Emulsion chamber with big radiation length for detecting neutrino oscillations

*Corresponding author. Tel.: 095-237-0079.E-mail address: [email protected] (A.E. Asratyan).1Now at Kansas State University, Manhattan, USA.

Nuclear Instruments and Methods in Physics Research A 450 (2000) 1}11

Emulsion chamber with big radiation length for detectingneutrino oscillations

A.E. Asratyan*, G.V. Davidenko, A.G. Dolgolenko, V.S. Kaftanov,M.A. Kubantsev1, V.S. Verebryusov

Institute of Theoretical and Experimental Physics, Moscow 117259, Russia

Received 25 October 1999; accepted 4 January 2000

Abstract

A conceptual scheme of a hybrid-emulsion spectrometer for investigating various channels of neutrino oscillations isproposed. The design emphasizes detection of s leptons by detached vertices, reliable identi"cation of electrons, and goodspectrometry for all charged particles and photons. A distributed target is formed by layers of low-Z material,emulsion}plastic}emulsion sheets, and air gaps in which s decays are detected. The tracks of charged secondaries,including electrons, are momentum-analyzed by curvature in magnetic "eld using hits in successive thin layers ofemulsion. The s leptons are e$ciently detected in all major decay channels, including s~Pe~mm6 . The performance ofa model spectrometer, that contains 3 t of nuclear emulsion and 20 t of passive material, is estimated for di!erentexperimental environments. When irradiated by the m

lbeam of a proton accelerator over a medium baseline of

S¸/EmT&1 km/GeV, the spectrometer will e$ciently detect either the mlPm

sor the m

lPm

%transitions in the

mass-di!erence region of *m2&1 eV2, as suggested by the results of LSND. When exposed to the neutrino beam ofa muon storage ring over a long baseline of S¸/EmT&10}20 km/GeV, the model detector will e$ciently probe the entirepattern of neutrino oscillations in the region *m2&10~2}10~3 eV2, as suggested by the data on atmospheric neu-trinos. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 14.60.Pq; 14.60.Fg

Keywords: Neutrino oscillations; s leptons; Nuclear emulsion

1. Introduction

Oscillatory transitions among neutrinos of di!er-ent #avors have emerged as a major topic of par-ticle physics [1,2]. An accelerator experiment using

muon antineutrinos from l` decays at rest over ane!ective baseline of ¸/E&1 m/MeV, LSND at LosAlamos, has reported a positive signal in the chan-nel m6

lPm6

%with a probability of &3]10~3 [3].

A consistent, albeit less signi"cant, signal was alsoobserved in the CP-conjugate channel m

lPm

%us-

ing muon neutrinos from p` decay in #ight [4].When combined with the upper limits imposed byother accelerator and reactor experiments [5,6], theLSND data suggest that the m6

lPm6

%oscillation is

0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 2 4 7 - 3

driven by a mass di!erence squared, *m2, betweensome 0.3 and 2.3 eV2. The transitions m6

lPm6

%and

mlPm

%in this region of *m2 will soon be explored

with higher sensitivity by the BooNE experiment atFermilab [7].

Qualitatively, an analogy with the quark sectorsuggests that (i) the m

sshould pick the largest con-

tribution of the heaviest mass state m3, and that (ii)

the mixings between the neighboring neutrino #a-vors (i.e., m

%%m

land m

l%m

s) should be the stron-

gest. Therefore, a mass di!erence as large as*m2&1 eV2 should primarily manifest itself in them

lPm

schannel. The existing upper limits on the

e!ective mixing sin2 2h for the transition mlPm

s[8}11] largely come from experiments with smalle!ective baselines of S¸/EmT;1 km/GeV, andtherefore are much less compelling for*m2&1 eV2 than for larger values of the massdi!erence.

At neutrino energies well below the s threshold(Em(2 GeV), the transition m

lPm

sin the required

region S¸/EmT&1 km/GeV will be indirectlyprobed by BooNE through m

ldisappearance [7].

However, a convincing mlPm

ssignal can only be

demonstrated by detecting the CC collisions of them

sin appearance mode. For this, an appropriate

environment is o!ered by the `medium-baselinealocation on Mount Jura [12] in the m

lbeam of

CERN-SPS (¸"17 km and SEmT"27 GeV by#ux). The CC collisions of the m

smay be identi"ed

either topologically [12,13] or by detecting thedetached vertex of the produced s in a hybrid-emulsion spectrometer [9]. The latter option isconsidered in this paper. For the òriginalatransition m

lPm

%to be observed and investigated

in the same experiment, that will have very di!erentsystematics compared to either LSND [3] andBooNE [7], secondary electrons must be e$cientlydetected and momentum-analyzed.

The atmospheric evidence for neutrino oscilla-tions driven by a mass di!erence of 10~2}10~3 eV2

[14}19] will be initially probed by the long-base-line experiments operating in the neutrino beams ofproton accelerators [12,20,21]. Large angular di-vergences of these beams dictate that, for the sakeof statistics, "ne instrumentation of the detectorand/or the magnetic analysis be compromised fora multi-kiloton "ducial mass. Therefore, unraveling

the pattern of oscillations in this mass region willprobably require a much more intense beam gener-ated by a muon storage ring with a straight sectionthat points towards the detector [22]. Assuminga l~ ring, the beam fully consists of muon neu-trinos and electron antineutrinos whose energyspectra and angular spreads are precisely known.That the original beam contains neither m6

lnor

m%allows to identify the parent neutrino by the sign

of the lepton produced by the oscillated neutrino:negative and positive leptons are unambiguous sig-natures of the m

land m6

%parents, respectively.

Therefore, the transitions mlPm

%, m

lPm

s, m6

%Pm6

land m6

%Pm6

scan be tagged independently.

The unique properties of this neutrino beam maywarrant a "nely instrumented detector of relativelysmall mass, that would be sensitive to a broadrange of neutrino transitions driven by a massdi!erence of 10~2}10~3 eV2. For this, several con-ditions are obligatory. Selecting the m

sand m6

scolli-

sions by the detached vertex of the s is only possiblein a hybrid-emulsion apparatus. In order to sup-press the background to s decays arising from an-ticharm production in CC collisions of the m6

%, the

detector should identify the electrons as reliably asthe muons. For the transitions m

lPm

sand m6

%Pm6

sto be reliably discriminated, all charged secondar-ies (and not just muons) should be sign-selected andmomentum-analyzed in the detector. The latter willalso allow extra kinematic handles for further sup-pressing the background to s decays (such as p

Tof

a decay particle with respect to s direction andtransverse disbalance of the event as a whole withrespect to incident neutrino).

In this paper, we propose a conceptual scheme ofa hybrid-emulsion spectrometer for detecting andidentifying the neutrinos of di!erent #avors by theirCC collisions, as required for probing variouschannels of neutrino oscillations. The design em-phasizes detection of s leptons by detached vertices,reliable identi"cation and sign selection of elec-trons, good spectrometry for all charged secondar-ies, and reconstruction of secondary photons andp0 mesons. We also estimate the performance ofthe proposed apparatus in a medium-baseline ex-periment using the neutrino beam of the protonmachine CERN-SPS and in a long-baseline experi-ment in the neutrino beam of a muon storage ring.

2 A.E. Asratyan et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 1}11

Fig. 1. Schematic of the assumed "ne structure of the target,showing the 1-mm-thick carbon}copper plates (960#40 lm),emulsion}plastic}emulsion sheets (50#100#50 lm), and driftspace in which s decays are selected (4800 lm).

2. The detector

Building on the ideas put forth by Ereditato et al.[23], we implement the principle of èmulsioncloud chambera: layers of thin emulsion are onlyused as a tracker for events occurring in passivematerial. But in contrast to Ref. [23], we aim atconstructing a distributed target with low densityand large radiation length [13], so that eithermuons, hadrons, and electrons can be momentum-analyzed by curvature inside the target itself. Ac-cordingly, the target should largely consist of low-Zmaterial like aluminum or carbon in the form ofcarbon-"ber composite. Apart from narrow gapsinstrumented with drift chambers that provide anelectronic `blueprinta of the event, the target isbuilt as a compact homogeneous volume in ambi-ent magnetic "eld. Owing to relatively weak mul-tiple scattering in low-Z material, the successivelayers of thin emulsion may act as an èmulsionspectrometera for analyzing the momenta ofcharged secondaries by curvature in magnetic "eld.In untangling the topologies of neutrino events, thedetector will operate very much like a bubblechamber. The proposed apparatus is therefore re-ferred to as the Emulsion Bubble Chamber (EBC).

Speci"ed below is a tentative structure of thedistributed target that has been assumed in oursimulations of detector response. (The actual de-sign, including the choice of low-Z material, mustof course be carefully optimized for a particularexperiment.) The "ne structure of the target is de-picted in Fig. 1. The 6-mm-thick basic element ofthe structure is formed by a 1-mm plate of passivematerial, by an emulsion}plastic}emulsion sheet(referred to as the ES) with a total thickness of 0.2mm, and by a relatively large drift space of 4.8 mmin which s decays are selected. In our tentativedesign, the passive plate is largely carbon (960 lm),but also includes a thin layer of denser substance(40 lm of copper) in order to boost the geometricacceptance. The ES is a 100-lm sheet of transpar-ent plastic coated on both sides with 50-lm layersof emulsion. The medium formed by successiveelements has a mean density of o"0.49 g/cm3 andan e!ective radiation length of X

0"52.6 cm.

(These values of o and X0

are very similar to thoseof a bubble chamber with neon}hydrogen "lling

[24,25].) Note that the element does not feature thesecond ES downstream of the gap, as originallyforeseen in Ref. [23]: the idea is to detect the kinkor the trident using track segments in the ESs oftwo successive elements, as allowed by relativelyweak Coulomb scattering in low-Z material of theintervening passive plate.

On the technical side, the air gap can be createdby thin and rigid `bristlesa on the upstream face ofthe carbon-composite plate that has been manufac-tured in `brush-likea form. (This is only possiblebecause we have just one ES per element.) Thepositions of the thin bristles will be tabulated, andsecondary vertices that match these positions willbe dropped. Another technical option for the driftspace is 5-mm-thick paper honeycomb.

For a s that emerged from the passive plate andsu!ered a one-prong or a three-prong decay in thegap, the decay vertex can be reconstructed from thetrack segments in two successive ESs. As soon asthe "tted secondary vertex lies within the inter-vening passive plate, the candidate event mustbe dropped because of the high background fromreinteractions. In principle, a s that decayed beforereaching the gap can be detected by impact param-eter, but this possibility is not considered here. By

A.E. Asratyan et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 1}11 3

Fig. 2. The depth of the primary vertex in the carbon}copperplate for all s events (a) and for those events in which the s hasdecayed in the drift space (b). Here and in subsequent "gures, theenergy spectrum of incident s neutrinos is assumed to be propor-tional to the spectrum of muon neutrinos from the CERN-SPSaccelerator.

adding a thin layer of denser material (copper)downstream of low-Z material (carbon), we slightlycompromise the radiation length for geometric ac-ceptance: thereby, the fraction of s leptons thatreach the air gap is e!ectively increased. That theproportion of copper events is boosted by the geo-metric e!ect is illustrated in Fig. 2. Here and below,the spectrum of incident s neutrinos is assumed tobe proportional to the spectrum of muon neutrinosfrom CERN-SPS [13].

For the tracks to be found in emulsion at thescanning stage, planes of electronic detectors mustbe inserted into the continuous structure formed bythe basic elements. These detectors should allowa crude on-line reconstruction of the event asa whole, and therefore they must have su$cientspatial resolution, provide angular information,and be spaced by less than one radiation length X

0.

In our tentative design, a stack of 30 elements formsa `modulea with a total thickness of 0.34X

0, and

4-cm-wide gaps between adjacent modules are in-strumented by multisampling drift chambers.

In a medium- or long-baseline experiment wherethe occupancy of ESs will be relatively low, theaccuracy of èlectronica reconstruction should besu$cient for unambiguously "nding an energetictrack in emulsion. Therefore, one can scan backalong a sti! track, starting from the downstream ESof the module in which the collision occurred. Assoon as the layer of origin is reached, a few success-ive ESs of the nearby elements must be fully scan-ned over relatively small areas towards "nding thestubs of all other tracks associated with the primaryvertex. To re"ne the alignment of ESs, a su$cientnumber of sti! muon tracks must be fully recon-structed in emulsion. This "rst stage of emulsionscanning will yield a relatively small number ofevents featuring decay signatures (either a kink ora trident in the air gap just downstream of theprimary vertex). The second stage will be to scandown all tracks emerging from the primary vertexand to "nd and measure the conversions of second-ary photons in the distributed target. This willallow to analyze the momenta of the secondaries inthe èmulsion spectrometera and to identify elec-trons by change of curvature and by emission ofbrems.

We foresee that the scanning of emulsion willonly start after the full period of detector exposure.

3. Spectrometry for charged particles and photons

The response of the EBC detector is simulatedusing GEANT. In "tting a track, we assume thateach tracker traversed (either an ES or a driftchamber) provides two spatial points with resolu-tions of 2 and 150 lm, respectively. Because ofmultiple scattering, the "t is but marginally sensi-tive to increasing the spatial error in the ES from2 up to 8 lm. A uniform magnetic "eld of 0.7 T,that is normal with respect to beam direction, isassumed throughout. (Using a stronger "eld wouldboost the performance of the spectrometer, butmay be impracticable for a big air-core magnet thatwill house a multiton EBC.)

When found in emulsion, the track of an electroncan be identi"ed by a variation of curvature due toenergy losses in successive layers of the target.These losses must be accounted for in estimating


Fig. 3. The length along the beam direction of the track segmentused for analyzing the e~ momentum by curvature (see text).

Fig. 4. The ratio between the "tted and true momenta of theelectron from s~Pe~mm6 , R"p.%!4

%/p536%

%, for p

%'1 GeV (a), for

1(p%(5 GeV (b), and for p

%'5 GeV (c).

the momentum by curvature in magnetic "eld. Forthis, we use a simple algorithm that should betreated as preliminary and is not fully realistic.Namely, we "t a restricted segment of the e~ trackover which the actual loss of energy does not exceed30%.

Additionally, this segment is required to cross noless than "ve ESs. We compute an ìdeal traject-orya for a given value of electron momentum p

%, to

which the observed trajectory is then "tted, usingGEANT. In doing so, we switch o! multiple scat-tering and radiation losses, but instead reduce themomentum `by handa in each layer of the target bythe same amount *p that is treated as an empiricalparameter. The value of *p is selected so as toobtain (i) an unbiased estimate of electron mo-mentum (Sp.%!4

%/p536%

%T&1) and (ii) a reasonable

value of s2.For de"niteness, we use a Monte-Carlo template

of electrons with p%'1 GeV originating from

simulated decays q~Pe~mm6 (see further). For theseelectrons, the mean length of the selected tracksegment is close to 30 cm (see Fig. 3), which proves

to be su$cient for analyzing the momentum in theèmulsion spectrometera formed by successive ESsof the distributed target. Plotted in Fig. 4 is theratio between the "tted and true momenta,p.%!4%

/p536%%

, for the properly "tted electrons (s2(3).The same ratio is then separately shown for tworegions of electron momentum: 1(p

%(5 GeV

and p%'5 GeV. We estimate that of all the elec-

trons with p%'1 GeV, nearly 90% can be reliably

detected, identi"ed, and sign-selected (that is, re-constructed with s2(3 and dp

%/p

%(0.40). On

average, the e~ momentum is measured to a pre-cision of some 11%.

Treating the length of the track segment as a freeparameter of the "t, which is a more realistic ap-proach that is not attempted in this paper, willfurther improve the momentum resolution andboost the fraction of sign-selected electrons. A goodspectrometry for electrons will allow EBC to detectand discriminate the CC collisions of electron neu-trinos and antineutrinos and, at the same time, toselect electronic decays of the s almost as e$cientlyas the muonic ones. A comparison between the two


Fig. 5. The ratio between the "tted and true momenta, R"p.%!4p

/p536%p

, for the p~ with pp'1 GeV originating from the decay sPp~m.

leptonic modes will provide an important handleon self-consistency of any s signal.

The spectrometry of muons and charged pions ismuch less a!ected by energy loss in matter, but forpions the momentum resolution is slightly de-graded by hadronic reinteractions. For the pionswith p

p'1 GeV originating from the simulated

decays s~Pp~m, Fig. 5 shows the ratio betweenthe "tted and true momenta, p.%!4

p/p536%

p. The mean

uncertainty on pion momentum is seen to be closeto 7%.

Neutral pions can be reconstructed from photonconversions in the distributed target. (Here, we as-sume that the potential length in the detector ismuch larger than X

0"52.6 cm.) In estimating the

energy of a photon that has converted in the target,we count only those conversion electrons that have"red at least one drift chamber and, therefore, canbe found in emulsion and then momentum-ana-lyzed by curvature. We also assume that the pri-

mary vertex has already been "tted, so that thedirection of the photon is precisely known. Forillustration, we consider the p0 mesons originatingfrom the decay s~Pp~p0m. The measured invari-ant mass of the two detected photons from p0Pcc,as plotted in Fig. 6, shows a distinct p0 signal. Theactual size of the mass window for selecting thep0 candidates will be dictated by the level of combi-natorial background in a particular analysis; forpurely illustrative purposes, we assume a mass win-dow of 115(m

cc(155 MeV. Thus estimated de-

tection e$ciency for p0 mesons is close to 0.26.

4. Detecting the leptonic and semileptonicdecays of s

For the s leptons emitted in either the deep-inelastic and quasielastic m

sN collisions, we gener-

ate the two leptonic decays, s~Pl~mm6 and


Fig. 6. For the decay sPp~p0m followed by p0Pcc, the measured invariant mass of the two photons that have been detected byconversions in the distributed target.

s~Pe~mm6 , and three semileptonic (quasi-)two-body decays: s~Pp~m, s~Pp~p0m that ismediated by the resonance o~Pp~p0, ands~Pp~p`p~m that is mediated by the resonancea~1Pp~p`p~. The threshold e!ect for s produc-

tion in neutrino}nucleon collisions and polariza-tion of the s, that a!ects the angular distribution ofdecay products in the s frame, are accounted for.Further details on our s generator can be found inRef. [26].

For de"niteness, we again assume that s neu-trinos incident on EBC have the same energy spec-trum as muon neutrinos from CERN-SPS [13](SEmT"27 GeV by #ux), and that the distributedtarget is magnetized by a uniform "eld of 0.7 T that

is perpendicular to the beam direction. The decayproducts of the s are propagated through the targetand then reconstructed from hits in emulsion, asexplained in the previous section. In estimating thedetection e$ciencies for di!erent decay channels ofs, we adopt the (quasi-realistic) selection criteriathat are listed below.

f The s must have decayed in the drift gap. This isnecessary for reconstructing the detached vertexfrom track segments in the upstream and down-stream ESs.

f The momentum of either charged daughter mustexceed 1 GeV, and its emission angle must liewithin 400 mrad of the beam direction. These


Table 1The detection e$ciencies, or acceptances, for those decay chan-nels of s that have been simulated in the detector

Decay channel Branching Acceptance Acceptancefraction times

branching

s~Pl~mm6 0.174 0.40 0.070sPe~mm6 0.178 0.36 0.064s~Pp~m 0.113 0.40 0.045s~Pp~p0m 0.252 0.30 0.076s~Pp~p`p~m 0.094 0.27 0.025

selections are suggested by the fact that soft andbroad-angle tracks are poorly reconstructed inemulsion.

f For a one-prong decay to a charged daughterd (either a muon, electron, or pion), the kinkangle must be su$ciently large: h

s$'20 mrad.

This lower cut re#ects the experimental uncer-tainty on the kink angle that is close to 5 mrad.

f All charged daughters of the s must be mo-mentum-analyzed by curvature and reliablysign-selected (that is, reconstructed in the de-tector with s2(3 and dp/p(0.40). This will "xthe sign of the s and, on the other hand, willallow kinematic handles for rejecting the decaysof charmed and strange particles.

f In a one-prong decay, pT

of the charged daughterwith respect to the s direction must exceed 250MeV. This is aimed at rejecting the decays ofstrange particles.

For those s decay channels that have actuallybeen simulated in the detector, the estimated detec-tion e$ciencies (or acceptances) are listed in Table 1.(Note that here we do not require the p0 froms~Pp~p0m to be reconstructed.) For these decaychannels of s, the acceptance-weighted branchingfractions add up to some 0.28. Approximately ac-counting for the other one- and three-prong chan-nels, we estimate that nearly 32% of all s leptonsproduced in passive material will be detected inEBC by visible kinks or tridents.

The (quasi-)two-body decays s~Pp~m,s~Po~m, and s~Pa~

1m may provide an extra

kinematic handle for discriminating the s~ against

the background of anticharm. In a decay s~Ph~m,`transverse massa is de"ned as M

T"

Jm2h#p2

T#p

T, where m

hand p

Tare the h~ mass

and transverse momentum with respect to q direc-tion. The two-body kinematics dictate that the un-smeared M

Tdistribution should reveal a very

distinctive peak just below M.!9T

"ms, see the up-

per plots of Fig. 7. The MT

technique for identifyingmassive parents by two-body decays in emulsionwas proved by discriminating the relatively raredecay D`

s(1968)Pl`m against a heavy back-

ground from other decays of charm [27]. Observ-ing the high-M

Tpeak in a detector requires good

spectrometry of charged pions and, for the decays~Pp~p0m in particular, good reconstruction ofp0 mesons. The bottom plots of Fig. 7 show thesmeared M

Tdistributions for the reconstructed

decays s~Pp~m, s~Pp~p0m, and s~Pp~p`p~m(the p0 for s~Pp~p0m has been selected in themass window 115(m

cc(155 MeV, see Fig. 6).

The Jacobian cusps of the original MT

distributionslargely survive the apparatus smearings of EBCand, therefore, will provide distinctive signaturesof the s.

5. Sensitivity to neutrino oscillations

Neutrino oscillations driven by *m2&1 eV2 [3]can be e$ciently probed at a location on MountJura near CERN, that is irradiated by the existingwide-band m

lbeam of the CERN-SPS accelerator

(SEmT"27 GeV by #ux) over a `mediuma baselineof 17 km [12]. At this location, an EBC detectorcan be deployed in the existing magnet of the NO-MAD detector [13] that delivers a magnetic "eld ofup to 0.7 T. In the large internal volume of thismagnet, 3.5]3.5]7.5 m3, one might deploy a dis-tributed target with a volume of 3.0]3.0]6.6 m3,that will consist of 30 modules (see Section 2). Thetarget has a total thickness of 10 radiation lengths,and contains nearly 20 t of passive material and3 t of standard emulsion (or a lesser amount ofdiluted emulsion, that may be warranted by a rela-tively low occupancy at a medium-baseline loca-tion). The magnetic volume will also house a fewdrift chambers downstream of the target. Asphoton conversions and electrons will be e$ciently


Fig. 7. Transverse mass MT"Jm2

h#p2

T#p

Tfor the (quasi-)two-body decays s~Ph~m with h~"p~ (left-hand column),

h~"a~1Pp~p`p~ (middle column), and h~"q~Pp~p0 (right-hand column). The unsmeared M

Tdistributions for all events in each

channel prior to any selections are shown in the top row. The smeared distributions for detected events are shown in the bottom row.

detected in the distributed target, no extra electro-magnetic calorimeter is foreseen. The design of themuon system is not discussed in this paper.

At the Jura location, the rate of ml-induced CC

collisions has been estimated [12] as 843 events perton of target per 1019 protons delivered by CERN-SPS. Assuming 1020 delivered protons which cor-responds to 3}4 years of operation, the proposeddetector will collect nearly 1.7]105 CC events.Even if the probability of the m

lPm

stransition is as

small as 0.3% [3], we will detect some 80 s eventswith a negligibly small background from the decaysof strange and anticharm particles. (This predictionassumes a s detection e$ciency of 32%, as esti-mated above for the same beam, and takes into

account the threshold e!ect in s production.) Alter-natively, a zero signal will allow to exclude (at 90%CL) an area of the parameter plane (sin2 2h

ls, *m2

ls)

that is depicted in Fig. 8.As demonstrated above, EBC will very e$ciently

identify, sign-select, and momentum-analyzeprompt electrons from CC collisions of electronneutrinos and antineutrinos. The energy of incidentm%

(m6%) will be estimated to better than 10%. If at

the Jura site the transition mlPm

%indeed occurs

with a probability of some 0.003 as in LSND [3], inan exposure of 1020 protons on target we will detectand reconstruct some 460 m

%NPe~X events due to

oscillated neutrinos against a background of nearly860 CC events due to the original m

%component of


Fig. 8. Null-limit sensitivity to the mlPm

stransition (at 90%

C.L.) of a 20-t EBC detector deployed at 17 km from CERN-SPS and at 732 km from a muon storage ring with E

l"50 and

100 GeV, assuming 1020 protons delivered by CERN-SPS and2.2]1021 negative muons injected in a ring with a straightsection of 25%. The shaded area on the right is the region ofparameter space for m

lPm

ssuggested by a combined analysis of

Kamiokande and Superkamiokande data [29]. Also illustratedare the best upper limits on sin2 2h

lq for *m2lq(10 eV2, as

imposed by E531 [9] and CDHS [8].

the beam [12]. As the original m%

component of thebeam is substantially harder than the m

lcompon-

ent, a mlPm

%signal will e!ectively reduce the mean

energy of m%NPe~X events.

A very intense and collimated beam of a muonstorage ring [22] will allow long-baseline experi-ments with relatively small but "nely instrumenteddetectors. For purely illustrative purposes, we as-sume an EBC detector as sketched above (20 t ofpassive material plus 3 t of standard emulsion) thatis irradiated by a l~ ring with parameters as fore-seen in Ref. [22] (that is, 7.5]1020 injected muonsper year of operation and a straight section thatamounts to 25% of the ring circumference). Wealso assume a `nominala distance of 732 km (eitherfrom CERN to Gran Sasso or from Fermilab toSoudan) that results in e!ective baselines ofS¸/EmT&20 and 10 km/GeV for the stored-muonenergies of E

l"50 and 100 GeV, respectively.

Then, given unpolarized muons with El"50 (100)

GeV in the ring, in the absence of oscillations thedetector will annually collect some 5.5]103(4.4]104) and 2.4]103 (1.9]104) CC collisions ofmuon neutrinos and electron antineutrinos, respec-tively. Note that the expected event rate forE

l"100 GeV is as high as in the CERN-SPS

beam at Jura location.Taken together, the data on atmospheric

[14}19] and reactor [28] neutrinos favor a mlPm

stransition with almost maximal mixing and witha mass di!erence in the range *m2

ls&

10~2}10~3 eV2 [29}31]. For de"niteness assumingsin2 2h

ls"1 and *m2

ls"5]10~3 eV2, we estimate

that in three years of operation a 20 t EBC willdetect a signal of some 50 (170) s~ leptons forE

l"50 (100) GeV with a negligible background of

m6%-produced anticharm. (Note that the E

l-depend-

ence of the s signal largely re#ects the increase ofp(m

sNPs~X) with neutrino energy: in the region

*m2(eV2);SEmT/¸ (GeV/km), the #ux of oscil-lated neutrinos is virtually independent of E

l.) Al-

ternatively, a zero s~ signal will e!ectively excludea large area of parameter space for the transitionm

lPm

s; see Fig. 8. The transition m6

%Pm6

swill be

simultaneously detected with a comparable sensi-tivity, and the two transitions will be reliably dis-criminated by the charge of s. Using the detached-vertex information, the transitions m

lPm

%and

m6%Pm6

lwill be separated from the transitions

mlPm

sand m6

%Pm6

sinvolving the leptonic decays

s~Pe~mm6 and s`Pl`mm6 , respectively. We mayconclude that, in this experimental environment,even a relatively small EBC detector will e$cientlyprobe the entire pattern of neutrino oscillations inthe region *m2&10~2}10~3 eV2.

6. Summary

A conceptual detector scheme is proposed forstudying the various channels of neutrino oscilla-tions. The hybrid-emulsion spectrometer will detectand discriminate the neutrinos of di!erent #avorsby their CC collisions. The design emphasizes de-tection of s leptons by detached vertices, identi-"cation and sign-selection of electrons, andspectrometry for all charged particles and photons.


A distributed target is formed by layers of low-Zmaterial, emulsion}plastic}emulsion sheets, and airgaps in which s decays are detected. Target mod-ules with a mean density of 0.49 g/cm3 and a radi-ation length of 52.6 cm, that are similar to those ofa bubble chamber with neon-hydrogen "lling, arealternated by multisampling drift chambers thatprovide electronic tracking in real time. The tracksof charged secondaries, including electrons, are mo-mentum-analyzed by curvature in magnetic "eldusing hits in successive thin layers of emulsion andin drift chambers. Electrons are identi"ed bychange of curvature and by emission of brems.Photons are detected and analyzed by conversionsin the distributed target, and neutral pions arereconstructed. The s leptons are e$ciently detectedand sign-selected in all major decay channels, in-cluding s~Pe~mm6 .

At a medium-baseline location on mount Jura inthe existing neutrino beam of the proton machineCERN-SPS, the detector will be sensitive to eitherthe m

lPm

sand m

lPm

%transitions in the mass-

di!erence region of *m2&1 eV2, as suggested bythe results of LSND. At a long-baseline location inthe neutrino beam of a muon storage ring, evena relatively small spectrometer of the proposed typewill e$ciently probe the entire pattern of neutrinooscillations in the region *m2&10~2}10~3 eV2

that is suggested by the data on atmosphericneutrinos.

Acknowledgements

This work was supported in part by the CRDFfoundation (Grant RP2-127) and by the RussianFoundation for Fundamental Research (Grant 98-02-17108).

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Emulsion chamber with big radiation length for detecting neutrino oscillations