MONOLITH: a next generation experiment for athospheric neutrinos

3 Pages·0.15 MB·English

by  P. Antonioli

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MONOLITH: a next generation experiment for atmospheric neutrinos
P. Antonioli a∗
aIstituto Nazionale di Fisica Nucleare, Sezione di Bologna,
Via Irnerio 46, 40126 Bologna, Italy
[email protected]
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0
0 MONOLITH is a massive magnetized tracking calorimeter, optimized for the detection of atmospheric muon
2 neutrinos, proposed at the Gran Sasso laboratory in Italy. The main goal is to establish (or reject) the neutrino
oscillation hypothesis through an explicit observation of the full first oscillation swing (the “L/E pattern”). Its
n
performance, status and prospects are briefly reviewed.
a
J

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1. INTRODUCTION neutrino sample, the measurement of the cosmic

1
muon spectrum at TeV energies and the use of
v Although the evidence for neutrino oscillation
the detector as a muon charge-ID detector for a
0
is growing year after year, the final proof that
4 beam coming from a ν-factory.
the observed anomalies are due to oscillation is
0
1 still outstanding. Alternative models (neutrino Mass 35 kt
0 decay, potential decoherence effect, influence of Magnetic Field 1.3 T
1 large extra-dimensions) to explain the Super- Dimensions (30.0×14.5)m2×13.1 m
0
Kamiokande (SK) atmospheric neutrinos results
Space resolution 1 cm
/
x have been proposed. On the experimental side, Time resolution 1 ns
e as pointed out also in recent Neutrino 2000 con- GSC counters 54000 m2
-
p clusions “there is as yet no direct evidence of an External Veto 1500 m2
e oscillation pattern” [1] and the oscillation para- p resolution ≈ 8% (FC), ≈ 20% (PC)
h µ
: meters in the atmospheric sector are still mea- Eh resolution 90%/pE (GeV)⊕30%
v sured with a poor precision.
Xi The MONOLITH experiment has been de- Table 1
signed exactly to reach these physics goals, that MONOLITH main parameters. FC: fully con-
r
a is prove ν oscillations through the observation of tained events, PC: partially contained. The Ex-
the oscillation pattern and improve the measure- ternal Veto consists of scintillator counters.
ments of parameters affecting the oscillations (if
oscillation is the pysical case). The experimen-
tal method that is proposed is designed to over-
come the limits of current atmospheric neutrino 2. APPARATUS ANDEXPERIMENTAL
experimentsthroughasystematicfreeandMonte APPROACH
Carlo independent measurements of the parame-
To explicitly detect an oscillation pattern in
ters. MONOLITH will also test the (now disfa-
the L/E spectrum of atmospheric muon neutri-
vored) hypothesis of mixing with sterile neutri-
nos,theenergyE anddirectionθ oftheincoming
nos searching for matter induced effects. Fur-
neutrino haveto be measuredineachevent. The
ther items of research, not discussed here, are
latter can be estimated from the direction θ of
the useofthe hugesampleofeventscomingfrom µ
the muon produced from the ν charged-current
the CNGS beam to complement the atmospheric µ
interaction. The neutrino energy E can be ob-
∗onbehalfoftheMONOLITHCollaboration tained by means of energy measurements of the
2
muon E and of the hadrons produced in the in- length,asoriginallyproposedin[3]. Thismethod
µ
teraction (E ). In order to make the oscillation is to a large extent insensitive to uncertainties
h
pattern detectable, the L/E ratio has to be mea- in the knowledge of atmospheric neutrino fluxes
suredwithaFWHMerrorsmallerthanhalfofthe and neutrino cross sections as well detector in-
modulation period. The resolution on L/E im- efficiencies. It is based on the up/down symme-
provesathighenergies,mostlybecausethemuon try of the neutrino fluxes, which holds at typical
direction gives a better estimate of the neutrino MONOLITH energies (E > 1.5 GeV). In this
th
direction. Therefore the ability to measure high framework, the atmospheric neutrinos represent
momentum muons (in the multi-GeV range) in an ideal case for a disappearance experiment.
partially contained events, which is rather lim-
ited in the on-going atmospheric neutrino exper- 3. DETECTION OF THE L/E PATTERN
iments, is particularly rewarding.
A complete description of the detector struc- The event selection has been tuned to obtain
ture can be found in [2]. The apparatus consists theL/Eresolutionrequiredtodetectthepattern.
of two modules, a stack of 125 horizontal 8 cm Muonsarereconstructedorviaenergylossinthe
thick ironplateseachwithasurfaceof15.0×14.5 iron or via magnetic analysis. Minimal require-
m2, interleaved with 2.2 cm gaps housing the ments have been applied to the reconstructed
sensitive elements. The main detector parame- muonmomentum(pµ >1.5GeV)and,foroutgo-
ters and resolutions are summarized in Table 1. ing muons, to the track-length l (l > 4 m). Ad-
Taking into account needs of such a large area ditional cuts (for details see [2]) on combinations
detector and fast mass production, Glass Spark of the observables Eµ, Eh and θµ are applied to
Counters [2], derived from resistive-plate cham- ensure the required L/E resolution and on visi-
bers, have been chosen as active elements. The ble vertex coordinates, muon direction, external
magnetization of iron plates is obtained through vetoinformationto rejectthe cosmicmuonback-
a vertical slot crossing all the stack: each field ground. With these cuts, we expect in 4 years
line lays entirely inside an individual plane. of data taking (see Fig. 1) to select for phy-
To avoid systematic effects and Monte Carlo sicalanalysis ≈ 1200events (80% fully contained
dependencies, we use the ratio between the mea- and 20% partially contained) of down-going νµ
sured down-going neutrino flux as a near un- (in case of no oscillation this number is the same
oscillated reference flux and the upward neutri- for up-going).
nos flux as a far oscillated one. Thus we asso-
ciateforeachdownwardneutrinoitsmirror path-
Figure 2. The L/E spectrum of upward muon
Figure 1. Selected events (left) and overallselec- neutrinoevents(hatchedarea)andtheL/E“mir-
tion efficiency and selectionefficiency for FC and rored” spectrum of downward muon neutrino
PCcontainedeventsasafunctionofL/E (right). events (open area) (left); their ratio with our
The ν survivalprobability using present best-fit best-fit superimposed (center); results of the fit
µ
value of SK is superimposed. (68%, 90% and 99% C.L. shown) (right).
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The gain in acceptance at small values of L/E the measurement of the oscillation parameters.
due to the magnetic field analysis is noteworthy: These qualitativeandquantitativeimprovements
as shown in Fig. 1 the inclusion of partially con- qualify MONOLITH as a next generation neu-
tainedeventsincreasestheMONOLITHselection trino oscillation detector.
efficiencybymorethanafactortwointhe region Wefinallynotethatthebasicparametersofde-
wherethefirstsemi-periodoftheoscillationisex- tectorsthatareproposedforneutrinobeamsfrom
pected according to the SK results. The cost of muon storage rings look very similar to those of
the magnetization, when compared to the whole MONOLITH(so that the expectedperformances
cost,is around8%, but, in practice, has an effect should roughly apply to MONOLITH). The use-
similar to that of doubling the total mass. ful life of the detector might be extended accord-
In Fig. 2 we show, for a particular choice of ingly.
oscillation parameters, how MONOLITH detects Acknowledgements. I wish to address my
theoscillationpatternusingtheratiobetweenup- warmestthankstoStefanoRagazziandFrancesco
going and down-going events. Note that around Terranova who provided me useful material to
∆m2 ≈ 10−3eV2 MONOLITH can measure the prepare this talk and Rosario Nania for helpful
∆m2 value with 6% precision. discussions.
An accurate analysis [2] of the capability of REFERENCES
MONOLITHtoconfirmtheoscillationhypothesis
withrespecttootheralternativemodelshasbeen 1. J.Ellis,“SummaryofNeutrino2000”,CERN-
performed. Around current SK values, MONO- TH-2000-265,hep-ph/0008334,Aug. 2000.
LITH is able to reject the decay hypothesis at 2. MONOLITH Proposal, LNGS P26/2000,
95% C.L. or better with 99.5% efficiency. In Fig. CERN/SPSC 2000-031, SPSC/M657, Aug.
3the expectedallowedregionsofthe ν →ν os- 2000.
µ τ
cillation parameters after 4 years of data taking 3. P. Picchi and F. Pietropaolo, “Atmospheric
with MONOLITH are shown for four different Neutrino Oscillations Experiments”, CERN
values of ∆m2. It is worthwhile to note that the preprint SCAN–9710037,Sep. 1997.
progress in the knowledge of the ∆m2 expected 4. M. Aglietta et al., LNGS-LOI 15/98,
by MONOLITH with respect to SK, is quantita- CERN/SPSC 98-28,SPSC/M615,Oct. 1998.
tivelysimilarto the one done bySK withrespect 5. MONOLITH Progress Report, LNGS-LOI
to Kamiokande. 20/99,CERN/SPSC 99-24, Aug. 1999.
4. STATUS AND PROSPECTS
TheMONOLITHCollaborationcountsnow85
physicists and 17 institutions. After preliminary
detector studies [4,5], a Proposal [2] has been
recently submitted to the LNGSC. Cosmic ray
and beam tests have been carried out during the
last two years at LNGS and CERN/PS to study
up/downdiscriminationandhadronicenergyres-
olution. Assuming approval in early 2001, the
first module is foreseen to be operational by the
end of 2004 and the detector to be completed by Figure 3. Left: expected allowed regions of
the end of 2006. νµ − ντ oscillation parameters for MONOLITH
The superior L/E resolution of the detector after four years of exposure: the results of the
willallow detectionof the firstoscillationperiod. simulationfor∆m2 =0.7,2,5,30×10−3eV2 and
MONOLITHwillthusprove(ordisprove)theos- maximalmixingareshownRight: exclusioncurve
cillation hypothesis. It will also highly improve assuming no oscillation.