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Questions tagged [standard-model]

A model of the basic particles and forces featuring six quarks, three charged leptons, three massless neutral leptons and four fundamental force carrying bosons. The twelve fermions are arranged into three generations, while the bosons serve to explain the electromagnetic interaction plus the strong and weak nuclear forces (and the Higgs mechanism). Do NOT use this tag for the standard model of cosmology, etc..

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109
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4answers
14k views

Why do electrons, according to my textbook, exist forever?

Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
95
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7answers
37k views

What's inside a proton?

What constitutes protons? When I see pictures, I can't understand. Protons are made of quarks, but some say that they are made of 99% empty space. Also, in this illustration from Wikipedia, what's ...
77
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5answers
10k views

How literally should you take “The Higgs boson gives other particles mass”?

A standard phrase in popular discussions of the Higgs boson is that "it gives particles mass". To what extent is this a reasonable, pop-science, level of description of the Higgs boson and it's ...
70
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6answers
9k views

Why do we think there are only three generations of fundamental particles?

In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of ...
67
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7answers
16k views

Why are protons heavier than electrons?

Our teacher told us that protons are nearly 1800 times heavier than electrons. Is there any known reason as to why this is so? Or is this just an empirical value, one we do not know the reason to?
51
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2answers
2k views

Symmetries of the Standard Model: exact, anomalous, spontaneously broken

There are a number of possible symmetries in fundamental physics, such as: Lorentz invariance (or actually, Poincaré invariance, which can itself be broken down into translation invariance and ...
50
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4answers
22k views

Why do electron and proton have the same but opposite electric charge?

What is the explanation between equality of proton and electron charges (up to a sign)? This is connected to the gauge invariance and renormalization of charge is connected to the renormalization of ...
45
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4answers
2k views

What is needed to claim the discovery of the Higgs boson?

As I understand the Higg's boson can be discovered by the LHC because the collisions are done at an energy that is high enough to produce it and because the luminosity will be high enough also. But ...
40
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2answers
1k views

Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and ...
38
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5answers
7k views

Why are muons considered to be “elementary particles” in the Standard Model?

According to this article, a muon decays into one electron and two neutrinos. According to this article, elementary particles or fundamental particles are particles "whose substructure is unknown, ...
38
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3answers
10k views

Are W & Z bosons virtual or not?

W and Z bosons are observed/discovered. But as force carrying bosons they should be virtual particles, unobservable? And also they require to have mass, but if they are virtual they may be off-shell, ...
37
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1answer
2k views

How close does a particle-antiparticle pair need to be for annihilation to happen?

I've most often seen the statement that the annihilation of a particle and its antiparticle occurs when they 'collide' with one another. So in other words when they get very close to one another right?...
36
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6answers
10k views

Is the Higgs boson an elementary particle? If so, why does it decay? [duplicate]

The Higgs boson is an excitation of the Higgs field and is very massive and short lived. It also interacts with the Higgs field and thus is able to experience mass. Why does it decay if it is ...
34
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7answers
3k views

How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles?

How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles? It would appear to a layman such as myself that these ...
33
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6answers
4k views

Could gravity be an emergent property of nature?

Sorry if this question is naive. It is just a curiosity that I have. Are there theoretical or experimental reasons why gravity should not be an emergent property of nature? Assume a standard model ...
33
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1answer
3k views

Why is there no theta-angle (topological term) for the weak interactions?

Why is there no analog for $\Theta_\text{QCD}$ for the weak interaction? Is this topological term generated? If not, why not? Is this related to the fact that $SU(2)_L$ is broken?
32
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4answers
11k views

Why is gravity so hard to unify with the other 3 fundamental forces?

Electricity and magnetism was unified in the 19th century, and unification of electromagnetism with the weak force followed suit, bringing into play the electroweak force. I've been told that ...
31
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2answers
6k views

How does string theory reduce to the standard model?

It is said that string theory is a unification of particle physics and gravitation. Is there a reasonably simple explanation for how the standard model arises as a limit of string theory? How does ...
31
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3answers
2k views

Why do we need complex representations in Grand Unified Theories?

EDIT4: I think I was now able to track down where this dogma originally came from. Howard Georgi wrote in TOWARDS A GRAND UNIFIED THEORY OF FLAVOR There is a deeper reason to require ...
30
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11answers
7k views

Theoretically, could there be different types of protons and electrons?

Me and my friend were arguing. I think there could theoretically be different types of protons, but he says not. He says that if you have a different type of proton, it isn't a proton, it's something ...
29
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1answer
1k views

A dictionary of string - standard physics correspondences

Motivated by the (for me very useful) remark ''Standard model generations in string theory are the Euler number of the Calabi Yau, and it is actually reasonably doable to get 4,6,8, or 3 generations'...
27
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3answers
3k views

Do standard model particles actually exist or merely usefully describe behaviors of a medium?

I've read about how sound propagation can be modeled as phonon particles moving and interacting. I understand that this is a useful mathematical construct to describe the behavior of longitudinal ...
27
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2answers
850 views

Finding the vacuum which breaks a symmetry

I will start with an example. Consider a symmetry breaking pattern like $SU(4)\rightarrow Sp(4)$. We know that in $SU(4)$ there is the Standard Model (SM) symmetry $SU(2)_L\times U(1)_Y$ but depending ...
26
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5answers
5k views

Why should the Standard Model be renormalizable?

Effective theories like Little Higgs models or Nambu-Jona-Lasinio model are non-renormalizable and there is no problem with it, since an effective theory does not need to be renormalizable. These ...
26
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2answers
3k views

What is (meant by) a non-compact $U(1)$ Lie group?

In John Preskill's review of monopoles he states on p. 471 Nowadays, we have another way of understanding why electric charge is quantized. Charge is quantized if the electromagnetic $U(l)_{\rm em}$...
26
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2answers
3k views

What happens to matter in a standard model with zero Higgs VEV?

Suppose you reset the parameters of the standard model so that the Higgs field average value is zero in the vacuum, what would happen to standard matter? If the fundamental fermions go from a finite ...
25
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5answers
3k views

Where is the evidence that the electron is pointlike?

I'm writing a piece about the electron, and I'm having trouble finding evidence to back up the claim that the evidence is pointlike. People tend to say the observation of a single electron in a ...
25
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3answers
15k views

What is the decay width and why is it given in energy units?

I'm reading Thomson, Modern Particle Physics, and in chapter 16 author says that the decay width of the Z boson is $\Gamma_Z =2.452 \pm 0.0023 \,\mathrm{GeV}$. He also says the total width of the ...
24
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3answers
3k views

How (or when) do gluons change the color of a quark?

I know a baryon is only stable when it contains a quark of each color. And as far as I know, the gluon essentially changes the color of a quark and moves onto the next, and this is what holds the ...
23
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2answers
3k views

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

I have often seeen statements on physics.SE such as, The only consistent theory of everything which we know of to date (2013) is string theory. Why exactly is this so? Adding the Loop Quantum ...
23
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4answers
22k views

Is the Higgs boson not responsible for most mass?

The video "Your Mass is NOT from Higgs Boson?" argues that almost all the mass that we are made of doesn't come interaction with the Higgs field, but rather that most of a nucleus's mass comes from ...
23
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1answer
6k views

Origin of electric charge

Baryons have charges that are the result of a polynomial calculation of their building blocks (quarks)'s fractional charges. But what gives these quarks electric charges? What interactions do they ...
23
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1answer
2k views

Are there massless bosons at scales above electroweak scale?

Spontaneous electroweak symmetry breaking (i.e. $SU(2)\times U(1)\to U(1)_{em}$ ) is at scale about 100 Gev. So, for Higgs mechanism, gauge bosons $Z$ & $W$ have masses about 100 GeV. But before ...
22
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3answers
5k views

Can neutrinos “hit” electrons?

I understand that particles interact via the fundamentals forces of nature. For example photons interact with matter because they carry the change in the electromagnetic field. Neutrinos, on the other ...
22
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5answers
16k views

Why don’t photons interact with the Higgs field?

Why don’t photons interact with the Higgs field and hence remain massless?
22
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4answers
9k views

Books for particle physics and the Standard Model

I know classical physics, quantum mechanics, special relativity, and basic nuclear physics. I would like to get into some particle physics. Where can I find a good introduction? It might be useful ...
22
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4answers
2k views

How come “smaller, weaker” particles are more massive (have higher energies)?

Something has always struck me as counter-intuitive: when reading about high-energy experiments such as the LHC, they are always looking for stuff on a really small scale with MASSIVE energies. I ...
22
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8answers
2k views

What Do We Get From Having Higher Generations of Particles?

Background: I have written a pop-science book explaining quantum mechanics through imaginary conversations with my dog-- the dog serves as a sort of reader surrogate, popping in occasionally to ask ...
20
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3answers
3k views

Mixing of quarks, neutrinos… and leptons?

This is a quite simple question: quarks do mix (through the CKM matrix), neutrinos do mix (through the PMNS matrix). Then why do charged leptons not mix?
20
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2answers
3k views

Evidence of why the Standard Model is a successful theory of particle physics

When discussing physics with laypersons, I'm often in the situation where I have to explain what the Standard Model is, and why it's a successful theory of particle physics. To help in such ...
19
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3answers
4k views

Why are gluons believed to be massless?

Earlier questions under a similar title referred to the short range of the strong force. My question is completely different. I'd like to know why gluons are considered massless in the Standard Model (...
19
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2answers
3k views

The physics community's take on noncommutative geometry

Connes's noncommutative geometry program includes an approach to the Standard Model that employs a noncommutative extension of Riemannian metric. In recent years I've heard physicists say that this ...
19
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2answers
3k views

Neutrino oscillations versus CMK quark mixing

I wish to describe in simple but correct terms the analogy between the Cabibbo–Kobayashi–Maskawa (CMK) and Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrices. The CMK matrix describes the rotation ...
19
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2answers
1k views

If the LHC doesn't find the Higgs Boson, what would be the implications for the Standard Model?

What would be the implications to the Standard Model if the Higgs Boson hadn't been found with the LHC? Also, if the Higgs Boson had not been found with the LHC, would it have been successfully ...
19
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3answers
5k views

Why does the weak force distinguish left and right handedness?

I'm wondering why the weak interaction only affects left-handed particles (and right-handed antiparticles). Before someone says "because thats just the way nature is" :-), let me explain what I find ...
19
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3answers
4k views

Are electrons just incompletely evaporated black holes?

Imagine a black hole that is fast-approaching its final exponential throws of Hawking evaporation. Presumably, at all points in this end process there there will remain a region that identifiably ...
18
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2answers
5k views

What does a $\rm SU(2)$ isospin doublet really mean?

What do we really mean when we say that the neutron and proton wavefunctions together form an $\rm SU(2)$ isospin doublet? What is the significance of this? What does this transformation really doing ...
18
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2answers
2k views

Is the Standard Model consistent (UV complete)?

This is a question about the self-consistency of the Standard Model - which I believe is the same as asking whether it is UV complete - in other words, can it be used to predict experimental results ...
18
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2answers
244 views

Values of SM parameters at one certain scale

The general question is: What are the values of Standard Model parameters (in the $\bar{MS}$ renormalization scheme) at some scale e.g. $m_{Z}$? As its parametrization in Yukawa matrices is not unique ...
17
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3answers
5k views

If electrons were positive and protons were negative, would life be different? [duplicate]

This was a question on a worksheet during my first week in a class on Electromagnetism. The answer is essentially: No. Life would be no different if electrons were positively charged and protons ...