Search for νμ — ντ oscillations in the CHORUS experiment

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PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 55C (1997) 419-423

Search for uu - u~ oscillations in t h e C H O R U S e x p e r i m e n t CHORUS Collaboration Talk presented by Ghislain Gr~goire a aInstitute of Nuclear Physics, Universit~ Catholique de Louvain, B- 1348-Louvain-la-Neuve, Belgium The CHORUS collaboration is searching for vu - vT oscillations through the appearance of r neutrinos in the intense uu beam at CERN. The detection technique and the performances of the various subdetectors are briefly described. The present status of a pilot analysis on data accumulated during 1994 confirms expectations as far as automatic measurements are concerned.

1. I n t r o d u c t i o n The properties of neutrinos are suspected to be linked to several important physics issues: the observed deficit of solar neutrinos w.ith respect to the predictions of standard solar models; the problem of atmospheric neutrinos where the observed ratio of v~- versus v~-flavors does not agree with the theoretical expectations; the possible contribution of massive neutrinos as constituants of hot dark m a t t e r in the Universe. A key point towards an explanation of these issues is a possible non-zero mass of the neutrinos. As far as previous observations are concerned, the present status of knowledge on neutrino masses is best summarized in a simplified model by exclusion plots expressing the oscillation probability from flavor a to flavor/3 as a function of a mixing angle 0 ~ and the difference A m 2 of the squares of the mass eigenstates

P(v~ --+ v~) = sin2(2Oa~) sin2(1.27~-~-~- - ) where L is the path length (in km) available for oscillation and E the neutrino energy (in GeV) (Figure 1 ). The CHORUS experiment is looking for the appearance of u~ neutrinos in the CERN Wideband v~ neutrino beam. Should vu - v~ oscillations occur then vr neutrinos are expected to be observed through the reaction v~ + N ~ T - + X followed by the decay of the T - lepton into one of the following modes: 0920-5632/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII: S0920-5632(97)00238-7

T - --+ # - ~ . v r ( 1 7 . 4 % )

T- ~ h-(nTr°)v~(49.8%) r - ~ 7r-~r+Tr- (nlr°)u~ (14.9%) These modes cover about 80% of all possible r lepton decays. They are unambiguously identified using the characteristic T decay topology and the corresponding missing transverse momentum carried away by the unobserved vr particle. 2. T h e C E R N W i d e B a n d n e u t r i n o b e a m The neutrino beam is obtained from the interactions of 450 GeV protons with a beryllium target. The charged secondaries, essentially pions, kaons and protons, are focussed (positive charges) or defocussed (negatives) by pulsed toroidal beam elements before reaching first an evacuated decay tunnel and then a 400-m long filter made of iron and earth. The average u, energy is 27 GeV. The energy spectrum of the dominant v~ neutrinos is shown in figure 2 together with the spectra of the contaminants ~ , v~ and ~ . The production of Ds mesons in the beryllium target also contributes to an intrinsic T-Ur contamination. We estimate its contribution to our data sample to be 3.3 x 10-cur charged current interactions per v u charged-current interaction. The average length L for oscillation is about 600 m.

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ENERGY/GeV Figure 1. Exclusion plot: the area to the right of the curves are excluded by the various experiments. At large Am 2 CHORUS expects to improve the present best limits (E531) [1] on sin2(2Bur) by a factor of about 30.

Figure 2. Energy spectra of the various components of the CERN Wide Band neutrino beam. Upper part: the ratios between the different components are u u : uu : ve : ue = 1.0 : 0.05 : 0.01 : 0.007. Lower part: computed energy spectrum of the vT contamination [2] .

3. The experimental setup The detector combines photographic emulsions and high resolution electronic detectors to identify neutrino interactions(Figure 3). Four massive (200 kg each) stacks of photographic emulsions act as a sensitive active target with a spatial resolution of the order of 1 micron. Each stack is followed downstream by 3 thin interface emulsion sheets and 1 or 3 interface fiber hodoscopes defining spatial coordinates along charged particle tracks(Figure 4). These coordinates can then be used to predict the vertex positions back in the emulsions. Further identification of the tracks is performed by detector elements located downstream: an ironfree spectrometer, a calorimeter and a muon spectrometer successively. At the end of 1994, after approximately 150

days of exposure, one of the four stacks was removed and replaced by a new one in order to start a pilot analysis. All four stacks were then removed at the end of the equivalent 1995 run. They were replaced by new emulsions for the 1996 and 1997 runs.

3.1. The target trackers Since they are located quite close to the target emulsions, the target trackers require a good two-track resolution and a high accuracy in predicting the vertex position in the emulsions. In total the target trackers have 32 planes arranged in six projections. Each plane consists of 7 staggered layers of 500 p m scintillating fibers covering a surface of 1.6 x 1.6m 2. The scintillation sig-

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nais are amplified by optoelectronic chains and recorded by CCD cameras. At the position of the impact point of the track on the interface emulsion sheets the accuracy obtained is about 220 #m for the transverse coordinates and about 3 mrad on the angle of the tracks. The construction and performances of the trackers are described elsewhere [3]. 3.2. T h e iron-free s p e c t r o m e t e r The momentum and charge of low momentum secondary particles from neutrino interactions are determined with the help of a pulsed air-core magnet. Its main purpose is to identify essentially hadrons and low-energy muons. Two additionai scintillating fiber hodoscopes are located downstream this spectrometer to perform charge and momentum reconstruction. Up to 10 GeV/c the probability to assign a wrong charge sign is measured to be less than (0.6 4-0.4)%. A detailed description of the iron-free spectrometer is given in [4]. 3.3. T h e c a l o r i m e t e r The calorimeter has 3 sections: the electromagnetic part and the first hadronic section are of the compensating type with scintillating optical fibers embedded in a lead matrix. In the third section layers of lead alternate with layers

Figure 4. Schematic view of the emulsion target, interface emulsion sheets CS and SS and a few scintillating fiber tracker planes.

of scintillators. The electromagnetic part has a thickness of about 22 radiation lengths while the thicknesses of the hadronic parts about 2.2 and 1.9 hadronic interaction length deep respectively. The calibrations of the different sections were performed with test beams; the intrinsic resolu13% for the electromagnetic section tions are ~ = ~7~ and ~ = -32% ~ for the hadronic parts with constant terms of the order of 1% in all cases. A detailed description of the calorimeter is given in [5]. 3.4. T h e m u o n s p e c t r o m e t e r It is located immediately downstream of the calorimeter. It is made of 6 magnetized iron toroids 50 cm thick each, separated from each other by 3 drift-chamber planes and 8 streamer tube planes. The latter improve the momentum resolution for low energy particles. The measured resolution, mostly dominated by multiple scattering, is about 15% for 20 GeV/c muons. 4. B a c k g r o u n d and s e n s i t i v i t y

Background events are generated by processes which simulate the decay topology of a ~- lepton. The background events fail into 3 cate~

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gories: (i) the large angle scattering of muons or pions without visible recoil inside the emulsion stacks ("white kinks"); (ii) the production and decay of negative kaons within the emulsions; (iii) the production of D - mesons followed by a decay with a negative muon and neutrals. This last mechanism originates only from the small ~, contamination in the beam and contributes to the background only if the positive muon from the charged-current P~ vertex escapes detection. Expressed as a fraction of the recorded chargedcurrent events the estimated contributions to the background are 3 × 10 -s for the first two mechanisms and 3 x 10 -7 for the last one. For the Ncc = 4.2 × 105 charged-current events recorded during the 1994-1995 runs (Table 1 ) we estimate the background to be about 1 event. On the other hand, on the basis of the present best limits[l] for the oscillation parameters (Am 2 = 40eV2; sin2(29~r) < 5 × 10-3 obtained by E531) we determine the number of expected candidate events taking into account all efficiencies (scanning, cuts ... ) and the relative charged current cross-sections (Table 2 ). The number of background events shown there does not include the effect of further kinematical cuts: for instance any T- lepton should balance a shower in the transverse plane while charmed mesons and white kinks tracks are in the shower. This additional cut should reduce the background contribution by an additional factor 5. After reconstruction of the impact points of tracks in the interface emulsion sheets these tracks are followed upstream into the emulsion target. At each stage, passing from the successive interface sheets to the bulk emulsion the accuracy to follow tracks improves and the scanning area can be reduced. All tracks are followed until they stop -presumably at the vertex of the events. The scanning of the interface and bulk emulsions is currently underway in Japan, Korea, Italy, Turkey and Russia. Fully automatic computer-steered microscopes in Japan are used here for the first time in an experiment. Video images are digitized and hits and tracks are reconstructed using digital image processing techniques. The estimates of the scan area, scanning time and tracking efficiencies are given in Table 3.

5. P r e s e n t s t a t u s of t h e analysis For the pilot analysis performed on the 1994 data we selected events with a single negative muon having an energy greater than 2 GeV. This implies that the results presented here deal only with 18% of the expected T-- candidates. At the time of this conference about 7600 vertex positions were found in the bulk emulsion. The impact parameter analysis and the search for kinks did not show any r candidate. To get an estimate of the sensitivity reached so far we took into account the various efficiencies for location and kink finding evaluated by numerical simulation. These efficiencies can be confirmed by an analysis of uu induced D + events. We reach then a preliminary limit of s i n 2 (28) < 10-2 , a factor of two above the E531 limit. The nearly negligible background also implies that the sensitivity of this experiment improves linearly with the statistics: the CHORUS experiment thus expects to reach its goal in improving significantly the present estimates of the oscillation parameters.

6. Summary CHORUS has taken data successfully for three years now. The emulsions exposed during 1994 and 1995 were developed. Scanning of the emulsions has been done for a selected sample of events with a single negative muon. Data analyzed so far confirm the feasibility of the automatic emulsion analysis. The new emulsions exposed during 1996 will be exposed again in 1997, thus doubling the presently available data sample. 7. A c k n o w l e d g m e n t s

This status report reflects the work of very many people including the accelerator staff and all technical personnel essential to the smooth operation of the experiment. Their contribution to the success of this experiment is gratefully acknowledged.

REFERENCES 1. N. Ushida etai, Phys. Lett. B 206 (1988) 375.

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G. Grigoire/Nuclear Physics B (Proc. Suppl.) 55C (1997) 419-423

Table 1 Statistics accumulated during the 1994-1995 runs 1994 Protons on target 0.9 x 10 TM % on tape 77% Main triggers 400000 Charged current events Ncc 150000

1995 1.1 x 10 TM 88% 547000 270000

Table 2 Efficiency, expected candidate and background events for the 1994-1995 runs T- decay mode Branching ratio Efficiency N~ #-~,v~ 17.4% 0.098 23 h - (n~r°)v~ 41.8% 0.046 29 ~r-lr+Tr - (nTr°)u~ 14.9% 0.065 12 Total : e x B.R. 82.1% 0.0496 64

Background 0.23 0.38 0.60 1.21

Table 3 Scanning area, measuring time and tracking efficiency for the pilot analysis of 1994 data Scanning step Scan area Time Present tracking ( # m x #m) (minutes) efficiency Fiber tracker to CS interface emulsion 1200 x 1200 2 87% CS emulsion to SS interface emulsion 600 x 600 1 80% SS emulsion to bulk emulsion 120 x 120 3 80% 2. B. Van de Vyver, P. Zuchelli, CERN-PPE/96-113 (submitted to Nucl.Instrum. Methods). 3. P. Annis etal, Nucl. Instrum. Methods A 367 (1995) 367. 4. F. Bergsma etal, Nucl. Instrum. Methods A 357 (1995) 243. 5. E. di Capua etal, CERN-PPE/95-188 (submitted to Nucl.Instrum. Methods).