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200

Imagine you are an electron. You have decided you have lived long enough, and wish to decay. What are your options, here? Gell-Mann said that in particle physics, "whatever is not forbidden is mandatory," so if we can identify something you can decay to, you should do that. We'll go to your own rest frame--any decay you can do has to occur in all reference ...


55

Strictly speaking, it is indeed incorrect that neutrinos travel at "close to the speed of light". As you said, since they have mass they can be treated just like any other massive object, like billiard balls. And as such they are only traveling at nearly the speed of light relative to something. Relative to another co-moving neutrino it would be at rest. ...


40

Last (?) Edit: The "problem" is solved: it was mainly a problem in the timing chain, due to a badly screwed optical fibre. A high level description of the problem is given here and a more detailed explanation of the investigation is here. List of possible systematic biases I thought it might be a good idea to list the possible systematic biases which could ...


38

The statement is true for decays, where lifetimes can be measured. It is not true for interactions though. A suicidal electron meeting a positron has a good probability to disappear, together with the positron, into two gamma rays, at low energies. Electron-positron annihilation It is intriguing that this is not true for neutrinos. If an electron ...


37

To maintain lepton number as a conserved quantity. Consider, in detail, what's going on in a beta decay (well, I'm going to ignore the nuclear context). The reaction is then $$ n \longrightarrow p^+ + e^- + \nu \,,$$ where you should take the symbol $\nu$ to mean some neutrino (without prejudice about matter-type or anti-matter-type for the moment). There ...


34

This was a reference to the apparent measurement that neutrinos travel faster than light. FTL travel can be used to travel back in time (though the procedure for doing so is somewhat involved). Sadly the apparent superluminal speed turned out to be due to experimental errors: a fibre optic cable attached improperly, which caused the apparently faster-than-...


29

Dark matter can be hot, warm or cold. Hot means the dark matter particles are relativistic (kinetic energy on the order of the rest mass or much higher), cold means they are not relativistic (kinetic energy much less than rest mass) and warm is in between. It is known that the total amount of dark matter in the universe must be about 5 times the ordinary (...


27

It's very hard to imagine that there is any sensible model consistent with OPERA's results. (Aside from models of unaccounted-for systematic uncertainties in the experiment.) We know that we live in a world described to very high precision by Lorentz-invariant quantum field theory, so the most sensible way to look for Lorentz violation is to start with such ...


26

Cute question! For a neutrino with mass $m$ and energy $E\gg m$, we have $v=1-\epsilon$, where $\epsilon\approx (1/2)(m/E)^2$ (in units with $c=1$). IceCube has detected neutrinos with energies on the order of 1 PeV, but that's exceptional. For neutrinos with mass 0.1 eV and an energy of 1 PeV, we have $\epsilon\sim10^{-32}$. The time of flight for high-...


25

I am afraid that one has to go to a "very unusual segment" of theoretical literature if he wants any papers about superluminal neutrinos. Guang-jiong Ni has been authoring many papers about superluminal neutrinos a decade ago: http://arxiv.org/abs/hep-ph/0103051 http://arxiv.org/abs/hep-th/0201077 http://arxiv.org/abs/hep-ph/0203060 http://arxiv....


24

The detector that took that image--Super Kamiokande (super-K for short)--is a water Cerenkov device. It detects neutrinos by imaging the Cerenkov cone produced by the reaction products of the neutrinos. Mostly elastic scattering off of electrons: $$ \nu + e \to \nu + e \,,$$ but also quasi-elastic reactions like $$ \nu + n \to l + p \,,$$ where the neutron ...


24

The experimental detection of slow neutrinos is indeed a big problem, but one that is very important. The cosmic neutrino background is at a temperature of around 2K and likely to consist of non-relativistic neutrinos for plausible neutrino rest masses - with a density of around 340 cm$^{-3}$ (all flavours). It is at this low temperature for precisely the ...


22

Depends on the detection technology. Yes Cerenkov based detectors (SNO and Super-Kamiokande for instance, as well a many cosmic ray neutrino detector) are direction sensitive, and this is one of the design considerations that drive the use of this tricky technique. The best results come from quasi-elastic reactions like $\nu_l + n \to l^- + p$. The ...


22

They are probaby talking about supernovae, like how SN1987A was first detected by neutrinos before the light arrived. In that case neutrinos and photons are both produced in the core of the supernovae explosion, but they have dense clouds of gas to get through before they get to empty space and travel freely to us. Since the neutrinos are weakly interacting ...


21

Is this even remotely possible? Well... "possible," yes, but kind of like how tunneling through a brick wall is "possible": while you can't definitively prove it impossible, you'd feel pretty safe saying "this will never happen." Relativity is really well-tested, and it's really hard to conceive of a way that neutrinos could travel faster than light ...


21

You have a few longer answers which were already updated, but here is a concise statement of the situation in mid-2014: An independent measurement by the ICARUS collaboration, also using neutrinos traveling from CERN to Gran Sasso but using independent detector and timing hardware, found detection times "compatible with the simultaneous arrival of all ...


21

There are other neutral particles with antiparticles, such as the neutron and the $K^0$ meson. In those cases we have a microscopic theory that says those particles are made of quarks: for instance, the $K^0$ is made of a down quark and an anti-strange quark, while its antiparticle the $\bar K^0$ is made of a strange quark and an anti-down. The neutrino is ...


21

At tree level, a matter-antimatter annihilation reaction doesn't just produce gamma rays, nor can you exclude neutrinos in the final state. Even the simplest such reaction can—given enough energy—produce a variety of particle-pairs. However, those pairs are subject to two subsequent processes: If the particles are not stable, they will decay towards ...


20

Before I answer, a couple caveats: As Adam said, the universe isn't going to start behaving any differently because we discovered something. Right now it seems much more likely (even by admission of the experimenters) that it's just a mistake somewhere in the analysis, not an actual case of superluminal motion. Anyway: if the discovery turns out to be ...


20

The neutrinos are coming straight at us. Indeed, their interactions with anything along the way are minimal at best. The reason the image is so big is that the angular resolution of the detector is rather poor (compared to, say, an optical telescope). This is not unexpected when it comes to neutrino telescopes. The details of how the detector work are ...


20

The cross-section for neutrino interactions is energy dependent. For solar neutrinos at $\sim 0.4$ MeV, which would likely dominate any neutrinos likely to interact (the cosmic background neutrinos have way low energies) , the cross-sections are $\sigma \sim 10^{-48}$ m$^2$, for both leptonic processes (elastic scattering from electrons) and neutrino-...


18

No. The decay products of a certain particle are not equivalent to its constituents. This is evident especially in the context of fundamental particles: quarks can decay into other particles, but that does not mean that a quark is not elementary (see my answer to this question). Nuclei are made of neutrons and protons, which in turn consist of quarks and ...


17

Neutrinos have mass and travel slightly below light speed, therefore an inertial frame for the neutrino exists, while it doesn't exist for a massless photon which travels exactly at $c$. We don't know the masses of the neutrinos, but neutrino oscillations tell us that the three neutrino families must have a mass difference. For all we know, one of the three ...


17

This is how the north pole looks: The sea ice at the North Pole is typically around 2 to 3 m (6 ft 7 in to 9 ft 10 in) thick. and this is how the south pole looks : The ice is estimated to be about 2,700 metres (9,000 ft) thick at the Pole, so the land surface under the ice sheet is actually near sea level.2 This is the ice cube neutrino ...


16

The historical formulation of the SM involved one Higgs doublet and only renormalizable couplings, the latter being due to the focus at the time on achieving a renormalizable formulation of the weak interactions. With these restrictions neutrinos are massless and do not oscillate. To get neutrino masses you need to extend this framework either by adding non-...


15

The calculation is done for 1987A here. Basically, the neutrinos' fractional speed increase from the new paper is $2.48\pm0.28\pm0.30\times10^{-5}$ (statistical / systematic errors, respectively) . SN1987a was $166\,912\pm10.1$ ly away, so multiplying the fraction by the travel time gives $4.14\pm0.97$ years. In reality, we got the neutrinos a few hours ...


15

Short answer: Unknown Slightly longer answer: the situation you describer would obtain if neutrinos were Majorana particles (and thus not Dirac particles). It is favored by theorists because it feeds into a nice explanation of why the neutrinos are so light by comparison to the other massive particles. Experiments are underway that might settle the ...


14

The 3 neutrino families ($e$, $\mu$, $\tau$) are usually called neutrino flavors. Neutrino flavor oscillation requires that the mass eigenstates of neutrinos are not equal and that the mass eigenstate is also not a flavor eigenstate. Since a neutrino is always produced in a flavor eigenstate (i.e. associated with an $e$, $\mu$, $\tau$), this flavor ...


13

The answer is yes. Neutrinos will travel faster than light in a medium with a refractive index ($n$) greater than one (which is the case of air). Indeed the speed of light in that medium will be $v_{\text{medium}}=c/n$ where $c=2.998\times10^8$ m/s and $n>1$. Then, because neutrinos interacts only very weakly (only through the weak nuclear force) with ...


12

It is technically impossible to measure the speed of such a particle directly; and it all depend on "which" neutrino you are talking about. The speed is related to the momentum and the momentum to the energy. So you can have a neutrino of some MeV of total energy, another one of some GeV, etc. But in any cases, the answer will be "very very close" to c. ...



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