CERNCOURIER
NEWS
A NALYSIS
CERNCOURIER.COMCERN COURIER JULY/AUGUST 2019 7
The open symposium of the European
Strategy for Particle Physics (ESPP),
which took place in Granada, Spain, from
13–16 May, revealed a vibrant field in flux
as it grapples with how to attack the next
big questions. Opening the event, chair
of the ESPP strategy secretariat, Halina
Abramowicz, remarked: “This is a very
strange symposium. Normally we discuss
results at conferences, but here we are
discussing future results.” More than
10 different future-collider modes were
under discussion, and the 130 or so talks
and discussion sessions showed that ele-
mentary particle physics – in the wake of
the discovery of the Higgs boson but so
far no evidence of particles beyond the
Standard Model (SM) – is transitioning
into a new and less well-mapped realm
of fundamental exploration.Plain weird
Theorist Pilar Hernández of the Uni-
versity of Valencia described the SM as
plain “weird”. The model’s success in
describing elementary particles and their
interactions is beyond doubt, but as an
all-encompassing theory of nature it falls
short. Why are the fermions arranged
into three neat families? Why do neutri-
nos have an almost imperceptibly small
mass? Why does the discovered Higgs
boson fit the simplest “toy model” of
itself? And what lies beneath the SM’s
numerous free parameters? Similar
puzzles persist about the universe at
large: the mechanism of inflation; the
matter–antimatter asy mmetr y; and the
nature of dark energy and dark matter.
While initial results from the LHC
severely constrain the most natural
parameter spaces for new physics, said
Hernández, the 10–100 TeV region is an
interesting scale to ex plore. At the same
time, she argued, there is a shift to more
“bottom-up, rather than top-down”,
approaches to beyond-SM (BSM) phys-
ics. The new quarries includes a xion-like
and long-lived particles, and searches
for hidden, dark and feebly-interacting
sectors – in addition to studying the
Higgs boson, which has deep connec-
tions to many puzzles in the SM, with
much greater precision. “Particle physicsLong view
Discussions during
a morning coffee
break at the
ESPP update.Eu ropE a n Str atEg y
Granada symposium thinks big
could be heading to crisis or revolution,”
said Hernández.
The accelerator, detector and com-
puting technology needed for future
fundamental exploration are varied
and challenging. Reviewing Higgs-
factory programmes, Vladimir Shiltsev,
head of Fermilab’s Accelerator Physics
Center, weighed up the pros and cons
of linear versus circular machines.
The former includes the International
Linear Collider (ILC) and the Compact
Linear Collider (CLIC); the latter a future
circular electron–positron collider at
CERN (FCCee) and the Circular Electron
Positron Collider in China (CEPC). Lin-
ear colliders, said Shiltsev, are based on
mature designs and organisation, are
ex pandable to higher energies, and draw
a wall-plug power similar to that of the
LHC. On the other hand, they face chal-
lenges including their luminosity and
number of interaction points. Circular
Higgs factories offer a higher luminosit y
and more interaction points than lin-
ear options but require R&D into high-
efficiency RF sources and superconduct-
ing cavities, said Shiltsev.
For hadron colliders, the three cur-
rent options – CERN’s FCC-hh (100 TeV),
China’s SppC (75 TeV) and a high-energ y
LHC (27 TeV) – demand next-generation
superconducting dipole magnets. Akira
Yamamoto of CERN/KEK said that while a
lepton collider could begin constructionin the next few years, the dipoles nec-
essary for a hadron collider might take
10 to 15 years of R&D before construction
could start.
The symposium also saw much dis-
cussion about muon colliders, which
offer an energy-frontier lepton collider
but for which it was widely acknowledged
the technology is not yet ready. Concerning
more futuristic acceleration technologies
based on plasma wakefields, impressive
results at facilities such as BELLA at Berke-
ley and AWAKE at CERN were on show.Thinking ahead
From colliders to fixed-target to astro-
physics ex periments, said Francesco Forti
of INFN and the University of Pisa, detec-
tors face a huge variety of operating con-
ditions and employ technologies deeply
entwined with developments in industry.
Another difficulty, he said, is how to han-
dle non-standard physics signals, such as
long-lived particles and monopoles. Like
accelerators, detectors require long time
scales – it was the very early 1990s when
the first conceptual design reports for the
LHC detectors were written.
In terms of data processing, the chal-
lenges ahead are immense, said Simone
Campana of CERN and the HEP software
foundation. The high-luminosity LHC
(HL-LHC) presents a particular challenge,
but DUNE, FAIR, BELLE II and other ex per-
iments will also create unprecedented
data samples, plus there is the need to
generate ever-more Monte Carlo samples.
At the same time, noted Campana, the
rate of advance in hardware performance
has slowed in recent years, forcing the
community to towards graphics process-
ing units, high-per formance computing
and commercial cloud services. Forti and
Campana both argued for better career
opportunities and greater recognition
for physicists who devote their time to
detector and computing efforts.
The symposium also showed that the
strategic importance of communications,
education and outreach is becoming
increasingly recognised.
Discussions in Granada revealed a
community united in its desire for a post-
LHC collider, but not in its choice ofR StromNormally
we discuss
results at
conferences,
but here we
are discussing
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