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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

View Blog | Read Bio

Getting to the Bottom of the Higgs

Updated Friday, January 31, 2014: Candidate event of Higgs boson decaying to bottom quarks has been added at the bottom.

CMS has announced direct evidence of the Higgs coupling to bottom quarks. This is special.

Last week, the Compact Muon Solenoid Experiment, one of the two general purpose experiments at the CERN Large Hadron Collider (LHC), submitted two papers to the arXiv. The first claims the first evidence for the Higgs boson decaying directly to tau lepton pairs and the second summarizes the evidence for the Higgs boson decaying directly to bottom quarks and tau leptons. (As an aside: The summary paper is targeted for Nature Physics, so it is shorter and more broadly accessible than other ATLAS and CMS publications.) These results are special, and why they are important is the topic of today’s post. For more information about the evidence was obtained, CERN posted a nice QD post last month.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig 1. Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the ATLAS detector.

There is a litany of results from ATLAS and CMS regarding the measured properties of the Higgs boson. However, these previous observations rely on the Higgs decaying to photons, Z bosons, or W bosons, as well as the Higgs being produced from annihilating gluons or being radiated off a W or Z. Though the top quark does contribute to the Higgs-photon and Higgs-gluon interactions, none of these previous measurements directly probe how fermions (i.e., quarks and leptons) interact with the Higgs boson. Until now, suggestions that the Higgs boson couples to fermions (i) proportionally to their masses and (ii) that the couplings possess no other scaling factor were untested hypotheses. In fact, this second hypothesis remains untested.


Fig. 2: Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

As it stands, CMS claims “strong evidence for the direct coupling of the 125 GeV Higgs boson” to bottom quarks and tau leptons. ATLAS has comparable evidence but only for tau leptons. The CMS experiment’s statistical significance of the signal versus the “no Higgs-to-fermion couplings” hypothesis is 3.8 standard deviations, so no rigorous discovery yet (5 standard deviations is required). For ATLAS, it is 4.1 standard deviations. The collaborations still need to collect more data to satisfactorily validate such an incredible claim. However, this should not detract from that fact that we are witnessing phenomena never before seen in nature. This is new physics as far as I am concerned, and both ATLAS and CMS should be congratulated on discovering it.

Event display of a candidate Higgs boson decaying into a tau lepton and anti-tau lepton in the CMS detector.

Fig. 3: Event display of a candidate Higgs boson decaying into a bottom quark and anti-bottom quark in the ATLAS detector. HT to Jon Butterworth for the link.

The Next Step

Once enough data has been collected to firmly and undoubtedly demonstrate that quarks and leptons directly interact with the Higgs, the real tests of the Standard Model of particle physics start up. In the Standard Model, the strength at which a fermion interacts with the Higgs is proportional to the fermion mass and inversely proportional to the ground state energy of the Higgs field. There is no other factor involved. This is definitively not the case for a plethora of new physics models, including scenarios with multiple Higgs bosons, like supersymmetry, as well as scenarios with new, heavy fermions (heavy bottom quark and tau lepton partners). This is definitely a case of using newly discovered physics to find more new physics.

Happy Colliding.

– Richard (@bravelittlemuon)

PS I was unable to find an event display of a Higgs boson candidate decaying into a pair of bottom quarks. If anyone knows where I can find one, I would be very grateful.

PSS Much gratitude toward Jon Butterworth for providing a link to Higgs-bbar candidate events.


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  • Amir Livne Bar-on

    Can you explain why the search is for interaction with B quarks and not with T quarks? I didn’t understand the reason from the summary paper.

    From what I read here in QD in the past, the Higgs will interact with it much more strongly, and in fact a lot of the Higgs bosons in LHC are created through virtual T quarks. So why can’t we detect them on the output side of the process?

    Thanks, Amir

  • Hi Amir,

    That is a great question! The Higgs boson does couple more strongly to the top quark than the bottom quark by roughly a factor of 40! There is no doubt about that. However, a single top quark (175 GeV) is much heavier than a Higgs boson (125 GeV), whereas the Higgs is about 30 times heavier than a bottom quark (4 GeV). By conservation of energy, a particle cannot decay to two real particles if the decay products are *heavier* than the parent particle. A Higgs cannot decay to a pair of bottom top quarks because it would require much more energy than a Higgs boson has (at least 350 GeV is needed); it cannot even decay into one real and one virtual top. On the other hand, there is more than enough energy to make a pair of bottom quarks. The situations where two virtual top quarks are involved include the Higgs-photon and Higgs-gluon interactions, but, again, these are *indirect* measures of the Higgs coupling to quarks. For all we know, new particles can be participating in these processes as well.

    I hope this answered your question.


    Update Friday, January 31, 2014: Updated to reflect that a Higgs boson cannot decay into a pair of *top* quarks. The largest decay mode for the SM Higgs at 125 GeV is to a pair of bottom quarks.

  • Roger Jones

    You meant ‘A Higgs cannot decay to a pair of *top* quarks’ – an easy typo in the context, but just in case it causes any confusion. It should be clear from what you go on to say.

  • Hi Richard, nice article.

    There are some ATLAS VH(H->bb) candidates here if they are any use to you.


  • TheFox

    Hi Richard

    “A Higgs cannot decay to a pair of bottom quarks because…”

    I think that should say top quarks, unless I’ve misunderstood.


  • Hi Roger,

    Great eye. I apologize for the typo; you are absolutely correct.


  • Hi Fox,

    Great eye. I apologize for the typo; you are absolutely correct.


  • Tienzen (Jeh-Tween) Gong

    An excellent report.

    @Richard: “… This is definitively not the case for a plethora of new physics models, including scenarios with multiple Higgs bosons, like supersymmetry, …”

    John Ellis proclaimed “I’m not giving up on supersymmetry, …” (see http://www.math.columbia.edu/~woit/wordpress/?p=6601 ). Without a SUSY-Higgs boson, how many SUSY models are still viable? How strong is your statement above?

  • Hi Tienzen,

    This is a good question. Without a second set of Higgs bosons, the supersymmetric standard model cannot work, period.

    In the regular SM, the Higgs is used twice to give masses the up-type fermions (neutrinos and the up-quarks) and the down-type fermions (electrons and down-type quarks). However, the up-type fermions and the down-type fermions have different sets of quantum numbers. To match the quantum numbers with only one Higgs, we “conjugate” the Higgs (flip all the quantum numbers, e.g., “+1” becomes “-1”).

    This charge conjugation is forbidden in the supersymmetric potential, which stipulates at the interactions in SUSY. This means we have to introduce a brand new Higgs to match the quantum numbers. In other words, if no other Higgses exist, there is no SUSY.

    On the other hand, considering that there are four spin-1 bosons and 12 spin-1/2 fermions, I hardly think that there is only one spin-0 boson in the Universe. Secondly, scalars are super popular in many new physics scenarios. I have no doubt more scalars exist; what their charges and masses are, however, is quite a curiosity.


  • Disha Bhatia

    So what you are trying to say is that although their is a strong evidence for Higgs directly decaying to bottom and anti bottom and tau and anti tau, but we can’t treat it as a discovery unless their is a 5 sigma standard deviation seen.
    This is what confuses me.
    I have the notion of the standard deviation as a measure of how far the answer can lie from the mean value. Larger sigma meaning larger uncertainty from the measured value. So from this I don’t understand why 5 sigma is preferred over lower values of sigma.

  • Colin Bernet

    Hi Disha,

    5 sigma means that there is a probability of 1/1744278 that the observed data has been produced by background processes, assuming that the Higgs does not couple to fermions (in this particular case). That’s almost as low as one chance in 2 million, and the convention adopted by the scientific community is that this probability is small enough to claim a discovery.

    If you just count events selected under certain conditions, you could get 5 sigma in the following way. With simulations, you first estimate how many events you expect from background processes, and let’s say you expect 5 events for 1 year of data taking. Now, if instead you observe 20 events with the amount of data collected during this period, the excess over the background expectation is more than 5 sigma.

    This view is over-simplified, as we fold in many variables (not only event counts) when we compute these probabilities. Also, we check that the excess is compatible with the expected Higgs boson properties.