Our Higgs

Attendees of the Phenomenology 2013 symposium at the University of Pittsburgh. Look for the bowler hat.... [Credit: the website doesn't list a photographer name, so please help if you can]

Attendees of the Phenomenology 2013 symposium at the University of Pittsburgh. Look for the bowler hat…. [Credit: the website doesn’t list a photographer name, so please help identify if you can]

On July 4, 2012 — one year ago tomorrow — much of the world united in celebrating a result in fundamental science: the discovery of the Higgs boson. Even publications and television shows that rarely cover anything scientific ran Higgs boson pieces. A few commenters (including me) cautioned that this discovery was of a “Higgs-like particle”, but over time and with more data, that caution gave way to assurance: we confidently say now that researchers at the Large Hadron Collider (LHC) found the Higgs boson.

Or did they? The successful theory of particles and their interactions – the creatively named Standard Model – provides a single Higgs boson, but other models may contain more than one Higgs boson, or other types of Higgs particles in addition to the boson. The discovery of a Higgs boson at the LHC leads to the obvious next question: which Higgs, meaning as predicted by which theory?

As a result, particle physicists sometimes jokingly call this the “definite article problem”: is this the Higgs – meaning the Standard Model Higgs boson – or a Higgs, out of a possible list? My friend Richard Ruiz of the University of Pittsburgh (again, with tongue partly in cheek) says that whichever theory it is, it’s our Higgs – meaning it’s the particle we have available, at least until the LHC resumes operations in two years. (Like me, Richard is American, but I like to imagine “our Higgs” spoken with an English accent and inflection, as someone might speak affectionately of “our mum”.) Whether or not our Higgs corresponds to various theoretical models is not up to us.

To get a handle on the current thinking of the high-energy particle physics community – the things particle physicists say to each other without TV cameras present – I attended the Phenomenology 2013 Symposium at the University of Pittsburgh. My background is in cosmology rather than particle physics, but I speak enough of the language to get by. (Unless otherwise noted, the people quoted in this article participated in the symposium. Any errors of course are my own.)

Particle fields forever

In the typical approach to modern physics, particles are manifestations of underlying fields – an ambient froth filling the Universe that mostly averages out to nothingness. Both matter and their interactions are described by quantum fields, though in the Standard Model (SM) there’s a distinction. Elementary particles of matter – quarks, electrons, neutrinos – are fermions, particles that resist being piled together, while forces are carried by bosons, which can occupy the same space without penalty. Fermions can also combine to make composite objects, which can be bosons; that’s a point I’ll return to.

Then we have an exception: the Higgs field, which manifests as an electrically neutral boson, yet doesn’t carry a force in the usual sense. Instead, the Higgs field influences certain particles – electrons and quarks, but not photons or neutrinos – to give them mass: resistance to acceleration. Unlike the other fields, the Higgs field is always present, even when it doesn’t manifest as a particle. In other words, it has nonzero energy even in vacuum, acting as a medium through which particles move. (It’s still not certain that the Higgs boson found at the LHC has any connection to the Higgs field, but that’s not particularly relevant to most of what I have to say here.)

One version of the Higgs mechanism explaining how some particles acquire mass, typeset in Comic Sans. What, do I have to have a reason?

One version of the Higgs mechanism explaining how some particles acquire mass, typeset in Comic Sans. What, do I have to have a reason?

The CMS and ATLAS detectors at the LHC measured the Higgs boson mass to be about 126 billion electron-volts (GeV), a result consistent with measurements at Fermilab in the United States. (For comparison, a proton has a mass of 938 million electron-volts. Physicists use energy units for mass instead of switching back and forth using E = mc2.) That mass sets the value of the Higgs field energy, but it also partly dictates what sorts of particles the Higgs boson can decay into.

Those decay products are the key to identifying the Higgs boson in the first place, as the particle decays so rapidly that it doesn’t trigger the detectors. A boson decay will produce different types of particles than a fermion decay, the spin of the original particle will dictate the spins of its children, and a charged particle decaying will be distinguishable from a neutral particle. The Higgs boson at the LHC appears to be a neutral, spinless boson – precisely as predicted by the SM.

A related mystery is why the Higgs field energy isn’t either zero or really huge, the two most likely options. In fact, the Higgs mass seems to lie within an interesting range, where the vacuum of the Universe is “metastable”. If that’s the case, the Universe will be stable for a long period of time (much longer than its present 13.8 billion-year lifetime), then possibly expand rapidly enough to empty out large patches of the cosmos of matter. However, that condition isn’t certain, and depends on properties of the heaviest quark, the top quark, which are currently not well measured. University of Florida physicist Pierre Ramond (one of the founders of string theory) pointed out how strange Higgs boson mass lies in that region, metaphorically perched near a value where the vacuum would be unstable and cosmic expansion would be so rapid as to prevent any galaxies from forming.

A thumbnail sketch of particle physics

The SM is remarkably successful, though necessarily incomplete. The theory establishes the relationships between the masses of some particles, but the masses themselves must be determined experimentally. As Goran Senjanović of the Abdus Salam International Centre for Theoretical Physics (ICTP) pointed out, the theory was developed to understand the weak force, not as a theory of all forces. (Gravitation is described by a field theory – the general theory of relativity – but its quantum particle description is still elusive. The so-called Grand Unification Theory (GUT), a SM extension which joined the strong force to the electromagnetic and weak forces, predicted proton decay, which so far has not been seen experimentally.) As such, the SM only has one big failure: it predicts that neutrinos should be massless, in contradiction to a variety of experiments. The absence of spots for dark matter, massive neutrinos, the hierarchy of some particle masses, and gravity has inspired many extensions and alternatives to the SM.

The Higgs boson is one piece of the SM that was predicted, but missing from any experiment for about 40 years. If the LHC had failed to find it in the mass range allowed by the SM, that would have marked a strong sign of new physics. Since we know the SM is not the last word, many physicists expected – or at least hoped for – clear deviations from predicted behavior.

So far, these expectations have been disappointed. The Higgs boson from the LHC exhibits no noticeable differences from SM predictions. This has led to some convoluted theorizing, and not a little soul-searching among the physicists I spoke to at the symposium.

Symmetry and Supersymmetry

The relationships between the particles and interactions within the SM are based on fundamental symmetries – and how said symmetries may be violated. For example, you can rotate a white billiard ball by any angle around any axis running through the center, and things will look the same. However, if you paint a small red dot on it, that symmetry is broken: the ball now has a special axis, that passes through the red dot.

The Higgs field is like that red dot, breaking the symmetry between the electromagnetic and weak forces. However, there’s an additional symmetry, suggested by the laws of relativity. When applied to particle physics, this symmetry is known as supersymmetry (or SUSY, pronounced SOOsee). Unlike the SM, SUSY a set of theories with a variety of predictions, though all contain a plethora of new predicted particles – including dark matter candidates and additional Higgs particles.

While many versions of SUSY exist, many physicists have focused on the ones that are closest to the SM for the sake of introducing the fewest additional particles. The simplest is known as the Minimally Supersymmetric Standard Model (MSSM), which among other things predicts a Higgs boson mass, given certain other experimental inputs. However, experiments at the LHC and other colliders have failed to turn up predicted particles from the MSSM, and a Higgs boson with 126 GeV mass is barely consistent with the model. It’s not dead yet, but the MSSM is harder to take seriously now.

On the other hand, as Ramond noted, “There’s nothing special about the MSSM – it’s just good engineering.” That’s in the sense that leverages the successful features of the SM, adding as little as possible, but that doesn’t necessarily imply it’s the best modification. For example, the Next-to-Minimal Supersymmetric Standard Model (NMSSM) doesn’t have the same problems, though it suffers from an even clumsier name; though it isn’t quite as simple as MSSM, it allows a Higgs boson mass of 126 GeV. Ramond punned on a 1931 Cab Calloway song, “It Looks Like Susie”: it looks like SUSY, it must be SUSY, I’m sure it’s SUSY … but I don’t know.

Farther afield

Some graduate students I spoke to after Ramond’s talk were more pessimistic. (They spoke to me on condition of anonymity, since their professors control their stipends.) SUSY may or may not still be viable – not least since it comes in many flavors – but the LHC’s Higgs boson certainly doesn’t look SUSY-ish. Certainly rumors that the LHC had found a second Higgs particle, which might have helped SUSY’s case, weren’t based on good evidence.

Fermions can combine to make bosons, which is the case for particles known as mesons. In particular, the pions are bosons made of two quarks, and serve as messengers for the force binding protons and neutrons into atomic nuclei. Perhaps the Higgs boson behaves the same way: rather than being a fundamental particle, as predicted in the SM and SUSY, it’s a composite of unknown components.

If that’s the case, then it should decay at least sometimes into something new – particles that haven’t shown up in detectors. Despite the agreement with the SM, there’s still room for that possibility, since the LHC detectors aren’t as sensitive to certain decay modes involving fermions. The hope is small, but it’s hope nevertheless.

But what if the next round of experiments continues to confirm the Standard-Model nature of the Higgs? That doesn’t sound the death-knell for SUSY or its competitors, but it’s still a problem. Hope for a non-SM Higgs is hope for an entry into the new territory where we may possibly begin understanding dark matter or why particle masses are ordered the way they are; it might even lead to discovering new fundamental forces. A SM Higgs, however, leaves us searching for new physics elsewhere – possibly revealing a vast desert between the physics we know, and the physics we know exists but haven’t reached yet.


5 Responses to “Our Higgs”

  1. 1 Kudzu July 3, 2013 at 16:35

    I would note that as far as I am aware the weak field can also produce electrically neutral bosons and we don’t see a weak force. The Higgs field has always seemed weak to me in that respect at least.

    • 2 Matthew R. Francis July 3, 2013 at 17:29

      Are you talking about the Z boson? That’s one of the three carriers of the weak force (along with the two charged W bosons). The Higgs mechanism explains why those bosons are massive, while the other gauge bosons (photons and gluons) are massless.

      • 3 Kudzu July 4, 2013 at 00:32

        Yes, the Z boson. The mass of a force’s particle determines the range of the force (to some extent, it’s a good thing the strong force contains itself I think.) According to Matt Strassler the Higgs and Weak fields do exert forces but they are very short range due to the massiveness of the particles involved.

  2. 4 aa. sh. July 4, 2013 at 06:54

    Now the supreme question :
    Do our little spot of known physics in the vast ocean of complete physics give us the ” feeling ” that a supreme set of rules , principles and laws are a necessary obligatory fact ?

  1. 1 The 2013 Nobel Prize in physics: the Higgs boson | Galileo's Pendulum Trackback on October 8, 2013 at 19:55
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