Dark spectroscopy at lepton colliders
Yonit Hochberg,
1,2,*
Eric Kuflik,
1,2,†
and Hitoshi Murayama
3,4,5,‡
1
Department of Physics, LEPP, Cornell University, Ithaca, New York 14853, USA
2
Racah Institute of Physics, Hebrew University of Jerusalem, Jerusalem 91904, Israel
3
Ernest Orlando Lawrence Berkeley National Laboratory, University of California,
Berkeley, California 94720, USA
4
Department of Physics, University of California, Berkeley, California 94720, USA
5
Kavli Institute for the Physics and Mathematics of the Universe (WPI),
University of Tokyo Institutes for Advanced Study, University of Tokyo,
Kashiwa 277-8583, Japan
(Received 28 June 2017; revised manuscript received 5 December 2017; published 22 March 2018)
Rich and complex dark sectors are abundant in particle physics theories. Here, we propose performing
spectroscopy of the mass structure of dark sectors via mono-photon searches at lepton colliders. The energy
of the mono-photon tracks the invariant mass of the invisible system it recoils against, which enables
studying the resonance structure of the dark sector. We demonstrate this idea with several well-motivated
models of dark sectors. Such spectroscopy measurements could potentially be performed at Belle II,
BES-III and future low-energy lepton colliders.
DOI: 10.1103/PhysRevD.97.055030
I. INTRODUCTION
The existence of dark matter (DM) is by now well
established, though its exact identity is unknown.
Theoretical proposals for its particle nature span many
orders of magnitude in mass, with various possible mech-
anisms for setting its relic abundance. Often, DM is part of a
larger dark sector, comprised of a wealth of resonances, and
can exhibit rich dynamics. Moreover, complex dark sectors
can arise in models beyond the Standard Model (SM),
irrespective of candidates for dark matter. The possibility to
experimentally study the structure of dark sectors is, there-
fore, an extremely relevant and important task.
In the context of dark sectors, expansive attention has
been devoted to dark photons that are kinetically mixed
with the SM hypercharge [1]. Constraints from beam
dumps, fixed-target experiments, B-factories, stellar envi-
ronments and colliders have been widely studied in the
literature (see e.g. Refs. [2–39], and Ref. [40] for a recent
review of this topic). Experiments such as BABAR and
those at LEP or the LHC have access to the light states of a
dark sector by searching for events with missing energy.
Such searches are generically sensitive to dark sector states,
but do not directly probe the spectrum of the dark sector,
which can often be rich with dark meson resonances. (For
works on measuring different dark sector properties at
colliders, see Refs. [41–45].)
Here, we propose a method to probe the resonance
spectrum of a dark sector. The idea is simple, and we first
draw an analogy from QCD. At an e
þ
e
−
machine, the
resonance structure of QCD can be mapped by scanning
the center of mass energy of the collision. Similarly, the
resonance spectrum can be studied by looking at e
þ
e
−
→
γ þ hadrons events at a fixed center of mass energy
collision. There, the mono-photon energy traces the mass
of the system it recoils against, thus performing spectros-
copy even at fixed center of mass energy. As the observa-
tion of the resonances is not required, such a measurement
can easily be performed on a dark sector, where the
resonances may be invisible. A schematic description of
this proposed dark spectroscopy is given in Fig. 1.
FIG. 1. Mono-photon production at a lepton collider.
*
yonit.hochberg@cornell.edu
†
kuflik@cornell.edu
‡
hitoshi@berkeley.edu, hitoshi.murayama@ipmu.jp
Published by the American Physical Society under the terms of
the Creative Commons Attribution 4.0 International license.
Further distribution of this work must maintain attribution to
the author(s) and the published articl e’s title, journal citation,
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3
.
PHYSICAL REVIEW D 97, 055030 (2018)
2470-0010=2018=97(5)=055030(7) 055030-1 Published by the American Physical Society