Of Particular Significance

Author: Matt Strassler

The Constancy of the Heavens — Verified Anew

This is a post about constancy and inconstancy, one of my favorite topics. And about how alcohol can make you smarter.

There are many quantities that we call “constants of nature”. Of course, anything we call a “constant” is merely something that, empirically, appears to be constant, to the extent we can measure it. Everything we know comes from observation and experiment, and our knowledge is always limited by how good our measurements are.

We have pretty good evidence that a number of basic physical quantities are pretty much constant. A lot of evidence comes from the constancy of the colors of light waves (i.e. the frequencies of waves of electromagnetic radiation) that are emitted by different types of atoms, which appear to be very much the same from day to day and year to year and even across billions of years (neat trick! will describe that another time), and from here to the next country and on to the moon and to the sun and across our galaxy to distant galaxies. For example, if the electron mass changed very much over time and place, or if the strength of the electromagnetic force varied, then atoms, and the precise colors they emit, would also change. Since we haven’t ever detected such an effect, it makes sense to think of the electron mass and the electromagnetic force’s strength as constants of nature.

But they’re not necessarily exactly constant. One can always imagine they vary slowly enough across time or place that we wouldn’t have noticed it yet, with our current experimental technology. So it makes sense to look at very distant places and measure whatever we can to seek signs that maybe, just maybe, some of the constants actually vary after all.

[I wrote a paper in 2001 with Paul Langacker and Gino Segre about this subject (Calmet and Fritzsch had a similar one). This followed the observational claims of this paper (now thought false) suggesting the strength of electromagnetism varies across the universe and/or with time. A lot of what follows in this post is based on what I learned writing that old paper.]

Suppose they did vary? Well, the discovery of any variation whatsoever, in any quantity, would be a bombshell, and it would open up a door to an entirely new area of scientific research. Once one quantity were known to vary, it would be much more plausible that others vary too. For instance, if the electron mass varies, why not the W particle’s mass, which affects the strength of the weak nuclear force, and thereby radioactivity rates and the properties of supernovas? If the electromagnetic force strength varies, why not that of the strong nuclear force? There would be interest in understanding whether the variation is over space, over time, or both. Is it continuous and slow, or does it occur in jumps? One can imagine dozens of new experiments that would be proposed to study these questions — and the answers might reveal relations among the laws and “constants” of nature that we are currently completely unaware of, as well as giving us new insights into the history of the universe.

So it would be a very big deal. [Though I should note it would also be puzzling: even small variations in these constants would naively lead to large variations in the “dark energy” (i.e. cosmological “constant”) of the universe, which would potentially make the universe very inhomogeneous. However, we don’t understand dark energy, so this expectation might be too naive.] Since there’s no story about it on the front page of the New York Times, you can already guess that no variation’s been found. But a nice new measurement’s been done. (more…)

POSTED BY Matt Strassler

ON December 18, 2012

Two Higgs Bosons? No Evidence for That

[Note Added: this afternoon the author of the Scientific American article made a few corrections. I leave it to you to judge for yourself whether he addressed the issues raised here.]

There’s been a little silliness floating around (sadly, in Scientific American, whose article contains at least two factual errors) unscientifically speculating that ATLAS’s new results on the Higgs-like particle, from data collected at the Large Hadron Collider [LHC], suggest there are two such particles rather than one. The mass measurement of this particle using the data when it decays to two photons, 126.6±0.3±0.7 GeV/c², is different, by 2.7 standard deviations, from the mass measurement obtained from its decays to two lepton/anti-lepton pairs, 123.5±0.9+0.4-0.2 GeV/c². So… huh… gee… maybe there are two Higgs-like particles, a lighter one which rarely decays to two photons and a heavier one which rarely decays to two lepton/anti-lepton pairs?

[Note Added: I should emphasize, lest anyone blame ATLAS for this implausible line of speculation, that in the ATLAS presentation last week, which was one of several presentations that morning, these two mass measurements were presented simply and responsibly, as results from data. Not a single speculative word was said about there being a hint of two Higgs particles. I don’t know who got the ball rolling on that idea, but it wasn’t ATLAS. And it’s not a plausible idea: see below.]

Take a deep breath. For not only would the two types of particles somehow have to be magically and implausibly arranged to mimic, at first glance and to a rough extent, a single Standard Model Higgs particle (the simplest possible type of Higgs particle), there’s another experiment, which unfortunately the writer of the Scientific American article neglected to consult.

ATLAS’s mass measurement from the events with two lepton/anti-lepton pairs also disagrees with CMS’s mass measurement obtained from the same type of events: 126.2±0.6±0.2 GeV/c². Two similar experimental detectors, same measurement, moderate disagreement. Nature is nature; there’s no way that ATLAS can be making one type of particle all the time, while CMS is making a different one all the time. So there is no evidence here, taking ATLAS and CMS together, favoring the existence of a separate particle with a mass of about 123.5 GeV/c² that decays to two lepton/anti-lepton pairs.

What is behind these discrepancies, then? ATLAS and CMS each have scarcely a dozen of these two lepton/anti-lepton events, and their extraction of the Higgs particle’s mass from each event is somewhat uncertain, which is why many events are required for a good mass measurement. When you still have small amounts of data, funny statistical fluctuations will often occur. We’ve seen this before; back in 1989, when the Stanford Linear Collider (SLC) produced its first few Z particles at the Stanford Linear Accelerator Center, the plot of the Z particle’s mass gave a double resonance peak, instead of the single peak that was expected. A brief moment of speculation occurred, but with more data the anticipated single peak structure emerged. I’ve heard at least one other similar story from an earlier decade. In fact ATLAS and CMS had a 2 GeV mass discrepancy when the first Higgs hints came in; that was just an effect of statistics. Combine a fluctuation of this form with a minor detector calibration problem, and you’ll get discrepancies like this.

Multiple types of Higgs particles are certainly possible; people have considered this scenario for decades, and I’ve written about it here, for instance. Efforts to search for a second type of Higgs particle have been going on since the discovery of the first one. But let’s not manufacture one out of thin air by looking selectively at the data; that’s not how reliable science gets done.

POSTED BY Matt Strassler

ON December 17, 2012

Another Higgs Update from CERN

At the CERN laboratory today, there’s an ongoing report to the CERN council that oversees the lab, and this includes talks from the Large Hadron Collider [LHC] accelerator operators, and from the experimentalists who built and operate the detectors (ATLAS, CMS, LHCb, ALICE, TOTEM) that are designed to detect and interpret the debris from the LHC’s proton-proton collisions. Among the results being presented today are some measurements of the properties of the Higgs-like particle whose discovery was announced in July, including ones that were notably missing from the HCP conference presentations in Kyoto last month.

Here are some highlights, to be fleshed out in more detail later, if warranted.

LHC accelerator operations report:

An excellent year. In the best week the LHC produced 1.35 inverse femtobarn (fb) of data for both ATLAS and CMS; as of December 5, LHC produced 23.2 inverse fb for the year per experiment (note each experiment will have somewhat less recorded, due to normal losses), slightly above the target for the year.

Biggest problems: beam instability (much bigger problem in 2012 than 2011); stray high-energy particles affecting electronics in the tunnel; dust falling out of the beampipe into the beam, potentially a significant problem for 2015.

After the 2013-2014 shutdown, what will be the likely running conditions in 2015? The current intention is to go to 25 nanoseconds between collisions (the design) rather than the 50 nanoseconds used in 2012, and to start at 13 TeV per collision.

ATLAS: New two-photon and four-lepton measurements; new spin and parity measurements.

Two photons: new categories of events added, with either 1 lepton or of two jets at low invariant mass, characteristic of production of a Higgs with a Z or W.

This channel now shows 6.1 standard deviation significance (3.3 expected) by itself: discovery of a Higgs-like particle in a single decay mode. Mass 126.6±0.3±0.7 GeV/c²

Signal remains high: 1.8 ± 0.3 + 0.29 – 0.21 times the Standard Model expectation. (But note the expectation depends somewhat on the assumed mass of the Higgs; ~~not sure yet which mass was taken here.~~ 126.6 GeV was assumed.)

Four leptons: 4.1 standard deviations, signal strength 1.3 ± 0.4 times Standard Model expectation. Mass 123.5±0.9+0.4-0.2 GeV/c² — notably lower than two photons.

The two mass measurements are 3 GeV apart (surprising but not impossible given the amount of data; or perhaps there is a technical problem somewhere, though I’m sure they looked very, very hard for one). They are compatible only at 2.7 standard deviations. Combined mass: 125.2 +- 0.3 +- 0.6 GeV/c²

Spin (from photons): spin 2 disfavored at the 91% level; compatible with spin 0.

Spin (from leptons): spin 2 disfavored only at the 85% level; compatible with spin 0.

Parity (from leptons): Exclusion of odd-parity spin-0 particle at 99%.

CMS:

First limit on Higgs decay to Z + photon: although still ~20 times the Standard Model expectation, this limit is good enough to rule out various non-standard interpretations of the Higgs-like particle.

No update of Higgs decay to two photons. This is too bad. It’s somewhat exciting that ATLAS’s result on Higgs decay to two photons remains somewhat high compared to expectations. But the excess is still not yet 3 standard deviations. Deviations of this size do come and go. And we don’t have confirmation from CMS. So the situation remains tantalizing but unfortunately not yet very convincing. We may not learn anything more from CMS or ATLAS til March, when they have analyzed the full 2012 data set.

I did not catch anything new from LHCb or ALICE; generally I haven’t had time to cover ALICE’s research program here. TOTEM, a special purpose detector for measuring things I haven’t discussed on this website, is still getting rolling.

POSTED BY Matt Strassler

ON December 13, 2012

Testing, Testing: 12/12/12 12:12:12

This is a modified version of last year’s 11/11/11 article, in case you missed it.

Today is a special day — at least if you are fond of the number 12, and especially so if you’re willing to buy in to one of the oldest human pseudo-scientific pursuits: numerology. Oh, don’t get me wrong, I love numbers and I always have. When I was five years old I was mesmerized when my parents’ car reached 99,999.9 miles, and I think 12:34:56 on 7/8/90 is just a cool a time as anybody else does. But I do this with a sense of humor.

Unfortunately it happens that a few influential people attempt serious and consequential numerology involving the calendar — predicting disaster and convincing people to sell their homes and give away their belongings. Now that makes me mad. Outraged, in fact — because it’s often obvious from the way these predictions are generated that those who made them don’t understand much about the calendar, about time, about history and about astronomy or physics… and yet they speak with authority, an authority they haven’t earned and don’t deserve.

So as we celebrate this one-two-of-a-kind moment, let’s also remember, and enjoy, just how absurd it really is. Let us even count the ways. (more…)

POSTED BY Matt Strassler

ON December 12, 2012

Two Days of Polchinski Puzzles

One of the most prominent theoretical physicists of our time, Professor Joe Polchinski of the University of Santa Barbara, who has made lasting contributions to our understanding of quantum field theory, of gravity, and of string theory, gave a couple of talks at the Institute for Advanced Study in Princeton this week. The two presentations manifested a certain amusing (anti-)parallel; the first was on a puzzle that was thought to have been mostly solved 20 years ago, but turns out to have only been partly resolved; the second was related to a puzzle that was thought to have been solved last year, but turns out to have been partly solved over 20 years ago.

In the middle of all of this, it was announced that Polchinski was one of several people awarded one of these new-fangled Fundamental Physics Prizes that are getting lots of attention — specifically, one of the Frontiers Prizes, if you’re keeping score. You can read about that elsewhere. Here we’ll try to keep our focus on the science. (more…)

POSTED BY Matt Strassler

ON December 12, 2012

New Attempt at Atomic Article

It took me over six months, following my article on molecules, to write the sequel, on atoms. These are just two in a series, intended to introduce the structure of matter to novice readers who want to learn what particle physics is about. Atoms aren’t the main focus; future articles will focus on electrons, on protons and neutrons, on quarks, and on the forces that hold these objects together. But the essay on atoms might be the hardest of the set to write (at least I hope so). The long delay reflects the challenges involved, and as my readers’ wise and helpful criticisms of Friday’s first version confirmed, I didn’t meet them on my first try.

So after some thought, I’ve made another attempt. Critique still welcome from anyone who wants to make suggestions.

Aside from the fact that I fell into a couple of pedagogical traps that anyone who’d taught chemistry would have known about, I also struggled to describe atoms briefly, clearly and accurately because their features are determined by quantum mechanics — that weird but fundamental behavior of our world that we don’t encounter in daily life but is essential to the structure of matter. What’s profoundly confusing to the non-expert (and somewhat confusing even for experts) is that electrons are, on the one hand, best described in many circumstances as point-like particles (much smaller than atoms, and smaller even than atomic nuclei) yet around atoms they are in some way spread out in a very non-particle-like fashion. Well, indeed, thinking of elementary objects like electrons as “particles” will get you into trouble; for one thing, they are really “quanta” of quantum fields, and in most circumstances they behave much more like waves. And yet it is essential to explain that one can try to measure their size — essentially by forcing them, through an appropriate experiment, to reveal whether they, like baseballs, rocks and dumplings, have internal structure.

Ok, I can’t even figure out how to write this paragraph clearly. There needs to be a way to explain this issue, one that is both moderately intuitive and based on accurate and clear physical reasoning…

POSTED BY Matt Strassler

ON December 10, 2012

New Article on Atoms

I have a number of loose ends to tie off on this site, and one of them is my set of articles for novice readers about the structure of matter, which when complete will introduce the basic particles out of which we and all of ordinary matter are made. A while ago I wrote an article about molecules; today I finished the next article, on atoms [Note added: A revised version is now ready; thanks to readers for helpful criticisms of the original]. Future articles will explore how atoms work in more detail, and the subatomic particles out of which they are made. As always, comments (especially on the clarity of the writing, as well as typos) are welcome.

POSTED BY Matt Strassler

ON December 7, 2012

My Version of the TIME Higgs Paragraph

A few commenters have complained that I’m too hard on science journalists, who have a tough job; it’s hard to explain difficult concepts in a few words. To paraphrase them: “if it’s so easy, you do it! Rather than merely complain about the erroneous TIME magazine paragraph on the Higgs boson, write your own explanation of the Higgs particle for readers of TIME magazine.”

Well, first of all, I have never once suggested science journalism is easy; far from it! A big part of the challenge is to find ways to explain complex ideas that are simple, compelling and accurate (and not two out of three.)

Second, I have written an article suitable for non-expert readers; it’s just over a page long, and is called Why the Higgs Particle Matters. It’s gotten about 30,000 hits; some people seem to really like it, so try it out on your friends.

And third, for those who point out that the above-mentioned article is much longer than a paragraph, and that I shouldn’t be so critical of the TIME journalist who had to fit so much into a such a small space, here is my version of the TIME paragraph: six sentences rather than five, but scarcely longer. I have borrowed the style and the feel of the TIME journalist’s writing, and I have removed some inaccurate content and replaced it with different accurate content.

Take a moment to thank the Higgs field for all the work it does, because without it, you’d explode. This field pervades the universe and supplies electrons (and many other particles) with their mass, thus preventing ordinary matter from disintegrating into a ghastly vapor. It was in the 1960s that British physicist Peter Higgs (and a few others) first posited the existence of this field. But it was not until last summer that two huge teams of researchers at Europe’s Large Hadron Collider at last sealed the deal by discovering a new particle — the Higgs boson — which confirms the Higgs field exists. You see, the particle is a consequence of the field wiggling a bit; and just as sound, a ripple in the air, can’t be heard unless there’s air in the room, there wouldn’t be Higgs particles to discover unless Higgs and friends were right all along about their famous field. Now the Higgs — as most particles do — decays in an instant to other particles, so it wouldn’t be able to attend the award ceremony; however, the scientists would surely be happy to appear in its stead.

Although not everything I’ve written here is 100% accurate — that would indeed be impossible in a paragraph for a wide readership — I believe none of it is fundamentally wrong (but my colleagues should feel free to complain!) Yes, science journalism is difficult; but is it really inevitable that profound errors concerning the science must appear in articles for the public?

POSTED BY Matt Strassler

ON December 5, 2012

TIME for a Little Soul-Searching

Yes, it was funny, as I hope you enjoyed in my post from Saturday; but really, when we step back and look at it, something is dreadfully wrong and quite sad. Somehow TIME magazine, fairly reputable on the whole, in the process of reporting the nomination of a particle (the Higgs Boson; here’s my FAQ about it and here’s my layperson’s explanation of why it is important) as a Person (?) of the Year, explained the nature of this particle with a disastrous paragraph of five astoundingly erroneous sentences. Treating this as a “teaching moment” (yes, always the professor — can’t help myself) I want to go through those sentences carefully and fix them, not to string up or further embarrass the journalist but to be useful to my readers. So that’s coming in a moment.

But first, a lament.

Who’s at fault here, and how did this happen? There’s plenty of blame to go around; some lies with the journalist, who would have been wise to run his prose past a science journalist buddy; some lies with the editors, who didn’t do basic fact checking, even of the non-science issues; some lies with a public that (broadly) doesn’t generally care enough about science for editors to make it a priority to have accurate reporting on the subject. But there’s a history here. How did it happen that we ended up a technological society, relying heavily on the discoveries of modern physics and other sciences over the last century, and yet we have a public that is at once confused by, suspicious of, bored by, and unfamiliar with science? I think a lot of the blame also lies with scientists, who collectively over generations have failed to communicate both what we do and why it’s important — and why it’s important for journalists not to misrepresent it. (more…)

POSTED BY Matt Strassler

ON December 3, 2012

Particles are People Too

Much has already been written about the hilarious proposal that the recently-apparently-discovered particle called the Higgs Boson (you know, [what’s a boson?] the one sometimes called “God Particle” by people who don’t know much about particles) be selected as Time magazine’s “Person of the Year” for 2012. Well, I hope it wins, so that, as justice requires, it can finally be allowed to donate unlimited amounts of its money to U.S. election campaigns, just like U.S corporations, which, as you know, are people too. And I think this would also mean that Higgs bosons could finally get married, though I’m not entirely sure about the legality of whether they could marry each other… (recall the Higgs particle is its own anti-particle.)

But the hilarity became that much greater when Time magazine listed its various nominees and (as many have pointed out) described the Higgs particle in five glowing sentences, each of which contains at least one spectacular scientific or historical error. (Indeed, when I saw this, I assumed I was reading the Onion — maybe that’s where Time, following the lead of China’s famous newspaper Particles’ Daily, got this description? — though the Onion has been strangely quiet on the subject of the Higgs up to now.) The Higgs boson is candidate #18 out of 40, and I encourage you, in order to fully appreciate the company that it now keeps in the upper echelons of our society, to look at #17 and #19. Nevertheless, it’s worth noting that Time avoids the term “God Particle”, which would be praiseworthy under better circumstances.

So — a quiz for the reader. Can you identify five God-Particle-worthy errors in this piece from Time? Extra credit if you can find six or more. It’s an open-book test; you can use this website for insights.

The Higgs Boson

By Jeffrey Kluger

Monday, Nov. 26, 2012

Take a moment to thank this little particle for all the work it does, because without it, you’d be just inchoate energy without so much as a bit of mass. What’s more, the same would be true for the entire universe. It was in the 1960s that Scottish physicist Peter Higgs first posited the existence of a particle that causes energy to make the jump to matter. But it was not until last summer that a team of researchers at Europe’s Large Hadron Collider — Rolf Heuer, Joseph Incandela and Fabiola Gianotti — at last sealed the deal and in so doing finally fully confirmed Einstein’s general theory of relativity. The Higgs — as particles do — immediately decayed to more-fundamental particles, but the scientists would surely be happy to collect any honors or awards in its stead.

POSTED BY Matt Strassler

ON December 1, 2012

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The Higgs Boson

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