The so-called Hoyle state in 12C (7.654 MeV, 0+) is an excited state of 12C characterized by a well-pronounced 3α cluster nature. Many possible cluster configurations, such as a linear-chain arrangement of 3α particles, bentarm, triangular or gas-like structures were theoretically conjectured within the previous decades, but, also due to the existence of ambiguities in experimental data, it is still impossible to draw a firm conclusion. Its direct α-decay partial width is accurately predicted by models but requires better experimental constraints. We performed a high-precision and high-selectivity investigation of its α-decay by using the 14N(d,α2)12C reaction at 10.5 MeV incident energy with the aim of distinguishing between sequential and direct decay patterns. We are able to significantly improve the state of the art upper limit for direct 3α decays by placing the new limit at 0.043%. Such a low limit allows to give fundamental constraints to theoretical models attempting to describe the structure of the Hoyle state.

1.
W.
von Oertzen
,
M.
Freer
, and
Y.
Kanada-En’yo
,
Phys. Rep.
432
,
43
113
(
2006
).
2.
M.
Freer
and
H.
Fynbo
,
Prog. Part. Nuc. Phys.
78
, p.
1
(
2014
).
3.
F.
Hoyle
 et al,
Phys. Rev.
92
, p.
1095c
(
1953
).
4.
F.
Hoyle
,
Astrophys. J. Suppl. Ser.
1
, p.
121
(
1954
).
5.
C. E.
Rolfs
and
W. S.
Rodney
,
Cauldrons in the Cosmos: Nuclear Astrophysics
, 5th ed. (
The University of Chicago Press
,
Chicago and London
,
1997
).
6.
C.
Cook
,
W.
Fowler
,
C.
Lauritsen
, and
T.
Lauritzen
,
Phys. Rev.
107
, p.
508
(
1957
).
7.
T.
Akahori
,
Y.
Funaki
, and
K.
Yabana
,
Phys. Rev. C
92
, p.
022801
Aug (
2015
).
8.
C.
Angulo
 et al,
Nucl. Phys. A
656
, p.
3
(
1999
).
9.
N. B.
Nguyen
,
F. M.
Nunes
,
I. J.
Thompson
, and
E. F.
Brown
,
Phys. Rev. Lett.
109
, p.
141101
(
2012
), arXiv:1112.2136 [nucl-th].
10.
N. B.
Nguyen
,
F. M.
Nunes
, and
I. J.
Thompson
,
Phys. Rev. C
87
, p.
054615
(
2013
), arXiv:1209.4999 [nucl-th].
11.
O.
Kirsebom
 et al,
Phys. Rev. Lett.
108
, p.
202501
(
2012
).
12.
M.
Freer
,
Rep. Prog. Phys.
70
, p.
2149
(
2007
).
13.
H.
Morinaga
,
Phys. Rev.
101
,
254
258
Jan (
1956
).
14.
Itoh
 et al,
Phys. Rev. C
84
, p.
054308
Nov (
2011
).
15.
Y.
Kanada-En’yo
and
H.
Horiuchi
,
Progress of Theoretical Physics Supplement
142
,
205
263
(
2001
).
16.
A.
Tohsaki
,
H.
Horiuchi
,
P.
Schuck
, and
G.
Röpke
,
Phys. Rev. Lett.
87
, p.
192501
Oct (
2001
).
17.
S.
Ishikawa
,
Phys. Rev. C
90
, p.
061604
(
2014
), 1412–4176.
18.
M.
Freer
 et al,
Rev. Mod. Phys.
in press, – (
2018
), arXiv:1705.06192 [nucl-th].
19.
M.
Freer
 et al,
Phys. Rev. C
49
, p.
R1751
(
1994
).
20.
A.
Raduta
 et al,
Phys. Lett. B
705
, p.
65
(
2011
).
21.
J.
Manfredi
,
R. J.
Charity
,
K.
Mercurio
,
R.
Shane
,
L. G.
Sobotka
,
A. H.
Wuosmaa
,
A.
Banu
,
L.
Trache
, and
R. E.
Tribble
,
Phys. Rev. C
85
, p.
037603
Mar (
2012
).
22.
T.
Rana
 et al,
Phys. Rev. C
88
, p.
021601
(R) (
2013
).
23.
M.
Itoh
 et al,
Phys. Rev. Lett.
113
, p.
102501
(
2014
).
24.
DellAquila
,
D.
 et al,
EPJ Web Conf.
165
, p.
01020
(
2017
).
25.
W.
Koenig
 et al,
Il Nuov. Cim.
39
, p.
9
(
1977
).
26.
D.
Dell’Aquila
 et al,
Nucl. Instrum. Meth. Phys. Res. A
877
, p.
227
(
2018
).
27.
D.
Dell’Aquila
 et al,
Phys. Rev. C
93
, p.
024611
(
2016
).
28.
D.
Dell’Aquila
 et al,
EPJ Web of Conf.
117
, p.
06011
(
2016
).
29.
R.
Dalitz
,
Philos. Mag.
44
, p.
1068
(
1953
).
30.
D.
Dell’Aquila
 et al,
EPJ Web Conf.
184
, p.
01005
(
2018
).
31.
D.
Dell’Aquila
 et al,
Phys. Rev. Lett.
119
, p.
132501
(
2017
).
32.
C.
Spitaleri
 et al,
Phys. Rev. C
95
, p.
035801
(
2017
).
33.
I.
Lombardo
 et al,
Phys. Lett. B
748
, p.
178
(
2015
).
34.
I.
Lombardo
 et al,
Bulletin of the Russian Academy of Sciences: Physics
78
, p.
1093
(
2014
).
35.
J.-J.
He
,
I.
Lombardo
,
D.
Dell’Aquila
,
Y.
Xu
,
L.-Y.
Zhang
, and
W.-P.
Liu
,
Chin. Phys. C
42
, p.
15001
(
2018
).
36.
W.
von Oertzen
,
Zeit. Phys. A
357
, p.
355
(
1997
).
37.
I.
Lombardo
,
D.
Dell’Aquila
,
G.
Spadaccini
,
G.
Verde
, and
M.
Vigilante
,
Phys. Rev. C
97
, p.
034320
Mar (
2018
).
38.
R.
Smith
,
T.
Kokalova
,
C.
Wheldon
,
J. E.
Bishop
,
M.
Freer
,
N.
Curtis
, and
D. J.
Parker
,
Phys. Rev. Lett.
119
, p.
132502
Sep (
2017
).
39.
H.
Zheng
,
A.
Bonasera
,
M.
Huang
, and
S.
Zhang
,
Physics Letters B
779
,
460
463
(
2018
).
40.
J.
Refsgaard
,
H.
Fynbo
,
O.
Kirsebom
, and
K.
Riisager
,
Physics Letters B
779
,
414
419
(
2018
).
41.
G.
Cardella
 et al,
Nucl. Instr. and Meth. A
799
, p.
64
(
2015
).
42.
D.
Dell’Aquila
,
Il Nuov. Cim. C
39
, p.
272
(
2016
).
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