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Press release: The Nobel Prize in Physics -

In becoming massive, the W and Z absorb parts of the Higgs field. The remaining Higgs field has quantized vibrations that we call the Higgs boson that are analogous to vibrations on the lake itself. This effect bears close analogy with the theory of superconductivity that we will meet in Unit 8.

Weak Nuclear Force and Standard Model of Particle Physics

In a sense, the photon in that theory picks up a mass in the superconducting material. Not only do the weak force carriers pick up a mass in the Higgs phase, so do the fundamental fermions—quarks and leptons—of the Standard Model. Even the tiny neutrino masses require the Higgs effect in order to exist.

That explains why physicists sometimes claim that the Higgs boson is the origin of mass. However, the vast majority of mass in our world comes from the mass of the proton and neutron, and thus comes from the confinement of the strong interactions. On the other hand, the Higgs mechanism is responsible for the electron's mass, which keeps it from moving at the speed of light and therefore allows atoms to exist. Thus, we can say that the Higgs is the origin of structure. There is one important parameter in the electroweak theory that has yet to be measured, and that is the mass of the Higgs boson.

Throughout the s and onward, a major goal of the experimental particle physics community has been to discover the Higgs boson. The LEP experiments searched for the Higgs to no avail and have put a lower limit on its mass of Giga-electron-volts GeV , or roughly times the mass of the proton. For the Standard Model not to produce nonsense, the Higgs must appear in the theory at energies and therefore at a mass below 1, GeV. However, there have been stronger, more indirect ways to narrow in on the Higgs.

When LEP and other experiments were testing the electroweak theory by making various measurements of the mixing angle, the theory calculations needed to be very precise, and that required the computing of more complicated Feynman diagrams. Some of these diagrams included a virtual Higgs particle, and thus the results of these calculations depend on the existence of the Higgs. Though the effects of virtual Higgs bosons in Feynman diagrams are subtle, the experimental data is precise enough to be sensitive to the mass to the Higgs.

Thus, though never seen, as of , there is a prediction that the Higgs boson mass must be less than roughly times the proton mass. With a successful high-energy run of the Large Hadron Collider, and with the support of a full analysis of data from the Tevatron experiments at Fermilab, we should know a lot about the Higgs boson, whether it exists, and what its mass is by All rights reserved.

Legal Policy. Teacher resources and professional development across the curriculum Teacher professional development and classroom resources across the curriculum. About Us Video Series Prof. The Basic Building Blocks of Matter 2. The Fundamental Interactions 3. Gravity 4. We can then make a model of EWSB along the following lines: Spontaneous breaking of supersymmetry gives mass to some new particles at very short distances, and this in turn gives mass to the supersymmetric partners of SM particles.

Radiative corrections involving those mass terms can then induce a potential energy function that can favor a nonzero vacuum value for the Higgs field. All three fields receive mass from supersymmetry breaking.

Press release: The 1999 Nobel Prize in Physics

Arrange that these mass 2 terms are all positive, approximatey equal, and of TeV size. All three mass 2 terms receive these negative contributions, but the correction to the Higgs mass is largest, because of the factor of 3 from QCD color flowing around the loop. The and mass terms also receive positive corrections from diagrams involving the supersymmetric partner of the gluon. This calculation creates a potential energy function with a negative mass 2 for the. It explains why this scalar field—and no other—obtains a vacuum expectation value.

Ultimately, this model of electroweak symmetry breaking is testable.

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If all of the pieces fit together, we could then claim to understand EWSB at the same level at which we understand the appearance of superconductivity in metals. Supersymmetry produces a calculable theory of the Higgs mass term in the following way: At the level at which the symmetry is exact, this symmetry strongly constrains the Higgs potential.


If the symmetry is then softly broken, new terms appear as calculable radiative corrections. These latter terms give the negative mass 2 that drives electroweak symmetry breaking.

Other types of models can implement this same philosophy in different ways. Another familiar principle that can keep a particle mass at zero is local gauge invariance. A gauge field in 5 dimensions has 5 components,. The last component A 5 transforms as a scalar field in 4 dimensions. The symmetry can be broken by boundary conditions in the 5th dimension.

After appropriate symmetry breaking, the fields will be the gauge fields of the weak interaction , and will combine with another U 1 to provide the weak interaction U 1 gauge field. This group has a subgroup , such that one can interpret one as the weak interaction isospin gauge group. A vecuum expectation value of one of these bosons breaks to a diagonal group.

For bosons, the energy is typically minimized when and maximized when. However, for fermions, it is the reverse: the energy is minimized for. A somewhat formal way to understand this is to recall that the functional integral representation of the thermodynamic partition function for fermions uses fermion fields that are antiperiodic around a compact Euclidean direction. The reason for this is that the phase factor 38 is nonlocal over the 5th dimension.

Quantum fluctuations smaller than the full size of the 5th dimension see only a part of the integral in 38 and cannot distinguish this from a local gauge transformation. Each of these states gives a quadratically divergent contribution to the Higgs boson mass 2.

But, by what seems to be a miracle, the sum of these contributions is finite. By measuring the properties of these particles, we could in principle extract their couplings to the Higgs field and directly verify the cancellation of divergences and the generation of a finite, negative Higgs mass 2. There is a third way to construct a model in which the Higgs boson begins as a massless particle and acquires negative mass 2 by radiative corrections. This symmetry breaking produces Goldstone bosons, one for each broken global symmetry direction.

It is easy to arrange that some of these Goldstone bosons form a multiplet that transforms as under. We can identify this multiplet with the Higgs scalar doublet. We can now add gauge interactions and other weak couplings. These will lead to radiative corrections that will generate a nonzero potential function for the Goldstone boson fields and drive these fields to acquire nonzero vacuum expectation values.

A theory of Goldstone bosons is described by a Lagrangian that is invariant under the original global symmetry. The global symmetry may be nonlinearly realized, but still there are significant constraints that come from this structure. In particular, these models also require new heavy quarks with charge , with vectorlike couplings to the weak interactions.

The cancellation of ultraviolet divergences implies relations between the Higgs couplings of the new heavy quarks and that of the top quark, and these could eventually be tested experimentally The three types of models described in this section illustrate in different ways the possibility of dynamical explanations of the state of spontaneously broken symmetry required for the theory of the weak interactions.

These models are not simple modifications of the SM. They require large numbers of new particles that must eventually be discovered by accelerator experiments at high energies. If the arguments for physics beyond the SM are so compelling, and if the particle spectra expected are so rich, then why haven't we found evidence for these particles? I think that every theorist who puts forward arguments similar to those above is troubled by this question. In all three models above, the new particles introduced to explain the Higgs potential would be expected to have masses at the scale of hundreds of GeV.

An Elementary Primer on Elementary Particles and their Interactions

Nevertheless, the situation is different now than in earlier eras of particle physics. All of the models reviewed in Section 5 have the property that the new particles that they predict have vectorlike coupling to the gauge group and do not rely on the Higgs mechanism to obtain mass. It is likely that any other extension of the SM that we might consider would also have this property.

The known particles of the SM fill complete multiplets, leaving no place there for additional chiral particles. Further, any additional chiral quarks would have a major effect on the couplings of the Higgs boson. For example, if we add a new heavy quark of mass M to QCD and measure its effects at a scale , any new terms are suppressed by. This follows from the fact that the QCD Lagrangian is already the most general renormalizable Lagrangian one can write that contains the known quarks and has the QCD gauge symmetry. The shifts of quark masses and are visible only if we can independently measure these parameters at energies above the heavy quark mass.

If we cannot, we cannot know that these shifts have taken place. Then the only new and observable terms are of order. For a theory with chiral interactions and spontaneously broken symmetry, the situation can be quite different. The strong statement of decoupling requires that the heavy particles are complete gauge multiplets.

A sickness and a cure

At previous stages of our knowledge, our description of particle physics included some members of gauge multiplets but not others, for example, s but not c , b but not t , or the longitudinally polarized W boson but not the Higgs boson. Examples are provided by the c and t contributions to K — and B — mixing, the top quark loop contribution to precision electroweak observables such as , and the top quark loop contribution to the Higgs coupling to two gluons.

But now that the full SM particle content has been discovered, we have returned to the situation in which new particles added to the model should have vectorlike couplings and virtual effects suppressed as. Thus, it is quite plausible that new particles outside the SM might be present in nature but have only minor effects on observations at currently explored energies. When we finally reach the new particles thresholds, we will turn a corner, and a new realm of physics will come into view. Large multiplets of new particles will suddenly appear.

The reality of these particles will become obvious. The most powerful way to search for physics beyond the Standard Model, now more than ever, is to search for new thresholds at the highest energy accelerators. It is exciting that, this year, the LHC will finally be running close to its design energy. Over the next fifteen years, the LHC will open up a territory in which to search for strongly interacting particles about 3 times greater than that currently explored, and a territory for particles with only electroweak interactions—and signatures appropriate to hadron colliders—about 4 times larger than the current one.

Each step to higher energy is now a major technical, social, and political endeavor.