Particle physics is the study of the fundamental forces of nature as they are embodied in the interactions of elementary and composite particles at high energies and short time and distance scales.
4
votes
0answers
120 views
Trace of stress tensor vanishes ==> Weyl invariant
You often see in textbooks the statement that ${T^\mu}_\mu = 0$ implies Weyl invariance or conformal invariance. The proof goes like
$\delta S \sim \int \sqrt{g} T^{\mu\nu} \delta g_{\mu\nu} \sim ...
7
votes
0answers
351 views
About defining “baryons” and “mesons”
I want to understand the proof of the claims (of the construction as well as of its uniqueness) of gauge singlet states given around equation 2.13 (page 10) of this paper.
Also does the listing of ...
0
votes
0answers
43 views
What exactly is the spin of a particle? [duplicate]
Possible Duplicate:
What is spin as it relates to subatomic particles?
I'm having a hard time grasping the concept of spin, my textbook describes it very vaguely:
Stable matter contains ...
6
votes
3answers
158 views
Compton Scattering
Compton Scattering essentially states that when a photon of a given wavelength hits an electron the energy level of the electron changes and the photon has its wavelength changed. This seems to be ...
0
votes
0answers
55 views
How is the hierarchy problem consistent with the decoupling theorem?
One the one hand we have the hierarchy problem in it's various forms, in my understanding in it's most serious form one could state it as the observation that if there is a heavy mass scale M in ...
2
votes
0answers
50 views
Chirality when moving around legs in Feynman diagrams
Assuming one has the following term in a Lagrangian:
$$
g (\overline{A_R} B_L)(\overline{C_R}D_L)
$$
where A,B,C,D correspond to spin 1/2 Dirac particles and the subscripts $R$ and $L$ denote left- ...
3
votes
1answer
124 views
Why don't alpha particles have magnetic moments?
As I understand, particles such as the neutron, whilst having no external charge still possess a magnetic moment due to the underlying charges of its components.
By that logic why does the alpha ...
0
votes
0answers
38 views
How to calculate the scatterng angle of a particle collision [closed]
z0 decays to electron + positron pair as below. The momentum of z0 is 150 Gev/c and its mass is 90 Gev. Find the scattering angle of the electron
3
votes
1answer
50 views
Single electron non-perturbing detector
I am designing an experiment where I need to trigger the release of an electron by a radioactive source (Sr-90).
The easy way to do it is to use a thin scintillator right after the source collimator. ...
3
votes
2answers
163 views
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation?
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation ? If so , why ? Im confused by this.
2
votes
2answers
107 views
How much energy is carried away by neutrinos in matter-antimatter annihilation?
Some people say that neutrinos carry away most of the energy, some others say just a fraction. So what is the truth ? what is the percentage of energy lost due to neutrinos ?
0
votes
1answer
79 views
Optimal methods for mapping out molecules, atoms and nuclei and their energy levels?
I'm wondering if it would be possible to map out all the different types of molecules, atoms and nuclei and their energy levels on one page (even if in a generalised way)? But perhaps I'm referring to ...
2
votes
1answer
34 views
Question about linacs
Why are the electrodes of a linac connected to an alternating voltage? Within an electrode the electron moves with a constant speed, and once it is outside of the electrode it accelerates uniformly, ...
0
votes
1answer
116 views
Explosion of a Neutron Bomb
Watching "The Dark Knight Rises", Bane announces halfway into the movie at the stadium that what they have is a neutron bomb. But then at the end of the movie there is an actual nuclear explosion ...
5
votes
1answer
113 views
What is the first appearance of the MV (McLerran-Venugopalan) initial condition?
First a quick introduction for the unfamiliar: in saturation physics (my research field), a lot of theoretical work centers on the BK (Balitsky-Kovchegov) equation, which is a differential equation ...
1
vote
1answer
330 views
Is all matter made of virtual particles?
This article in New Scientist says that all matter is actually virtual particles popping in and out of existence and nothing more. is this correct?
4
votes
1answer
157 views
Trying to understand “recursion at the lowest level of matter”
I'm reading the book Gödel, Escher, Bach: an Eternal Golden Braid and on Chapter V, Hofstadter talks about different examples of recursive structures and processes. By page 142 of the 20th anniversary ...
0
votes
1answer
127 views
Is the idea of dividing the universe into particles anything more than an untrue convenience? [closed]
In theory, we speak of a particle as having properties.
In reality, the measurement of any property is just an interaction between the target to be measured, and the measuring apparatus, where the ...
3
votes
2answers
103 views
Is there something like Hawking radiation that makes protons emit component quarks?
If Hawking radiation can escape from black holes, could quarks perhaps become separated from protons despite it being "impossible" for that to happen?
3
votes
1answer
122 views
“Hard wall”/ “soft wall”
I have encountered those terms in various places. As I understand it, "soft wall" can correspond to a smooth cutoff of some spacetime, while "hard wall" can be a sharp one, which is be described in ...
0
votes
1answer
82 views
Is the heat required to alter the Higgs field an 'absolute heat'?
I have read and heard that manipulating the Higgs field would require heating up a local geometry to ridiculous temperature. I am trying to understand if there are stars or places in the universe ...
3
votes
2answers
212 views
Why does $\mathcal L = -\frac14 F^{\mu\nu} F_{\mu\nu}$ imply Photons are massless?
The Lagrangian $\mathcal L = -\frac14 F^{\mu\nu} F_{\mu\nu}$ with $F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu$ results in the four-potential's equation of motion
$$ \underbrace{\partial^\mu ...
0
votes
0answers
28 views
Energy of split of nucleus [closed]
How much energy must be supplied to the nucleus of lithium isotope Li-7 (3 protons, 4 neutrons) to separate them into individual protons and neutrons? The atomic weight of lithium is $1.1524 \times ...
2
votes
1answer
56 views
Neutrino mass as counted in Dark Matter
If I try to add up neutrino masses (let's assume 1 eV rest mass equivalent each) to count as DM, do I use the rest mass or relativistic mass?
4
votes
1answer
148 views
Labelling representations using isospin and hypercharge
Can someone explain how isospin and hypercharge, can be used to label representations? What is the meaning of the term singlet, doublet etc in this context? In particular how can I use it to label ...
1
vote
3answers
143 views
Do particle pairs avoid each other? Please end my musings
Can you explain what happens when a particle and its antiparticle are created. Do they whiz away from each other at the speed of light or what? I suppose that they don't because otherwise they would ...
1
vote
1answer
93 views
What are “hidden valley sectors”?
In this end of the year article, Prof. Strassler mentioned that hidden valley sectors could lead to some still open loopholes concerning the experimental discovery of supersymmetry and other BSM ...
3
votes
1answer
99 views
Why are some extra dimension theories known as strongly coupled and others as weakly coupled?
I was looking at pdf file of the presentation of a conference talk. The speaker discusses two types of "mechanisms" for stabilizing the weak scale and calls them "weakly coupled" and "strongly ...
2
votes
4answers
176 views
The building blocks of energy
I have a couple of related questions that have been bothering me for a while. They might sound unscientific, but here is goes:
What are the building blocks of energy? What does energy consist of? Is ...
3
votes
1answer
62 views
Will a gas keep forever in a “perfect” flask?
I've been wondering about the porosity of materials, I know that, for example the air comes out of tires/balloons because (besides having huge gaps on the rim contact area/knot) they are made of a ...
1
vote
0answers
42 views
Conservation laws in mSUGRA model
Can somebody list all the quantum numbers (beside R-parity) that are conserved in vertex for SUSY particles in mSUGRA model?
6
votes
3answers
195 views
Is there a simple way to compute some physical constant from Feynman diagram statistics?
I've been playing around writing some software to generate Feynman diagrams for QED, respecting the vertex "rules" described here, and avoiding creating isomorphic duplicates.
So from a starter ...
2
votes
0answers
114 views
Do all the particles acquire mass in the Standard Model due to the Higgs mechanism only?
I know that a mass term for an intermediate boson is not compatible with the gauge symmetry. But in principle a mass term for the electron field does not violate a gauge symmetry. However to build an ...
3
votes
0answers
56 views
Can the mass of a SUSY particle depend on the process it participates in?
I believe that mass is property of every particle,as well as spin etc.Now I'm interested in SUSY particles in cMSSM model.Can it be,that mass of a SUSY particle (at one point in five parameter space) ...
0
votes
0answers
17 views
Hamiltonian of the charged current in SM (related to the lorentz invariance)
recently when I was studying the scatterings which involves a vector boson (like W boson) as an intermediate particle, I saw that the propagator is not Lorentz invariant, I read that there is another ...
4
votes
1answer
73 views
Parity of a decay
If a particle of unknown intrinsic parity decays into 2 particles each with negative intrinsic parity, does that necessarily imply that the original particle also has negative parity?
1
vote
1answer
130 views
Lower bound for the spin in particle decay process
Is my logic right?
Suppose there is a particle $p$ that can either decay into $ \{$a spin-1 and a spin-0 particle$\}$ or two spin-0 particles, then the lowest possible spin of $p$ is 2. This is ...
2
votes
1answer
99 views
What is the spatial mode of light or the spatial mode of a massive particle?
I'm extremely confused by what physicists mean by the spatial mode of light. I am also equally if not more confused by what the spatial mode of a massive particle is. Can anyone help me out by ...
2
votes
2answers
157 views
Identification of extended quantum objects?
In some physics theories like string theory we have notion of spatially extended quantum objects - strings, membranes etc. Assuming that such objects exist, how would they appear in experiments ? More ...
0
votes
1answer
310 views
How do I start learning particle physics? [closed]
I am 16 at the moment. I am really interested in physics. Especially particle physics. Can someone please tell me how to start learning the subject. like what to learn first. like which fundamental ...
10
votes
2answers
127 views
What is the 'bump' near $M_{\mu\mu}\approx 30\text{ GeV}$
In this (attached) Summer 2011 plot from CMS (twiki page), they have a plot of the dimuon invariant mass spectrum across 3 orders of magnitude in energy. There seems to be a 'bump' near ...
4
votes
1answer
180 views
How are forces related to decays?
How are decays related to forces, what is meant by particle X decays through the, say, strong force?
The way I understand forces is by how they change the acceleration of particles with the right ...
4
votes
2answers
195 views
Assuming that extra dimensions will not be visible at LHC, what motivation will still remain to study them?
Many physicists believe that there is little possibility of observing extra dimensions at LHC so that some extra dimension models originally designed to solve hierarchy problem (ADD/Randall-Sundrum) ...
6
votes
1answer
53 views
Measuring nucleons using electron beams
sorry if the question is too elementary. From: The Britannica Guide to Particle Physics:
The sizes of atoms, nuclei, and nucleons are measured
by firing a beam of electrons at an appropriate target. ...
5
votes
1answer
100 views
Mechanisms of mass generation for Dirac neutrinos
If neutrinos are Majorana particles, one way of explaining their small masses is the seesaw mechanism.
Now say I'd like my neutrinos to be Dirac, for symmetry to the quark sector. What mechanisms ...
6
votes
3answers
189 views
Origin of lepton/quark generations?
What theoretical explanations exist for the fact that there are three generations of leptons and quarks?
I'm not so much asking why there are exactly 3 generations, but rather what makes electron, ...
3
votes
1answer
117 views
Will Randall-Sundrum extra dimension scenario become defunct if not supported by LHC?
The Randall-Sundrum extra dimension scenario had been one of the most extensively studied class of theories. This offered a solution to the hierarchy problem. However, if this picture is not supported ...
6
votes
6answers
561 views
What is the meaning of the word “particle” in particle physics?
I want to use Matt Strassler's definition of the word "particle" as a specific example:
Matt Strassler writes:
(1) "...all the elementary “particles” (i.e. quanta) of nature are
quanta of waves ...
2
votes
2answers
108 views
What is difference between the different 'flavours' of neutrinos?
Moreover, how-come scientist know that muon-neutrino are different from electron-neutrino when they didn't even know what the difference was? Did they interact differently with other particles?
4
votes
2answers
223 views
About free quarks and confinement
I simply know that a single free quark does not exist. What is the reason that we can not get a free quark?
If we can't get a free quark then what is single-top-quark?
