Physics Letters B 800 (2020) 135112
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Physics Letters B
www.elsevier.com/locate/physletb
Wino potential and Sommerfeld effect at NLO
Martin Beneke, Robert Szafron
∗
, Kai Urban
Physik Department T31, James-Franck-Straße 1, Technische Universität München, D–85748 Garching, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
13 September 2019
Received
in revised form 19 November 2019
Accepted
20 November 2019
Available
online 28 November 2019
Editor:
J. Hisano
We calculate the SU(2)×U(1) electroweak static potential between a fermionic triplet in the broken phase
of the Standard Model in the one-loop order (NLO). The one-loop correction provides the leading non-
relativistic
correction to the large Sommerfeld effect in the annihilation of wino or wino-like dark matter
particles χ
0
. We find sizeable modifications of the χ
0
χ
0
annihilation cross section and determine the
shifts of the resonance locations due to the loop correction to the wino potential.
© 2019 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
.
1. Introduction
It is by now well-known that the Sommerfeld effect due to the
electroweak Yukawa force [1–3]can lead to a dramatic enhance-
ment
of the annihilation cross section of two dark matter (DM)
particles if their mass is in the TeV range. Contrary to the classic
Sommerfeld effect for massless gauge boson exchange in QED and
QCD, which rises as 1/v as the relative velocity of the annihilat-
ing
particles decreases, the enhancement due to the Yukawa force
saturates at small velocities, except near isolated resonances. These
occur at dark matter mass values, when a zero-energy bound-state
develops in the spectrum. The phenomenon is quite general and
also appears for lighter DM, if there is a force carrier with even
smaller mass [4]. Furthermore, if the DM particle is part of a mul-
tiplet
with a small mass splitting, the effect depends sensitively on
the mass difference [5].
Its
main interest is nevertheless due to the fact that it is a
generic feature of the classic WIMP DM particle, where it arises
from the well-established Standard Model (SM) interactions. Thus,
it appears in the so-called minimal models [6] and for TeV scale
MSSM WIMPs (see, for example, [7–9]). The Sommerfeld effect is
particularly important for the annihilation rates and relic density
of the pure wino, an electroweak triplet of fermions of which the
electrically neutral member is the DM particle [2,3,6,10–12], or a
mixed but dominantly wino state [13,14]. The pure wino (“wino”
in the following) model has become a test case for the quantitative
understanding of large electroweak corrections in the annihilation
of TeV scale DM particles. In view of the possible detection or ex-
*
Corresponding author.
E-mail
address: robert.szafron@cern.ch (R. Szafron).
clusion of the wino particle through measurements of high-energy
cosmic rays, the wino annihilation rate into photons is of partic-
ular
interest. Here electroweak perturbation theory breaks down
due to electroweak Sudakov logarithms, which must be summed
in addition to the Sommerfeld corrections. Recent work on ex-
clusive
and semi-inclusive photon yields has shown that Sudakov
logarithms can be controlled with 1% accuracy with NLL’ resum-
mation
[15–17]. At this level of precision, the treatment of the
Sommerfeld effect should be revisited, since, up to the present, all
calculations have been done with the tree-level exchange poten-
tial,
which corresponds to the leading-order (LO) approximation in
non-relativistic effective field theory (EFT) for the DM particle [8,
18,19].
In
this paper we compute the one-loop corrections to the wino
potential and discuss its effect on the wino pair annihilation cross
section to photons, χ
0
χ
0
→ γ + X. We recall [1–3] that the LO
potential is given by the matrix
V
LO
(r) =
⎛
⎜
⎜
⎝
0 −
√
2 α
2
e
−m
W
r
r
−
√
2 α
2
e
−m
W
r
r
−
α
r
−α
2
c
2
W
e
−m
Z
r
r
⎞
⎟
⎟
⎠
.
(1)
The IJ entry refers to the non-relativistic scattering of wino two-
particle
states I → J with I, J = 1, 2 referring to χ
0
χ
0
and χ
+
χ
−
,
respectively. The above matrix describes the scattering of electri-
cally
neutral two-particle states in a
1
S
0
spin-angular-momentum
configuration, since the spin-1 configuration is forbidden due to
the Majorana nature of the χ
0
. One might expect the one-loop
correction to the potential to be small due to the smallness of the
SU(2)×U(1) couplings. However, we shall see that over most of the
interesting wino mass range from 1 to 10 TeV, the effect on the an-
https://doi.org/10.1016/j.physletb.2019.135112
0370-2693/
© 2019 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
.