Tag Archives: photons

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.

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!

So What Is It???

So What Is It? That’s the question one hears in all the bars and on all the street corners and on every Twitter feed and in the whispering of the wind. Everybody wants to know. That bump seen on the ATLAS and CMS two-photon plots! What… IS… it…?


The two-photon results from ATLAS (top) and CMS (bottom) aligned, so that the 600, 700 and 800 GeV locations (blue vertical lines) line up almost perfectly. The peaks in the two data sets are in about the same location. ATLAS’s is larger and also wider. Click here for more commentary.

Well, to be honest, probably it’s just that: a bump on a plot. But just in case it’s not — just in case it really is the sign of a new particle in Large Hadron Collider [LHC] data — let me (start to) address the question.

First: what it isn’t. It can’t just be a second Higgs particle (a heavier version of the one found in 2012) that is just appended to the known particles, with no other particles added in.   Continue reading

Neutrinos From That Recent Gamma-Ray Burst?

[NOTE ADDED: A reader forwarded a message that IceCube did not see any neutrinos with energies above 1 TeV = 1000 GeV from this GRB. Maybe this is not quite the final word (there would still be sensitivity, with some effort, to neutrinos in the 100 GeV – 1000 GeV range) but clearly the neutrino signal isn’t striking, and it is probably not there at all.  But as I’ve suggested below, even a non-observation might have significant implications for the science; the question is, how many neutrinos would the standard speculations about how GRB’s work have led you to expect at IceCube?  If a reader can provide that info, I’d appreciate that.]

The very recent report of a powerful and long-lived gamma-ray burst (GRB), and questions and remarks by my readers (thank you!), have motivated me, both as a scientist and a blogger, to try to understand whether we should have observed neutrinos from this GRB. This is forcing me to catch up on the related subjects of GRB’s, searches for high-energy neutrinos, and the highest-energy cosmic rays. I’m certainly not caught up yet; there are decades of research out there, and I’m quite far behind on developments over the past three or four years. But here are some of the basics that I believe I understand. Still, be cautious with the content of this post, both because I’m not an expert and because this is a very active area of research in which some fraction of the more speculative stuff will surely turn out to be wrong.   I will try to refine this post with a more detailed and corrected article sometime later, perhaps once we know whether neutrinos from this GRB were or were not observed.

GRBs that last more than a few seconds are widely believed to be associated with an exceptional form of Type II (or “core-collapse”) supernova, though this is not known for certain. In these types of GRBs, there are (at least) two sources of photons (everything from gamma-rays to visible light to radio waves) and two sources of neutrinos. It is important not to confuse the different sources! Continue reading

Review of the Higgs-to-2-Photon Data

Since it’s been the main news story of the last week, perhaps it would be useful to do a quick summary of what the CMS and ATLAS experiments at the Large Hadron Collider [LHC] have been saying, over the past fifteen months, about their search for the process in which a Higgs particle is produced and decays to two photons.

Before we start, let me remind you that in statements about how uncertain a measurement is (and all measurements have some level of uncertainty — no knowledge is perfect), a “σ”, or “sigma”, is a statistical quantity called a “standard deviation”; a 5σ discrepancy from expectations is impressive, 3σ intriguing; but anything less than 2σ is very typical, and indicative merely of the usual coming and going of statistical flukes and fluctuations of real data around the truth. Note also that the look-elsewhere effect has to be accounted for; but usually a 5σ discrepancy without the look-elsewhere effect is enough to be convincing. And of course a discrepancy may mean either a discovery or a mistake; that’s why it is important that two experiments, not just one, see a similar discrepancy, since it is unlikely that both experiments would make the same mistake.

Ok: here are the results as they came in over time, all the way back to the inconclusive hints of 15 months ago.

December 2011:

  • ATLAS (4.9 inverse fb of data at 7 TeV): excess 2.8σ (where 1.4σ would be expected for a SM Higgs); less than 2σ after accounting for “look-elsewhere effect”.
  • CMS: (4.8 inverse fb of data at 7 TeV): excess just over 2σ (where 1.4σ would be expected for a SM Higgs); much less than 2σ after accounting for “look-elsewhere effect”.

July 2012:

  • ATLAS: (reanalyzing the 7 TeV data and adding 5.9 inverse fb of data at 8 TeV): signal 4.5σ (where 2.4 was expected for a SM Higgs); 3.6σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.9 ± 0.5 (about 1.8σ above the SM prediction)
  • CMS (reanalyzing the 7 TeV data and adding 5.3 inverse fb of data at 8 TeV): signal 4.1σ (where 2.5 was expected for a SM Higgs); 3.2σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.6 ± 0.4 (about 1.5σ above the SM prediction)

November/December 2012:

  • ATLAS: (increasing the 8 TeV data to 13.0 inverse fb): signal 6.1σ (3.3 expected for SM Higgs); 5.4σ when look elsewhere is accounted for; best estimate of size of signal divided by that for a SM Higgs: 1.8 ± 0.4 (about 2σ above the SM prediction)
  • CMS: No update

March 2013:

  • ATLAS: (taking the full 7 and 8 TeV data sets): 7.4σ (4.1 expected for a SM Higgs); best estimate of size of signal divided by that for a SM Higgs: 1.65 ± 0.30 (slightly more than 2σ above the SM prediction)
  • CMS: (taking the full 7 and 8 TeV data sets) uses two different methods as a cross-check, one of them complex and (on average) more powerful, the other simpler but (on average) less powerful. For the best estimate of size of signal divided by that for a SM Higgs: one method gives 0.8 ± 0.3 and the other gives 1.1 ± 0.3. Both of these are within 1σ of the SM prediction and within 2σ of the CMS July result.

To understand how consistent the two new CMS results are with each other, you have to consider how the two studies are correlated (since they are selecting events for study from the same pile of data.)  Because the two methods select and discard candidate events in two different ways, they don’t include the exact same data.  CMS’s simulation studies indicate that about 50 percent of the background events and 80 percent of the signal events are common to the two studies. In the end, the conclusion (see the figure below) is that the two results are consistent at 1.5σ (and at 1.8 if one considers only the 8 TeV data) — in other words, reasonably consistent with one another.

You can also ask how consistent are the new results compared to the old ones from July. When you observe that the uncertainty on the July result was very large (1.6 ± 0.4 times the Standard Model prediction, i.e. a 25% uncertainty at 1σ, 50% uncertainty at 2σ) it should not surprise you that CMS claims that their new results are both consistent with the old ones at below the 2σ level.

Slide from Moriond-QCD conference talk presenting CMS's results, and looking at the compatibility of the two results with each other (top two lines in the table) and each of the two results with the previous published results.  Note the conclusion in the last line.

Slide from Moriond-QCD conference talk presenting CMS’s results, and looking at the compatibility of the two results with each other (top two lines in the table) and of each of the two new results with the previous published results. Note the conclusion in the last line.

Meanwhile, all of the ATLAS results are closely compatible with each other. This is more what one would naively expect, but not necessarily what actually happens in real data. Of course ATLAS’s results aren’t giving a consistent mass for the new particle yet, whereas CMS’s are doing so… well, this is what happens with real data, folks.

The real issue is whether ATLAS’s measurements and CMS’s measurements of the two photon rate are compatible with each other. Currently they are separated by at least 2σ and maybe as much as 3σ (a very rough estimate), which is not unheard of but is somewhat unusual. Well, whether the cause is an error or a statistical fluke or both, it unfortunately leaves us in a completely ambiguous situation. On the one hand, CMS’s results agree with the Standard Model prediction to within about 1σ. On the other hand, ATLAS’s results are in tension with the Standard Model prediction by a bit more than 2σ. We have no way to know which result is closer to the truth — especially when we recall that the uncertainty in the Standard Model prediction is itself about 20%. If ATLAS and CMS had both closely agreed with the Standard Model we’d be confident that any deviations from the Standard Model are too small to observe; if they both significantly disagreed in the same way, we’d be excited about the possibility that the Standard Model might be about to break down. But with the current results, we don’t know what to think.

So as far as the Higgs particle’s decays to two photons, we’ve gotten as much (or almost as much) information as we’re going to get for the moment; and we have no choice but to accept that the current situation is ambiguous and to wait for more data in 2015. Of course the Standard Model may break down sooner than 2015, for some other reason that the experimenters have yet to uncover in the 2011-2012 data. But the two-photon measurement won’t be the one to crack the armor of this amazing set of equations.  (For those who got all excited last July; you were warned that the uncertainties were very large and the excess might well be ephemeral.)

Quantum Physics Is Very Real

Just ask the Nobel Prize committee: is quantum physics some sort of speculative new science? (A smart educated woman asked me, just a week ago, `What do you think about that quantum physics stuff?’, as though it were in the same category as theories of consciousness, speculations about the origin of life, and string theory.) No way: it’s all over your computers and cell phones; it’s in many modern light bulbs; it’s the laser that reads the prices at the grocery store and your ticket at a concert; it’s the heart of the best timepieces and the eyes of the best microscopes; it’s what makes solids solid and liquids flow, and powers chemical reactions and radioactivity; it’s probably playing a big role in biology that we’re just starting to understand; and it’s sunshine and moonlight and the glowing auroras borealis and australis.  It’s the foundation and fabric of your world.

And though it may be bizarre, it is by no means abstract.  Maybe in the early 1930s one could still say it was abstract; but already for many decades particle physicists have passively observed individual particles, one at a time, behaving in quantum mechanical ways.  Today scientists can control individual quantum objects, things whose behavior can only be predicted by accepting the odd rules and counter-intuitive implications of our quantum world.  In particular, physicists have learned to capture and manipulate individual photons (particles of light), atoms, and ions (atoms with an electron removed or added, to make them electrically charged — see the Figure below.)  It is for their work advancing these capabilities, making possible new classes of experiments and opening up the potential for new technologies, that Serge Haroche and David Wineland have won the Nobel Prize for 2012.  Read about it here (brief press release or summary for non-technical readers)… using your preferred quantum-mechanical device.

Light emitted from three individual ions of Beryllium, trapped and held in place for an extended period of time. (National Institute of Standards and Technology image gallery.)