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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Non-existence of gluon singlets: Any recent theoretical progress?

An unanswered question from last year (2012) on gluon singlets asked whether there is any theoretical explanation for the experimental absence of the ninth or colorless (singlet) gluon. This is the ...
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0answers
50 views

the effects of an ln-prime transformation to physical models

I have rather a "toy" type of modelling-problem that appeared to me along a book I am writing on number theory. I would be outmost thankful for any concrete or inspirational answers, including ...
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0answers
36 views

Limits on stable charged particles

Are there generic model-independent limits on massive stable charged particles (say 10-500GeV)? I mean SU(2)/color singlets with just a hypercharge. For example the LEP search (CERN-EP/99-075) was ...
6
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538 views

What goes wrong when one tries to quantize a scalar field with Fermi statistics?

At the end of section 9 on page 49 of Dirac's 1966 "Lectures on Quantum Field Theory" he says that if we quantize a real scalar field according to Fermi statistics [i.e., if we impose Canonical ...
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1answer
174 views

Anti-particle problem for Dirac sea

According to the Dirac hole theory we know that Dirac sea is completely filled with negative energy, called vacuum. We will need $2mc^2$ or greater to get electron and a positron by incident photon. ...
3
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1answer
205 views

Complex masses for Dirac and Weyl spinors

I'm trying understand how to rotate Dirac fields to absorb complex phases in masses. I have a few related questions: With Weyl spinors, I understand, $$ \mathcal{L} = \text{kinetic} + ...
7
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1answer
291 views

Precise statement of Mermin–Wagner theorem

Roughly speaking, Mermin-Wagner theorem states that continuous symmetries cannot be spontaneously broken at finite temperature in systems with sufficiently short-range interactions in dimensions ...
4
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2answers
380 views

How to directly calculate the infinitesimal generator of SU(2)

We commonly investigate the properties of SU(2) on the basis of SO(3). However, I want to directly calculte the infinitesimal generator of SU(2) according to the definition $$X_{i}=\frac{\partial ...
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0answers
104 views

Categorizing solutions to Hierarchy problem

We know that no gauge symmetry can prevent a term $m_\phi^2|\phi|^2$ for a scalar field, and that, given the quadratic loop corrections, the natural scale is $m_\phi \sim M_P$. This is related to the ...
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1answer
144 views

can be exist the negative mass? [duplicate]

I'm not sure about this but I guess there must be negative masses in the universe because of the symmetry. If the gravity is one of the main forces in nature it must has negatives mass to be able to ...
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1answer
328 views

what is the magnetic quadrupole operator?

To find magnetic or electrical moments in quantum theory we must calculate the expectation value of an appropriate operator. the dipoles operator are similar and is easy to find but the magnetic ...
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1answer
66 views

Interpreting the results

I have preformed the muon lifetime experiment at my uni's lab, and got the data. It's text file with 8190 numbers. My TDC unit was set so that the time gates were at 10 $\mu s$, and it has 8192 ...
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3answers
571 views

Resonances in high energy physics

I still do not understand what a resonance precisely is. Is it exactly the same as a particle? Or only an excited state? And why does it make a peak in some diagrams? And which diagrams?
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1answer
313 views

Can 3 photons be combined to give a spin-0 projection?

Motivation: The neutral pion decays to 2 photons ($\pi^0\to\gamma\gamma$) most of the time. For the decay of the neutral to 3 photons ($\pi^0\to 3\gamma$) we have an upper limit on the branching ...
4
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1answer
159 views

Asymptotic Freedom - Qualitative Explanation

I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of asymptotic freedom. The lecture notes basically say that a quark may experience no force/be "unbound" ...
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2answers
3k views

What happens in electron-electron collisions?

What are the results of high energy electron electron collisions? Are other particles created?
4
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1answer
137 views

Are electrons simple? Do they have any inner structure? [duplicate]

The plank length is far smaller than the classical electron radius. Could the electron have structure?
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1answer
140 views

Why Planck scale is so important?

I know that Planck scale is the scale where both, gravity and quantum effects are relevant simultaneously. Are there more reasons?
3
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1answer
101 views

Parametrization of $U(N)$ non-linear sigma model

The motivation of this question actually comes from this (really old) paper of Weinberg. He considers a theory of massless pions. They have a chiral $SU(2)_{L} \times SU(2)_{R}$ symmetry. The pions ...
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0answers
113 views

List of cross sections?

Sometimes I need to look up a certain cross section, say the inclusive Z production cross section at $\sqrt{s}$ = 7 TeV. Is there a place where 'all the' cross sections are tabulated ...
5
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1answer
85 views

Caveats when using event-by-event reweightings?

A fairly common technique in experimental particle physics is event-by-event reweighting. The idea is that you have a sample of background-model events, either from a Monte Carlo simulation, or from a ...
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103 views

Matrix element approximation

In the formula for the decay width of $\Upsilon(4S)$ to B-mesons from $\text{e}^+\text{e}^-$ collisions: $$\Gamma_{\Upsilon(4S)\to B\bar{B}}=\frac{\left|\underline{P}_B \right|}{8\pi ...
6
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0answers
271 views

Calculating the branching ratio of higgs for decay to two photons? [closed]

I need to use the three lowest order Feynman diagrams to first calculate the squared matrix element to put into Fermi's golden rule formula and then from there get the branching ratio of higgs decays ...
2
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1answer
385 views

Baryon wave function symmetry

If a baryon wavefunction is $\Psi = \psi_{spatial} \psi_{colour} \psi_{flavour} \psi_{spin}$, and we consider the ground state (L=0) only. We know that the whole thing has to be antisymmetric under ...
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2answers
176 views

Does it make a sense to speak about age of electron or atom?

It's possible that this question is too soft or even quite senseless for this forum, but I will ask nevertheless. Everyday (macroscopic) things, like a grandfather's pendulum clock or the grandfather ...
14
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0answers
387 views

Status of experimental searches for tachyons?

Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for neutrinos. Although the OPERA claim ...
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1answer
426 views

is the nature of particle beam weapons in science fiction true to the reality of particle physics?

I am referring to the use of specific particle types such as an antiproton beam, positron beam, meson beam or muon beam for example in the likes of shows like star trek. I was curious if a beam of a ...
3
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1answer
782 views

Infinite reflection of light and the conservation of energy / momentum

First off, I confess I'm no physicist, but I have been asking people with a more extensive knowledge this one question, without a definitive answer so far. Basically, I'm playing around with the idea ...
14
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2answers
275 views

Infrared-free QED and Higgsless standard model phenomenology

This is one of those "what if" fantasy world type questions. I like hard sci-fi so please no "well, you changed one thing about the world so now anything goes." :) What if the Higgs had no vev? That ...
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1answer
416 views

Elementary question on pion-proton scattering cross-section

Is $E_k$ the energy of the outgoing or incoming pion? The first peak is supposed to be a delta baryon. What does the graph tell us, experimentally? A pion of kinetic energy x comes in, then we ...
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1answer
71 views

Entangled photons creating particle pairs

Two gamma ray photons are entangled and travel through space. The first gamma photon interacts with a low frequency photon and creates an electron positron pair. What happens to the other gamma ...
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1answer
292 views

Parity and Particle Exchange Operators

I would be grateful if somebody could help me brining the parity operator and the particle exchange operator together. Suppose, there is a two-proton system, where one proton is sitting at $+r$ on ...
5
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1answer
232 views

Earliest example of naturalness/fine-tuning arguments

The notion of naturalness is important in particle physics, especially supersymmetry. I was a little surprised, then, that the idea, or at least the name, is apparently only ~30 years old ('t Hooft, ...
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2answers
502 views

Quantum tunneling is faster than light travel?

Quantum tunneling is faster than light travel ? My reasoning is that the particle cannot be detected inside the tunnel so if it travels from A to B it must be instantly going from A to B , hence ...
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146 views

Field content and symmetry groups of Minimal Composite Higgs Models

I'm trying to teach myself the Composite Higgs Model, both its theory and its LHC phenomenology (particularly the 4DCHM). Unfortunately, I'm struggling; the literature is contradictory and/or omits ...
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2answers
1k views

Transfer of electron energy to atoms (heating up of matter by absorption of photons)

If an electron absorbs a photon to get exited to a higher energy level, it should either come back to same state or any other lower state by emitting the required photon. How then can there be a net ...
4
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2answers
824 views

What is the Standard Model equation which unified Strong, Weak, and Electromagnetism?

I always heard that three atomic force in small scale structure (Strong, weak and electromagnetic) are unified through the standard model, but I've never seen its unified equation. What is single ...
2
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4answers
156 views

What is the process that gives mass to free relativitic particles?

When a free particle move in space with a known momentum and energy then what is the physical process that gives mass to that free (relativistic) particle? What is role does the Higgs field in that ...
2
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1answer
210 views

Proton-proton collisions

I have a question about proton-proton collisions at the LHC. Firstly, the 4-momentum $p^\mu=(E/c,\vec{p})$ can be represented as $p^\mu =(m_T \cosh \Psi, p_T \cos \phi , p_T \sin \phi, m_T c \sinh ...
3
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3answers
358 views

Second baryon octet

Let's temporarily ignore spin. If 3 denotes the standard representation of SU(3), 1 the trivial rep, 8 the adjoint rep and 10 the symmetric cube then it's well-known that 3 x 3 x 3 = 1 + 8 + 8 + 10 ...
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1answer
364 views

Single photon's effect on conservation of momentum?

When your looking at basic Compton theory you find that if you shoot a stream of photons at a particle (usually atoms or electrons), then you have the basic laws of conservation of momentum. The ...
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1answer
409 views

Which was first, energy or matter in the creation of our universe?

Was it the Big Bang or was it something else that gives us our universe in its present condition? Did it all begin with just pure energy that eventually evolved into simple atoms of matter, that ...
2
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0answers
118 views

Parton Distribution Functions, average cross section of the nucleon

Say we are given the scattering cross section for neutrinos from $d$ and $\bar{u}$ quarks as $\frac{d\sigma^{d}}{dQ^2}=\frac{G_F^2}{\pi}$, $\frac{d\sigma^{\bar{u}}}{dQ^2}=\frac{G_F^2}{\pi} (1-y)^2$, ...
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1answer
2k views

What is Transverse Energy?

What is transverse energy? Why we use transverse total energy instead of energy and transverse momentum in place of Total momentum in the particle detectors?
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3answers
1k views

Does a photon have mass? [duplicate]

I have seen questions assume photons have no mass. But I have not seen any questions that directly ask whether or not photons have mass. If photons have no mass, then how do they occupy space? How ...
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1answer
197 views

Facts About Quarks Electric Charge [duplicate]

Quarks have the unusual characteristic of having a fractional electric charge. here there is a new model that suggests maybe an up Quark has no electric charge and infact down Quark has electric ...
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0answers
170 views

Mirror Matter Hypothesis?

What is the current state of the hypothesis of mirror matter today? Are there any experimental data or theoretical arguments that exclude it by now, or is it still considered viable among physicists? ...
3
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1answer
106 views

Convolving Parton Distribution Functions

I have the cross-sections as a function of $\sqrt{s}$ for a process with a $u$-quark and $u$-antiquark in the initial state (eg.: $u \bar{u} \to e^- e^+$). I have a standard parton distribution ...
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2answers
472 views

What particles carry various forms of energy?

If I didn't get this wrong, light or heat energy consists of photons and they in turn effect electrons' behavior and thus responsible for chemical and electrical energy. What kind of similar particle ...
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3answers
1k views

Quantum mechanics - how can the energy be complex?

In section 134 of Vol. 3 (Quantum Mechanics), Landau and Lifshitz make the energy complex in order to describe a particle that can decay: $E = E_0 - \frac{1}{2}i \Gamma$ The propagator $U(t) = ...