CERN Courier has a special issue this month celebrating what they consider as the 100th anniversary of Quantum Mechanics.
Of course, the focus here is predominantly on elementary/particle physics. And yet, many of the most obvious demonstration and manifestation of quantum mechanics can be found not in particle physics, but in condensed matter physics. The Schrodinger-Cat type demonstration using SQUIDs, and the clearest manifestation of the effect of coherence can be seen in condensed matter experiment. To quote Carver Mead's article[1]:
Although superconductivity was discovered in 1911, the recognition that superconductors manifest quantum phenomena on a macroscopic scale (4) came too late to play a role in the formulation of quantum mechanics. Through modern experimental methods, however, superconducting structures give us direct access to the quantum nature of matter. The superconducting state is a coherent state formed by the collective interaction of a large fraction of the free electrons in a material. Its properties are dominated by known and controllable interactions within the collective ensemble. The dominant interaction is collective because the properties of each electron depend on the state of the entire ensemble, and it is electromagnetic because it couples to the charges of the electrons. Nowhere in natural phenomena do the basic laws of physics manifest themselves with more crystalline clarity.
Zz
[1] C.A. Mead, PNAS v.94, p.6013 (1997); or you may be able to access it here.
First of all, I'm old! I started being in a student in physics since the early 1980's (do your own math). During all of that time when I have paid attention to physics, I've seen a lot of major milestones, including the discovery of High-Tc superconductors, discovery of exoplanets, the cold-fusion debacle, etc...etc.
The one thing that pops up every now and then is the claim of the possible discovery of this "fifth force". Honestly, even back in the 1980's, there were already such claims being made. None of the have amounted to anything as far as I can tell. Therefore, you can understand my "Oh no, this again?" reaction when I read the latest claim of the possible detection of the Yukawa particle as an indication of the existence of this fifth force (that article contains a link to the actual PRL paper that you can download).
This is not a knock on this work, heavens no. But the publicity surrounding this makes it sound as if this has not happened before. I guess it is not surprising that people have short memory, which is why mistakes are often repeated.
I'm going to wait a year and revisit this post and see if we have gone beyond first based on this discovery.
The big news of the week that got all the media coverage is the result that came out of Fermilab's Muon g-2 experiment that confirmed an earlier result from Brookhaven more than a dozen years ago. Fermilab even announced it like.
However, as with any scientific discovery or announcement, one has to take a deep breath and let the process works itself out before we put our stamp of validity to it. This is because there is a theoretical calculation that has also been published along with this result that basically recalculates what the Standard Model predicts as the magnetic moment of a muon, and they found that the new calculation produces a result consistent with the experiment. In other words, there is no new physics if this calculation is verified, because the old Standard Model does, in fact, predicted this new result.
One of the major difficulties in physics is that in many situations, we do not have a simple equation that we can plug-and-chug to get numbers out. In fact, this is why predicting the weather is difficult, because the non-linear differential equations that need to be solved to get the number out can only be done numerically, i.e. it has to be done via some numerical algorithm.
This is made worse when there are a gazillion interactions involved in a system. So one ends up making simplifying models or adopt calculational techniques to allow us to get to some numerical answers. We benchmark the technique to known values and known systems to make sure that it gives accurate and sensible answers, but as we push the boundary even more, there is no guarantee that that calculational technique will work all the time.
The author of the theoretical paper used a calculational technique called lattice QCD. This is a known calculational model that has been described in simple terms in the link I provided above. It appears that using this method, the Standard Model does provide a value for the muon magnetic moment that is consistent with the experiment. If this is true, then it means that the old calculation of the magnetic moment was incorrect in the first place, and that there is discrepancy between what the Standard model predicts, and what the experiment measures.
While this is good news for the Standard Model and is another evidences of why it is an amazing theory, those who are looking for new physics beyond the Standard Model will obviously not be jumping for joy. But that isn't the issue here and not what I want to highlight. Rather, it is the constant reminder that in science, and especially in such exotic areas of physics, every discovery or new ideas must not be overblown or overhyped, because those require multiple verification over a period of time. It is not a situation for instant gratification. A lot of hard work is still to come because we have seen way too many times where something that was touted turned out to not be valid.
This announcement received a lot of media coverage. I just hope that this is a valid "new physics" and not just something that turned out to be what the old theory did predict.
We have had reports of the discovery of possible tetraquarks and pentaquarks before (i.e. particles with 4 quarks and particles with 5 quarks, respectively). There is an extensive overview of the experiment and theory in this article. So the announcement out of LHCb is not that new. What is new is that this could possibly be a new type of tetraquark made up of 4 heavy quarks.
“Particles made up of four quarks are already exotic, and the one we
have just discovered is the first to be made up of four heavy quarks of
the same type, specifically two charm quarks and two charm antiquarks,”
says the outgoing spokesperson of the LHCb collaboration, Giovanni
Passaleva. “Up until now, LHCb and other experiments had only observed
tetraquarks with two heavy quarks at most and none with more than two
quarks of the same type.”
That should clear up very much of what the brouhaha is. I probably would have glanced over this had it not be the fact that I stumbled onto another news reports of this discovery, but with a different tone that could be misleading.
First of all, let's look at how CERN produced its news release. The first paragraph read like this:
The LHCb collaboration has observed a type of four-quark particle never
seen before. The discovery, presented at a recent seminar at CERN and
described in a paper posted today on the arXiv preprint server, is likely to be the first of a previously undiscovered class of particles.
Notice that it says "... a type of four-quark particle ...". This means that there are already other four-quark particles, and that this discover is for a new type that has not been observed before.
Now, compare that to the reporting done by two (count 'em) particle physicists on The Conversation (a place that I go to regularly) on the same discovery. Here is what they wrote:
The LHCb collaboration at CERN has announced the discovery of a new exotic particle: a so-called “tetraquark”. The paper
by more than 800 authors is yet to be evaluated by other scientists in a
process called “peer review”, but has been presented at a seminar. It
also meets the usual statistical threshold for claiming the discovery of
a new particle.
If you don't know any better, by reading the first sentence alone, you'd think that this is the first ever discovery of a tetraquark, which would be false.
Certainly, if you read the article further, you'd come across the passage that clarifies what this discovery is:
All tetraquarks and pentaquarks that have been discovered so far
contain two charm quarks, which are relatively heavy, and two or three
light quarks – up, down or strange. This particular configuration is
indeed the easiest to discover in experiments.
But the latest tetraquark discovered by LHCb, which has been dubbed
X(6900), is composed of four charm quarks. Produced in high-energy
proton collisions at the Large Hadron Collider, the new tetraquark was observed via its decay into pairs of well-known particles called J/psi mesons,
each made of a charm quark and a charm antiquark. This makes it
particularly interesting as it is not only composed entirely of heavy
quarks, but also four quarks of the same kind – making it a unique
specimen to test our understanding on how quarks bind together.
So this is not the first discovery of a tetraquark, but rather a discovery of a type of tetraquark, which is what the CERN article implied.
I know I'm being picky, but I've always said that communication between scientists and the general public is extremely tedious. Often times, what you wrote is not what they understood! And once something or some impression has stuck into their heads, it is very difficult to change that. Having a misleading idea immediately imprinted at the very beginning of an article is a horrible thing to do, even if the rest of the article is accurate. At worse, the reader holds on to the original misleading idea, and at best, the reader becomes confused with conflicting understanding. In the world where a lot of people have attention deficit and all they care about are quick bites of news, the message conveyed in the very first paragraph, or even the very first line, is all that they read and get.
The latest report on T2K results has been published[1], and it looks good for the upcoming long neutrino baseline experiment at DUNE and T2HK. The result may suggest that these two upcoming experiments may finally nail down CP violation in neutrinos, which will be a substantial advancement in our understanding on why there are more mater than antimatter in our universe.
The discovery of substantial leptonic CP violation would be
groundbreaking. Its observation, together with evidence that a quantity
known as lepton number has been violated (that is, not conserved), would
provide strong circumstantial evidence for leptogenesis as the origin
of the matter–antimatter imbalance.
I suppose if I want to be accurate, I should say it is the electron antineutrino, since they measured this from beta decays, but nowadays, we don't have a clear cut idea of the difference between the neutrino and its antiparticle. For all we know, they can possibly also be a Majorana particle.
I'll be giving this report to my students in the general physics class, and see if they can convert the 1.1 eV into "kg". :)
Another Don Lincoln video, and this time, it is on a topic that I had a small involvement in, which is neutrino detection.
My small part was in the photomultiplier photocathode used for detection of Cerenkov light that is emitted from such a collision between the "weak boson" and the nucleus. We were trying to design a photodetector that has a large surface area as compared to the current PMT round surface.
In any case, this is a good introduction to why neutrinos are so difficult to detect.
Sabine Hossenfelder is probably doing a "book tour", since this talk certainly addressed many points that she brought up in her book.
As I've said many times on here, I don't disagree with many things that she brought up. I find the trend of foundational physics to even think about discarding experimental verification to be very troubling. I'm just glad that the field that I'm in is still strongly experimental.
This new Don Lincoln's video is related to the one he did previously on the PIP-II upgrade at Fermilab. This time, he tells you how they make neutrino beams at Fermilab.
The D0 meson is made of a charm quark and an up antiquark. So
far, CP violation has only been observed in particles containing a
strange or a bottom quark. These observations have confirmed the pattern
of CP violation described in the Standard Model by the so-called
Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix, which characterises how
quarks of different types transform into each other via weak
interactions. The deep origin of the CKM matrix, and the quest for
additional sources and manifestations of CP violation, are among the big
open questions of particle physics. The discovery of CP violation in
the D0 meson is the first evidence of this asymmetry for the charm quark, adding new elements to the exploration of these questions.
If confirmed, this will be another meson that has exhibited such CP violation, and adds to the argument that such symmetry violation could be the source of our matter-antimatter asymmetry in this universe.
CP violation is an essential feature of our universe, necessary to
induce the processes that, following the Big Bang, established the
abundance of matter over antimatter
that we observe in the present-day universe. The size of CP violation
observed so far in Standard Model interactions, however, is too small to
account for the present-day matter–antimatter imbalance, suggesting the
existence of additional as-yet-unknown sources of CP violation.
Do you ever want to know about US Fermi National Accelerator Laboratory, or Fermilab?
Don Lincoln finally has made a video on everything you want to know about Fermilab, especially if you think that they don't do much anymore nowadays now that the Tevatron is long gone.
As someone who has visited there numerous times and collaborated with scientists and engineers that this facility, it is a neat place to visit if you have the chance.
Hey, you should read this blog post by Tommaso Dorigo. It touches upon many of the myths regarding particle physics, especially the hype surrounding the name "god particle", as if that means something.
I've touched upon some of the issues he brought up. I think many of us who are active online and deal with the media and the public tend to see and observe the same thing, the same mistakes, and misinformation that are being put in print. One can only hope that by repeatedly pointing out such myths and why they are wrong, the message will slowly seep into the public consciousness.
Chad Orzel has posted a fun piece that really tries to clarified all the brouhaha in many circles about a "crisis" that many are presuming to be widespread. The crisis in question is the lack of "beyond the standard model" discovery in elementary particle physics, and the issue that many elementary particle theorists seem to think that a theory that is based on solid foundation and elegance are sufficient to be taken seriously.
I find this very frustrating, because physics as a whole is
not in crisis. The "crisis" being described is real, but it affects only
the subset of physics that deals with fundamental particles and fields,
particularly on the theory side. (Experimental physicists in those
areas aren't making dramatic discoveries, but they are generating data
and pushing their experiments forward, so they're a little happier than
their theoretical colleagues...)
The problems of theoretical high energy physics, though, do not
greatly afflict physicists working in much of the rest of the
discipline. While this might be a time of crisis for particle theorists,
it's arguably never been a better time to be a physicist in most of the
rest of the field. There are exciting discoveries being made, and new
technologies pushing the frontiers of physics forward in a wide range of
subfields.
This is a common frustration, because elementary particle physics is not even the biggest subfield of physics (condensed matter physics is), but yet, it makes a lot of noise, and the media+public seem to pay more attention to such noises. So whenever something rocks this field, people often tend to think that this permeates through the entire field of physics. This is utterly false!
Orzel has listed several outstanding and amazing discoveries and advancements in condensed matter. There are more! The study of topological insulators continues to be extremely hot and appear to be not only interesting for application, but also as a "playground" for exotic quantum field theory scenarios.
I've said it many times, and I'll say it again. Physics isn't just the Higgs or the LHC. It is also your iphone, your MRI, your WiFi, your CT scan, etc....etc.
I can't believe it. I'm reporting on Ethan Siegel's article two days in a row! The last one yesterday was a doozy, wasn't it? :)
This one is a bit different and interesting. The first part of the article describes our understanding of where mass comes from for matter. I want to highlight this because it clarify one very important misconception that many people have, especially the general public. After all the brouhaha surrounding the Higgs and its discovery, a lot of people seem to think that all the masses of every particle and entity can be explained using the Higgs. This is clearly false as stated in the article.
Yet if we take a look at the proton (made of two up and one down quark)
and the neutron (made of one up and two down quarks), a puzzle emerges.
The three quarks within a proton or neutron, even when you add them all
up, comprise less than 0.2% of the known masses of these composite
particles. The gluons themselves are massless, while the electrons are
less than 0.06% of a proton's mass. The whole of matter, somehow, weighs
much, much more than the sum of its parts.
The Higgs may be responsible for the rest mass of these fundamental
constituents of matter, but the whole of a single atom is nearly 100
times heavier than the sum of everything known to make it up. The reason
has to do with a force that's very counterintuitive to us: the strong
nuclear force. Instead of one type of charge (like gravity, which is
always attractive) or two types (the "+" and "-" charges of
electromagnetism), the strong force has three color charges (red, green
and blue), where the sum of all three charges is colorless.
So while we may use the Higgs to point to the origin of mass in, say, leptons, for hadrons/partons, this is not sufficient. The strong force itself contributes a significant amount to the origin of mass for these particles. The so-called "God Particles" are not that godly, because it can't do and explain everything.
The other interesting part of the article is that he included a "live blog" of the talk by Phiala Shanahan at occurred yesterday at the Perimeter Institute, related to this topic. So you may want to read through the transcript and see if you get anything new.
The latest and most accurate experiment to detect any hint of an electric dipole moment of an electron has revealed that there isn't any.
Now, the Advanced Cold Molecule Electron
Electric Dipole Moment, or ACME, search, based at Harvard University,
has probed the electron’s EDM with the most precision ever — and still found no sign of smooshing, the team reports online October 17 in Nature.
The finding improves the team’s last best measurement (SN Online: 12/19/13) by a factor of 10 to find an EDM of 10-29 electron
charge centimeters. That’s as round as if the electron were a sphere
the size of the Earth, and you shaved less than two nanometers off the
North Pole and pasted it onto the South Pole, says Yale University
physicist David DeMille, a member of the ACME team.
This improves upon the previous measurement that I mentioned a year ago. Looks like if any theory predicts the possible structure of an electron, they have some severe constraints to overcome.
In this video, Fermilab's Don Lincoln tackles less about physics, but more about history and classification of our current Standard Model of elementary particles.
Another giant in our field, especially in elementary particle physics, has passed away. Burton Richter, Nobel Laureate in physics, died on July 18, 2018.
Richter’s Nobel Prize-winning discovery of the J/psi subatomic particle,
shared with MIT’s Samuel Ting, confirmed the existence of the charm
quark. That discovery upended existing theories and forced a
recalibration in theoretical physics that reverberated for years. It
became known as the “November Revolution.” One Nobel committee member at
the time described it as “the greatest discovery ever in the field of
elementary particles.”
He would be shortchanged if all the public ever remembers him is for his Nobel Prize discovery, because he did a whole lot more in his lifetime.
I decided to modify a bit the title of the Symmetry article that I'm linking to, because in that article, the possible link between the Higgs boson and dark matter is made. This allows for the study of the decay of the Higgs to be used to detect the presence of dark matter.
The Standard Model not only predicts all the different possible
decays of Higgs bosons, but how favorable each decay is. For instance,
it predicts that about 60 percent of Higgs bosons will transform into a
pair of bottom quarks, whereas only 0.2 percent will transform into a
pair of photons. If the experimental results show Higgs bosons decaying
into certain particles more or less often than predicted, it could mean
that a few Higgs bosons are sneaking off and transforming into dark
matter.
Of course, these kinds of precision measurements cannot tell
scientists if the Higgs is evolving into dark matter as part of its
decay path—only that it is behaving strangely. To catch the Higgs in the
act, scientists need irrefutable evidence of the Higgs schmoozing with
dark matter.
So there you have it.
If you are not up to speed on the discovery of the Higgs (i.e. you've been living under a rock for the past few years), I've mentioned a link to a nice update here.
If it is covered in CNN, then it has to be a big-enough news. :)
I mentioned earlier that the g-2 experiment at Fermilab was about to start (it has started now), which is basically a continuation and refinement of what was done several years ago at Brookhaven. In case the importance of this experiment escapes you, Don Lincoln of Fermilab has written a piece on the CNN website on this experiment and why it is being done.
If you are not in science, you need to keep in mind this important theme: scientists, and definitely physicists, like it A LOT when we see hints at something that somehow does not fit with our current understanding. We like it when we see discrepancies of our results with the things that we already know.
This may sound odd to many people, but it is true! This is because this is why many of us get into this field in the first place: to explore new and uncharted territories! Results that do not fit with our current understanding give hints at new physics, something beyond what we already know. This is exploration in the truest sense.
This is why there were people who actually were disappointed that we saw the Higgs, and within the energy range that the Standard Model predicted. It is why many, especially theorists working on Supersymmetry, are disappointed that the results out of the LHC so far are within what the Standard Model has predicted.
The old muon g-2 experiment that was at Brookhaven was taken apart, and rebuilt at Fermilab. Now, after the logistic challenge of moving the huge magnet from there, and after the long hard work of rebuilding the facility, the muon g-2 is now about ready to start its run.
The facility is now better than ever, and physicists are hoping that there will be an anomaly in the measurement, indicating new physics beyond the Standard Model.
In 2013, the g-2 team lugged the experiment on a 5000-kilometer odyssey from Brookhaven to Fermilab, taking the ring by barge around the U.S. eastern seaboard and up the Mississippi River.
Since then, they have made the magnetic field three times more uniform,
and at Fermilab, they can generate far purer muon beams. "It's really a
whole new experiment," says Lee Roberts, a g-2 physicist at Boston
University. "Everything is better."
Over 3 years, the team aims to collect 21 times more data than during
its time at Brookhaven, Roberts says. By next year, Hertzog says, the
team hopes to have enough data for a first result, which could push the
discrepancy above 5 σ.
