LETTER of INTENT the OPERA Emulsion Detector for a Long

Home , OPERA experiment

LNGS-LOI 8/97

Septemb er 8, 1997

LETTER OF INTENT

The OPERA emulsion detector for a long-baseline

neutrino-oscillation exp eriment

H. Shibuya

Toho University, Funabashi, Japan

K. Hoshino, M. Komatsu and K. Niwa

Nagoya University, Nagoya, Japan

S. Buontemp o, A. Ereditato, G. Fiorillo, P. Migliozzi and P. Strolin

Universit a Federico I I and INFN, Naples, Italy

G. Romano

Universit a di Salerno and INFN, Salerno, Italy

Y. Sato

Utsunomiya University, Utsunomiya, Japan

ABSTRACT

We present a new detector concept OPERA, a massive iron/emulsion target for a long-

baseline neutrino-oscillation search. The exp eriment can p erform an app earance search for

oscillation in the parameter region indicated by the atmospheric neutrino anomaly.

It exploits nuclear emulsion for the unambiguous detection of the decay of the pro duced

in charged currentinteractions and features a detector target with relatively small mass

200 ton and dimensions. The very low background is an essential ingredient to its high

discovery p otential and promises a unique role in the clari cation of the exp erimental sce-

nario. OPERA can run in the Gran Sasso Lab oratory in the prop osed neutrino b eam from

CERN. 1

1. Intro duction

One of the crucial questions in the present scenario of particle physics is whether the neu-

trino has non-vanishing mass. A massive neutrino would b e the direct indication of physics

beyond the Standard Mo del, representing a fundamental milestone in particle physics. More-

over, it would have profound implications for cosmology and astrophysics, giving a clue in

the explanation of the dark matter puzzle [1]. The interesting mass range is out of reachof

the measurements from decay kinematics. The only way to assess this issue is to search for

neutrino oscillation, whichmay o ccur for massive neutrinos.

In a rst approximation oscillation phenomena o ccur b etween two neutrino avours and

are describ ed bytwo parameters: the mixing angle sin 2 and the mass squared di erence

m . The sensitivity of the exp erimental searches to these parameters dep ends on the neu-

trino energy and on the distance L of the detector from the neutrino source. For exp eriments

at high-energy accelerators one usually de nes short L 1 km, medium L 10 km or

long L 1000 km baseline.

Presently, there are three exp erimental indications for neutrino oscillation. Matter-

enhanced oscillations [2] can explain the solar neutrino de cit with parameters m = 0.5{

5 2 2 2

1.6 10 eV and sin 2 = 0.4{1.2 10 [3]. If one attributes to neutrino oscillation

the apparent de cit of atmospheric muon neutrinos, as measured in the Kamiokande ex-

2 2 2 2

p eriment, a small m 10 eV and a large mixing angle sin 2 > 0:5 emerge [4];

b oth p ossibilities of and oscillation are contemplated. In addition, the re-

cent claim from the LSND exp eriment [5] suggests the existence of oscillation with

2 2

m 1eV .

Within the conventional two- avour oscillation scheme, two indep endentm are avail-

able as oscillation parameters. The three m indicated ab ovewould thus require a fourth

sterile neutrino [6]. Recently, several phenomenological analyses of the present data have

b een published [7][8][9][10][11][12][13] where the full mixing matrix of a three- avour general

approach is exploited. Such three- avour analyses incorp orate the exp erimental observa-

tions to a larger extent by attributing at least two of the phenomena to the same mass

di erence. The contemplated mass/mixing patterns result in interesting predictions for the

exp eriments.

The next generation solar and atmospheric neutrino exp eriments are exp ected to sub-

stantially contribute, with new data, to the exp erimental scenario on neutrino mixing. SNO

[14] will p erform the measurement of the ratio of charged to neutral currentevents. Both

SNO and Sup er Kamiokande [15] will b e able to observe e ects that can discriminate b e-

tween neutrino oscillation schemes, such as time variation of the solar signal and sp ectrum

distortion of the recoil electrons from neutrino interactions. The Borexino exp eriment [16] 2

can address the energy dep endence of the solar de cit. The atmospheric = anomaly

has b een con rmed by preliminary results from Sup er Kamiokande [17][18].

Sp eci c app earance searches are pursued by accelerator exp eriments. The evidence for

oscillation rep orted by LSND will so on b e veri ed by the upgraded KARMEN ex-

p eriment [19]. In the channel, the two running exp eriments at CERN, CHORUS

[20] and NOMAD [21], using only a fraction of the available data have recently rep orted

[22][23] no evidence for oscillations at the level of the present b est limit [24]. These are short-

2 4

baseline exp eriments, therefore sensitive to small mixing angles sin 2 2 10 and

relatively large m . A p ositive signal will call for a new dedicated exp eriment with higher

sensitivity, to con rm the discovery and to learn more. In case of a negative search, theo-

retical sp eculations incorp orating the observation of the solar neutrino de cit and particle

physics arguments [25][26][27] suggest to further extend the investigation to smaller mixing

angles. A mass ranging from 1 to 10 eV is predicted, whichwould then b e a natural can-

didate for the \hot" comp onent of the dark matter. A new exp eriment I213/TOSCA has

b een recently prop osed at CERN [28] which is able to increase the sensitivityby more than

one order of magnitude with resp ect to the searches presently under way. Finally, similar

in design to CHORUS, but exp ecting a total data sample one order of magnitude larger,

COSMOS [29] is approved for a short-baseline searchatFermilab. Within the three- avour

analysis, TOSCA and COSMOS have sucient sensitivity in mixing to prob e a prop osed

solution of all oscillation evidences and, in this framework, the LSND signal [9].

A complementary approach requires to explore the region of oscillation parameters in-

dicated by the atmospheric neutrino anomaly. This parameter region is not accessible by

short-baseline exp eriments. Long medium-baseline exp eriments are needed to explore the

small m region favoured by the existing data. Two forthcoming reactor exp eriments,

CHOOZ [30] and Palo Verde [31], will lo ok for disapp earance with a sensitivityin the

3 2

mass di erence of 10 eV . A new generation of accelerator-based exp eriments is b eing

planned. A neutrino b eam will b e sent from KEK to the Sup er Kamiokande detector, ab out

250 km away [32]. AtFermilab, the exp eriment MINOS [33] is approved. MINOS will use a

b eam from Fermilab to the Soudan Mine, at 730 km distance, to test the hyp othesis of neu-

2 2 3 2

trino oscillation with m 10 10 eV . Long-baseline exp eriments [34][35][36]have

b een prop osed using the future b eam from CERN to the Gran Sasso Lab oratory [37][38],

at 731 km from the source. A medium-baseline exp eriment has also b een prop osed recently

[39], whichwould use the present SPS b eam and a liquid Argon TPC detector of 400 ton

lo cated in the Jura mountain, 17 km from CERN, to explore neutrino-oscillation in the

2 2

region of m 1eV .

The technique of nuclear emulsion, which nds its rst large scale application in the

active target of the CHORUS exp eriment, can b e further improved for future os-

cillation exp eriments [40][41][42][28]. In [43] ideas for emulsion exp eriments suitable for

app earance oscillation searches were presented. In particular, the conceptual design

1 2

of a massive detector OPERA able to explore with high sensitivity the lowm region

2 3 2

10 10 eV was outlined. In OPERA, emulsions are used as high precision trackers,

unlike CHORUS or TOSCA where they comp ose the active target. The extremely high

space resolution of the emulsion cop es well with the p eculiar signature of the short-lived

lepton, pro duced in interactions, and the direct observation of the decay top ology allows

background reduction to a very low level.

As p ointed out in the classic b o ok byPowell, Fowler and Perkins [44], after the impressive

work for the developmentofemulsion plates with high grain densityitwas generally realized

that the metho d had features of great interest for exp eriments on the search for unstable

8 10

particles with mean lifetimes of the order of 10 {10 s. Fiftyyears after the discovery

of the pion through the observation of its decay in the emulsion exp osed to cosmic rays

[45], emulsions app ear again imp ortant for the observation of particles with lifetimes of

12 13

10 {10 s and for the study of related pro cesses.

Emulsion exp eriments b ene t nowadays from the impressive progress in the eld of

computer controlled microscop es, read-out by CCD cameras, with automatic pattern recog-

nition and track reconstruction. After its pioneering work [46], the Nagoya group in the

CHORUS Collab oration has pro duced second generation automatic systems ab out 10 times

faster [47]. A new automatic system has also b een designed by the Salerno group [48]. More

improvements are exp ected from the intense R&D programs under way. Fig. 1 depicts,

schematically, the op eration of an automatic microscop e. Fig. 2 shows the CCD image of a

neutrino interaction vertex and Fig. 3 displays a neutrino event in the CHORUS emulsion

target, as reconstructed by the computer.

As outlined in [49][50], the OPERA concept is app ealing for a long-baseline oscillation

search. The prop osed b eam from CERN to the Gran Sasso Lab oratory o ers such an

opp ortunity. The exp eriment will b e able to explore the parameter region indicated by the

atmospheric neutrino de cit, searching for the unambiguous signature of oscillation.

OPERA will p erform a app earance search, through the background-free detection of the

decay of the lepton pro duced in the interaction with a relatively small-mass target

200 ton. Thanks to the capability of the emulsion in identifying electrons and to the

small contamination of in the b eam, OPERA will also accomplish a oscillation

e e

search with go o d sensitivity. Fig. 4, taken from [13], shows the and avour

conversion-probability implied by [11] and [12] for a baseline of ab out 730 km. A sp ectacular

signal is predicted by the Cardall and Fuller scheme [12], in particular in the app earance

channel.

The prop osed exp eriment is complementary to the short-baseline search envisageable

with TOSCA whichmay run in the same b eam line. The present exp erimental situation is

such that explorations of the di erent regions of oscillation parameters domain are needed

Oscillation Pro ject with Emulsion-tRacking Apparatus. 4

to e ectively contribute to the clari cation of the global scenario.

2. The OPERA detector

Fig. 5 shows the exp erimental set-up intro duced in reference [51] to detect -pairs from

the decay of energetic neutral-pions from cosmic rays. The diverge in the air gap and

then pro duce cascades in passing through the sandwich of lead and emulsion. The op ening

angle of the -pair and the development of the cascade o er means to estimate the energy

of the neutral pion [44][52][53]. This device, now generally called Emulsion Cloud Chamber

ECC is very economical in the use of emulsion with resp ect to pure emulsion targets. The

ECC has b een used for several also large scale applications [54] and recently revised and

prop osed for long-baseline exp eriments searching for oscillation [55][43].

In general, the ECC provides a massive target made up of a sandwich of dense passive-

material plates and thin emulsion sheets Fig. 6. Employing iron as a target, with the

emulsion used only for tracking, the target mass can b e increased bytwo orders of magnitude

or more, as compared to other emulsion exp eriments like CHORUS and TOSCA. However, in

the ECC, the direct observation of the decay \kink" p erformed by CHORUS and TOSCA

is replaced by an impact parameter measurement.

The OPERA concept is an evolution of the ECC. The idea is to add a \drift space"

between consecutiveemulsion sheets. As further discussed in the following, this \empty"

space b etween the ES allows the direct detection of the decay kink, sup erior to the impact

parameter measurement p erformed with the conventional ECC. This results in a substantial

background reduction.

2.1 The target

The detector describ ed here is a 200 ton iron/emulsion target sub divided in 92 mo dules

Fig. 7. Each mo dule, whose dimensions orthogonal to the b eam direction are ab out

3 3m , consists of a sequence of 30 sandwiches, each comp osed of a 1 mm thick iron plate

followed by an emulsion sheet ES, a \drift space" of 2.5 mm, and another ES Fig. 8.

An ES is made up of a pair of emulsion layers 50 m thick, on either side of a 100 m

plastic base. The drift space can b e realistically lled with very low density material. Along

the b eam axis, the total thickness of one mo dule is ab out 12 cm. Transverse to the b eam

direction, each mo dule could b e sub divided into elements, e:g : with a 30 30 cm area. The

insets of Fig. 7 show a target mo dule and its elements. Fig. 9 shows the supp ort structure

of the detector elements allowing for their replacement.

When neutrinos hit the iron plates, primary particles are pro duced, some of which, in

turn, mayinteract or induce showers downstream in the detector. Charged particles give 5

two track segments in each ES. The 1 m granularity of the emulsion layers ensures ample

redundancy in the measurement of particle tra jectories.

If a is pro duced, most likely it will decay within a few millimeters. Its decay can b e

detected by measuring the angle formed by the charged decay daughter of the with resp ect

to the direction, this decay kink angle b eing due to the invisible neutrinos pro duced in

the decay Fig. 8. For decays in the drift space, the directions of the tracks b efore

and after the kink are reconstructed directly in space by means of the rst pair of ES

downstream of the iron plate where the primary vertex o ccurs.

The 50 m thickness of the emulsion layers is large enough to achieve high tracking

eciency and angular resolution 5 mrad and is suciently small to allow for industrial

pro duction, like commercial photographic or X-rayemulsion. The thickness of the plastic

base b etween the twoemulsion layers determines the angular resolution. The use of a thicker

base 200 m maybeenvisaged, if this is compatible with the request of minimizing the

amount of material in each sandwich. Feasibility and optimization studies will havetobe

p erformed by constructing prototyp e mo dules and exp osing them to charged particle and

neutrino b eams.

The whole target must b e kept at constant temp erature 10{15 C in order to protect

the emulsion against thermal deformations and \fading" see Section 3.2.

2.2 Event lo cation and electronic detectors

Obviously, the emulsion sheets have no time resolution. Therefore, electronic detectors

are needed to correlate the o ccurrence of neutrino events to that target element where the

interaction o ccurs and to guide the scanning. For a long-baseline application the detector will

naturally b e placed underground, shielded against non-p enetrating particles and low energy

muons from cosmic ray showers. Together with the low neutrino b eam ux, determined by

the long distance from the source, and with the absence of b eam halo muons, this leads

toavery low density of background tracks \stored" in the emulsion and removes the need

for a precise lo cation of individual tracks in the ES by electronic trackers. Therefore, these

detectors mayhave a mo derate space resolution a few millimeters.

The electronic trackers are placed b ehind each 12 cm target mo dule. They detect the

particles pro duced by the neutrino interaction and, hence, lo cate the ES where scanning

must start. Ab out 1 cm of this ES is scanned in corresp ondence with the detected inter-

action and all the found track segments are measured. These are then extrap olated and

searched for in the upstream ESs, until the eventvertex is reached. This \scan-back" pro ce-

dure is p erformed for secondary tracks, correlating segments in ES which are closely spaced

by the thin iron plates and the \empty" drift gaps. The metho d is intrinsically very ecient

to nd the p osition of the eventvertex. 6

The accuracy in the relative alignment of the ES which comp ose a target element can b e

of the order of a few hundred microns by means of a suitable mechanical supp ort structure.

Due to the very low density of tracks, a further micrometric precision in the alignmentmay

be achieved by using the tracks of any particle crossing the mo dule. In particular, one can

consider those tracks b elonging to the event itself.

The tracking detectors have the additional task of muon identi cation with good ef-

ciency and angular acceptance. For this purp ose, the tracking sections are arranged in

planes with transverse dimensions suciently larger than those of the emulsion. Honeycomb

chamb ers [56]maybeenvisaged, with a design similar to those installed in the CHORUS

apparatus [57].

Honeycombchamb ers can b e made up of several layers of closed hexagonal cells, with

a size of approximately 1 cm and a pitch of 1.2 cm. Each cell has a thin 20 m sense

wire at ab out 2000 V p otential di erence w:r:t: the cell wall. This wall is comp osed of

preformed conductive plastic. The op eration of these chamb ers is similar to straw tub es.

The di erence is that the construction of large planes is easier, since wires can b e p ositioned

when the cells are still op en.

Three orientations maybe foreseen, rotated by 60 degrees with resp ect to each other.

Each orientation contains three wire planes. This is sucient to p erform pattern recognition

and to obtain three-dimensional information. According to an electronic design available

for the read-out of these chamb ers [57], each wire is connected to its own multi-hit TDC.

The precision which can b e reached in large systems is 200 m p er plane. The number of

wires is 300000 for our application. Fig. 10 showsatypical multi-trackevent reconstructed

by the CHORUS honeycombchamb ers [57].

To increase the muon detection eciency and, consequently, the background rejection,

muon detectors could also be added around the target. Honeycomb chamb ers or limited

streamer tub es may b e used.

The momentum of the charged -decay pro ducts can b e determined byamultiple scat-

tering measurement in the emulsion. The resolution achievable with this metho d is weakly

dep endent up on momentum. It ranges from 10 at 1 GeV/c to 20 at 30 GeV/c, according

to preliminary estimates. However, as we will see in Section 3.4, in the prop osed exp eri-

ment there will be practically no need for momentum measurement, due to the very low

background level. A magnetized-iron muon sp ectrometer is placed b ehind the iron/emulsion

target, with the purp ose of measuring the charge of forward muons.

The electronic detectors also act as the active comp onent of a ne-grained calorimeter,

where the 12 cm thick iron/emulsion mo dules play the role of the absorb er material. Each

target mo dule is nearly 2 radiation lengths thick. Therefore, a calorimetric measurement

of the events can be p erformed, helping in the kinematical analysis of candidate events.

E GeV . Monte Carlo simulations predict an electromagnetic energy resolution of 35= 7

To complete the energy measurement for events originating downstream in the target, the

muon sp ectrometer is calorimetrized.

The overall dimensions of the exp erimental apparatus, shown schematically in Fig. 7,

are 3 3 25 m . We stress that the relevant parameter in the target design is the

mass. One can envisage to mo dify the ab ove transverse and longitudinal dimensions in

order to optimize the exp eriment p erformance and to cop e with the actual logistic of the

exp erimental hall.

2.3 Background rejection in OPERA

In the prop osed search for oscillation, all the ma jor decaychannels are studied.

For the single prong decays into a charged hadron, whichhave the largest branching ratio

BR = 50, an imp ortant source of background is p otentially given by hadron reinterac-

tions. One of the primary hadrons of the eventmay, in fact, reinteract in the vertex iron

plate giving pro ducts invisible in the emulsion, hence faking the decayofthe .

Given the average numberofcharged hadrons p er event 4, the iron plate thickness

and the numb er of neutral current NC events collected by the exp eriment, one can predict

the numberofbackground events from hadron reinteractions N r eint: 4 0:050=10:5

N =0:02 N , where the term in bracket is the ratio b etween one half of the iron

NC NC

plate thickness and the nuclear collision length. It is evident that, even in the case of a

relatively low statistics exp eriment, the reinteraction background can not b e disregarded.

The novel feature of the OPERA concept is the 2.5 mm drift space b etween consecutive

iron plates Fig. 8. By requiring that the decay o ccurs in this gap, one rejects the reinter-

action background. This, at the cost of a somewhat reduced signal eciency, b ecause of the

loss of the decaying in the iron plate where the primary vertex o ccurs \short decays".

Also the small fraction of \long decays" is rejected, i:e: those events where the kink o ccurs

in the following iron plate or further on.

The OPERA concept also allows for a reduced background to the muonic decays. The

requirement that the decay do es not o ccur in the iron eliminates those charged current CC

charm events whichmay fake a large impact parameter of the muon, as shown in Fig. 11.

Following arguments similar to those presented in Section 3.4, we can estimate the p otential

+ + 4

background induced by D mesons to b e N D 4 10 N , where N is the

bg CC CC

total number of charged currentevents. A factor two higher numberofevents due to D

0 3

pro duction and decay is predicted N D 10 N .

bg CC

Both the reinteraction and the charm backgrounds describ ed ab ove are signi cantinan

exp eriment employing a \standard" ECC detector. In that case, in particular, one maybe

led not to use the valuable muonic decay channel of the [55][43], which has very clear 8

top ology but is to o much a ected bycharm background Fig. 11.

Wehave seen that removing background events with a kink in the iron, a lower eciency

is obtained for the signal. With resp ect to this, a denser target material would b e advan-

tageous. With higher densitye:g : tungsten or lead alloy, thinner foils could b e used 500

m, increasing the detection eciency at constantemulsion mass. Another p ossibilityis

to still use 1 mm thick but more massive plates. In the latter case, one would have a more

compact detector, reducing the amountofemulsion and the numb er of electronic detectors.

For a denser material alternative to iron, b oth the ab ove options havetobe evaluated in

relation to the technical feasibility, to the p ossible deterioration of the detector p erformance

and to the cost.

3. OPERA at the Gran Sasso Lab oratory

Several p ossibilities can b e contemplated for the oscillation scenario, invoked to explain

the atmospheric de cit. There could be a pure or conversion or a

combination of the two. In b oth cases, by using a high-energy accelerator neutrino b eam,

longmedium-baseline exp eriments have to be used to explore the interesting oscillation

2 2 2

parameter region of lowm 10 eV .

A neutrino b eam from the CERN SPS to the Gran Sasso at 731 km distance is under

study [34][37][38][39]. The sensitivity to small mixing angles is, at long-baseline, relatively

p o or, given the low neutrino ux due to the very long travel-distance and to the divergence

of the neutrino b eam. Therefore, the target has to b e massive.

The OPERA detector app ears very attractive for an app earance exp eriment at the Gran

Sasso. In the search for oscillation, the high detection eciency of OPERA, together

with its negligible background and the direct observation of the kink, allow to limit the

target weight and dimensions, still retaining good sensitivity in the interesting domain

of the oscillation parameters. It uses the 200 ton target previously describ ed, while all

the exp eriments prop osed so far for long-baseline oscillation-exp eriments [32][33][34][35][36]

feature very large target-mass 1 kton or more.

In references [55] and [43] the use of a standard, large mass 1 kton ECC was envis-

aged to search for oscillation with a long-baseline exp eriment. The detection is

accomplished by means of an impact parameter measurement, without top ological informa-

tion ab out the presumed decayvertex. For that detector the relatively high event yield

which allows for a short run is counterbalanced by a higher background. In addition,

a large quantity of emulsion is needed. The OPERA detector seems b etter suited for an

exp eriment at the Gran Sasso, given its p eculiar characteristic of detecting decay kinks in

the drift space. Similar exp erimental considerations apply to the neutrino b eam from the

Fermilab Main Injector sent to the Soudan mine, where the MINOS exp eriment is planned 9

[33]. OPERA could also run in that b eam at a long-distance p osition.

Besides the detection capability, which is the distinctive feature of the detector, also

prompt electrons signature of oscillation may b e identi ed by exploiting the prop-

erties of the emulsion [58][59]. The electron identi cation is practically feasible due to the

small numberofevents of the exp eriment, and leads to a search with go o d sensitivity see

Section 4.

3.1 The neutrino b eam

Concerning the features of the neutrino b eam from the CERN SPS to the Gran Sasso

Lab oratory, we refer to published studies [34][37][38][39][60]. Several b eam designs exist,

di ering in the primary proton energy, in the extraction mo de, and in the fo cusing magnets

con guration. As a basis for the exp eriment design, in this pap er we will take the scheme

describ ed in references [39] and [60], where neutrinos mainly are pro duced by a 450 GeV

proton b eam. The most relevantcharacteristics of this b eam at the Gran Sasso lo cation are

given b elow, evaluated over a surface of 9 m .

The present CERN SPS can pro duce neutrinos by accelerating 2.5 10 p ot/year,

with a 200 days run, 3 10 p ot/ 14.4 s cycle and 70 eciency [42]. This is a realistic

statement, given the current p erformance of the machine in supplying the CHORUS and

NOMAD exp eriments. For a future b eam to the Gran Sasso, the SPS should achieve a

comparable p erformance [61]. Obviously, higher b eam intensity, as requested for the TOSCA

exp eriment [28], will allow a faster reach of the sensitivity goal and a higher discovery

p otential.

The exp ected neutrino ux and the number of interactions for the di erent neutrino

sp ecies of the b eam is given in Table 1. The mean energy of the is 30 GeV, while the

average energy of the contaminating the b eam is 37 GeV. The energy sp ectra of muon

and electron neutrinos are given in Fig. 12. The relative contamination of the b eam is

0.4 0.1.

We observe that for the b eam line design of reference [37] a factor twolower neutrino

ux is predicted for the Gran Sasso lo cation.

3.2 Neutrino run and interactions

The maximum time of exp osure of the emulsion dep ends on two ma jor parameters: the

density of integrated background tracks and the \fading" pro cess, i:e: the partial loss of

the latent image prior to the development. In the case of a short-baseline lo cation, as

in CHORUS and TOSCA, the integration of b eam halo muons, of muons asso ciated with 10

e e

19 2 10 8 7 7

Flux =10 pot m 1:1 10 1:3 10 5 10 0:5 10

Charged curr. ev ts=10 pot k ton 1022 16 5 0.7

Table 1: Exp ected neutrino uxes and event rates at the Gran Sasso lo cation.

the SPS test b eams and of cosmic rays limits the exp osure time to ab out 2{3 years. Also

fading, which to some extentmay b e tuned by mo difying the emulsion comp osition, plays

an imp ortant role.

Supp ose that wewere to run OPERA at the Gran Sasso Lab oratory. Halo muons are

absent and the cosmic background is negligible. In addition, the low event rate has an

imp ortant exp erimental implication. The emulsion sheets in the few target elements hit

by neutrinos can be develop ed and replaced during the run, say after each year of data

taking see Section 5. Therefore, fading is also not a limitation. The conclusion is that it

is realistic to foresee a long run 4 years, or more by regularly replacing a small fraction of

the target elements. This implies an ecient use of the investment represented by the b eam

facility and by the exp erimental apparatus.

Under the previous assumptions on the b eam line design and accounting for a total of

10 p ot, the exp ected number of interactions in 4 years is 2044 CC, 640 NC and 10

interactions induced by the contaminating the b eam. The amountof interactions and

e e

the uncertainty on its estimate contribute to set the sensitivity limit for a oscillation

is 30. search, which will also b e discussed here. The number of events pro duced by

The ab ove estimates are valid in the case of absence of oscillation and for a 200 ton target.

3.3 Detection eciency for the decay

Due to the low statistics, in the prop osed exp eriment no kinematical cuts need to be

applied to reduce the number of events to b e scanned, so leading to high eciency for the

signal and to an e ective use of the detector mass.

The signal of the o ccurrence of neutrino oscillation is the charged currentinteractions

of neutrinos in the detector target

N ! X:

The detection of the decayofthe lepton in the nal state identi es this reaction. The

can b e detected through its decay mo des into single hadrons, muons and electrons 11

2 2 2 2 2 2 2 2

m =0:008 eV m =0:01 eV m =0:1 eV m 1 eV

56 3 59 2 61 1 62 1

Table 2: Eciency to detect a kink inside the drift space as a function of m .

gap

! h n

! e

and into three charged pions plus neutrals

+ 0

! n :

In OPERA, the decays in the vertex iron plate \short kinks" are rejected, as well as

the small fraction of \long kinks" o ccurring in the iron plate following the vertex plate. One

retains decays in the drift space b etween ES. In this way, one p erforms a direct kink detection

instead of an impact parameter measurement, with consequent background reduction and

improved signi cance of the signal. As already noted, the price to pay for such an imp ortant

gain is a reduction in the detection eciency. The oscillation probability dep ends on

2 2

m L=E . This means that small m values corresp ond to low energy oscillated

and, hence, to low energy leptons. Since the decay length of the is related to its energy

through the factor, the eciency of the ab ove cut is a function of the m value

gap

Table 2. The discarded events with decay in the iron will b e available for sp ecial analyses.

An additional source of ineciency is determined by the rejection of small angle kinks

< 15 mrad. This is given by the angular resolution in the measurement of the kink angle

by means of two ES, which give four space track-segments. Ab out 95 of the decays

havea kink angle larger than the 15 mrad. Taking also in account errors on the relative

alignment of the ES within a target element, we obtain an overall kink detection eciency

= 90.

k ink

Due to edge e ects, a ducial volume cut of 5 has to b e applied. This intro duces an

additional eciency term = 95.

fid

The decay into single charged hadron plus neutrals has the highest BR 50. In

the whole kinematical domain accessible by OPERA, the overall detection eciency of this

channel = BR ranges from 24 to 26.5, according to the m domain.

h gap k ink fid 12

Similar considerations apply to the muon and electron decay of the . The contributions of

these twochannels to the signal are nearly the same. The BR is 18 for b oth the decay

mo des, and an eciency ranging from 8.5 to 9.5 is obtained. The global detection

eciency = + + of OPERA is high. It amounts to 45.5 41 in the high

tot h e

low m region.

The three-pion decaymodehasavery distinctive top ology. The three track segments of

the daughter pions match in the drift space with the primary track. The BR of this channel

is 14 and an eciency 5:5 is estimated. However, the exp ected background for

this channel is comparable to the one of the single prong decay mo des altogether. For this

reason we do not include it for the present sensitivity estimate.

In Fig. 13 and Fig. 14 we show the display of a simulated event with the decaying into

muon, as reconstructed in the OPERA detector.

3.4 Background

Backgrounds and rejection metho ds in OPERA are similar to those in CHORUS, which

exp ects less than 2 background events p er 500000 CCs for the single pion, three pion and

muonic decaychannels altogether. This numb er is further reduced to well b elow one event

by the kinematical analysis of the candidate events at the eventvertex [20]. Such analysis

is only p ossible by exploiting the features of the emulsion.

The OPERA concept of detecting kinks in the drift space allows to make the reinteraction

background irrelevant. The main p ossible background sources in OPERA are:

1. prompt pro duction in the primary proton target and in the b eam dump;

2. decay of pions and kaons pro duced in CC and NC interactions;

3. 1-prong decayofcharmed particles.

1 The rate of pro duction by the interaction of 450 GeV protons in the Be target and

in the downstream iron b eam dump has recently b een evaluated in [62] and [63] for the

present CERN Wide Band Beam. Following the metho d of calculation of [62], for 10 pot

events over the OPERA ducial volume. This number is we estimate 3 10 CC

completely negligible.

2 The decay in the drift space of pions and kaons pro duced in NC neutrino interactions

can fake the top ology of a candidate event. Both charge states can contribute to this

background. In fact, the single charged-prongs from decay are negative, but the charge of

the =K daughter may only b e measured for the muonic decay, with the muon detected by

the downstream sp ectrometer. CC interactions are an additional source of this background

if the primary muon is undetected. 13

Both NC and CC contributions have b een evaluated by Monte Carlo simulations, where

wehave taken into account K and decays into h; e; plus neutrals. A 90 kink nding

eciency has b een assumed, with 95 eciency in identifying the primary muon. For

forward going muons a charge determination eciency = 70 is obtained by means of

the downstream sp ectrometer. We assume that the charge of electrons and hadrons is not

measured.

The exp ected numberofbackground events from pions and kaons is 0.02, for an inte-

grated numberofCCinteractions N = 2044.

3 Charmed particles are pro duced in CC and NC neutrino interactions through the reac-

tions

a N ! cX

b N ! ccX

c N ! cc X:

The ab ove pro cesses may constitute a background to the oscillation signal if one fails to

detect the primary muon a-b or the charm partner b-c.

The most relevant background source is given by single charm pro duction reaction a.

Charmed mesons have mass and lifetime similar to those of the . If the primary muon

is missed, a D meson decaying in one charged particle plus neutrals can fake a genuine

event, since the p ositive sign of the D daughter may only b e measured for the muonic

channel.

The level of the ab ove background is estimated by the following expression

N N D

N single charm = BRD 1 1 N

bg ch gap k ink fid CC

N N

CC c

where the factors are, resp ectively, the probability to pro duce charm in CC interactions

4, the D pro duction probability 25, the BR for the considered decaychannel, the

charge detection ineciency 30 for muons and 100 for hadrons and electrons, the

probability for the D to decay inside the drift gap 60, the kink detection eciency, the

ducial volume eciency, the probability not to identify the primary muon, and the total

number of charged currentevents N = 2044.

For 1-prong hadronic D decay BR = 24 we obtain 0.12 background events. Similar

calculations lead to 0.02 events for the muonic channel BR = 10 and 0.05 events for the

electron channel BR = 9. In total, one is left with 0.19 background events from single

charm pro duction by .

The rate of asso ciated charm pro duction in NC interactions has b een directly measured

by the E531 exp eriment on the basis of one observed event [64]. They also set a 90 CL on

the CC pro duction rate, consistent with the two rates b eing equal. Indirect measurements

exist for the CC pro cess, coming from the exp erimental data on prompt same sign dimuons.

Unfortunately, these measurements strongly dep end up on the momentum cut applied to the

second muon and on the statistical subtraction of the non-prompt meson decay background.

In the following we assume N cc=N =0:13, as determined in [64].

The contribution of reactions b and c to the background in OPERA is then

N D N cc

BRD 1 1 1 N N cc=

ch gap cc k ink fid CC

N N cc

N cc N D

N cc= BRD 1 1 N :

ch gap cc k ink fid CC

N N cc

CC CC

Taking into account a tracking eciency of the asso ciated charmed meson in most cases

D or D =0:55, and = =0:3, the sum of the twochannels amounts to 0.02

cc NC CC

events.

In the previous calculations wehave neglected the contribution of charm pro duced by

, and , due to their small relative abundance in the b eam Table 1.

e e

In total, OPERA will b e left with 0.23 background events with decay top ology, for the

three decay mo des with one charged prong altogether. We stress that for the analysis

describ ed ab ove the momentum measurement in the emulsion is not used.

Candidate events will undergo a further kinematical analysis, meant to exclude the

charm background at a higher con dence level. With emulsion, the background rejection

is particularly ecient as it can b e based on the observation of the short track b efore the

decay. A \vertex" kinematical analysis may be exploited, with high rejection power and

go o d signal eciency.

The direction of all primary and secondary particles at the vertex can be measured.

The electrons may b e identi ed by following their tracks in the ES, and by detecting their

electromagnetic interactions [58][59]. Photon conversions from neutral pions can b e searched

for in the ES further downstream of the vertex plate. The calorimetric information of the

electronic detectors could also be used. The exp ected di erence in the kinematics of

decays with resp ect to charm events is then exploited. Even not accounting for momentum

and energy measurement, the determination of the pro duction angle will p ermit to givea

further background rejection keeping high eciency for the decays [20][28], so leading to

avery low background level, for the prop osed exp eriment. 15

In conclusion, even in the hyp othesis of more p essimistic assumptions ab out eciencies

and acceptances, OPERA will be a background free exp eriment in searching for

oscillation.

3.5 Sensitivity to oscillation

The overall detection eciency amounts to 45.5 for high m . The ratio of

tot

cross-sections for tau and muon neutrinos is 0.54, averaging over deep inelastic and quasi-

elastic events. The total exp ected number of CC events is 2044. With no observed events

this leads to the following oscillation probability

P < 2.3/N / 4.5 10 90 CL.

CC tot

In the conventional two- avour mixing scheme, the corresp onding limit on the mixing angle,

for large m ,is

2 3

sin 2 =2P < 9 10 90 CL.

The minimum m , for full mixing, is given by the following relation

3 2 2 2

P /1.27 Lkm 2.5 10 eV . m eV =

The exp eriment, therefore, will be sensitive to the low m region corresp onding to the

atmospheric neutrino anomaly in the hyp othesis of conversion, as rep orted by the

Kamiokande exp eriment [4] and recently con rmed by Sup er Kamiokande [17][18].

2 3 2

Assuming that oscillation o ccurs with parameter values m =8 10 eV and

sin 2 = 0.9 i:e: the Kamiokande favoured values in the standard two- avour scheme,

a total of 55 CC interactions would b e exp ected in the target. Since the overall eciency

to detect a decay in this parameter region is 41, OPERA would observe ab out 22

interactions. Given the absence of background and the unambiguous exp erimental signature,

this is a large numb er which quali es the discovery p otential. Preliminary results from Sup er

2 3 2

Kamiokande [18] suggest m =5 10 eV .

The ab ovenumb ers are valid in the usual two- avour neutrino-mixing scheme. An even

more sp ectacular signal would b e detected by OPERA in the hyp othesis of the three- avour

mixing scheme describ ed in [12]. In this case, the huge oscillatory e ect in the

channel, shown in Fig. 4, would pro duce ab out 180 events detected by OPERA.

4. Search for oscillation

OPERA will also be sensitive to oscillation. The primary vertex tracks in

muonless events will be measured to lo ok for the o ccurrence of CC interactions. The

electron identi cation is p erformed by following its track in the ES downstream of the

vertex plate [58][59]. Given the small iron plate thickness 0.06 radiation lengths, the

probability for the electron to start showering in the rst plate is negligible. In Fig. 15 we

show the result of a Monte Carlo simulation of the electron identi cation capabilityofan

ECC by means of multiple energy-loss measurements. The identi cation eciency is given

as a function of the material thickness traversed electron track-length. This eciency is

assumed to b e 75, conservatively for our con guration.

In OPERA there are two ma jor sources of background electrons. The rst is given by

the contaminating the b eam, with a ux ratio / of ab out 200. According to

e e e

Table 1, it yields 10 events p er 2044 CC interactions. Accounting also for a 95 ducial

volume eciency, this corresp onds to 9 detected events. The other source is represented

by the electrons from the Dalitz decay of neutral pions pro duced in NC interactions

0 + 0

! e e and from photon to electron conversion in the iron plate ! 2 followed

by N ! e e N . Selecting neutrino interactions with one electron p ositron identi ed,

we estimate ab out 35 background events from neutral pions. In this estimate wehave not

considered the p ossible application of kinematical cuts. In particular, simulations show that,

by rejecting events with electron energy lower than 5 GeV, one removes 90 of induced

events, with 90 eciency for the signal.

The statistical errors on the ab ovenumb ers of background events determine the sensi-

tivity in the measurement of the oscillation parameters, together with the systematic errors

due to the uncertainty 5 on the estimate of the content of the b eam and to the un-

certainty 20 on the numb er of electrons pro duced by . These are preliminary guesses,

and more studies will b e needed.

In conclusion, we obtain 45 exp ected background events with a total uncertaintyof10

events. The 90 CL single-sided upp er limit on this number is ab out 15 events. In the

case of a negative search, accounting for 2044 CC events, an electron identi cation eciency

of 75 and a ducial volume eciency of 95, OPERA would achieve a 90 CL limit in

2 2 2

the mixing parameter sin 2 2 10 , for large m . For full mixing, the minimum

2 3 2

detectable m is 3 10 eV . This result is describ ed by the exclusion plot shown in

Fig. 16. OPERA can then also test the hyp othesis of oscillation as an explanation

of the atmospheric muon neutrino de cit and compare this result with the outcome of the 17

oscillation search.

5. Emulsion and scanning

Ab out 10 ton of emulsion are needed for the OPERA emulsion sheets, with a densityof

silver bromide the active comp onent of the emulsion a factor twolower than in the emulsion

target of short-baseline exp eriments. This is p ermitted by the muchlower background level

in the long-baseline lo cation and by the fact that automatic scanning devices do not require

particularly high emulsion \grain" densities.

Handling of such a large quantityofnuclear emulsion is a challenge. However, several

hybrid exp eriments were run for more than 20 years b oth at CERN and Fermilab. This

resulted in a progress of the technique. The main problem to be faced is the pro duction

of raw material emulsion gel, with high and uniform sensitivity and with go o d stability,

within a reasonable p erio d 6 to 12 months and at a convenient cost. Contacts with

imp ortant companies have b een established and several tests will b e p erformed jointly with

them. Some p oints are still op en, but the residual problems could b e solved in due time.

Concerning the emulsion cost, a reduction of at least a factor two with resp ect to the present

one seems achievable.

Another imp ortant issue is the preparation of the emulsion b efore the exp osure p ouring

and assembling into emulsion sheets. The most serious problem is represented here by

the time needed for p ouring, i:e: for pro ducing the nal emulsion from gel. With the

presently available facilities, the pro cedure would take to o long and would require to o much

manp ower. We are studying alternative solutions, either based on a new lab oratory, with

greater resources and mostly automated, or giving the job to the pro ducers of gel, after

having carefully agreed up on the sp eci cations and under sup ervision. Therefore, one may

think of emulsion sheets pro duced by companies or by dedicated machines, similar to those

employed for photographic lm pro duction, with consequent substantial cost reduction. It

is envisageable to run these machines at the exp eriment site, so reducing the p ossibilityof

integrating cosmic background after the p ouring pro cedure.

Both OPERA in a medium baseline lo cation [43] and TOSCA [28] rely on the capability

of p erforming high-sp eed automatic scanning of the emulsion sheets or target, given the

large number of events to be scanned up to several million. The situation for the long-

baseline exp eriment here prop osed is much more favourable. The total number of events

to be scanned, corresp onding to a four years' run, is ab out 2700. However, the scanning

pro cedure is more complicated and time consuming than for TOSCA. In OPERA the elec-

tronic detectors lo cate events rather than individual tracks. For eachevent, several emulsion

sheets must b e scanned over a larger area 1cm and some of the tracks have to b e fol-

lowed upstream to the primary vertex. We can estimate in 30{60 minutes the time needed 18

to completely scan one event. The scanning load is, anyhow, very limited, and would be

compatible with the much higher load given by the TOSCA exp eriment.

Given the small rate of events of the order of 3 per day the event analysis can be

conducted quasi-on-line. One can p erio dically remove and replace those target elements

where the eventvertex o ccurs, and p erform the emulsion scanning. This scheme allows a

fast analysis, with some complication of the detector set-up. The target is, in fact, a fully

mo dular structure, allowing the automatic access to the target elements Fig. 7 and Fig. 9.

We observe that the numb er of elements to b e dismounted after one year running 650

is small as compared to the total numb er of target elements 9800.

6. Tests, construction, running and time schedule

We are planning to construct target element prototyp es and to test them with charged

particle b eams and, p ossibly, with the present CERN Wide Band neutrino Beam which will

be op erational in 1998. One could then pro t from the intense b eam and reconstruct a

sizeable amount of neutrino events. These would allow to assess the design p erformance of

the OPERA target.

Among the aims of these tests we mention the study of the eventvertex nding eciency,

the particle identi cation in the emulsion, the angular resolution and the kink detection

capability through the study of charm events. The test results will also allow to optimize

the mechanical design of the target elements and of the \spacers" to b e used to ll the drift

space b etween the ES.

We are presently investigating the p ossibility to use a lead alloy as passive target material.

As already p ointed out, this would allow either a reduction of the detector dimensions, and of

the amountofemulsion, or an increase of the detector mass, at constant detector dimensions

and emulsion mass.

The physics goal of the prop osed exp eriment is complementary to that of the high-

sensitivity short-baseline search p erformed by TOSCA. Also technically, the two exp eriments

could b ene t from each other in many resp ects. The emulsion technology is in common and

b oth exp ertise and infrastructures could be eciently shared. The electronic detectors

honeycombchamb ers in a tentative approach could b e the same, and also in this case one

can think of the same \industrial" pro duction facility. The extra work which a long-baseline

exp erimentwould imply for the emulsion and scanning facilities set-up for CHORUS, and

whichwould b e used by TOSCA, is very limited due to the muchlower numberofevents.

A p ossible time schedule for OPERA at the Gran Sasso could be as follows. In 1997

and 1998 Monte Carlo simulations and test b eams on prototyp e mo dules will allow the

feasibility of the exp eriment to b e assessed. With a detailed Prop osal and its approval by 19

1999, the years 1999-2001 would be devoted to the construction of the apparatus and to

a rst technical run. The commissioning of the exp eriment will t with a neutrino run in

2002. Four years' running 2002-2005 will allow to reach the goal sensitivity. The rst

physics results could come after the rst year of data taking, thanks to the \quasi" on-line

emulsion analysis and to the negligible physics background.

7. Conclusions

We have presented the conceptual design of a novel detector OPERA exploiting the

technique of nuclear emulsion, to be used for a new oscillation search. OPERA

is an evolution of the so called Emulsion Cloud Chamber ECC. The detector consists

of a massive target made up of a sandwich of dense passive-material plates and of thin

emulsion sheets for high-resolution tracking. The use of the passive material iron in the

present design allows to build a 200 ton detector.

In this Letter of Intentwe prop ose to exploit the OPERA detector, already envisaged for

a medium baseline application [43], for a background-free app earance exp eriment searching

for oscillation at the Gran Sasso Lab oratory in the prop osed neutrino b eam from the

CERN SPS. The high detection eciency of OPERA, together with its negligible background

and the direct observation of the decay kink, allows to use a target mass ab out one

order of magnitude lower than for other exp eriments though yielding go o d sensitivityinthe

interesting domain of the oscillation parameters.

In the conventional two- avour mixing scheme, and for a required number of 10 protons

2 3 2

on target, OPERA will b e sensitiveuptom 2.5 10 eV 90 CL, well b elow the

2 2

indication from the atmospheric neutrino anomaly. The limit on sin 2 for large m

will b e 9 10 90 CL. These limits are graphically shown in the exclusion plot of

Fig. 17.

The discovery p otential of OPERA is high. Thanks to the absence of background,

single events are meaningful. If oscillation would o ccur with parameters m =

3 2 2

8 10 eV and sin 2 = 0.9, the exp eriment would detect the app earance of 20

unambiguous charged current interactions. In the phenomenological scenario outlined in

[12] with a three- avour mixing scheme, OPERA would detect a sp ectacular signal. In this

case, the huge oscillatory e ect in the channel would pro duce ab out 180 detected

events.

OPERA would also b e sensitiveto oscillation, due to its capability of identifying

electrons and to the low contamination of in the b eam. The exp ected limits 90

3 2 2

CL, in the absence of a p ositive signal, will b e 3 10 eV in m for full mixing and

2 2 2

2 10 in sin 2 for large m . Further studies are needed to assess in particular

this issue. 20

The quoted p erformance will b e obtained with a four years' run of the exp eriment. The

limited numberofevents ab out 2700 in total will p ermit to remove p erio dically the small

fraction of target elements hit by neutrinos and p erform the emulsion scanning \quasi" on-

line. Therefore, the rst physics results will b e obtained so on after the start of the running

p erio d, and regularly up dated, as in the case of a fully electronic detector.

The OPERA exp eriment can thus contribute to clarify some crucial asp ects of the present

scenario on neutrino oscillation, thanks to its sensitivity, discovery p otential and time sched-

ule. In particular, it will allow to understand the interpretation of the neutrino atmospheric

anomaly in terms of neutrino oscillation with a fast, unambiguous and background-free

app earance search.

OPERA is complementary to TOSCA, which in a short-baseline lo cation will explore

the small mixing angle domain not accessible by OPERA. The scanning load required by

OPERA is much lower than for TOSCA. Both exp eriments may b ene t from \p ositive

interferences" in the use of common resources, exp ertise and infrastructures.

The imp ortance of the physics case, the unclear exp erimental situation and the neces-

sity that any result be con rmed in this eld indicate that indep endent and sp ecialized

explorations are needed for a conclusive clari cation of the scenario. A healthy europ ean

research programme is necessary to participate in the world-wide enterprise on neutrino

physics together with those pro jects approved and partly funded in USA and Japan.

The actual realization of OPERA will have to b e rmly addressed by means of further

detailed studies and simulations. Tests on detector prototyp es will help in the assessment

and optimization of the exp erimental p erformance. The de nition of a complete Prop osal,

incorp orating the results of these studies, can b e achieved with the joint e ort of a scienti c

Collab oration to b e established and made so on op erational. We wish to favour the creation

of such a Collab oration and wewould welcome the interest of exp erimental groups of the

international neutrino community.

Acknowledgements

Weacknowledge the kind collab oration of V. Palladino, F. Pietropaolo and S. Sorrentino

for the neutrino b eam simulations. We also thank A.G. Co cco and R. Listone for their con-

tribution to the detector simulation. We are indebted to P. Picchi for the useful discussions

and for the careful reading of the pap er. 21

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[64] N. Ushida et al., Phys. Lett. 206 B2 1988 375. 24 reconstruction by hardware automatic microscope video processor TOMOGRAPHIC IMAGE CCD (1–3 sec/event) TO VIDEO PROCESSOR

50 m focal depth 5

(one view

512

512 pixels

150m 120 )

frame (formed by adjacent views)

emulsion track ( matching grains)

Figure 1: Schematics of the op eration of an automatic microscop e, as used in CHORUS. 25

Figure 2: The CCD image of a typical neutrino event in CHORUS. The dimensions of the

view are ab out 120 100 m , with a fo cal depth of a few micron. 26

Figure 3: Reconstructed neutrino event as measured in the CHORUS emulsion. Shown are

the grains, with reconstructed tracks, in the emulsion layers on b oth sides of the plastic

base. To indicate the scale, the sensitivevolumes emulsion layers have a thickness of 350

m each, and the plastic base 100 m. The black dots identify the so-called \scan-back"

track, namely, the one extrap olated from the dowstream electronic trackers. A background

track, clearly not related to the event, is also displayed. 27 1.0 (a) 0.8 AP

0.6

µ P

0.4

0.2 CF 0.0 1 10 100 E (GeV)

1.0 CF (b) 0.8

0.6

µτ P

0.4

0.2 AP

0.0 1 10 100

E (GeV)

Figure 4: Probabilities for a and b app earance in the b eam from CERN to

Gran Sasso or Fermilab to Soudan mine, versus the neutrino energy. \CF" \AP" stands

for the Cardall & Fuller [12] Acker & Pakvasa [11] oscillation scheme. 28

Figure 5: The set-up made of emulsion layers and passive material, as used in [51] from

reference [44]. 29 Emulsions ντ Iron

e- h- µ−

τ-τ− ντ

Plastic

1 mm 1 mm

50 µm

Figure 6: Schematic structure of the ECC target. The use of the impact parameter technique

is also shown. 30

)

a 1 3 m

10 cm

(

F u e

12 cm

F E 3 m 3 m µ 2.5 mm Drift space 50+100(base)+50 Emulsion 3.9 mm spectrometer Sandwich (x30) (x92) µ Fe/Emulsion Fe

1 mm ~25 m ~25 30 cm

Element

30 cm 30 12 cm 12

General view

Figure 7: The OPERA detector: elements, mo dules and general view. 31 Iron ES ES Iron

τ- e,-µ,- h-

ν τ

2500 µm drift space

1000 50 100 50 (µm)

Figure 8: Schematic structure of the OPERA target. The decay kink is directly recon-

structed in space, by using four track segments in the emulsion sheets. 32

Figure 9: Detail of a mo dule in OPERA: the supp ort structure of the detector elements

allows for their replacement. 33 Run 6800 Event 34Proj. Zenith

-84

-86

-88

-90

-92

-94

-96

-98

250 252 254 256 258 260 262 264 266

Figure 10: Tracks segments reconstructed in the CHORUS honeycomb chamb ers. The

circles represent the drift time measurement. 34 p ντ τ-

µ−

o D νµ

µ−

Figure 11: Signal and charm background for short decays rejected in OPERA in the muon

channel. 35

Figure 12: Exp ected black circles and op en circles energy sp ectra at the Gran

Sasso lo cation. 36

Figure 13: Event displayofamuonic decay in OPERA Monte Carlo simulation. 37 π+

π+

τ−

µ−

π−

Figure 14: Detail of the vertex region of the event shown in the previous Figure. 38 Electron identification efficiencies IDrate(%) IDrate(%)

Energy(Gev) Energy(Gev) IDrate(%) IDrate(%)

Energy(Gev) Energy(Gev)

Figure 15: The electron identi cation eciency is rep orted as a function of the electron

energy for di erent track lengths Monte Carlo simulation. 39 3 10 2

/eV 2 2 10 LSND

∆

-1 10

atm. ν OPERA -2 10 (Gran Sasso)

-3 10

-4 -3 -2 -1 10 10 10 10 1 2

sin 2Θµe

Figure 16: Sensitivity of OPERA in the search for oscillation. The 90 CL con-

tour limit is shown together with the LSND signal and with the region indicated by the

atmospheric neutrino anomaly. 40 3 2 10 /eV 2

m E531

∆ 2 10 CHORUS TOSCA

-1 10

atm. ν

-2 10 OPERA (Gran Sasso)

-3 10 -6 -5 -4 -3 -2 -1 10 10 10 10 10 10 2

sin 2Θµτ

Figure 17: Sensitivity of OPERA in the search for oscillation, as compared to other

exp eriments. 90 CL contour limits are shown, together with the region indicated by the

atmospheric neutrino anomaly. 41