A First Course in String Theory
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What sector(s) can be combined with a left-moving NS- to form a consistent closed string sector?
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There are no mass levels in NS+, R+, or R- that can match those in NS-. So NS- can be paired only with NS-:
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— Me@2018-06-26 07:36:41 PM
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2018.06.26 Tuesday (c) All rights reserved by ACHK