While science, or more accurately, the scientific
method, is clearly the most successful approach
ever devised to understand the physical world,
it's worth remembering that absolute certainty
is never possible, although some things are
more certain than others.
For instance, while we're incredibly confident
that the Earth isn't flat and that cigarettes
are extremely bad for you, researchers like
my colleagues and I are used to encountering
ideas and claims that are far less solid.
It's entirely normal for discoveries to appear
and disappear and sometimes reappear again.
A recent announcement by an experiment at
the LHC has the potential to have this elusive
character.
This interesting story involves a subject
called a pentaquark.
A pentaquark is a particle containing five
quarks or, more accurately, four quarks and
an antimatter quark.
A particle with these properties might have
been discovered.
However, to understand this claim a bit better,
we need to remind ourselves about the theory
of quarks and what we know about them.
Quarks are particles found inside protons
and neutrons.
They were independently proposed in 1964 by
George Zweig and Murray Gell-mann.
While Zweig called these particles aces, it
is Gell-mann's label that has stuck, when
he used a word from a phrase from James Joyce's
Finnegan's Wake.
Over the last half a century, we've discovered
six types of quarks.
They are called up and down, charm and strange,
and top and bottom.
Up and down quarks are those that are found
inside protons and neutrons and the others
are made in collisions using particle accelerators
like the LHC.
Quarks have been observed to combine in only
a few different configurations.
For instance, one configuration is when three
quarks combine to make a particle of a class
called a baryon, while the other common configuration
is when a quark and an antimatter quark combine
to make a class of a particle called a meson.
Baryons and mesons are very common.
For instance, protons and neutrons are examples
of baryons and the most commonly produced
particle in one of our big accelerators is
a pi meson.
If you add up all the baryons and mesons we've
discovered, the number is far more than a
hundred.
The reason that quarks arrange themselves
into groups of three or a matter/antimatter
pair originates in the fact that they carry
a kind of charge, called the strong nuclear
charge, or what physicists call color.
You should be aware that we are using this
word rather loosely, as it's only an analogy.
In the same way that if we add red, green
and blue, we get white, if we add the three
quark charges, we get a particle with no strong
charge.
Similarly, antiquarks have an opposite charge,
so a quark and an antimatter quark can cancel
out and make a particle with no strong charge.
However, there are other predicted configurations
of quarks that have resisted discovery so
far.
One option is to have two quarks and two antimatter
quarks- what we call a tetraquark- and another
is to have four quarks and an antimatter quark-
and that is called a pentaquark.
Claims of the discovery of pentaquarks have
surfaced before.
For instance, in 2002, scientists claimed
that they had found a pentaquark, consisting
of two up quarks, two down quarks and a strange
antiquark.
They called this particle a Theta plus.
With the announcement of the Theta plus, other
scientists dug through their data and found
other objects that were other possible pentaquarks.
On the other hand, other scientists dug through
different data and found nothing.
It was an exciting couple of years, until
2005, when physicists at the Thomas Jefferson
National Accelerator Facility in Newport News,
Virginia repeated the original measurement
with far more data and found no evidence for
the Theta plus.
Pentaquarks appeared to have been a subatomic
will o' the wisp.
Now I don't wish to be critical of the scientists
involved in this incident.
It's how science is done on the edge of knowledge.
Further, it's an important point that while
scientists made the initial announcement,
other scientists provided the definitive measurement
that killed the initial claim.
Perhaps even more important is that the original
scientists accepted that their claim had been
refuted.
Science worked the way it was supposed to.
So what about these new pentaquark claims?
Well, the LHCb experiment has announced evidence
that they believe supports a pentaquark with
two up quarks, a down quark, a charm quark
and an antimatter charm quark.
Actually, they claim that they've found two
different kinds of pentaquarks.
I think it's important to say that the measurement
didn't start out as a search for pentaquarks.
In fact, they were studying a heavy baryon,
called the Lambda−sub-b, in an effort measure
its lifetime.
They were looking for a specific manner in
which it decayed, specifically into a charged
K meson, a proton and another meson called
a J/psi.
This is all fairly garden variety stuff, although
it requires a top notch detector to do it.
Just like any good careful scientists, they
looked over the data to make sure that it
all hung together and that it couldn't have
occurred by accident.
They looked at the angles at which the particles
came out and their energies.
They mathematically combined the energies
and the momenta of the particles to see if
it made sense and this was when the surprise
began.
They found that when they looked at the proton
and the J/psi meson that the data showed unexpected
features.
There were bumps in the data.
So, in particle physics, bumps are interesting.
They can mean a discovery.
So they looked harder at them.
After a ton of work, they concluded that it
looked like an unexpected particle was being
produced- well, two, actually.
They compared the properties of the new particle
to all of the hundreds of known mesons and
baryons and found no matches.
That was when things began to be a bit more
exciting.
While I'm not a member of that experiment
and wasn't part of the closed door conversations,
I am 100% certain that the level of critical
discussion was very, very, high.
In the end, the researchers announced that
the new data looked like a possible pair of
pentaquarks.
So how should you view this announcement?
First, you should realize that the scientists
involved are world class.
This isn't some announcement from a couple
of amateurs.
Further, they are keenly aware of the history
of pentaquarks and I'm totally sure that their
internal discussions and cross-checks were
thorough.
This is an announcement that must be taken
seriously.
There are a couple of questions that need
to be answered.
The first question is "did they find two new
particles?"
Here the situation is on stronger ground.
The size of the bumps are big, and big bumps
means more certain.
Scientists define a measure of their certainty
called the "significance," where more significance
means more certain.
The minimum significance for a discovery is
5 and the two new particles have a significance
of 9 and 12 respectively.
It really does look like LHCb found something.
But are they pentaquarks?
That's a bit harder.
Further, there are two different kinds of
configurations that could exist.
One is what I might call a true pentaquark,
which is when all four quarks and one antiquark
are all jumbled together in a single bag,
so to speak.
However another possibility is what we might
call a nuclear molecule, although that terminology
can be confusing.
In the molecule case, a J/psi meson and proton
were made, but they were made stuck together,
like two marbles connected with Velcro.
Either configuration is possible and the data
does not yet pick one over the other.
A troubling question is the fact that the
LHCb pentaquark candidates are super heavy
because they contain charm quarks.
Why are there not lighter pentaquarks?
Is it because charmed pentaquarks are easier
to make?
Or are they more stable?
Nobody really knows, although the scientific
community is mulling over the possibilities.
Finally, there is the question of confirmation.
The LHCb collaboration is an excellent group
of people, no doubt about that, but reputation
is not enough in research.
The rest of the community must be skeptical,
and, in the words of President Reagan:
Trust but verify
Have other experiments confirmed the announcement?
The answer to that question is "not yet,"
but they are definitely looking.
The situation remains uncertain.
But it won't be for long.
That's the beauty of science.
We don't know the answer to all questions.
But this is one question for which the answer
will become clear in the next months and possibly
a year or two.
And when we know, we'll let you know.
