/sci/ - Science & Math

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Anonymous Wed Feb 15 16:00:04 2012 No.4368523 [Reply] [Original]

Using the pair annihilation cross section, derive the total unpolarized cross section for <span class="math">\gamma + \gamma \to e^{+} + e^{-}[/spoiler]. Your answer should be expressed in terms of the velocity of the produced particles in the center-of-mass frame.

>>	Anonymous Wed Feb 15 16:13:41 2012 No.4368562 Bump, please guys This bitch is due in four hours and i'm so sleep deprived it looks like its in Greek

Anonymous Wed Feb 15 22:17:59 2012 No.4369927
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...you honestly expect any sane person to write this entire thing out for you?

the derivation is usually to get the invariant amplitudes for the photons, you can use crossing symmetry to do a transform, evaluate at the electron momenta, and since you are given the (unpolarized?) pair annihilation cross section you should obtain the breit-wheeler formula, it should be:
<div class="math">\bar{\sigma}_{(\gamma \gamma \to e^{+} e^{-})} = \frac{\pi}{2} \frac{a^{2}}{m_{0}^{2}}(1-v^{2})\left [ (3-v^{4}) ln \frac{1+v}{1-v} - 2v(2-v^2) \right ]</div>
take the nonrelativistic and ultrarelativistic limits so you can express in terms of v, and you're done.

(USER WAS BANNED FOR THIS POST)

>>	Anonymous Wed Feb 15 23:01:41 2012 No.4370037 >>4369927 love you too, mod!

>>	Anonymous Wed Feb 15 23:08:29 2012 No.4370050 >>4369927 FUCK YES! DOWN WITH THE PONIES! MODS ARE GODS!

>>	fag !SHjzCSZsqE Wed Feb 15 23:10:44 2012 No.4370054 >>4369927 MODS = GODS you guys should ban ponyfags more often

>>	Anonymous Wed Feb 15 23:10:44 2012 No.4370056 File: 21 KB, 1233x1233, herpafuckingderp.jpg [View same] [iqdb] [saucenao] [google] >>4370050 here, have some pixels.

>>	Anonymous Wed Feb 15 23:11:52 2012 No.4370057 >>4370056 reported

>>	Anonymous Wed Feb 15 23:12:52 2012 No.4370061 >>4369927 Lol wat?

>>	Anonymous Wed Feb 15 23:13:01 2012 No.4370064 Banning for ponies? MODS = FAGS

Anonymous Wed Feb 15 23:14:08 2012 No.4370065

>>4370064
Reported for violating global rule 8:
>Complaining about 4chan (its policies, moderation, etc.) on the imageboards can result in post deletion and banishment. The administrator will address your questions, comments, complaints, and concerns via e-mail.

Enjoy your ban.

>>	Anonymous Wed Feb 15 23:14:16 2012 No.4370067 File: 28 KB, 352x279, gtfo.png [View same] [iqdb] [saucenao] [google] >>4369927 ponyfag GTFO

>>	Anonymous Wed Feb 15 23:15:25 2012 No.4370072 >>4370065 Reported for faulty moderation. Enjoy your ban

>>	Anonymous Wed Feb 15 23:16:25 2012 No.4370075 >>4370067 reported for reddit faggotry, enjoy your ban.

>>	Anonymous Wed Feb 15 23:17:32 2012 No.4370077 >>4370072 >faulty moderation Get_a_load_of_this_fag.jpg

>>	Anonymous Wed Feb 15 23:18:49 2012 No.4370079 >>4370075 reported for rageface? seems a little extreme

>>	Anonymous Wed Feb 15 23:20:49 2012 No.4370082 >>4370079 reported for ignorance and faggotry, enjoy your ban

>>	Anonymous Wed Feb 15 23:20:56 2012 No.4370085 >>4370082 wut?

>>	Anonymous Wed Feb 15 23:21:04 2012 No.4370087 File: 78 KB, 301x277, 1307812760036.png [View same] [iqdb] [saucenao] [google] Mods can go choke on a million dicks because they're retarded underage fuckbags

>>	Anonymous Wed Feb 15 23:22:04 2012 No.4370089 >>4370087 0/10, too obvious. Not reported, enjoy your time here on /sci/!

>>	Anonymous Wed Feb 15 23:23:20 2012 No.4370091 >>4370089 wut?

>>	Anonymous Wed Feb 15 23:24:28 2012 No.4370092 >>4370091 Reported for posting "wut?", enjoy your ban.

>>	Anonymous Wed Feb 15 23:28:52 2012 No.4370100 >>4370092 reported for reporting people for dumb reasons

Anonymous Wed Feb 15 23:29:00 2012 No.4370105
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>>4369927
YEAH LET'S BE FUCKING MORONIC ASPIE SHITSTAINS AND BAN ACTUALLY HELPFUL AND KNOWLEDGABLE POSTERS JUST BECAUSE THEY USE A PONY PIC

MAKES TOTAL SENSE
LOL JUST KIDDING

MODS ARE TRIPLE NIGGERSHITS AND THEY DESERVE TO HAVE THEIR FUCKING HEADS RIPPED OFF

>>	Anonymous Wed Feb 15 23:32:16 2012 No.4370111 >>4369927 here. please don't do this, guys. you'll just shit up the board and get banned.

>>	resident /sci/ brony Wed Feb 15 23:34:40 2012 No.4370123 File: 243 KB, 419x316, my-little-pony-friendship-is-magic-brony-party-soft.gif [View same] [iqdb] [saucenao] [google]

>>	Anonymous Wed Feb 15 23:43:37 2012 No.4370159 File: 37 KB, 493x335, getty_rf_photo_of_boy_writing_on_blockboard.jpg [View same] [iqdb] [saucenao] [google]

>>	Anonymous Thu Feb 16 00:55:01 2012 No.4370376 File: 98 KB, 900x726, Isay.jpg [View same] [iqdb] [saucenao] [google] >>4369927 >(USER WAS BANNED FOR THIS POST)

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