Do neutrinos exist?
The Cowan-Reines Experiment
Do neutrinos exist? And if they do exist, what makes the neutrino
physicists think they actually interact with ordinary
matter at all?
These seem like ridiculous questions given
the amount of time and money that has been spent on neutrino research. How could they not be detectable? Billions of dollars and entire lives have
been dedicated to the study of neutrinos.
Nobel prizes have been handed out.
However, unlike photons, neutrons,
electrons, protons, and an entire zoo of unstable particles, direct observation
of neutrinos has never been achieved.
Rather, neutrinos are postulated to exist based on assumptions about their
interactions with other particles. So, why
did the neutrino become solidified in the hearts and minds of the physicists
long before they discovered a single one?
After all, neutrinos have some very unusual
characteristics: they travel at speeds indistinguishable from light, and yet,
they seem to have mass. They can carry
large amounts of energy, and yet, are not directly detectable. And they supposedly oscillate between three
different neutrino flavors. Or
more. From an outsider’s perspective,
neutrino theory is about as head-scratching a collection of hypotheses as you will
see in the realm of physics.
No one ever accidentally found a neutrino
and then said "hey, something keeps popping up in our experiment, what is it?"
Rather, there was an intense urgency to discover the neutrino.
And that was because it solved the problem of
the "missing" energy and spin of beta decay. If you have an electron ejected from a
nucleus, then obviously you need another spin ½ particle to conserve angular
momentum.
Therefore, the neutrino was quickly
assigned lepton status, just like its cousin, the electron.
As such, the discovery of the neutrino was
met with almost universal acceptance, despite the questionable results and data
tinkering from many of the neutrino experiments. The normally skeptical community of
physicists quickly embraced the sketchy experimental evidence, primarily
because it maintained sacred conservation rules.
And the fact that it has mass, travels at
the speed of light, violates certain parity rules, changes "flavors", and
contains large amounts of energy, all the while being directly undetectable,
well, I guess that was easier to live with than any other theory to explain the
continuous energy spectrum of beta decay.
But is there really a neutrino, or have we
all been subjected to one of the greatest episodes of scientific groupthink
since the geocentric model of the universe?
The necessity for the neutrino
But let’s go back to the original problem
that brought about the need for the neutrino.
And that was the energy associated with the three main types of
radiation. Alpha, beta, and gamma. Alpha particles are emitted by large atoms,
such as uranium. However, the kinetic
energy of the emitted alpha particles falls into discrete levels, analogous to the
above graph on the left.
Atoms such as Cobalt-60 emit both beta and
gamma radiation. But the energy of the
emitted gamma radiation falls into discrete levels, something like the above graph
on the right. However, with beta decay,
the kinetic energy of the emitted particles is continuous, and looks more like
the graph below.
If there ever was an original sin with
regards to neutrinos, Wolfgang Pauli committed it. The problem Pauli wanted to solve was the
phenomenon of the continuous energy spectrum of beta decay. Why was beta radiation continuous, while
alpha and gamma were discreet?
Let’s take a closer look at the continuous
spectrum of beta decay. Carbon-14 has 6
protons and 8 neutrons, and one of those neutrons will eventually release an
electron, and decay into a proton, creating Nitrogen-14, with 7 protons and 7
neutrons.
And in the figure below, we see exactly
the problem faced by Pauli. On the
vertical axis is the fraction of electrons measured during the decay events, and
the horizontal axis is their kinetic energy.
0.16 MeV is the maximum energy associated with the electron during
decay, however, it is only achieved by a very small fraction of the electrons.
Why are the electrons showing such a broad
range of energy during the decay? What
happened to the discrete nature of radiation?
Shouldn’t they all decay with discreet energies?
And with this simple presumption, the
neutrino was born.
Carbon-14 Beta decay energy spectrum
Pauli wrote a famous letter, almost
embarrassed to be proposing the existence of the neutrino that would carry off the
remainder of the missing energy. Therefore,
the total energy of the electron combined with the neutrino was a constant, and
the conservation of energy was saved.
Okay, obviously it is easy to see an
immediate issue with Pauli’s proposal.
This new particle would also have to be continuous in its energy
distribution to make the total energy of beta decay a constant value. So now we have not just one particle with a
continuous energy distribution, the electron.
With the neutrino, we now have two.
But no problem with that. As long as the
electron and neutrino balanced Pauli’s need for discrete energy accounting, it
was a go. And Pauli was thinking big
with this new particle. He thought it
might be about the same mass as the electron, but had
no electrical charge.
But if it was so big, why hasn’t anyone
seen it yet? Thus, Pauli’s embarrassment
in his letter, which kicked off the search for the neutrino (which he calls a "neutron"):
"But so far
I do not dare to publish anything about this idea, and trustfully turn first to
you, dear radioactive people, with the question of how likely it is to find
experimental evidence for such a neutron if it would have the same or perhaps a
10 times larger ability to get through [material] than a gamma-ray."[1]
In other words, Pauli was trolling for a
particle, any particle, that could resolve this continuous nature of the energy
spectrum of beta decay. But was a new
particle necessary?
There were other explanations. Niels Bohr proposed that energy conservation
was statistical. It wasn’t going to be
the same from decay to decay. And at the
time of Pauli’s proposal, there were other particles involved in beta decay,
like the recoiling nucleus, that might account for the continuous energy distribution
of the emitted electrons.
After all, later experiments have determined
that the energy of radial electron emission during beta decay was higher than
electrons emitted tangentially. The
angle of emission is playing a role in the energy of the emitted electron.
Further, beta decay sometimes emits
photons. And interestingly, the energy
associated with these photons is also not constant. Bales et al.[2]
tracked photons with energies varying from 0.4 keV all the way to 782 keV.
In the nucleus, beta-minus decay (emission
of an electron) is more likely to occur as the number of neutrons in a nucleus
exceeds the number of protons, and beta-plus decay, also known as positron
emission, is more likely as the number of protons exceeds the number of
neutrons.
So there are dynamic Coulomb forces in the nucleus that contribute heavily
to initiating beta decay, which indeed could also be contributing to the continuous
beta decay spectrum. But positing the
existence of a neutrino instead of some risky new theory on how the nucleus operates
is a heck of a lot easier.
Pauli did something that would be repeated
often in the face of an inexplicable experimental result: propose the existence
of a new particle.
And hopefully they will find it. But despite Pauli’s lofty reputation, the
neutrino was initially met with much resistance. And for his new theory to quickly climb up to
the top of the physics pops, Pauli needed support from another capo.
What Weak force?
Pauli, in the very famous letter about his new
neutrino, said:
"I admit that my remedy
may seem almost improbable because one probably would have seen those neutrons,
if they exist, for a long time. But
nothing ventured, nothing gained..."[3]
And sure enough, Pauli placed his bet. Fortunately for him, one of the giants of his
day, Enrico Fermi, bought hook, line, and sinker into Pauli’s neutrino. There was a very good reason for Fermi’s enthusiasm, which
emerged in the previous decade, in Paul Dirac’s quantum theory of radiation.
In this theory, Dirac’s electromagnetic force
was caused by the exchange of a photon between two interacting electrons. So, if two electrons bumped into each other,
they would exchange a photon, and repel each other in proportion to the energy
of that photon.
Fermi loved Dirac’s theory so much he used
it as the basis for his own theory of beta decay. But instead of Dirac’s theory of photon
exchange mediating electromagnetic interactions, Fermi’s theory was contorted
into a force that causes a particle to transform into some other particle. And electric charge is the main thing that
was transformed, although spin could also be mediated by this new weak force.
And according to Fermi’s theory of beta
decay, the cause of neutrons converting into protons was this new "weak"
force.
But Fermi had a problem. Instead of the large distances in which
electrons could interact, Fermi’s weak force had to be local, for a very good
reason. A weak force that could
transform particles from great distances was catastrophic to the universe.
And just like Pauli, Fermi had an
existential problem. Why hadn’t this
neutrino shown its face? Strange that
such a numerous particle that can carry away large quantities of energy would
go completely unseen.
So, goodbye to Pauli’s electron sized
neutrino. In Fermi’s mind, the neutrino had
to be next to nothing. It had to be so
non-interactive with ordinary matter that no one has yet to see one. And that meant its rest mass had to be next
to nothing.
Can you prove it exists? Probably not.
But can you prove it doesn’t?
So, Fermi stuck us with this new force
that was quite a bit different from the other forces, which were attractive and/or
repulsive. Instead, this new force
transformed particles into other particles, and its range was a fraction of
that of the strong force. And thus, its
name, "weak".
But Fermi was not done sampling from
Dirac. Analogous to how light quanta
behaved in radiation theory, Fermi proposed that the number of electrons and
neutrinos emitted during beta decay were not constant.
It is important to note that both neutrons
and protons experience beta decay. Beta-minus
decay, or the decay of neutrons, emits electrons. And beta-plus decay, or the decay of protons,
emits positrons. So naturally, there was
proposed to be a different neutrino emitted corresponding to each type of beta decay:
an antineutrino for beta-minus decay, and a regular neutrino for beta-plus
decay.
The physicists were far from done with the
ever complexifying theory of the neutrino.
We also have to deal with something called neutrino
"handedness", as according to the neutrino physicists, there are left-handed
neutrinos and right-handed antineutrinos.
Okay, we’ll give the neutrino physicists
their due. Life is not easy. After all, when you can’t directly detect a
neutrino, how do you assemble experimental evidence on it? Neutrinos could be doing almost anything as
far as they knew, and proposals for their behavior are about as wild as
anything you will see in physics.
So how did the physicists detect them if
they couldn’t directly see them?
Let’s start by looking at the two aforementioned processes where neutrinos are supposedly
created. Beta-minus decay and beta-plus
decay.
In the above diagram, we
see the two varieties of beta decay.
Beta-minus decay starts with a neutron.
In this example, the neutron is bound in a Carbon-14 nucleus and decays
into a proton, electron, and antineutrino, creating Nitrogen-14. On the other hand, beta-plus decay starts
with a proton. In the above case it is
bound in a Carbon-10 nucleus, which decays into a neutron, positron, and
neutrino, creating Boron-10.
Note that both these
processes start with a single particle (proton or neutron) which each decay
into three new particles. But there is a
slight problem. In beta-minus decay, the
neutron has more mass than the proton and electron combined, so there is
nothing else needed to add to the decay to get the neutron to eject the
electron. And indeed, neutrons not bound
to any nucleus decay very quickly, in about fifteen minutes.
But in beta-plus decay, the
proton has less mass than the resultant particles, which are the neutron and
positron. So, what gives? And indeed, free protons do not decay on
their own into a neutron and positron.
Only protons bound in nuclei do that.
So, we can certainly make the argument that beta-minus decay and
beta-plus decay are not mirror images. Beta-plus decay steals mass-energy from somewhere,
presumably from its constituent nucleus.
It is relatively easy for
physicists to set up detectors to capture the emitted electrons and positrons
from beta decay. But no such luck with
directly detecting the ghostly neutrinos.
But, where there’s a will,
there’s a way, and it didn’t take long after Fermi’s theory for Bethe and Peierls, in 1934, to come up with the "cross section" for
something called inverse beta decay.
Inverse beta decay? What the hell is that?
While it would be logical
to presume so, inverse beta decay is not to be confused with beta-plus decay,
which is a nucleus converting a proton into a neutron and ejecting a positron
and neutrino.
So, what is this inverse
beta decay that became so important in the detection of neutrinos?
Inverse beta decay: antineutrino hits a proton,
creating a neutron and positron
As seen above, inverse beta decay is not
exactly inverse. Unlike the one particle
in and three particles out interaction of beta decay, inverse beta decay is two
particles in and two particles out. An
antineutrino hits a proton (bound in a nucleus) and ejects a neutron and a
positron.
Okay, you might rightly ask how this process
would be used to detect neutrinos? Well,
it’s all in the timing of photon detection, and not in the detection of
neutrinos.
In the above process, we are told that an
antineutrino interacts with a proton, which ejects a positron, creating a
neutron which is subsequently captured by a nucleus which releases photons.
The positron interacts with an electron,
releasing two photons. And that’s
it. As you might notice, detection of a
neutrino has nothing to do with directly detecting neutrinos. It’s all about detecting photons.
This entire process indeed seems like a
substantial leap of faith, because there are a multitude of physical processes
that create photons. And those processes,
also known as "noise" or "background", would have to be filtered out during the
experiment.
So, to detect a neutrino, the physicists
need just a few things: a "cross section", which is akin to the probability of
interaction the neutrino has with a proton; a positron that will quickly
interact with an electron and emit high energy photons; and an event where a
nucleus captures a neutron and emits other high energy photons.
And high energy photons are easy to
detect.
Now all the neutrino experimentalists have to do is see how closely in time these high energy photons
correlate with each other. It was like
taking candy from a baby.
The Cowan and Reines
Neutrino Experiments
In
reality, it was the exact
opposite. Despite the uneasy feeling one
gets from inferring the existence of neutrinos by using time-correlated photon
detection, the experiments on inverse beta decay were finally executed by Frederick
Reines and Clyde Cowan in the 1950s.
Their first experiment, in 1953, known as "The Hanford Experiment", used
neutrinos purportedly created during nuclear fission from a reactor located in
Hanford, Washington.
They
were using the "delayed coincidence" technique, which involved intercepting
gamma rays from positron annihilation and neutron capture as proof that a
single neutrino caused the chain of events. And after months
of tinkering with their shielding techniques and scintillation solutions, they
could not eliminate the background noise, although they claimed the neutrino signal they did detect wasn’t far from passing the ordinary
tests of statistical significance.
Regardless, Cowan summed up
their thoughts on this inconclusive experiment:
"Neutrons
and gamma rays from the reactor, which we had feared most, were stopped in our
thick walls of paraffin, borax and lead, but the cosmic ray mesons penetrated
gleefully, generating backgrounds in our equipment as they passed or stopped in
it. We did record neutrino-like signals but the cosmic
rays with their neutron secondaries generated in our shields were 10 times more
abundant than were the neutrino signals. We felt we
had the neutrino by the coattails, but our evidence would not stand up in
court."[4]
But Reines and Cowan were
sure the neutrino existed. They turned
their attention to the reactor at Savannah River in South Carolina, where they finally
struck gold. Their famous 1956 Savannah
River experiment was reported in both Science
and Nature magazines, and of course
they went on to describe why they believed it was the neutrino that they
detected, and not the background.
It was because of the
strange thing known as the "neutrino cross section".
Here’s what Reines and
Cowan reported in Science in 1956:
"A reactor-power-dependent signal was observed which was
(within 5 percent) in agreement with a cross section for reaction 1 of 6.3x10-44
cm2. The predicted cross section (8) for the
reaction, however, is uncertain by ±25 percent.’’[5]
That was a spectacular
result. I mean, a cross section of 6.3x10-44 cm2 is small. Really
small. This was a startling agreement
between theory and experiment. One of
the most startling results in the history of physics.
After all, it would be a
miracle to get this close without both the theory and the experiment being
correct. What are the odds that a cross
section on the order of 10-44 would be both theoretically and
experimentally derived to such a high level of agreement?
That can only happen if both
were correct. The theory and the
experiment matched. Neutrinos existed,
and they proved it. A stunning
achievement in both the theory and the experiment!
You have
to feel some sympathy for the position that Reines and Cowan were
in. Most interested physicists agreed
there had to be a neutrino, and there were some big names in this group. Pauli, Fermi, Bethe. These were the big boys.
If Reines and Cowan couldn’t
match the neutrino’s theoretical cross section, then maybe they were bad experimental
physicists?
But they hit right on it. Within 5% of 6.3x10-44
cm2. And the congratulations followed for
everyone. A case of champagne and Nobel
prizes all around. It was a Hollywood
ending.
Or was it? Was this really the correct theoretical neutrino cross
section? What the hell were Reines and
Cowan looking at? Are you kidding? The neutrino cross section of 6.3x10-44 cm2 wasn’t really the correct theoretical cross
section after all?
Then how did Reines and Cowan
match it so closely? How is it possible
to match an incorrect theoretical calculation, unless
you were trying to match it?
Indeed. After Cowan
and Reines’s 1956 boast, published in Science
and Nature, about matching the
neutrino’s theoretical cross section, there is a terrifying problem that haunts
all experimental physicists that have just proven a theory to be correct.
The theory changed on
them. In 1957, Lee and Yang came up with
a new estimate of the neutrino cross section that effectively doubled the
neutrino size. And unfortunately for Cowan
and Reines, after publishing their great achievement, they had yet to publish
the details about how they did it.
They just said that they
discovered the neutrino by matching the theoretical cross section within 5%. Talk about karma.
And since Lee and Yang’s
new left-handed antineutrino was based on the preliminary findings of another
neutrino-related experiment regarding the weak force, Reines and Cowan had a
big problem. There were other experiments
out there that were giving information about neutrino cross sections.
Do you stick to your guns
and tell Lee and Yang to piss off? That seems like a reasonable response. After all, Reines and Cowan did the
experiment to the best of their ability, and those were their results. This is unbiased physics, after all. There are no politics in physics. So, screw Lee and Yang.
But nope. Reines and Cowan caved, and when their
detailed version of their experiment appeared in 1960, the experimental cross
section they had estimated had doubled from their initial press release in
1956.
It miraculously now matched
Lee and Yang’s new cross section. This mysterious
adjustment, according to Reines, was because:
"our analysis grossly overestimated the
detection efficiency with the result that the measured cross section was at
first thought to be in good agreement with [the pre-parity violation]
prediction."[6]
Okay, perhaps this was an
honest mistake. Or Reines was indeed
tinkering with the estimated "detection efficiency" to make the experiment land
near the theoretical cross section at that time. But Reines just didn’t land on it once. He landed on it twice.
This post hoc retrofitting of
the experimental data to better match the theory is something we will see in
our other case studies. We will also
see a case where the theory was retrofitted to fit the experimental data before the
experimental data was released.
So, the early neutrino
experiments of Reines and Cowan would never hold up in court, and he and his
associates should quite correctly be dismissed as arbiters of neutrino
detection.
But they weren’t. Reines went on to bigger and better things,
becoming a senior leader in the Irvine-Michigan-Brookhaven neutrino
collaboration, and the probable target of Dr. John LoSecco’s
critique of IMB management.
In any event, the Nobel
committee was also suspicious of Reines, and waited 40 years to recognize him,
long after neutrinos were a consensus fact.
Please send comments to Charles Brack at brack@wtfphysics.com
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[1]Wolfgang Pauli, Open Letter,
1930. https://fermatslibrary.com/s/the-proposal-of-the-neutrino
[2]Bales
et al. Precision Measurement of the Radiative Beta Decay of the Free Neutron.
Phys. Rev. Lett. 16, 242501 (2016) https://arxiv.org/pdf/1603.00243.pdf
[3] Pauli, 1930. https://fermatslibrary.com/s/the-proposal-of-the-neutrino
[4] Cowan, C. L. 1964. Anatomy of
an Experiment: An Account of the Discovery of the Neutrino. Smithsonian
Institution Annual Report: 409.
[5] C. L. Cowan, Jr., F. Reines,
F. B. Harrison, H. W. Kruse, A. D. McGuire. Detection of the Free Neutrino: a
Confirmation. Science. 20 July 1956, Volume 124, Number 3212.
[6] Reines, F. 1979. The Early
Days of Experimental Neutrino Physics. Science 203 (4375): 11.
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