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.

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How do particles get their charge?

How does an electron get its charge? And how can it maintain that charge for very long (infinite) periods of time? And how come a neutron has no charge since and a proton does? They are both made of ...
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22 views

Exercise: Calculating Q-value for $D_0 \to \chi_s^-e^+\nu_e$

Question Is the following methodology correct in calculating the Q-value (change in kinetic energy) for the $D^0 \to \chi_s^-e^+\nu_e$ decay process? Steps The change in kinetic energy is ...
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3answers
135 views

Why is this particle interaction forbidden?

Why is this interaction forbidden? $$\nu_e+\bar{\nu}_e\rightarrow K^+ + K^-$$ Lepton number is conserved, charge is conserved, baryon number is irrelevant since these are mesons. Energy is conserved ...
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1answer
1k views

Why do physicists say that elementary particles are point particles?

For example, an electron, it has mass and charge, but is considered to have point mass and point charge, but why? Why are they assumed to have charge and mass in a single infinitely small point in ...
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76 views

Why do particle physicists use the $h\rightarrow{\gamma\gamma}$ decay mode rather than $h\rightarrow{b\bar{b}}$?

Just wondering, the $h\rightarrow{\gamma\gamma}$ decay mode is often quoted as being most sensitive to measuring the mass of the Higgs particle, why isn't the $h\rightarrow{b\bar{b}}$ decay channel ...
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16 views

Difference between 'merging' and 'matching' procedure

For combining scattering processes with different jet-multiplicities in the final state, higher order corrections and parton shower, merging/matching procedures need to be applied to avoid double-...
2
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1answer
39 views

How come two protons create top-antitop quark pair? (A question regarding CERN courier May 2016)

I am not very well versed in particle physics lingo but as much as I know $p$ stands for proton and $t$ stands for top-quark. Then, how could this be possible? I hope I am wrong about what is ...
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1answer
66 views

How are the particles produced at the very early universe?

We know that according the standard model of cosmology $\Lambda {\rm CDM}$, there is a primordial plasma after the inflation. And then we study the non-equilibrium evolution of this plasma such as ...
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2answers
269 views

From the viewpoint of field theory and Derrick's theorem, what's the classical field configuration corresponding to particle? Is it a wavepacket?

In the framework of QM, we have known that particle, like electron, cannot be a wavepacket, because if it is a wavepacket then it will become "fatter" due to dispersion and it's impossible. However ...
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2answers
81 views

What particles do these Feynman diagram lines represent?

What particles could be represented by A and B (labels I have added myself - this is not a homework question)? For A, it looks like it's supposed to be a photon, but I was under the impression that ...
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1answer
60 views

A few questions on quarks, gluons and the strong force

I wanted to answer for myself some pretty fundamental questions pertaining to the strong interaction, color charge and quarks, but I'm sort of stumbling on a few. So I wanted to find the reason/...
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23 views

Semileptonic decay amplitude

I'm trying to do past exam papers and I am absolutely failing understand some things. I would very much appreciate any insights/intuition you have. The part I'm struggling is here: In the first ...
2
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1answer
79 views

Supersymmetry and grand unification

I came across this diagram in my introductory particle physics class. Given the fact that SUSY can make the plot look suspiciously nice, what can we do with it? Also, electromagnetism has been ...
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0answers
32 views

Why can an electric chargeless Z-particle decay in three photons?

The Z-particle that is supposed to convey the weak force has no electrical charge. So how is it possible the particle can decay in three photons. Because the weak and e.m. force are more or less ...
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1answer
83 views

How to understand the idea of functional renormalization group?

I have been looking at how to use the functional RG method in many-body systems, but I don't quiet get the idea of it, it look different from Wilson's RG approach (eg. why shall we integrate out the ...
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0answers
21 views

B meson mixing and CP violation

From my understanding of CP violation in the weak interation only the $V_{ub}$ and $V_{td}$ terms in the CKM matrix contain the complex phase terms which introduce CP violation. In B meson mixing, ...
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0answers
33 views

Electromagnetic Field caused by accelerating and oscillating charge particles?

Will accelerating and oscillating charges will cause EM waves of different shapes ? As EM waves are always shown as sinusoidal Electric and Magnetic Fields perpendicular to each other. Does it mean ...
2
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2answers
146 views

The mass of the proton times its charge radius is very close to 4ħ/c. Is this a coincidence?

The claim is $r_p m_p = 4 L_0 M_0 = 4\hbar/c$, where $r_p$ and $m_p$ are the proton's charge radius and mass, and $L_0$ and $M_0$ are the Planck length and mass. Using the muon measurement $r_p=0....
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38 views

Colliding beams vs static target

In this problem I need to find the threshold energy of a positron-electron collision that creates a Z boson (the reverse of the following picture; Z mass 92 GeV) in two distinct cases: collision ring ...
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45 views

Are quadrilinear weak boson couplings possible?

$ZZ\rightarrow ZZ$, $ZZ\rightarrow W^+ W^-$, etc. Are these interactions possible?
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34 views

Feynman Diagram of $\nu_\mu + p \rightarrow \nu_\mu + p + \pi^0$

According to my class notes, the following interaction is possible $$\nu_\mu + p \rightarrow \nu_\mu + p + \pi^0$$ I tried drawing a Feynman diagram and got the below, but don't know what happens ...
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0answers
18 views

Conversion from PPBV to PPBM [closed]

What is the conversion between PPBV and PPBM? I have a database that includes values of Parts-per-billion-by-volume (PPBV) as a function of altitude. I've been asked to convert this to "parts-per-...
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1answer
30 views

For processes with multiple Feynman diagrams is the cross section increased?

For example in electron, positron annihilation $e^- + e^+ \rightarrow 2\gamma$ has 2 diagrams, whereas, $e^- + e^+ \rightarrow 3\gamma$ has 6 possible diagrams. This suggests, $\frac{\sigma_{2\gamma}}...
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0answers
19 views

Decay kinematics of Higgs boson

So I'm looking at a two-body decay question for the $h\rightarrow\gamma\gamma$ decay channel for the Higgs Boson. I am asked to find the invariant mass of the Higgs boson. I'm not too sure how to ...
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9 views

Question pertaining to the branching ratio of Ka0ns

I'm a little lost for part c of this question: I understand the $\mathcal{BR}$ (branching ratio) for species $i$ is $\mathcal{BR}=\Gamma_{i}/\Gamma_{T}$ (total Decay width). I know that $$\...
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19 views

Invariant amplitude in QED in terms of Mandelstam variables [closed]

So I'm having a little trouble with this question: These are my workings thus far: I just can't seem to get certain terms to vanish. I have the 1/4 factor for going between $8e^4$ to $2e^4$. Is ...
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1answer
41 views

Why is the energy shift due to a 'sagging' potential negative and independent of box size?

Consider a box of width $L$ and the composed of the following potential $$V(x)=\frac{V_0x(x-L)}{L^2}, x\in[0,L]$$ and $V(x)=\infty$ elsewhere. Using perturbation theory - with a square box as the ...
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1answer
37 views

How would one go about drawing a Feynman diagram of color screening and anti-screening in QCD?

Title really explains this one. I'm not sure how to put together a diagram of the lowest order for colour screening and anti-screening in QCD. Seems to be a lack of diagrams for this in the literature!...
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50 views

How to draw Feynman diagram for K^- + p -> Sigma^0 + pi^0 — superposition in diagram

I'm trying to draw the Feynman diagram for the reaction $$K^- +p \rightarrow \Sigma^0 + \pi^0$$ I have two problems: How to draw the $\pi^0 = (u\bar u + d\bar d)/\sqrt 2$, a superposition of ...
3
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1answer
48 views

Pion decay: why is the positron electron neutrino decay suppressed?

I'm reading through M. Thomson's Modern Particle Physics to better understand why the positron electron neutrino decay channel is suppressed relative to the muon and muon neutrino decay route. I know ...
2
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1answer
41 views

What experimental measurement could be used to show that a neutrino is a Majorana and not a Dirac particle?

I've just been reading something on the concept that neutrinos could be Majorana particles and not Dirac fermions. I was wondering what experimental measurement could show/prove that neutrinos are in ...
2
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0answers
59 views

Explanation of graphs from CERN [closed]

I have a few questions about these kinds of graphs What is the name of this type of graph? What does the width of the peak mean? If the points are data points, how was the curve created/predicted? ...
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37 views

Gravitational entropy [duplicate]

In the beginning (big bang), mass was evenly distributed (if you consider an elementary particle as an extended object, like a circle on a very thin cylinder, with a radius near the Planck length, but ...
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1answer
20 views

Semileptonic decay of neutral D meson: branching ratios?

I am looking for the branching ratio of the semileptonic decay of the neutral $D^0$ meson: BR($D^0 \to \pi^+e^-\bar\nu_e$) = ? According to the PDG book the branching ratio for $D^0 \to \pi^-e^+\...
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1answer
44 views

What determines the color of photon that is emitted from an exited atom?

I understand the principle of how light is emitted from an atom. What I don't know is why neon atom is red and copper is green when exited? Is is the distance between the electron to protons or the ...
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0answers
30 views

Are alpha and beta rays also generating electromagnetic rays?

To discover radio waves in 1886, Hertz used electric sparks between two iron balls. These sparks caused radio waves which generated a current between other balls. The oscillating charges caused a ...
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3answers
112 views

Why is $np$ always equal to $n_i^2$?

For you guys who studied semiconductor physics must be familiar with the equation: $$np=n_i^2$$ I can understand why this is true for the intrinsic case (the broken bonds would always provide ...
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1answer
47 views

Fine structure constant and unit conversion [closed]

In a paper I'm reading, the author writes down the following formula: $$\Gamma=\dfrac{\alpha^2}{576\pi^3}\dfrac{\left(4+z\right)^2}{z}\dfrac{m^5}{m^2_\pi f_\pi^2}$$ $\Gamma$ is a function of $m$ (in ...
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1answer
100 views

How do I calculate the differential cross section with respect to the transversal momentum?

First of all, sorry for my English, my first language is German. My problem is: I calculated the matrix element of the quark-gluon-Compton process (q+g -> gamma + q). With the kinematics of ...
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0answers
47 views

I want to know about the quantization of mass [duplicate]

Is mass quantized? If yes, then why do we see all amounts of mass? Well , can it be said quantized just for smaller masses?
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1answer
36 views

Differential cross-section for a 2-particle process in the LAB frame

This should really be a straightforward calculation, but somehow, I keep confusing myself and failing over and over again. I did the calculation so many times that I don't even know what I'm looking ...
3
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1answer
135 views

How do (and don’t) particles emerge from fields?

I am aware of the following field- and particle-like notions: QFT particle, a unit of excitation in (the Fock space of) a QFT; SR field, an extremal $A = A(\mathbf x)$ of a Lorentz-invariant action; ...
28
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3answers
4k views

How can an electron fake a jet?

This is a question for experimentalists. I have seen in several ATLAS papers (see for example chapter 4 in arXiv:1602.09058, 6th paragraph), that after objects have been correctly identified, any jet ...
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0answers
16 views

Conjugation Operator General Question

I’m sitting here reading my particle physics book, and the Conjugation Operator is defined as: $C = -(-1)^{S+1}(-1)^{L}$, where $L = $ relative angular momentum for the $q\overline{q}$ pair for a ...
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0answers
46 views

Neutrino interaction probability [closed]

Just a quick question, if a single 1GeV neutrino (muon neutrino) were fired at a block of iron with a given density, $\rho$, and the neutrino-nucleon interaction cross section is $\sigma$, what would ...
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0answers
59 views

What is more fundamental $E=\hbar \omega$ or $E=c\hbar k$?

Let us say I have a massless particle (not necessarily a photon) then in a medium where $\omega \ne c k$ (e.g. a dispersive medium) what is the energy of the particle? As my title indicates are think ...
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1answer
12 views

Adjoint Dirac equation (in momentum form) from Dirac eq in momentum form method

I just wanted to check the method I have formulated for the derivation for the adjoint Dirac equation using Gamma matrice notation. This is a problem from the very excellent "Modern Particle Physics" ...
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1answer
32 views

Motion of an electric charge within an electric field with two electric charges

Assume that air resistance and gravity are negligible and the only significant force acting in the scenario is the electric force. There are two electric charges, both with an equal and positive ...
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1answer
53 views

Sequence of topics for studying quantum theory (Research strategy) [duplicate]

I am trying to learn about the higher level basics of physics. I want to cover up all that is included in particle physics/quantum physics. Now being in high school, I only know partially about ...
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1answer
29 views

Taking Measurements of Quantities in QM

I have a quick question relating to Annihilation and Creation operators, and in taking observables in general. Let's say, for instance, that I prepare a particle so that I consider the projection of ...