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A SEARCH FOR NEUTRON-ANTINEUTRON OSCILLATIONS USING FREE NEUTRONS
H. Prosper
To cite this version:
H. Prosper. A SEARCH FOR NEUTRON-ANTINEUTRON OSCILLATIONS USING FREE NEUTRONS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-185-C3-187.
�10.1051/jphyscol:1984331�. �jpa-00224045�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6ment au n03, Tome 45, mars 1984 page C3-185
A
SEARCH FOR NEUTRON-ANTINEUTRON OSCILLATIONS USING FREE NEUTRONS
H.B. Prosper*
C ER N -ILL - Padova - R AL -
SussexCollaboration
Resume - Une experience pour mettre en Qvidence de possibles oscillations neutrons - antineutrons a CtC effectu6e sur un parcours de 6 m B l'interieur d'un blindage magnetique au moyen d'un faisceau (HI81 de neutrons froids du reacteur de I'ILL. Une limite g > 0,8 lo6 sec. 2 90% de confiance peut 8tre donnee B la pQriode r N ~ de superposition neutron - antineutron.
Abstract - A search for neutron-antineutron oscillations in a 6-metre long magnetically shielded region has been carried out using the HI8 cold neutron beam at the ILL reactor. A limit g > 0.8 x lo6 secs at 90% confidence level can be set for the neutron-antineutron mixing time
T ~ ~ .The simplest Grand Unified Theories predict a complete absence of particle phenomena within the energy range lo2
-t1015 GeV. This rather unlikely situation has prompted the construction of more elaborate GUT'S which allow, amongst other things,processes in which the baryon number can change by two units, a consequence of which is the allowance of mixing between the neutron and antineutron states, that is, they allow neutron-antineutron oscillations /1,2,3/. Estimates for the mixing time range from
-lo5 to -107 seconds. The experiment reported here was carried out to investigate the aforementioned range for
TNW.THE MIXING TIME AND BACKGROUND
Given a neutron beam of intensity N neutrons/sec one would expect to detect on average N antineutrons/sec within the beam after it has travelled for a time t, where
and A E = ~ / T ~ ~ ~ is the energy gap between the neutron and antineutron states arising from their differential interaction with some external field. The oscillation time
rose becomes identical to the mixing time T~a,rhodulusa factor of 2a, provided it can be arranged that rose >> 2Trtor that BM << h/2uNt if, as is the case for a free flight neutron beam, the main source of perturbation is due to the presence of a magnetic field BM. This condition leads to the following simple perturbation inde- pendent form for
which when averaged over the time of flight squared distribution of the beam gives the total average number of antineutrons expected per second. If we observe a beam of total intensity 0 neutrons/sec for a time T with a detector whose antineutron detection efficiency is
Ethe mixing time - T~~ is given by
where, the probing time, Tp - 0<t2> is a measure of the intrinsic sensitivity of the experiment.
*~uth&rford Appleton Laboratory, Chilton, Didcot, Oxon 0x11 OQX, U.K.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984331
C3-186 JOURNAL DE PHYSIQUE
An important consideration in this experiment is the question of the background level. In general <R>
=<E> - <B> where <E> and <B> are respectively the average number of events, inclusive of background, and the average number of background events . Let us assume, as in fact appears to be the case for the current generation of experiments,that we are in the regime in which <8> << 1. When <B>
=0 che limit
on <N> is independent of the observation time T and as a consequence the limit on
~~n grows in proportion t o e . Suppose, however, that one finds E events of which B are background events and with B - E. (Of course for a given experiment E and B are in general not equal to <E> and <B> respectively). Under these circumstances the best that we can do is to say that the value of <R> must be less than or equal to the expected fluctuations in the quantity E-B, that is,
<N> ,(
CWB or
<N> ,( C6 from which it follows that
7
where
E* = EandC is a constant dependent upon the confidence level. We may
LVLS
therefore conclude that the presence of a significant level of background effective- ly reduces the antineutron detection efficiency by the factor w h e ~ K is the upper limit for <N> in the case of zero background. Moreover, provided we are still in the regime in which <N> << 1 this efficiency falls like l/flwhich implies that the limit on rNm improves only as fl However, in order to avoid this unattractive behaviour we need only reduce the background to a level commensurate with the desired limit. The condition is simply that <B> << eTTp/riE.
EXFERIMENTAL LAY OUT
The experiment was performed in three phases. In the first phase a simple lead- scintillator calorimeter detector was used to measure the energy of the particles
am dump
, , . , Ground
.
Figure 1
produced in a L ~ beam dump (fig. 1). ~ F At the end of phases 1 and 2 the detector was modified in the light of experience gained in order to reduce further the level of background. Indeed, the background rate was reduced from -102 events/day, in phase 1, to -10-2 eventstday in phase 3. At the lower level the background rate was consistent with the expected rate of interactions in the target of neutral cosmic ray hadrons arriving with the right centre of mass energy.
The large reduction in the background rate was achieved by replacing the detector of phase 1 with an array of limited treamer tubes forming a tracking chamber /4/
with which particle multiplicity could be measured in phase 2, whilst in phase 3
the target for antineutrons was no longer the beam dump but was instead a 120 car- bon foil placed 15 cm upstream of it. The magnetic shield consisted of three concen- tric layers of mumetal, 6.4 m in length having an average diameterof0.7 m. A triple layer mumetal end cap was placed immediately downstream of the dump. The neutrons whose average wavelengths was 24.5 traversed an average distance of 4.3 m in the region within which the magnetic field intensity was <0.5 mG. In order to keep the probability of antineutron-gas interactions below - 10-3 the magnetically shielded region was evacuated down to a level of 10-5 Torr.
RESULTS AND CONCLUSIONS
In phase 1 a limit of 0 . 8 ~ 1 0 ~ secs was reached. The results for phases 2 and 3 are summarised below
The total neutron intensity 0 was 1.5~109 neutron/sec and <t2> was calculated to be 6.9x10-~ s2. The data were taken with the reactor on, field< 0.5 mG, reactor off and with the reactor on but with a 40 mG field in the evacuated region in order to measure the background by suppressing the oscillation amplitude. Combining the data from phases 2 and 3 allow a limit on of > 0.8~106 secs to be set with a con- f idence level of 90%.
In conclusion we report that no evidence for neutron-antineutron oscillations has been found and if the phenomena exists the mixing time is greater than about 106sec.
Phase
2 3
REFERENCES
E.
0.23 0.32
/1/ LAXACKER P., Phys. Rev. 72 (1981) 185
/ 2 /
