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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106 views

Little confusion in drawing Feynmam diagram

If the arrows of both the outgoing solid lines of the Feynman diagram corresponding to the bhabha scattering of $e^+$ and $e^-$, are just reversed, will it not describe same thing? Doesn't both imply ...
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227 views

Gauge fermions versus gauge bosons

Why are all the interactions particle of a gauge theory bosons. Are fermionic gauge particle fields somehow forbidden by the theory ?
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53 views

Fusion of two neutrinos

What are the results of fusing an electron neutrino and a muon neutrino? If you have an answer, then why? I looked this up and searched for stuff on Google, Wikipedia and a few other sites, but all ...
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41 views

Particle physics conservation law checking tool

I'm just starting out with simple particle physics, and I'm doing a ton of exercises where I have to check if a certain reaction is allowed, from the point of lepton/baryon/energy conservation and ...
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1answer
202 views

Parity of proton is 1?

I have found from Wikipedia that "a parity transformation is the flip in the sign of spatial coordinates". Now when we operate parity operator, does that mean we are taking any physical entity at ...
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49 views

A question about particle scattering

For massless spin-1/2 fermions say $N$ I am using the spinors as given here say - http://theory.fnal.gov/people/ellis/Calctools/spinor.pdf - the $u_{+}(k)$ and $u_{-}(k)$ on page 2. So the $u_{+}(k)$ ...
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1answer
52 views

About muons, taus and the number of generations

there! Why cannot consider the muon or the tau just like excited states of the electron (and similarly, with the (u,d) (c,s) (t,b) pairs? I guess that it is related to the absence of decays like ...
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2answers
110 views

Do atoms have an outter shell or boundary?

Do atoms have boundaries or an outer shell? If so, how do photons react to it? Also, if not, then does the atom always have the same set of electrons or do the electrons keep on changing? What I mean ...
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1answer
148 views

Two distinguishable particles in a box

I would like to determine the number of energy states two free, distinguishable particles in a box of length $L$ have. I would then like to determine the number of states two free, indistinguishable ...
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1answer
110 views

Sterile Neutrinos and Supersymmetry

Just read a Scientific American review of a recent article which suggests evidence for sterile neutrinos: ...
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2answers
543 views

Does a muon detector on Earth's surface correctly measure the mean lifetime of a muon?

Just a simple question. Does a muon detector on Earth's surface correctly measure the mean lifetime of a muon? I would think the answer is no because most muons detected are created about 15 km above ...
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2answers
139 views

Do neutrinos not couple to the Higgs field?

I was reading the CernCourier, my favorite source of message on Higgs and friends. I was rather shocked, when I saw this: "The mechanism by which neutrino mass is generated is not known." What? ...
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353 views

Did CDMS identify dark matter?

A recent paper by the CDMS collaboration (PRL here and free text here) makes this statement in the abstract: This blind analysis of 140.2 kg day of data taken between July 2007 and September 2008 ...
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36 views

Range Of An Interaction

Why is the Compton wavelength $\lambda_c=\frac{\hbar}{mc}$ used as a sensible measure for the range of an interaction, where m is the mass of the corresponding mediator?
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53 views

Isospin and weak Isospin

What is the difference between isospin and weak Isospin? What is the difference between hypercharge and weak hypercharge?
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2answers
188 views

If nature exhibits symmetry, why don't up and down quarks have equal magnitude of electric charge?

I always hear people saying symmetry is beautiful, nature is symmetric intrinsically, physics and math show the inherent symmetry in nature et cetera, et cetera. Today I learned that half of the ...
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67 views

Calculating the Neutron Stopping Power of complex materials

Is there a fast and convenient way of calculating the neutron stopping power of materials, consisting of multiple elements (e.g. doped crystals) without the need for Monte Carlo Simulation, that is ...
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1answer
128 views

Seesaws and Dark Energy

Lawrence Krauss and James Dent recently proposed a mechanism for producing the observed scale of dark energy. This proposal was inspired by the see-saw mechanism that produces light yet non-zero ...
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1answer
204 views

Orbital angular momentum of photon

People talk about orbital angular momentum (OAM) of photons. Is there some physical example that cannot be explained without assuming that photons have non-zero OAM? Does different photons have ...
5
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1answer
121 views

$\tau$ pair production question

There's a question on my homework about the process $e^{-} e^{+} \rightarrow \tau^{+} \tau^{-}$. Specifically, it is claimed that the minimum energy required of the colliding positron and electron ...
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1answer
115 views

What is the difference between QFT and elementary particle physics?

I'm a little unclear as to how QFT differs from Elementary particle physics. They both use pictorials of Feynman graphs, is it that Elementary particle physics assumes the point particle perspective, ...
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44 views

Looking For The Derivation Of Gellmann-Nishijima Relation

How is the Gellmann-Nishijima relation $Q=I_3+\frac{Y}{2}$ derived? Either a derivation or a link will be helpful.
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2answers
502 views

Why is electric charge conserved?

We have long been taught that electric charges are neither created nor destroyed. But somehow it is okay to destroy two oppositely charged particles at once ! Why is that so? Let's just take a look ...
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2answers
217 views

Why absence of electron is called hole?

I am having hard time in understanding the concept of holes: If there is no electron than how can it be a hole? For a moment lets assume absence of electron is termed as hole but how can this absent ...
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1answer
108 views

Why did it take a long time to discover top quark?

In high school physics, I learned that it took a long time before the top quark was discovered. One of the reasons that was given in my book was that the top quark has a large mass, much larger than ...
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17 views

Accelerating cavity in synchotron- how does it actually work? [duplicate]

I wanted to know how a radio frequency accelerating cavity actually works. I know that an electric field is used to accelerate the charged particles. However, is this done by using metal plates with ...
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1answer
77 views

Same U(1) charge for the SU(2) doublet

Consider the symmetry $SU_L(2)\otimes U_Y(1)$. The entries of $SU_L(2)$ doublet will have same U(1)-charge. How can this be shown mathematically?
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1answer
125 views

Massless neutrinos and Chirality

The massless neutrinos can be represented by two component Weyl spinors. Then how does one say that it is an eigenstate of the chirality operator $\gamma^5$, which is a $4\times 4$ matrix and can act ...
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1answer
113 views

No valid Feynman diagram for processes

This will likely be easy for anyone experienced in particle physics, but I'm not. I'm asked to explain why it is impossible to construct a valid Feynman diagram using Standard Model vertices for the ...
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109 views

Neglecting mass at asymptotic spacelike momenta

What is the rational/reason for neglecting masses at asymptotic non-exceptional space-like momenta. I have come across this as a first fix for being able to extract information from the ...
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4answers
156 views

Observed composition of UHE cosmic rays

How much is known about the composition of ultra high energy cosmic rays (say $E>10^{20}\text{ eV}$)? I get the impression that the particles are often assumed to be protons or other heavier ...
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1answer
154 views

Photons and proper time

Why is there no proper time without inertial frame? In question n°95054 I learned that there is no proper time zero and no proper distance zero for photons because they are no inertial frames. That ...
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2answers
272 views

How come the universe is made of matter and not antimatter?

Antimatter is like matter on opposite day: it has the same properties as the stuff that makes up planets, stars and galaxies, but one vital piece is different—its charge. The universe supposedly ...
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1answer
87 views

Multi-Fermion interactions induced by integrating-out Yukawa-Higgs terms?

Suppose one considers a multi-component free fermions field theory with field $\psi_{q_i}$ with a give global symmetry (such as U(1)). We can say that every component of fermions carry some U(1) ...
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72 views

Interesting identity on $SU(3)$

In arXiv:hep-ph/1307.5414 Grabovsky use an interesting identity which is not derived in the paper: ...
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2answers
225 views

Oil drop experiment and quantization of charge

How to systematically show that the resulting charges in oil drop experiment are integers multiplied by $e$ in other word how to extract $e$ from the data?
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212 views

Does the fact that protons and neutrons have larger mass than electrons mean they're bigger in size?

and so if a proton is so larger than an electron doesn't that mean it has a shape? What would be the shape of a subatomic particle? are they spherical?
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140 views

Wavefunction of a Baryon

How to write the total wavefunction of a Baryon including space part, spin part, isospin part and color part such that the net wavefunction is anti-symmetric? What is the difference in wavefunctions ...
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1answer
195 views

Can gravity be reversed?

I'm wondering if any set of hypothetical conditions could result in us gravitating in a different direction. Specifically a reversal of gravity on the surface of Earth (or at least part of it), so ...
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222 views

Why does the spiral of a positron have a larger radius than that of an electron in this picture in a bubble chamber?

The smaller spiral is caused by an electron The bigger spiral is caused by a positron However, they have the same mass and magnitude of charge. So, during this pair production, why does the positron ...
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96 views

Could sphaleron-induced proton decay also cause vacuum decay?

I will say right away that I don't mean standard-model sphalerons, I mean the sphalerons of some extension of the standard model. The reason to even think about this is last year's paper by Frampton ...
3
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1answer
115 views

Sign of mass of an anti-particle

When deriving the Lagrangian for Spin $\frac{1}{2}$ particles we are naturally led to using $\Psi$ and $\bar{\Psi}$. The Euler-Lagrange equations lead us to two wave equations: \begin{equation} ...
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1answer
220 views

Why is the charge on protons == to charge on electrons? [duplicate]

I am not a expert on physics, just another high schooler, so sorry if the question is obvious. This is something I've been wondering about for a while. Why is the charge on a proton equal but ...
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55 views

What is the smallest distance measured that the muon is considered pointlike?

What is the smallest distance measured that the muon is considered pointlike. I know for the electron it is clost to 0.001 of a femtometer. It seems to be assumed that this is the same for the muon. ...
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1answer
163 views

Why positronium can annihilate in vacuum?

I thought that the annihilation process of positronium cannot take place without a third-party particle. This can be directly derived from energy & momentum conservation: energy conservation: ...
2
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1answer
61 views

Interaction muons with Iron

I would like to know why $\mu^+$ muons can easily penetrate a solid metal such as Fe with negligible interactions while $\pi^+$ mesons lose their energy a lot faster when traveling through the metal. ...
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1answer
90 views

Can a positron beam cut through metal?

Can a positron beam be used to cut through metal?
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35 views

flavor splitting of quark masses

It seems pretty clear that flavor symmetry splits the quark masses. It seems extremely odd that a global gauge symmetry of the Standard Model fields could or would produce such extreme splitting as ...
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3answers
669 views

Do particle accelerators produce (dangerous) radiation?

I was under the impression that particle accelerators were pretty harmless, but some article said that they produce harmful radiation when you're in the tunnel. Given that the Internet... isn't always ...
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1answer
115 views

Quantum corrections to massless fermionic field

in QED the corrections to electron propagator change the bare electron mass from $m_0$ to $m=m_0+δm=m_0+∑(\not{p}=m)$ (Peskin, formula 7.27). This is the consequence of the fact, that the quantum ...