Tau-sterile neutrino mixing in NOMAD

HAL Id: in2p3-00010929

http://hal.in2p3.fr/in2p3-00010929

Submitted on 12 Mar 2002

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Tau-sterile neutrino mixing in NOMAD

P. Nedelec

To cite this version:

P. Nedelec. Tau-sterile neutrino mixing in NOMAD. International workshop on tau lepton physics 6,

Sep 2000, Victoria, Canada. pp.37-42. �in2p3-00010929�

Tau-sterile neutrino mixing in NOMAD

P. NEDELEC

LAPP, IN2P3-CNRS, Cheminde Bellevue,BP110,

F-74941,Annecy-le-Vieux

abstract

Forseveralyears,theKARMENexperimentwasreportingananomaloussignalwhichwas

interpreted as a possible existence of heavy sterile neutrinos. The NOMAD experiment

has the capability to search for such an isosinglet neutrino. It is assumed that the (

s )

couples to the

produced in the SPS neutrino beam at CERN. Analysis of NOMAD

data leads to a single candidate event, compatible with the background expectations.

This allows alimittobe set forthe rst time onthe mixingstrength between sterile and

tau neutrinos, in the mass range 10to 190 MeV.

Talk given at the TAU2000 Workshop,18-21 September 2000, Victoria,Canada

Tau-sterile neutrino mixing in NOMAD

P.Nedelec a

a

Laboratoired'AnnecydePhysiquedesParticules,

chemindeBellevue,74941Annecy-le-VieuxCEDEX,France

For several years, the KARMENexperiment was reporting ananomaloussignal whichwas interpreted as a

possibleexistence ofheavysterile neutrinos. TheNOMADexperiment hasthecapabilityto searchfor suchan

isosingletneutrino. Itisassumedthatthe(

s

)couplestothe

producedintheSPSneutrinobeamat CERN.

AnalysisofNOMADdataleadstoasinglecandidateevent,compatiblewiththebackgroundexpectations. This

allows alimittobe setforthe rsttimeonthemixingstrength betweensterile andtauneutrinos,inthemass

range10to190MeV.

1. Introduction

In the Standard Model (SM) of Particle

Physics,neutrinosonlyinteractviatheirSU(2)

L

left-handed states. Inseveral modelsbeyondthe

SM,likeGrandUniedTheoriesandSuperstrings

inspired models [1], heavy isosinglet neutrinos

(

s

)notcouplingtotheWandZbosonsarenat-

urallyintroduced. Suchsterileneutrinosarealso

introduced in other models developed to solve

baryo/leptogenesisin theUniverse,likeleft-right

symmetric or see-sawmodels. Their masses are

predictedin theGeV- TeVrange. Light sterile

neutrinos, m

< eV, are introduced [2] to ex-

plain in acoherentframework the solar, the at-

mospheric (SuperK) and LSND neutrino exper-

iment's results. The dierent experiments ex-

plaintheirobservationsviaaneutrinooscillations

mechanism. This implies three dierent m 2

.

SinceLEPhaslimitedthenumberofligthneutri-

nosto3,afourthsterileneutrino

s

isanelegant

wayto solvetheneutrinopuzzle.

Recently the KARMEN experiment was re-

portingaboutananomalyinthetimestructureof

itsevents,whichwasoriginallyinterpretedasan

indicationof a heavyneutralparticle in the few

tensMeV range[3]. More recently aKARMEN

representativereportedon\nomoreindicationof

anoscillation"[4].

The NOMAD experiment at CERN, has the

capabilitytolookforsuchheavyneutralparticle

formasses uptofewa100MeV [5]

The main question concerning a new heavy

neutrinoistoknowifitcouplestoordinaryneu-

trinosandifso,whatare themixing parameters

U

li

. Leptonicdecayandneutrinoexperimentsset

limitsonjU

es j

2

andjU

s j

2

. ConcerningjU

s j

2

,no

experimentallimitsexistinthe100MeVmassre-

gion. Recently,cosmologyandastrophysicalcon-

siderations were used to constrain jU

s j

2

in this

massregion[9].

2. The KARMEN anomaly

The KARMEN experiment studies the inter-

actions of neutrinos in a large tank lled with

56 tonsof liquid scintillator. Neutrinosare pro-

ducedbystopping800MeVprotonsinamassive

target,producingpions. The areabsorbedby

thetarget whilethe +

decayatrest (DAR)via

+

!

+

. The lowmomentum +

also stops

in thetargetandthendecaysvia +

!e +

e .

The specic time structure of the beam (two

pulses, 100ns long 225ns apart) allowsfor dis-

criminationbetweenneutrinoscomingfromde-

cayingatrest( =26ns)andtheneutrinoscom-

ing from the subsequent decay ( = 2.2 s),

asindicated onFigure 1. Due to the- chain,

theneutrinobeamcontainsidenticalintensitiesof

,

and

e

. Theducialregionofthedetector

is situated between16.6 m and19.8 m from the

Figure1. TheKARMENexperimentalsetup

hugeshielding system,made outof7000 tonsof

steelequippedwithtwolayersofvetocounters,in

order tosuppressbackgroundsfrom beamcorre-

latedspallationneutrons,fromhadronsfromcos-

mic radiation and to reduce cosmic muon ux.

This conguration is know as KARMEN1. In

1996athird,largearea,vetosystem(300m 2

)was

installedtofurther improvetheshielding(KAR-

MEN2experimentalconguration).

Inthetimewindow0.6-10.6s,afterthebeam

isincidentonthetarget,oneexpectsasignaldis-

tribution corresponding to the 2.2 slifetime of

the muon superimposed on a at cosmic back-

ground. The KARMEN1 data exhibit a bump-

likedistortion inthetimespectrumaround3.1-

4.1s,theso-calledanomaly,asshownonFigure

2. Thisexcesswasintepretedascomingfromthe

decayofslow-moving,neutral,weaklyinteracting

Figure 2. Timedistribution ofKARMENevents

withtheanomalybump

particle Xinside theducial volume,where Xis

produced in arare decay ofpions:

+

!

+

X.

The mass of this new particle was estimated to

bem

X

33:9MeV.Ananalysisincludingarst

part of the KARMEN2 data sample conrmed

theaccumulationofevents. However,anupdated

analysis of allthe KARMEN2data, collectedso

far (March 2000) revealed no moredistortion in

thetimedistribution. Basedontheincompatibil-

itybetweenthetwodataperiods,theKARMEN

experiment concluded that the +

!

+

X hy-

pothesishadto berejected.

In NOMAD we have initiated an analysis to

look forsuchaneect. Letmerstdescribethe

principleofthesearch.

3. Principle ofthe search

AsschematicallyshownonFigure3,thesterile

neutrino

s

is produced in the D

s

!

s decay

through

s

mixing. The D

s

meson is pro-

duced in thep+Beinteractions attheneutrino

target. The

s

propagates to NOMAD through

thedownstreamneutrinobeamshieldingwithout

signicantattenuationifitisalong-livedparticle.

Occasionally a

s

decays in the NOMAD detec-

tor via the process : ! e +

e . An excess

ν

ν _τ U

|

|

| U

|

0 00 1 11

00 0 11 1 0000

00000000 00000000 0000

1111 11111111 11111111 1111

00000000000000000000000 00000000000000000000000 11111111111111111111111 11111111111111111111111

0 00 00 00 00 00 00 0 00 00 00 00 0

1 11 11 11 11 11 11 1 11 11 11 11 1

00000000000000000000000 11111111111111111111111 00

0000 0000 0000 0000 0000 0000 00 0000 0000 0000 0000

11 1111 1111 1111 1111 1111 1111 11 1111 1111 1111 1111

000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000 000000

111111 111111 111111 111111 111111 111111 111111 111111 111111 111111 111111 111111 111111 111111 111111

00000000000 11111111111 00000000000000000

11111111111111111

Proton beam, 450 GeV/c Target

D

τ

Steel, earth shielding

Z

e e

-

NOMAD detector s

+_

Figure3. Principleofasterileneutrino

s

production,propagationinthebeamdumpandinteractionin

theNOMADdetector.

of e +

e pairs events above the expected back-

groundwould indicate thepresence of a

s

intheneutrinobeam.

4. The NOMAD experiment

The NOMAD detector is described elsewhere

[6]. Itconsistsofdierentsub-detectorsinstalled

in a 0.4 T dipole magnet: a set of drift cham-

bers(DC)asactivetarget(2.7tons),ahighper-

formancetransitionradiationdetector(TRD)[7]

forparticle identication andacalorimetricsys-

temforenergymeasurements. Outsidethemag-

neticvolume,ahadroniccalorimeterandasetof

muon chambers complete the experiment. The

electronidentication is provided mainly by the

TRDwhichhasarejectionpowerforisolatedelec-

trons greater than 10 3

at 90% eÆcency for the

electronin themomentumrange1-5GeV[8].

5. Eventselection

The data used for this

s

!

e

+

e analysis

correspond to the full

VT1T2data sample col-

lected during the period 1996-1998. This data

setcorrespondstoatotalof4.110 19

protonson

target(pot's). Thesignaleventsarecharacterised

byae +

e pairwithnoother activityin thede-

tector. Theselection criteriaforthe selectionof

thesignalarethefollowing:

Event in NOMAD ducial volume: two

themidentiedasanelectronbytheTRD;

E

pair

>4GeV;invariantmassm

pair

<95

MeV.

Calorimetric activity: no photons with

E

> 0.4 GeV, except if compatible with

ae +

ore bremsstrahlungphoton.

No other activity: E

HCAL

<0.4 GeV; no

muonintheevent.

After this selection 207 events are collected.

Because ofthehighcollinearityof thee +

e pair

withtheneutrinobeamforthesignal,anangular

cut isapplied to further reduce the background.

One denes the variable C 1 cos

(e +

e ) ,

where

(e +

e )

istheanglebetweentheneutrino

beam direction and the total momentum of the

reconstructed e +

e pair. The cutapplied to se-

lect signalin thereferenceboxisC<210 5

.

The accuracy ofthe colinearitydetermination

for simulationeventswascheckedwith asample

of

CCdata having ae +

e pair comingfrom

aphotonconversion,producedfarfromthemain

vertex.

6. Backgrounds

Themainbackgroundcomesfromneutrinoin-

teractionsinthesurroundingmaterialoftheNO-

MAD target: the return yokes, the front pillar

and the magnetic coil, producing a single 0

in

eventsarecharacterisedbyae e pairinthetar-

getregioncomingfromthe 0

decay,withverylit-

tlehadronicactivity. This backgroundhas been

estimated,basedonafullsimulationofneutrino

interactionsin theoveralldetector.

Figure 4. Distribution of C 1 cos

(e +

e )

fordata and Monte-Carlo. Shaded area(narrow

peakontheleftside)representstheregionofthe

expectedsignal

7. Resultsand limits

In order to avoid human bias in the selection

criteriaablindanalysishasbeenperformed. This

means that data and Monte-Carlo events were

comparedinaregionwherenosignalinexpected,

i.e. outside the box asshown on Figure4. Out-

side thebox,19 eventsareobservedand 20 4

arepredicted.

Two dierent methods are used to estimate

the background in the signal box, the rst one

is based on Monte-Carlo, while the other relies

only on the data themselves. The two methods

give consistent results: N MC

bk g

= 2:5 +0:9

0:8

(stat)

0:6(syst) events from the MC and N Data

bk g

=

2:4 +0:9

0:9

(stat)0:7(syst) events from the data,

wherethesystematicerrorincludesuncertainties

and on thecoherent productioncrosssection

(25%). Uponopeningthebox,wehavefoundone

event passing the selectioncuts. This is consis-

tentwiththeexpectedbackgroundandhenceno

evidence for isosinglet neutrino decayshas been

found.

We can then determine the 90% C :L: up-

perlimitforthecorrespondingmixingamplitude

jU

s j

2

as afunction of the

s

massby using the

followingrelation:

N

s!e +

e

<N up

s

!

e

+

e

(1)

The 90% C :L: upper limit for the expected

number of signal events, N up

s

!

e

+

e

, is com-

puted within the frequentist approach of ref

[10] by taking into account the uncertainties in

the background estimate [11]. This results in

N up

s!e +

e

=2:1events.

Foragivenux(

s

),theexpectednumberof

s

!

e

+

e decaysoccuringwithintheducial

length L of the NOMAD detector located at a

distance L 0

fromtheneutrinotargetisgivenby

N

s!e +

e

= Z

(

s

)exp( L 0

m

s

=p

s

s ) (2)

[1 exp( Lm

s

=p

s

s )]

(

e +

e

=

tot

)"AdE

s /jU

s j

4

where p

s

is the

s

momentum,

e +

e

;

tot are

the partial and total mass dependent

s -decay

widths,and"isthee +

e pairreconstructioneÆ-

ciency. TheacceptanceAof theNOMADdetec-

tor was calculated tracing

s

's produced in the

Be-targetthrough thebeamdump to thedetec-

tor taking all relevant momentum and angular

distributions intoaccount[12].Theux(

s )of

sterile neutrinos canbe relatedto the ux of

presentin theSPSneutrinobeam[5].

The nal 90% C :L: upper limit curve in the

(m

s

;jU

s j

2

)planeisshownin Figure5together

with the Big Bang Nucleosynthesis (BBN) and

supernovaeSN1987alowerlimitsobtainedinref.

[9]. Forthe mass range m

s

m

the shapeof

theNOMADlimitcurveisexplainedbythecon-

tributionfromthe

s

!

0

decaytothe

s de-

cay rate: as the ! 0

decaychannelopens

10^-6 10

-5 10^-4 10^-3 10

-2 10^-1

0 20 40 60 80 100 120 140 160 180 200 SN1987a

NOMAD

BBN

νs mass (MeV) Coupling |Uτs|2

Figure5. ExclusionregionsforjU

s j2vsm

s

up,thebranching ratioBR(

s

!

e

+

e )drops

rapidlyasm

s

increases.

Our data can also be used to restrict one of

the interpretations of a time anomaly observed

by the KARMEN experiment. From our limit

curve in Figure 5 we can give for each mass

valuealimitonthecouplingstrength. Thislimit

can be converted to a lifetime constraint. For

m

s

=33:9MeV the lifetime ofsuchaneutrino

has to be greater than ' 10 2

seconds, as in-

dicated on Figure 6. The BBN lower limit of

ref. [13]constrainsthe

s

lifetime tobelessthan

0.1 sec. This results in a small window around

BR ( +

! +

+X)'10 12

leftuntested. Note,

that therecent PSI result on the search for the

33.9 MeV particle in +

!

+

+X decay cor-

responds toBR ( +

!

+

+X)<6:010 10

at

95%C :L: [15].

8. Conclusion

In NOMAD we havecarried out a search for

isosinglet heavy neutrinos (

s

) produced in the

D

s

!

decay through

s

mixing, fol-

lowed by adecay ! e +

e in the detector.

10^-16 10^-15 10^-14 10^-13 10^-12 10^-11 10^-10 10^-9 10^-8

10^-1 1 10 10²10³10⁴10⁵10⁶10⁷10⁸10⁹ NOMAD

KARMEN curve

BBN PSI

Lifetime τ(µs)

Branching Ratio

Figure 6. ExclusionregionsforBr vslifetime

Theexcellentcapabilitiesoftheapparatusforthe

identication and themeasurementof thee +

e

pairdirectionallowsubstancialsuppressionofthe

background from standard processes. A single

candidateevent,consistentwithexpectationswas

found. Thisallowstoderivationoflimitsonmix-

ing strength jU

s j

2

in the masse range 10-190

MeVto bemade. Theresultsobtainedareused

to constrain theinterpretation oftheKARMEN

anomaly intermsofararedecay +

! +

+X,

where X hasamassof33.9MeV andiseithera

isosingletneutrino

s

oranew,short-livedweakly

interacting neutralfermion. A 90 % C :L: lower

limit on the

s

lifetime

s

> 0:017 sec is ob-

tained. Thewindow0:017<

s

<0:1s between

the BBN upper limit and present lower limit is

still openforfutureexperimentalsearches.

9. acknowledgements

I would like to thank my NOMAD colleagues

with whom we made this nice experiment and

particularlyL.DiLellaandS. Gninenkoforvalu-

ablediscussions.

1. R.N. Mohapatra and P.B. Pal, in "Mas-

siveNeutrinosin PhysicsandAstrophysics",

WorldScientic,Singapore,1991.

2. O.V.MazurinandE.A.Porai-Koshits(eds.),

Phase Separation in Glass, North-Holland,

Amsterdam,1984.

3. J-M Levy and F.Vannucci, A consistentin-

terpretation oftheKARMEN anomaly, hep-

ph/0003194(2000)

S.N. Gninenko and N..K Krasnikov, Exotic

muon decays and the KARMEN anomaly,

hep-ph/9804364v4(1998).

4. K.Eitel,LatestresultsoftheKARMEN2Ex-

periment,hep-ph/0008002(2000).

5. P.Astieretal (NOMADCollab.),Searchfor

MeV isosinglet neutrinosmixing to tau neu-

trinos,CERN-EP-xxx(2000)

6. J.Altegoeretal(NOMADCollab.),TheNO-

MAD experiment at the CERN SPS, Nucl.

Instr.andMeth.A404(1998)96.

7. G.Bassompierreetal,Alargeareatransition

transition detector for the NOMAD exper-

iment, Nucl. Instr. and Meth. A403 (1998)

363.

8. G. Bassompierre et al, Performance of the

NOMAD transition radiatio detector, Nucl.

Instr.andMeth.A411(1998)63.

9. A.D. Dolgov, S.H. Hansen, G. Raelt,

andD.V.Semikoz,Preprinthep-ph/0008138,

SubmittedtoNucl. Phys.B.

10. G.J. FeldmanandR.D. Cousins,Phys.Rev.

D 57(1998)3873.

11. A.StuartandJ.K.Ord,"Kendall'sAdvanced

TheoryofStatistics",vol.2,"ClassicalInfer-

enceandRelationship,5thed., OxfordUniv.

Press,NewYork,1991, p.861,Eq.(23.4).

12. M.C. Gonzalez-Garcia and J.J. Gomez-

Cadenas,Phys.Rev.D 55(1997)1297.

13. A. D. Dolgov, S. H. Hansen, G.Raelt, and

D.V.Semikoz,Nucl.Phys.B580(2000)331.

14. V. Barger, R.J.N. Phillips and S. Sarkar,

Phys.Lett.B352(1995)365;

(E)ibid.B 356(1995)671.

15. M. Daumet al., Phys. Rev.Lett. 85 (2000)

16. F. Atchison,M.Daum,P.R. Kettle,C.Wig-

ger, Preprinthep-ex/0008073.