Theoretical particle physics

Theoretical particle physics

Represented by Theory group:

Faculty : De-Chang Dai, Xiao-Gang He, Xiangdong Ji
Postdoc:, Wei Chao, Jianhui Zhang
Student : Gang Guo, Guan-nan Li, Bo Ren, Dong Xu
Graduated Ph.D students: Bo Ren (Shaoxing University)
Former postdoc: Yi Cai (University of Melbourne, Australia)
Where is the nearest accelerator in your place?
In Chicago (Tevatron )? In Switzerland(LHC)?
You may be right, but the most likely answer is the cloud over your head. The electric charges in the cloud generate a strong electric field. Once the electric field is strong enough (2 to 3 MeV/m), the electrons are discharged from one of the cloud and start a complicated collision process. As one may notice the bright light and then the thunder. Surprisingly, lightning generates also positrons. A positron is an anti-particle of an electron. It is the first antiparticle made by mankind. It had never been found or recognized before Pauli Dirac predicted it in 1928. There are many natural accelerators, like quasar. To study the physics at man-made accelerators will help to understand the nature.

Fig. 1: Lightning: The nature accelerator. The photo is made by Sebastien D'ARCO and Koba-chan at Toulouse (France)

What is studied in particle physics( the Standard Model)

Molecules and atoms are the basic elements of familiar substances that we can see and feel. We see a water drop and we know it is made of water molecules. Even though a molecule or an atom is very small, they still can be seen under a transmission electron microscope (TEM). Molecules are made of protons, neutrons and electrons. In 1900's, these particles were still considered to be fundamental particles. It remained to be true until the invention of accelerators. It is surprising that a host of new particles have been generated by these accelerators. By 1960, hundreds of new particles have been found and a new question appears – could these particles be fundamental particles? Through a series of studies, the answer finally has been found. There exist two kinds of fermion particles, leptons and quarks, and a set of forces that allow fermion particles to interact with each other. To be precise the “forces” are being transmitted through exchanging gauge bosons. The combination of these particles form protons, neutrons and other particles. Today the standard model is the theory that describes the particles and the interactions between them.

Fig. 2: Everything is made of elementary particles. These include leptons and quarks.

Mission Accomplished

Although the Standard model is proposed to describe all particle phenomena, not all the particles had been found while it was proposed. The most famous and probably the most important missing particle is the Higgs boson. This particle gives the other particles mass. Since it is very important, the media start to call it God particle after the title of Leon Lederman's book. Before 2011, there are no hints of its existence. Some Higgsless alternative models have been proposed for its absence. However some hints start to appear in 2011 and finally CERN announces the evidence of discovering a boson with properties consistent with the expected Higgs. If the discovery is confirmed in the future, we finally have a complete standard model.

Fig. 3: CERN reports that Higgs particles probably have been found.

Search for New Physics

While someone is happy to celebrate the accomplishment, someone is still looking for the new physics. The new physics includes phenomena that have not been understood and phenomena that have not been seen. The new physics can be found through three methods. The first one is from collision events in an accelerator. This method gives a direct evidence of the unknown physics. The second one is from underground Lab. The cosmic ray includes all kinds of possible particles in the universe. If there are new particles, the cosmic ray must have them. Unfortunately, we have not found it. This could be that there is no new particle, the interaction is too weak, or the noise is too high. If the noise is too high, then a shield is needed. Hence, most labs have been built underground to suppress the noise. If the interaction is too weak, then more targets are needed. That is why larger detectors are built. The last one is through satellites. If the process of new physics is a rare event on the earth, we should look at other locations to find them. The first hint of existence of dark matter is found in this way.

Fig. 4: There are three methods to find the hint of new physics.

Dark matter

In 1932, Jan Oort studied stellar motion in the local galactic neighborhood and found the mass of the galaxy must be more than the visible material. It is called the “missing mass problem” at that time. This is the first evidence of the existence of dark matter. About 40 years later, Vera Rubin et al. studied the galactic rotating curve and found the strong evidence of invisible mass. The finding shows either there is an invisible matter or the Newtonian gravity (or General Relativity) is wrong. Today this problem has not been fully settled down. However, the bullet cluster collision provides an almost direct evidence of dark matter. Most of the modified gravity theories can not have this feature. To further understand the nature of dark matter, it is important if the dark matter can be detected by earth bond experiment. There are a lot of efforts on this front. At SJTU, a dedicated dark matter search experiment, PANDAX is under way. Our theory group is also involved in this effort. In fact professor Xiangdong Ji is leading this experiment, although he has been a theorist.

If the dark matter real exists, then particle physics should be able to describe the phenomena. However, the standard model is not able to do so, because there is no candidate in the standard model which has the same characteristics as the dark matter has. Therefore, a theory beyond the standard model must be considered.


Fig. 5: The bullet cluster: Bullet cluster is indeed two clusters. They collide and pass through each other. The baryon matter interacts with each other and leaves a bullet-like shock wave. The dark matter go through each other without slowing down.

Model Building

Even though there are so many indirect evidences of the existence of dark matter, a direct dark matter survey is still needed to confirm and settle down the problem. However, because of the absence of the knowledge of dark matter's physical characteristic, we need possible candidate to guide experiments to find the particles. An specialist in model building will be very helpful in the survey. Professor Xiao-Gang He is an expert on the field.
Dark matter can decay to regular matter through interaction in most of models. The annihilation of dark matter makes it possible to detect dark matter. The annihilation also change the universe evolution. For example it is proposed that in the early universe the dark matter density may high enough and act as the first energy source before regular matter starting the nuclear reaction.

Fig. 6: A dark star: The heat of the star is generated by dark matter annihilation.

Quantum Chromodynamics

One of the cornerstones of the standard model of particle physics is Quantum chromodynamics (QCD). It describes the strong interaction that binds quarks and gluons together to form hadrons. QCD has many interesting properties that are not present in quantum electrodynamics, among which a crucial one is the asymptotic freedom. At short distance scales the interaction between quarks and gluons becomes weak, and quarks and gluons behave as asymptotically free particles; while at long distance scales the interaction becomes strong, leading to quark confinement in hadrons. The confinement makes the comparison of QCD predictions with experimental measurements at colliders very complicated, since it is hadrons, not free quarks and gluons that are involved in the collision. Unravelling the structure of hadrons is a central problem of QCD. Many theoretical and experimental efforts have been devoted to exploring the quark and gluon (or parton) distribution functions in hadrons. A related problem is the spin structure of nucleons, namely how the contribution to the nucleon spin is distributed between quarks and gluons inside the nucleon. Prof. Xiangdong Ji is an expert on these fields.

Fig. 3: QCD .

CP violation and Flavor physics

Apart from finding new particles, ordinary matter still has some special phenomena that have not been well understood. Two of them are CP violation and Flavor Physics. CP is a product of two symmetries. C is for Charge conjugation, which transforms a particle into its anti-particle. P is for parity, which creates a mirror image of a physics system. The idea of parity symmetry is that the equations of particle physics are invariant under mirror inversion. Therefore, the mirror image of a reaction should be at the same rate as its original one. This intuitive idea turns out to be invalid. In 1956, Tsung-Dao Lee and Chen-Ning Yang proposed several experimental tests on parity symmetry. In the same year, Chien-Shiung Wu carried out the experiment and demonstrated that the weak interaction violate the Parity symmetry. Since the parity symmetry is violated, Lev Landau proposed CP - symmetry as the true symmetry between matter and antimatter. In 1964, James Cronin and Val Fitch discovered CP-violation in the decays of neutral kaons. Today, to understand CP-violation becomes very important in cosmology, because it may explain why the matter dominates over antimatter in the present universe.

As we already mentioned, particles are divided into two kinds of particles, quarks and leptons, in the standard model. Each kinds include six flavors of particles. Flavor physics studies how different flavor particles exchanging. These include the quark decays and the neutrino oscillations. In general flavor exchange involves weak interaction. However, the neutrino oscillation does not involve weak interaction. Its origin may or may not come from the mass eigen states being different from the weak eigen states. Although this is the preferred explanation, the full picture has not been settled down yet. The neutrino oscillation has three mixing angles. The last mixing angle has been measured by Daya bay experiment. Professor Jiang-Lai Liu is one of the leading researcher of the experiment. Professor Xiao-Gang He and professor Xiangdong Ji have several studies on the field.
Fig. 7: Daya bay neutrino oscillation experiment
Theory beyond the standard model

Since there are many phenomena that the Standard Model can not explain, various extensions of the Standard Model have been developed. These models are called “theories beyond the standard model”. The most famous one is probably the Supersymmetry(SUSY). In SUSY, every particle has its own “superpartner”. For a type of boson, there exists a type of fermion with the same mass and internal quantum numbers and vice versa. Since the number of particle types suddenly becomes twice as many as ordinary particle types, it provides a dark matter candidate naturally. Apart from dark matter candidate, the introduction of SUSY also helps to develop the Grand Unified Theory (GUT), in which at high energy the three gauge interaction of the Standard Model are merged into one single interaction. The merging happens at GUT scale (a few orders below the Planck mass), which is far beyond the reach of any foreseen collision experiment. Although it can not be directly detected, the GUT effect can be detected through proton decay, the properties of the neutrinos, or elementary particle's electric dipole moments. It may explain why the left-handed neutrino is so light. There are many other theories beyond the standard model, like superstring, extradimension, RS-II, technicolor...

Fig. 8 : Every particle has its own partner in SUSY.

New Physics and universe evolution

In general, the introduction of a new model will change the evolution of the universe. For example it may change the Big Bang Nucleosynthesis and primordial element ratio. Apart from the effect from the new models, some effects from old model could be overlooked. For example, the neutron star is considered to be the final stable state before a star becomes a black hole. However, it is proposed that if the pressure inside the star is sufficiently high due to the star's gravity, the individual neutrons break down into their constituent quarks. This star should be called “quark star”, instead of “neutron star”. If the pressure is higher, strange quarks appear, and it should be called “strange star”. If the pressure is even higher, the electroweak phase transition may happen and quarks become neutrinos and energy can be taken away by the neutrinos. Therefore, a condensed star can be a good place to test our theory. Professor De-Chang Dai has some studies on the field.

Fig. 9: The left hand side is a regular neutron star. The quarks are constrained in a neutron. The right hand side is a quark star. The quarks are not constrained in a small region and the whole star becomes a quark sea.

A Cartoon From PHD Comics