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NEUTRON-ANTINEUTRON OSCILLATIONS
M. Baldo-Ceolin
To cite this version:
M. Baldo-Ceolin. NEUTRON-ANTINEUTRON OSCILLATIONS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-173-C3-183. �10.1051/jphyscol:1984330�. �jpa-00224044�
JOURNAL DE PHYSIQUE
Colloque C 3 , supplement a u n ° 3 , Tome 4 5 , m a r s 198<f page C3-173
NEUTRON-ANTINEUTRON OSCILLATIONS
M. BaJdo-Ceolin
Dipartim ento di Fisica "G. Galilei", Universita di Padova, Padova, Italy and Jstifcufco Nazionale di Fisica Nucleare, Sezione di Padova, Padova, Italy
Résumé - Le projet de la nouvelle expérience à l'ILL pour la détection de T^^ jus- qu'à 2-108 secondes est discuté d'après les résultats expérimentaux déjà obtenus à Grenoble.
Abstract - The project for the new Grenoble experiment aiming at detecting T^ up to 2*108sec is discussed on the basis of the experimental results already obtained at the high flux reactor in Grenoble.
INTRODUCTION
With the successful outcome of electroweak unified theory a new fundamental step has been accomplished toward the general aim of physics, the finding of deeper unity and simplicity in the laws of Nature. At present, the search for grand unified theories appears to open (in a gratifying accord with cosmology) new interesting pos- sibilities, as the question of nuclear matter stability and the determination of neu-
(2) trmo masses .
In the general frame of the unified theories, an appropriate six-quarks coupling might exist, leading to AB=2 transitions in nuclei, namely the processes (np)->pions'3).
According to this hypothesis, there must be a AB=2 neutron-antineutron mixing, cha- racterized by a mass splitting^ *
where r is the AB=2 decay rate and M is the nucleoli mass.
An interesting consequence of this assumption is that an initially pure neutron beam becomes a neutron-antineutron mixture after a finite time, and neutron oscillat- ions arise with the characteristic transition time
In this context it should be noted that the n-n oscillation process depends on the AB=2 coupling constant to the first order: so that^4) o n e would expect T ^ 106sec
30 . • osc if T = 1 0 years: the experimental search for neutron oscillations appears then as an excellent means to investigate the existence of AB=2 processes.
Actually the theoretical models leave a rather large incertitude in the value to be expected for T . The so-called "left-right symmetric" models, however, suggest
• -, i- osc . . . 7 (z.\
mainly for the neutron oscillation time a value T - 10 secVJ'.
Moreover, m the quark-lepton picture the simplest term in the effective Lagran-osc gian that can induce AB=2 processes is
and therefore, if neutron-oscillation processes are observed in the experimentally accessible region up to T ^ 10 sec, this will open a new physics in the mass range
k c OSC
M ^ 1CTi106GeV.
x
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984330
In the following, i will first briefly consider the neutron-oscillation phenome- nology and subsequently discuss the experimental problem of the search for n-;i oscil- lations, with a special emphasis on Grenoble experiment and its perspectives.
NEUTRON OSCILLATION PHENOMENOLOGY
The AB=2 interaction produces a mixing among neutron and antineutron states, so that n and n are no more "two distinct states", but belong to "one two state system"
whose time evolution (in the c.m.s.), assuming CP invariance and neglecting decays, is governed by the equation
with
where non diagonal terms represent the n n transition energy, - while diagonal terms the effective neutron and antineutron masses: M = m + V M = m + V-, m is the neu-
n n n
tron(antineutr0n) mass, and V , V-, the nuclear andyor e~lctroma~netic potential for n n
neutrons and antineutrons in the surrounding medium.
To the eigenstates of eq. (2)
belong the masses
where
6m 1
tg20 = - and AE = -(V - V-)
AE 2 n n
The time evolution of a neutron state
is described by the equation
and the probability of finding an antineutron component at time t, in a state that at t = 0 was a pure neutron state, is given by
Eq. (4) shows that the probability ~(n,t) oscillates with amplitude A = (G~/AM)' and angular frequency w = AM, where
Free and quasi Free Neutrons
In the case of free neutrons the n-n energy splitting AE is equal to zero and -
eq. (4) reduces to the equation
or simply to
when actually observable times t << T are considered.
The antineutron component in a free neutron state builds-up as the square of the time .
Moreover, eq. (5) shows that any experiment looking for neutron oscillations will require a very large number of neutrons: if T % 107sec, an initial number of at least
1016 neutrons are needed in order to produce a n + n transition within a time interval t 0,l sec.
In practice, neutrons are never free, AM is much larger than 6m, and neutron oscil- lations are strongly suppressed, the probability p(n,t) going to zero as (6mlA~)~.
However, real neutrons behave as free neutrons for values of t and AM such that ( 7 )
Eq. (6) represents the "quasi free neutron condition": by it p(n,t) still grows as t2, while for t > (Am)-' p(n,t) oscillates between ( 6 m / A ~ ) ~ and 0 with the angular fre-
quency w = (AM)-'. -
Thus in order to reach an appreciable value for P(n,t) it is imperative to make AE as small as possible.
This condition can be fulfilled primarily using neutron beams propagating in a regic properly evacuated (in order to avoid nuclear interactions) and shielded against any external magnetic field.
It appears experimentally possible, for example, to reach propagation time t % 1 sec, if the neutron beam propagates in vacuum, in a region where proper shield-
-1 8
ing reduce the earth magnetic field, (AE % 10 MeV), by a factor >lo5.
Summarizing the above discussion we may conclude that an experimental search on neutron oscillations requires at first:
i) a very intense neutron source;
ii) a propagation region free from any kind of interactions.
Neutrons in Nuclear Matter
Since the condition i) is largely satisfied in the deep underground experiments searching for AB=l nucleon decays, the possibility has been explored(8) that the same experiments might also detect n-n oscillations in nuclei, thus allowing a measurement of the interesting parameter 6m = (T )-I.
The argument may be summarized as follows. In the case of neutrons in nuclei BE OSC -2 L
may be evaluated to 1 0 ~ 1 1 0 ~ ~ e ~ : from eq. ( 4 ) , for 6m 5 10 MeV, it follows that neu-
- 6 0
trons in nuclei would oscillate with an amplitude A < 10 and an angular frequency w % AE % 1oz4sec-l.
The effect ?f neutron oscillations would thus result, in a rough approximation, in an antineutron state in nuclei with a "quasi constant" probability
which, considering the n annihilation cross-section, a - % AE, would give rise to an- nihilation events at a rate r % (6m/A~)~. AE.
ann -31
From proton-decay type experiments it resulted r < 10 years(9). These results, due to the presence of unavoidable background effects, appear to be the experimental ann limit of the present generation experiments.
Moreover, as it is well known, there are substantial uncertainties in relating
free and bound n-n mixing arising from uncert in y in the n optical potential.
Furthermore, it has been pointed out recentl?lof that the strength of n : transit-
ions needs not to be the same for neutrons isolated or contained in nuclear matter, and consequently r does not constrain the value of the free neutron oscillation time. ann
NEUTRON OSCILLATION EXPERIMENTS
Following the neutron oscillations assumption, a state initially composed of pure neutrons should become, within finite time, a mixture of neutrons and antineutrons. In order to test this hypothesis, one has to detect the antineutrons bringing them to an- nihilate in nuclear matter.
The antineutron signature will be typical energy release of * 2 GeV, distributed over several pions (5 in average) with total momentum p + 0.
Once the "quasi free condition" is satisfied, the constraint which defines the quality of a neutron-oscillation experiment may be deduced from eq. (5) as
- r = JN'E t =
OSC v
- 1
where: I, the neutron current in n sec , depends on the power of the neutron source;
E, the fraction of annihilation events which can be unambiguously identified, depends on the properties ("qualityt') of the detector; t = - L v l , is the time in sec of the "quasi free propagation"; v, the neutron velocity in m sec ; ~ t for a given source, depends ~ , upon neutron energy and annihilation target area. -
An important peculiarity of n-n oscillation experiments carried out with neutron beamslies in the fact that the background can he measured directly. The background is mainly due to neutral cosmic rays interactions that the detector would not be able to discriminate from annihilation processes. However, the antineutron yield may indeed be suppressed at will by a magnetic field B applied in the propagation region (see eq. 4) leaving the cosmic ray background to be measured separately.
THE GRENOBLE EXPERIMENT
The Grenoble experiment was planned in 1980 as an exploratory search, without a special emphasis on sensitivity, but with the definite purpose of singling out the main factors to play upon in planning an experiment apt to reach an actually significant sen- sitivity in T . The experimental result obtained (1 1)
osc
r >106sec at90%C.L.
OSC
which so far is the first and only experimental result on neutron oscillations, shows the practical possibility of measuring T up to 10~+10~sec.
In the following, I will discuss some details concerning the possibility to raise OSC
the present sensitivity by two orders of magnitude.
The experimental set-up is sketched in Fig. 1.
a) The Beam. Cold neutrons were transported to the experimental area along a total re- flecting guide (H18) made of 40 elements, each 25 cm long, with 3x20 cm2 cross- section, arranged on a curve of 25 m radius. The direct radiations from neutron source were that way switched off and, at the same time, the lower energy neutrons were selected: the effective neutron temperature was reduced to %1,5" K, correspond-
-1
ing to an average velocity v 1. 160 m sec . The beam intensity was I = 1 .5.109n
- 1
sec . Peculiar adavantages of this arrangement are:
i) the guide secures a pure neutron beam,
h o d o s c o p ~ b a r r e l hodoscope v e t o c o u n t e r s , I
Fig. 1 - Experimental set-up.
ii) the low neutron velocity contributes directly to the overall sensitivity of the experiment, the measurable limit of T OSC being = 7 1 .
b) The Propagation Region. Neutrons propagate under the "quasi free condition" along a straight section 4.5 m long, followed by a vessel 2.7 m long with a diverging conic shape in order to match the divergence of the incoming beam, avoiding colli- sions on the walls anci so conserving the full initial current. The vessel ends on a 6 ~target, where the neutrons are dumped. This target, i ~ 54 cm diameter, 0.3 cm thick, glued on an AL plate 0.3 cm thick, was supported by an AL disk, 1.4 cm thick, which closed the system into a vacuum tight enclosure protecting the whole pro- pagation region.
The straight guide and the drift vessel were evacuated to a residual gas pressure torr, and shielded against the earth magnetic field by a triple p-metal layer, down to B % gauss.
An auxiliary magnetic field of a few tenth of a gauss, generated by suitable coils, could be switched on along the oscillation region. The average neutron propagation time, from their last collision inside the guide to the 6 ~target, was evaluated i ~
- 2 t = 2 . 7 . 1 0 sec where
LQ+L
t = - v Lv
> t = -
v v v
L o being the average free propagation length inside the straight section of the guide and L the length of the actual vessel.
Thus one obtains t v > t at a small additional expense.
v
c) The annihilation Target. Two different approaches have been adopted for the annihi- lation target. At first the neutron dump itself was used for this purpose. But this solution had an inherent drawback: while the expected annihilations would take place in a very thin layer at the surface of the target, the whole thickness of the 6 ~ i ~ and the AL supporting plates was able to act as a source of background events, which could not be geometrically separated from the true annihilation events.
Therefore as a second solution an almost immaterial target was adopted: a 10C um Car- bon foil which was thick enough to annihilate practically 100% of the expected ; com-
ponent, still being transparent to the neutrons, was placed 15 cm upstream of the neutron dump.
The neutron beam cross-section at the targets, evaluated on the basis of the guide shape and beam divergence, is shown in Fig. 2: more than 90% of the expected anni-
hilation events should lie inside an area of 32x20 em2 around the center of the an- nihilation target.
It seems worthwhile to underline that the annihilation area required with the describ- ed set-up
depends only on L and is therefore less than half the area that would be required if the neutrons propagated along the total length (L,+L v ) in a divergent vessel.
The important point is that detector volume and background effects are proportionally v reduced.
51 and 52
c o u n t e r s . Vacuum vessel
Fig. 2 - Lego-plot of the neutron Fig. 3 - The calorimeter.
beam at the target.
d) The Detector. In the first part of the experiment the main emphasis was on detection of the annihilation energy. The detector (Fig. 3) placed in front of the neutron target acted as a calorimeter: it consisted of 5 modules 16 cm high, 80 cm long, co- vering an area of 80x80 cm2, each consisting of twenty lead layers and twenty plastic scintillator layers, each 0.5 cm thick.
The detector covered a A f i / 4 1 ~ % 25%, of the total solid angle.
Successively a fine grain detector was introduced whose major aim was at spatial re- solution in order to allow particle identification, track pattern recognition, and vertex reconstruction (see Fig. 1).
It surrounded the annihilation target, AQ/4.rr > 0.7, and consisted of:
- limited streamer tubes(' ') (.9x.9 cm2 cross-section) arranged in planes (1~1.5 m2), interleaved with A% or Fe plates, 0.5 cm thick. Each tube plane provided two coor- dinates for any crossing particle;
- layers of scintillation counters inserted between the metal shields, in front and between the limited streamer tube planes.
Fig. 4 shows two typical events: the vertex reconstruction has an average resolutiod of a few cm3.
e) The Cosmic Ray Veto System. The whole apparatus was shielded against cosmic rays by means of anticoincidence counters, covering an area of about 30 m2. The overall ef-
ficiency of the anticoincidence system was measured to be greater than 99.95%.
No shielding material was placed outside, in order not to overload the floor of the experimental hall.
f) Collecting Data. Data were taken alternating 24 hours runs with magnetic field off
Fig. 4 - Two typical events as seen on the display.
(quasi free condition) and with magnetic field on (which depresses the oscillation probability by a factor %lo6). Control runs were also made with reactor off.
g) Experimental Results. The experiment may be subdivided in three phases.
Phase 1: 6 ~target as annihilation target and calorimeter as detector. i ~ Phase 2: 6 ~target as annihilation target and tracking detector. i ~ Phase 3: Carbon foil target and tracking detector.
For each phase the three sets of experimental data, collected in the three conditions field off, field on and reactor off, were analyzed and compared. The results turned out to be identical, so leading to the conclusion that there was no evidence of neu- tron oscillations. Furthermore:
1) in phase one, where the energy was detected primarily, possible background and noise from reactor and neutron associated radiation have been considered in detail, with the conclusion that, in our conditions, they are negligible(' 3). The total ef-
fective running time was %45 days equally distributed in the three conditions;
2) in phase two, data were taken during 110 days effective time, 45 "field-off". The collected data, visualized on a display, were scanned for possible annihilation events. In order to have good identification, events with
a) three or more tracks, at least one in backward and one in forward direction, the latter traversing at least 8 tube planes;
b) the vertex in the annihilation area within the experimental resolution, were selected.
A candidate event was found, well compatible with the expected background N = 1.8k0.4 events. B
A careful study of cosmic ray interactions satisfying the above topological select- ion criteria was done. Events with vertex inside a cilinder, 10 cm long, starting from the full target area, %20 kg in mass, were considered.
The background event rate was so experimentally determined to be less than 1 event per kilogram of target mass per 107sec running time;
3) during phase three, in order to avoid background events from cosmic ray interactions in the matter adjacent the target, the annihilation target was moved 15 cm upstream the neutron dump. No further candidate was found in -65 days of effective running time. In phase three, indeed, on the basis of phase two results, less than 0.1 back- ground event with the vertex in the C annihilation target was expected during the
complete d a t a t a k i n g .
Adding up t h e r e s u l t s from phase two and t h r e e we o b t a i n e d a s a n t i n e u t r o n y i e l d
- N 2.3 -1 6
R = - 5 = 6.10 a t 90% C.L.
N I . 5 * 1 0 ~ . 1 0 ~ . ~
where E , t h e e f f e c t i v e d e t e c t i o n e f f i c i e n c y , r e s u l t e d E = 0.27 from a Monte C a r l o c a l c u l a t i o n based on e x p e r i m e n t a l d a t a from a t e s t r u n i n a n a n t i p r o t o n beam a t CERN PS.
Some c o m e n t s may b e a p p r o p r i a t e a t t h i s p o i n t :
i ) T h e cosmic r a y background (which was measured t o be l e s s t h a n 1 e v e n t p e r kg t a r g e t mass, p e r 1 0 7 s e c ) c a n b e e l i m i n a t e d u s i n g a t h i n i s o l a t e d t a r g e t and a d e t e c t o r e n s u r i n g good v e r t e x r e c o n s t r u c t i o n and e n e r g y r e s o l u t i o n . i i ) A l i m i t e d s t r e a m e r t u b e d e t e c t o r a p p e a r s w e l l s u i t e d t o p r o v i d e t h e s e con-
d i t i o n s .
i i i ) A c o l d n e u t r o n beam p r o p a g a t e d i n a s l i g h t l y b e n t g u i d e c o n t r i b u t e s v e r y c o n s i d e r a b l y t h e e f f i c i e n c y of t h e system.
THE PROPOSED GRENOBLE EXPERIMENT
The n e x t s t e p we a r e p r o p o s i n g f o r t h e Grenoble experiment aims a t measuring T O S C
up t o % 1 0 8 s e c . I t i s based o n t h e g e n e r a l c o n s i d e r a t i o n s d i s c u s s e d above:
a ) a c o l d n e u t r o n beam t r a n s p o r t e d t o t h e e x p e r i m e n t a l a r e a by a curved g u i d e and t h e n p r o p a g a t e d i n t h e q u a s i f r e e c o n d i t i o n ;
b) a t h i n i s o l a t e d a n n i h i l a t i o n t a r g e t ;
c) a f i n e g r a i n d e t e c t o r w i t h h i g h s p a t i a l and energy r e s o l u t i o n ; d) a n e f f i c i e n t cosmic r a y s h i e l d .
It w i l l t a k e a d v a n t a g e of t h e new c o l d s o u r c e and a s p e c i a l l y d e s i g n e d new n e u t r o n g u i d e .
Furthermore a s i m p l e d e v i c e i s e n v i s a g e d , based on n e u t r o n r e f l e c t i o n p r o p e r t i e s w i t h i n a guide(''), i n o r d e r t o k e e p dimensions of t h e a n n i h i l a t i o n t a r g e t and t h e ex- p e r i m e n t a l a p p a r a t u s w i t h i n r e a s o n a b l e v a l u e s .
This d e v i c e c o n s i s t s of a g u i d e w i t h s l i g h t l y d i v e r g e n t w a l l s which a l l o w s somehow t o f o c u s ( o r a c t u a l l y t o p a r a l l e l i z e ) t h e n e u t r o n beam, s o r e d u c i n g i t s c r o s s - s e c t i o n a t t h e t a r g e t a s shown i n F i g . 5. While t h e g l a n c i n g a n g l e of a n e u t r o n t r a j e c t o r y a t t h e w a l l s o f a p a r a l l e l s i d e d g u i d e remains c o n s t a n t a l l a l o n g , and t h e same happens f o r i t s a n g l e O w i t h t h e a x i s o f t h e g u i d e , a r e f l e c t i o n a t t h e w a l l s o f a d i v e r g e n t g u i d r e d u c e s 0 by 26, where 6 i s t h e d i v e r g e n c e
of t h e w a l l s . So, i f Bin i n d i c a t e s t h e d i - v e r g e n c e o f a n e u t r o n e n t e r i n g t h e g u i d e ,
t h e d i v e r g e n c e of t h e n e u t r o n a t t h e e x i t 5 o
ed i n t h e p r e v i o u s l y d e s c r i b e d e x p e r i m e n t , a new e x p e r i m e n t a l s e t - u p h a s been design-
e d , a s d e s c r i b e d i n t h e f o l l o w i n g para- -..
-
-75 -50 -25 o 25 50 75 c mgraphs and s k e t c h e d i n t h e F i g . 6.
F i g . 5 - The n e u t r o n beam c r o s s - s e c t i o n a t t h e a n n i h i l a t i o n t a r g e t . a f t e r n r e f l e c t i o n s , w i l l be
2 5
O = O . - 2 1 1 6 . o u t I n
The r e q u i r e d t a r g e t a r e a A needed t o 0 c o n t a i n t h e beam w i l l b e c o r r e s p o n d i n g l y - 25
d e c r e a s e d by a c o n s i d e r a b l e f a c t o r .
On t h e b a s i s of t h e e x p e r i e n c e a c q u i r - -50
. . .
. .
- ,--:.;:.,>.,. . . .,.. ':
4,'L:;.,
; -::.As*.: ,L.k'2-... -,. ,.
?;f>%.*:7;
- .-' , .::,&$;; '-'-;. *> . .. .
- .. . - .- .. -.zr:*.:-..,: . . -. ,*-. . .
-
1) Neutron Beam and Guide. Cold neutrons will be transported to the experimental area by the new guide H 52 (1.60 m long, R = 5000 m). The outcoming beam will conrain 3.3.10''n sec -1 at an average T ' 15°K and no spurious radiation.
2) Propagation Region. The quasi free propagation region will consist of i) a 30 m long straight guide, with a divergence of 1.3 mrad;
ii) a divergent vessel 1.30 m long, ending with $ = 130 cm;
- 6 - 4
where a gas pressure P < 10 torr and a residual magnetic field B < 10 gauss warrant the quasi free condition.
The average time interval from the last neutron reflection in the divergent guide to the end of the quasi free propagation region will be
Lo+L
t = - = v 0.1 sec.
<v>
3) Annihilation Target. A 1.100 pm thick Carbon foil will be placed down-stream the ma- gnetically shielded region. The beam cross-section at the target (see Fig. 5) will be .~70x100 cmZ, and the corresponding vessel diameter $I = 130 cm.
The important point is that the annihilation target will be far from any other ma- terial.
4) Annihilation Detector. It is shaped as a box surrounding the target with a solid angle AS1/41r ' 1. The walls of the box will consist of limited streamer tube planes and scintillation counter plates, immediately outside the propagation vessel. The ensemble (see Fig. 6) works as:
i) a charged particle vertex detector (10 planes in each side) with no material in between;
ii) a neutral particle vertex detector (10 planes in each side) interleaved with .5 cm AX plates and a . 5 cm Pb plate in front;
iii) a calorimeter able to absorb the whole energy emitted in annihilation processes, consisting of 16 limited streamer tube planes interleaved with Fe plates of in- creasing thickness.
d n t l c o ~ n ~ ~ d r n c e
5 n, c metal S h i e l d c o u n t e r s
-
beam durn?
Fig. 6 - Longitudinal and cross- sectional view of the experimental set-up.
A Monte Carlo calculation shows that %80% of the annihilation processes will be fully
contained in this ensemble and the vertex reconstructed within few centimeters 5) Cosmic Ray Veto and Shield. The experiment will be protected by proper material
against cosmic ray neutrals and by a veto system against charged penetrating par- ticles.
The main characteristics of the planned experiment are summarized in Table I, and compared with those of the already completed Grenoble experiment.
TABLE I
-
OLD EXPERIMENT NEW EXPERIXENT
Neutron Beam
I = 1.5.10'n see-' I = 3.3.1011n sec-I
R = 2 5 m R = 5000 rn
/ch,Z = 25 J 5 7 = 8 :
Propagation Region
1) 4.5 m long straight guide 0 30 m long straight divergent guide 2) 2.7 m drift vessel 2) 30 m drift vessel
t = 0.027 sec t = 0.1 sec
Annihilation Target
1) 'L~F dump Carbon foil 1.100 "ID thick 2) C foil *to0 urn thick
Neutron beam cross-section Neutron beam cross-section at the target 36x20 em2 at the target 50x100 cm2 vessel 4 = 54 cm vessel 4 = 130 cm
Detector
Limited streamer tube planes Limited streamer tube
+ scintillation counter plates planes + scintillation counter plates (1~1.5 mZ) x 30 tube planes (5x4 m2) x 144 tube planes
Good vertex reconstruction Good vertex reconstruction Good energy measurement
Background
T ".1c7sec
Sensitivity
r 1.106sec
OSC r *2.10'sec
OSC
As a conclusion we may state that:
a) on the basis of the analysis of the cosmic ray background reported in the pre- vious paragraph, less than 1 background event in 108sec would be expected. There- fore, the experiment can be run for a year effective time with no background, so- to reach a sensitivity in T up to 2-10~sec;
osc
b ) in this experiment one expects %3 annihilation events per day if r = 107sec,
and still 10 events if T = 108sec. OSC
OSC
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