Of Particular Significance

The Day of the Higgs!!!! DISCOVERY!!! History Made!

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 07/04/2012

[This paragraph was my introductory paragraph from this morning, before the presentation. Details of the presentation follow.] IndependHiggs Day has dawned, warm and hazy.  Yesterday, at the CERN laboratory that hosts the Large Hadron Collider [LHC], all the discussion at lunch, and over afternoon coffee and evening beer, centered on the Higgs particle (here’s my Higgs FAQ, my video clips for non-experts about how the Higgs search is done, and a recent article for non-experts explaining its larger importance;more detailed background on the Higgs is linked from here), and on today’s presentation to update the search for it.  The mood is ebullient and excited; everyone is anticipating a very big step forward, perhaps a definitive one.   Gone is the uncertain mood of December, when CERN preceded its presentations with a warning that the results shown then would not be conclusive — a warning that is notably absent this time.   It’s a party atmosphere, and a media circus; many luminaries are here, invited by the director to witness the event.  The press will fill an entire auditorium all their own; physicists will be spread out over perhaps a dozen auditoriums.  It’s been suggested that CERN should have rented a soccer stadium in Geneva; the small size of the main auditorium at the lab reflects how much smaller particle physics experiments were just 30 years ago.

What I will be watching for in the presentations today is the following (to be updated when I know the answers): Here’s what happened in the presentation today:

[8:50 Professor Higgs has entered the room.]

[9:00 CERN Director General Rolf Heuer very brief opening remarks]

First, CMS (presented by spokesperson Joseph Incandela, from the University of California Santa Barbara): (5.2 inverse fb of data, with 5.6 for muon analyses, improvements on many fronts)

  • Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons? Yes!! 4.1 sigma, a bit above standard model expectation though not too much
  • Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs? Yes! 5 or 6 events above background, 3 sigma, a bit below expectation but quite consistent with standard model
  • Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino?   (a) not at all, but very low statistics (b) not incompatible, but still low statistics (c) preliminary technique, yes
  • TOTAL 4.9 sigma, EXPECTED 5.9 sigma ;
  • MASS 125.3 +- 0.6, preliminary
  • Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011?  Yes
  • Does the evidence in favor of a new particle support the idea that this is a Higgs particle?  Absolutely!!!!  As I have explained elsewhere, an observation of a signal in both photons and leptons, with even roughly the ratio predicted by the Standard Model, is a very strong argument in favor of the new particle being a Higgs particle.
  • Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)?  No signs of (statistically significant) non-Standard Model behavior

Next, ATLAS (presented by spokesperson Fabiola Gianotti, from CERN): (up to 5.9 inverse fb, and various improvements, some quite important)

  • Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons?  Yes!! 4.5 sigma , 3.6 sigma with look-elsewhere effect over 110-150 GeV: (cross-section almost twice as big as expected! but only 2 sigma excess); mass about 126.5 GeV
  • Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs?  Yes!! 6 or  7 events above background, 3.4 sigma (2.5 sigma after look-elsewhere over 110-140), cross-section again a bit large, 1.3 +- 0.6 of Standard Model expectation
  • Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino? Nothing from 8 TeV collisions shown today
  • MASS 126.5 GeV (from photons only; leptons are lower; no error bar given, so measurement of mass is not complete) with 1.2 of standard model production rate
  • Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011?  Yes
  • Does the evidence in favor of a new particle support the idea that this is a Higgs particle?  Absolutely!!
  • Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)?  No significant deviations from Standard Model though two-photon rate is currently 2 sigma high.  Two sigma is not enough to be excited about.  This is especially since the Standard Model prediction for the overall cross-section is a very tricky calculation; its current precision is estimated at 15%, but perhaps one might worry about that estimate.


  • Is what CMS observes largely consistent with what ATLAS observes?  Absolutely!!
  • Are there any notable discrepancies between the two experiments that might undermine confidence in their results? None!  The discrepancy of about 1.2 GeV in the masses quoted is not official, since ATLAS does not yet have a final mass measurement with a proper uncertainty estimate.  Even if I take the ATLAS estimate as if it were a mass measurement, if I put a similar uncertainty on it as CMS has quoted, the discrepancy between the two experiments is below 2 sigma, not enough to cause concern at this time, especially given the strength of the signal in each experiment separately.


[Heuer celebrates a global success… A standing ovation for all…]

[No one has any questions; the data is so clean and so convincing!]

[Some nice words from the inventors of the idea…]

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81 Responses

  1. Excellent explanation and discussion! The only thing missing from your site, Prof.Strassler, is an explanation about that bizarre unit called a “barn”! Why don’t you just express luminosity in a unit that could be readily understood by all, instead of this inverse femtobarn you keep talking about? Like the number of collisions per square centimeter, for instance?

  2. Laymans question if you will endulge me Proff. Strassler.

    I assume the energy level of the LHC will be increased to 7 TeV per beam, regardless of last days events. And more data will be accumulated.

    In what way (if any) do you expect the “picture” to change when more data is gathered, and higher energy levels are reached?
    I can imagine the graphs will become even more clear, but could there still be surprises?


    Michel Beekveld

    1. Remember we have many more measurements to make on the Higgs particle, so increasing the energy will increase the number produced will improve those measurements.

      But also, if there are heavy particles with masses 10 times or so heavier than the Higgs that we haven’t yet discovered, increasing the energy will make it easier for us to discover them.

      So many surprises are possible, yes.

  3. Matt, something is confusing me.
    In the Higgs Field article you said the Eletromagnetic and the Weak forces appear in a non-zero Higgs Field. Also, the Higgs Boson doesnt interact with the photon (atleast not directry, otherwise I think it would have mass), so, my question is:
    The photon interacts with the Higgs Field but not the Higgs Boson? If thats not the case, how does it work?
    Its a bit confusing…

  4. Thanks Matt for this site and all the clear-headed explanations. One simple (hopefully) question. The Higgs particles observed at LHC could be said to be man-made since we set up the collisions which produced the particles. Is it likely that they occur in any natural environment, or that there was a time when they did occur naturally? If so are they likely to be (or have been) rare or fairly common?

    1. Maybe we should say “human caused” rather than “human made”? Anyway — yes, if the universe was once as hot as we believe that it was, there was a time when (a) Higgs particles were common, but had a slightly different mass because (b) at that time the Higgs field was zero on average, not non-zero as it is today.

      So the type of Higgs particle we made over the past two years — the one that has mass of 125 GeV/c-squared and is a ripple in a non-zero Higgs field has never been common. And since its lifetime is so short you’ll never find one hanging around.

      Of course that last part is true of charm quarks, bottom quarks, top quarks, tau leptons, and W and Z particles.

      1. Matt, what is the reason for thinking the Higgs Field had a zero VEV in the early universe?
        And could it’s value change again for some reason?

        1. The reason it was zero had to do with the very high temperature. It is similar to the fact that if you heat up a magnet it will eventually lose its magnetization. Think of the magnetization M as like the Higgs field H — they are both 0 at high temperature, and then as the temperature cools there is a phase transition and both become non-zero.

          For the Higgs, the temperature of the phase transition would have been somewhere around a thousand million million degrees Centigrade.

          There are two reasons the Higgs field might change again: if something heats up the universe in the distant future (not expected) or if it is possible for the universe to “tunnel” to a more stable configuration (a quantum mechanical process whose likelihood we can only speculate about; don’t worry, extraordinarily unlikely [and at worst, instantaneous and painless.])

  5. Do they have enough now to award Nobel in October 2012? Or do we need more confirmation of the properties?

    It will be most interesting how the Swedes handle this. Do they just prioritize the theorists based on publication date – even though Englert(Brout) has no boson and issues within their PRL paper? Clearly PH is in…but do they snub GHK who many think had the most advanced paper with the field, boson, and Goldstone failure demnostration? Seems like the rule of three for Nobel Prize is more trouble than it is worth.

  6. Perfect Matt: which attribute can the academy indicates to Peter Higgs to the Nobel prize, even all people knowing that he deserves it? I also agree too, but in which ownership the prize should be given ? thanks.

    1. I would also like very much the opinion of Prof. Strassler on this paper.

    2. Many people predicted some Higgs mass but these authors are the only ones that didn’t add (really) new physics to the Standard Model and they predicted the Higgs mass with only a few GeV of margin AND THEY GOT IT EXACTLY RIGHT!

    3. None as yet. One has to keep in mind that there are many theories that predict 126, or something in a range that includes 126. The problem is to figure out which one (if any) of these theories is right; that requires a lot more work. Only then we can talk about who gets credit.

  7. They speak a lot about an a different Higgs particle, not the expected. But whom can guarantee me that this particle has no different characteristics suggested by the reference about Higgs mass? Imagine if they give the Nobel prize to Peter Higgs and then we discover that this particle, although been there, is not exactly predicted by the Higgs theorys? Is There any chance of that?

    1. Higgs and the others deserve a prize for suggesting the basic idea. None of them actually suggested the Standard Model Higgs that we know now — that was Weinberg. What they did (or what some of them did in part, at least — history is murky and complicated) was suggest a general mechanism that would allow particles like the W and Z to become massive and predicted a new scalar particle. Even if the new particle was not the simplest Higgs, and instead a more complicated one, the basic idea would still be right, and those folks would deserve their prize.

      Moreover, you could argue that for their contribution of their idea to the progress of the field, they deserve a big prize anyway.

  8. I think this is wonderfully exciting, in a thousand years time they will talk about the Higgs been discovered in 2012. But the obvious fear is that I wonder if this is the end of the road for what humans will find out about the fundamental nature of reality. Sure there is still much to do in areas like astrophysics and other fields. But from what I understand, if it is just a standard model Higgs then any new fundamental physics could be at such high energy scales, that it would be practically impossible to ever experimentally probe it.

  9. would you be able to talk about what would this new machine could do which the lhc cannot do and how long it would take to make ?


    It is almost certain that there will be new calls for a dedicated machine focused on studying the Higgs. This will now receive a huge amount of interest from around the globe and, if built, will complement the studies of the Large Hadron Collider.

    1. I think it’s a little premature (though not much). Let’s talk about this in a month or two after the dust settles a little bit. There are a couple of important things to check first.

  10. History is not made in a day, at least not in science. Remember the detection of faster-than-light neutrinos not too long ago? We just have to wait and see whether these results can be consistently reproduced in the future.

    1. They were already reproduced though. there were two different experiments, CMS and ATLAS, using very different detectors, that came to roughly similar conclusions. Besides which, nobody in the physics world really believed the FTL neutrinos thing — it was very suspicious for many, many reasons (don’t you remember all those blog posts?) but the higgs result was almost exactly was what expected, with no major reasons for suspicion.

      1. First of all, I am neither familiar enough with the theoretical nor the experimental side of the project to allow myself my own judgment regarding the trustworthiness of the results. But I think it is a dangerous attitude to generally consider expected results more trustworthy than unexpected ones. Wishful thinking is never a good thing in science. A healthy scepticism should be present at all times, in particular when it comes to announcing sensational results to the public (and considering the long time it took to find any kind of evidence for the Higgs Boson, this is at least as sensational as the ‘detection’ of faster-than-light neutrinos). And in both cases one can not really ignore the fact either that these were/are CERN affiliated experiments and scientists.


        1. I agree with you that expected results should be treated with great caution. But these were treated with great caution. We have two experiments with approximately 5 sigma excesses; if we had only one, I would be much more cautious. Healthy skepticism must be present, but there is a point at which it becomes unhealthy, and in my opinion, and in the opinion of every colleague I have spoken to so far (dozens now) you’ve crossed that line.

          When the OPERA result was announced, almost every non-OPERA participant believed it should not have been announced at all, especially at CERN, and almost everyone believed it was wrong, and said so. There were disagreements within OPERA as to whether the result should have been announced. Moreover, despite the fact that the neutrinos came from CERN, NONE of the OPERA experimenters are CERN staff. The two situations are completely different and should not even appear in the same discussion.

      2. Thomas:
        I’m not a physicist either, and I’m just expressing a philosophical opinion here, but I definitely do think expected results should be treated differently than unexpected ones. Another way to say this is “extraordinary claims require extraordinary evidence”.

        An ordinary claim is consistent with patterns that have been observed in the past, and therefore has the weight of all those observations behind it. An unexpected claim is an exception to those patterns. Exceptions do exist, but they occur (by definition) less often than things that fit the usual pattern. Therefore, the prior probability is weighted in favor of the ordinary claim. So it’s almost like it already has some evidence behind it. The unexpected claim needs enough evidence to more than cancel out this advantage if we are to believe it.

        1. I agree with this point of view. But this was an *expected* result at some level — the theory combined with existing data did predict a lightweight Higgs — so it is a little tricky here to decide where you want to put the weight. In any case, the data is convincing on its own without priors; that’s what is so nice about having a bump on a smooth background, it’s a purely statistical question without need for strong priors.

    2. History is not made in a day, that is true. But this result was not made in a day. It was years in the making. And we expected a result of a definite sort around this time.

      You have asked whether these results can be consistently reproduced… as you should. But the ATLAS result is convincing evidence by itself, confirmed by CMS; or you can say that the CMS result is convincing evidence by itself, confirmed by ATLAS. Or you can say that the photon data is convincing, confirmed by the lepton data. Or you can say that the 2011 data from ATLAS and CMS is moderate evidence, and the 2012 data from ATLAS and CMS clinches the deal. No matter how you chop this data set into halves or quarters, it looks very convincing. So I think we have, in fact, satisfied your criterion. Finally, unlike many other measurements at these colliders, this was a particularly easy and clean measurement of peaks over smooth backgrounds. It is the combination of all of these things that explains why I’m prepared to say, with confidence, that this is a discovery.

  11. Professor Strassler, I’m excited today as I’m sure all are who follow physics closely or peripherally. I do have one question for you, as I gather you seem to be one who respects the scientific process greatly. What does it mean that great discoveries are now reported first via press conferences rather than being first published? Not to temper the excitement, just a thought. Merry Higgsmas in July!

    1. Good question. I don’t think there is harm in discussing unpublished results via press conferences in principle, when the data is firm, and the press conference is PRECEDED by a scientific presentation at which opportunities for tough questions are permitted. We’ve been doing this kind of pre-publication scientific presentations for a long time in this field without noticeable harm; the press conferences are necessary because of public interest.

      I think the question of when data is ready for public viewing is very much a judgment call and has to be dealt with on a case by case basis. In this case the argument is very strong. Notice these two experiments both have very strong evidence, and that one therefore confirms the other. Also, the evidence is of the cleanest sort; there will be very few questions that could upset the evidence from any referee of the papers. You saw that today after the talks; normally there are lots of questions (there were many after the OPERA result was reported) but there was almost nothing to ask today, because any question would really have been nit-picking about a very solid result. On top of this, the results would have been leaked widely had it gone for publication without presentation to scientists, and there would be even greater confusion in the media had CERN refused to talk about it (including concerns of “what are you hiding?”. So I think there wasn’t much choice in this case, and also there wasn’t any harm in it. Had the results been much murkier and harder to be sure of, I might have felt differently.

    1. I’m aware; there was a brief comment added to yesterday’s post. By comparison with today it’s a small amount of corroborating evidence, but it will come up in later discussion… Make no mistake, the Europeans were way ahead; the US threw away its leadership in this research field.

  12. I agree the evidence for a new boson coupling to the 3/4 of the mediators (glue, photon, Z0) is convincing. I wonder if the systematic error on the theoretical evaluation (given merely the SM) of the coupling particularly to glue could describe a higher production cross section.

    The next step is proving the couplings to fermions, which is a strong argument for 10X more LHC data. The Tevatron result on coupling to bbar is kind of suggestive already, though.

    CMS did a nice job with the 4lepton kinematics. Noticeably absent was the correlation in azimuthal angle between the decay planes of each lepton pair, which might untangle the parity of this bump. Something for next summer

  13. Congratulations to everyone involved in this Egyptian Pyramid-scale enterprise, another proud moment in human history! Thank you Professor Matt Strassler for your well-appreciated explanations about what is going on for experts and non-experts alike! Now all we have to do is explain why the various Yukawa couplings have the particular values they do!

  14. Well, it was not really unexpected, was it ?
    Congratulations for the great discovery to all participating in this succesful effort! Matt, where were you, when they needed you ? 🙂

    1. No, it wasn’t unexpected in a sense — but the evidence was stronger than most people expected because (a) they got a bit lucky in a couple of ways, and (b) they got smarter. More on this at a later time.

  15. The LHC is another successful example of what we humans can achieve by working together to reach a common goal. Like the Olympic Games, these large scale experiments help unite peoples of different cultures for a common good.

    I congratulate all involved in this fantastic endeavor and hope that we can learn not only of the new physics bu also further global cooperation in shaping our global village into one which all people can live a decent and prosperous life.

    Thank you professor Strassler for you time, effort and enthusiasm in this celebration of physics.

  16. Kind of exciting to read this! Congrats to all, and thanks to Professor Strassler for the work explaining this stuff to a lowly engineer-


  17. I was under that impression that the size of the bump in the two photon channel was a shocker and cannot be explained by tweaking the Standard Model. I guess it could just me statistics…

    1. It’s just not statistically significant. And CMS sees something smaller. And theorists cannot calculate the overall production rate to better than 15% accuracy — and that accuracy estimate is an *estimate*, maybe it is too small. Hard to get excited just yet. We’ll see more in coming months.

    1. Your question is not complete.

      When an real electron absorbs a real photon, it has to DO something: it will quickly emit a photon; or perhaps it will move into a new energy state in an atom; or it may pop off the atom to which it is attached. Whichever of these occurs, there may be a short while where there is a virtual electron (which is not a particle at all, really, and can have any mass) but in the end, when an electron proper emerges from all the activity, it will have the same mass that it always has.

      If the photon itself is virtual rather than real, it is possible, in some circumstances, for the electron to absorb it whole and not do anything else other than recoil. But in this case, too, the electron’s mass is the same as always. And the virtual photon (not really a particle at all) can have any mass.

  18. So the Higgs gives all other particles mass but its own mass is just inherent? Dr. Leo Stein, that seems unsatisfying. It reminds me of the assertion that everything needs a creator except “god.”

    1. Strassler himself:

      “In particular, as you can see in Figs. 3 and 7, the Higgs particle itself does not get all of its mass from the non-zero Higgs field — and the strength of its interaction with itself is not directly related to its mass. [There is a correlation, but not proportionality.] This is not unusual. In other articles on this site, you will see many other examples of conjectured particles, such as those that arise in the speculative theories known as supersymmetry and as warped extra-dimensions, that get their masses in other ways.”


      As for your claim, it seems circular without any way to break it. If a particle mass is inherent because it appears spontaneously, it is unsatisfying to you because it is “inherent”. If it doesn’t appear spontaneously, it is unsatisfying to you because it has “a creator”.

  19. Clear signs of a new particle….a boson , that is a confirmation..
    The higgs …. that is NOT a confirmation..
    So when do you expect confirmation of the later statement ?
    congratulations for the former , patience for the later.

    1. That it is a type of Higgs is not confirmed, but the evidence is very, very strong. The observation of the particle decaying to both two photons and two lepton-antilepton pairs, with roughly the expected ratio of probabilities, constitutes this evidence. I have explained why in http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-standard-model-higgs/seeking-and-studying-the-standard-model-higgs-particle/ (search for the section marked “How do we even know this new particle actually is a Higgs particle of some type (not necessarily the Standard Model version of the Higgs particle)”? I’ll explain this again, more carefully, tomorrow.

    1. In the simplest model, it has an inbuilt potential in the action which makes it massive. This is not allowed for electrons and their ilk for technical reasons, which is why the Higgs interaction is needed to give them a mass; but a scalar Higgs can get is mass from a potential.

  20. What could mean a too large diphoton signal and no excess in the tau-tau channel (CMS)? Also why do ATLAS and CMS differ by 1 GeV in the mass?

    1. Probably it means there isn’t enough data yet to make statistically meaningful measurements of the ratios of decay rates. See above. As for the mass measurement, ATLAS’s measurement isn’t complete yet and there could be some systematic shifts that still have to be tracked down. If it were 2 GeV I might worry, but not at 1 GeV, and not with such strong evidence from both experiments.

  21. Do we know what range they’ve excluded? I ask, wondering about the uncertainties for the mass, but also not knowing if there could be more than 1 particle there.

  22. This is a day óf Particular Significance, congratulations to you all! I hope mankind uses the potential for good reasons.

    Superb website, keep up!

  23. Yeah, Happy Higgs day and congratulations to all people involved … 😀

    Thanks a lot for this exciting live report Prof. Strassler !

    Cheeeeeeeers :-)))

  24. I’m really wondering what will happen to the hierarchy problem. both super symmetry and extra dimensions failed in 1 TEV. So what is the solution?

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