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CERN | Geneva | Switzerland

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New baby boson is born, weighing in at about 126 GeV

Today, in a special seminar held at CERN, the two collaborations operating large multipurpose detectors, CMS and ATLAS, presented new and convincing signals from the Large Hadron Collider (LHC) that could be coming from the Higgs boson.

This fundamental particle was predicted nearly fifty years ago in the framework of the Standard Model, the theoretical model that describes just about everything observed so far in the world of particle physics. Without the Higgs boson, this model however failed to explain how particles acquired their mass.

The seminar was webcasted live to the world and the ambiance was both festive and serious. Or should I say seriously festive.

To find this new particle, physicists sifted through billions of events, looking for more events having well defined characteristics than what is expected from other well-known processes described by the Standard Model. These we refer to as the background. An excess of events indicates something new is also present.

But particle physics follows statistical laws and what you get is never exactly what you expect, within a certain margin of error. Evaluating this margin is crucial to being able to make correct statements.

Imagine the following. In a large bag, mix a thousand blue marbles and a thousand red marbles. Then blindly draw ten marbles out of the bag, how many red ones will you get? Seven? Five? None? All these answers are probable, except 5 is more likely than 7 which is also more probable than none.

And if you draw 100 marbles, you are more likely to get closer to 50% red marbles. The same occurs in particle physics: the statistical fluctuations get smaller once you have a larger data sample. Hence, the error margin on the presence or not of the Higgs boson is smaller when adding the 6 inverse femtobarns (fb-1) of data collected in 2012 to the 5 fb-1 collected in 2011 (this is just how we measure the amount of data).

We cannot look for the Higgs directly since it decays into smaller, more stable particles. Hence, we find events potentially containing a Higgs boson by looking at its decay products.

The two decay channels that give the most precise measurements if a Higgs has a mass around 125 GeV are when a Higgs boson decays into two photons or when it decays into two Z bosons, each one breaking apart into two electrons or two muons. This is called the four-lepton channel, since both electrons and muons are leptons. As can be seen on the plot below, Higgs decays to a pair of b quarks or WW are more frequent but much less precise to find the exact mass and the background in these channels is also very large, making it harder to see anything.

The many different ways a Higgs boson can be produced and decay. At 125 GeV (shown by the vertical dashed line) many channels are possible. The vertical logarithmic scale shows which process occurs more often as a function of the Higgs boson mass (the horizontal axis). Any decay involving quarks (qq or bb), tau leptons τ or neutrinos ν is less precise since some of the fragments are totally or partially lost. Fewer events will happen where a Higgs boson decays to two photons γγ or four leptons l+l-l+l- but they are easier to reconstruct since nothing is lost.

If the debris we find really comes from a Higgs boson breaking apart, once you recombine them, they will all cluster at the same mass and we will see an excess of events above the background at this particular mass value.

In the two-photon channel, CMS sees an excess of events that corresponds to 4.0 times the error margin on the expected number of events coming from the background. This is what we call a 4 sigma variation. For ATLAS, the excess corresponds to a 4.5 sigma deviation at a mass of 126.5 GeV.

Here are the 59059 events selected by ATLAS in the two photon channel. The small bump at 126.5 GeV corresponds to the 170 events that could be coming from the Higgs boson. The bottom insert shows what is left after subtracting the background, estimated in the upper plot by the red curve, making the Small excès more visible.

Then both experiments also see an excess of events in the four-lepton channel. It ranks at 3.4 sigma for ATLAS and 2.5 sigma for CMS. The most probable mass is 125 GeV for ATLAS and 125.5 for CMS.

Here we can see under the peak drawn in red the excess of events attributed to Higgs bosons decaying into

The other channels, although less sensitive, offer good cross-checks. When each experiment adds up the probabilities for all channels and data analyzed so far, CMS obtains a total probability of 4.9 sigma while ATLAS sees 5.0 sigma. Taking into account that statistical fluctuations are always possible if you look at all the mass values in the range of 110-150 GeV, then the global significance is slightly reduced to 4.7 sigma for ATLAS.  The most probable mass being measured at 126.5 GeV in ATLAS and 125.3 GeV in CMS.

The local probability that the observed excess of events found by ATLAS comes from a statistical fluctuation of the background is less than a chance in thirty million as shown by the solid curve. The dashed curve shows how strong the excess should be if it is produced by a Standard Model Higgs boson. Hence, the observed excess is slightly stronger than what is predicted by the Standard Model, but still within errors.

In particle physics, an experiment needs a 5 sigma excess to claim the evidence for a new particle. But since both experiments have it independently, it is clear that a new boson has just been discovered.

So welcome, little boson. But is it really the Higgs boson predicted by the Standard Model? That is too early to tell even though the chances are excellent. We must first check if this new particle is produced and decays exactly as the theory predicts for a Higgs boson in all possible channels.

Much work remains to be done and hopefully, other exciting discoveries will also come in due time as more data becomes available. Meanwhile, champagne is in order!

One event with two muons (tracks in red) and two electrons (tracks in green) found by CMS.

One of the four muon event selected by the ATLAS search. This could be coming from a Higgs boson decaying into four muons shown by the four red lines representing their tracks in the detector. There is no way to tell if this one in particular comes from a Higgs boson or some background event.

Pauline Gagnon

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27 Responses to “New baby boson is born, weighing in at about 126 GeV”

  1. David Dewar says:

    Many, many congratulations to all involved present and past. A magnificent achievement and an inspiring cooperation/collaboration across the world.

  2. carlos says:

    Wow! This is the closest i have ever been to understanding the Higgs Boson. Most appreciated.

  3. Bill Pierce says:

    Could the major implications of finding the Higgs boson be in the area of metaphysics? The concept of there being nothing but energy, seen and unseen, all one in being and that things are thoughts and thoughts are things? Could this nullify most ideas of religion and worship? This is all very interesting.

  4. Ashwini Dubey says:

    Brilliant article, all the findings well explained. Exciting times I’m sure. And as said above, great to see such Global co-operation….

  5. What a day to be alive !! Feeling blessed to be part of this generation.. History in the making and i’m getting to watch it.. Way to go guys..

  6. Mike says:

    Terrific explanation, but think you left in a couple of XX placeholders …

  7. Michael Rally says:

    Can somebody explain why the Atlas data shows a periodicity from 130 – 160 GeV?

  8. Diego Forero says:

    Congratulations and thank you for to explain this hystorical discovery

  9. Paul says:

    A great achievement.
    For a musical accompaniment, can I propose Colin Matthew’s ensemble work “Broken Symmetry” [Decca 474316-2]

  10. Shah Yasser Aziz says:

    Tons of cangratulations people! You have finally reached your goal! Kudos!

  11. Robert Andrew says:

    Congratulations to all.
    Our grandchildren will learn about this date in history when dark energy is the world’s major energy source and the planet is clean and unpolluted

  12. Petes says:


  13. Randy Hermatz says:

    I do not recall being invited to the baby shower for this little boson. What should I get him? Is he registered?

  14. See at http://universalrule.info in this way there are many great things to be discovering through physics. See into- http://www.twitvid.com/UENCI found multimedia

  15. Vasco says:

    Congratulations to all parts involved througout 5 decades at least. It is a great day for Science and one can say, if there are two main achievements for humanity, definitely is the Man on the moon and the search for the Higgs!

    Great to be a scientist and even more, humanity.


  16. Josh Forrest says:

    Congratulations, this is a wonderful step toward a more complete understanding of the incredible universe that we live in. Well done and thank you to all involved :).

  17. DIPRO SINHA says:

    A benign exploration of PHYSICS,has just pleads for the new way to the future discoveries & gives a pace to the forth coming change over,,, I think…the collaboration of the scientists around the world just coagulate the total & spontaneous emotions of us to us.

  18. siddhartha says:

    very nice ….appreciated….
    The one who has witten this will go in HEAVEN …(think he is alrdy in heaven when he heard about that “NEW PARTICLE”not sure about GOD )

  19. Matthias von Arx says:

    One question however: How is ist possible, that in two channels (gamma-gamma, ZZ) the observed significance lies quite a bit above SM-expectation and in the other (more complicated) channels (eg. tau-tau) the observed significance lies quite a bit below SM-expectation? If I understood correctly, this is the case for both experiments. Therefore it is unlikely due to statistical fluctuations. What coud be the reason?

  20. Enr says:

    Thanks for this simple explanation for non-specialists! :-)

  21. Mikko Kondratjeff says:

    This is the most important historic discoveries which will have impact on all applied physics. I’m very excited to see how this will affect atmospheric science.

  22. Congratulations to the team. Their Cooperation Made Them to achieve this feat hope This Continues and the People (We) will gwt more good results and clear Explainations.

  23. Zeynab Milani says:

    absolutely great news, thanks for the article.

  24. S K Sur says:

    The heavy sibling of photon appears to be underweight – is he/she premature?

  25. pauline gagnon says:

    Thanks. Indeed, this could only happen because so many people contributed.

  26. fluidic says:

    Great Achievement!

    I don’t know why it is hard for me to believe that it is THE HIGGS BOSON, but rather I see it as the greatest bull’s eye hit of the outstanding ingenuity of Peter Higgs, our greatest of predictor scientist. I believe that what Peter Higgs predicted came true with an incredibly small lifespan decay of lower than 1.0 X 10-33secs. The 125-126GeV displays matter’s perfect-liquid to perfect-liquid (RHIC) transformation decay that forms, generates, and pushes strange and new particles and other electromagnetic radiations as a result of its ultra-rapid unstable lifespan under the ultra energy collision densities, which experimentally from two independent detectors CMS and ATLAS displayed itself at around this beautiful very tight range 125-126GeV.

    Congrats to ALL who contributed, especially to Peter Higgs.
    Fluidic, Dequantizer

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