The 2013 Nobel Prize in Physics

Hans Peter Beck, Uni Bern

 

Award

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2013 to François Englert and Peter W. Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider".

 

 

Figure 1: The Higgs potential resembles a Mexican hat.

The Brout-Englert-Higgs mechanism

Back in 1964, Robert Brout together with François Englert and, independently, Peter Higgs proposed a solution to a problem that has deeply plagued the understanding of weak and electromagnetic interactions. If elementary particles, the fundamental building blocks of all matter in the universe shall be described by quantum field theory obeying basic gauge symmetries, then all gauge bosons and also all fermions necessarily have to be massless. Evidently this is in clear violation to the observed mass spectrum of particles. The solution proposed by Brout, Englert and Higgs was to break these underlying basic symmetries in a specific way, such that the initial state of the universe obeys to such symmetries but quickly finds its ground state, away from its initial symmetric state [1-3]. This is similar to a pen standing on its tip, which presents a fully symmetric initial state under rotations around the pen’s vertical axis. However, such a carefully prepared pen falls quickly in an arbitrary direction and consequently will find itself lying flat on the ground pointing to a now specific, but otherwise fully arbitrary direction, evidently breaking the initial rotational symmetry. Translating back to quantum field theory, the role of the standing pen is taken up by a two component complex scalar field, the Higgs field φ, postulated such to pervade throughout the universe. Further, this field φ needs to live in a potential with the potential well away from its initial state. A potential with the shape of a Mexican hat has exactly such properties as is illustrated in Fig 1.

A two component complex scalar field has four free parameters, as is easily recognized when adding up real and imaginary parts for both components. These correspond to four degrees of freedom that are added to the theory. When the field φ finds its ground state, three of these four free degrees freeze out and become the transverse polarization modes of the two oppositely charged W-bosons and the Z-boson. The mere existence of transverse polarization of a spin-1 vector particle is equivalent for it having gained mass, as Lorentz invariance for mass-less particles prohibits otherwise. One degree of freedom stays, which gives rise to an excitation mode of the field φ above its ground state; this excitation is exactly what yields the Higgs boson. Brout, Englert and Higgs have been first to formulate ideas on how to allow for massive gauge particles in quantum field theories. More effort by people like Gerald Guralnik, Carl Hagen and Tom Kibble was needed to understand the properties of this mass generating mechanism [3]. Only after the ground-breaking work of Sheldon Glashow, Abdus Salam, Steven Weinberg [4-6], and of Gerard ‘t Hooft with Martinus Veltmann [7], symmetry breaking in a unified electro-weak theory could finally be settled. Electro-weak unification with a broken symmetry became a fundamental corner stone of the now very well established Standard Model of particle physics, with the other being quantum chromodynamics, the theory of strong interactions with unbroken symmetry. Unfortunately, Robert Brout passed away on May 3rd, 2011 and therefore could no longer be recognized by the Royal Swedish Academy who awarded two of the three discoverers of the Brout-Englert-Higgs (BEH) mechanism: François Englert and Peter Higgs and formally recognized the ATLAS and CMS experiments at CERN’s Large Hadron Collider, who have confirmed the discovery of a new boson with properties consistent with those of the Standard Model Higgs boson on July 4th, 2012, see Fig 2.

 

Figure 2: François Englert, Peter Higgs and CERNs Rolf Heuer at the announcement of the discovery of a new boson in a seminar at CERN on 4th July 2012. Picture © CERN.

Higgs Hunting

The BEH mass-generating mechanism allows for massive gauge bosons, today recognized as the W and Z bosons, the carriers of electro-weak interactions. Fermions also gain their mass via interacting with the all-pervading Higgs field φ, with massive fermions interacting more strongly with φ than less massive ones. In consequence, the interaction strengths of fermions with the Higgs boson H turns out to be proportional to their measured mass. Mass is thus a dynamic property of elementary particles due to their interaction with the Higgs field φ. Mass is truly distinct from other intrinsic properties of particles like charge or spin. This is also true for the Higgs boson itself, which owes its mass to its own interaction with the Higgs field.

The so completed Standard Model predicts all properties of the Higgs boson, except its mass, which however is bound from above and below from theoretical considerations. These are based on the simple fact that the potential well of the Mexican hat shaped potential needs to be finite and has to have its lowest value smaller than its value at its origin. Hunting the Higgs became a search over a wide range of possible Higgs masses between few tens of GeV to few hundreds of GeV. The dominant production and decay modes of the Higgs boson vary widely with the assumed Higgs mass, which became a free parameter throughout the searches. The search strategies have been tailored specifically for low, medium, and high Higgs masses. Direct searches of the Higgs boson and precision measurements of the W boson and top quark masses, which relate to the Higgs mass via quantum loop corrections, at the SPS, HERA, LEP and Tevatron accelerators were able to narrow down the still allowed Higgs range. These results were important ingredients in the design and building of the LHC, to definitely find the Higgs boson or to exclude it for good. An experimental confirmation of the non-existence of the Higgs boson would have been an equally fundamental result as its discovery. The all-pervading Higgs field would have been puffed away as the lumiferous ether was puffed away after the precision measurements of Michelson and Morley concluded in the ending 19th century giving way for a new relativistic interpretation of space and time. The Higgs boson, however, exists, and with it the Higgs field φ, pervading space throughout the universe.

 

The Higgs discovery

The Large Hadron Collider, the 27 km circumference underground particle accelerator at CERN, straddling the Franco-Swiss border between the Jura-mountains and lake Geneva, collided protons on protons head on over the last three years at an initial center of mass energy of 7 TeV until the end of 2011 and of 8 TeV from April 2012 onwards. The ATLAS and CMS experiments, located at opposite sites of the LHC, were able to individually measure and decide online on every of the almost two quadrillion collisions (25 fb-1 × 70 mb = 1.75 × 1015 minimum bias collisions) offered to each of them whether the collision is worth while for further offline analysis. Those collisions that were retained underwent careful reconstruction and analysis to subsequently decide whether it contains a viable Higgs candidate. Less than a million Higgs bosons are expected of being produced in all these collisions, but only a few thousand of them decay in such a way to make a distinct feature in the experiments. There are three prominent decay channels for Higgs searches, where a Higgs decays either to a pair of high energetic photons, to a pair of Z bosons that further decay to two charged lepton pairs, and finally to a pair of W bosons that further decay each to a charged lepton and an escaping neutrino. All other decay channels of the Higgs are either too rare to be useful for discovery or too difficult to extract a non-ambiguous signal from the overwhelming background collisions at the LHC. ATLAS and CMS found clear evidence for a new boson at a mass of 125-126 GeV [9-10] that is produced in proton-proton collisions at the LHC and that decays into the various channels under consideration at about the correct rates. Figure 3 shows a Higgs candidate event, where the Higgs decays via two Z bosons into one pair of oppositely charged electrons and a second pair of oppositely charged muons. Figure 4 shows a Higgs candidate event, where the Higgs decays via an internal loop involving W-bosons and top quarks to two photons. Spin and parity as well as the coupling strengths of this new boson to W- and Z-bosons as well as to fermions have been inferred by both experiments and these agree with the for a Standard Model Higgs boson predicted values, within the today achievable precision [11-13]. The Swiss participation in the ATLAS and CMS experiments are with the Universities of Berne and Geneva participating in ATLAS, and the University of Zürich, ETH Zürich and PSI participating in CMS.

 

Figure 3: Event display of a H → ZZ* → 2e2µ candidate event measured with the ATLAS detector. Muon tracks are colored red, electron tracks and clusters in the liquid argon calorimeter are colored green. The larger inset shows a zoom into the tracking detector. The smaller inset shows a zoom into the vertex region, indicating that the four leptons originate from the same primary vertex. Picture © CERN.
Figure 4: A typical candidate event including two high-energy photons whose energy (depicted by red towers) is measured in the CMS electromagnetic calorimeter. The yellow lines are the measured tracks of other particles produced in the collision. The pale blue volume shows the CMS crystal calorimeter barrel. Picture © CERN.

Standard Model or Beyond Standard Model?

All experimental results as of today indicate that indeed a Higgs boson has been found, which was reason enough for the Royal Swedish Academy of Sciences to award the 2013 Nobel Prize to Englert and Higgs. Whether the Standard Model is indeed the final answer to the mass generating mechanism or whether new physics has been found with the discovery of this new boson is still open to further precision measurements at the LHC. The actual planning for running the LHC foresees providing collisions to all its experiments over the next one and a half decades, increasing in both center of mass energy and collision rate allowing for a rich and far reaching physics programme. Today, about 1% of the prospected data volume has been gathered and that already gave experimental proof of the existence of a Higgs boson and therefore of an all-pervading Higgs field φ. The remaining 99% of data foreseen to be collected at the LHC will allow measuring the properties of this new boson with high precision, which will establish whether this new boson is indeed the last missing piece of the Standard Model or whether it is a door opener for new physics territory beyond the Standard Model, which could help answering open questions about the nature of Dark Matter or other ingredients of the universe.

 

References

[1]Broken Symmetry and the Mass of Gauge Vector Mesons
François Englert and Robert Brout
Phys. Rev. Lett. 13, 321-323 (1964)
Received 26 June 1964; published in the issue dated 31 August 1964
[2]Broken Symmetries, Massless Particles and Gauge Fields
Peter W. Higgs
Phys. Lett. 12, 132-133 (1964)
Received 27 July 1964; published in the issue dated 15 Sept. 1964
[3]Broken Symmetries and the Masses of Gauge Bosons
Peter W. Higgs
Phys. Rev. Lett. 13, 508-509 (1964)
Received 31 August 1964; published in the issue dated 19 October 1964
[4]Global Conservation Laws and Massless Particles
Gerald S. Guralnik, Carl R. Hagen and Thomas W. Kibble
Phys. Rev. Lett. 13, 585-587 (1964)
Received 12 October 1964; published in the issue dated 16 November 1964
[5]Partial-symmetries of weak interactions
Sheldon L. Glashow
Nucl. Phys. 22, 579–588 (1961)
Received 9 September 1960; published in the issue dated February 1961
[6]Electromagnetic and weak interactions
Abdus Salam and John C. Ward
Phys. Lett. 13, 168–171 (1964)
Received 24 September 1964; published in the issue dated 15 November 1964
[7]A Model of Leptons
Steven Weinberg
Phys. Rev. Lett. 19, 1264–1266 (1967)
Received 17 October 1967; published in the issue dated 20 November 1967
[8]Renormalizable Lagrangians for massive Yang-Mills fields
Gerard ’t Hooft
Nucl. Phys. B 35, 167–188 (1971)
Received 13 July 1971; published in the issue dated 1 December 1971
[9]Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC
ATLAS Collaboration
Phys. Lett. B 716, 1-29 (2012)
Received 31 July 2012; published in the issue dated 17 September 2012
[10]Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC
CMS Collaboration
Phys. Lett. B 716, 30-61 (2012)
Received 31 July 2012; published in the issue dated 17 September 2012
[11]Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC
ATLAS Collaboration
Phys. Lett. B 726, 88-119 (2013)
Received 4 July 2013; published in the issue dated 7 October 2013
[12]Evidence for the spin-0 nature of the Higgs boson using ATLAS data
ATLAS Collaboration
Phys. Lett. B 726, 120-144 (2013)
Received 4 July 2013; published in the issue dated 7 October 2013
[13]Observation of a new boson with mass near 125 GeV in pp collisions at √s=7 and 8 TeV
CMS Collaboration
JHEP 06 (2013) 081
Received 19 March 2013; published in the issue dated 20 June 2013

 

 

[Released: November 2013]