: ν
τ
+N→τ
−
+ X. Given the
lifetime of the τ
−
and the energies considered here, the τ
−
will travel about 1 mm before
decaying. The spatial resolution of NOMAD, while good, is not sufficient to recognize
the non-zero impact parameter associated with such tracks. Instead, the decays of the τ
−
will be identified using kinematic criteria, based on a precise measurement of the missing
transverse momentum in the final state.
In order to be sensitive to a large fraction of the τ
−
decay modes and to be able to
select events with high acceptance and low background, the NOMAD detector must be
able to:
– Measure the momenta of charged particles in the drift chamber target with good
precision.
– Identify and measure electrons and photons.
– Identify and measure muons.
– Achieve a high level of rejection against tracks which fake electrons and muons.
As will be discussed below the NOMAD detector is well on the way to achieving
these goals.
In addition to searching for neutrino oscillations, the large sample of data in a
detector with a target density of a hydrogen bubble chamber will permit NOMAD to
explore many other processes involving neutrinos.
The next section of this paper describes the NOMAD subdetectors. The following
section documents its excellent performance.
2 THE DETECTOR AND BEAM
The NOMAD detector [1] is shown schematically in Fig. 1 (side view) and Fig. 2
(top view). It consists of a number of subdetectors most of which are located in a dipole
magnet [2] with a field volume of 7.5 ×3.5 ×3.5m
3
. Moving downstream along the beam
direction we find a veto counter, a front calorimeter, a large active target consisting of drift
chambers, a transition radiation detector, a preshower, an electromagnetic calorimeter,
a hadron calorimeter, and an iron filter followed by a set of large drift chambers used
for muon identification. Upstream and downstream of the transition radiation detector
1
