Higgs Boson for Dummies

Transcription

1 Faculty of Education, University of Ljubljana 1st South-Eastern European Meeting on Physics Education 2012 September 11, 2012

2 For dummies? I am not supposed to start from the Higgs Lagrangian which I would not do anyway.

3 What is the Higgs boson? A particle that gives mass to elementary particles of the Standard model an idea proposed by Peter Higgs in Paper rejected in Phys. Lett. "of no obvious relevance to physics" Heisenberg: "You do not understand the rules of physics."

4 Questions: What is mass?

5 Questions: What is mass? What are the elementary particles?

6 Questions: What is mass? What are the elementary particles? What is the mechanism that generates mass of particles?

7 Questions: What is mass? What are the elementary particles? What is the mechanism that generates mass of particles? How (new) particles are discovered?

8 Questions: What is mass? What are the elementary particles? What is the mechanism that generates mass of particles? How (new) particles are discovered? How do we know that the particle discovered at CERN is indeed the Higgs boson?

9 What is mass? One of the basic physical quantity; related to two dierent concepts: Inertial mass m a F F = m a a is the acceleration of the body

10 What is mass? One of the basic physical quantity; related to two dierent concepts: Inertial mass m a F F = m a a is the acceleration of the body Gravitational mass m weight: F = m g F g is the strength of the gravitational eld

11 Relativity Equivalence principle: both masses are equivalent

12 Relativity: mass and energy Mass and energy are equivalent: E = mc 2, c = m/s. A body at rest has energy due to its (rest) mass; also, a moving body acquires larger mass compared to its rest mass m 0 : m = m 0. 1 v2 c 2 The Higgs boson provides nonzero rest mass (m 0 )

13 Relativity: light bending The photon (light) has nonzero mass due to its energy; it is deected in the gravitational eld, e.g. of the Sun:

14 Massless particles Some particles, e.g. the photon (light), the neutrino ν... travel with the speed of light c. m 0 = m 1 v2 c 2 = 0, if v = c. Hence Particles that travel with the speed of light have zero rest mass.

15 Massless particles Some particles, e.g. the photon (light), the neutrino ν... travel with the speed of light c. m 0 = m 1 v2 c 2 = 0, if v = c. Hence Particles that travel with the speed of light have zero rest mass. Vice versa: Massless particles cannot rest; they always travel with the speed of light.

16 Massless particles Some particles, e.g. the photon (light), the neutrino ν... travel with the speed of light c. m 0 = m 1 v2 c 2 = 0, if v = c. Hence Particles that travel with the speed of light have zero rest mass. Vice versa: Massless particles cannot rest; they always travel with the speed of light. Strange behaviour? Not at all; according to Higgs, there is nothing wrong with massless particles; what is strange is that the "normal" bodies rest or travel with the speed less than the speed of light.

17 Microscopic world Matter consists of building blocks m body N p m proton + N n m neutron, m proton m neutron, m electrons m proton

18 Elementary particles But: m proton = m u quark + m u quark + m d quark. In fact: m u quark m d quark m electrons m proton. m proton W kinetic c 2.

19 Standard model (Periodic table of elementary particles) m c quark m proton m b quark 4 m proton m t quark 180 m proton m τlepton 4000 m electrons 2 m proton

20 Standard model (Periodic table of elementary particles) m c quark m proton m b quark 4 m proton m t quark 180 m proton m τlepton 4000 m electrons 2 m proton Are heavy quarks and leptons composite particles? No, there is no evidence whatsoever for their internal structure.

21 Standard model (Periodic table of elementary particles) m c quark m proton m b quark 4 m proton m t quark 180 m proton m τlepton 4000 m electrons 2 m proton Are heavy quarks and leptons composite particles? No, there is no evidence whatsoever for their internal structure. How do we then explain their large masses? Answer: the Higgs mechanism

22 What is the role of bosons? Classical explanation of long range forces (e.g. electro-magnetic force): A charged particle creates a eld in the surrounding space. Quantum explanation (Feynman): a charged particle emits a photon (a boson, in general) and a second particle absorbs it.

23 Elementary interactions gluon is the exchange boson of the strong interaction between quarks d weak bosons (W ±, W 0, Z) carry the weak interaction g u Two complementary interpretation: Bosons are elementary excitations of the eld; on the other hand, the eld is a condensate of bosons, e.g. the electron is surrounded by a cloud of the so called virtual photons.

24 Higgs eld Higgs assumed the existence of a new eld, the Higgs eld, that lls all of space and has no external source. The Higgs boson is an elementary excitation of the eld. The source of the Higgs eld is the Higgs eld itself. In the alternative picture, the Higgs bosons in the condensate attract each other. The resulting potential energy of the system has its minimum at a non-zero value of the eld.

25 Mass generation All elementary particles are massless and therefore move with the speed of light. But most of them bounce o the Higgs bosons in the vacuum and hence eectively move with a nite velocity. Their kinetic energy is transformed into the rest energy (mass). Some particles including the Higgs boson itself interact more frequently than the others; it means they are more massive. Photons, gluons, neutrinos do not interact at all; they are massless more precisely, their rest energy is zero.

26 Production of the Higgs boson In order to observe a free Higgs boson, a huge amount of energy has to be transferred to the vacuum (i.e to the Higgs eld). A particle in quantum mechanics is described as a wave with frequency ν = E/h (h is the Planck constant). The largest probability to excite an oscillation is at the resonance i.e. when the transferred energy is equal to the energy (mass) of the particle.

27 Discovery of new boson At the proton collider at CERN two proton traveling in opposite direction collide and produce a shower of particles, mostly quark-antiquark pairs, which in turn annihilates and produce long lived particle that are nally detected and analyzed by two independent experimental groups.

28 Two "cleanest" events

29 Conclusion So far the observations are consistent with the observed particle being the Standard Model Higgs boson. The particle decays into at least some of the predicted channels. Moreover, the production rates and branching ratios for the observed channels match the predictions by the Standard Model within the experimental uncertainties. However, the experimental uncertainties currently still leave room for alternative explanations. It is therefore too early to conclude that the found particle is indeed the Standard Model Higgs. [PDGLive. Particle Data Group. 12 July 2012.]

30 Spontaneous breaking of chiral symmetry The reason for introducing the Higgs eld actually lies in the observation that the equations of motion preserve the chiral symmetry while in nature this symmetry is violated. The symmetry requires that the helicity, i.e. the projection of particle spin onto the direction of motion, is a good quantum number, and the elementary particles are supposed to be either left- or right-handed. This solution is however not realized in nature: if one observer sees a right handed electron then for another observer, moving with the velocity greater than the electron velocity in the same direction, the electron has opposite helicity. The helicity is preserved only for massless particles moving with the speed of light; massive particles violate the symmetry.

31 Spontaneous breaking of chiral symmetry The situation in which the underlying laws are invariant under some symmetry while the solution is not is called "spontaneous symmetry breaking" and the Higgs mechanism is a model that describes such breaking.