Experimental search for neutron-antineutron transitions with free neutrons

Volume 156B, number 1,2

PHYSICS LI-TTERS

E X P E R I M E N T A L SEARCH FOR N E U T R O N - A N T I N E U T R O N WITH FREE N E U T R O N S

13 June 1985

TRANSITIONS

CERN- ILL- Padova- Rutherford-Sussex Collaboration G. F I D E C A R O , M. F I D E C A R O , L. L A N C E R I , A. M A R C H I O R O CERN, CII- 121l Gene~'a 23, Switzerland

W. M A M P E Institute Laue- LangeL,in, 156X, F-38042 Grenoble Cedex, France

M. B A L D O - C E O L I N , F. M A T T I O L I , G. P U G L I E R I N Dipartimento di Fisiea G. Galilei. 35131 Padua, lta O, and Istituto Nazionale di Fisica Nueleare, Sezione di Padova. 35131 Padua, Italy

C.J. B A T T Y , K. G R E E N , H.B. P R O S P E R , P. S H A R M A N Rutherford Appleton l~aborato~.', (,'hilton, Didcot. Oxon OXI10QX, UK

J.M. P E N D L E B U R Y

a n d K.F. S M I T H

School of Physical and Mathematical Sciences, University of Sussex. Brighton BNI 9QH. UK

Received 11 March 1985

The observation of neutron-antineutron transitions would be direct evidence for bar2,,on number violation. For the first time an experiment has been carried out to search for this phenomenon with neutrons in free flight. The experiment using the research reactor at the Institut Laue-Langevin in Grenoble has set a lower limit to the oscillation time r,n of 106 s.

1. I n t r o d u c t i o n . Baryon number non-conserving interactions may induce n e u t r o n - a n t i n e u t r o n mixing and consequently n e u t r o n - a n t i n e u t r o n oscillations, which would occur as a first-order process through a AB = 2 interaction. The theoretical implications have been discussed phenomenologically [1] and in the context o f unified theories where, for example, the l e f t - r i g h t symmetric model o f Mohapatra and Marshak [2] predicts oscillation times as low as 105 s. The mixing is characterized by a mass splitting between pure baryon states e = (filHIn) ,

and a corresponding n e u t r o n - a n t i n e u t r o n oscillation time rnfi = l/e. 122

Experimental limits for AB = 2 processes have been given by the experiments measuring nuclear stability lifetimes [3]. These can be interpreted in terms o f n e u t r o n - a n t i n e u t r o n oscillation times [4,5] to give a 90% confidence level lower limit to rnfi in the range ( 2 . 7 - 1 1 ) × 107 s. It must be stressed however that this limit rests on nuclear model assumptions [4, 5 ] and that "rnfi may be determined in a model independent way only by searching for n e u t r o n - a n t i neutron oscillations with free neutrons [6]. The experiment reported here is the first using free neutrons and with oscillation limits rnfi of order 106 s is sensitive to mass scales in the region of 105 GeV through the simplest effective lagrangian [7,8] for A B = 2 processes of 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Volume 156B, number 1.2

PHYSI('S LFTrt.,RS

hef f ~- M~5(qqq-qqc0 + h.c.

2. Experimental method. As a consequence of a AB = 2 interaction an initially pure neutron state (B = +1) will in time acquire an antineutron (fi) component (B = - 1 ) with probability P(fi, t)

~l;'Z)l

e(fi, t) = [ez/(e 2 +

sin2((e 2 + AE2)I/2t),

(1)

where 2 A E is the energy difference between the n and fi states due to external field perturbations. These external interactions may be magnetic, acting through the equal but opposite n and fi magnetic moments, or nuclear through the differing n and fi strong interaction properties. For totally free neutrons A E = 0 and then P(fi, t) : sin2(et) = (t/7nfi)2

,

if t ' ~ r n ~ .

(2)

Although the condition that neutrons are free is never satisfied in nature and AE is much larger than e, neutrons can be considered as free for a time (t) such that AE.t ,~ 1 ("quasi free neutron" condition). In this case eq. (1) reduces to the free neutron relation of eq. (2). In practice the experiment seeks to observe the number of antineutrons N(fi, t) in an initially pure beam of neutrons of intensity On s - I after a free flight time t per neutron during a data recording time T N(fi, t) = (~n(t/Znr~)2T'n, where 77 is the efficiency for detecting antineutron events. The experimental requirements are thus to maxiraise ~bn and t whilst subjecting the neutrons to minireal magnetic and nuclear field perturbations, such that AE.t < 1. The fi an~plitude growth can however be suppressed by the application of an external magnetic field (H) as H -2. These relations are the basis for experimentally observing the phenomena and verifying the validity of any observed signal.

3. The experiment. The experiment was carried out using the 57 MW research reactor at the Institute Laue-Langevin (ILL) in Grenoble. Cold neutrons,

13 June 1985

moderated in liquid deuterium at 25 K, were transported from a position close to the reactor core to the experimenr, d area by reflection through a curved system of neutron guides of 3 × 20 cm 2 cross section which eliminate all q,'s and fast neutrons coming directly from the reactor. The beam intensity was 1.5 × 109 neutrons/s with a mean wavelength of 24.6 A corresponding to an energy of 1.4 X 10 - 4 eV and velocity 161 m/s. A diagram of the experimental arrangement is shown in fig. 1. Neutrons from the exit of the curved guide were transmitted in vacuum through a straight guide 4.5 m long and then drifted freely in a vessel 2.7 m long into the beam dump. The mean squared time of flight of the neutrons from their last reflection in the neutron guide to the beam stop was (t 2) = 6.8 X 10 - 4 s 2. To reduce external energy perturbations due to neutron-nucleus collisions to a negligible level the beam guide and vacuum vessel were evacuated to a pressure of less than 10 -5 Torr. The magnetic field in the region of the beam was reduced to a value of less than 10 - 3 G by a triple-layer/~-metal shield surrounding both the straight neutron guide and the vacuum vessel. The resulting external perturbing energy AE was less than 6 X 10 -15 eV for all neutrons. A coil wound outside the vacuum vessel, but inside the shield, could be used to provide a magnetic field which would suppress any n-fi oscillations if they existed. After drifting freely the beam hit the dump well inside the target area of 32 X 50 cm 2. This was determined separately from the main experiment by gold foil activation measurements and by direct n + 10B -', a + 7Li reaction profiles. In the early stages of the experiment (phase I) the dump was made of 3 mm thick polyurethane sheet enriched in B4C and covered at the centre by an additional 20 cm diameter disc of 6LiF (enriched to 95%). In the later stages of the experiment (Phases I1 and III) the whole beam area was covered by 6LiF. For the latter case, capture of the incident beam of 1.5 X 109 neutrons/s led to radiation from the beam stop [9] o r a l . 5 X 105 neutrons/ s with energies up to 16 MeV and 6 X 104 "t's/s with energies up to 8 MeV. Antineutrons generated by n il transitions are expected to annihilate in the target with a cross section of 104-105 barns [10] to give [11] on average 5 pions. In the first stage of the experiment (phase I) a cal123

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13 June 1985

orimeter (not shown in fig. I)was used to detect events from the beam stop with a large energy release; the latter being taken to be an indication of a possible antineutron annihilation. Tile energy sampling calorimeter consisted of a total of 40 sheets of alternate 5 mm thick plastic scintillator and lead covering an area of 80 × 80 cm 2 and ~25% of the total solid angle from the target region. Cosmic rays were used to calibrate the calorimeter system and to monitor its gain stability. To reduce the number of background events due to cosmic rays the whole apparatus was shielded by an array of 60 plastic scintillation counters covering a total area of 30 m 2. The overall efficiency of the veto system was measured to be greater than 99.95%. To prevent possible antineutron events being rejected by self-veto, due to the annihilation products being detecied by the anticoincidence shield, a 20 cm thick layer of cast iron was placed between the anticoincidence shield and the apparatus. It was not possible to place further shielding outside the anticoincidence counters because of restrictions on the permissible reactor floor loading. The analysis of the calorimeter data [ 12] indicated that in order to reduce the number of background events, there was a need for a track-recording detector in order to recognize and reject events produced by cosmic rays. Therefore the calorimeter was subsequently replaced (phases I! and IlI) by a relatively fine grain detector whose main feature was to give spatial resolution in order to allow track pattern recognition and vertex reconstruction. This detector consisted of an array of limited streamer tubes [ 13] of 0.9 X 0.9 cm 2 cross section arranged in modules of four planes, interleaved with AI plates 5 mm thick and covering 1.5 X 1.0 m 2. Two of these modules were placed downstream of the target and four around the target in a square box geometry. The two modules immediately downstream of the target were followed by a further module consisting of ten planes of similar streamer tubes interleaved with 5 mm thick Fe plates which acted as a coarse energy calorimeter. Also in the forward direction, placed immediately after the first/~-metal shield, was a 12 X 12 element (48 X 4 X 0.6 cm 3) scintillation counter hodoscope to give additional positional information. Further scintillation counters (labelled T and B in fig. I) placed before the A1 and Fe streamer tube modules

Volume 156B. number 1.2

PIIYSICS I.E'I'TERS

were used as part of the trigger system. The trigger was a coincidence between counters T, B and the two hodoscope planes in anticoincidence with the cosmic ray veto counters. Laterally, surrounding the first/a-metal shield was a "barrel" hodoscope of scintillation counters with 56 elemcnts (100 × 4 X 0.6 cm3). The lateral streamer chamber modules, already mentioned, were mounted outside the magnetic shield (see fig. 1). In the last phase (III) of the experiment the B counters were removed to incrcasc the detection solid angle. An additional set of scintillation counters, placed behind the lateral sets of streamer chamber modules were also included in the trigger. This together with a chamber requirement allowed for either forward or lateral charged particle tracks to trigger the system. In phases I and 11 of the experiment any antineutron component in the neutron beam would be cxpected to annihilate in the beam stop. Analysis of the phase I1 data showed that much of the background came from the interaction of neutral cosmic rays in the region around the beam stop. This background is, of course, proportional to the quantity of material in the interaction region. For the third phase a 125/am thick carbon target was installed 15 cm upstream from the beana stop. Any antineutrons would annihilate in this target whilst the neutron beam would pass through, largely unaffected. In this way it was hoped to reduce the background from the region around the antineutron annihilation target.

13 June 1985

4. Data selection. Preliminary results from phase I enabled an oscillation time limit rN~ ~ 105 s to be set and this has been reported elsewhere [12]. In phase 11 and I11 approximately 200 d w o r t h of data were taken, shared al,l~ost equally between signal data and background data. The latter was taken with either the reactor off or the reactor on but with a,1 fi sign',d suppressing magnetic field applied to the neutron beam. The two background data sets in each experimental phase were treated as one in this analysis since no difference was found between reactor on background and reactor-off background event rates once fi selection criteria were imposed. The experimental conditions and data taken are shown in table 1. All the events recorded were subjected off-line to a simple program filter. The scanning requirement was one track in the forward direction crossing >/8 planes of streamer tubes plus 2 other tracks elsewhere crossing at least 3 planes, one of which was backward of the fi annihilation region. The further requirement that the/>3 tracks meet at a vertex within a loose fiducial volume (Ixl. lyl ~< 25 cm; Izl ~< 10 cm; where x , y are transverse to the neutron axis and z is along that axis from the fi annihilation target) reduced the initial 1.2 X 106 triggers to 57 events which warranted detailed analysis. The vast bulk of the triggers were either random coincidences between the trigger planes or else cosmic ray interactions clearly originating in material outside the fi target region.

Table 1 Experimental conditions and data. Conditions/data

Phase II

Phase I11

fi annihilation "target neutron flux (n sec-1) [¢~n] neutron observation time 1/2 (s) signal data time [T] (s) background data time (s) no of event triggers no of selected events no of candidate events - signal background fi detection efficiency [r~l 90% confidence level limit (events) nh oscillation limit rnfi (s)

6LiF (1.5 -+ 0.2) X 109 (2.6 ± 0.1) X 10-2 3.9 X 106 4.9 X 106 396 099 35 2 5 (0.20 -+0.03) 1.9 ~6.5 X IOs

12C (1.4 ± 0.2) × 109 (2.5 -+ 0.1)x 10-2 5.5 x 106 4.2 x 106 805 459 6 1 2 (0.30 -+0.03) 2.1 ~8.2 x l0 s

-

combined result r2nfi= (1/<~>)[(¢bnT~)ll+ (OnTr~)lll] gives rnfi/> 1.3 X 106 s with = 1.3 events at 91~o CL

125

Volume 156B, n u m b e r 1,2

PItYSICS LI.XFERS

The tracks o f these events (47 from phase II and 10 from phase III) have been measured by physicists and reconstructed to a common vertex (x,y, z). The consistency o f measurement and orthogonal view agreement shows an average reconstruction accuracy of-+45 mm in all three coordinates. In table 2 are listed the vertex coordinates of those 41 events, whose coordinate measurements agree within -+10 cm.

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Table 2 List of the selected events. Vertex coordinates.

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These events are equally distributed between signal and background conditions and there is no evidence to suggest the presence of a real fi signal. Figs. 2 and 3 show the distribution of the measured vertices as axial (z) and radial (r) coordinates for the 41 selected events. The figures show clearly a peaking of event vertices around z = 0 and radially in proportion to the area. This reflects the distribution in mass o f the neutron beam dump, vacuum tank walls and/a-metal shields. The events are thus consistent with a background dependent on the mass of material around the neutron beam. The axial event rate at large z value (z > 10 cm) in phase II is also in agreement with the event rate in phase ili and consistent with the hypothesis that these events are due to secondary interactions in the material which give reconstructed vertices in the vacuum region. The expected distribution of fi

Volume 156B. nt, mber 1.2

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A combined analysis for the two phases with 3 signal events and 6.6 normalised background events gives 1.3 fi events as the 90% confidence level limit [ 141. This sets a limit to the oscillation time rnfi > 1.3 X l06 s (90% CL). A likelihood ratio test [15] has also been performed on an enlarged event sample lying in the full fi region ( r < 200 mm and Izl < 100 mm). Taking into account the expected axial and radial distributions for fi annihilation and background events, a 90% upper limit o f 3.0 fi events and an oscillation time r n f i > 0.9 X 106 s has been obtained. These two alternative analyses of the same data are self-consistent and therefore in conclusion we set an overall 90% confidence limit of 7"nil > 1 × 10 6 s .

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13 June 1985

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This result is the first data published on n e u t r o n antineutron oscillations with free neutrons and at a level some thousand times rarer than neutron/3-decay shows no evidence for AB = 2 processes.

r (ram)

Fig. 3. Integral distribution of selected events as a function of the radial distance from the beam axis. A uniform distribution of 0.020 -+ 0.003 events/cm2 is consistent with the data (dotted line). The event selection criteria reduces the acceptance for r > 250 mm. The full line represents the integral probability distribution of annihilation vertices derived from the neutron beam prof'fle. event vertices, weighted by the annihilation probability over the neutron beam profile and smeared by the measurement resolution, is shown in fig. 3. Accordingly final selection cuts of Ixl < 100 ram; lYl < 150 ram; Izl < 90 mm are expected to contain (90 -+ 5)% of possible fi events. This leaves 7 events in the phase 1I and 3 events in the phase 11I data sets.

5. Results. The results are summarised in table 1. The efficiency of the detection system to fi annihilation events was estimated separately for the two phases through a Monte Carlo process using experimental 0 - n u c l e o n annihilation cross sections and a simulation of the experimental apparatus. The validity o f this process was verified by measuring the detection efficiency of a simplified form of the apparatus to ~12C annihilations in the K23 stopping ~ beam at CERN. The calculated and measured efficiencies cross check to within 10% o f their values.

We thank the directors and staff at the Institute Laue-Langevin in Grenoble for their support during the experiment. The experiment would not have been possible without the skilled technical assistance from our laboratories; A.I. Kilvington, C.A. Baker, J. Moir and A.G.D. Payne from RAL; M. Renevey and P. Dechelette from CERN; L. Visentin, D. Filippi, R. Pavanello and G. Testa from Padova and the scanning teams from CERN and Padova. The experiment was funded jointly by CERN, the UK Science and Engineering Research Council and the lstituto Nazionale di Fisica Nucleare, Italy.

References [1] V.A. Kuzmin, JETP Lett. 12 (1970) 228; S.L. Glashow, in: Proc. Neutrino 1979, VoL 1, p. 518. [2] R.N. Mohapatra and R.E. Marshak, Phys. Rev. Lett. 44 (1980) 1316. [3] M.L. Cherrey et al., Phys. Rev. Lett. 50 (1983) 1354; G. Battistoni et al., Phys. Lett. 133B (1983) 454; T.W. Jones et al., Phys. Rev. Lett. 52 (1984) 720. [4] C.B. Dover, A. Gal and J.M. Richard, Phys. Rev. D27 (1983) 1090. [5] W.M. Alberico, A. Bottino and A. Molinari, Phys. Lett. 114B (1982) 266; WaM. Alberico et al., Nucl. Phys. A429 (1984) 445. 127

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PHYSICS LETTI-RS

[6] P.K. Kabir, Phys. Rev. Lett. 5 1 (1983) 231. [7] S. Weinberg, Phys. Rev. Lett. 43 (1979) 1566; Phys. Rev. D22 (1980) 1694. [8] F. Wilczek and A. Zee, Phys. Rev. Lett. 43 (1979) 1571; Phys. Lett. 88B (1979) 311. [9] M.A. Lone et al., Nucl. lnstrum. Methods 174 (1980) 521. [10] A.S. ll'inov et al., Sov. J. NucL Phys. 36 (1982) 513. [11] L.E. Agnew et aL, Phys. Rev. 118 (1960) 1371; H.J. Besch et al., Z. Phys. A292 (1979) 197. [ 12] G. Fidecaro, Proc. Intern. Conf. on Neutrint~ physics and astrophysics Vol I (Hawaii, 1981) p. 264;

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M. Baldo-Ceolin, Proc. ICOBAN Meeting (Bombay, 1982) p. 197. [131 G. Battistoni et al., Nucl. Instrum. Methods 176 (1980) 297. [ 14] H.B. Prosper, The distribution of the difference of two Poisson variates, Nucl. lnstrum. Methods, to be published; and ILL internal scientific report-84PR 14S. [ 15 ] M. Baldo-Ceolin, F. Mattioli and G. Puglierin, Padova internal report PD8416; M. Fidecaro and ll.B. Prosper, CERN EP internal report 85-03.