Hello,
it’s been almost a decade since the Quantum Universe posed nine questions: a challenge to particle physics.
And a great decade it was for particle physics! The existence of the aether (a.k.a. the Higgs field) has been established by observing its excitations. The mixing angle Theta13 has been measured to be large, enabling further work to precisely test the unitarity of the neutrino mixing matrix. Stunning, unambiguous signs of the new state of matter – Quark-Gluon Plasma – have been established.
What are the questions that you’d like to see answered in the next decade or two?
Start the discussion here, or post your list at the Snowmass 2013 page. These questions will form a focus for our discussions at “Snowmass on the Mississippi,” to be held at the University of Minnesota from July 29 to August 6.
DPF Executive Committee
(1) Can we explain the patterns of quark and lepton masses and mixings?
Why are the quark and lepton patterns different?
Does the lepton pattern point to a new mass scale?
Are neutrinos their own antiparticles?
Will we need to know about dark matter before we can answer?
Are there sterile neutrinos?
(2) Are there new symmetries and forces beyond SU(3)xSU(2)xU(1)?
Is the standard model embedded in a grand unified theory?
If so, can we learn about the stages in which it is broken?
Are there hints at present of accessible extra gauge symmetries?
Is supersymmetry just around the corner?
At what scale is left-right symmetry broken?
Does the proton decay? Do neutrons oscillate into antineutrons?
(3) Is there a hierarchy problem?
Why is the Higgs light but the flavor-violation scale heavy?
Does supersymmetry solve the problem?
Is the Higgs composite? If so, is anything else composite?
Is vacuum metastability telling us something?
(4) What is the nature of dark matter?
What can we learn if dark matter interacts purely gravitationally?
Is dark matter represented by a single WIMP?
If not, what is the pattern of organization of dark particles?
If there is a complex dark world, can it help us understand (1)?
(5) What can we learn from string theory?
What is the scale of the extra dimensions in string theories?
What insights does it provide to solving strong-coupling problems?
Are there any experimental tests of quantum gravity?
Does it illuminate gauge symmetries beyond SU(3)xSU(2)xU(1)?
Is one of its predictions low-energy supersymmetry?
(6) What is responsible for the apparent acceleration of the Universe?
Is there a time-independent cosmological constant?
If not, can one measure more than its first time derivative?
Is there a theory for the cosmological constant or its equivalent?
(7) What flavor symmetries are preserved up to what mass scale?
What do we learn at each level of studying rare processes?
What are the theoretical limitations accompanying each process?
Do flavor-diagonal rare processes (edm,g-2,…) enjoy any advantage?
Are there processes to which we have forgotten to pay attention?
(8) What can we learn from astrophysical sources?
What do we learn from gamma rays of 100 GeV and above?
What is the source of ultra-high-energy cosmic rays?
Can we detect (and learn things from) neutrinos above a PeV?
What will we learn from the next supernova explosion in our Galaxy?
What can we learn about non-gravitational dark matter interactions?
(9) What is the source of the Universe’s matter-antimatter asymmetry?
Are there plausible electroweak-scale mechanisms?
What are the constraints associated with a leptogenesis mechanism?
Relation between leptogenesis and CP-violating neutrino oscillations
(10) How do we provide answers to these questions?
What accelerators and underground installations are required?
What advances in instrumentation and detectors are required?
What are the present and future needs in computing?
How do we communicate the importance of these questions?
Jon has listed an excellent set of questions. I would add two of a more theoretical nature:
a) Is naturalness a useful or useless concept? Is any concern about the little hierarchy problem warranted in the absence of an understanding of the big hierarchy problem?
b) Is there a useful non-perturbative formulation of string theory?
1) Why are there three generations?
2) Is there physics beyond the Standard Model?
3) If so, what is it?
4) Why do we live in a matter dominated Universe?
5) What is dark matter?
6) What is dark energy?
7) Can gavity be quantized?
8) Theorists love supersymmetry. Does Nature?
9) What is the source of symmetry breaking in the electro-weak sector?
10) Why do quarks and leptons have the masses they have?
1. Is the 2012 Higgs Boson an excitation of the Higgs Field responsible for fermion mass?
what are the couplings of Higgs Bosons to fermions?
what is the coupling of Higgs to itself?
is the Higgs a composite particle?
are there additional Higgs Bosons?
what protects the mass of the Higgs Boson?
is the longitudinal polarization of the W and Z due to a primordial partner of the Higgs Boson?
2. What is the nature of Neutrinos?
what are neutrino masses
what are neutrino flavor and symmetry properties
are neutrinos their own antiparticles?
3. How many fundamental forces are there in Nature?
is SU(3)xSU(2)xU(1) a subgroup of higher order groups in Nature?
what is responsible for higher order symmetry breaking?
are there additional spin 1 fields?
is Supersymmetry correct?
do the forces of nature unify at high scale? What is that scale?
4. How many fundamental particles are there?
are there more than 3 generations of leptons and quarks?
is the doublet structure of fermions the final story?
what is the weak mixing among them?
are quarks composite?
5. Is there a shortest distance in Nature?
are there more than 3 dimensions of space?
is it possible to definitively test string theory?
6. Is there a quantum of Dark Matter?
7. What is responsible for the apparent asymmetry between matter and antimatter?
8. Is the proton stable?
9. What is the quantum of Gravity?
1. Was the Universe one entity at the Big Bang?
2. Do all particle constituents of the Universe have a common origin?
lepton-quark symmetry: grand-unification
boson-fermion symmetry: supersymmetry
triplication of chiral families: family symmetry
3. What causes some symmetries to break?
4. Are there extra dimensions?
new space dimensions
fermionic dimensions
5. Why does the Universe contain more matter than antimatter?
baryogenesis or leptogenesis?
does the proton decay?
are neutrinos their own antiparticles?
6. What mechanism engenders Inflation?
7. What is dark matter?
8. Why is the universe accelerating?
9. Is space-time locality fundamental?
will it break down
will quantum mechanics fail?
what is quantum gravity?
what is a quantum mechanical black hole?
1. Why is there T-violation? What are the phases in CKM/PMNS and why do they have the values (nearly maximal for CKM) they do? Are there other sources of T-violation?
2. How many space dimensions are there?
3. How many generations are there and why?
4. Why is the CKM matrix perturbative and the PMNS matrix elements large? Why aren’t they exactly diagonal or all equal?
5. Is the neutrino mass set by the SM Higgs? If not, what determines the neutrino masses and mass differences? Is there underlying physics that explains both the neutrino mixing matrix and the CKM matrix?
6. What sets the Higgs mass?
7. What sets the quark masses?
8. What sets the lepton masses?
9. Are protons stable?
10. Why is alpha_S~1, alpha_EM ~.01, and G_F ~ 10^{-5}?
Contemporary physics seems to have reached a dead end.
The reasons might be that most physicists take the attitude that all proposed statements about physical concepts must be verifiable by experiments. This closes the door to investigations that concern subjects that are only deducible and can fundamentally not be observed.
A second reason is that in 1936 Garret Birkhoff and John von Neumann suggested a suitable foundation to physics, which indicates that nature’s physics is fundamentally countable. Some decades later Constantin Piron added to this foundation that nature’s numbers are taken from a division ring. Contemporary physics took other directions, which close the door to investigations that explore the undercrofts of physics.
String theory and loop quantum gravity deviate even further from the suggested foundation.
When these deviations are turned back, the lower hierarchy of physical objects appears far richer than has been discovered with contemporary methodology.
1) Are there other ways of formulating a theory equivalent to the Standard Model, but using other frameworks than QFT.
Something akin to Lagrange’s or Hamilton’s reformulations of the Newtonian theory. A new formulation (perhaps modelled purely in computer code, as cellular automatons etc.) could allow us to gain new insights, beyond higher precision perturbation calculations.
2) Can the elementary particle fields be manipulated by other means than scattering. Is it possible to engineer techniques similar to what solid state physics is to quantum mechanics, but for particle (and to some extent nuclear) physics.
This could not only transform the way we study nature, but also make it applicable to practical use outside our labs.
3) In science we as humans are “biased” in how we model our observations through mathematical notation, and our expectations on where to look for phenomena. With the advent of computing and “Deep Learning” we now for the first time have the ability to get a pair of second eyes on our Universe.
I suggest a project akin to the European “Human Brain Project” where a computer program is allowed to data mine as much experimental data as possible, and infer its own analytical model bottom up, and see if it catches new structures we haven’t discovered ourselves.
4) Gain enough experimental sensitivity to measure the emergence of gravity on particles.
To combine particle physics with gravity, studying how gravity “forms” at low masses could be a way to gain new insight, it could connect to Dark Matter as well?
5) Is Dark Matter, particles at all, a collective field effect or something completely different but still relevant to particle physics?
Besides these questions I agree with the classical orthodox questions raised by other posters.
I have never had a satisfactory explanation for the following questions:
(a) What is (or what do we understand is meant by) a dimension and dimensionality
(b) how does ‘dimensionality’ relate to physical phenomena and the fundamental characteristics of entities (our reference here are the fundamental particles) that ‘inhabit’ or derive from the dimensional (manifold) framework.
We seem to have a lot of theories (string, QM, GR) making use of dimensional infrastructures, manifolds etc., but none that explain the stuff of dimensionality (not simply the pervasive ‘vacuum potentials’ within) itself.
(1) So… Where do dimensions come from and why are they so inherently related to the measurable properties of particles and field?. Do dimensions exist independent of the entities within them or are they ‘part and parcel’ : another kind of duality as implied by current thinking but not, it seems to me, directly espoused
(2) Can a future theory be formulated to predict, and future experiments verify,the properties (symmetries etc.) resulting from such a theory? Would such a theory reveal all the potential forces of nature rather than us blindly stumbling upon them? I am of course thinking of Dark Energy and, I suspect, there are others to discover.
1) Is antimatter repulsive to other antimatter (as in anti-gravity)?
2) If so, is the magnitude of anti-gravity for antimatter the same as the magnitude of gravity for matter?
3) If so, since antimatter doesn’t aggregate gravitationally, does the universe really contain more matter than anti-matter?
(continuation)
2) Does matter interact gravitationally with antimatter?
(continuation)
Thie previous question should be numbered 4).
Note anything that moves through space-time moves on a unique world-line, so antimatter wouldn’t “fall up” in a matter-dominated gravitational field. The effect of antimatter on the world line would be opposite that of matter if 1) is true.
5) Can the space-time curvature of dark matter actually be non-uniformities in the expansion of space by the dark energy?
I thank the DPF for stimulating the discussion of large issues for the field and for seeding that discussion with excellent questions. My own questions are of course variations on these common themes.
1. Can new very high gradient methods for accelerating high energy particle beams be realized? Can they be used for compact,t lower-energy accelerator applications?
2. How far can we go toward building massless trackers, high resolution calorimeters without dead regions, and detectors with much higher rate tolerance?
3. Our present measurements indicate the need for some high mass scale (the strong CP problem, neutrino masses, suppression of FCNCs, metastability of the Higgs potential at large scales, etc.). Can we provide the information that illuminates the character and location of this new scale?
4. What CP-violating mechanisms are causing the baryon asymmetry in the universe?
5. What is the nature of the particles in the dark sector and how do they connect to the visible spectrum of the standard model?
6. Is the proton unstable, and does it matter if it is?
7. Are neutrinos Majorana particles and thus fundamentally different from charged leptons and quarks?
8. Does grand unification occur, and what new physics enables it? Is there any deep reason to have such unification?
9. Does dark energy signal the breakdown of general relativity, or is there a new field at work in the universe?
10. Do our familiar notions of space-time need revision with extra dimensions or new symmetries between fermions and bosons?
11. Is it possible to motivate large societal investment in questions of such little practical import as those above?
Colleagues:
These are just about all good suggestions. It strikes me that there is however something missing, to which Paul Grannis #11 indirectly refers. It is fine to list many theoretcal challenges, but some are not practical and some are just not cost effective and some will not tell us anything definitive even if we get there. I will not start naming projects, but billions are being, or proposed to be, spent on big science projects that will not pay. Somehow this fine wish list needs to be tempered with (much as I hate the term) cost/benefit. We in HEP may ignore this but Congress will not.
Maybe a #12 would be something like: Explore and promote multiple use physics projects which have a multi-disciplinary or practical payoff. (OK, I cite our interest in geo-neutrinos and in remote reactor detection as examples of applications for with embedded fundamental science… I am sure there are more.).
1. What is dark matter?
2. What is dark energy?
3. What is the origin of baryon asymmetry?
4. What is the origin of all elements?
5. Are there any new fundamental particles?
6. Are there any new fundamental forces?
7. Are there any new space-time-vacuum structures?
8. Is there any particle theory beyond quantum field theory?
9. Is there any gravity theory beyond general relativity?
10. Is there any link between quantum field theory and general relativity?