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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Is there a theoretical upper bound on the mass any new particles can have?
One possible outcome of the collision experiments at LHC is the discovery of new elementary particles with large mass. Is there a theoretical way to derive an upper bound on the mass of elementary ...
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5answers
426 views
What Would be the Experimental Signature of Composite Leptons?
So far, the quarks and leptons appear to be fundamental particles. But they're complicated enough that there's always been some speculation that they might be composite.
What experimental evidence ...
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96 views
Status of the little hierarchy problem
What is the current thinking on the little hierarchy problem in light of a potential Higgs mass above 120 GeV? A few years ago, at least, I remember various phenomenologists saying that this at least ...
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295 views
What is meant by the phrase “the mass is protected by a symmetry”?
In a particle physics context I've heard this phrase used. I guess it means that the mass of a particle is less than you'd naively expect from $E=mc^2$ after computing the momentum uncertainty ...
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255 views
Weak contribution to nuclear binding
Does the weak nuclear force play a role (positive or negative) in nuclear binding?
Normally you only see discussions about weak decay and flavour changing physics, but is there a contribution to ...
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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. ...
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209 views
Dynamical supersymmetry breaking and Witten index
Witten index, defined as ${\rm Tr}(-1)^F$, makes know if supersymmetry is spontaneously broken or not for a given model. But it is known that supersymmetry can be also broken dynamically and one can ...
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1answer
183 views
Weinberg angle measurement methods
I was reading up on the history of $W/Z$ bosons today and I got a little puzzled. I always assumed that people measured $M_Z$ and $M_W$ and then derived the Weinberg angle. But it appears that they ...
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338 views
What is a quark condensate?
What is a quark condensate? is it a bound state between 2 quarks? can we have 3(or more)-quarks condensate?
What mediates the interaction between the constituents of the condensate?
Are the ...
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1k views
What is the significance of the QCD scale parameter $\Lambda$?
I see that it appears as a constant in the relation for the running of the strong coupling constant. What is its significance? Does it have to be established by experiment? Is it somehow a scale for ...
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1answer
408 views
Proof of Yang's theorem
Yang's theorem states that a massive spin-1 particle cannot decay into a pair of identical massless spin-1 particles. The proof starts by going to the rest frame of the decaying particle, and relies ...
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1answer
43 views
Which arguments for $m_u \approx 0$ are still in the market?
The RPP note on quarks masses has traditionally carried, and it is still there, the comment that
It is particularly important to determine the quark mass ratio mu/md,
since there is no strong CP ...
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1answer
187 views
Has anyone actually produced any braided anyons (quasi-particles) yet? (quantum computer)
Quasiparticles theoretically can form braids on a 2D surface. The braids apparently are quite stable for reasonably long periods, allowing a superposition state more time before it decoheres through ...
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1answer
325 views
Fundamental particles with spin > 1
I am in undergraduate quantum mechanics, and the TA made an off-hand comment that currently no one knows how to describe fundamental particles with spin > 1 without supersymmetry. I was curious and ...
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1answer
408 views
What's with Mandelstam's argument that only linear regge trajectories are stable?
While thinking about how to answer a "describe string theory" question, I remembered an old argument of Stanley Mandelstam's that linear Regge trajectories implies stability. I never fully understood ...
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3answers
175 views
supressing certain decay paths and enhancing others with interference
In a scattering reaction, there are many possible final states for the products, each with different production rates.
Question: Is there a way in which we could in general supress certain rates ...
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202 views
How is the distance between Geneva detector and the Gran Sasso one measured?
Does anyone know how is the distance between detectors in the now famous neutrino experiment measured? Also, how was the time of flight measured?
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749 views
How do Leptons arise from Lambda decay?
I have a question for an assignment:
Use your understanding of the quark model of hadrons and the boson model of the weak nuclear interaction to explain how leptons can arise from lambda decay, ...
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1answer
458 views
What is the difference between 'running' and 'current' quark mass?
When looking at the PDG, there is a difference between the 'running' and the 'current' quark masses.
Does anyone know which is the difference between these two?
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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
161 views
SUSY, ways to boost Chargino/Neutralino production?
Does anybody know a good reference that works out the equations for the Chargino/Neutralino production cross section in SUSY? I'm trying to understand if there are any tricks for boosting the ...
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0answers
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status of +4/3 scalar as explanation of $t\bar t$ asymmetry
One of the early proposals for the Tevatron asymmetry on $t \bar t$ was a "fundamental diquark" with a charge (and hypercharge) +4/3, either in a triplet or a sextet colour. I am interested on the ...
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208 views
Is there an explanation for the 3:2:1 ratio between the electron, up and down quark electric charges?
I understand that the NNG formula relates $Q$, $I_3$, and $Y$ and can be derived in QCD; does this unambiguously predict the electric charge ratios without making assumptions about the definitions of ...
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What defines the mass of elementary particle?
The electron is particle.
The mass of electron is $9.10938215(45)\times 10^{−31}\, {\rm kg}$.
But why is the mass exactly what it is?
What in physics defines the mass of elementary particle?
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Do Killing spinors know global information?
The conformal Killing spinor equations on $R\times S^3$ in Minkowski signature are
\begin{equation}
\nabla_\mu \epsilon=\pm \frac{i}{2}\gamma_\mu\gamma^0\gamma^5\epsilon
\end{equation}
whose solution ...
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584 views
Kinetic Energy vs. Potential energy with regards to creating particles
So I know that when you collide particles with high enough kinetic energy, (kinetic energy = at least the rest mass of the particles you are making), you get particles.
How come potential energy ...
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The observation of a non-SM resonance at 38 MeV
Was reported here. Of course if this is real it is very exciting. It leads me to the question: given that it took so long to find this resonance at a meager 38 MeV, is it possible that all SUSY ...
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What do you think about teaching Standard Model in school? [closed]
Here is a scan from an old Soviet textbook for school children:
It shows the table of quarks and antiquarks of different generations, colours, spins.
The book also includes similar tables of gluons ...
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2answers
2k views
Why is pseudorapidity defined as $-\log \tan \theta/2$
Why the log? Is it there to make the growth of the function slower?
As this is a common experimental observable, it doesn't seem reasonable to take the range from $[0,\infty)$ to $(-\infty,\infty)$ ...
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143 views
If helicity can suppress weak decay of $\pi^{\pm}$, doesn't that make chiralty observer dependent?
I have been under the impression that chirality is at least in part a way to sneak in a Lorentz invariant version of helicity for particles that mass. Flip Tanedo seems to espouse this view in a ...
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4answers
662 views
Is there the smallest particle that can be guaranteed to be unable to be broken down into smaller particles?
Is there the smallest particle that can be guaranteed to be unable to be broken down into smaller particles?
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1answer
177 views
Spin of Higgs boson 125 GeV
Can someone please explain to me why (according to decay of Higgs boson into 2 photons) Higgs boson cannot have s=1?
Thanks in advance
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518 views
Concept of a point particle in quantum mechanics
A point particle is usually thought of as structureless and without dimension. However, given that Heisenberg's uncertainty principle prohibits us from knowing the position of a particle exactly, what ...
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427 views
Is there a maximum number of types of elementary particles?
Doing a Google search i found a paper called The maximum number of elementary particles in a super symmetric extension of the standard model.
It claims in the abstract that the upper bound is 84 (i ...
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Is it possible to destroy proton in proton-proton collision?
Or in proton-electron collision.
To destroy is to turn into other particles, not baryons.
In context of the baryon asymmetry.
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Higgs Boson: The Big Picture
First, please pardon the ignorance behind this question. I know a fair amount of math but almost no physics.
I'm hoping someone can give me a brief "big picture" explanation of how physicists were ...
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228 views
How does Annihilation work?
I'm wondering why matter and antimatter actually annihilates if they come into contact. What exactly happens? Is that a known process? Is it just because of their different charges? Then what about ...
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353 views
Intrinsic structure of electron
The electron contains finite negative charge.The same charges repel each other.What makes electron stable and why does it not burst? Is it a law of nature that the electron charge is the smallest ...
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Can the process $u\overline{u} \rightarrow s\overline{s}$ be mediated by the EM interaction?
I’m working on a homework problem which asks for the dominant contribution (e.g. EM, strong, or weak) to the process $p + \overline{p} \rightarrow \Lambda + \overline{\Lambda}$).
I know that the EM ...
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301 views
Production of antiproton at the Tevatron
The Tevatron is a proton-antiproton collider; it collides a beam of protons against a beam of antiproton.
I can understand how we obtain the protons, but for the antiprotons ? How are they produced ?
...
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Do other particles besides scalars admit tachyonic solutions?
Do other particles besides scalars admit tachyonic solutions? For example fermions or gauge-boson tachyons? The picture in my head is that a tachyonic scalar simply rolls off some unstable potential ...
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What is the fastest process or shortest time in nature?
We know about some events that happen very quickly. For example, the dielectric relaxation time is about $10^{-14}\, \mathrm{seconds}$.
I'm interested in other processes that switch extremely fast ...
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96 views
What maintains quark spin alignments in baryons?
What maintains quark spin alignments in baryons?
The $uud$ proton and $udd$ neutron are both spin 1/2, implying that two of their spin 1/2 quarks are always parallel and the other is always opposed.
...
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2answers
249 views
Was the Higgs mass correctly predicted by asymptotic safety of gravity?
This paper was published in Phys Lett B in 2009, and predicted the Higgs mass to be 126 GeV based on the asymptotic safety of gravity. Is this prediction taken seriously by the theory community, or is ...
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2answers
411 views
How do we distinguish between virtual particle exchange and particle decay?
The difference between virtual particles and unstable particles is discussed at length in this question (namely, virtual particles correspond to internal lines in Feynman diagrams and are not ...
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386 views
Can the spin of a photon change during its “life”?
Or is the spin set in one of two possible states at its moment of creation and does not change for the rest of the duration of its "life"?
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What's the difference between “evidence of a new particle” and “discovery of a new particle”?
Today’s exciting press release from Tevatron on the Higgs boson keeps its head cool and say that physicists saw a “hint” of the Higgs boson because the signal is barely above the two-sigma level. In ...
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130 views
Are there any reasonable attempts at explaining the OPERA result
as there's so many papers on the opera result and it's a struggle flicking through them all combined with my limited knowledge meaning I cannot well differentiate silly nonsensical papers from ones ...
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Do neutrinos of any flavor get trapped in black holes?
This question has been bothering me a bit. I know that neutrinos have super small mass and they interact via the weak force. Since they have a non-zero mass, they should be affected by black holes and ...
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Why is there a linear relationship between charge and isospin?
So the title basically says it all. Something that's really bothering me is the fact that the Gell-Mann Nishijima relationship can be extended to provide a linear relationship between charge and all ...
