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exa mpl es of a wea k forc e inter act ion . T o calcu lat e the deta ils of

ot he r , mo re co mp le x nuc le ar pr oc es se s in vo lv in g th e wea k fo rc e,

we must ﬁrst fully understand how it operates in neutron decay .

Dis ce rni ng the ex act r ate of neut ron de ca y woul d als o help

tes t the big b ang th eory fo r the ear ly evo lut ion of t he cos mos .

Acc or din g to the theo ry , whe n th e uni ver se was abou t one seco nd

old, it consisted of a hot, dense mixture of particles: protons, neu -

tro ns , ele ctr ons , an d ot her s. A t th is ti me , the tem per at ure of t he

universe was roughly 10 billion degrees—so hot that these par ti -

cle s we re to o ene rget ic to bin d tog ethe r int o nuc lei or at oms .

Aft er abou t thr ee minu tes , the univ ers e expa nd ed an d coo led to a

tem pe rat ure w her e pro ton s and n eut ron s cou ld st ick t oget her t o

mak e the s imp lest a tom ic nu cle us, de ute riu m ( the h ea vy iso top e

of hydrogen). F rom here other simple nuclei were able to form—

deu te riu m co ul d ca pt ure a p rot on to mak e an iso top e o f h eli um,

two deu teri um nuc lei co uld jo in toge the r t o c reat e h eavie r h eli -

um, and small numbers of larger nuclei formed, up to the ele -

men t l ith ium (al l th e he av ier ele men ts a re t hou ght to h a ve b een

pro du ced in sta rs ma ny mil lio ns of ye ars la ter ).

Thi s pro ces s is know n as big bang nucl eos ynt hes is. If , whi le

the uni ver se was losi ng heat , ne utr ons had d eca ye d at a ra te that

was much faste r tha n the un ive rse co ole d, th ere wo uld hav e been

no neutrons left when the universe reached the right tempera -

tur e to fo rm nuc le i—o nly th e pro ton s wou ld ha ve re mai ned , and

we wou ld hav e a cos mos mad e al most ent ire ly of hyd rog en. On

the oth er han d, if the neut ron lif etim e were muc h lo nge r than the

time required to cool su  ci ent ly for big ban g nucl eos ynt hes is,

the univ ers e woul d hav e an over ab und anc e of heli um, wh ich in

turn would have a  ecte d the form ati on of the hea vi er ele men ts

in vol ved in the evol uti on of sta rs and ulti ma tel y lif e. Thus , the

balance between the universal cooling rate and the neutron life -

tim e was quit e cri ti cal f or the crea ti on of th e el eme nts tha t ma ke

up ou r plan et and eve rythi ng on it .

F ro m as tr o no mi c al d a ta w e c an mea s ur e th e co s mi c ra t io of

he l iu m to hy dr og en, as we l l as th e am o un ts of de ut e ri um and ot he r

li g ht el em en t s th a t ex is t thr o ug ho u t th e u ni ve r se . W e wo ul d lik e to

se e if th es e me a su re m en ts ag re e w it h th e nu m be rs pr ed ic te d by bi g

ba n g th e ory . The the o re ti ca l p re di ct i on , ho wev er , de p en ds on t he

pr e ci se val u e of th e neu tro n li fe ti me . Wi th ou t a rel i ab l e va lu e fo r it ,

ou r ab il i ty to ma k e th is co m pa ri s on is l im it e d. On ce th e ne ut r on

li f et im e is k no wn mo re pre c is el y , we c a n co mp ar e t he o b se rve d

r at io f r om as tr op h ys i ca l ex p er im e nt s wit h th e pre dic te d v al ue

from theor y . If the y agree, we gain further conﬁdence in our stan -

da r d b i g b a ng sc e na ri o fo r ho w th e un i ve rs e ev ol v ed . Of co u rs e, if

th ey di sa gr e e, thi s mod e l mi gh t ha ve to be al t er ed . F or in st a nc e,

cert ain d iscr epan cies mig ht i ndic ate the exist ence of n ew ex otic

part icle s in the univ erse such as an extra typ e of neut rino , which

cou ld have int erfer ed in the pro cess of nucl eosy nthe sis.

One way to resolve the di  erence between the beam and bot -

tle results is to conduct more experiments using methods of com -

parable accuracy that are not prone to the same, potentially con -

fou nd ing syst ema tic err ors . I n add iti on to co nti nui ng the be am

and bot tl e pr oje cts, scie nt ists in se ver al ot her gro ups worl dwi de

are w ork ing on al ter nat ive me tho ds of meas uri ng the neut ron

lifetime. A group at the J apan Proton Accelerator Research Com -

ple x (J- P ARC) in T okai is deve lop ing a ne w beam expe rim ent th at

will de tect the electrons rather than protons produced when neu -

tro ns de cay . I n anot her ve ry exci tin g devel opm ent , gro ups at IL L,

the P eters bur g Nucl ear Phy sic s Ins tit ute in Rus sia , Los Alamo s

Na ti ona l Labo ra tory , the T e chn ica l U niv ers it y of M un ich a nd th e

J oh ann es Gute nbe rg U niv ers it y Ma inz in Germ an y plan to use

neu tr on b ot tle s th at con ﬁne ult ra col d ne utr ons wit h m agn etic

ﬁelds rather than material walls. This is possible because the neu -

tro n, tho ugh elect ric all y neu tra l, beh av es as th oug h it is a smal l

mag net . The numb er of neut ro ns ac cid ent all y lost thro ug h the

sides of such bot tles should be quite di  erent from tha t of previ -

ous measurements and thus should produce quite di  erent sys -

tematic uncertainties. W e fer vently hope that, together , continu -

ing bo tt le and beam exp eri men ts and thi s next gene ra tio n of

measurements will ﬁnally solve the neutron lifetime puzzle.

Time

Number of

neutrons going

through trap

Measured slope

Neutron beam

(known intensity)

passes through

Count the number of decays within the time interval

Trap

+ – +

Proton Electrodes

The Beam M ethod

In  ontras t to the ottle ethod, the ea tehniue looks not f or neutrons

ut for one o f their deay pr oduts, pr otons. ientist s dir et a str ea

of neutr ons through an eletr oagneti trap ade of a agneti eld

and r ing-shaped high-oltage eletr odes. he neutr al neutrons pass r ight

through, ut if one deay s inside the trap , the resul ting positi ely har ged

prot ons ill get stuk. he r ese arhers kno ho  any neutrons  ere in

the ea, and they kno ho long the y spent pas sing thr ough the tr ap ,

so by c ounting the pr otons in the tr ap the y can measure the number of

neutrons that deay ed in that span of tie. his easur eent is the deay

ra te, hih is the slope of the deay ure at a gi en point in tie and

hih allo s the s ientists to alulate the a er age neutron lifetie .

MORE T O EXPLORE

easur eent o f the eutron ifetie sing a rai tational  rap and a o -

 eperatur e  olin oating. A. S erebr ov et al. in Physics Lett ers B , V ol. 605,

Nos. 1 –2, pages 72– 78; January 6, 2005 .

he eutr on ifetie. Fr edE.  Wietfeldt andGeor ey L. Gr eenein  Reviews of Modern

Physics , V ol. 83, No . 4, Article No. 1 173; O ctober–Dec ember 201 1.

Ipro ed eterination of the eutr on ifetie . A. T . Yue et al. in Physic al R eview

Letter s, V ol. 1 1 1, No. 22, Article No. 222501; November 27 , 2013.

FROM OUR ARCHIVES

ltr aold eutrons. R. Golub , W . Mampe, J. M. P endlebury and P . Ager on; June 19 79 .

he  rot on adius r ole. Jan C. Ber nauer and Randolf Pohl; F ebruary 2014.

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