N ELSEVIER Nuclear Physics B (Proc. Suppl.) 77 (1999) 220-224

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PROCEEDINGS SUPPLEMENTS

CHORUS results

Osanm Sa te a CHORUS Col labora t ion

aDepar tmen t of Physics , Nagoya Universi ty, 464-8602, Nagoya, J A P A N

The CHORUS experiment aims to detect neutrino oscillation u~, -+ u~, especially at the dark matter mass scale ,,, 10 eV.

The CHOIR, US apparatus has been installed in the CEIRN \Vest Area and exposed to the SPS \)Vide Band neutrino bemn for about 150 days per year from May '94 to November '97.

The emulsion stacks were replaced every 2-year period and developed. A limit on u, --+ u~ oscillation has recently been published from the analysis of a subsmnple of neutrino interactions, taken in '94 and '95 [11 [2]. This paper contains an update of the result using the statistics taken in '96.

At present the u~ interaction search has been performed with 66,304 "lt*" events and 7,081 "0p" events in nuclear emulsion. No neutrino oscillation signal was found, which translates into a mixing angle linlit s i n 2(20) <_ 1.3 x 10 -a (90%CL at large Am 2, of the order of 100 eV2).

1. T h e C H O R U S e x p e r i m e n t

The neut r ino oscil lation signal is searched as tau neut r ino events produced 1) 3, an or igimdly u~- free heron. The CERN wide band neut r ino bemn contains main ly u, (94~7c) and the backg round coming fl'om the In'erupt Vr in the beall l is es- t imated as low as 3.3 × 10 -~ ur CC in te rac t ion per u , CC interact ion.

The average beam energy of u~, is 27 GeV, the distance fi'om neut r ino beam source to the CHO- RUS a p p a r a t u s is abou t 600m.

770 kg of nuclear emulsion are used a.s neu t r ino target. Thanks to the very sha rp spa.ce resolu- tion in nuclear ennllsion (sul)-im~), the "I- decay followed by the u~ interact ion can l)e de tec ted di- rectly t)y the short distance decay topology (<_ 3mm).

The ta rge t se tup is a sandwich s t ruc tu re of nuclear emulsion and of scint i l la t ing fiber t rack- ers (Target Tracker) . The Target Tracker recon- structs the neut r ino events and gives angle and posit ion informat ion about the tra.(ks, to be used for emulsion scamting.

An air core magnet behind tit(' ta rget se tup measures the lnolnentunL of the tra.cks. An elec- t romagnet ic and ha(Ironic ca lo r imete r measures the energy of the event. The mos t downs t r eam

0920-5632 /99 /$ - see front matter © 1999 Elsevier Science B.V. PII S0920-5632(99)00421-1

I)art is a rouen spec t rometer . A background fl'ee identif icat ion of u~ intera( '-

t ions can be achieved 1)y combining the d a t a from tile electronic de tec to r to the topological informa- tion in the emulsion.

Detai led per formance on the a p p a r a t u s are lie- scr ibed elsewhere [3].

2. T h e e v e n t s e l e c t i o n

In 85c/c of the cases the tau decays in to a sin- gle charged par t ic le ("kink" topology), while the remaining 141/c show a charged-3-prong topology.

Theretore the cur ren t analysis focuses on the following channels :

7 - - ---+ I t - n t- ~ , -}- lee (Be ~,- 18cZ ,) r - --~ b - + Iz~ ° + u , ( B r ,-., 50(/,)

The first channel will t)e searched a m o n g the events with a negat ive llltlOll identif ied 1)v the electronic de tec to rs (1-p sample) while the search for the second channel will use the events with no rouen associated to the ver tex (0-1t sample) .

hi the first class, only the p - of n lon len tu iu lower than 30 O e V / c have been analyzed , hi the 0p. class, all negat ive pa r tMes with a recon- s t ruc ted n lo lnen tum between 1 and 20 G e V / c have been considered as possibh' tau decay prod-

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O. Sato/Nuclear Physics B (Proc. Suppl.) 77 (1999) 220-224 221

Table 1 Neutrino events numbers

lit,,, (Year) I 1004 I

l ip Prediction 66,911 i/~ S('alIIled frac 63(/, ̀ lp Located 18,286 17,890

0/i Prediction 17,731 27,841 011 S('anned frac 50% 29% 0p Located 3,401 3,680

1995 1996 1997 II All

110,916 17.9,669 151,105 458,601 340{c 56(Z , 0(ff 33V

0 66,304 30,128

32,548 I 37,929 116,049 0(7~' [ 0(Z' 15~Z

0 0 7,081

u('t,

3. The scanning sys tem

Th(' tracks passing the previous selection are scanned by automatic microscope systems from the downstream emulsion in the stack where the vertex is predicted.

An emulsion plate, the size of which is 36cm x "~2ci,, al,d the thickness is 790p.m (350pm of emul- sion on both side of a 90 p m thick acetyh:ellulose- l)ase) is set on the microscope stage.

The movement of the 3 axes(x,y,z) is controlled t)y ('omi)uter and a high refl'esh rate CCD takes microsc,)pe images a.round the predicted track lo- cation.

The ol)jective lens magnification x50 ,,takes the size of one CCD view as 150 ttm x 120pro of emulsion. For one track, 16 fl'alneS of CCD pixel data are taken changing the depth of the focal plane (every 6 pro). A device called Track Selec- tor, developed at Nagoya University, recognizes a track fl'om the CCD 16 frames pixel inforlnation.

The pri,wiple of the Track Selector is to ad(l lin- ear offsets to the CCD pixel positions (calculated fro,,, the give,, angle ilfformation) and overlay the pulse height of the 16 frames.

If the track predicted by the scintillating fiber tracker is there, a big pulse height peak appears. The track finding efficiency of the Track Selector is above 98(/~ , for track angles up to 400 mrad. The speed of the track recognition is 0.3 second per microscope view(150 ltm x 120/tin).

4. Event location and Decay search

One stack of emulsion module consists of 36 plates with a total thickness of 3cm, correspond ing to one cascade length.

All tra.cks are scanned fl'om downstream up to the vertex (sea.n-back). Scamdng is done at the upstream surface of every plate. Only 100pro depth CCD pixel information, represented by white spots in figure 1, are used to judge whether the track element exists. The plate in which the track disappears (i.e. the tra.ck elemeltt is not found in two successive plates, represented by grey spots in figure l) is called the Vertex Plate. The definition of the Vertex Plate is the starting point of the (lecay search.

For most of the events, one or more predicted tracks exist in addition to the scan-back track. They are searched in the plate downstream of the Vertex Plate. The presence of the neutrino intera,etion vertex in the Vertex Plate can then I)e confirmed if at least one track i~ fi)und whi('h h~s a small impa('t parameter with the sean-l)ack track.

The procedure used tbr the d('cay search is fie- tailed below. It takes into ~wcount the different situations illustrated in figure 1. If a decay topol- ogy is found, it will havc to satisfy the cuts given in Table 2 to be accepted as a 1/~ signal.

4.1. Long flight wi th Small angle decay search

If the r (leeay angle is small (<~25mrad), the pr imary vertex will 1)e found by the automatic procedure at the Vertex Plate (bot tom type in Figure 1). In this case, the ca,i[didate ~/~ interac- tions are in the small impact parameter sample.

222 O. Sato/Nuclear Physics B (Proc. Suppl.) 77 (1999) 220-224

Table 2 Definition of candidate ~,~ interactions in CHORUS

Applied cut Main snpprcssed event type No other lepton (t t or e) at primary vertex Kink decay is found in emulsion Charge negative (decayed particle) Flight length < 3 or 5 plates downstream of Vertex plate Decay transverse momentum > 250 MeV/c

le~, CC+charm decay hadron interaction let, CC+charln decay hadron interaction/decay hadron interaction/(lec~\y

° . _

V~

2~

plate n+l n n-I n-2 n-3

0 7901am ~ . S c a n b a c k Locat ion

Figure 1. Decay types searched for : The plate n is the Vertex Plate

The decay possibility is checked by transverse mo- lnentum (pt), which is calculated from the track momentum and the difference between the angles at the most upstream plate angle and a.t the most dmvnstream plate angle.

If pt is bigger than 250 MeV/c, the events are scanned by eye and re-mea.sured carefully. No sig- nal event has been found by this manual scanning and all the large pt values could be attributed to the mea,surement inaccuracy of tile automatic

scalmil,g system.

4.2. Short flight decay , Long flight with Large angle decay search

The salnple of events with large impact paraln- eter contail~s r decays of the type shown in tile top and middle part of Figure 1.

In these cases the r decay is ill the Vertex Plate and the primary vertex can be found ill the sanle plate or in a inore upstream plate. To hnnt a de- cay in the Vertex Plate, fllll depth infonnation of the Vertex Plate is taken by means of 48 frames of CCD pixel infornlation (Image Data). Tracks are followed in tile Image Data and if an angle differ- elite is detected in the emulsion (pt>250MeV/c), the event is checked by eye.

The "decay " position and the topology are careflllly checked. Almost all hadron interactions make fragment of nuclei or Auger electron blob at the "decay " position. They Call be e~Lsily found because of the sub-micron space resolution of nu- clear elnulsion.

If a r decay candidate is found in the \:ertex Plate, the primary vertex is searched by human eye check and the topological and kinematical cri- teria on Table 2 are applied. Up to now, no signal event was found.

4.3. Long flight with Large angle decay search

Tile linage analysis doesn't work near tile base and near the emulsion surface. To recover de- cay finding efficiency, we developed another type of decay analysis. In the case of Long flight with Large angle decay, we Call detect the r track at the upstream surface of Vertex Plate and detect the r daughter track in the downstream

O. Sato/Nuclear Physics B (Proc. Suppl.) 77 (1999) 220-224 223

plate(middle type in Figure 1). A general search for all angle tracks is per-

formed on the ui)stream surface of tile Vertex Plate to find tile parent(could be r) aud to calcu- late the impact parameter with the decay daugh- ter (mea.sured ill downst ream plate).

When the impact parameter is small enough (I.P < 15pro), they are considered as R E A L par- ent and daughter , and scamled manually. The manual check proce(lure is tile same as in tile pre- vious section. As a result, no signal event wa.s found in this procedure.

5. E f f i c i e n c y c h e c k

Tit(" kink linding efficiency has been evaluated by Monte Ca.rlo simulation. The validity of this cMculation can be checked by looking at the ob- served samI)les of hadr(m interactions and charm decays.

From the (Ip event sample, 21 hadron interac- tions haw' been observed by the decay search pro cedure. The result is in good agreement with the Monte Carlo exl)cctatio], 24 + 2.

In a pa.rtial sample of 2tt events, 25 charged charm muonic decays were found. This statistics is also in good agreement with the Monte Carlo expecta t ion 22.8 ± 3.9•

6. C u r r e n t l i m i t for n e u t r i n o o s c i l l a t i o n

66,304 '" ltd' events and 7081 "Otd' events have been found in emulsitm and tile r decay s('arch was pcrforule(l.

The sensitivity to p, ~ u~ oscillation can be d('rived from tile numbers listed in Tabh ' 3 and the following relations :

Number of l,~ events in tile ease of fifll mixing :

N [

~" : CC interaction cross section ratio JI r ;~,r : C H O R U S Acceptaace ratio

Br : Branching ra.tio for search qa.int. : kink finding efficiency

~ l l l l i ] ) e r O f observed e v ( ' l l t s :

N . o ~ . ~ = -% • P ( , ' , -+ ",) P(p, + p-~) = sh,2(20). .s 'ht2(1.27&m 2L)

O :mixing angle ~,n 2 :squared m~uss difference (eV 2) L :neutrino flight length (kin.) E :neutrino energy (GeV)

No neutr ino oscillatiov signal event has been observed. This yMds a lilnit on the mixing pa- ralneter .s'in2(20) < t . 3 . 10 a at large squared mass difference (of the order of 100 eg 2), where the second fa.ctor in the above expression for the neutrino oscillation probabil i ty averages to 1/2.

The neutr ino oscillation parameters are shown in Figure 2.

O \ "0

<~

J J )

1C 2 /

(

[ I I O R C S

1 :?

N O M .4.1) [ T [ ~ . -" i-- I H ~ ~ T F T VITTI

!i es31 ...""

.i )::

C I ) H S " " - - . J 5

- - .

,H±d ~±, t a ~ u l • L2 , 1 a M _ _ L ~ J , L L H I

2 o i 1 [ i 1 ' ;

• 2 . . . .

Figure 2. C H O R U S exclusion plot (u, --~ p,)

7. C o n c l u s i o n s

The C H O R U S (letector r('corded 600,000 nctt- trit,o events in nuclear enmlsiolL target after 4 years exposure to neutr ino l,cam (CERN SPS).

224 O. Sato/Nuclear Physics B (Proc. Suppl.) 77 (1999) 220-224

Table 3 S u m m a r y of the events present ly analyzed bv ( ;HORUS

. . . . . . l p event

Number of ana lyzed event 66,304 1

[l N,tml,( 'r of p,, C C c v c n t A:,.(. ] 66,304 [ ~: 0.530 I cry,

a, 1.08 A~

Br 0.18

I/kin k (I.426 Number of tes ted u. CC event N~ 2910 N u m b e r of observed lZr CC event :Y, ob., 0

Cross section rat io

Accep tance rat io

Bi 'an(hi l lg rat io kink finding efficiency

Ott event

7,081

31,333 0.530

0.43

0.50 0.234

835 0

U 1) to now about 30 % of the events which were rc( 'onstruct( 'd by the Target 'Tra.('k('r hay(' [)c('it sc'anlt('(1 l)v a.UtOlUatize(l sCanlting systenls and found in nutIca.r emulsion.

From tit(' de ta i led decay seaxch a.t the \ : ( ' r tcx l)late, no I/r cV('lff was foultd.

\Vi th the sam(' pro( 'edure, several cha.rm decays a.n(l ha(Iron i l l te ra( t ions were found. Since their topology in emulsion is very simila.r to tau de(ay, these events wet(' usc(l to check the Monte Carlo ,wMuation of kink finding efficiency.

From a.nalvz('d sample of 66,304 "ltd" ( 'vents ;tlld 7,081 "'()l d' , a ltCW limit on the neut r ino os- cil lati(m lm.ramctcrs was derived. We cx( ludcd the poss ib i l i ty of s / ,2 (20) > 1.3. 10-:* ~,~.(,~, ~,t large A m 2 (90%CL).

A b o u t T()(fi of the events a.rc still to 1)e ana.- lvzed. Thanks to tit(' ( 'ontimtous iml)rOV<'m('nts (ff a u t o m a t i c sea.rating devices and rc( 'ons t ru( t ion a lgor i thms, this analysis will 1)c COml)h't('d with all. iml)rovc(l cfli('iclwy.

If no signal events will be found a.t the ('rid of t he analysis , t It(: sensi t iv i ty will rca.(h ,s'it~ 2 (20) =: 9. 10-* a.t larg(' _kin e.

4. Van de Vyver ,B. et al., Nucl . Ins t .Meth A,785 91-99(1997)

5. Ya.Zel 'dovic and I .D.Novikov, I le la t iv is t ic As- troldlysics,Nauka,Mos('ow 1967

6. Harar i ,H. Phys .h ' t t .B 216 413-418(1989) 7. Ushida, N. ct al.,E531 Col labora t ion

,Phys . I~ev .Let t .57 2897-2900(1997)

R E F E R E N C E S

[. E .Esku t ct a l . ,CHORUS Colla.I)oration, Phvs .Le t t . /3~/24 '20'2 (1998)

2. E .Eskut et aI . ,CHORUS Col labora t ion , Phys .Lc t t . B~.%/ 205 (1998)

3. E .Esku t et a l..CHOIRUS Col labora t ion , Nuc l . Ins t .Meth A~O1 7 (1997)