Neutron Lifetime Anomaly and Big Bang Nucleosynthesis Tammi Chowdhury1and Seyda Ipek1 1Carleton University 1125 Colonel By Drive Ottawa Ontario K1S 5B6 Canada

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Neutron Lifetime Anomaly and Big Bang Nucleosynthesis

Tammi Chowdhury1, ∗and Seyda Ipek1, †

1Carleton University 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada

We calculate the Big Bang Nucleosynthesis abundances for helium-4 and deuterium for a range

of neutron lifetimes, τn= 840 −1050 s, using the state-of-the-art Python package PRyMordial.

We show the results for two diﬀerent nuclear reaction rates, calculated by NACRE II [1] and the

PRIMAT [2] collaborations.

I. INTRODUCTION

The primordial abundances of light elements are the

earliest measurements of the history of our universe.

During the Big Bang Nucleosynthesis (BBN) era, cor-

responding to temperatures T∼O(MeV), light elements

such as helium, deuterium, tritium, lithium and beryl-

lium were produced as a result of nuclear reactions pre-

dicted in the Standard Model (SM). The ﬁnal abundances

depend on parameters such as the number of relativistic

species (Neﬀ ), baryon-to-photon ratio η, neutron-proton

mass diﬀerence and the neutron lifetime τn. These pa-

rameters can all be calculated within the SM or mea-

sured in SM processes, except η, whose value can be

determined from BBN and the cosmic microwave back-

ground (CMB) individually. Given the SM predictions

for BBN abundances, the primordial abundance measure-

ments can then be used to constrain new physics beyond

the SM.

There has been a consistent discrepancy between two

diﬀerent methods of measuring the neutron lifetime in

the last decades, suggesting possible new physics. As

will be described in the next section, “bottle” experi-

ments give τn= 878.4±0.5 s [3] while “beam” exper-

iments ﬁnd a higher value of τn= 888.45 ±1.65 s [4].

Neutron lifetime is an important ingredient in calculat-

ing the SM predictions of BBN abundances. Thus, BBN

measurements can be compared to theoretical expecta-

tions to extract the neutron lifetime. This possibility has

been studied in the literature [5,6]. We revisit this idea

in light of newest neutron lifetime measurements, BBN

observations as well as updates on nuclear rates that go

into BBN calculations. We use a recent Python package

PRyMordial [7]. (See [8] for details about the pro-

gram.) Our results are shown in Figure 1.

II. NEUTRON LIFETIME ANOMALY

In the SM the neutron decays to a proton, an elec-

tron and antineutrino via electroweak interactions, n→

∗Tammi.Chowdhury@carleton.ca

†Seyda.Ipek@carleton.ca

p e−¯ν1. The SM decay rate is [9]

τn=4908.7(1.9) s

|Vud|2(1 + 3g2

A),(1)

where Vud = 0.97370 ±0.00014 is an element of the

Cabibbo-Kobayashi-Maskawa (CKM) quark mixing ma-

trix and gA≡GA/GV≃1.27 is the ratio of axial and

vector couplings.

Neutron lifetime is measured using two experimen-

tal methods. In the bottle method ultracold neutrons

(UCNs) are trapped in a container. After a time com-

parable to the expected neutron lifetime, the remaining

neutrons are counted. In the beam method a slow neu-

tron beam decays in an experimental volume. The decay

products, protons and electrons, and the remaining neu-

trons are captured. Given the neutron ﬂux, the beam

method measures the amount of neutrons that decayed

into protons in a decay volume. This then can be trans-

lated to a neutron lifetime, assuming neutrons only decay

in this way. (See [10] for a historical review of neutron

lifetime measurements.) These two methods give

τbottle

n= 878.4±0.5 s τbeam

n= 888.45 ±1.65 s ,(2)

with more than 4σdiscrepancy between the two mea-

surement methods. This discrepancy could be due to

unaccounted-for systematic errors in one or both meth-

ods or it could be due to new physics beyond the SM [11–

15]. For example, if neutron decays to dark matter, beam

experiments, which count the decay products, would in-

terpret that as a longer (SM) neutron lifetime while bot-

tle experiments, which are blind to diﬀerent decay chan-

nels, will still measure the total neutron lifetime.

Very recently space-based measurements of the neu-

tron lifetime emerged [16,17]. Neutrons are freed from

the surface of planetary objects by galactic cosmic rays.

These neutrons thermalize with the atmosphere with ve-

locities of a few km/s. Spacecrafts that are O(1000 km)

above the surface of the planetary object can be sen-

sitive to the neutron lifetime by measuring the neutron

ﬂux from the object’s surface, given its elemental compo-

sition. NASA’s MESSENGER spacecraft performed such

1A small fraction, about 10−3, of decays produce ﬁnal states with

a photon. Even more rarely there will be decays in which electron

binds to the proton to form a hydrogen atom.

arXiv:2210.12031v2 [hep-ph] 19 Oct 2023

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NeutronLifetimeAnomalyandBigBangNucleosynthesisTammiChowdhury1,∗andSeydaIpek1,†1CarletonUniversity1125ColonelByDrive,Ottawa,OntarioK1S5B6,CanadaWecalculatetheBigBangNucleosynthesisabundancesforhelium-4anddeuteriumforarangeofneutronlifetimes,τn=840−1050s,usingthestate-of-the-artPythonpackagePRyMordia...

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Neutron Lifetime Anomaly and Big Bang Nucleosynthesis Tammi Chowdhury1and Seyda Ipek1 1Carleton University 1125 Colonel By Drive Ottawa Ontario K1S 5B6 Canada

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