In the NUSEX experiment, during 2.8 years of operation, 31 fully contained events have been collected; 3 among them are nucleon decay candidates, while the others have been attributed to upsilon interactions. Limits on nucleon lifetime and determinations of upsilon interaction rates are presented

Battistoni, G.

Bellotti, E.

Bologne, G.

Campana, P.

Castagnoli, C.

Chiarella, V.

Ciocio, A.

Cundy, D. C.

Dettorepiazzoli, B.

Fiorini, E.

English

NASA Technical Reports Server

271 HE6.2-6
Nucleon Decay andAtmospheric Neutrinos
in the ttont BlancExperiment
O.Bettistoni1,E.Bellotti2, O.Bolocj__3, P.Gempege1,C Cast 1i3,VChiee'elle1,A.Ciocio3.
D.C.Cund£4, B.e' Ettorr'_Piazzoli_E.Fiorini2, P.Oeieottlq_,E.l_occi'.C Liouor'i2, I).
Mennocdli"_,O.P.Murte_1,P.Neari2, O.Nicoletti!,P.Picchi3, I'1.d.Price4, A.Pullia2, 5.
Regezzi2, I'1.Rollier,2,O.5aevedra3,L.Sett81, L.Tresetti!, L.Zenotti2
1leboretoriNezionelidell'INFN,Fresceti,Italy
2DipertimentodiFisiceclell'Universite'nd8ezionedewINFN,Milan,Italy
31stitutodiCosmogeofisicede1CNRendIstitutodiFisiceGenerale,Turin,Italy
4CERN,EuropeanOrganizationf r'NuclearResee-ch,Geneva,Switzerland.
Abstract
In NU5EXexperiment, d_Jring2.8 years of operation, 31 fully contained
events have been collected; :3amongthem are nucleondecay candidates,
while the others havebeenattributed to _ interactions.Limits on nucleon
Ilfetlme anddeterminations of t) Interaction rates are presented.
The Detector.
NUSEXdetector Is a digital tracking calorimeter, plannedto study the
stability of nucleon.
It has beenrunnlng slnce June 1st, 1982 In a cave aside the Mont Blanc
Tunnel, .coveredby a mlnlmum overburdenof =-5000 hg cm-2 5. R.,wlth a
live time of 2.5 years, that Is about86_ of solar operation time.
The detector ls a sandwich of 136 Iron plates lcm x 3.5 x 3.5m2,
Interleaved by layers of plastic streamer tubes of the resistive cathode
type.
Eachplaneof tubesIsformedbyan arrayof320 tubes,3.5m longand I
cm2cross-sectlon,alignedwlththetunnelaxls.
Tubesaremade ofextrudedPVC;a highreslstlvltyvarnlshcoversthe
cathodewalls;anodesare_I00lJmwlres,andtubesareoperatedat3.9kV
wlthanAr-CO2-Penthane(I+2+I)gasmlxture.
Positionof eachhitinthedetectorIsrecordedthroughtwo setsofAI
strips(eachsethasa Icm pltch),mountedxternallytothetubes,which
are respectively parallel (320 strips) and orthogonal(288 strips) to the
wires.
Total mass is 150 tons; a detailed description of the detector will be
shortly publishecl(I)
https://ntrs.nasa.gov/search.jsp?R=19850027829 2020-03-20T17:10:33+00:00Z
272
i 6.2-6
Data are collected via CAMACserial readoutbya PDP 11/60 computeron
magnetic tape.
Minimum trigger patterns are: either 4 contiguousplanes hlt or 3,2
contiguous planes hit or 2+2,2 contiguous planes hit, plus any
combinationof higherplanemultiplicity.
Data set for each event consists of the coordinatesof each tube fired
together with time of firing relative to trigger start, recordedby 100 ns
TDC's.
Time informationallows identification of p decays(11stoppinginside the
detector) in 17%of the cases.
A trigger to detect magneticmonopoleshasalso beenimplementedand is
describedin (t)
Data concerning nucleon decay and atmospheric neutrinos.
An extimate of rate andtopologiesof o Interactions in our detector has
beenobtained throughan exposureof a test moduleto a _)p(_p)beam In
CERN,designed to slmulate atmospheric_) spectra in the energy reglon
around1 Gev(4).
Data thus collected havebeen used to computebackgroundsto nucleon
decaythat will be quotedfurther, consideringalso the presenceof _)e's In
atmospheric_)flux.
Data from NU$EXexperiment have been selected according to the
foollowingcharacteristics:
1
I-. Vertex of the event must be insidethe volumeof the detector;
2-. Tracks comingfrom vertex muststop Insidevolumeof the detector;
3-. Noincomingvisible track pointingto the vertex is required.
We guarantee thus to collect only events due to _) interactions in the
detector, andpossiblyclueto nucleondecay.
31 events satisfy the above quoted features; they are uniformly
distributed in the detector volume and cannot be ascribed to neutron
interactions (_.< 10"6cm-2 s-1 st-l); the first I0 amongthem havealready
beendiscussedin detail elsewhere(2.3)
Neutrino rates.
Trigger requirements put an effective threshold both for p ande at 250
MeV total energy.At this energy trigger efficiency is 40%andrapidly falls
below that value.
Supposingthat all 31 eventsare o Interactions, we can Identify 21 events
273 HE6_-6
as op interactions, 9 as _o Interactions, while 1 ts ambiguous( _p- _e).
Correcting for detection efficiency (trigger x containment efficiency)
event by event, we get the total v interaction rate in our detector, above
the thresholdof E_stble= 200 HeV:
Rv= 152 -*20 _/(Kton x yr)
andtheratio fgoand_ Interactions:
R(ool_)=0.28+ 0.0g+_0.02
wherethefirsterm IntheerrorIsduetostatisticsand thesecondtoue
_ ambiguity.A plotofvisiblenergydistributionofdatauncorrectedfor
detectionefficiencyisreportedinfig.I.
Fig. I. Visible energy
distributionof events,not
corrected for detection
® 5. I__ ._ efficiencies.
I Shadedareasare °e events.0
>
® White areas are _pevents.
Y ii::!::__:.il 1 _, event of 3.7 GeVis out of
_.:i:!:!:!
,_ "::i:i:i: scale.
":':"':"'":':':i i:_ i i:i:i:i:i: !::| •
1.0 2.0
Evisible (GeV)
Nucleon stability.,
• At present we have 3 events that can be considered nucleondecay
candidates. •
Eventn. 1, that has beenwidely discussedelsewhere(2), is a protondecay
candidate, fitting well the hypothesis:
p--> it++K'O
I
. --> TI+TI-
withE - 1.0 + 0.2 GeV,P - 0.4 *_0.2 GeV/c, pp- 0.38 _+0.15 GeV/c, Mm_=
. 0.55 +_0.08 GeV/c2, PK= 0.3 _*O.! GeV/c. Errors on total energy and
momentumare mainly clueto candidate p track going almost parallel to
iron plates.Other decaymodesalso fitted (2)are p --> o + K* andp --> 3p.
Backgroundcomputed from test run Is 2 events In 403 _ interactions
(correspondingto _-35yearsof datatakingwlthNUSEX),leadingtothe
expectedbackgroundrate:
B = O.I_+0.07
274
HE6.2-6
Second candidate is event n. 22, which is a _ 2 back to back showers
event; best fit is given by a 3 showers hypothesis with:
EI=370_+ llOMeV el2= 160o ; E2=550+_ 140MeV 813= 1400
E3= i40_+80MeV e23=50o ; Etot= 1060_+200 MeV; P =250MeV
consistentwitha P -->e+ + no decaymode.Expectedbackgroundfrom
analysisofpn eventsin_)testrunandelectronsof 1.5GeV frome beam
testsimulatingthetopologyofoureventgivesarate:
B = 0.25,_0.07
Eventn.30 isa candidateforn -->p+il-decay;kinematicalfitisvery
good,both fortotalenergy(.85GeV) and formissingmomentum (.35
GeVlc).Howeverinthischannelfromourtestwe expect0.6background
events.
Among otherchannelswe haveeventscompatiblewithnucleondecayonly
in p -->_ + II+;7 eventshavebeencollected,buttheexpectedbackground
fromquasi-elasticop is6.LimitreportedinTableIhasbeencomputedby
backgroundsubtraction.
Table I
Decoy mode N. of condidotos Background Lifetime/branching ratio
(yr x 1031) 90X C. L.
p --7 e+ er0 _ I 0.25 > 0.6
n --7 a+ w- 0 >0.8
N--> e 'n" > 1.0
p --7 p+ wo 0 > 0.7 "
n --> p+ zr- I 0.6 > 0.3
N-->u _ 70.8
p-->Uw . < 7 6 >0.2
n --> IBIr0 0 > I.I
p --7 '8 K+ 0 > 0.8
n -->I_ K0 0 > 0.9
p --> p+ K0 I O.I > 0.7
BeLe,.ce.o.¢.
I-.G.Battlstonletal."The NUSEXDetector"tobesubmittedtoN.I.M.
2-.G.Battistonietal.Phys.Left.I18B (1982)461
3-.G.Battistonletal.Phys.Lett.133B (i983)454
4-.G.Battistonietal.Nucl.Instr.& Methods219 (1984)300


Nucleon decay and atmospheric neutrinos in the Mont Blanc experiment

Nucleon decay and atmospheric neutrinos in the Mont Blanc experiment

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