# Tag Info

## Hot answers tagged particle-physics

12

Creating anti-protons is straightforward in principle because any high energy collision produces a shower of protons, antiprotons and various types of pions. The pions decay in a few nanoseconds, so you just have to wait for the pions to decay then separate the antiprotons from the protons. At Fermilab a 120GeV proton beam was collided with a nickel target ...

10

Spin is best understood as an intrinsic angular momentum. It is probably easier to understand the concept for a charged particle. A classical charged particle moving along a circle has an angular momentum and the "circuit" has a magnetic moment. Further, the two are proportional to each other. It is experimentally found that a charged particle like an ...

9

Detectors at particle colliders are layered like onions around the collision vertex. The CMS detector at CERN First there are charged particle sensitive detectors where charged particles leave tracks because of ionisation, but mass density is low so strong interactions do not happen often; their momentum can be measured by the curvature in the ...

9

Gluons and photons are similar in the sense that they are both massless gauge bosons. They do, however, correspond to different gauge symmetries: photons arise due to $\mathrm{U}(1)$ symmetry, while gluons follow from $\mathrm{SU}(3)$. This leads to a different number of particles: there is only one photon, while there are eight different gluons, ...

7

This reaction is not possible, but for a nontrivial reason. As you note, the non-conservation of strangeness forces this to be a weak interaction, but there is nothing stopping baryons from partaking in weak interactions (as in e.g. this reaction). However, no charge is exchanged in the process, so it would be due to a weak neutral current, and these do ...

7

An elementary particle is defined as an irreducible representation of the Poincar\'e group. These were classified by Wigner in 1939. This was done via the little group construction. The important representations are (metric signature $(-,+,+,+)$ $p^2 = 0$, $p^0 < 0$ - The little group is ISO(2). All finite dimensional representations of this group are ...

7

The total angular momentum of a meson is the sum of the spins of the two quarks and their orbital angular momentum. Excited states can have $L>0$ and therefore $J>1$.

6

From the perspective of fundamental quantum field theory, gluons and photons are quite similar. Both of them are gauge bosons, meaning that their existence is required by a mathematical mechanism called local gauge invariance. However, as particles, there isn't any particular connection between them. For instance, there's no reason they both have to be ...

4

The four quantum field theories (QCD, QED, QFD, and EWT) unite quantum mechanics and special relativity. They are all fully understood, complete, and proven. In your quote for the standard model, there are not four distinct field theories, the electroweak has united the electromagnetic and the weak in one field theory, the electroweak theory. The ...

4

The criterion for gravitational radiation is (conjectured to be, pending direct evidence) a changing quadrupole moment in the mass distribution, so an accelerating mass distribution does not always radiate, but can do so if the acceleration changes the quadrupole moment. This is in contrast to electromagnetic radiation, which occurs when the charge ...

3

Spin arises from the need to represent the rotation group $\mathrm{SO}(3)$ upon our Hilbert space of states. We need such a representation because the rotations (together with space translations) correspond to the non-relativistic changes of reference frames. Since states are only determined up to rays in the Hilbert space, the true space of states on which ...

2

The number of gauge bosons is restricted by symmetry: a given theory with a certain gauge invariance admits as many gauge bosons as there are generators of the corresponding gauge group. For example, there is one generator for $\mathrm{U}(1)$, resulting in the existence of a photon. $\mathrm{SU}(3)$ admits eight generators, which yield eight gluons. This is ...

2

A theory with N=2 supersymmetry, where particles have two superpartners, has mirror symmetry built in. Nir Polonsky wrote some papers about an N=2 extension of the standard model (e.g.). The main problem for such a theory are the chiral Yukawa interactions between fermions and the Higgs field, which give fermions their mass in the SM. The mirror symmetry of ...

2

There experiment which has measured the most stringent limit on neutron to anti-neutron oscillations (i.e. produce neutrons, let them fly for some time and then look if you find anti-neutrons) has used a 130 micrometer thick and 110 cm diameter carbon foil. This target had a probability greater than 99% for anti-neutrons to interact (and thus produce ...

2

There are different layers of reconstruction, at each step the amount of data is reduced with the goal of inferring the momenta, type and direction of the particles produced first in the collision: pulse shape reconstruction: the electronic signals caused by particles interacting with the detector cells are digitized at a rate of 40 MHz at LHC. Some ...

2

The expression refers to the situation where the particle, such as an atom, containing charged constituants, is coupled to the (quantised) electromagnetic field. Then, if the atom is in an excited electronic state, it can decay to a lower state by emitting a photon (a quantum of the electromagnetic field). This process is known as radiative decay.

2

The weak force acts on particle that have weak hypercharge, just as electromagnetism acts on objects with electrical charge and gravity acts on objects with mass. All the quarks and leptons have weak hypercharge, so the weak force act on them. The description you have been given of beta decay is incomplete because it does not conserve the overall lepton ...

2

Gamma decay is the emission of photons. You are thinking of $\beta$ decay. When the particle is decaying, if it emits a $W^{-}$boson, it will subsequently decay and create an electron ($e^-$), and an electron antineutrino ($\overline{v}_e$), the antimatter particle to an electron neutrino. It will also flip a neutron into a proton. This is known as ...

2

A very brief, although somewhat incomplete, answer would be that charge is related to a local symmetry, therefore with a gauge field that acts in the whole spacetime, even if the charges are inside the event horizon you could use the electromagnetic field to probe it. Lepton and Baryon number, or other flavour related quantities are related to global ...

2

Yes, it is allowed. This is a typical flavor changing neutral process. In the Standard Model, this kind of process is highly suppressed, because the neutral bosons ($\gamma$, $Z$) always couple to quarks of the same flavor at tree-level. So, a vertex connecting two quarks of different flavors with a neutral gauge boson only appears at higher orders in ...

2

The simplest answer lies in a combination of Gauss's law and Birchoff's theorem. These say, alternately, that the electric field and gravitational field of a spherically symmetric charge distrubtion only depends on the charge and mass-energy enclosed in the spherical shell${}^{1}$. Therefore, if I have a spherically symmetric distribution of charged ...

2

Since John is not addressing positrons one should know that positrons are easily created once a photon has more energy than twice the mass of the electron, in electron positron pairs. This can be seen clearly in this bubble chamber picture: where the positron is shown in purple on the right. One knows they are electrons (positrons) because of the ...

1

$R$ Parity is a discrete $Z_2$ symmetry while an $R$ symmetry is a global continuous symmetry. If you use a $Z_2$ symmetry to build your model then each field can just be either odd and even, that's it. If you impose a continuous symmetry then there are an infinite number of possible choices of $R$ charges. From a model building perspective, a continuous ...

1

I'm trying to give a less technical answer. It's not rigorous but should give you the idea how spin and the regular rotation related. Maxwell's equations say in order to have magnetic field, you need a ring current. This can be achieved by giving angular momentum to charged particles. This can be orbital or simply because the particle is spinning. This was ...

1

Simply because it is usually taught from historical, heuristic and pragmatic point of view, rarely from axiomatic point of view (e.g. Wightman axioms, as mentioned in a comment by ACuriousMind). This is because it is taught to be useful, as most QFT calculations boil down to scattering and decay amplitudes, and as Sean Carroll said: Heuristic QFT, on ...

1

From a decoherence point of view, fields are more fundamental as they give rise to particle-like behavior from the wave behavior if interactions with the environment are strong. In the end though, quantum mechanics only describes correlations between macroscopic changes in detectors (or other materials), so whatever kind of ontology you want to take in the ...

1

Strictly speaking the prefix semi means half, but it's often used in the sense of partial. A good example of this would be semiconductor. So semileptonic just means partially leptonic.

1

For the massless case, one needs to show that $W^\mu = \lambda P^\mu$. Equation (10.53) provides a basis for an arbitrary four-vector and then expands $W^\mu$ in that basis. Imposing the two conditions $W\cdot P=0$ and $W \cdot W=0$ completes the proof by showing that all other "components" in that basis vanish.

1

There are two things that define a particle physics model (at low energies). The first one is the gauge group G we want the model to be symmetric under. For the Standard Model (SM) we set this to $G=SU(3)\times SU(2)\times U(1)$ (for good experimental reasons!). This will uniquely determine the number of gauge bosons needed to make the model consistent. The ...

1

Classical physics describes the movement of the center of gravity of extended bodies, which, when poorly taught, in the mind of the student becomes equivalent with "classical physics being a theory of point particles". That, of course, is utterly false, even on the level of the classical description. A center of gravity is a vector, not a point. ...

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