Of Particular Significance

Author: Matt Strassler

Everybody’s a Critic

I wanted to make a few assorted comments about the OPERA experiment’s painful climb-down, and about yesterday’s widespread response to it, which bothered me a lot. You may want to read my initial post from yesterday, and also my attempt to sharpen the main question OPERA left unanswered in my second post. [ALSO: look ahead to the next post, in which many of the confusions that were still present at the time of this post were resolved.]

Over the past day I’ve learned enough to be ~~pretty~~ convinced ~~(but not certain)~~ that the situation that we are in is case (e) [or a version of case (d)] as described in yesterday’s post: that probably the previous OPERA experimental data is tainted and we can draw no conclusions from it at all. It’s not that they found a problem that shifts their data so that it is consistent with Einstein’s relativity [case (b) from yesterday], and they can say that the neutrinos travel as expected. (Press reports that said so are just wrong.) It’s that they found a problem that means their data from last year can’t be interpreted at all… at least, not at the moment, and maybe not ever. If true, this would indeed mean that there is no longer any data from OPERA that can be used to measure neutrino speeds to good accuracy, and we’re back where we were before OPERA ran in the first place: with no reason to think there’s anything amiss with Einstein’s relativity equations. As for OPERA, the only way forward is to rerun the experiment (apparently in ~~March-April-~~May.)

[The NY Times article that appeared today (which attributes OPERA, a non-CERN experiment, to CERN; what has journalism come to these days?) has some additional details, but if you read it carefully, those details don’t change anything written in this post. See the first comment at the very end of this post.]

Ok, some comments. (more…)

POSTED BY Matt Strassler

ON February 24, 2012

Synopsis of the OPERA Situation

I thought I’d better try to clarify what the logical issues are regarding OPERA before things get too confusing. [Read the previous post first.] What’s going on is sketched in the Figure below.

a) In September (and confirmed in November) OPERA claimed to have found that, compared to expectations, neutrinos from CERN seemed to be arriving early, by about 60 nanoseconds, with a six sigma significance. That is sketched in the figure as a bar, centered at 60 nanoseconds, with a width of 10 nanoseconds in each direction (so if you made the bar six times larger its bottom would reach zero nanoseconds.) Such a result is convincing — convincing that either they made a mistake or they’ve discovered a new effect in nature. The new effect in this case — neutrinos going faster than the universal speed limit — is very implausible for many reasons that I’ve covered in the past, but such an experimental result cannot simply be ignored.

OPERASynopsis — (a) What OPERA said originally. (b) What yesterday's newspaper article claimed OPERA was now saying. (c),(d),(e) Three possible interpretations of OPERA's statement. In each case, a bar whose center lies at the most likely result of the measurement, and whose width indicates the degree of uncertainty, is shown. If we take OPERA's statement at face value, we still don't know if it is inconsistent with zero but less certain than before, consistent with zero, or simply unknown (in which case the previous measurement must be discarded.)

b) On February 22nd, a news report said that OPERA had found a mistake — a problem with the connection in an optical fiber — that explained the 60 nanosecond shift. That would mean that the result was now — with confidence — consistent with Einstein’s relativity.

But the OPERA statement contradicts b). Unfortunately it doesn’t clarify whether we have the situation c), d) or e):

c) The uncertainty on the result is larger than it was before, although the result is still not particularly consistent with what you would expect from Einstein’s relativity. However, the reduced confidence in the result would mean that instead of it being an impressive result it would become a much less impressive one, much easier to ignore.

d) The uncertainty on the result is so much larger than it was before that the result is actually consistent with Einstein’s relativity, though so weak that it can’t distinguish at all between zero and the original non-zero value that OPERA claimed.

e) There is no sensible estimate of the uncertainty on the result. The result simply cannot be used and will have to be discarded.

From the point of view of the plausibility of the original OPERA result, (b), (c), (d) and (e) are all bad. From the point of view of the experiment’s credibility, though, case (b) would look the best: the experimenters could at least say that they figured out what they did wrong and knew what it was. Cases (c), (d) and (e) are increasingly bad; they indicate less and less knowledge of how their own experiment worked. [But see the first comment below, which criticizes the way I expressed this, and emphasizes that a lack of knowledge is not always the fault of the experimenters.] However, the situation might change over time; right now they might be in case (d) [because they’re still working on understanding what happened] but soon it might switch to case (b), or back to case (a), once they have it straight. So we do have to let things play out a little bit, and not jump to conclusions.

But I think it is clear we are not currently in case (b). Because if we were, why would one would emphasize the re-running of the experiment? There would be no need.

POSTED BY Matt Strassler

ON February 23, 2012

OPERA in Question

[UPDATE: Some extensive comments added below.]

[UPDATE: Journalists and Bloggers: PLEASE NOTE: OPERA is not a CERN experiment. The CERN laboratory does not deserve the bad press it is getting (though they certainly unwisely put themselves in a position to receive it.)]

~~Many of you are probably already aware of various rumors running around~~ By now you are all aware of yesterday’s initial report that the OPERA experiment — famous for announcing that neutrinos traveling from CERN to the Gran Sasso Laboratory in Italy arrived 60 nanoseconds (billionths of a second) earlier than expected, thus suggesting that neutrinos can travel slightly faster than the speed of light — had found a problem with a cable connector that exactly explains the 60 nanosecond timing shift.

But the immediate source of this rumor was a science journalist, and the article was based on an anonymous source who is not described as being in the OPERA experiment. And the details quoted in the article didn’t add up, in my view. Given the number of wrong reports and rumors that I have read over the past months about this experiment, my reaction was to wait.

I didn’t have to wait long. ~~Presumably~~ Perhaps to avoid misinformation from hitting the headlines, ~~it appears that~~ OPERA has released a statement that indicates that the article from earlier ~~today~~ yesterday is not true. [Update: To be clearer, I probably should have written, “true in its details.”] But this statement itself contains big news. (more…)

POSTED BY Matt Strassler

ON February 23, 2012

What to Watch in the Sky This Week: Beauty in Motion

Why does the sight of the Moon draw our gaze and silence our voices? What is it about the planets, those exceptionally bright points of light that wander among the stars, that we instinctively find so beautiful? Is it perhaps that they make us dream of faraway, unreachable places? Is it that they are beacons in the night, nature’s candle flames, helping keep fear at bay, and offering us hope amid darkness? Or is it perhaps that they seem to float — we do love things that float, whether they be autumn leaves, balloons, clouds or birds — suspended in the sky, in apparent defiance of the force of gravity which keeps us pinned to the Earth?

This last thought offers a certain delicious irony… for in truth the planets and the Moon, in their procession above our heads, obey gravity’s dictates.

The next few days, weather permitting, will give us a chance to contemplate these questions. Our planet’s natural satellite, on its monthly trip around the Earth, will pass three of the brighter planets in the sky, creating one lovely spectacle after another. Of course, the Moon really passes nowhere near the planets, just as your outstretched hand, when it blocks your view of the Moon, is nowhere near the Moon itself. It is all a matter of perspective — of geometry, of cavernous spaces, of the play of light, and of the elegant choreography of the solar system. But this perspective is not something we sense easily. Our eyes can perceive no depth for objects so far away, and so our brains form a two-dimensional picture from the three dimensions of the universe, projecting the Moon, the planets and the stars, at extraordinary distances from one another, onto a psychologically flat black screen of the night sky. It takes great mental effort to see things as they are, and not as they appear. This, too, is worthy of contemplation.

First, on the night of February 22nd, just after sunset (don’t be late!), one of the most delicate of nocturnal sights awaits: Click here to read more…

POSTED BY Matt Strassler

ON February 21, 2012

How Do We KNOW a Proton Is So Complicated? (Data!)

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!

POSTED BY Matt Strassler

ON February 17, 2012

A Few More Facts About Protons

[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.

pdfs_linear — 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: (more…)

POSTED BY Matt Strassler

ON February 15, 2012

The Benefits of 8 TeV Collisions Over 7 TeV.

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.

POSTED BY Matt Strassler

ON February 14, 2012

Why a Lightweight Higgs is a Sensitive Creature — Part 2

[Note added: It is official — as expected, at this year’s Chamonix workshop, where the Large Hadron Collider’s [LHC’s] future is planned out each year, it was decided that the LHC’s energy will be increased by 14% next year (from 3.5 TeV energy per proton and 7 TeV energy per collision in 2010-2011 to 4 TeV per proton and 8 per collision.) Also the time between collisions will remain at 50 nanoseconds. I’ll have some things to say about the pros and cons of this decision, in particular the challenges for the experiments, over the next few days.]

On Monday last week, I gave you half the explanation as to why a lightweight Higgs particle is a sensitive creature, one that is easily altered by new phenomena — by particles and/or forces that we might not yet know about. It all had to do with an analogy between a violin string (or a guitar string or a xylophone key) and the properties of the Higgs particle. Today, on the same webpage as the first half, I have provided the second half of the story. (If you have already read the first half, just look for the boldface words “The Diverse Modes of a Higgs’ Demise”, which separate last week’s prose from the new stuff.) I’ve also added, for particle physicists and for those laypersons who want to go a little deeper, a short quantitative discussion of my main points.

Also: I will have the honor to be interviewed on Wednesday 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. In short, he’s a bright and interesting dude whom you should consider following on Twitter, or in whatever way floats your boat in the ocean of social media. For this reason I suspect that the conversation is going to be a lot deeper and more interesting than the average interview, with the interviewer making at least as many interesting comments about the topic as the interviewee.

POSTED BY Matt Strassler

ON February 13, 2012

This Week’s Step Forward in the Search for the Higgs Particle

I’ve been busy with some pressing work in service of the triggering strategy at the Large Hadron Collider [LHC] experiments for the last few days… (and if you understand what the trigger does, you know that stuff having to do with triggering pretty much takes priority over almost anything else, including sleep.) So my apologies that I’ve been a little slow to sum up this week’s updated results on the search for the Higgs particle. Today I hope to make amends.

In Tuesday’s post I reported that the ATLAS and CMS experiments at the LHC had updated their preliminary results on the Higgs search presented on December 13th, through the release of documents intended for publication [so-called “preprints,” intended for submission to a journal for peer review.] In updates to that post, I highlighted two issues which I found particularly interesting in comparing the updated information to the presentations in December. The first of these represents additional evidence from CMS, which strengthens their case for a signal of a Higgs-like particle with a mass around 124 GeV/c². The second of these involves the lack of any improvement in the concordance between ATLAS and CMS, which one might have hoped for, and in whose absence the results still remain almost as inconclusive as they were back in December. Today I want to explain these in a bit more detail. (more…)

POSTED BY Matt Strassler

ON February 9, 2012

Some Higgs News NOW

We’ve been expecting this: the ATLAS and CMS experiments at the Large Hadron Collider [LHC] ~~are going to be making public shortly~~ have made public the publishable form of their papers (preprints, which one submits to a journal for the purpose of peer review) on the search for the Standard Model Higgs particle (the simplest possible type of Higgs particle.) These papers are based on the preliminary results that we heard about on December 13th, which I discussed in some detail here. Here is a page where you can get all the ATLAS and CMS papers.

~~I’ll report after reading them. There will probably be~~ I see no major changes from December 13th, though there ~~will be~~ are interesting minor ones. That’s because the measurements are very sensitive and precise. [UPDATE 1: The two-photon measurement from CMS includes something new: they separate the small number of events that have two additional jets out from the rest. This subset of the data is sensitive to both the g g –> Higgs and q q –> q q Higgs production processes, and it shows an excess at 124 GeV/c², consistent with the existence of a Standard Model-like Higgs at that mass.] [UPDATE 2: See below]

(more…)

POSTED BY Matt Strassler

ON February 7, 2012

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