Higgs Papers Out

By Sean Carroll | August 3, 2012 11:44 am

We were all transfixed by the Higgs seminars on July 4, but the work was nowhere near over for the experimentalists — they had to actually write up papers describing the results. And of course taking the opportunity to do a little more analysis along the way.

Now the papers have appeared on the arxiv. (Via lots of places, e.g. symmetry breaking and Matt Strassler.) Here’s ATLAS:

Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC

The ATLAS Collaboration
(Submitted on 31 Jul 2012)

A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb^-1 collected at sqrt(s) = 7 TeV in 2011 and 5.8 fb^-1 at sqrt(s) = 8 TeV in 2012. Individual searches in the channels H->ZZ^(*)->llll, H->gamma gamma and H->WW->e nu mu nu in the 8 TeV data are combined with previously published results of searches for H->ZZ^(*), WW^(*), bbbar and tau^+tau^- in the 7 TeV data and results from improved analyses of the H->ZZ^(*)->llll and H->gamma gamma channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0 +/- 0.4(stat) +/- 0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10^-9, is compatible with the production and decay of the Standard Model Higgs boson.

And here’s CMS:

Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC

The CMS Collaboration
(Submitted on 31 Jul 2012)
Results are presented from searches for the standard model Higgs boson in proton-proton collisions at sqrt(s)=7 and 8 TeV in the CMS experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at 7 TeV and 5.3 inverse femtobarns at 8 TeV. The search is performed in five decay modes: gamma gamma, ZZ, WW, tau tau, and b b-bar. An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, gamma gamma and ZZ; a fit to these signals gives a mass of 125.3 +/- 0.4 (stat.) +/- 0.5 (syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.

No huge surprises, I would say, but a few tiny tweaks to the earlier announcements. ATLAS now includes an analysis of decays into two W bosons, which wasn’t discussed in the July 4 seminar. Their overall significance is now 5.9 sigma, and CMS has 5.0 sigma, both a touch higher than before. (And at this point the search for more sigmas becomes less urgent; nobody really doubts that they’ve seen something.) The seminar results hinted at a discrepancy between the observed rate of decays into two photons, and the predicted rate in the Standard Model; sadly, the significance of this discrepancy has gone down just a bit. “Sadly,” of course, because such a discrepancy could be a sign of new particles contributing to the decays. They could still be there, but we’ll need more data to say anything for sure.

For the book I’m writing (did I mention that?) I fought to include two relatively technical-looking plots — the “bumps” in the two-photon events near the mass of 125 GeV. My winning argument was simple: “This is what we paid $9 billion for!” So here are the relevant updated plots from these papers, three for each experiment.

CMS two-photon events:

ATLAS two-photon events:

CMS 4-charged-lepton events (via ZZ):

ATLAS 4-charged-lepton events (via ZZ):

CMS 2-tau events:

ATLAS 2-charged-lepton events (via WW):

We’re certainly not sure that this is “the” Higgs boson as predicted by the Standard Model, and both papers are careful not to fall into that trap. In my book I was intentionally not so careful. I did take care to explain that, while we’ve found a new particle, we don’t know for certain that it’s the SM Higgs. But it’s certainly Higgs-like, and if further investigation reveals that it differs in interesting ways, so much the better. So I’m happy to call it “the discovery of the Higgs boson” until we get firm evidence that it’s something else.

CATEGORIZED UNDER: arxiv, Higgs, Science, Top Posts
  • Dan

    They should print the first graph into a t-shirt with the subtitle, I paid 9 billion dollars and all I got was this complicated graph.

  • Steven

    Reddit was here!!

  • http://reddit.com jkjacoby

    I came here from Reddit and now somehow I feel dumber for not being able to read this.

  • Sigh

    People who’re saying “Reddit was here” etc.. You’re doing it wrong, you’re the kind of people who ruin the reddit community

  • chrd

    so what now?

  • http://reddit anthony

    i agree with jkjacoby

  • dicky

    Science is awesome!!!

    Much better than the dark ages where scientists were killed for daring to prove how the universe works.

    We are the 99%

  • Jorge

    > Much better than the dark ages where scientists were killed for daring to prove how the universe works.

    What? lol Catch up on your History.

  • Pdelic

    If the Higgs is proven, then what?

  • meh

    what the hell is reddit?

    anyway, the important thing is that if you’re here from reddit: BUY SEAN CARROLL’S BOOK and catch the most recent episode of Through the Wormhole narrated by Morgan Freeman on Science Channel.

  • Erich

    It’s really a very simple graph. Perhaps you need to refine your terminology.

  • Stefan

    Wish I could understand it. I am nonetheless excited. Arrogant and pretentious ‘redditors’ can leave.

  • Sili

    I fought to include two relatively technical-looking plots

    I hate editors.

    And/or the book-reading public.

  • McStabbin

    I think i get it. the spikes in the graph show something happened other then the predicted outcome? Could be a diffrent boson?

  • Edward

    I need to take two aspirins and can someone please call me tomorrow!!!

  • http://nervousnation.com polytic

    This IS profound weather you comprehend it or not and no, it cost you pennies for this amazing project, its multinational. We will all be thankful in a century.

  • http://reddit.com Reason

    First of all, you did not single handedly pay 9 billion dollars for this. This was a joint operation between many countries.

    Second, this discovery does not end here as we predict that there are many new particles to be discovered now that we discovered the higgs boson and learned its attributes.

    Almost all news sources and ‘experts’ will tell you how this particle ‘gives’ other particles mass but they dont tell you how interesting this particle is. The particle will randomly disappear, where it goes nobody knows. Some scientists say the particle moves to different dimensions but I dont know too much about that statement. Certainly an interesting discovery and IMO worth it plus more.

  • Joe


  • s johnson

    What is the mass of the new particle? 125.6-125.8 GeV?

  • Chris

    One thing that has been bugging me is that the Higgs decays into two Z bosons. The Higgs is 126 GeV and the Z is 91 GeV. The daughters are heavier than the parent. Or is this similar to the muon decay which uses a W boson by violating energy conservation temporarily?

    • https://plus.google.com/118265897954929480050/posts Sean Carroll

      Chris– That’s a great question, I’m actually writing a post that addresses it. Short answer: the Higgs decays into two virtual Z bosons, which then decay to leptons. To be virtual, a particle must be hidden inside the decay process, just serving as a stepping-stone between what comes in and what goes out. And when a particle is virtual, it doesn’t need to have the mass of a real particle. In particular, it can be lighter. So there’s no problem with the Higgs decaying into particles that would be heavier than it if they were real, as long as those particles immediately decay into something lighter.

  • Chris Greene

    I seem to recall there was some talk that there were fewer tau-antitau pairs than expected. Is that still there? CMS seems to have a spike at around the 100-120 range, was it ATLAS that had a lower than expected amount? Was it a spike, but not a big enough one?

  • eyesoars

    The last sentence of the CMS paper reads: “The decay to two photons indicates that the new particle is a boson with spin different from one.” Photons, IIRC, are spin 1, and two photons generated by a decay could (?) have a total spin of 2 or 0. (?) Is this what is meant by the statement?

  • Pingback: Higgs Boson Research Papers, Get Em’ While They’re Hot! - NBitWonder()

  • MKS

    It’s so cool to see the formation of a belief of Sean’s more-or-less as it happens and it must be frustrating to ‘change modes’ from doing the science and being oh so precise and conditional and then trying to communicate that information with non-scientists in English, which is another language entirely with less precision…not to mention the media with their economic need to find a narrative ASAP

  • http://www.theprocarios.org Mike Procario

    On the 4th of July, I got up well after the seminars were done, so I searched on line for the talks. I found the CMS talk and showed the gamma-gamma mass plot to my wife and said, “There’s the Higgs Boson.” She replied that she preferred fields of study that were more concrete. I pointed out the a bump in a mass plot is about as concrete as it gets for a particle physicist. I was delighted to see the mass plot. The limit curves that had been published by ATLAS and CMS and before them CDF and D-Zero were really hard to explain to physicists much less no physicists.

  • kam

    what does this mean? does it have anything to do with quantum mechanics?


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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