# Tag Info

14

Elastic collisions do happen at the LHC. The TOTEM experiment measures the differential cross section (rate as a function of angle) for proton-proton elastic scattering at the LHC. Here is their latest result. They don't publish an estimate of the elastic cross section, but according to their data it must be at least 25 mb (millibarns) (my first version of ...

10

No, the virtual photon is not a particle, since a virtual particle is what one calls the internal lines in a Feynman diagram, and there are no asymptotic particle states associated to these lines, so a virtual particle is not a particle in the usual (or any other rigorous) sense. Therefore, the question is non-sensical because it is not clear what an ...

9

Yes, there are the quantum numbers Charm, Strangeness, Topness and Bottomness, which are conserved by strong and electromagnetic interactions, but not by weak interactions. Upness and Downness are simply the Isospin, which is also preserved for strong interactions, when the quark masses can be neglected, which is usually a very good approximation as ...

8

Anything that is not forbidden must happen. That's an important statement to keep in mind when approaching quantum physics. It doesn't mean that anything that can happen always happens, but it must happen at some time or another just like someone eventually has to win the lottery. That said, some protons do go through the LHC, ram into each other and ...

7

A shadow is a lack of light. Therefore, a shadow has no mass, for a shadow is not an object or energy. Shadows can go faster than light in certain cases because they are not objects. In the same way, a vacuum has no mass.

6

Your question asks why the "current quark masses" [see http://pdg.lbl.gov/2011/download/rpp-2010-booklet.pdf at page 21] of the quarks that make up a proton don't add up to the mass of the proton. The problem is that, for the light quarks, the "current quark masses" are very different from the "constituent quark masses" [see wikipedia]. "Constituent quark ...

6

elementary particles (e.g. protons) Protons aren't elementary particles, they're made of partons (quarks and gluons) in "soup". Below, $\lambda$ is the wavelength corresponding to the energy of the interaction via the usual de Broglie relation and $r_p$ is the radius of the proton. At low energy with $\lambda >> r_p$ the interactions are just ...

6

An early bit of evidence for the neutron as an uncharged constituent of the nucleus, with approximately the mass of the proton, actually comes from the exclusion principle, and the low-temperature heat capacities and excitation spectra of atomic gases. The argument is a little bit subtle, so you'll have to bear with me. First, we have the exclusion ...

6

When a radioactive element decays, part of its mass is converted to energy - no obvious need for antimatter anywhere. Instead, the energy is released because the binding energy of the sum of the fragments might be higher than that of the parent nucleus. However, to fully convert matter to energy you do need the antiparticle. Otherwise, you run into ...

5

It's risky to think about subatomic particles in a classical way, but maybe we can get something from it if we're careful. Electrons orbiting an atom in a state with well-defined angular momentum quantum number $\ell$ have wavefunctions described by the spherical harmonics. The $s$ shell, with $\ell=0$, has spherical symmetry; this state really is ...

4

Quarks, the constituents of hadrons/mesons, interact via the strong, weak and electromagnetic force. So hadrons/mesons do interact via all this forces, too. Even if the total net-carge is zero. Take for instance the neutron, which has zero electric charge. Still it has a magnetic moment which gives rise to electromagnetic interactions. It can also decay via ...

4

Charged hadrons, and neutral hadrons with nonzero magnetic moment, interact electromagnetically. A spinless, neutral hadron would not couple to the electromagnetic field at tree level, but the most obvious example of such a particle is the $\pi^0$, which decays electromagnetically to two photons. All particles with flavor participate in the weak ...

4

Energy is never created nor destroyed, and to say "X is converted into energy" is just meaningless. We don't convert things distinct from energy into energy, all we ever do is convert one form of energy into another. The badly posed question from your book probably intends to ask why we cannot convert the mass energy that any chunk of matter contains as per ...

3

The same kinds of things that happen when an electron collides with a proton. Electrons (both matter and antimatter) being leptons and (anti-)protons being baryons there is no annihilation issue, so this is primarily an electromagnetic scattering event (with a small admixture of weak scattering which will be just slightly different). In principle such an ...

3

Here is a nice late-undergrad or early-grad-school lab report on the determination of spin states in nuclear decays. The references to that paper (from 1940 and 1950) are relatively accessible, too. As your pullquote says, you get $J^{PC}$ from measuring angular correlations. If a particle at rest decays into two daughters, the angular correlation is ...

3

You should always apply energy conservation, and it ought to hold in all reference frames, including the frame in which the sigma is at rest. In the sigma's rest-frame, $$E_{\text{initial}} = E_\Sigma = m_\Sigma$$ and $$E_{\text{final}} = E_\Lambda + E_\pi \ge m_\Lambda + m_\pi$$ Thus we have that, $$E_{\text{initial}} < E_{\text{final}}$$ The ...

3

If we think as electrons around atoms classically, then electrons would irradiate electromagnetic energy, losing momentum and thus collapsing into the nucleus, and atoms couldn't exist. Therefore your question makes no sense. The correct description of an atom is using quantum mechanics, which means there is no orbits on atoms. There is only the solution of ...

3

Leptons and quarks are fermions. ( Fermions are particles with half integer spins.) You can, like the author has, divide them into three generations on basis of their masses. The Higgs boson is a boson. (Bosons are particles with integer spins.) The Higgs boson (which happens to be electrically neutral) is part of a completely different category of ...

3

It is the way one reads/writes Feynman diagrams, a particle going backwards in time is the antiparticle. The electron radiates a gamma, and continues to meet the positron , annihilating charge with another photon. Two real particles are needed for momentum conservation in the center of mass, and two photon vertices are the simplest case within the standard ...

3

That is true indeed. A hole has no physical existence. It is just the absence of an electron that creates the illusion of a positive charge at that point. You can find it in Boylested Electronic Devices and Circuit Theory that it's a theoretical thing.

3

This decay (occurring via the strong interaction) violates the charge conjugation since $J^{PC}(\pi^0) = 0^{-+}, J^{PC}(\rho^0) = 1^{--}, J^{PC}(\eta'^0) = 0^{-+}$. The charge conjugation transforms a particle in its anti-particle. In the case of the 3 particles involved in this decay, they are all their own anti-particle, and the effect of the charge ...

2

This is a bubble chamber antineutron event This picture, taken in the Berkeley 30-inch propane bubble chamber in 1958 Antiprotons enter from the top with momentum 684MeV/c . At the arrow one of the antiprotons in the beam disappears, shown with purple on the right. Then a vertex appears out of nothing where an annihilation event is obseved into ...

2

All of charge, lepton number and lepton-flavor number should be conserved in the reaction. Because the reaction is charged-current, the neutrino will be converted into a charged lepton and because it is a neutrino (and not an anti-neutrino) it must be a negatively charged lepton (to conserve lepton number). That means that the quark involved must be ...

2

There are already antiproton proton pairs experimentally created at LEP , Proton–antiproton pair production in two-photon collisions at LEP The reaction is studied with the L3 detector at LEP. The analysis is based on data collected at e+e− center-of-mass energies from 183 GeV to 209 GeV, corresponding to an integrated luminosity of 667 pb^−1. The ...

2

There are no such things as particles in the physical world. The correct description of "small things" in classical mechanics is that the dynamics of the motion of the center of mass of an extended object is the only relevant physical quantity while internal degrees of freedom like rotation, vibration, magnetization, temperature etc.. can be ignored. That ...

2

The experiments, both CMS and ATLAS report 2.5 and 3 sigma candidates, but not at the same spot/channel. The place to look is at Cern's document server , asking for "supersymmetric" for example in conjunction with CMS or Atlas. This general talk is about limits . Lubos Motl in his blog discusses an Atlas 3 sigma possible excess and there are links there. ...

2

Electrons don't gain mass when they hop up an energy level--we know this because we calculate the electron levels assuming a particular electronic mass, so if the electron mass changed, it would also have a different energy level. Photons are massless. Nothing that has mass can travel at the speed of light. Now, $E = mc^2$ is not a complete formula. The ...

2

Ok, the previous answer by Alchemist is totally reasonable, but I think we could add a bit of "what is real?" into this discussion without getting metaphysical. A hole is a perfectly well-defined mathematical concept, in the same way that an electron is a perfectly well-defined mathematical concept. The mathematical concept of an electron in the theory of ...

2

Major differences no, because what really collides are constituents of the protons (called partons): the valence quarks, the quarks and anti-quark due to quantum fluctuation and the gluons. The same thing happen with neutron. However, since in proton $u$ quarks carry more momentum than $d$ quarks (because proton contains 2 $u$ valence-quarks and 1 $d$ ...

2

The main force keeping the electrons around the nucleus is the electromagnetic one. Electrons do interact gravitationally and weakly but those are very much weaker forces. In principle if the masses were exactly inverted, it would just change the definition of positive and negative charge, which is arbitrary. Generally changes in mass affect the orbitals. ...

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