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Neutrino Oscillation Formula 
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#1
Jul514, 04:59 PM

P: 148

Hi all!
I am not sure how to prove mathematically that the expression for the probability that a neutrino originally of flavor α will later be observed as having flavor β [itex]P_{α \rightarrow β}=\left<\nu_{\beta}\nu_{\alpha}(t)>\right^2=\left\sum_{i}U_{β i}^*U_{α i}e^{iE_it}\right^2[/itex] (1) can be equivalently written as [itex]P_{α \rightarrow β}=δ_{αβ}4\sum_{i>j}Re(U_{β i}^*U_{α i}U_{β j}U_{α j}^*)\sin^2(\frac{Δm_{ij}^2L}{4E})+2\sum_{i>j}Im(U_{β i}^*U_{α i}U_{β j}U_{α j}^*)\sin(\frac{Δm_{ij}^2L}{4E})[/itex] (2) Whoever is not familiar with the notation and would still like to contribute "mathematically", all the variables and constants are explained perfectly in the wiki article: http://en.wikipedia.org/wiki/Neutrino_oscillation The proof has to be trivial, but here I am trying for two hours and still not able to show this for a general case. 


#2
Jul514, 07:11 PM

P: 1,058

which part of the formula is confusing you?
how to express [itex]E_{i} t[/itex] in terms of [itex]L, m_{i}[/itex]? This is done by some easy calculations, I think you can find them easy if you search... eg http://www2.ph.ed.ac.uk/~vjm/Lecture...s/PPNotes5.pdf eq 16.3 and also by saying [itex] t= \frac{L}{c} [/itex] the time they traveled from distance L up to reaching us. with c=1, and 16.3 you get your result. or how to write the sums? Those sums are in fact two sums, of i and j with the same expressions just change the i's to js and taking the conjugate... After that you can start breaking the sums in such a way that you'll get something as the final result... for example for i=j you will evenutally get delta because the exponentials will cancel each other out (remind i=j again), and the U matrices are unitary, so you will get the delta Kroenicker by summing them: [itex] (U^{\dagger} U)_{ab} = \delta_{ab} [/itex] for U unitary. The rest is more tedious work, but in general that's how you get the Re and Im part (depending of what i,j configurations you sum). 


#3
Jul514, 08:49 PM

P: 148

I wanna start with (1) and reach (2) step by step.
[itex]P_{α \rightarrow β}=\left\sum_{i}U_{β i}^*U_{α i}e^{iE_it}\right^2=\left\sum_{i}U_{β i}^*U_{α i}e^{iE_it}\right^2=\left\sum_{i}(Re(U_{β i}^*U_{α i})+iIm(U_{β i}^*U_{α i}))(\cos(E_it)i\sin(E_it))\right^2=[/itex] [itex]\left[\sum_{i}Re(U_{β i}^*U_{α i})\cos(E_it)+Im(U_{β i}^*U_{α i})\sin(E_it)\right]^2+\left[\sum_{i}Im(U_{β i}^*U_{α i})\cos(E_it)Re(U_{β i}^*U_{α i})\sin(E_it)\right]^2[/itex] Is this correct so far? How do we further simplify this expression and get the double sums? Also some trigonometry is involved for sure. 


#4
Jul614, 03:00 AM

P: 1,058

Neutrino Oscillation Formula
why don't you write the square explicitly?
[itex] \sum_i U_{β i}^* U_{α i} e^{iE_{i}t}^{2} = \sum_{i} \sum_{j} U_{β i}^* U_{α i} U_{ α j}^{*} U_{β j} e^{iE_{i}t} e^{+iE_{j}t} [/itex] [itex]= \sum_{i} \sum_{j} U_{β i}^* U_{α i} U_{α j}^{*} U_{β j} e^{i (ΔE)_{ij}t} [/itex] that's in general how you proceed... 


#5
Jul614, 04:23 PM

P: 148

So you simply write the square of the absolute value as the complex number times its conjugate and you get the Δm differences.
I see how for i=j, the Kronecker delta arises. Sorry but I am too bad at this and I still cannot work out the rest of the terms. Can you take me by hand or show the derivation in detail? 


#6
Jul714, 01:30 PM

P: 1,058

because in most of the cases this is an exercises, you should just keep track of your indices and what you get....
I guess could help you if you showed me what you did, where you've reached. In most of cases it's just a tobedonecarefully calculation, and at one point you have to use trigonometric identities (if I recall well)... One thing is for sure, after you found the case of delta, you need also to add to this the cases where i is not equal to j.... and from that see what you get for that term... 


#7
Jul714, 07:57 PM

P: 681

For the remaining terms (##i \neq j##), you have one term for ##i > j## and one term for ##j < i##. Consider how they are related by writing out both sums and then changing the summation indices in one of them. After that you can make use of the relations $$ z + z^* = 2 Re(z), \quad z  z^* = 2 Im(z) $$ (Also note that ##\Delta E_{ij} =  \Delta E_{ji}## ...) 


#8
Jul1814, 08:50 AM

P: 148

Thanks for your replies. I have finally worked out the derivation!



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