CERNCOURIER
CERN COURIER JULY/AUGUST 2019 13
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DIGEST
CERNCOURIER.COMThrough a beam dump darkly
The NA64 collaboration at CERN has
published world-best limits on dark
photons below a mass of 200 M eV,
excluding mixing strengths down to
10 –3, and below 10–5 for masses of the
order of 1 MeV (a rX iv:1906.00176).
Hypothetical dark-sector physics
could have an equally rich struc-
ture as the Standard Model, and the
dark-photon vector field is one pos-
sible extension that could mix with
the photon. A fixed-target exper-
iment at CERN’s SPS, NA64 (image
above) looks for missing energy in
the interaction of 100 GeV electrons
in an active beam dump.Circular reasoning
Following the European Strategy
Update in Granada (see p7), pro-
ponents of a 100 km future circular
collider in the vicinity of CERN have
released a “Frequently Asked Ques-
tions”-style document, which will
be regularly updated as the com-
munity debates the post-LHC future
(arXiv:1906.02693). Focused on the
first “FCC-ee” phase, the document
currently addresses 24 questions,
including “Why do we need at least
5 × 1012 Z decays?”, “Why do we want
FCC in Europe?” and quite simply
“Are there better ways to 100 TeV
than FCC-ee?”Dø nabs Zc±(3900)
Following first observations of the
charmonium-like state Z±c(3900) at
e+e– colliders in 2013, the Dø col-
laboration has published 5.4σ evi-
dence for its presence in the decay
chain of b-quarks in pp– collisionsM B r i c e/C E R N-P HO T O -2 0 161 1-278-(arXiv:1905.13704). Meanwhile, the
BESIII e+e– experiment in Beijing
has published 3.9σ evidence for its
decay to ρ±ηc (arXiv:1906.00831).
The Zc resonance’s quark con-
figuration is still unknown,
with models to describe its inner
structure including loosely bound
molecules of charm-meson
pairs, compact tetraquarks, and
hadro-quarkonium.Strange neutron stars
Researchers working on the STAR
detector at BNL’s Relativistic
Heavy-Ion Collider have measured
the binding energy of hypertriton- a hypernucleus consisting of a
proton, neutron and Λ hyperon.
They found a higher value than
the accepted 1973 measurement,
calling weakly-bound Λ-deuteron
models of the particle into ques-
tion (arXiv:1904.10520). The team
has also provided new constraints
on neutron-star interiors (see also
p16) and reported the first test of
matter–antimatter symmetry per-
taining to the binding of strange
quarks in a nucleus.
Hawking’s maths vindicated
Following the first observation of
Hawking radiation in 2016 (Nature
Phys. 12 959), researchers at the
Israeli Institute of Technology have
now quantitatively confirmed the
thermality of its spectrum and
measured its temperature using
an analogue black hole (BH) con-
sisting of a flowing Bose–Einstein
condensate (Nature 569 688). The
flow is supersonic within the ana-
logue BH and subsonic without:
the event horizon at the boundaryexcites the quantum vacuum and
gives rise to the emission of quanta,
known as Hawking radiation. The
group now plans to study the time
evolution of the analogue BH.In pursuit of Majorana
Researchers working on EXO-- a prototype liquid-xenon TPC in
Carlsbad, New Mexico – are step-
ping up their pursuit of neutrino-
less double beta decay (0νββ): the
most sensitive probe for the pos-
tulated Majorana nature of neutri-
nos, whereby a neutrino is its own
antiparticle. The full EXO-200 data
set allowed a 90% confidence limit
on the half life of 0νββ in^136 Xe of
3.5 × 1025 years (arXiv:1906.02723),
approaching the world-best limit
o f 10.7 × 1025 years set by KamLand–
Zen in 2016. Comparable limits in
other isotopes where single-beta
decay is suppressed have been set
for^76 G e (8.0 × 1025 years, GERDA)
and^130 Te (1.5 × 1025 years, CUORE).
Attention is now turning to nEXO, a
planned tonne-scale successor that
will push the sensitivity up by two
orders of magnitude to ~10^28 years.
The rarest decay
The XENON1T experiment, located
1.4 km below the Gran Sasso massif
in northern Italy, has observed the
rarest decay process ever seen, with
a half-life a trillion times longer
than the age of the universe: the
two-neutrino double-electron
capture of^124 Xe (Nature 568 532).
The measurement is a step towards
the search for neutrinoless dou-
ble-electron capture, which, like
neutrinoless double-beta decay,
would establish the Majorana
nature of the neutrino and give
access to the absolute neutrino
mass. The detector’s active mass
is now being trebled to boost its
primary dark-matter search.Got an axion to grind?
Researchers are turning to cre-
ative proposals to search for
axions – cold dark-matter can-
didates originally postulated to
resolve the strong CP problem in
QCD. Berkeley theorists propose
employing superconducting RFcavities: axions would be con-
verted to photons in the magnetic
field of a gapped toroid inspired
by the ABRACADABRA and DM
Radio experiments (arXiv:1904.
0724). Meanwhile, cosmologist
Lawrence Krauss proposes to
use atomic clocks to look for the
effect of an axion background
on photon propagation in the
vacuum (arXiv:1905.10014). Sim-
ilarly inventive proposals pub-
lished recently seek to extend
the search for generic low-mass
dark matter downwards, for exam-
ple using nuclear recoils in lab-
grown diamond crystals (Phys Rev
D 99 123005).EDM search goes pear-shaped
Permanent electric dipole
moments (EDMs) of particles are
excellent testbeds for the Standard
Model. Nonzero EDMs imply that
both P and T, and by implicationCP, are violated. Nuclear physicists
have thus been seeking to measure
atomic EDMs since the 1980s, with
pear-shaped nuclei the most prom-
ising candidates. More correctly
termed a static octupole distor-
tion, the pear shape was first seen
at CERN’s ISOLDE facility in radi-
um-isotope^224 Ra in 2013 (Nature
497 199). However, Peter Butler
of the University of Liverpool and
coworkers, using beams from the
upgraded HIE-ISOLDE, have now
established that radon-isotopes(^224) Rn and (^226) Rn do not possess
static pear shapes in their ground
states, and are thus not promis-
ing candidates to have measura-
ble atomic EDMs (Nat. Commun. 10
2473), inducing the team to switch
focus to other isotopes.
The NA64 experiment.
Nuclei with octupole distortions may
bear fruit in EDM searches.
Jeff Steinhauer’s group has measured
the Hawking temperature.
Technion
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