My Structure of Matter series has been on hold for a bit, as I have been debating how to describe protons and neutrons. These constituents of atomic nuclei, which, when combined with electrons, form atoms, are drawn in most cartoons of atoms as simple spheres. But not only are they much, much smaller than they are drawn in those cartoons, they hide within them a surprising commotion, one that cannot be anticipated from the relatively simple structures of atoms and of nuclei.
As I’ve described in my new article, along the lines of this short article and this more detailed one that I wrote some time ago in the context of the Large Hadron Collider, the story that scientists tell the public most often, that “a proton is made from two up quarks and a down quark”, is not in fact the full story — and in some ways it is deeply misleading. The structure of protons and neutrons is so entirely unfamiliar, and so complicated, that scientists neither have a simple way of calculating it, nor an entirely agreed-upon way to describe it to the public, or even to physics students. But I believe my way of describing it will be satisfactory to most particle physicists.
The new article is not entirely complete; it is perhaps only half its final length. I’ll be adding some further sections that cover some subtle issues. But since I suspect many people won’t feel the need to read those later sections, the completed part is written to stand on its own. If you like, take a look and let me know if you have questions, suggestions or corrections.
Yesterday’s Quiz Question: when was the first Higgs particle produced by humans? (where admittedly “Higgs” should have read “Higgs-like”) got many answers, but not the one I think is correct. Here’s what I believe is the answer.
[UPDATE: After this post was written, but before it went live, commenter bobathon got the right answer -- at 6:30 Eastern, just under the wire! Well done!]
The first human-produced Higgs particle [more precisely, the Higgs-like particle with a mass of about 125 GeV/c2 whose discovery was reported earlier this month, and which I’ll refer to as “`H”– but I’ve told you why I think it is a Higgs of some sort] was almost certainly created in the United States, at the Fermilab National Accelerator Center outside Chicago. Back in 1988 and 1989, Fermilab’s accelerator called the Tevatron created collisions within the then-new CDF experiment, during the often forgotten but very important “Run Zero”. The energy per collision, and the total data collected, were just enough to make it nearly certain that an H particle was created during this run.
Run Zero, though short, was important because it allowed CDF to prove that precision mass measurements were possible at a proton collider. They made a measurement of the Z particle’s mass that almost rivaled the one made simultaneously at the SLC electron-positron collider. This surprised nearly everyone. [Unfortunately I was out of town and missed the scene of disbelief, back in 1989, when CDF dropped this bombshell during a conference at SLAC, the SLC's host laboratory.] Nowadays we take it for granted that the best measurement of the W particle’s mass comes from the Tevatron experiments, and that the Large Hadron Collider [LHC] experiments will measure the H particle’s mass to better than half a percent — but up until Run Zero it was widely assumed to be impossible to make measurements of such quality in the messy environment of collisions that involve protons.
Anyway, it is truly astonishing that we have to go back to 1988-1989 for the first artificially produced Higgs(-like) particle!! I was a first-year graduate student, and had just learned what Higgs particles were; precision measurements of the Z particle were just getting started, and the top quark hadn’t been found yet. It took 23 years to make enough of these Higgs(-like) particles to convince ourselves that they were there, using the power of the CERN laboratory’s Large Hadron Collider [LHC]!
[Perhaps this remarkable history will help you understand why I keep saying that although the LHC experiments haven’t yet found something unexpected in their data, that absolutely doesn’t mean that nothing unexpected is there. What’s new just may be hard to see, waiting to be noticed with more sophisticated methods and/or more data.] Continue reading
Posted in Higgs, LHC Background Info
Tagged #Tevatron, atlas, CDF, cms, DZero, gluons, Higgs, LHC, LHCb, proton
Yes, it’s true what you’ve read; the CMS experiment at the Large Hadron Collider has found a new particle. However, this isn’t one to get excited about. Or rather, it’s the particle that’s excited, not the rest of us. It’s a nice result; a neat result; but this particle is a slightly more massive version of a hadron that we already knew about, a composite object similar to a proton, built out of more fundamental particles we discovered over 30 years ago. So in the grand scheme of things, this is minor news; no big mysteries to resolve here. Nevertheless, congratulations to CMS! Finding such particles always involves reconstructing them from their decay products, and since this one decays in a very complicated way, the result represents a technical tour-de-force!
This is a similar story to one from last December, when ATLAS announced that it had found, with confidence, a new particle. I explained to you then that there are particles and there are particles; Continue reading
This is a follow-up especially aimed at those non-experts who got really excited by my recent posts on the internal structure of the proton (here, here and here), in which I described the proton as being a lot more complicated than just two up quarks and a down quark, emphasizing the presence of many gluons and of many quark/anti-quark pairs in addition to those three quarks that everyone talks about.
Following those posts, I got a lot of very good questions. I’ve been absorbing them and thinking about how to answer them effectively. I had taken you as far as I knew how to go without hitting technical barriers. You probably noticed I was very careful to address certain issues and not others — answering certain questions and avoiding others. And many of you, intelligently, asked the questions I didn’t answer. So now you get to find out why I didn’t answer them in the first place. [You asked!] Continue reading
Among the bridges that I hope to build, as I develop this website, is one connecting what we know today about nature with how we know it. After all, you’re reading my depiction of nature, based on how I think nature works. I can try to assure you that my depiction is the mainstream viewpoint at the forefront of the research field — but you may still wonder if this website is legitimate, or if I might just be full of hot air, or if I might simply be mistaken. Well, my confidence in what I’m saying doesn’t come from having trained at some fancy university or my degree or from having been in the business for over 20 years. It comes from the data… in short, from nature itself.
So it’s important, I think, to link the data to the ideas and concepts, when it’s possible to do that.
You’ve heard the famous statement that “a proton is made from two up quarks and a down quark”. But in this basic article, and this somewhat more advanced one, and in Wednesday’s post where I went into some details about what we know about proton structure, I’ve claimed to you that protons are actually chock full of particles, most of which carry a tiny fraction of the proton’s energy, and most of which are gluons, with a lot of quarks and antiquarks. [If this sounds unfamiliar, you should read those articles and posts before reading this one, which is a follow-up.] And I claimed that these complications make a big difference at the Large Hadron Collider [LHC].
So should you take my word for this? You don’t have to. Let me show you evidence. From LHC data. Here’s an article defending the main claim’s of Wednesday’s post. It’s a near-final draft, still needing some proofreading perhaps, and probably some clarification, but I think it is fully readable now. Enjoy it (and please feel free to give me feedback on its clarity, so I can improve it), or wait for the final version next week, as you see fit. And have a great weekend!
[Reminder: I’ll be interviewed today at 5 p.m. Eastern time, at http://www.blogtalkradio.com/virtuallyspeaking/2012/02/15/matt-strassler-tom-levenson-virtually-speaking-science , which you can listen to either live or later. My interviewer, Tom Levenson, is an eminent science journalist who has written fascinating and surprising books on Einstein and on Newton, among others, won awards for his work on television (e.g. NOVA), has a great blog (and also posts here), and is a professor of science writing at MIT. Should be fun!]
Since a number of readers were surprised to learn, from yesterday’s article about the benefits of increasing the energy of the protons at the Large Hadron Collider [LHC], that protons are very complicated and have a lot more in them than just two up quarks and a down quark, I thought I’d put up a plot or two that gives some indication of how particles are distributed inside a proton. Caution: the answers you get, and the physical intuition you obtain, depends in some subtle ways on exactly what you ask, so you should pay some attention to precisely which question I’m answering below. The details matter.
Two plots, differing only in the range for the vertical axis, showing the relative likelihood of striking a gluon or an up or down quark or antiquark carrying a fraction x of the proton's energy. At low x gluons dominate (and quarks and antiquarks become equally likely, and numerous, though far less so than gluons), while quarks dominate (but are very rare) at moderate x. Plotted using a Mathematica package (Trout and Olness, 2000) based on CTEQ5L results; somewhat out of date, but accurate enough for today's purposes.
The two plots in the Figure show exactly the same thing, just with a different vertical scale, so that certain things that are hard to see on one plot are clearer on the other. And what they show is this: if a proton is flying toward you in a Large Hadron Collider [LHC] proton beam, and you strike something inside that proton, how likely are you to have hit an up quark, or down quark, or gluon, or up antiquark, or down antiquark, that carries a fraction x of the proton’s energy? From these plots we can learn: Continue reading
Yesterday, a commenter asked me a very good question that I realized I hadn’t yet addressed on this site. Answering it gives us a chance to look at real data from the Large Hadron Collider [LHC], and to see what differences will arise the machine’s energy is increased from 7 TeV to 8.
The protons that are smashed together at the LHC are made from many quarks, gluons and antiquarks. The proton-proton collisions take place at a definite energy: 7 TeV = 7000 GeV in 2011, 8 TeV = 8000 GeV in 2012. But what we’re mainly interested in — what can really create new physical phenomena for us to observe — are the collisions of a quark in one proton with an antiquark in the other proton, or the collision of two gluons, etc. These “mini-collisions” carry only a fraction — typically a very small fraction — of the total proton-proton collision energy. How high a fraction can they carry? and what are the motivations for increasing the energy from 7 TeV per collision to 8 TeV? Click here for the answer.