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Making muon rings round neutrino factories

15 March 2000

A new way of making neutrino beams has caught the attention of physicists
worldwide.

Almost every day, fresh results steadily fuel the progress of science. Less
frequently, major breakthroughs in experimental techniques revolutionize the way
in which this research is done. Examples of such breakthroughs in particle physics
include the development of accelerators in the 1950s, and of colliding rings in the
late 1960s, finally culminating with CERN’s proton-antiproton collider, using
beam-cooling techniques and opening up a new energy regime.

Although
there is still a long research and development road to be negotiated, the first major
breakthrough in particle physics experimental techniques for the 21st century looks
to be the advent of a new type of machine – the muon storage ring – and using it to
provide neutrino beams.

Making accelerators with muons seems crazy at
first. Machine builders so far have had the wisdom to store and accelerate particles
that are abundant – like the protons and electrons naturally found in matter – or, if
not abundant, that at least have the good taste to be stable, like positrons and
antiprotons.

It takes at least 30 min to fill CERN’s LEP collider and
accelerate its beams of electrons and positrons. How could one do such a thing
with unstable muons, with their combined inconveniences of being rare and having
a lifetime of a mere 2.2 ms?

Progress in
accelerator techniques has made this challenge at least conceivable, to the extent
that there has been discussion of muon collider rings (CERN
Courier
December 1997 p1) as a serious future option in the US and at
CERN.

When, following the inspiration of the US muon collider
collaboration, European physicists started looking at this new route, the obstacles
appeared overwhelming, with many new problems to solve
simultaneously.

A breakthrough came with the realization that muon decay
could be turned into an advantage – muon storage rings would be an abundant
source of neutrinos. Coming at the same time as the new awareness of neutrino
oscillations (CERN CourierSeptember 1998 p1), this step forward met with
a thunderclap of enthusiasm.

The muon storage ring as neutrino source,
nicknamed “neutrino factory”, requires a much lower density of particles and
should thus be easier to build than a muon collider. The decay of muons into
electrons provides the only known source of high-energy electron-type neutrinos –
a unique and powerful new physics tool.

This led the prospective study
group mandated by the European Committee for Future Accelerators (ECFA) to
propose a three-step approach to muon storage rings, the first being the
construction of a neutrino factory (Autin, Blondel and Ellis 1999). This has led to a
series of international workshops – Lyon in July 1999 and Monterey, California, in
May 2000. Neutrino-factory research and development is now a well recognized
and supported project at CERN and further afield, with ECFA-supported study
groups investigating the very rich physics opportunities.

Beams
from rings

The key requirement is a very intense proton accelerator, delivering
several megawatts of beam power. These protons will be used to create pions,
which will be magnetically collected. Designing a target to withstand so much
power more than once is beyond what has been achieved so far and will require
either a liquid jet target or a very large rotating wheel to dissipate the
heat.

Pion collection is optimized for rather low momentum – about 300
MeV/c. These pions rapidly decay into muons of similar momentum. At this point
the “beam” is about 1 m across and the haphazard momentum spread 100% –
more like a big, hot potato than a beam.

The design challenge is to shrink
the momentum spread to 5% and the beam size to a few centimetres within a few
microseconds to shape the muons into an acceptable beam.

This requires
two crucial elements. The first, “phase rotation” (“monochromatization”), uses
variable longitudinal electric fields of a few million volts per metre to slow down
the fastest particles and accelerate the slow ones. This needs either high-gradient,
low-frequency radiofrequency cavities or an induction linac, with considerably
improved performance compared with what has been achieved so far.

The
second crucial beam element is cooling. Beam cooling was a key feature of
CERN’s antiproton project, converting the largest possible number of rare particles
produced from a target into a smooth beam. While antiprotons are stable and can
be stored almost indefinitely, muons need fast action. However, as muons choose
not to interact strongly with nuclear matter, one can use cooling via ionization
energy loss, reducing the momentum in three dimensions. Followed up by
reacceleration in the beam direction via a longitudinal electric field, the net result
will be a decrease of transverse momentum. Simulations are promising, but this
technique has yet to be demonstrated in practice.

This initial conditioning is
followed by a series of fast accelerators to take the muon beam to high energy. If
well designed, the system spares enough muons after decays or acceptance losses
so that, from the original 1016 protons per second, 1014
high-energy muons per second can be injected into a storage ring, where during a
few hundred turns, positively charged muons, for example, will decay into
electrons, accompanied by electron-type neutrinos and muon-type
antineutrinos.

Storage ring geometry

The intentionally long,
straight sections of the storage “ring” generate a large flux of collimated neutrinos,
particularly electron-type ones, with properties very different to those of traditional
laboratory neutrino beams (which are mainly composed of muon-type neutrinos).
The geometry of the storage ring is left to the designer’s imagination. Bow-tie,
triangular and trombone ring configurations have been proposed.

Whatever
the geometry, very intense neutrino beams would be available right next to the
storage ring, opening a new era of neutrino physics. However, what has made
everyone really excited is the prospect of firing neutrino beams through the Earth,
serving several underground experiments in several continents and providing
different neutrino flight paths – “baselines” – for the study of neutrino
oscillations.

For a long time the three neutrino types (electron-, muon- and
tau-) were considered massless, and thus immutable.

Following indications
from solar neutrinos as early as 1975, experiments studying neutrinos produced by
the decay of cosmic-ray pions and muons in the atmosphere finally confirmed in
1998 that neutrinos undergo transmutations. The observed signals can only be
understood if neutrinos starting out as muon-type in the upper atmosphere change
into another type in transit – probably tau neutrinos. This neutrino “oscillation” can
only be understood if the particles have a mass.

Although these masses are
probably tiny – a fraction of an electron volt – the consequences are considerable.
As neutrinos are one of the most common particles in the universe, their total mass
could provide a significant fraction of the whole mass of creation. From the particle
physicists’ point of view, neutrinos are very interesting. Since they do not feel
electromagnetic or strong forces, one hopes they could provide cleaner clues to the
origin of mass.

In quantum mechanical language, neutrinos produced in a
weak decay or interacting via weak interaction are well defined – the well known
electron, muon and tau neutrino “flavours”. However, if they have mass, neutrinos
also feel the mysterious “Higgs” force that generates masses, and the neutrino
states emerging with well defined masses need not be the same as those with well
defined flavours.

The three flavour neutrinos are therefore mixtures of the
three mass neutrinos, and a matrix of parameters connects the two triplets.
Moreover, as usual in quantum mechanics, this mixing has time-dependent phases,
so that any one neutrino flavour turns into another as time passes – as one type of
neutrino disappears, another “appears” to take its place. This is what is meant by
neutrino oscillations.

Information on these oscillations is still scanty, but
atmospheric neutrino experiments tells us that a muon neutrino of 1 GeV probably
turns into a tau neutrino after about 500 km. Experiments with electron neutrinos
from nuclear reactors show that these particles are reluctant to oscillate on this
timescale. The disappearance of solar neutrinos, which set out as electron-type,
shows that these particles have a much longer oscillation timescale. However, solar
neutrinos are somewhat ambiguous, since neutrinos produced deep in the stellar
interior have to travel through the Sun before emerging into the vacuum of space,
and one does not know where the oscillation takes place.

New “long
baseline” experiments, firing neutrino beams at detectors hundreds of kilometres
distant, are setting out to explore these oscillations in more detail (CERN
Courier
January p1). However, these experiments are based on conventional
synthetic neutrino beams, composed mainly of muon-type particles, and are
expected to validate and sharpen the pattern derived from the combined findings of
atmospheric neutrino experiments and reactor neutrino experiments, although
surprises cannot be excluded.

Crucial information should come from new
reactor and solar neutrino experiments – Kamland (CERN CourierApril
1999 p22), Borexino (CERN CourierOctober 1998 p12) and SNO
(CERN CourierJuly 1998 p1) – sensitive to the disappearance of electron
neutrinos suggested by the solar neutrino experiments.

New
neutrino physics

With the neutrino factory, and as new results from solar, reactor
and accelerator experiments become available, physicists can plan a much more
systematic investigation of neutrino mass differences and mixings. The key is the
high-intensity flux of electron neutrinos from a neutrino factory. With this, any
appearance of muon neutrinos from oscillation of the electron neutrinos would give
an immediately recognizable neutrino interaction signature, producing a muon of
opposite sign to that of the original muon beam.

Comparing results using
beams of positively and negatively charged muons would contrast the behaviour of
electron neutrinos and their antineutrinos. As neutrinos pass through matter, they
encounter atomic electrons. The interactions of electron neutrinos and antineutrinos
with these electrons are different, and would lead to a matter-induced
asymmetry.

Depending on whether the transmutation into muon neutrinos
of electron neutrinos and antineutrinos are enhanced or suppressed by matter, one
would be able to distinguish between the two mass scenarios shown in the
figure.

CP violation with neutrinos

Comparing oscillation rates
for electron neutrinos and antineutrinos would open another possibility, which until
recently had been almost unthinkable. By comparing the transformations of, say,
electron-neutrinos into muon-neutrinos with the process in the reverse direction,
and with the corresponding rates for antineutrino transformations, physicists could
for the first time be able to investigate delicate CP and time symmetry violations
for the neutrino sector.

Such effects have been well explored in the quark
sector, using the neutral kaon system. CP violation unambiguously differentiates
particles and antiparticles, implying that what is called matter and what is called
antimatter is not a heads-or-tails call. This is one of the necessary ingredients to
explain how a matter-dominated universe evolved from a Big Bang that
supposedly produced equal amounts of matter and antimatter.

CP violation
is deeply connected to the violation of time reversal symmetry, when a “film” of a
particle interaction run backwards would look different.

Neutrino physicists are very excited at these prospects. However, such
experiments would require very long baselines (in excess of 3000 km) and
preferably two different baselines to unravel different processes. This leads to a
vision of a truly world machine with intercontinental beams.

These new
neutrino sources are of world-wide interest and a whole network of detailed
working groups has been set up to attack the problems. A crash study at Fermilab
will shortly make its recommendations, while a wider study involves other US
laboratories.

In Europe, CERN has set up a neutrino factory study group
with specialized subgroups looking at specific machine components (proton driver,
targets, accumulator rings, etc). Other groups, under the sponsorship of the ECFA,
look at physics objectives. These studies involve specialists from many European
laboratories.

By the time this year’s Neutrino Factory meeting in Monterey
takes place in May, these plans should have progressed significantly and hopefully
give insight on how difficult the construction of a neutrino factory will be, on how
long it would take to design and build. A similar effort is necessary to understand
what detectors could be built to best take advantage of these fascinating beams.
This is certainly a line of physics that will take us well into this century!

Further reading
B Autin, A Blondel and J Ellis,
Prospective Study of Muon Storage Rings at CERN,CERN 99-02, ECFA
99-197.

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