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First Big Results from LHC at 13 TeV

A few weeks ago, the Large Hadron Collider [LHC] ended its 2015 data taking of 13 TeV proton-proton collisions.  This month we’re getting our first look at the data.

Already the ATLAS experiment has put out two results which are a significant and impressive contribution to human knowledge.  CMS has one as well (sorry to have overlooked it the first time, but it isn’t posted on the usual Twiki page for some reason.) Continue reading

An Overdue Update

A number of people have asked why the blog has been quiet. To make a long story short, my two-year Harvard visit came to an end, and my grant proposals were turned down. No other options showed up except for a six-week fellowship at the Galileo Institute (thanks to the Simons Foundation), which ended last month.  So I am now employed outside of science, although I maintain a loose affiliation with Harvard as an “Associate of the Physics Department” (thanks to Professor Matt Schwartz and his theorist colleagues).

Context: U.S. government cuts to theoretical high-energy physics groups have been 25% to 50% in the last couple of years. (Despite news articles suggesting otherwise, billionaires have not made up for the cuts; and most donations have gone to string theory, not particle physics.) Spare resources are almost impossible to find. The situation is much better in certain other countries, but personal considerations keep me in this one.

News from the Large Hadron Collider (LHC) this year, meanwhile, is optimistic though not without worries. The collider itself operated well despite some hiccups, and things look very good for next year, when the increased energy and high collision rate will make the opportunities for discoveries the greatest since 2011. However, success depends upon the CMS experimenters and their CERN lab support fixing some significant technical problems afflicting the CMS detector and causing it to misbehave some fraction of the time. The ATLAS detector is working more or less fine (as is LHCb, as far as I know), but the LHC can’t run at all while any one of the experimental detectors is open for repairs. Let’s hope these problems can be solved quickly and the 2016 run won’t be much delayed.

There’s a lot more to say about other areas of the field (gravitational waves, neutrinos, etc.) but other bloggers will have to tell those tales. I’ll keep the website on-line, and will probably write some posts if something big happens. And meanwhile I am slowly writing a book about particle physics for non-experts. I might post some draft sections on this website as they are written, and I hope you’ll see the book in print sometime in the next few years.

Completed Final Section of Article on Dark Matter and LHC

As promised, I’ve completed the third section, as well as a short addendum to the second section, of my article on how experimenters at the Large Hadron Collider [LHC] can try to discover dark matter particles.   The article is here; if you’ve already read what I wrote as of last Wednesday, you can pick up where you left off by clicking here.

Meanwhile, in the last week there were several dark-matter related stories that hit the press.

There has been a map made by the Dark Energy Survey of dark matter’s location across a swathe of the universe, based on the assumption that weak signals of gravitational lensing (bending of light by gravity) that cannot be explained by observed stars and dust is due to dark matter.  This will be useful down the line as we test simulations of the universe such as the one I referred you to on Wednesday.

There’s been a claim that dark matter interacts with itself, which got a lot of billing in the BBC; however one should be extremely cautious with this one, and the BBC editor should have put the word “perhaps” in the headline! It’s certainly possible that dark matter interacts with itself much more strongly than it interacts with ordinary matter, and many scientists (including myself) have considered this possibility over the years.  However, the claim reported by the BBC is considered somewhat dubious even by the authors of the study, because the little group of four galaxies they are studying is complicated and has to be modeled carefully.  The effect they observed may well be due to ordinary astrophysical effects, and in any case it is less than 3 Standard Deviations away from zero, which makes it more a hint than evidence.  We will need many more examples, or a far more compelling one, before anyone will get too excited about this.

Finally, the AMS experiment (whose early results I reported on here; you can find their September update here) has released some new results, but not yet in papers, so there’s limited information.  The most important result is the one whose details will apparently take longest to come out: this is the discovery (see the figure below) that the ratio of anti-protons to protons in cosmic rays of energies above 100 GeV is not decreasing as was expected. (Note this is a real discovery by AMS alone — in contrast the excess positron-to-electron ratio at similar energies, which was discovered by PAMELA and confirmed by AMS.)  The only problem is that they’ve made the discovery seem very exciting and dramatic by comparing their work to expectations from a model that is out of date and that no one seems to believe.  This model (the brown swathe in the Figure below) tries to predict how high-energy anti-protons are produced (“secondary production”) from even higher energy protons in cosmic rays.  Newer versions of this models are apparently significantly higher than the brown curve. Moreover, some scientists claim also that the uncertainty band (the width of the brown curve) on these types of models is wider than shown in the Figure.  At best, the modeling needs a lot more study before we can say that this discovery is really in stark conflict with expectations.  So stay tuned, but again, this is not yet something that in which one can have confidence.  The experts will be busy.

Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

Figure 1. Antiproton to proton ratio (red data points, with uncertainties given by vertical bars) as measured by AMS. AMS claims that the measured ratio cannot be explained by existing models of secondary production, but the model shown (brown swathe, with uncertainties given by the width of the swathe) is an old one; newer ones lie closer to the data. Also, the uncertainties in the models are probably larger than shown. Whether this is a true discrepancy with expectations is now a matter of healthy debate among the experts.

Added a short section to my Quantum Tunneling page

To fill in another important detail that will be important later, I added a short section to the end of my article about quantum tunneling. Specifically, suppose you have an electron, placed in one of two traps, such that the electron can tunnel from one trap to the other.  What happens if one of the traps is deeper than the second?

This difference between the traps leads to a bias — the electron tends to end up in the deeper trap, because it is harder for it to tunnel back to the shallow trap than it is to tunnel into the deeper one.  This simple fact has implications for the entire universe, as I’ll describe in a few days.

A Disturbing Story on Fukushima — Can You Help Confirm It?

[Note Added: I have been unable to confirm the story described below from any source other than the original one — the lawyer who stands to benefit from it.  At this point, based on remarks by my readers, I’m inclined to think the story is implausible.]


It’s been a little quiet on the blog, because I’ve been working very hard, with a dozen colleagues, to finish a monster project that will appear later today on the public physics archive (arXiv) for professional theoretical physicists to present their work to the their colleagues.  More on that tomorrow.

But in the meantime, I have come across a very disturbing update to a story regarding the Fukushima nuclear plant disaster— nothing to do with what is going on there now, but something that happened immediately after the accidents following the earthquake and tsunami in March 2011.  The latest update, though widely reported around the internet, is currently attributed only to a lawyer for plaintiffs… hardly a reliable source of information.  Nevertheless, the story might be true, and I’m looking for corroboration, if one of you has come across any.

The story is that dozens of sailors from the US aircraft carrier Ronald Reagan (ironies, anyone), who I believe were right off the coast of Japan following the quake to help out with disaster relief, were exposed to radioactive sea water.  Some were diving into the sea to help rescue people, and many were bathing in and even drinking desalinated sea water — and taking the salt out of seawater does not remove radioactive atoms of iodine, caesium, etc.   Apparently it was a short but significant time before somebody realized the water was not safe.  And now dozens of sailors — more than 1% of the total number on board (last year it was eight, and this summer the lawsuit apparently grew to more than 50) are suing TEPCO (the Japanese electric company) after suffering a variety of diseases, including various cancers, eye and thyroid problems, etc.  So says their lawyer, anyway.

Does anyone reading this know anything else about this story?  In particular, does anyone know someone who was on the ship?

A certain number of people get sick every year, just by chance; assuming the story is true, is this particular cluster of illnesses a chance event, or a real effect of radiation exposure? This is one of those situations where you could do a real scientific test, if the Navy would let someone do it.  You could compare disease rates on the Ronald Reagan to disease rates for sailors who served on the same types of ships operating in other parts of the world, and see if they are significantly larger.  The populations are plenty big enough for such a study… But will anyone be able to find out the truth when the truth becomes a football in a lawsuit?

Quantum Field Theory, String Theory and Predictions (Part 4)

This post is a continuation of three previous posts: #1, #2 and #3.

When the Strong Nuclear Force is Truly Strong

Although I’ve already told you a lot about how we make predictions using the Standard Model of particle physics, there’s more to the story. The tricky quantum field theory that we run into in real-world particle physics is the one that describes the strong nuclear force, and the gluons and quarks (and anti-quarks) that participate in that force. In particular, for processes that involve

  • distances comparable to or larger than the proton‘s size, 100,000 times smaller than an atom, and/or
  • low-energy processes, with energies at or below the mass-energy (i.e. E=mc² energy) of a proton, about 1 GeV,

the force between quarks, gluons and anti-quarks becomes so “strong” (in a technical sense: strong enough that it makes these particles rush around at nearly the speed of light) that the methods I described previously do not work at all.

That’s bad, because how can one be sure our equations for the quarks and gluons — the quantum field theory equations of the strong nuclear force — are the correct ones, if we can’t check that these equations correctly predict the existence and the masses of the proton and neutron and other hadrons ( a general term referring to any particles made from quarks, anti-quarks and gluons)?

Fortunately, there is a way to check our equations, by brute force. We simulate the behavior of the quark and gluon fields on a computer. Sounds simple enough, but you should not get the idea that this is easy. Even figuring out how to do this requires a lot of cleverness, and making the calculations fast and practical requires even more cleverness. Only expert theoretical physicists can carry out these calculations, and make predictions that are relevant directly for the real world.  Don’t try this at home.

The first step is to simplify the problem, and consider an imaginary world, an idealized world that is simpler than the real world. Since the strong nuclear force is extremely strong inside a proton, the electromagnetic and weak nuclear forces are small effects by comparison. So it makes sense to do the calculation in an imaginary world where the strong nuclear force is present but all other forces are turned off. If you put those unimportant forces in, you’d have a much more complicated computer problem and yet the answers would barely change. So including the other forces would be a big waste of time and effort.

Here we use an imaginary world as an idealization — a bit like treating the earth as a perfect sphere. Obviously the earth is not a sphere — it has mountains and valleys and tides and a slight bulge at the equator — but if you’re computing some simple properties of the earth’s effect on the moon, including these details will waste a lot of your time without affecting your calculation very much. The art of being a scientist requires knowing what you need to include in your calculations, and knowing what not to include because it makes no difference.  In fact we do this all the time in particle physics; gravity’s effect on measurements at the Large Hadron Collider [LHC] is tiny, so we do our calculations in an imaginary world without gravity, a harmless simplification.

Here’s another idealization: although there are six types (often called “flavors”) of quarks — up, down, strange, charm, bottom and top — the last three are heavier than a proton and consequently don’t play much of a role in the proton, or in the other low-mass hadrons that I’ll focus on here. So the imaginary, idealized, simplified world in which the calculations are carried out has (see Figure 1)

  • Three “flavors” of quark fields: up, down and strange, each with its own mass, and each with a charge (analogous to electric charge in the case of the electric force) which is whimsically called “color”.  Color can take three values, whimsically called “red”, “green” or “blue”. These fields give rise to both the quark particles and their antiparticles, called anti-quarks, which carry anti-color (anti-red, anti-blue, anti-green);
  • Eight gluon fields (each carrying a “color” and an “anti-color”.) [You might have guessed there’d be nine; but when color and anti-color are the same there are some little subtleties which aren’t relevant today, so I ask you to just accept this for now.]

So now we have a quantum field theory of three flavors of quarks with three possible colors, along with corresponding anti-quarks, and eight gluons which generate the strong nuclear force among the quarks, antiquarks and gluons. This isn’t the real world, but it is close enough to give us very accurate answers about the real world. And this is the one the experts actually put on a computer, to see if our equations do indeed predict that quarks, antiquarks and gluons form protons and other hadrons.

Fig. 1: The fields of the stripped-down world in which calculations of proton and other hadron masses are done. Up, down and strange quark fields (responsible for both quarks and anti-quarks) interact with gluon fields (responsible for gluon particles.) Each of the eight quark fields has a ``charge'' (named, whimsically, red, green or blue) and each gluon field has a color and an anti-color.

Fig. 1: The fields of the stripped-down world in which calculations of the proton mass and other hadron masses are done. Up, down and strange quark fields (responsible for both quarks and anti-quarks) interact with gluon fields (responsible for gluon particles.) Each of the eight quark fields has a “charge” (named, whimsically, red, green or blue) and each gluon field has a color and an anti-color.

Does it work? Yes! In Figure 2 is a plot showing the experimentally measured and computer-calculated values of the masses of various hadrons found in nature. Each hadron’s measured mass is the vertical location of a horizontal black line; the hadron’s symbol appears below that line at the bottom of the plot. I’ve written the names of a few of the most famous hadrons on the plot:

  • the spin-zero pions,
  • the spin-1 rho mesons and omega meson,
  • the spin-1/2 “nucleons”, meaning the proton and the neutron, and
  • the spin-3/2 Delta particles.

The colored dots represent different computer calculations of the masses of these hadrons; the vertical colored bars show how uncertain each calculation is. You can see that, within the uncertainties of the calculations, the measurements and calculations agree. And thus we learn that indeed the quantum field theory of this idealized world

  • predicts that hadrons such as protons do exist
  • predicts the ones we observe, without a lot of extra ones or missing ones
  • predicts correctly the masses of these hadrons

from which we conclude that

  • the quantum field theory with the fields shown in Figure 1 has something to do with the real world
  • we were wise to choose the imaginary world of Figure 1 for our study, because clearly the idealizations we made didn’t affect our final results to an extent that they caused disagreements with the real world

    Fig. 2: The masses of various hadrons, whose names appear at bottom and whose measured masses appear as grey horizontal lines, as calculated by computer: each colored dot is a calculation, whose  uncertainty is shown by a vertical bar.  I have written the names of some famous hadrons.

    Fig. 2: The masses of various hadrons, whose names appear at bottom and whose measured masses appear as grey horizontal lines, as calculated by computer: each colored dot is a particular calculation, whose uncertainty is shown by a vertical bar. I have written the names of some famous hadrons.

All looks great! And it is. However, I’ve lied to you. I haven’t actually told you how hard it is to obtain these answers. So let me give you a little more insight into what you have to do to obtain these calculations. You have to go off into even more imaginary worlds.

How the Calculation is Really Done: Off In Imaginary Worlds

The imaginary world I’ve described so far is still not simple enough for the calculation to be possible. The actual calculations require that we make predictions in worlds very different from our own. Two simplifications have to do with something you’d think would be essential: space itself. In order to do the calculation, we have to imagine

  • that the world, rather than being enormous, is made of just a tiny little box — a box only large enough to hold a single proton or other hadron;
  • that space itself, rather than being continuous, forms a discrete grid, or lattice, in which the distances between points on the grid are somewhat but not enormously smaller than the distance across a proton.

This is schematically illustrated in Figure 3, though the grids used today are denser and the boxes a bit larger.  The size of a proton, relative to the finite grid of points, is indicated by the round circle.

Fig. 3: The calculations are done in a world whose space is a small grid.  This picture of a 4 x 4 x 4 grid is a cartoon to make the idea clear; today, grids of 32 x 32 x 32 are not unusual.

Fig. 3: The calculations are done in a world whose space is a small grid. Note, however, that this picture of a 4 x 4 x 4 grid is a cartoon to make the idea clear; with modern computers, grids of 32 x 32 x 32 are not unusual.

Advances in computer technology are certainly helping avoid this problem… the better and faster are your computers, the denser you can take your grid and the larger you can take your box. But simulating a large chunk of the world, with space that is essentially continuous, is way out of reach right now. So this is something we have to accept, and deal with.  Unlike the idealizations that led us to study the quantum field theory in Figure 1, choosing to study the world on a finite grid does change the calculations substantially, and experts have to correct their answers after they’ve calculated them.

And there’s one more simplification necessary. The smaller are the up and down and strange quark masses, the harder the calculation becomes. If these masses were zero, the calculation just would be impossible. Even with the real world’s quark masses (the up quark mass is about 1/300 of a proton’s mass, the down quark 1/150, and the strange quark about 1/12) calculations still aren’t really possible — and they weren’t even close to possible until rather recently. So calculations have to be done in an imaginary world with much larger quark masses, especially for the up and down quark, than are present in the real world.

Fig. 4: Two types of imaginary worlds arise here.  First, the real world is stripped down, with all irrelevant particles and forces dropped, giving the red imaginary world.  Then this world's space is made into the grid of Figure 3, and the up, down and strange quark masses are raised.  In this purple imaginary world, calculations are easier, but give very wrong answers; only by extrapolating (Figure 5) are the predictions extracted.

Fig. 4: Two types of imaginary worlds arise here. First, the real world is stripped down, with all irrelevant particles and forces dropped, giving the red imaginary world. Then this world’s space is made into the grid of Figure 3, and the up, down and strange quark masses are raised. In this purple imaginary world, calculations become practical, but they give incorrect answers; only by extrapolating (Figure 5) are useful predictions extracted.

So since we can’t calculate in the real world, but have to calculate in a world with a small spatial grid and heavier quarks, how can we hope to get reasonable answers for the hadron masses? Well, this is another place where the experts earn our respect. The trick is to learn how to extrapolate. For example:

  • Do the calculation for fields in a small box.
  • Then do the calculation again in a medium-sized box (which takes a lot longer.)
  • Then do the calculation in a larger box (still small, but big enough that it uses about as much computer time as you can spare.)

Now, if you know how going from a small to medium to larger box should change your answer, then you can infer, from the answers you obtain, what the answer would be in a huge box where the walls are so far away they don’t matter.

The experts do this, and they do the same thing for the space grid, computing with denser grids and extrapolating to a world where space is continuous. And they do the same thing for the quark masses: they start with moderately large quark masses, and they shrink them in several steps. And knowing from theoretical arguments what should happen to the hadron masses as the quark masses change, they can extrapolate from the ones they calculate to the ones that would be predicted if the quark masses were the real-world ones. You can see this in Figure 4. As the up and down quark masses are reduced, the pion mass gets smaller, and the “nucleon” (i.e. proton and neutron) masses becomes smaller too. (Also shown is the Omega hadron; this has three unpaired strange quarks, and you can see its mass doesn’t depend much on the up and down quark masses.) The experts take the actual calculations (colored dots), and draw a properly-shaped curve through all the dots. Then they go to the point on the horizontal axis where the quark masses equal their real-world values and the pion mass comes out agreeing with experiment, and they draw a vertical black line upward. The intersection of the black vertical and blue curved line (the black X mark) is then the prediction for what the proton and neutron mass should be in the real world. Well, you can see that the black X is pretty close, within about 0.030 GeV/c², to what we find in experiments: 0.938 and 0.939 GeV/c² for the proton and neutron mass.  And this is how all of the results shown in Figure 2 are obtained: extrapolating to the real world by calculating in a few imaginary ones.

Fig. 5: Calculations (colored dots) are done with larger quark masses than in the real world, and one must extrapolate to the smaller quark masses of the "real" world (black dotted vertical line) to make predictions (black X's).  "N" stands for "nucleon", meaning both protons and neutrons.

Fig. 5: Calculations (colored dots) are done with larger quark masses than in the real world, and the results are as much as 50% too large.  One must extrapolate to the smaller quark masses of the “real” or “physical” world (black dotted vertical line) to make predictions (black X’s). “N” stands for “nucleon”, meaning both protons and neutrons.

The Importance of Such Calculations

This is a tremendous success story. The equations of the strong nuclear force were first written down correctly in 1973. Calculations like this were just becoming possible in the mid-1980s. Only in the 1990s did the agreement start to become impressive. And now, with modern computer power, it’s become almost routine to see results like this.

More than that, these methods have become essential tools. There are many important predictions made for experiments which are partly made with the methods I described in my previous post and partly using these computer calculations. For example, they are extremely important for precise predictions of the decays of hadrons with one heavy quark, such as B and D mesons, which I have written about here and here. If we didn’t have such precise predictions, we couldn’t use measurements of these decays to check for unknown phenomena that are absent from the Standard Model.

But There’s So Still Much That We Can’t Compute

Despite all this success, the limitations of the method are profound. Although computers are fine for learning the masses of hadrons, and some of their other properties, and quite a few other interesting things, they are terrible for understanding everything that can happen when two protons (or other hadrons) bump into each other.  Basically, computer techniques can’t handle things that change rapidly over time.

For example, the data in Figure 6 show two of the simplest things you’d like to know:

  • how does the probability that two protons will collide change, if you increase the energy of the collision?
  • what is the probability, if they collide, that they will remain intact, rather than breaking apart into a spray of other hadrons?

We can measure the answer (the black points are data, the black curve is an attempt to fit a smooth curve to the data.) But no one can predict this curve by starting with the quantum field theory of the strong nuclear force — not using successive approximation, fancy math, brute force computer simulation, string theory, or any other method currently available. [Experts: there are plenty of attempts to model these curves (look up “pomeron”.) But the models involve independent equations that can’t actually be derived from or clearly related to the quantum field theory equations for quarks and gluons.]

Fig. 6: The probability for two protons to collide (upper data points) and to collide without breaking (lower data points) as a function of the collision energy.

Fig. 6: The probability for two protons to collide (upper data points, “total”) and to collide without breaking (lower data points, “elastic”), as a function of the energy of one proton as viewed by the other proton.  Data are taken from many experiments, including the LHC at the far right.  The curve shows an attempt to fit the data, but this data cannot currently be predicted starting from the equations for quarks and gluons.

At the LHC, when a quark from one proton hits a quark from another proton, we can predict, using the successive approximation (“perturbative”) methods described in my previous post, what happens to the quarks. But what happens to the other parts of the two protons when the two quarks strike each other? We can’t even begin to predict that, either with successive approximation or with computers.

My point? The quantum field theory of the strong nuclear force allows us to make many predictions. But still, many very basic natural phenomena for which the strong nuclear force is responsible cannot currently be predicted using any known method.

Stay Tuned. It’s going to get worse.

Continued here

Courses, Forces, and (w)Einstein

This week and next, I’m very busy preparing and delivering a new class (four lectures, 1.5 hours each), for a non-technical audience, on the importance of and the discovery of the Higgs particle.  I’ll be giving it in Western Massachusetts (my old stomping grounds).  If it goes well I may try to give these lectures elsewhere (and please let me know if you know of an institution that might be interested to host them.)   Teaching a new class for a non-technical audience requires a lot of concentration, so I probably won’t get too much writing in over that period.

Still, as many of you requested, I do hope soon to follow up last week’s article (on how particle physicists talk about the strength of the different forces) with an article explaining how both particles and forces arise from fields — a topic I already addressed to some extent in this article, which you may find useful.

Now — a few words on the flap over the suggestion that math Ph.D. and finance expert Eric Weinstein, in his mid-40s, may be the new Albert Einstein.  I’ve kept my mouth shut about this because, simply, how can I comment usefully on something I know absolutely nothing about?  (Admittedly, the modern media, blogosphere and Twitter seem to encourage people to make such comments. Not On This Blog.) There’s no scientific paper for me to read.  There’s no technical scientific talk for me to listen to.  I know nothing about this person’s research.  All I know so far is hearsay.  That’s all almost anyone knows, except for a few of my colleagues at Oxford — trustworthy and experienced physicists, who sound quite skeptical, and certainly asked questions that Weinstein couldn’t answer... which doesn’t mean Weinstein is necessarily wrong, only that his theory clearly isn’t finished yet.  (However, I must admit my expert eye is worried that he didn’t have ready answers to such basic questions.)

What I do know is that the probability that Weinstein is the new Einstein is very low.  Why?  Because I do know a lot about how very smart people with very good ideas fail to be Einstein.  It’s not because they’re dumb or foolish. Continue reading

Additional Atomic Articles

I’ve been adding to my series of layperson’s articles on The Structure of Matter, which eventually will serve as an introduction to particle physics for those coming to this site for the first time. You might recall that in early December I supplemented my older article on molecules with an article on atoms. I got some terrific reader feedback, in the form of incisive constructive criticism, which allowed me to greatly improve the latter article. Well, readers, you’ve got another chance to help me out if you would like to — or you can just enjoy the read. I have three new articles (two of them short) which were put up over the last few weeks. These are:

Incidentally, the next stage in this series will be to describe electrons, and then I will turn to atomic nuclei, to the neutrons and protons that they contain, and eventually to the quarks and gluons that make up the neutrons and protons.