The 2016 Data Kills The Two-Photon Bump

Results for the bump seen in December have been updated, and indeed, with the new 2016 data — four times as much as was obtained in 2015 — neither ATLAS nor CMS [the two general purpose detectors at the Large Hadron Collider] sees an excess where the bump appeared in 2015. Not even a hint, as we already learned inadvertently from CMS yesterday.

All indications so far are that the bump was a garden-variety statistical fluke, probably (my personal guess! there’s no evidence!) enhanced slightly by minor imperfections in the 2015 measurements. Should we be surprised? No. If you look back at the history of the 1970s and 1980s, or at the recent past, you’ll see that it’s quite common for hints — even strong hints — of new phenomena to disappear with more data. This is especially true for hints based on small amounts of data (and there were not many two photon events in the bump — just a couple of dozen).  There’s a reason why particle physicists have very high standards for statistical significance before they believe they’ve seen something real.  (Many other fields, notably medical research, have much lower standards.  Think about that for a while.)  History has useful lessons, if you’re willing to learn them.

Back in December 2011, a lot of physicists were persuaded that the data shown by ATLAS and CMS was convincing evidence that the Higgs particle had been discovered. It turned out the data was indeed showing the first hint of the Higgs. But their confidence in what the data was telling them at the time — what was called “firm evidence” by some — was dead wrong. I took a lot of flack for viewing that evidence as a 50-50 proposition (70-30 by March 2012, after more evidence was presented). Yet the December 2015 (March 2016) evidence for the bump at 750 GeV was comparable to what we had in December 2011 for the Higgs. Where’d it go?  Clearly such a level of evidence is not so firm as people claimed. I, at least, would not have been surprised if that original Higgs hint had vanished, just as I am not surprised now… though disappointed of course.

Was this all much ado about nothing? I don’t think so. There’s a reason to have fire drills, to run live-fire exercises, to test out emergency management procedures. A lot of new ideas, both in terms of new theories of nature and new approaches to making experimental measurements, were generated by thinking about this bump in the night. The hope for a quick 2016 discovery may be gone, but what we learned will stick around, and make us better at what we do.

A Flash in the Pan Flickers Out

Back in the California Gold Rush, many people panning for gold saw a yellow glint at the bottom of their pans, and thought themselves lucky.  But more often than not, it was pyrite — iron sulfide — fool’s gold…

Back in December 2015, a bunch of particle physicists saw a bump on a plot.  The plot showed the numbers of events with two photons (particles of light) as a function of the “invariant mass” of the photon pair.  (To be precise, they saw a big bump on one ATLAS plot, and a bunch of small bumps in similar plots by CMS and ATLAS [the two general purpose experiments at the Large Hadron Collider].)  What was that bump?  Was it a sign of a new particle?

A similar bump was the first sign of the Higgs boson, though that was far from clear at the time.  What about this bump?

As I wrote in December,

  “Well, to be honest, probably it’s just that: a bump on a plot. But just in case it’s not…”

and I went on to describe what it might be if the bump were more than just a statistical fluke.  A lot of us — theoretical particle physicists like me — had a lot of fun, and learned a lot of physics, by considering what that bump might mean if it were a sign of something real.  (In fact I’ll be giving a talk here at CERN next week entitled “Lessons from a Flash in the Pan,” describing what I learned, or remembered, along the way.)

But updated results from CMS, based on a large amount of new data taken in 2016, have been seen.   (Perhaps these have leaked out early; they were supposed to be presented tomorrow along with those from ATLAS.)  They apparently show that where the bump was before, they now see nothing.  In fact there’s a small dip in the data there.

So — it seems that what we saw in those December plots was a fluke.  It happens.  I’m certainly disappointed, but hardly surprised.  Funny things happen with small amounts of data.

At the ICHEP 2016 conference, which started today, official presentation of the updated ATLAS and CMS two-photon results will come on Friday, but I think we all know the score.  So instead our focus will be on  the many other results (dozens and dozens, I hear) that the experiments will be showing us for the first time.  Already we had a small blizzard of them today.  I’m excited to see what they have to show us … the Standard Model, and naturalness, remain on trial.

The Summer View at CERN

For the first time in some years, I’m spending two and a half weeks at CERN (the lab that hosts the Large Hadron Collider [LHC]). Most of my recent visits have been short or virtual, but this time* there’s a theory workshop that has collected together a number of theoretical particle physicists, and it’s a good opportunity for all of us to catch up with the latest creative ideas in the subject.   It’s also an opportunity to catch a glimpse of the furtive immensity of Mont Blanc, a hulking bump on the southern horizon, although only if (as is rarely the case) nature offers clear and beautiful weather.

More importantly, new results on the data collected so far in 2016 at the LHC are coming very soon!  They will be presented at the ICHEP conference that will be held in Chicago starting August 3rd. And there’s something we’ll be watching closely.

You may remember that in a post last December I wrote:

  “Everybody wants to know. That bump seen on the ATLAS and CMS two-photon plots!  What… IS… it…?

Why the excitement? A bump of this type can be a signal of a new particle (as was the case for the Higgs particle itself.) And since a new particle that would produce a bump of this size was both completely unexpected and completely plausible, there was hope that we were seeing a hint of something new and important.

However, as I wrote in the same post,

  “Well, to be honest, probably it’s just that: a bump on a plot. But just in case it’s not…”

and I went on to discuss briefly what it might mean if it wasn’t just a statistical fluke. But speculation may be about to end: finally, we’re about to find out if it was indeed just a fluke — or a sign of something real.

Since December the amount of 13 TeV collision data available at ATLAS and CMS (the two general purpose experiments at the LHC) has roughly quadrupled, which means that typical bumps and wiggles on their 2015-2016 plots have decreased in relative size by about a factor of two (= square root of four). If the December bump is just randomness, it should also decrease in relative size. If it’s real, it should remain roughly the same relative size, but appear more prominent relative to the random bumps and wiggles around it.

Now, there’s a caution to be added here. The December ATLAS bump was so large and fat compared to what was seen at CMS that (since reality has to appear the same at both experiments, once enough data has been collected) it was pretty obvious that even if it there were a real bump there, at ATLAS it was probably in combination with a statistical fluke that made it look larger and fatter than its true nature. [Something similar happened with the Higgs; the initial bump that ATLAS saw was twice as big as expected, which is why it showed up so early, but it gradually has shrunk as more data has been collected and it is now close to its expected size.  In retrospect, that tells us that ATLAS’s original signal was indeed combined with a statistical fluke that made it appear larger than it really is.] What that means is that even if the December bumps were real, we would expect the ATLAS bump to shrink in size (but not statistical significance) and we would expect the CMS bump to remain of similar size (but grow in statistical significance). Remember, though, that “expectation” is not certainty, because at every stage statistical flukes (up or down) are possible.

In about a week we’ll find out where things currently stand. But the mood, as I read it here in the hallways and cafeteria, is not one of excitement. Moreover, the fact that the update to the results is (at the moment) unobtrusively scheduled for a parallel session of the ICHEP conference next Friday, afternoon time at CERN, suggests we’re not going  to see convincing evidence of anything exciting. If so, then the remaining question will be whether the reverse is true: whether the data will show convincing evidence that the December bump was definitely a fluke.

Flukes are guaranteed; with limited amounts of data, they can’t be avoided.  Discoveries, on the other hand, require skill, insight, and luck: you must ask a good question, address it with the best available methods, and be fortunate enough that (as is rarely the case) nature offers a clear and interesting answer.

 

*I am grateful for the CERN theory group’s financial support during this visit.

Spinoffs from Fundamental Science

I find that some people just don’t believe scientists when we point out that fundamental research has spin-off benefits for modern society.  The assumption often seems to be that it’s just a bunch of egghead esoteric researchers trying to justify their existence.  It’s a real problem when those scoffing at our evidence are congresspeople of the United States and their staffers, or other members of governmental funding agencies around the world.

So I thought I’d point out an example, reported on Bloomberg News.  It’s a good illustration of how these things often work out, and it is very rare indeed that they are discussed in the press.

Gravitational waves are usually incredibly tiny effects [typically squeezing the radius of our planet by less than the width of an atomic nucleus] that can be made only with monster black holes and neutron stars.   There’s not much hope of using them in technology.  So what good could an experiment to discover them, such as LIGO, possibly be for the rest of the world?

Well, Shell Oil seems to have found some value in it.   It’s not in the gravitational waves themselves, of course; instead, it is in the technology that has to be developed to detect something so delicate.   http://www.bloomberg.com/news/articles/2016-07-07/shell-is-using-innoseis-s-sensors-to-detect-gravitational-waves

Score another one for investment in fundamental scientific research.

 

LIGO detects a second merger of black holes

There’s additional news from LIGO (the Laser Interferometry Gravitational Observatory) about gravitational waves today. What was a giant discovery just a few months ago will soon become almost routine… but for now it is still very exciting…

LIGO got a Christmas (US) present: Dec 25th/26th 2015, two more black holes were detected coalescing 1.4 billion light years away — changing the length of LIGO’s arms by 300 parts in a trillion trillion, even less than the first merger observed in September. The black holes had 14 solar masses and 8 solar masses, and merged into a black hole with 21 solar masses, emitting 1 solar mass of energy in gravitational waves. In contrast to the September event, which was short and showed just a few orbits before the merger, in this event nearly 30 orbits over a full second are observed, making more information available to scientists about the black holes, the merger, and general relativity.  (Apparently one of the incoming black holes was spinning with at least 20% of the maximum possible rotation rate for a black hole.)

The signal is not so “bright” as the first one, so it cannot be seen by eye if you just look at the data; to find it, some clever mathematical techniques are needed. But the signal, after signal processing, is very clear. (Signal-to-noise ratio is 13; it was 24 for the September detection.) For such a clear signal to occur due to random noise is 5 standard deviations — officially a detection. The corresponding “chirp” is nowhere near so obvious, but there is a faint trace.

This gives two detections of black hole mergers over about 48 days of 2015 quality data. There’s also a third “candidate”, not so clear — signal-to-noise of just under 10. If it is really due to gravitational waves, it would be merging black holes again… midway in size between the September and December events… but it is borderline, and might just be a statistical fluke.

It is interesting that we already have two, maybe three, mergers of large black holes… and no mergers of neutron stars with black holes or with each other, which are harder to observe. It seems there really are a lot of big black holes in binary pairs out there in the universe. Incidentally, the question of whether they might form the dark matter of the universe has been raised; it’s still a long-shot idea, since there are arguments against it for black holes of this size, but seeing these merger rates one has to reconsider those arguments carefully and keep an open mind about the evidence.

Let’s remember also that advanced-LIGO is still not running at full capacity. When LIGO starts its next run, six months long starting in September, the improvements over last year’s run will probably give a 50% to 100% increase in the rate for observed mergers.   In the longer run, the possibility of one merger per week is possible.

Meanwhile, VIRGO in Italy will come on line soon too, early in 2017. Japan and India are getting into the game too over the coming years. More detectors will allow scientists to know where on the sky the merger took place, which then can allow normal telescopes to look for flashes of light (or other forms of electromagnetic radiation) that might occur simultaneously with the merger… as is expected for neutron star mergers but not widely expected for black hole mergers.  The era of gravitational wave astronomy is underway.

Giving two free lectures 6/20,27 about gravitational waves

For those of you who live in or around Berkshire County, Massachusetts, or know people who do…

Starting next week I’ll be giving two free lectures about the LIGO experiment’s discovery of gravitational waves.  The lectures will be at 1:30 pm on Mondays June 20 and 27, at Berkshire Community College in Pittsfield, MA.  The first lecture will focus on why gravitational waves were expected by scientists, and the second will be on how gravitational waves were discovered, indirectly and then directly.  No math or science background will be assumed.  (These lectures will be similar in style to the ones I gave a couple of years ago concerning the Higgs boson discovery.)

Here’s a flyer with the details:  http://berkshireolli.org/ProfessorMattStrasslerOLLILecturesFlyer.pdf

Pop went the Weasel, but Vroom goes the LHC

At the end of April, as reported hysterically in the press, the Large Hadron Collider was shut down and set back an entire week by a “fouine”, an animal famous for chewing through wires in cars, and apparently in colliders too. What a rotten little weasel! especially for its skill in managing to get the English-language press to blame the wrong species — a fouine is actually a beech marten, not a weasel, and I’m told it goes Bzzzt, not Pop. But who’s counting?

Particle physicists are counting. Last week the particle accelerator operated so well that it generated almost half as many collisions as were produced in 2015 (from July til the end of November), bringing the 2016 total to about three-fourths of 2015.

 

The key question is how many of the next few weeks will be like this past one.  We’d be happy with three out of five, even two.  If the amount of 2016 data can significantly exceed that of 2015 by July 15th, as now seems likely, a definitive answer to the question on everyone’s mind (namely, what is the bump on that plot?!? a new particle? or just a statistical fluke?) might be available at the time of the early August ICHEP conference.

So it’s looking more likely that we’re going to have an interesting August… though it’s not at all clear yet whether we’ll get great news (in which case we get no summer vacation), bad news (in which case we’ll all need a vacation), or ambiguous news (in which case we wait a few additional months for yet more news.)

The Two-Photon Excess at LHC Brightens Slightly

Back in December 2015, there was some excitement when the experiments ATLAS and CMS at the Large Hadron Collider [LHC] — especially ATLAS — reported signs of an unexpectedly large number of proton-proton collisions in which

  • two highly energetic photons [particles of light] were produced, and
  • the two photons could possibly have been produced in a decay of an unknown particle, whose mass would be about six times the mass of the Higgs particle (which ATLAS and CMS discovered in 2012.)

This suggested the possibility of an unknown particle of some type with rest mass of 750 GeV/c².  However, the excess could just be a statistical fluke, of no scientific importance and destined to vanish with more data.

The outlook for that bump on a plot at 750 GeV has gotten a tad brighter… because not only do we have ATLAS’s plot, we now have increasing evidence for a similar bump on CMS’s plot. This is thanks largely to some hard work on the part of the CMS experimenters.  Some significant improvements at CMS,

  1. improved understanding of their photon energy measurements in their 2015 data,
  2. ability to use 2015 collisions taken when their giant magnet wasn’t working — fortunately, the one type of particle whose identity and energy can be measured without a magnet is… a photon!
  3. combination of the 2015 data with their 2012 data,

have increased the significance of their observed excess by a moderate amount. Here’s the scorecard.*

  • CMS 2015 data (Dec.): excess is 2.6σ local, < 1σ global
  • CMS 2015 data (improved, Mar.) 2.9σ local, < 1σ global
  • CMS 2015+2012 data: 3.4σ local, 1.6σ global
  • ATLAS 2015 data (Dec. and Mar.): 3.6σ local, 2.0σ global to get a narrow bump [and 3.9σ local , 2.3σ global to get a somewhat wider bump, but notice this difference is quite insignificant, so narrow and wider are pretty much equally ok.]
  • ATLAS 2015+2012 data: not reported, but clearly goes up a bit more, by perhaps half a sigma?

You can read a few more details at Resonaances.

*Significance is measured in σ (“standard deviations”) and for confidence in potentially revolutionary results we typically want to see local significance approaching 5σ and global approaching 3σ in both experiments. (The “local” significance tells you how unlikely it is to see a random bump of a certain size at a particular location in the plot, while the “global” significance tells you how unlikely it is to see such a bump anywhere in the plot … obviously smaller because of the look-elsewhere effect.)

This is good news, but it doesn’t really reflect a qualitative change in the situation. It leaves us slightly more optimistic (which is much better than the alternative!) but, as noted in December, we still won’t actually know anything until we have either (a) more data to firm up the evidence for these bumps, or (b) a discovery of a completely independent clue, perhaps in existing data. Efforts for (b) are underway, and of course (a) will get going when the LHC starts again… soon!  Next news on this probably not til June at the earliest… unless we’re very lucky!