Tag Archives: atoms

Breaking News: Two Great New Measurements

Two new ground-breaking measurements reported results in the last 24 hours!  Here are very quick summaries.

A group of atomic physicists, called the ACME collaboration, has performed the best search so far for the electric dipole moment (EDM) of the electron.  Unfortunately they didn’t find the EDM, but the limit

  • |de| < 8.7  10-29 e cm

is 12 times stronger than the previous one.  While this is still a billion times larger than what is expected in the Standard Model of particle physics (the equations used for the known elementary particles and forces), there are various types of as-yet unknown particles and forces that could easily produce a much larger electron EDM, through new violations of T symmetry (or, almost equivalently, CP symmetry).  These effects could have been large enough to have been discovered by this experiment, so those types of possible phenomena are now more constrained than before.  Fortunately, there’s more to look forward to; the method these folks are using can eventually be improved by another factor of 10 or so, meaning that a discovery using this technique is still possible.

This morning the LUX dark matter experiment reported new results, and knocked everyone’s socks off.  They have understood their backgrounds from radioactivity much better and more quickly than most of us expected, using new calibration methods and a much better characterization of their backgrounds than has previously been possible.  Although they have a detector only a bit larger than XENON100 and have only run the detector underground for three months, compared to the year or so that XENON100 ran previously, their limits on the rate for a dark matter particle to hit a Xenon nucleus beats XENON100’s results by a factor of 2 for a dark matter particle of mass 1000 GeV/c², increasing to about a factor of 3 for a dark matter particle in the 100 GeV/c² mass range, and soaring to a factor of 20 for a dark matter particle in the 10 GeV/c² mass range.  Consequently, LUX pretty definitively rules out the possibility, hinted at by several dark matter experiments (as discussed in the second half of the article I wrote about this in April), of a dark matter particle in the 5 – 20 GeV/c² mass range.  (See the figure below.) While XENON100 seemed to contradict this possibility already, it didn’t do so by a huge factor, so there were questions raised as to whether their result was convincing. But the sort of ~10 GeV/c² dark matter that people were talking about is ruled out by LUX by such a large factor that finding ways around their result seems nigh impossible.   And again, there’s more to look forward to; by 2015 their results should improve by another factor of 5 or so… so they get another shot at a discovery, as will XENON1T, the successor to XENON100.

Congratulations to both groups for their spectacular achievements!

Results from the LUX paper, with labels added (hopefully correctly) by me; the shaded blue area is the range that LUX expected to reach, and the blue line their actual result, which exceeds the XENON100 result (red line).  The left plot shows the range 10 - 1000 GeV/c^2; the right plot is an inset showing details of the low-mass region, along with the hints of signals from DAMA/LIBRA, COGENT, CMDS and CRESST, all of which now appear entirely implausible.

Results from the LUX paper, showing excluded regions as a function of dark-matter particle mass (horizontal axis) and dark-matter/nucleus collision rate (vertical axis), with labels added (hopefully correctly) by me.  The shaded blue area is the range that LUX expected to reach, and the blue line their actual limit, which considerably exceeds the XENON100 result (red line) at all masses. The left plot shows the mass range 3 – 4000 GeV/c^2; the right plot is an inset showing details of the 5 – 12 GeV/c^2 mass region, along with the hints of signals from DAMA/LIBRA, COGENT, CMDS and CRESST, all of which now appear entirely implausible.

A Short Break

Personal and professional activities require me to take a short break from posting.  But I hope, whether you’re a novice with no knowledge of physics, or you’re a current, former, or soon-to-be scientist or engineer, or you’re somewhere between, that you can find plenty of articles of interest to you on this site.  A couple of reminders and pointers:

* If you haven’t yet seen my one-hour talk for a general audience, “The Quest for the Higgs Boson”, intended to explain accurately what the Higgs field and particle are all about, while avoiding the most common misleading short-cuts, it’s available now, along with a 20-minute question and answer session.

* If you want a slightly more technical and written discussion of the Higgs field and particle, complete with animated images, and suitable for people who may once have had a semester or two of university physics and math, try this series of articles first, and then go to this series.

* If you’d like to better understand the language of “matter”, “mass”, and “energy” that is everywhere in popular explanations of science, but eternally confusing because of how different authors choose to talk about these subjects, you might find some useful tips in these articles: #1, #2, #3, #4.

* If you need a reminder about what “ordinary matter” (i.e. things like pickles, people and planets) is made of, try this series, which goes all the way from molecules down to quarks.

* If you’re curious about what “particle/anti-particle annihilation” does and doesn’t mean, try this article.

* And here are the types of particles and forces of nature that we know about, and (for the moderately advanced reader) here’s how they’d be rearranged if the Higgs field were turned off.

Hopefully there’s something on that list that interests you, and many links within those articles to other things that may even interest you more.  Have fun exploring!  And stay tuned; I’ll be writing more in the near future…

What is the “Strength” of a Force?

Particle physicists, cataloging the fundamental forces of nature, have named two of them the strong nuclear force and the weak nuclear force. [A force is simply any phenomenon that pushes or pulls on objects.] More generally they talk about strong and weak forces, speaking of electromagnetism as rather weak and gravity as extremely weak.  What do the words “strong” and “weak” mean here?  Don’t electric forces become strong at short distances? Isn’t gravity a pretty strong force, given that it makes it hard to lift a bar of gold?

Well, these words don’t mean what you think.  Yes, the electric force between two electrons becomes stronger (in absolute terms) as you bring them closer together; the force grows as one over the square of the distance between them.  Yet physicists, when speaking their own language to each other, will view this behavior as what is expected of a typical force, and so will say that “electromagnetism’s strength is unchanging with distance — and it is rather weak at all distances.

And the strength of gravity between the Earth and a bar of gold isn’t relevant either; physicists are interested in the strength of forces between individual elementary (or at least small) particles, not between large objects containing enormous numbers of particles.

Clearly there is a language difference here… as is often the case with words in English and words in Physics-ese.  It requires translation.  So I have now written an article explaining the language of “strong” and “weak” forces used by particle physicists, describing how it works, why it is useful, and what it teaches us about the known forces: gravity, electromagnetism, the strong nuclear force, the weak nuclear force, and the (still unobserved but surely present) Higgs force. Continue reading

Why Do Protons and Neutrons Form Nuclei, and Why Are The Nuclei So Small?

The Structure of Matter series continues: last week’s article on the basics of atomic nuclei is now supplemented with an article discussing the “residual” strong nuclear force which binds protons and neutrons inside of nuclei.  It further explains why nuclei are so small compared to atoms.  Or rather, it explains it in part, because I have to also explain why protons and neutrons themselves are so small — which I will do soon enough.

As always, readers are encouraged to comment on things that don’t seem clear or correct.  And any nuclear physics experts who want to weigh in on my presentation — suggesting how it might be improved or extended, or identifying misconceptions on my part — are encouraged to speak up (publicly or privately as you prefer).

Meanwhile, we’re entering the March conference season, when many new results from the Large Hadron Collider [LHC] (based on analysis of last year’s data) and from other important experiments will start appearing.  Since the LHC’s proton-proton collisions went til December in 2012 (in 2011 they stopped in October) the time for LHC data analysis has been rather short.  I therefore think it likely that any really surprising results from the LHC will be delayed for extra scrutiny — and may not appear until late spring or summer, when there are other conferences.  But I could be wrong!  And one thing we’re all waiting for is the measurement by CMS (one of the two general purpose LHC experiments) of the rate for the recently discovered Higgs particle to decay to two photons.   However, we won’t see that result until CMS is absolutely confident in it.

Article on Atomic Nuclei

Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week.

Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to introduce non-experts to the basics of particle physics.  The first of these articles is done.  In it I describe why it was so easy (relatively speaking) to figure out that nuclei are made from certain numbers of protons and neutrons, and how it was understood that nuclei are very small compared to atoms.   Comments welcome as always!

A related article, which should appear later this week, will clarify why nuclei are so tiny relative to atoms, and describe the force of nature that keeps them intact.

The Puzzle of the Proton and the Muon

Fig. 1: A hydrogen atom consists of a tiny proton surrounded by an electron cloud, which is where the even tinier electron is to be found when sought.

Fig. 1: A hydrogen atom consists of a tiny proton “orbited” by an electron.

There’s been a lot of reporting recently on a puzzle in particle physics that I haven’t previously written about. There have been two attempts, a preliminary one in 2010 and a more detailed one reported just this month, to measure the size of a proton by studying the properties of an exotic atom, called “muonic hydrogen”. Similar to hydrogen, which consists of a proton orbited by an electron (Figure 1), this atom consists of a proton and a short-lived heavy cousin of the electron, called the muon (Figure 2). A muon, as far as we have ever been able to tell, is just like an electron in all respects except that it is heavier; more precisely, the electromagnetic force and the strong and weak nuclear force treat electrons and muons in exactly the same way. Only the first two of these forces should play a role in atoms (and neither gravity nor any force due to the Higgs field should matter either). So because we have confirmed our understanding of ordinary hydrogen with very high precision, we believe we also understand muonic hydrogen very well also.  But something’s amiss. Continue reading

Electrons and Their Properties

I’ve been quite busy with some physics research this week, but I have nevertheless managed to finish a new article on electrons, part of my Structure of Matter series, which aims (among other things) to introduce a non-expert to particle physics, step-by-step.  The completion of this article feels like a significant step for this website.  After all, the electron was the first subatomic particle and the first of the apparently-elementary particles to be discovered, about 115 years ago, and its discovery really gave birth to the field of particle physics we know today.  Moreover, it was the failure to describe the behavior of electrons within and outside of atoms that forced physicists to go beyond Newtonian views of physics processes, and introduce the theory of quantum mechanics.  Electrons, tiny as they are, are enormous in human life; they play a key role in all chemical reactions, including those that sustain our bodies.  Beyond that, they lie at the heart of much modern technology — electronics!  And there’s more.  So no particle physics website can be complete without an electron webpage.

Looking ahead, a question I sometimes get asked is whether I’m sure electrons (or any other elementary particles that physicists talk about) really exist.  After all, it is true I’ve never seen a picture of one taken with any sort of microscope!  Well, in answer to this question, I want to write an article on why we particle physicists are so confident that electrons (and atomic nuclei) exist… explaining the types of experiments and the types of logical reasoning that lead to this conclusion.  I suspect a lot of readers will find such an article interesting; after all, why should one take expert knowledge for granted just because it appears in a textbook or on a website?  Readers should demand to know where the knowledge came from — and a writer should be prepared to answer.

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.