Volume 112B, number 1 PHYSICS LETTERS 29 April 1982

LIMITS ON NEUTRINO-ANTINEUTRINO TRANSITIONS

FROM A STUDY OF HIGH-ENERGY NEUTRINO INTERACTIONS

A.M. COOPER, J.G. GUY, A.G. MICHETTE 1, M. TYNDEL and W. VENUS Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, England

Received 4 January 1982

We distinguish electron and positron tracks from v e and ve charged-current interactions in a large bubble chamber filled with a neon-hydrogen mixture. No evidence is seen for an excess of ~e interactions from primary fie sources or secondary v# ~ Ve or v e --* ~e oscillations. Upper limits on oscillation parameters, majoron emission and lepton-number violating *r and K decays are presented.

In the context o f gauge theories of grand unification or of lep ton-quark substructure even the best established conservation laws not directly implied by the gauge structure become suspect. Indeed the search for their violation, for example in proton decay or neutrino mix- ing, becomes of prime importance. In addition the ob- servation of neutrino mixing would imply massive neu- trinos which could be of cosmological significance. Thus it is particularly important to investigate all possibilities o f neutrino mixing.

One possibility that has been discussed theoretically from several viewpoints [ 1 ] is that neutrinos produced in normal decays could sometimes interact as antineu- trinos, perhaps antineutrinos o f a different fiavour, or else oscillate into states that do so interact.

Oscillations leave the helicity of a neutrino essentially unchanged; For such a process to occur with a probability higher than O(mv/E)2 therefore requires the presence of some right-handed leptonic charged current. Theo- retically such a current arises naturally in many grand unified theories and leads to neutrino mass and neutrino mixing. It could, for example, be due to a massive right- handed W boson or to mixing with mirror fermions with right-handed couplings. Experimentally it is admissible at the 10% level event in low-Q 2 meson decay ampli- tudes [2]. A scalar coupling mediated, for example, by charged Higgs exchange could also serve and is equally

I Now at Queen Elizabeth College, London.

admissable experimentally. Alternatively neutrino mass may be associated with

the majoron. Majoron emission at production or inter- action point would change the helicity o f the neutrino, thus changing the neutrino into an antineutrino of the same or different flavour [3].

This letter reports new experimental limits on v u Ve and v e -~ Ve oscillations and transitions. The data

are from BEBC filled with a 74 mole percent n e o n - hydrogen mixture and exposed to the CERN SPS wide- band neutrino beam. This beam has a small predicted flux of re" The predicted ratio of Ve : Ve : vu events is 1 : 30 : 1800 hence secondary vu -~ Ve or v e ~ Ve os- cillations cannot be masked by a large background of primary ~e- The same film has already been used to study4a_-e + events [4], to measure the vu total charged- ¢U~---eross section [5] and to search for vu ~ re, vu o v r and vu -~v x oscillations [6]. Thus the experimental details have all been reported previously excepting only the criteria used for distinguishing between v e and Ve events (i.e. between high-energy e - and e+).

The BEBC was operated to give thin continuous tracks and maximise the spatial resolution. As a result, and because o f their rapid energy loss in the heavy liq- uid, (X 0 about 42 cm) and the high magnetic field (3.5 Tesla), the charges of most electrons were evident on inspection. In the few cases of ambiguity due to early shower development the unambiguous part of the elec- tron track was measured precisely and reconstructed.

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Volume 112B, number 1 PHYSICS LETTERS 29 April 1982

In 2% of the cases the electron showered so early that its track could not be followed far from the vertex un- ambiguously, and measurement over this short unam- biguous length o f track gave a sagitta value with a frac- tional error exceeding 70%: these tracks were considered to be o f unidentified charge.

Using these criteria, 104 unpaired single e - or e*- events (v e candidates) and 18 unpaired single e + events (Ve candidates) were found on the film. These events were inside the kinematical cuts: electron energy above 1 GeV and corrected neutrino energy above 10 GeV. The same film was estimated to contain 6060 -+ 440 charged-current vu interactions over 10 GeV, a few of which would appear as background in the 1) e o r Ve sam- ples if they had semi-leptonic charm decays, close comp- tons, or asymmetric e+e - pairs. To remove this back- ground, events were discarded if they contained a # - candidate *l whose momentum or transverse momentum exceeded that of the electron. The loss o f true I) e and Ve events due to this cut was estimated as (3 -+ 2)% from the effect o f this cut when interacting rather than non- interacting negative tracks were considered as # - can- didates. The only other appreciable losses were due to event scan efficiency (2 + 2)%, electron identification efficiency (12 + 4)% and confused events (2 + 2)%. After all cuts the residual background was negligible (about 1 event) and there remained 89 v e candidates and n o Ve candidates (see table 1).

The calculated beam composition predicts 73 -+ 8 v e events and 2 -+ 1 fie e v e n t s inside the cuts from conven-

¢1 Data f rom the EMI were not consistent ly available and were not used. A # - candidate was defined as any negative or am- biguous sign particle leaving the chamber wi thout interacting.

Table 1 The numbers o f events with unpaired electron tracks found on the film with electron m o m e n t u m over 1 GeV and neutr ino energy over 10 GeV. Pt and P~2 are the transverse and longitu- dinal m o m e n t u m o f the electron (e) and # - candidate (L) with respect to the v beam direction.

e - and e +- e +

all events found 104 18 events with Pte ~> PtL 93 1

or P~e > P~L events with P t e > PtL 89 0

and P~e > P~L

tional sources, consistent with the numbers observed. Since no #e events from either conventional or uncon- ventional sources were observed, upper limits on uncon- ventional Ve sources are given below at a 90% confidence level corresponding to less than 2.3 observed events.

I n general, events from unconventional sources would not have the same energy spectrum as the v e events nor the same y distribution, and would consequently not have quite the same probability of being detected within the kinematical cuts. Monte Carlo simulations were used to evaluate these effects.

In order to express the results in terms of neutrino oscillations we assume that only two neutrinos have significant mixing. The mixing probability P(va, v#) of two observed weak charged-current eigenstates (va, v#) is then related to their flight path I in metres and energy E in MeV by P(va, v#) = sin220 sin2(1.27 lA/E) where Aii is the difference o f the squared masses of the mass eigenstates i and j and Oii their mixing angle [7]. We therefore suppose that an initial left-handed V#u beam may contain, after some flight path 1, a fraction of left-handed antineutrinos ~eL" The charged-current interaction cross section for such VL would be zero if V - A were exact. We further suppose a V + A current enables the VeL to interact with the usual cross section

5OO

I00

50

5

, _ 0 5

i I I I 0"2 0"4 0"6 0"8 I'0

a2sin 2 2e

Fig. 1. The 90% conf idence l~nits showing the a l lowed regions (below the curves) for the mixing angle and mass squared differ- ences for the oscillations shown.

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Volume 112B, number 1 PHYSICS LETTERS 29 April 1982

Table 2 Upper limits on the oscillation parameters for small and large mass squared differences A (at the 90% confidence level). See also fig. 1.

(re, Z3eL) (v#, ~eL) A

.P(v, ffeL) < 0.03 < 0.0005 small a,x sin 20 < 7 eV 2 < 0.7 eV 2 small

P(v, VeL) <0 .03 <0.0005 large a 2 sin 220 < 0.05 < 0.001 large

Table 3 Upper limits on lepton-number violating 7r and K decays (at the 90% confidence level).

Decay mode Limit on branching fraction \

rr+ "-+ n+ + ~e <0.0015 rr ÷ -~#+ + v e <0 .008 K+ "+#+ + ~e <0.0033 K ÷ --,/~+ + v e <0.012 K+ '+ e+ + no + ~e <0.003

for v# times a factor et2 where et could be as high as about 0.1 [2]. The results are shown in table 2 and fig. 1. Allowance for a possible V + A component in the neutrino product ion process would tighten the limits quoted.

The upper bound for production by v# of an e ÷ in- stead o f a # - , o ( v u + N ~ e + + X ) / o ( v u + N ~ # - + X) is 5 × 10 -4 at 90% confidence level for events with neutrino energy above 10 GeV, electron energy above 1 GeV and a mean beam energy for observed vv events of 46 GeV. From this a limit on the majoron coupling g2 e can be deduced. After allowing for the factor 0.8 loss in acceptance due to the missing energy carried by the majoron we found, following Barger et al. [3], g~e is less than 0.03 for a common neutrino mass m v of 10 eV, and m x = 100 keV.

The absence o f any excess ~e interactions may also be used to set limits on any zr or K decays producing these neutrinos. The ratio o f primary n -->/~ + v u and K -->/a + vg decays responsible for the observed vg charged-current events is 7 : 3. The limits on various decays of the parent 7r and K mesons which would violate normal lepton number conservation rules are given in table 3, assuming here that any losses of ve and v e due to oscillations are negligible.

In conclusion we see no evidence for n e u t r i n o -

antineutrino oscillations in the CERN SPS wide-band

neutrino beam. eta sin 20 for v. -+ ffeL and v e ~ VeL

are less than 0.7 eV 2 and 7 eV ~', respectively, at the 90% confidence level. We place limits on majoron emis- sion and limits on some rr and K decays which do not conserve usual lepton numbers.

We would like to thank R.J.N. Phillips for several

enlightening discussions.

R e f e r e n c e s

[ 1 ] J.N. BahcaU and H. Primakoff, Phys. Rev. D 18 (1978) 3463 ; Dan-di Wu, Phys. Rev. D23 (1981) 2038; Phys. Lett. 96B (1980) 311; V. Barger et at., Phys. Rev. Lett. 45 (1980) 692.

[2] M.A. Beg et al., Phys. Rev. Lett. 38 (1977) 1252; T.G. Rizzo et al., Brookhaven Laboratory report, BNL 29/16 (1981); K. Enqvist et al., University of Helsinki preprint, HU-TFT- 81-18 (1981); S. Nandi et al., Fermi Laboratory report, FERMILAB-Pub- 81/53-THY (1981).

[3] V. Barger et al., Madison University report, MAD-PH/15 (1981).

[4] O. Erriquez et al., Phys. Lett. 77B (1978) 227. [5] D.C. Coney et al., Z. Phys. C2 (1979) 187. [6] O. Erriquez et al., Phys. Lett. 102B (1981) 73. [7] S.M. Bilenky and B. Pontecorvo, Phys. Rep. 41C (1978) 225.

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