Physics Letters B 762 (2016) 421–425
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Finding the strong CP problem at the LHC
Raffaele Tito D’Agnolo
∗
, Anson Hook
School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
5 July 2016
Accepted
30 September 2016
Available
online 5 October 2016
Editor: G.F.
Giudice
We show that a class of parity based solutions to the strong CP problem predicts new colored particles
with mass at the TeV scale, due to constraints from Planck suppressed operators. The new particles are
copies of the Standard Model quarks and leptons. The new quarks can be produced at the LHC and
are either collider stable or decay into Standard Model quarks through a Higgs, a W or a Z boson. We
discuss some simple but generic predictions of the models for the LHC and find signatures not related
to the traditional solutions of the hierarchy problem. We thus provide alternative motivation for new
physics searches at the weak scale. We also briefly discuss the cosmological history of these models and
how to obtain successful baryogenesis.
© 2016 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
The Standard Model (SM) of particle physics provides an ex-
cellent
description of all known low energy phenomena. However,
there are several instances in the SM where our effective field
theory intuition fails spectacularly. These are the cosmological con-
stant,
the Higgs mass (the hierarchy problem), the neutron electric
dipole moment (the strong CP problem) and the Yukawa couplings.
These problems have motivated most of the work on extensions of
the SM that are currently being probed experimentally. The hierar-
chy
problem has been the main driving force behind searches for
new physics at the Large Hadron Collider (LHC). The reason is that
any dynamical explanation of the smallness of the Higgs mass re-
quires
TeV scale physics while the other problems do not. In this
paper, we note that certain solutions to the strong CP problem also
provide strong motivation for new physics at the LHC.
The
neutron electric dipole moment is proportional to
θ = θ +arg det Y
u
Y
d
(1)
where θ is the coefficient of the CP violating term in the QCD
action G
a
μν
˜
G
μν
a
and Y
u,d
are the Yukawa matrices. Current ex-
perimental
measurements of the neutron electric dipole moment
indicate that θ<10
−10
[1], with an order of magnitude uncer-
tainty
from theory [2,3]. This result is especially surprising given
*
Corresponding author.
E-mail
address: dagnolo@ias.edu (R.T. D’Agnolo).
that the Yukawa matrices are complex and have an order one CKM
phase, i.e. CP is badly broken in the SM. The smallness of θ is
called the strong CP problem.
There are two broad categories of solutions to the strong CP
problem. The first are solutions based on anomalous symmetries.
The most well known of these solutions are the axion [4–7] and
the massless up quark [8]. In the UV, these solutions have an
anomalous symmetry under which θ shifts, rendering it unphys-
ical.
In the IR, this anomalous symmetry is spontaneously broken
and a scalar field dynamically removes θ from the Lagrangian.
The second class of solutions are those which use Parity (P) or
Charge-Parity (CP) to set θ to zero in the UV. After P or CP is spon-
taneously
broken, care must be taken to reintroduce a large CKM
phase but a small θ . The most well known of the CP based solu-
tions
are the Nelson–Barr approach [9,10] and [11]. More recently,
a systematic approach to the mediation of CP violation to the SM
was done in Ref. [12]. The focus of this paper will be on the parity
based solutions [13].
The fact that the Strong CP problem can provide motivation
for new physics at the TeV scale was first observed in Ref. [14].
There it was shown that in the context of a massless quark so-
lution
to the strong CP problem, higher dimensional operators
combined with the stringent bounds on the neutron EDM can re-
quire
the existence of new colored particles at the TeV scale. In
this note, we show that a broad class of parity based solutions to
the strong CP problems are also subject to strong constraints from
higher dimensional operators and also predict colored TeV scale
physics.
http://dx.doi.org/10.1016/j.physletb.2016.09.061
0370-2693/
© 2016 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
.