You may have heard about a recent and very exciting measurement announced by researchers
at Fermilab – the accurate measurement of the mass of the W boson.
This measurement was both made with unprecedented precision and, even more interesting, it disagrees
with modern theory.
If this measurement is confirmed, it could be a solid hint that we need to revamp our
understanding of the laws of nature.
The prospect of breaking our theories is an exhilarating one, and I want to tell you all
about it.
(intro music)
The W boson is one of the particles that transmit
the weak nuclear force.
It comes in two versions, with an electric charge of plus one and the other negative
one.
We don’t distinguish between the two variants, so we just say “the W boson.”
The other particle that transmits the weak
force is called the Z boson.
It is electrically neutral, which means there’s only one version of it.
The W and Z bosons were postulated in the 1960s and they were discovered at the CERN
laboratory in 1983.
Our modern theory, which we call the Standard
Model, makes no predictions for the mass of either of these particles or, for that matter,
any of the subatomic particles.
But it does make predictions about how they are related.
For example, if you know the mass of a couple subatomic particles, you can predict the mass
of others.
I’ll have more to say about that later.
An accelerator called LEP operated at the
CERN laboratory from 1989 to 2000.
It produced some 18 million Z bosons, distributed between four distinct experiments.
These experiments measured the properties of the Z boson to exquisite precision.
It will be a long time before the LEP measurements are beat- indeed, they may never be surpassed.
The mass of the Z boson was determined to
be 91,187.6 plus or minus 2.1 million electron volts or MeV.
Don’t worry too much about the meaning of the units, because I’ll use the same units
throughout.
The important thing is the uncertainty, which is 2.1 MeV.
The recent announcement by Fermilab is a measurement
of the mass of the W boson with an uncertainty of 9 MeV.
Now that's much worse than has been achieved for the Z boson, but before this recent measurement,
but best uncertainty anyone could achieve for the W boson was about 20 MeV.
So this is a huge improvement, and it’s part of a multiyear effort to measure the
mass of the W boson as precisely as we have the Z boson.
Okay, so let’s talk about measuring the
mass of the W boson.
The LEP accelerator wasn’t a great place for making W bosons, so their measurements
were all in the 50 or 60 MeV range.
But the Fermilab Tevatron was a great place
to make W bosons.
The Tevatron operated from the late 1980s through 2011 and millions of W bosons were
generated.
There were two detectors located at the Tevatron,
called CDF and DZero.
The recent announcement was made by researchers using the CDF detector.
And, just so you know, I have been a member of the DZero experiment since 1994.
So, how do you go about measuring the mass
of the W boson?
Well, the first thing to note is that you don’t put a W boson on a scale and weigh
it.
For one thing, they don’t live very long.
They decay in about three times ten to the minus twenty five seconds.
Even traveling at the speed of light, they travel about a tenth the diameter of a proton
before they decay.
That’s long before they hit a detector.
So how do you study them?
Well, what you do is look at the particles
into which they decay and measure those particles' energy and momentum.
You then use the laws of energy and momentum conservation and work backwards.
This is how the mass of all short-lived subatomic particles are measured.
This particular image is from CDF and contains two W bosons from top quark decay.
W bosons can decay into two distinct paths.
They can decay into a quark and antimatter quark, or they can decay into a lepton, like
an electron or a muon, and a neutrino.
On the one hand, you can detect quarks pretty
easily in your detector, but it’s hard to precisely measure their energy and momentum.
So this type of decay isn’t used for precise measurements.
On the other hand, modern experiments can
measure the energy and momentum of electrons and muons very well.
So that’s attractive.
However, on the third hand, neutrinos pass through the detector without interacting,
so you have no direct information about the neutrino.
That’s a problem, but you can estimate the energy and momentum of the neutrino by measuring
everything else.
Since the momentum is conserved and the total momentum before the collision is zero, the
momentum of the neutrino should be the opposite of everything else.
You can then use the measurements of the electron
or muon and the estimate of the motion of the neutrino to determine the mass of the
W boson.
So that’s what CDF did.
They measured the mass of the W boson to be 80,433.5 plus or minus 9.4 MeV.
This uncertainty is half the size of the next
best measurement and that’s amazing, but that’s not the most exciting part.
What’s really exciting is that it disagrees with the theoretical prediction, which is
80,357 plus or minus 6 MeV.
For those of you who are fans of statistics,
this level of disagreement is seven standard deviations– so-called seven sigma.
Since the agreed upon threshold for a discovery is five sigma, this could be a big deal.
Has the CDF experiment broken the Standard Model?
Well maybe.
Now, as a member of the competition, I would ordinarily trash talk the other experiment,
but I really can’t.
While I will deny ever saying it, the CDF group is really world class and the person
leading the W mass measurement is widely recognized as one of the world experts in the field.
The result needs to be taken seriously.
Of course, what is needed is independent confirmation and that’s where things get tricky.
You see, this is an incredibly difficult measurement
to make at the required level of precision.
The tiniest mismeasurement or incomplete understanding of your detector can fool you.
You have to look at the data from every conceivable angle.
Let’s give some context.
The Fermilab Tevatron stopped operating in 2011 and this measurement just came out.
That gives you some idea of the difficulty that's involved.
So, what about a similar measurement by the
DZero experiment?
Will we have anything to say?
Sadly- no.
We released a measurement back in 2012 with an uncertainty of 23 million electron volts,
with every intent at making an improved measurement like CDF’s recent one.
However, after a careful evaluation of the data, DZero physicists determined that the
beam had damaged the detector enough so that there was no way to cut our uncertainties
in half.
So there will be no help from DZero.
What about the new kid on the block– the
Large Hadron Collider, currently the most powerful particle accelerator ever built?
The LHC hosts four detectors, two of them dedicated to studying high energy phenomena,
one called ATLAS and one called CMS.
Do they have anything to say?
Well, possibly.
But, again, as a needed reality check, although the LHC began operating in 2011, the CMS experiment
still hasn’t released a high precision measurement of the mass of the W boson.
And the ATLAS experiment did publish a paper back in 2018, but it only used data recorded
in 2011.
Furthermore, that measurement had an uncertainty of nineteen MeV.
A decade of recorded data hasn’t been published simply because of the difficulty in making
the measurement at the required level of precision.
So, what about the future?
This is the final CDF result and the DZero experiment decided to not pursue a precision
measurement.
And, while the CMS and ATLAS experiments will generate a seemingly limitless supply of W
bosons, the experimental conditions are much harsher than those at the Tevatron.
It's simply a harder measurement to perform at the LHC.
Getting a more precise measurement than was reported by CDF may be impossible at the LHC.
What we really need to find out if the CDF
measurement is accurate is a new electron/positron collider.
A high energy e plus e minus collider could answer this pretty easily.
And such a collider is already on the drawing boards, but the most wildly optimistic schedule
has it operating in 2040, and 2050 is more probable.
Either way, it’s far in the future.
So, given that the CDF W boson mass measurement
is here and will be around for years, let’s take a quick look at it and what it might
mean.
The plot you see here shows the situation.
The top four measurements are from LEP experiments.
Then there is the 2012 DZero measurement and the 2016 ATLAS measurement and the new CDF
measurement.
The length of the red bar shows you the uncertainty.
The grey bar is the prediction of the Standard Model that arises from combining precision
measurements of other variables.
And what you see is that the CDF measurement
seriously disagrees with theory.
Let’s suppose that this is real and not a mismeasurement.
This implies that the Standard Model is not self-consistent, which means that some unknown
physics needs to come into play.
One can explain the discrepancy by invoking supersymmetry, but direct searches for supersymmetry
have come up empty, so that’s probably not it.
But who knows?
That’s why science is exciting.
This new measurement could be the place where our current understanding of the laws of nature
unravel, requiring us to weave together a new theory.
Or it could just be an honest mistake made by excellent scientists trying their best.
Time will tell.
So this was a fun video about an exciting
paper.
What does it mean?
I don’t know.
Nobody does.
If you enjoyed the video, please like, subscribe, and share.
And watch more videos on this channel.
They’re all about physics and what’s not to like about that?
Because, well, of course- physics is everything.
(outro music)