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CERN | Geneva | Switzerland

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The mystery remains on the Higgs boson

Ever since the discovery of what might be the Higgs boson last July, physicists from the CMS and ATLAS experiments have been trying to pinpoint its true identity. Is this the Higgs boson expected by the Standard Model of particle physics or some “Higgs-like boson” befitting a different theoretical model?

To tell the difference, we must check all its properties, like how often this boson decays into different types of particles, and determine its spin and parity, two properties of fundamental particles.

Since the new boson has a short lifetime, it breaks apart immediately after being created. There are five ways a Standard Model Higgs boson should decay that we can study at the Large Hadron Collider (LHC): breaking into two photons, two W or two Z bosons, two b quarks or two tau leptons in well defined proportions.  We must check both the presence of and the rate at which each decay mode occurs.

Last summer, just after the discovery of the new boson, both experiments reported unambiguous observations in only three channels. Unfortunately, the data sample was still too small to really be able to check if the new boson could decay into a pair of b quarks or tau leptons.

With more data available, the two experiments have just shown results for all channels today at a conference held in Kyoto as shown on the two figures below.









The left figure is for CMS and the right one for ATLAS. The values “σ/σSM” and “μ” are equivalent and represent the ratio of what is seen to what is expected from the Standard Model. So if μ is exactly one for a given channel, it means that channel decays at the rate expected from the theory. A value of zero would imply this particular decay channel is not seen at all, contrary to expectation. If μ has any other value, it implies the new boson does not behave quite as predicted. But one must take into account the error margin (the horizontal bar) before drawing any conclusion.

Both experiments now measured decays into two b quarks and two tau leptons and the errors have gone down for several channels. For now, CMS obtains a combined value of 0.88 ± 0.21whereas ATLAS mesures 1.3 ± 0.3. Both are compatible with 1.

The confirmed presence of all five modes would be compatible with a spin-zero particle. Having in addition all the correct decay rates would make the new boson look much more like a Higgs boson but it would still not be quite sufficient. The new boson must also have positive parity as the Standard Model predicts.

The spin of a fundamental particle refers to its rotation on itself, as the name suggests. Parity has to do with flipping direction in space, exactly like what happens when we watch an event directly or through a mirror where the left and right directions are inverted. Particles with a positive parity look the same when you observe them directly or through a mirror.

The parity can be determined by looking at the direction taken by all fragments after the boson decays. Depending on its parity, its debris will fly in a preferred direction. For example, CMS measured all angles between the four electrons or muons when the “Higgs-like” bosons decay into two Z bosons, each one ending in a pair of electrons or muons. Then they compared the distributions with two standards: one for positive, one for negative parity as shown on the figure below.

The left curve in blue shows the probability one would measure for a particular point on the horizontal axis if the new boson had a negative parity. The right curve in pink shows the same for a particle with positive parity. The value measured by CMS (green arrow) indicates the new boson most likely has a positive parity as expected by the Standard Model.

CMS also started looking for other bosons with masses beyond 600 GeV, the current excluded limit. If new bosons turn up, it could mean we have found one of the five Higgs bosons predicted by supersymmetry, a new theoretical model, and not the single Higgs boson predicted by the Standard Model.

So where do we stand? With more than twice as much data as shown in July, scientists have moved from searching for this elusive particle to starting to measure its properties. Once the decay channels, decay rates, spin and parity are clearly established, we will be able to determine its identity.

It is still too early to tell but the new boson looks like, sings like and dances more and more like a Higgs boson. More certainty will come out next March at a winter conference with still more data and improved analyses. But it will take a long time to figure out beyond any doubt if the discovered boson was really the Standard Model Higgs boson.

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.




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  • Antonio Saraiva

    The Higgs doesn’t exist because the mass is an electric dipole moment:

    m = q.k/x (1-pi^3.alpha^2 /2)

    m-mass or an electric dipole moment;q-electron charge;
    k-Boltzmann constant(square meter);x-Compton wavelength;
    pi = 3.1415927; alpha = fine structure constant.

    kilogram = coulomb x meter

  • Pingback: Einsteinin aivot ja muita viikon uutisia « Merkintöjä()

  • The Higgs does not exist because the number of angels that can dance on a pin head is an even number that is positive semi-definite and less than the multiplicative identity.

    Or perhaps the reason is that Leprechauns are restricted to Ireland?

    How can we trust physicists who dare to do experiments that might contradict our notion on how the Universe works, especially when they torture cats in existentialist experiments?

  • Hello Gavin,

    I hesitated before approving your comment We try to keep a discussion to the point here. But given the excellent humor in your comment, it would have been a pity to trash it! Thanks for the excellent joke on Schroedinger’s cat!

    Cheers, Pauline

  • My understanding is that some particles have more mass than can be explained by the existing set of fundamental particles in the Standard Model – excluding the Higgs.

    Assuming that just one field/particle is responsible for the extra mass: then there are at least 2 values (one of which is mass) of the Higgs Boson, which can not be determined even approximately by the Standard Mode – except perhaps as a range. However, several properties such as electric charge and spin can be determined. So the current Higgs candidate was found by looking for a particle that matched the predicted properties. However, the particle itself could not be detected as it is so unstable, so the decay products where looked for – fortunately, the theory predicted frequency of the various decay modes – unfortunately, the particles in the decay modes are also produced by other factors. Hence a combination of phenomenally cunning mathematics and massive brute force computing must be applied to the extremely large number of collision observed in the LHC by the armies of experimental physicists working in consort with legions of theoretical physicists.

    A compounding factor is the assumption that there is just one Higgs to be ‘observed’. If there is more than one, then the properties of the one we have found will differ from those predicted on the basis of just one, and other Higgs are waiting to to be discovered with less well defined properties.

    So physicists are carefully investigating what properties this new particle actually has, such as the relative frequencies of the decay modes of the current Higgs candidate and the other characteristics like spin and parity. Also they are extending the search into other mass ranges; as several theories explaining the extra mass, require more than just one Higgs.

    I note that physicists are being extremely careful not to claim the current Higgs candidate is the one and only possibility.

    So I take exception when someone appears to cavalierly dismisses the current current Higgs candidate with one equation of apparently dubious parentage (I’m attempting to be diplomatic, but failing), that has a distinct lack of theoretical justification and total absence of any reference to experimental results. Even if the equation was well founded theoretically, it would still have to be firmly tied into experimental results – even then, a good explanation as to why the current current Higgs candidate that appears to be a good match but isn’t, would need to be given.

    The above is my current personal view, and did not involve the creation of spherical cows in pairs or otherwise (to the best of my limited knowledge).

    Since I am not a licensed physicist, please consult a properly certified physicist before allowing the above to alter your world view.

  • Dear Pauline,

    Take a look at the site http://www.primons.com and see that the Standard Model Higgs boson does not exist simply because quarks are composite and, thus, there should exist three spin zero Higgs-like bosons (two charged and on neutral).

  • Dear Mario,

    thank you for your reply. I had a look at you site and your theory about primons. Interesting but like any theory, it needs to be corroborated by experimental evidence. So far, such evidence has not been found despite what you claim, at least nothing that has been overwhelmingly accepted by the scientific community. But you have an interesting view point here. Thank you for sharing it.

    Cheers, Pauline