Neutrino Physics
It shocked me when knowing mass eigenstates are not necessarily interactive eigenstates. This results the massive neutrino oscillation, which has been observed by the SuperK and SNO, providing the first firm evidence of beyond the standard model.
Currently neutrino physics has vast experimental progress but still cannot address competely theoretical puzzles from the tiny but non-zero neutrino masses:
What is the origin of neutrino mass? why is it so samll?
Is it Dirac or Majorana-type?
Whether there is a large CP violation in the neutrino sector?
Can neutrinos provide a successful baryogenesis mechanism, and how to test it?
…
These question can be answered by the low-energy experiments, such as $0\nu \beta \beta$ decay, neutrino long and short baseline, neutron-antineutron oscillation, etc. The relatively lower costs means it would be one of the important directions in the future.
Besides, the potential discrepency of W-boson mass from SM also implies the incompleteness of the electroweak theory.
From both theoretical and experimental aspects, neutrino physics is a subject not to be missed. Moreover, I secretly believe it is probably one of the rare projects where theories and experiments parade abreast as of now.
Dark Sector
“What is this unseen flame of darkness whose sparks are the stars?”
I like this verse written by Tagore, and think it as the exact word for the Dark Sector, the unseen world floatting around us with the rare emergence through various (if possible) portals.
The very first evidence of such unseen world is the unmitigated discrepency between the observed and theortically predicted mass distribution of galactic clusters. By approximation, it accounts for 85% of the total mass. Thus, we, poorly consisting of 15%, are the dark side and further the minority for the dark side. Is the detection or awareness to us harder for them? Would the dark world rain? Are they receiving the signals from us and trying to study our world? I ponder them in sonder.
We have, so far, considered many possible candidates as DM, including dark photons, axions, sterile neutrinos, and PBHs. They do not interact via SM gauge bosons. Including them must extend the SM, finally leading us to go beyond it.
Nonetheless, thanks to the common spacetime shared by DM and us, we do not need to go far. One can readily discuss their behavior by QFT, gauge group, and GR.
Surely, we might need some new fundamental rules for exploration. It is an answer awaiting the right questions.
Duality and Entanglement Higher Dimensional Physics
Quantum entanglemtn was initiated by the EPR paradox, developed ably by many others, and now has increasingly become an important branch that seems able to unify many disciplines.
Besides the brandishing Quantum Information or Quantum Computer, I would like to say another gem I savored for a long time, the Reeh-Schlieder theorem, which states one can in principle create any states at any place by some local operators. It reveals the intrinsic entanglement-associated nature of QFT, which can be related to the necessity of the gauge (quote Xiaogang Wen).
For a large picture, AdS/CFT is a not-to-miss subject, which originates from the area entropy of BHs and the holography principle. There are many fascinating fruits from it, one the most salients (in my opinion) is the RT-formula, which states that the entanglement entropy of the boundary system is as same as the area entropy of the corresponding minimum-area volume. An implication would be that there will be no volume when there is no entanglement on the boundary. Somehow, gravity emerges from quantum.
The Hawking radiation itself proffers many clues to the entanglement, given the recent progress made by A. Wall and N. Engelhardtarea.Bell inequility and CHSH inequility can apply to BHs, giving the most precise description of firewall.
Other then such thrilling unification, another important and same fruitful aspect is AdS/CMT, the term coined by CMT theorests for studying the quantum materials from holography.
It is a win-win situation. CMT theorists benefit from many concepts of HEP, like the connection between QHE and gravity anomaly, and HEP theorists benefit from the useful and informative models, like the SYK model and the Ising model.
The recent child is Black Hole/Superconductor Correspondence, which opens the new vista to other mechanisms of superconductence, transcending the tranditional BCS paradigm.
Quantum liquids, quark-gluon plasma, strongly correlated electron systems and other quantum exotic matters awaiting both sides to collaborate to tackle.
Black Hole and GR
The beauty of GR: what is your feeling when you know that the actual inertia guy is who freely falls instead of you standing readily on the earth? For me, a bolt of electrons raced up my spine. GR likes marbles, among which the most saliant would be, personally, BHs, the simple and elegant objects marrying QM with GR.
GR itself does not answer the question that why gravity is so small,
To better understand BHs, we should dig deep into the geometery of spacetime, whcich is an active field. Although some one consider it as a kind of old school physics.
Aside, recall the discovery of DM, the role of GR in searching the physics beyond SM is indispensable, given only by GR we can predict and detect the large-scale structure which has been missing in the study of the quantum world for a long time. In the multi-messenger time, a solid understanding of Gr must be more and more important.
For example, the recent notable work is that S. T. Yau and collaborators have proved the supertranslation-invariant definition of angular momentum in the curved spacetime, which was a long-nagging question in GR. The result will not only pave the way for further study of GR but also benefit gravitational wave detection since the emissions are usually produced by rotating emergence.
BTW, highly recommend the book of S. Weinberg and the updating note of M. Blau. They are the best I ever seen.
