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CERN | Geneva | Switzerland

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How to tell a Higgs from another boson?

On July 4, when CERN announced “the observation of a new particle” and not the discovery of the Higgs boson, many wondered why be so cautious. It was simply too early to tell what kind of boson we had found. The Higgs boson is the last missing piece of the Standard Model of particle physics, a model that has enabled theorists to make extremely precise predictions. But to fully trust this model, it should have all its pieces. Who would want to complete a 5000-piece puzzle with the wrong piece?

Both the CMS and ATLAS experiments have been conducting several checks since July:

1) Are all possible decay modes predicted by the Standard Model observed?

2) Is each observed decay happening at the right rate?

3) What are the fundamental properties of the new boson?

The first checks (based on half the data now available) indicate that the new boson is compatible with being the Higgs boson. But the precision is still too low to tell as shown on the plots below (the signal strength and σ/σSM H are the same quantity).

The Higgs boson can decay in many ways and the plot shows which decays have been observed and at what rates. A signal strength (of 1 means the signal corresponds exactly to what is expected for a Higgs boson.  Zero would mean there is no signal seen for this particular decay channel. The black points represent the measured values and the horizontal bar, the error margin.

At this point, we cannot tell unambiguously if the first two measurements are more compatible with 0 (the decay does not exist) or 1 (yes, it decays at the predicted rate).  Both CMS and ATLAS need to analyze more data to say if the new boson decays into two b quarks (H → bb) and two tau leptons (H → ττ).

The other three decay modes, namely WW, two photons (H → γγ) and ZZ occur at about the rate or slightly more often than expected by the Standard Model.

The decisive test will come by measuring its spin and parity, two “quantum numbers” or properties of fundamental particles. The spin is similar to the angular momentum of a spinning object. But for fundamental particles, only discrete values can be used. For bosons (the particles carrying the various forces), these values can be 0, ±1, ±2 and so on. For fermions, the building blocks of matter like quarks and leptons (electron, muon, tau and neutrinos), it can only be +½ or -½.

Aidan Randle-Conde has compiled all possibilities on his blog. A particle with spin 1 cannot decay into two photons. Since we have seen the new boson decaying into photons, spin 1 is already ruled out in the table below. Moreover, a spin 2 boson could not decay into two taus, which is why it is so important to look for this decay in the latest data.

(from Aidan Randle-Conde’s blog)

The Standard Model predicts that the spin and parity of the Higgs boson will be 0+. To distinguish between 0+ and 0-, as well as 2+ and 2-, the only way is to carefully measure the angles at which all the decay products fly apart. So if we observe the new boson decaying into photons, we must measure the angle between the photons and the beam axis. If it decays into two Z, each one going into two electrons or two muons, we must carefully measure the angles of these four particles and their combined mass. Here is what Sara Bolognesi and her colleagues predict for Higgs bosons decaying into ZZ, WW or two photons. We must measure specific quantities, namely the mass and angles of the decay products, to distinguish them. If they match the red curve, we will know it is the Higgs boson, but it they look like one of the other curves, it will mean the new boson corresponds to a different theoretical model.

Each experiment now has about 14 fb-1 of data on tape and expects about 25 fb-1 in total by the end of the year. With the 5 fb-1 collected last year, it should be sufficient to unmask the new comer. “All” we need to do is measure these extremely complex quantities.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

For more info, see these two CERN news videos  on CERN YouTube (part 1 and part 2) on the Higgs boson spin.

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  • Joshua Thompson

    I have been having an ongoing discussion with a friend of mine about how one can identify the Higgs Boson. We are both lawyers, both interested in physics (at a layman’s level), and both have been following the CERN developments regarding the Higgs. Anyway, I sent him a link to this post, and he gave the following response. Can you address his questions? Thanks in advance.

    “Obviously this is over my head, but it doesn’t seem to really answer my question, which was, if a boson can have the same quantum state as another boson–same place, same spin, etc., at the same time, then how can it EXIST, in the sense of being a thing which is not another thing? Isn’t it then more of a quality than an entity?–because then if two bosons are occupying the same quantum state, then wouldn’t they really be the *same* boson? Or is it the case that they *can* do this, but then they indicate a higher energy level, and that’s how we detect it? If that’s the case, then a boson would seem to be more like an adjective than a noun–it’s just the higher energy output we measure. Which makes sense, I guess, if a boson is an energy carrying thing–that it would just somehow BE energy. But then, is that really “occupying the same quantum state”?”

  • http://www.cern.ch CERN

    Sorry for the late reply, I had missed your comment. Your friend seems to confuse a few things (or I am not getting his point right). Think of any particle. Let’s take photons, which are also bosons. They all have the same value of spin (which is 1). There are tons of photons but you won’t all find them at the same place and at the same time.

    Same for the Higgs boson. All Higgs bosons will have the same characteristics (spin 0, same mass (125 GeV if what we found is the Higgs boson), parity +, etc). It will also decay in very specific ways, each way having a given rate.

    Bosons are different from fermions (particles of matter with half spins like the electrons or quarks) in that they obey different statistical rules. Fermions cannot exit in the same quantum state. This is why one only finds 2 electrons (one with spin up, one with spin down) per atomic level as we learned in high school chemistry. This is why larger atoms with many electrons have many levels and many sub-levels, each one with at most 2 electrons.

    Now bosons can all exist in the same quantum state. You can put as many boson as you want in one given quantum state. True, they become absolutely indistinguishable. I suspect that unlike humans, they do not care about loosing their identity.

    Cheers, Pauline

  • http://rwc.arran@btinternet.com thefisherman22

    Regarding nutrinos, the expirament sending nitrions from CERN to the Italian lab beating the speed og light. As nutrinos are never seen perhaps they do travel faster than light!
    Reaserch in the USA when nutrinos were collected underground in a form of detergent may also support the assumption, that they do travel faster than light

  • Denis

    > if a boson can have the same quantum state as another boson–same place, same spin, etc., at the same time, then how can it EXIST, in the sense of being a thing which is not another thing?

    Two bosons in the same quantum state are indeed indistinguishable. However, there are still *two* bosons there.

    For example, in the future one of them can leave this state, while other can stay: say, two photons in the same quantum state enter a slice of some material, one is absorbed but other isn’t. Clearly, for that to be possible, there have to be two particles: it’s impossible to absorb a half of a particle.

  • http://www.cambridgecoaching.com.au Reza Bokat

    How can we measure the angle between photons?

  • http://www.cern.ch CERN

    Hello Reza,

    in the detector, we can see the two photons clearly. All that is needed then is to measure the angle between them like one measure any angle. Have a look at some of the event displays for Higgs bosons decaying into two photons taken by the ATLAS experiment: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/EventDisplaysFromHiggsSearches#H

    Cheers, Pauline

  • http://www.cern.ch CERN

    Hello,

    in fact, after careful cross-checks by other experiments and by the OPERA experiment itself, it turned out to be an experimental error. Neutrinos have a speed inferior to the the speed of light.

    Regarding the experiment in the Creighton Mine in Minnesota that you refer to, what was measured there was not the speed of the neutrinos but their number. Ray Davis measured that only a third of the neutrinos produced by the sun reached the Earth. The reason was that the electron neutrinos produced on the sun can change into other types of neutrinos on their way to Earth. The cleaning product used to detect them contained chlorine and it only reacts to electron neutrinos. To test this hypothesis, another experiment was built in Sudbury in Northern Ontario in Canada in the Inco Mine. There, they used heavy water, a substance that was sensitive to all types of neutrinos (electron neutrinos, muon neutrinos and tau neutrinos). They were able to show that the sum of the three types of neutrinos corresponded to the number of electron neutrinos produced on the sun. So this was a different problem.

    This other experiment implies that neutrino have masses. This allows them to mix into each other. Since they have mass, they should travel at less than the speed of light. This is also now proven after redoing this experiment in Italy with different detectors.

    I hope this helps, Pauline

  • Valery

    I read an article that claims that the top quark mesons should appear in the CERN data. The claim is that although the mean life time of the top quark is less than 10^-24, the time required to create a meson is nearly 10^-24 and many top quarks should leave enough time to create such meson.

    What do you think about this argument? The article continued and claimed that it is possible that the new particle is a top meson (consists of top and top-bar). It says further and claims that W,Z are top mesons as well.

  • http://www.cern.ch CERN

    Hello Valery,

    I am afraid the author of the article you read was misinformed. First of all, top quarks are not expected to form bound states. Second, such a particle would have specific decay modes, incompatible with the decay modes observed for the new boson and for the W and Z bosons.

    Cheers, Pauline

  • Valery

    Thank you Pauline. Can you read the first argument+answer of the link below, and tell me what is the author’s mistake? He claims that this is a proof that top quark mesons must exist. Thanks!
    http://nohiggs.wordpress.com/2012/10/09/why-top-quark-mesons-must-exist/

  • http://www.cern.ch CERN

    Hello again, Valery,

    I am no expert on the matter but I completely trust the judgement of Matt Strassler, a very reputed theorist from Rutgers University. In his blog, he answers the arguments brought by Mr. Conway. See http://profmattstrassler.com/new-start-here/ and search for “Larsen”. As I said before, it is believed that top mesons decay too fast to have time to form a bound state. And everything measured so far for the top quark, and the W, Z and new bosons are in agreement with the SM predictions. So I am happy to go with that.

    I read the blog from Conway. All I can say is: I am always suspicious of people claiming to have the answers to everything and claiming that everybody else is wrong. If his arguments were sound and published in scientific journals, other theorists would have listened or cross-checked his claims. If he cannot convince anybody, it is probably that his arguments are faulty as Matt Strassler argues. He simply claims the top quark lives long enough when all evidence points to the contrary. To me, all this sounds like a “conspiration theory” attitude, stating that the whole scientific community somehow has an agreement to hide something. I know this is not the way it works. Usually, people state their arguments and it progresses from there.

    I hope this will help you form your own opinion.

    Cheers, Pauline

  • Valery

    Thank you. I read Comay’s blog carefully and my impression is different.

    Eliyahu Comay is a physicist since the 1960s and he has many publications. One of the most recent publication is about the W boson equations. The blog (which is written by his son) says that Comay proved that the equations show that W is not elementary.

    In his blog Comay claimed that top quark mesons exist and in Matt Strassler blog he mentioned his claims. Matt Strassler said that he doesn’t have time to answer (he said he has better things to do). So my conclusion, for now, is there are no answers – all they have is a set of excuses. I hope that someone in CERN would answer to the specific points that were raised in Comay’s son blog blog or in Comay’s article and change my feeling about the whole thing…

    All the best

    Valery

  • Valery

    Hi Pauline

    I see that you prefer not to publish my previous comment. Can you edit the text in my comment? If it is possible, please change from:
    “particle physicists do not have answers – all they have is a set of excuses”
    to:
    “there are no answers”

    and publish it?

    Thank you. I am following Comay’s blog for several months and I do not understand why no one answers his questions. Matt Strassler wrote that he is silly and stupid. You wrote that you do not trust him. And according to his blog he solved in the past a problem that was open for 30 years (a paradox called “the hidden momentum” or something like that). I think that he deserves better “treatment” from the community. Instead of saying that he doesn’t understand physics, people in the community should answer his claims, using scientific arguments.

    That’s what I think.

    In the meantime, I posted a question in his blog about your comment regarding the decay channels of W,Z.

    Valery

  • Valery

    For anyone who is interested – Comay provided an explanation why the decay channels of W,Z and the new particle support his approach that they are mesons of top quarks.

    I do not have the tools to understand everything he says. Physicists can read about it here: http://nohiggs.wordpress.com/2012/10/09/why-top-quark-mesons-must-exist/#comment-1789

  • Ilya Kuryakin

    I refer to the earlier explanation regarding measurement of the angle between decay photons. As the photons are emitted simultaneously, there would be a coincidence requirement, so that only correlated photons are accepted. I presume that energy gating is also used. Similarly for other decay modes (WW, ZZ).

  • Tin

    Why do you refer to Aidan Randle-Conde blog when it is both wrong and misleading, as Frank Close has pointed out in the comments of that blog entry?