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)
