Physics Letters B 747 (2015) 299–304
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Parity violation in neutron capture on the proton: Determining the
weak pion–nucleon coupling
J. de Vries
a,∗
, N. Li
a
, Ulf-G. Meißner
a,b,c,d
, A. Nogga
a,b,c
, E. Epelbaum
e
, N. Kaiser
f
a
Institute for Advanced Simulation, Institut für Kernphysik, and Jülich Center for Hadron Physics, Forschungszentrum Jülich, D-52425 Jülich, Germany
b
JARA – Forces and Matter Experiments, Forschungszentrum Jülich, D-52425 Jülich, Germany
c
JARA – High Performance Computing, Forschungszentrum Jülich, D-52425 Jülich, Germany
d
Helmholtz-Institut für Strahlen- und Kernphysik and Bethe Center for Theoretical Physics, Universität Bonn, D-53115 Bonn, Germany
e
Institut für Theoretische Physik II, Ruhr-Universität Bochum, 44780 Bochum, Germany
f
Physik Department T39, Technische Universität München, D-85747 Garching, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
19 January 2015
Received
in revised form 25 May 2015
Accepted
28 May 2015
Available
online 3 June 2015
Editor:
W. Haxton
We investigate the parity-violating analyzing power in neutron capture on the proton at thermal energies
in the framework of chiral effective field theory. By combining this analysis with a previous analysis of
parity violation in proton–proton scattering, we are able to extract the size of the weak pion–nucleon
coupling constant. The uncertainty is significant and dominated by the experimental error which is
expected to be reduced soon.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
Although parity violation (PV) induced by the weak interaction
is well understood at the level of elementary quarks, its manifesta-
tion
at the hadronic and nuclear level is not that clear. This holds
particularly true for the strangeness-conserving part of the weak
interaction which induces PV in hadronic and nuclear systems.
The Standard Model predicts PV forces between nucleons. How-
ever,
their forms and strengths are masked by the nonperturbative
nature of QCD at low energies. Combined with the difficulty of do-
ing
experiments with sufficient accuracy to extract parity-violating
signals, hadronic PV is one of the least tested parts of the Standard
Model.
The
understanding of low-energy strong interactions has in-
creased
tremendously by the use of effective field theories (EFTs).
It has been realized that by writing down the most general inter-
actions
among the low-energy degrees of freedom that are con-
sistent
with the symmetries of QCD, one obtains an EFT, chiral
perturbation theory (χ PT), that is a low-energy equivalent of QCD.
Each interaction term in the chiral Lagrangian comes with a cou-
pling
strength, or low-energy constant (LEC), which needs to be
extracted from data or computed in lattice QCD. In contrast to low-
energy
QCD itself, χPT allows one to calculate observables in a
perturbative framework with expansion parameter p/
χ
, where p
is
the momentum scale of the process and
χ
∼ 1GeV, the scale
*
Corresponding author.
E-mail
address: j.de.vries@fz-juelich.de (J. de Vries).
where the EFT breaks down. Although nuclear physics is intrinsi-
cally
nonperturbative, the nucleon–nucleon (NN) potential can be
calculated perturbatively within χ PT. The resulting chiral potential
is then iterated to all orders to calculate NN-scattering and bound
state properties. This framework is usually called chiral nuclear EFT
(for recent reviews, see Refs. [1,2]).
The
success of chiral EFT in parity-conserving (PC) nuclear
physics has led to an analogous program in the PV sector [3–7].
One starts with the four-quark operators that are induced when
the heavy weak gauge bosons are integrated out. The next step
entails constructing a PV chiral Lagrangian which contains all in-
teraction
terms that transform under chiral symmetry in the same
way as the underlying four-quark operators. From the resulting
chiral Lagrangian one then calculates the PV NN potential and elec-
tromagnetic
current. In the final step the obtained PV potential and
current are applied, in combination with the PC chiral potential
and current, in calculations of nuclear processes. The PV LECs ap-
pearing
in the PV chiral Lagrangian can be fitted to some data and
other PV processes can then be predicted.
Although
this sounds like a good strategy, in practice this pro-
cedure
is complicated by the lack of data on PV processes. So
far, hadronic PV has only been measured in a handful of exper-
iments
(see Refs. [8,9] for recent reviews). The longitudinal ana-
lyzing
power (LAP), which would be zero in the limit of no PV,
has been measured for proton–proton scattering at three differ-
ent
energies [10–12], for proton–alpha scattering only at a single
http://dx.doi.org/10.1016/j.physletb.2015.05.074
0370-2693/
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.