ELSEVIER Nuclear Physics B (Proc. Suppl.) 66 (1998) 362-365

I | l [ l ~ u

PROCEEDINGS SUPPLEMENTS

Preliminary results from the NOMAD experiment on v, oscillations

J.-P. Meyer ~

a D A P N I A / S P P Bt. 141, CEA Saclay, 91911 Gif-sur-Yvette, F R A N C E

NOMAD is a short baseline neutrino oscillation experiment searching for v , -,~ v~ oscillations in the CERN SPS wide band neutrino beam. The experiment has been collecting data since July 1995 and the data taking will continue until at least the end of 1997.

A preliminary analysis based on the 1995 data sample allows NOMAD to set a 90 % confidence limits on sin2(20,,-) < 4. × 10 -3 for v , -,,* v~ oscillations at large Am 2.

1. I n t r o d u c t i o n

NOMAD[l] is an appearance experiment searching for v u ~ Vr oscillations in the CERN SPS wide band neutrino beam which is mainly composed of vu (see figure 1). Table 1 gives the relative abundances of the different neutrino beam components . Notice that the expected v~ contaminat ion coming from p + N , ) D, + X followed b y D ~ , ~ r ~ a n d r , ) v ~ + X i s w e l l below the sensitivity limit of this experiment.

The 7-- produced by a v~ charged current in- teraction (CC) on a nucleus of the active target (v~ + N -+ 7"- + X) is detected using kinemat- ical criteria in both leptonic and hadronic decay channels covering 85% of the 7-- decay modes (e , p ,rr ,p ,~- ~r ~r + + n~r°). Therefore the required detector performances are:

* a good measurement of charged particle mo- menta and total energy flow;

• identification of electrons and muons with high efficiency and purity.

For a two family mixing, the probability of ob- serving a Vr at a distance L in a v u beam is:

P ( v u "-0 vr) = s i n 2 ( 2 0 ) s i n 2 ( ~ -) (1)

with:

X(km) = 2.48Eu(GeV) a,,,~(~v~) (2)

where 0 is the mixing angle, X is the oscillation length determined by the neutrino energy E~ and

0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0920-5632(98)00064-4

the difference A m 2 between the squares of the neutrino mass eigenvalues. The NOMAD detec- tor is located at an average distance of 650 m from the neutrino source and its sensitivity is limited to the region A m ~ > 1 eV ~'.

N { : ~ I ~ T o ~ e t

. • ' ' I • • • I • ' ' I • ' • I ' ' ' I ' ' '

l o 7

E 101 :

1 0 2 : . . . . . . . . . . . . . . . . . . .

. . . . "L._..: - - 1 . : . , " ' - % . . . ,

- - " " . . . . . " " " ~ - - - '-"---I ,--, . . . . ~ l e ~ - - - . , - - * . - - , . - , .

1 0 : .. . . . . . . . "

ii ii i:ii IIIIII'- 1 • . . . - ~ . . . . . . ; ~ . -~_ . = , . ~ : . " .

1 0

1 0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

Nc, uLr~no E n e r g y (GeV)

Figure 1. Predicted energy spectra for the differ- ent components of the CERN Wide Band Neu- tr ino Beam

2. T h e N O M A D d e t e c t o r

Full details of the NOMAD detector are given in reference[2]. The 49 drift chambers (DC) act s imultaneously as tracking device and as low Z

J-P. Meyer~Nuclear Physics B (Proc. Suppl.) 66 (1998) 362-365 363

Table 1 Relative composition of the CERN Wide Band Neutrino Beam from Monte-( Jarlo calculation

Neutrino < E , > (GeV) Rel. composition 23.0 22.7 37.1 33.2

"-~ 35.

l.lp

lY¢t

l/e ~e

IJ r

1.00 0.07 0.01

0.003 5 x 10 -6

target, with a total fiducial mass of 2.7 tons over an area of 2.6 x 2.6 m 2. The chamber momentum resolution can be parametrized ms:

ap 0.05 0.008P p - : T r G W

where the momentum P is given in GeV/c and the track length L is given in meters.

The transition radiation detector (TRD) con- sists of nine modules made of a radiator followed by straw tubes filled with a mixture of xenon and methane. A pion rejection of 103 for a 90% electron efficiency is obtained in the momentum range 1 to 50 GeV/c.

The electron identification is completed by a preshower (PS) and a 19-X0 deep lead-glass elec- tromagnetic calorimeter (ECAL) which provide a combined extra rejection factor of 10-" to 103 de- pendiag on the momentum. The measured elec- tron energy resolution of the calorimeter has been parametrized as:

-,E) i:: = (1.04.-t= 0.01)%d, (3"_,'_,+0(~7)~

where E is the energy of the electron measured in GeV.

The DC, TRD, PS and ECAL are inside a mag- net providing a homogeneous 0.4 Tesla magnetic field.

Downstream from the magnet, an iron- scintillator sampling calorimeter (HCAL) was in- strumented in order to detect neutral hadrons and to provide a complementary measurement of the energy of charged hadrons.

Following the HCAL, two muon stations made of drift tubes and separated by an iron wall pro- vide an almost fully efficient detection device for muons above 4 GeV/c momentum.

Upstream from the magnet, the detector is cov- ered with scintillating counters used to veto resid- ual muons from the neutrino beam.

3. v, -,~ u, osc i l l a t ion s e a r c h

The analysis results presented below have been obtained using data taken by NOMAD during 95. For this period the delivered SPS beam corre- sponds to 0.62 1019 protons on target. The r - decay modes currently studied are presented in the following sub-section except for the r - --+ rr r 7r + decay mode which was still too prelim- inary at the conference time.

The transverse plane is defined as the plane or- thogonal to the beam direction. In this plane we define: fiapto, the projection of the lepton mo- " T

mentum;/57~adr°" the vectorial sum of the projec- tions of all the reconstructed hadrons inc!uding the neutrals but excluding the lepton; p~n,s, the transverse momentum which balances ~tepto,~ and *7"

fi~a&o,. Using these variables one can form two angles in the transverse plane: ¢ ~ the angle be-

tween /5~i~,~ and ~ad~o,; ¢i, th the angle between ~Tlevt o,~ f ~o~r o.

a n d

Note that for ue c c events (I)~ is peaked strongly a¢.,180 ° whereas in vet c events the correlation is weaker because of the extra de and Ur produced by the r - decay. The direction of the f i~ i~ vec- tor in u[ :c events is dominated largely by res- olution, making the dp~ distribution rather fiat, whereas in u~ 'C it is peaked at 1800 . The separa- tion between signal and background is enhanced by combining these two variables in the a~t ,~,,. plane.

Backgrounds and efficiencies are estimated c c events from data by replacing the muon in u,

with the appropriate particle. Fake u, neutral current (NC) events are created by removing the muon and fake uCe c events are formed by replac- ing the p by an electron and reweighting by the ue/u u flux ratio.

To improve the sensitivity, the da ta sample has been subdivided in the two following classes:

• events with at the most 3 primary tracks which are of simpler topologies and there- fore easier to reconstruct. Note also that

364 J-P. Meyer~Nuclear Physics B (Proc. Suppl.) 66 (1998) 362-365

this sample is enriched in quasi-elastic events so that one gains on the cross-section ratio c%/o" u which increases from 0.48 to 0.8.

• events with more than 3 primary tracks dominated by deep inelastic scattering events.

The analysis results of the different channels presented below are summarized in table 2.

3.1. T h e r --+ 7 r - ( K - ) v r + X ana ly s i s Events containing no identified #-e and no pri-

mary e + are selected. The events must also con- tain at least one primary negative charged track with momentum greater than 3 GeV/c and for which the extrapolation to the muon chamber

c c in- reaches the sensitive region. To reject vu teractions, the events must have no muon cham- ber hit in a road along the extrapolated track and the energy deposition into the ECAL must be smaller than the momentum measured in the drift chambers. Event having a transverse mass MT < 2.5GeV/c 2 are selected (M~ = (P~ + p ~ ) 2 _ ( f i ~ +/5~)2, where/5~ =/3~i ,~) . Remain-

- c o inter- ing background coming from vcu c and v u actions is removed by rejecting events in which the track with highest PT cannot be identified as a ninon because of geometrical acceptance or mo- ,nentuni threshold. "lb remove remaining back- ground coming from neutral current events we use

the variable QT = ~/(fi~,)'~ - (fi=" #tot È)-/PJot',

where Ptot is the total momentum of the event. This variable is expected to be small for NC events since the rr- candidate is taken from the hadronic jet, whereas it is large for lr- coming from r decay. Requiring QT > 1.7. GeV/c (see figure 2), all the background events are rejected with no candidate left in the data.

The same cuts have been applied to the channel r --+ p-Vr + X followed by p - --+ rr-~.0. Since the p- was not reconstructed, the efficiency is rather low in this channel. This will be improved in a future analysis taking into account the produced 7r ° and the p - mass constraint.

'03~ .,d,.-o-.~ v#- NC (Monte Corlo) O) 10 2 J ~ - "' ~ v,- NC (Dato 95)

0 0.5 1.b 2 2.b

qT

'02k ~*'~,,,,, v~- cc (Vorte CorJo) b)

:-}* • -L_" -:-_ ooioo r -q_ cor,oFH: I I l II

10-t-v,,. I .... I,, ,hli i':~' ~ I , I I I ,, I I I 0 0.5 1.5 2 2.5 .3

qT

Figure 2. a) Solid line: the QT distribution obtained from vu neutral current Monte-Carlo events. The full circles correspond to the QT dis- tribution obtained from u, events with removed p. b) Solid line: the QT distribution for the Vr Monte Carlo sample. Dashed line the QT dis- tribution for the V, neutral current Monte-Carlo events. The triangles are the QT distribution for the 1995 data.

3.2. T h e r --+ e-Feud- + X a n a l y s i s Events containing a primary electron identified

in the TRD, the preshower and the calorimeter are selected. Events from 3' conversions and 7r ° Dalitz decays in neutral current interactions are rejected by requiring that the invariant mass of the electron and any other primary positive track is greater that 0.1 GeV and that tile electron can- didate is isolated (QT, > 0.75 GeV/c) . The t.o- tal visible energy is calculated by summing up the energies of both charged tracks and neutral calorimeter clusters. Since the ve c c energy spec-

c c energy spectrum, the trum is harder than the v u events with total energy less than 40 GeV are se- lected. As for the previous analysis, the events compatible with the r mass (MT < 2.5 G e V / c 2) are selected. Finally we cut on the angles in the

~ _ ~ n plane (see figure 3).

3.3. T h e r --+ #-~UVT + X a n a l y s i s The events are selected in a way similar to

the r --+ e - & v r + X analysis but with stricter cuts because of the very large background from the v c c events. In addition an explicit cut on

J-P. Meyer~Nuclear Physics B (Proc. Suppl.) 66 (1998) 362-365 365

-, ~ ~, ~ ............................................ • O- "Q-

' i ' . " ; . - . " .:., .'.,.~/f~ , " ' .:::".'g:. i~:.

~s =- :.. ..~."

: • .'.." i~.~ i

~ "r--->e MC

as ~- ~ i i i i I i i i , I i i i i T ~

# 2 3

/

, ~ 95 dafo . \ = (prellmino~) a5 - .\ ; i I i I I . . . . I . . . . I ' ,

# 2 . t

0,,,h

Figure 3. ~I ,~-~ for Vr sinmlated charged current interactions followed by r - ~ e-g~Ur

z.~2 i '4

J ,J - -

: ) , \ ~ u, CC MC "'. \

o.s r k

p~, i~ , > 1.7 G e V / c is applied.

3.4. u u ~ uT osc i l l a t ion p r o b a b i l i t y No candidate event, was observed in the 1995

da ta sample, while the estimated total back- ground amounts to 0.6 event. The sensitivity of the experiment is then given by:

-\'~ (:~) P<"~<" < (X,,l<,;×(o~lo,',×~, , l~.,×~,,,

where N r is an upper limit on the possible num- ber of r decays, .,V u is the observed number of ucu :c

interactions inside the fiducial volume, e u is tile uucc identification efficiency.. (~rr /av) is the kine- matic suppression factor due to the difference in masses for the / t and r , B r i and ei are respectively the branching ratio and the selection efficiency for the i - t h decay mode of the r.

The uncertainty on the sensitivity is about 20% coming mainly from the uncertainty on the effi- ciency calculations. Due to this fact the upper limit on the number of possible r - candidates at 90% confidence level is increased[a] from 2.3 to 2.41. The preliminary limit on the probability of v u ""+ V r oscillations is then:

Table 2 Summary of the u u ~ Ur oscillation search. The visibility is the denominator of equation (3) for a single decay channel. Note that the r --+ p-uT analysis was obtained using the cuts optimized for the r --+ rr-vT analysis. F Decay mode Br ~(~T/c%) visib.

I v--+ r t - ( K - ) v T 0.378 0.68% 410 and r -+ p-Ur

r --+ e - a e U r 0.178 2.31% 659 r -+ p-u-uuT 0.174 0.44% 123

TOTAL 0.730 3.43% 1192

Posc(Vu "---+/Jr) < 2. X 1O -3

which corresponds to:

sin220uT < 4. x t 0 - 3

for large A m 2 at 90% C.L.

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

A preliminary analysis of the 1995 data sample results in a 90%-C.L. upper limit of 4. × 10 -3 on sin220uT for vu ~ Ur oscillations at large A m 2.

The experimental data from the 1996 run are cur- rently being analysed. The NOMAD detector is continuing data taking in 1997. The total num- ber of events available at the end of 1997 wilt be of tile order of 5 times the one used in the present analysis.

R E F E R E N C E S

1. NOMAD Collaboration. P. Astier et al., CERN-SPSLC/91-21 (1991); Add. 1, CERN-SPSLC/91-48(1991); Add. 2, CERN- SPSLC/91-53 (1991),

2. NOMAD Collaboration. J. Altegoer et al., CERN-PPE/97-059 (1997) to appear in Nucl. Instrum. Methods.

3. R.D. Cousins, V.L. Highland, Nucl. Instrum. Methods A 320,331 (1992).