CURRENT RESULTS FROM NOMAD NEUTRINO OSCILLATION EXPERIMENT

THE NOMAD COLLABORATION Presented by E. GANGLER

LPNHE - Tour33 RdC ,/, place Jussieu - 75005 PARIS

France

NOMAD is a short baseline neutrino oscillation experiment searching for neutrino oscillations in the CERN SPS wide band neutrino beam. The analysis of 1995 data sample gives no evidence for neutrino oscillation, thus setting an upper limit on the mixing angle of sin2 28 < 1.8 x 10-• for ,,,.. -+ "• and sin2 29 < 3. 7 x 10-• for ,,,.. -+ "• oscillations for large t:i.m2 at the 903 confidence level.

1 Introduction

The Neutrino Oscillation MAgnetic Detector (NOMAD, WA-96) experiment 1 was designed to search for vT appearance from v,.. -+ vT oscillations in the CERN SPS wide band neutrino beam. In the absence of oscillations the beam consists mainly of v,.. neutrinos with a small v. component of the order of 1% and a negligible (::::l 5 x 106) vT contamination 2

The NOMAD Experiment is sensitive to the cosmologically relevant mass range of Llm2 > leV2 and to mixing angles about one order of magnitude smaller than the best limit available at the time of its proposal (sin2 28 < 5 x 103 at large Llm2) 3•

The selection of potential vT charged current (CC) interactions relies entirely on kinematic criteria through the reconstruction of the r decay products. Background events from v,.. and v. CC or neutral current (NC) interactions are rejected by requiring the isolation of the T decay products from the other particles in the event and momentum imbalance in the transverse plane (missing PT) associated with the final state neutrino(s) from T decay. The detector must then measure precisely charged particle momenta and the total energy flow and must sufficiently identify electrons and muons.

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The analysis of the v.CC candidates coming from the low Ve contamination of the beam is relevant to search for vµ -+ v. oscillations, since an oscillation signal would manifest itself both as an excess of events and as a distortion of the total energy spectrum for that sample. The interest for this study has recently increased, following the LSND 4 collaboration claim for evidence for iiµ -+ iie oscillations. In the (sin2 28, �m2) plane, NOMAD is sensitive to a large fraction of the parameter region suggested by LSND for �m2 > 10eV2 •

2 The detector

Neutrino

�

z I metre

Front Calorimeter

Drift Chambers

TRD Modules Preshower

Electromagnetic

Calorimeter

Hadronic Calorimeter

Figure I: Side view of the NOMAD detector.

A side view of the NOMAD setup is shown in Fig. 1, and a detailed description of the detector can be found elsewhere 5• We describe briefly its main components. The NOMAD detector consists of a number of subdetectors most of which are located in a dipole magnet with a field volume of 7 .5 x 3.5 x 3.5 m3• The target part of the detector was designed to accommodate two conflicting requirements: to be as light as possible {low density and low atomic number materials) in order to allow the precise measurement of charged particle momenta and to minimize electromagnetic showers and hadronic interactions, and to be as heavy as possible in order to produce a significant number of neutrino interactions.

This conflict was resolved using an active target (2 .7 tons) of 44 3 x 3 m2 drift chambers (DC) perpendicular to the beam axis, with the target mass (mainly carbon) given by the chamber structure with an average density of 0.1 g/cm3. Placed inside the magnetic field of 0.4 T, these chambers provide a momentum resolution of � � � El:) 0·0�{P , where L i s the track P vL vL length in meters, and p is the momentum in GeV /c. This corresponds to a resolution of 3.5% for average length tracks with momenta up to 20 GeV /c. The drift chambers are followed by 9 transition radiation modules (TRD) 6 . Each module consists of a polypropylene radiator followed by a plane of straw tu bes. Five additional drift chambers are interleaved with the TRD

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modules. The electron identification by the TRD provide a 103 pion rejection factor for 903 efficiency on electrons. Two scintillator trigger planes, Tl and T2 are placed before and after the TRD. Together with a veto scintillator plane V, placed in front of the magnet, they provide the trigger VTl T2. A preshower (PS) consisting of a 1 .6 radiation length (Xo) lead converter and a horizontal and a vertical plane of proportional tubes follows the TRD an precedes an electromagnetic calorimeter (ECAL) 7 consisting of an array (10 Xo ) of lead glass Cerenkov counters. The energy resolution of the PS/ECAL system is u(E)/ E = 3.23/ JE{GeV) $ 13 together with an additional e/1r separation factor better than 102 for 903 electron efficiency.

An iron-scintillator hadronic calorimeter (HCAL)with an energy resolution of 100%/ ./E(GeV) is located just outside the magnet coil and is followed by two muon detection stations consisting of large area drifts chambers, the first after 8 and the second after 13 interaction length (>.o). They provide segments and hits which can be associated to extrapolated DC tracks.

3 The data sample

The results presented in the following are based on data collected during the first NOMAD run in 1995, for a total exposure of 0 .86 x 1019 protons on target (p.o.t.) . This yields a clean sample of 130000 events with an identified muon inside the fiducial volume which corresponds to 162000 v11CC events, given the muon efficiency of 80%. The overall data sample therefore includes these events and correspondingly smaller number of NC events and of events produced by other neutrino flavors .

The full NOMAD data sample collected so far includes about 390000 and 470000 v11CC for the 1996 and 1997 runs respectively, which are currently being analyzed . In addition, the 1998 run started in April .

4 Search for v11 --+ vT oscillations

The r appearance search of the NOMAD experiment is sensitive to about 88% of the r decays through both the leptonic (e-ii0vT 1 µ-ii11v.,.) and the hadronic ( 1-prong and 3-prong) channels 8 . The oscillation search i n each channel as obtained from samples of Monte-Carlo (MC) and data simulator (DS) events uses a set of kinematic criteria that strongly suppress the background and optimize the sensitivity to v11 --+ vT oscillations .

Two separate analyses are carried out for each channel: one using events with � 1 tracks at the primary vertex in addition to the r decay products (DIS analysis) and the other using events with at most two additional tracks (low multiplicity analysis, LM) . A cut on the hadronic momentum at PH > l .5GeV/c is then applied to make the two samples complementary, enriching the low multiplicity one in quasi-elastic and resonance events.

4.1 Background and efficiencies

Since the basic principle of the r appearance search is to discriminate against a possible signal from the tails of the background distributions of the reconstructed kinematic variables, it is crucial to provide a reliable estimate of the background and signal efficiencies for a given set of cuts. This is achieved by comparing the MC predictions with a data simulator technique involving the data themselves, from which the appropriate correction factor is extracted.

The proced ure starts from measured v11 CC events for which the identified muon (track and energy deposition) is replaced by an electron, a r which is subsequently decayed or a neutrino (i.e. just nothing) in order to simulate a v0CC, a vTCC or a NC event respectively. The systematic uncertainties coming from the above replacement a.re ma.inly related to the lepton quality and to the charge effects in the hadronic system for NC events and are reduced by applying exactly the

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same procedure to a Monte-Carlo sample of vµ event (MCS) . All the final signal and background efficiencies are then obtained through the formula:

tns t = tMc --

fMCS

which implies that efficiencies for the lepton reconstruction and an approximate estimate of the hadron reconstruction are obtained from the monte Carlo (tMc), while any difference between the data and the l\IC is taken into account through the ratio cns/t,\fCS·

Since the r -+ µ-iiµvT channel, which could contribute about to 10% of the total sensitivity, is dominated by the v,,CC background for which the above technique cannot be applied, this channel is not included in the final combined results .

. {2 T -+ e-VeVT decay channel

The electron candidate is defined as a negative charged track of p > l.5GeV/c which is well identified by the TRD (probability to be a 'Ir- < 10-3), PS (signal above m.i.p.) and ECAL (shower-shape and (E-P)/(E+P) cut) . The background contamination coming from photon conversions and Dalitz decays of 7ro is mostly rejected by the primary vertex association and by the requirement that the invariant mass formed between the electron candidate and any other track of opposite charge must be > 120MeV/c2•

Further reduction of neutral current background is obtained by imposing an isolation con­dition in the plane (B.h , q10p) , which require that both the spatial opening angle B.h and the transverse momentum q10p of the candidate with respect to the hadronic jet are large (Figure 2).

i ::� rnµ NC I ::; rnokeN

0.1 0.1 u u � ::: . ::: . :- I 1• .., . . .., . a a � . : · � . I O O 2 4 I B 10 O O 2 4 I a 10 � 10

.-r-----,,------'qu=c..•_(G_•V)� 11uP (GoV) ! � ! � � � 'ii o.a .. 0.1

0.1 o.• 0.4 0.3 0.2 0.1

'

i

e a to ql.EP (GeV)

0.1 0.1 0.1 O.• 0.4 0.3 ... 0.1

I I 10 llL£P (GoV)

-15 -10 -5 5 10 15 20 25 30 log llkollhood RaUo

Figure 2: On the left: the isolation cut in the plane (9,h, q1,p) for the T -+ .-;;,.,, DIS search: the selected region for the signal is the one bounded to the right by q1,p = 5GeV. On the right: the likelihood ratio r.,,, for the data (points), the background corrected with the DS method (hatched area) and the simulated T signal (solid line) is

shown on the right.

The remaining sample is the entirely dominated by primary electrons from v0 CC interac­tions. These are reduced by requiring consistency with the T mass (0.2 < MT < l .8GeV/c2) and by rejecting electrons with q/ep > 5GeV/c. A likelihood ratio £T/• is then evaluated under the T signal and the v. background hypothesis. It uses the distributions of the electron momentum, of the ratio of electron over the jet transverse momentum, of the transverse angles </>eh between

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electron and jet and <Pmh between jet and missing transverse momentum, of the missing trans­verse momentum itself, and of the total visible energy. A cut at log .Cr/e > 6.5 is seen to reject the v0CC background by two orders of magnitude while keeping 46% of the signal (figure 2).

4.s r --+ ir- (mr0)vr decay channel

A ir- candidate is defined as a leading primary track with a negative charge capable of reaching the muon detector and with momentum p > 3GeV/c. The rejection of vµCC and v0CC interac­tions in which the leading lepton is not identified is obtained by imposing that no track in the event with PT > O.SGeV/c and well isolated from the hadronic jet escapes the muon or electron detection acceptance. In addition, a further lepton veto based on TRD, PS, ECAL and on the muon chamber information is applied.

IO

0.5 2.5 3 a, (GeV)

Figure 3: The Qr distribution in the DIS T -t .-- (mr0)v, analysis for the 1995 data (points), the background (solid line) and the simulated signal (dashed line).

The remaining background is then mostly due to NC events where one of the tracks in the hadronic jet is mistaken for the hadron of the r decay. This is first reduced by the requirement that the missing PT of the event is < 2GeV and by a cut on the transverse mass at Mr < 2.5GeV/c2• A strong isolation condition on the transverse momentum Qr of the candidate with respect to the total visible momentum (Qr > 1 .7GeV/c) eliminates the remaining NC background (Figure 3) .

4.4 The limit for Vµ --+ Vr oscillations

We express the results of the measurements described above as a frequentist confidence interval 9 by optimally combining the measurements for each channel, taking into account the number of observed signal events, the expected background and its uncertainty, and the number of expected signal events if the oscillation probability were unity. This last quantity is given by

(1)

where N µ is the number of vµCC interactions, after a correction for muon identification efficiency, urfuµ is the kinematic suppression factor du to the difference in the r and in the µ masses and Br and E are respectively the branching ratio and the selection efficiency for the r decay

�07

Table 1: Summary of the T appearance search in the 1995 data sample. Nmas is the maximum number of expected signal events ace d" · 1 d DIS d LM f h DIS d I It" Ii · t pies respectively. or m11: to equation an an re er to t e an ow mu 1p c1 y sam

Cha.nnel Efficiency Ba.ckground Ca.ndida.tes Nma:r r -+ e DIS 4.6 ± 0.9 0.6!g:; 0 635 r -+ e LM 3.6 ± 0.6 o.5!g:� 0 218

T -+ ir(iru) DIS 1 .2 (0.3) 0.2!�:� 0 173 T -+ ir(ir0) LM 3.5 (0.5) o. 1!g:� 1 198

T -+ p DIS 0.8 ± 0.2 0.5!�:� 0 128 T -+ 3ir DIS 1.6 ± 0.3 0.4!g:1 0 122 T -+ 3ir LM 2.0 ± 0.3 0.4!g:: 0 108

TOTAL 2.7!9:8 I 1582 I

cha.nnel under considera.tion. The systema.tic uncerta.inty on Nmax is mostly rela.ted to the signa.l detection efficiency a.nd is a.bout 15%.

The confidence interva.I is ca.lcula.ted a.ccording to the prescriptions of reference9 by repla.cing the likelihood ra.tio with a. genera.lized form 10 to a.ccount for the uncerta.inties in the ba.ckground estima.tion . The resulting 90% confidence level upper limit corresponds to 3.3 events which lea.ds to a.n oscillation proba.bility:

Pm(v,. -+ vT ) < 3.3/1582 = 2. 1 X 10-3 which corresponds to sin2 29,.T < 4.2 x 10-3 for large �m2• Figure 4 shows the corresponding exclusion region in the plane (�m2, sin229).

10' CHORUS I ,.: No fl_ :' , · . " i\ ' ·'" .( t'.">·llARM II ccA�. =. :,

�"'�.·.··· ...... :- ··· .... :·.�

10

"····· ... l'DI is"··· ··· ... Figure 4: Region of the plane (�m2, •in228) currently excluded by NOMAD data at 903 CL (solid line). The

limits achieved by other experiments 3•1 1 are also shown.

5 Search for v,. -+ v, oscillations

This sea.rch relies entirely on the knowledge of the beam chara.cteristics since the v. appearance is inferred from a comparison between the observed and the expected spectra. The CERN wide­band neutrino beam 12 is obta.ined by bombarding a. Beryllium ta.rget with 450 GeV /c protons,

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thus producing many secondary particles (mainly pions and kaons). Positive particles are focused by a system of magnetic lenses and then decay in a vacuum tunnel. Residual charged particle are finally stopped by a thick shield placed before the experimental site, where only neutrinos can penetrate.

'• - __ .. _ --i-wca.vi

1 0

_, 10

NOMAD

LSND allowed region

sin'21'1 Figure 5: On the left: Parent particles contribution to the different beam components. On the right: the preliminary exclusion plot in the plane (Li.m2, sin2 29) corresponding to 1995 data only is shown together with

the LSND region.

A Monte-Carlo package has been developed to describe the entire beam line: it is based on the GEANT 13 library for the detectors and the material description and makes use of the · FLUKA 14 package to simulate hadronic interactions. The predicted energy spectra for the different beam components are showed in figure 5. The I</rr ratio constrains the v0 fraction in the beam and hence it is one of the most crucial parameter for this analysis. The uncertainty on this ratio results on a systematic uncertainty of 10% in the interesting kinematic region. A precise measurement of the K/rr ratio was recently obtained by the NA56 collaboration 15 and will be included in our analysis.

If oscillations occur with parameters in the large 6.m2 range of LSND solutions, a significant effect should be observed in the measured veCC total energy spectrum. For example, in case of oscillations with sin2 2ll = 0.006 and 6.m2 = 19eV2, the number of veCC would approximately double for 10 < E,, < 40 GeV. In order to reduce systematic uncertainties, it is preferable to study the ratio between the number of Ve and Vµ. CC as a function of the total energy:

Re (E,,) = [ # ve CC events ] (E,,) µ. # vµ. CC events

A statistical comparison of the measured Reµ. with expectations for different values of the oscillations parameters allows to compute an exclusion plot in the (6.m2, sin22ll) plane. A preliminary curve, which takes into account a 10% systematic error on the K/rr ratio, is shown in figure 5. The LSND allowed solutions are also drawn for comparison on the same plot.

6 Conclusion

An analysis of the 1995 data sample allows to set an upper limit on sin2 2ll < 4.2 x 10-3 for vµ. -7 vr oscillations at large 6.m2 (90% CL) 8 • The IIµ. -7 lie search in the same sample provides

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no evidence for oscillations, giving an upper limit sin2 20 < 1.8 x 10-3 at large /lm2 (90% CL) and excluding that the claimed LSND iiµ -+ iie oscillation signal is associated with oscillations for /lm2 > 10eV2• The NOMAD experiment is currently taking data. The search for oscillation is in progress using the 1996 and 1997 samples, which will increase the available statistics by about a factor 6.

References

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

2. M.C. Gonzales-Garcia and J .J . Gomez-Cadenas, Phys. Rev. D 55, 1297 (1997) . 3. E531 Collaboration , N. Ushida et al. Phys. Rev. Lett. 57, 2897 ( 1986) . 4. C. Athanassopulos et al . , LSND Collaboration Phys. Rev. C 54, 2685 (1996) , Phys. Rev.

Lett. 77, 3082 ( 1996) . 5. NOMAD Collaboration, J . Altegoer et al. Nucl. lnstrum. Methods A 404, 96 (1998). 6. G. Bassompierre et al., Nucl. Instrum. Methods A 403, 363 (1998);

G. Bassompierre et al . , LAPP-EXP-97-06 (1997), submitted to Nucl. /nstrum. Methods. 7. D. Autiero et al., Nucl. /nstrum. Methods A 372, 556 ( 1996);

D. Autiero et al., Nucl. Instrum. Methods A 373, 358 (1996) ; D. Autiero et al. , Nucl. Instrum. Methods A 387, 352 ( 1997).

8. NOMAD Collaboration, J. Altegoer et al. CERN-EP/98-57; to be published in Phys. Lett. B .

9. G.J. Feldman and R.D. Cousins Phys. Rev. D 57, 3873 ( 1998). 10. M.G. Kendall and A. Stuart, The advanced theory of statistics Vol. 2, Inference and

Relationship 3rd Ed .. (Hafner Publishing, new York 1973), p. 243 Eq. 24.4. 1 1 . CHORUS Collaboration, E. Eskut et al. CERN-PPE/97-149;

submitted to Phys. Lett. B; CHARM-II Collaboration, M. Gruwe et al., Phys. Lett. B 309, 463 (1993); CCFR Collaboration, K.S. McFarland et al. Phys. Rev. Lett. 75, 3993 { 1995); CDHS Collaboration, F. Dydak et al. Phys. Lett. B 134, 281 { 1984).

12. G. Acquistance et al . , CERN-ECP/95-14 (1995) . 13. GEANT 3.21, CERN Progr. Libr. W5013 (1994) . 14. A. Ferrari and p.R. Sala, Proc. MC93 Int. Conf. Tallahasee Florida (1993) . 15. SPY Collaboration , G . Ambrosini et al . , CERN-EP /98-18;

submitted to Phys. Lett. B.

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