67 votes
Accepted

Resolving Conflicting Reports on Fermilab $g-2$ Results

This seems rather incredible that these two seemingly conflicting announcements come on the same day. The pre-print for the Nature paper by the BMW group was placed on arXiv in 2020 around the same ...
ohwilleke's user avatar
  • 3,903
57 votes
Accepted

Is there any evidence that dark matter interacts with ordinary matter non-gravitationally?

There are some standing anomalies that could be explained by non-gravitational dark matter interactions. For example, Fermi-LAT is an indirect detection experiment (i.e. an experiment that looks for ...
knzhou's user avatar
  • 102k
42 votes
Accepted

Why can't the LHC detect heavy particles?

It's not detecting the particles that is hard, it's making them in the collisions. Although the LHC collision energy is 14TeV, collisions aren't between the protons but rather between individual ...
John Rennie's user avatar
36 votes

What news about the second Higgs mode (or the mysterious particle) is anticipated to be seen at LHC around 750 GeV?

One of the searches performed at the LHC consists in selecting events in which two high energy photons are produced ($\gamma\gamma$ channel) and in computing their invariant mass - the energy of the ...
Lucas Gautheron's user avatar
31 votes

Why do all fields in a QFT transform like *irreducible* representations of some group?

Gell-Mann's totalitarian principle provides one possible answer. If a physical system is invariant under a symmetry group $G$ then everything not forbidden by $G$-symmetry is compulsory! This means ...
Qmechanic's user avatar
  • 202k
30 votes

Why is Standard Model + Loop Quantum Gravity usually not listed as a theory of everything

One can pinpoint the technical error in LQG explicitly: To recall, the starting point of LQG is to encode the Riemannian metric in terms of the parallel transport of the affine connection that it ...
Urs Schreiber's user avatar
26 votes

Why do we need complex representations in Grand Unified Theories?

Charge conjugation is extremely slippery because there are two different versions of it; there have been many questions on this site mixing them up (1, 2, 3, 4, 5, 6, 7, 8, 9), several asked by myself ...
knzhou's user avatar
  • 102k
26 votes
Accepted

What gives mass to dark matter particles?

I think this question contains a misconception unfortunately caused by popular science descriptions of the Standard Model. The question seems to assume there needs to be some concrete source that ...
knzhou's user avatar
  • 102k
19 votes
Accepted

Why do all fields in a QFT transform like *irreducible* representations of some group?

This is only semantics. A reducible representation $\mathbf R$ of the symmetry group can be decomposed into a direct sum $\mathbf R_1 \oplus \cdots \oplus \mathbf R_N$ of irreducible representations. ...
Noiralef's user avatar
  • 7,238
16 votes
Accepted

Could someone explain the muon $g-2$ experiment problem?

The $g$ factor describes the magnetic moment of a spinning particle. The $g$ factor for a classically spinning particle is equal to 1, but in the "basic" (ie, non-interacting) quantum field ...
Andrew's user avatar
  • 48.8k
14 votes

Why do all fields in a QFT transform like *irreducible* representations of some group?

Irreducible representations are always determined by some numbers, labeling the representation, which correspond to the eigenvalues of some observables which are invariant under the (unitary) action ...
Valter Moretti's user avatar
13 votes
Accepted

Why should the modes of the linearized metric perturbation be "wavefunctions" of gravitons (in the Randall-Sundrum model)?

This is a slight abuse of terminology, related to talking about 'second quantization.' The word 'wave function' in this case really refers to the 'one particle wave function,' which happens to ...
Andrew's user avatar
  • 48.8k
13 votes
Accepted

How are strings of String theory different from particles in quantum field theory?

Strings are not quanta. They are not excitations of something, they are the fundamental objects from which standard string theory starts building its model. In quantum field theory, particles only ...
ACuriousMind's user avatar
  • 125k
12 votes
Accepted

What is wrong with a nonrenormalizable theory?

In the modern effective field theory point of view, there's nothing wrong with non-renormalizable theories. In fact, one may prefer a non-renormalizable theory inasmuch they tell you the point at ...
coconut's user avatar
  • 4,653
12 votes

Why can't the LHC detect heavy particles?

As a consequence of $E=mc^2$, to create a heavy particle (i.e. large $m$) requires a large amount of energy ($E$). Since the LHC only generates a finite amount of energy in the collisions, there may ...
lemon's user avatar
  • 13.2k
10 votes
Accepted

How to decompose the representation of $\rm SU(5)$?

Actually the Lie group $$G~:=~SU(3)\times SU(2) \times U(1)$$ is not a subgroup of $SU(5)$. However the standard model gauge group $G/\mathbb{Z}_6$ is a subgroup of the GUT gauge group $SU(5)$, cf. e....
Qmechanic's user avatar
  • 202k
10 votes
Accepted

What are the Feynman diagrams for neutrino oscillations?

There are none, and the question isn't really even sensible. Neutrino oscillation is not mediated by force carrying particles any more than any other change of quantum basis is. This is similar to ...
dmckee --- ex-moderator kitten's user avatar
10 votes

Does theory allow that one neutrino is massless and the other two not?

Experiments so far have pinned down the two mass differences of the three neutrino masses. The lowest one, $m_1$, may conceivably be zero, even though in physics massless states need some type of ...
Cosmas Zachos's user avatar
10 votes
Accepted

Do we have good reasons to believe that neutrino oscillations aren't the result of a new particle? If so, what are they?

The reason that no other Standard Model particles oscillate into each other is that they are defined not to. An "oscillation" is another name for an interaction in which one particle goes in ...
benrg's user avatar
  • 26.2k
10 votes
Accepted

Is the discovery of the Higgs boson tantamount to discovering the Higgs mechanism?

No, the discovery of the Higgs boson has little to do with validating the Higgs mechanism, or the breathtaking role of the Higgs field Yukawa couplings giving masses to fermions. There are several &...
Cosmas Zachos's user avatar
9 votes
Accepted

Does either Quantum Field Theory or the Standard Model of Particle Physics predict the maximum number of particles or fields that can exist?

There is a limit on the number of flavors in Quantum Chromodynamics, behind that limit Color Confinement can no longer exist. The Beta-function that describes the interaction strength at different ...
Darkseid's user avatar
  • 1,430
9 votes
Accepted

Indivisiblity of quarks

The former, although maybe we're wrong. It wouldn't be the first time.
J.G.'s user avatar
  • 24.8k
8 votes

Why should the modes of the linearized metric perturbation be "wavefunctions" of gravitons (in the Randall-Sundrum model)?

In physics, it is not out of the ordinary to quantize a composite system in stages: First quantize some of its components and then proceed to the rest. We do this all the time, sometimes unconsciously....
David Bar Moshe's user avatar
8 votes

What forbids off-diagonal elements in the kinetic terms of the Standard Model?

The most general Lagrangian consistent with the symmetries indeed has a non-diagonal kinetic term however the point is that you can usually redefine the fields such that the kinetic term is the ...
JeffDror's user avatar
  • 8,994
8 votes
Accepted

Why is the Standard model Higgs not a candidate of dark matter (in particular, a WIMP)?

Higgs would quickly decay to a mix of products, many of them electrically charged. Thus dark matter is neither Higgs nor former Higgs.
J.G.'s user avatar
  • 24.8k
8 votes

Why is it assumed supersymmetry in nature is ${\cal N}=1$?

It is because the Standard Model is a chiral theory, i.e. there are particles whose components of different chirality transform diferently under gauge symmetry. Only $N=1$ allows for chiral matter ...
Diracology's user avatar
  • 17.7k
8 votes
Accepted

If neutrinos are disfavoured as DM candidates why aren't axions?

The answer is that the axions are not relativistic, but rather extremely cold. Neutrinos are hot because they were in thermal equilibrium with the standard model heat bath before they decoupled. This ...
Ihle's user avatar
  • 1,018
8 votes
Accepted

$B$-$L$ global symmetry in the grand unification theories

In the standard model lagrangian, B and L are separately conserved global charges, and B-L, a vector like symmetry, is anomaly-free. GUTs, like the G-G SU(5) violate B and L, but preserve B-L. ...
Cosmas Zachos's user avatar
8 votes
Accepted

Why does the observation of $Z$-boson decay imply active neutrinos to be heavy?

The observation of Z-boson decay doesn't imply active neutrinos to be heavy. The cited white paper says The observed Z-boson decay width implies that any additional active neutrinos are quite heavy ...
Chiral Anomaly's user avatar
8 votes
Accepted

Is neutrino mass really physics "beyond the Standard Model"?

So my real question is, do any of the extension of the SM to add neutrino mass signal new physics? What does even count as new physics? I think it would be something like a fifth force. Not sure what ...
Cosmas Zachos's user avatar

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