268 NATURE PHYSICS | VOL 9 | MAY 2013 | www.nature.com/naturephysics

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In July 2012, CERN announced the discovery of a new elementary boson1,2, made by the two experimental

collaborations, ATLAS and CMS, at the Large Hadron Collider (LHC). Given the rather tenuous nature of the discovery, however, they were hesitant to pronounce it the long-sought Higgs boson, referring to it instead as a “Higgs-like particle”. Since then, the LHC has more than doubled the amount of data delivered to the experiments, and in March new results were presented at the annual ‘Moriond’ conference3, held at La Thuile in Italy. The time has come to say that we have indeed found maybe not the, but definitely a, Higgs boson.

So what makes a boson a ‘Higgs’ boson? Bosons have integer spin, whereas fermions have half-integer spin. In the past century, we discovered exactly three elementary bosons, all of which have one unit of spin: the photon, and the W and the Z, which are the carriers of the electromagnetic and weak forces, respectively. The quantum field theory associated with the photon is a vector field: an electric charge placed in the field gains potential energy and then, as it is accelerated, a momentum in a specific direction.

A Higgs boson is a special boson. It was proposed in 1964 to explain the breaking of the electroweak gauge symmetry, in particular the postulated existence of the massive W boson (the W and the Z were confirmed in experiments in the 1980s). The Higgs field, in its ground state, permeates the entire Universe. Such a symmetry-breaking boson field must necessarily be scalar, and thus the boson itself spin-0: if it were spin-1, like the other known bosons, particles would be accelerated in the ground state, violating momentum conservation. Embedded in the Higgs field, an elementary particle takes on mass; the stronger the particle’s interaction with the Higgs field, the larger its mass. And because the Higgs field is necessarily everywhere all the time, this mass becomes a fixed property of the particle.

Peter Higgs proposed the simplest extension of the standard model to include a Higgs boson (although he himself did not give the particle his name). In this version, there is one Higgs boson and its couplings

to the other particles have fixed values. In contrast, if supersymmetry is introduced — doubling the list of elementary particles by adding a super-partner for each particle in the standard model — the simplest model includes five different Higgs bosons. The couplings are also proportional to mass, but further parameters come into play, altering some standard-model relations.

When protons collide in the LHC, they couple to the Higgs field, excite it and produce Higgs bosons, at a rate that depends on how the Higgs couples to the quarks and gluons that exist inside a proton. The dominant process is shown in the Feynman diagram of Fig. 1a: two gluons couple to virtual quarks, and these fuse to produce the Higgs (so even though the gluons are massless, they can couple indirectly to the Higgs). The Higgs then decays into lighter particles, for which various modes are

possible. The ‘Higgs-like’ particle found last summer, having a mass of 125 GeV, can in principle decay into pairs of W bosons, or Z bosons, or pairs of fermions such as tau leptons or bottom quarks; but not top quarks, as these are too heavy. The other quarks (up, down, strange and charm) and leptons (muon and electron) are light and couple only feebly to the standard-model Higgs. Even though the photon is massless, the Higgs decay can still produce photon pairs (Fig. 1b,c) at a meaningful rate.

For the first announcement in July last year, the necessary five-sigma significance to claim a discovery was only achieved by combining the signal picked up in all three bosonic decay modes (WW, ZZ and two-photon). Now, using the increased dataset and improved analyses, the cleanest decay mode alone — the ZZ channel — has a significance of 6.6 sigma (in the data from ATLAS) and 6.7 sigma (from CMS). The other bosonic channels are around 4 sigma (except for the two-photon channel measured by ATLAS). Thus the new data fantastically confirm the discovery of a new particle — next, we need to say something about its properties.

Because it shows up as a resonant bump in the two-photon and ZZ channels, it is clear that the decaying particle has to have integer spin: a half-integer-spin fermion cannot decay to two bosons. Furthermore, a spin-1 particle cannot decay to two photons (according to the Landau–Yang theorem of quantum field theory). Thus the only options are spin-2 and spin-0. However, there is no consistent model for spin-2 that hasn’t already been excluded by other data; furthermore, a spin-2 model analysed by ATLAS is excluded at the 99.3% confidence level by the new data. Thus we are now sure we have observed a spin-0 particle.

Next we must consider the rates of decay in each channel. Overall, in the new data these rates agree very well with the standard model. Last summer, the main curiosity was the abnormally high two-photon rate, seen by both experiments, at the two-sigma level. This anomaly is still there in the ATLAS data, but has vanished in the new data from CMS. We must await further data to settle this,

PARTICLE PHYSICS

A Higgs is a HiggsIn the light of more data, the particle discovered at CERN last year is now confirmed to be a Higgs boson — but what kind of Higgs boson? And what might the discovery mean for theories that reach beyond the standard model?

Herbi Dreiner

a

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Gluon

Gluon

Higgs

Higgs

Higgs

Photon

Photon

Photon

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Figure 1 | Higgs production and decay at the LHC. a, The most probable way to produce a Higgs boson is by the fusion of two gluons from the colliding protons, through a loop of virtual top quarks (t). b,c, In similar fashion, the Higgs boson can decay through virtual W bosons (b) or virtual top quarks (c) to produce a pair of photons, which are picked up by the LHC detectors.

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NATURE PHYSICS | VOL 9 | MAY 2013 | www.nature.com/naturephysics 269

news & views

but given the overall agreement with the standard-model Higgs expectations, I would expect this anomaly to have been just a statistical fluctuation.

Another marked feature of the data, at the time of the update given at CERN in December 2012, was that the ATLAS two-photon and ZZ resonant peaks did not coincide, but sat at slightly different Higgs-mass values (a 2.5-sigma effect). In the new ATLAS data, the discrepancy is slightly reduced. These twin peaks are not seen in the CMS data, and in fact the ATLAS peaks lie either side of the CMS mass value. Again we must wait for further data, but the experiments should observe identical Higgs properties.

Other than these two slight anomalies in the ATLAS data, all measurements line up exactly with what is expected for a standard-model Higgs boson. Which is exciting — it is a momentous confirmation of a bold prediction made almost 50 years ago. This is the first elementary scalar particle ever observed, and evidence of a profound new mechanism for generating mass.

However, we have also been looking for signs of physics that is beyond the standard model, to answer some of the many remaining questions in particle physics. A good example of such physics is supersymmetry, so what do we learn

about it from the new data? Back in July 2012, with the first announcement of a 125-GeV Higgs-like particle and strict lower bounds from the experiments on the masses of predicted super-partner particles, supersymmetry looked to be in a tight corner.

Yet, if supersymmetry did exist in nature, and the signal at 125 GeV was due to the lightest of the five Higgs bosons of supersymmetry, then the 125-GeV boson we see would be expected to be very standard-model-like, having much the same decays as a lone standard-model Higgs would. In fact, now that we are observing all of these decays to be in such good agreement with the standard model, this actually improves the overall fit of supersymmetry to all of the data. This is, of course, far from a proof of supersymmetry — but the theory is also far from dead.

The LHC has now closed down for two years of repairs and upgrades, although this summer we can expect further updates on the Higgs analyses based on the data that have already been collected, in particular on the fermionic decay channels. CMS has observed the Higgs decay to two tau leptons at the 2.9-sigma level, ATLAS has yet to report its data on this channel. Both experiments should announce results for the Higgs decay to

two bottom quarks. These are crucial tests of the Higgs mechanism, as the couplings to fermions are fixed by the known masses of the tau and the bottom quark. If not this summer, then soon after we should also get results from combining the data from both experiments, effectively doubling the data set.

Two years from now, the LHC will run at higher energy and luminosity, as discussed at an open symposium4 organized by the CERN Council, in Kraków, Poland in September 2012. The increase in energy will extend the reach of the detectors to higher supersymmetric masses. For the Higgs, the signal-to-background ratio will improve significantly, leading to higher-quality measurements. We can expect to measure the couplings of the Higgs boson to other particles down to about the 5% level — a thorough testing of the standard model, and of what might lie beyond. ❐

Herbi Dreiner is at the Physikalische Institut, University of Bonn, D-53115 Bonn, Germany. e-mail: dreiner@uni-bonn.de

References1. http://www.atlas.ch/news/2012/

latest-results-from-higgs-search.html2. http://cms.web.cern.ch/news/

observation-new-particle-mass-125-gev3. http://moriond.in2p3.fr/4. http://espp2012.ifj.edu.pl/

Electron spin resonance1 (ESR), which is based on the resonant absorption of electromagnetic energy by an

electron spin, has diverse applications in chemistry, biology, medicine and physics. The technique also plays an essential role in spin-based quantum information processing as it enables the coherent manipulation of electron spins2,3. However, ESR is not easily scalable: it requires two perpendicular magnetic-field components: one static and one alternating. The static field is usually generated by a large magnet, whereas the alternating field is produced by a microwave generator3. The physical size of these devices makes it non-trivial to address individual spins packed in a small space. Efforts

have been made to eliminate the need for an oscillating magnetic field (ref. 4, for example) but these alternative techniques still require a static magnetic field. Writing in Nature Physics, Haruki Sanada and colleagues5 use a clever trick to demonstrate ESR in a semiconductor without applying any external magnetic field.

For ESR spin manipulation, a static field B0 is applied parallel to one of the two basis states of the electron spin operator — conventionally referred to as spin-up or spin-down — along the z axis. The spin precesses around the z axis at the Larmor frequency. In the rotating frame of this precession, a spin state can be represented by a static point on the surface of the

so-called Bloch sphere. An alternating magnetic field B1 applied perpendicular to B0 with a frequency matching the Larmor precession can rotate the spin around an axis perpendicular to the z axis. Then, by controlling the duration of rotation, the spin state can be rotated to any point on the Bloch sphere.

Is it possible to generate these magnetic-field components without an external magnetic-field source? It is known that the spin–orbit interaction — a relativistic effect between a moving charge and an electric field — can provide such an effective magnetic field. In certain materials, the symmetry of the crystal structure leads to an intrinsic spin–orbit interaction such

ELECTRON SPIN

The long and winding roadIn electron spin resonance techniques, spins are usually manipulated by applying external magnetic fields, but the same effect can be obtained by guiding electrons along a meandering path using surface acoustic waves.

Masaya Kataoka

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