12.07.2015 Views

Ivancevic_Applied-Diff-Geom

Ivancevic_Applied-Diff-Geom

Ivancevic_Applied-Diff-Geom

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

<strong>Geom</strong>etrical Path Integrals and Their Applications 1241there is no contraction between ∂ τ x and x. To cancel this term, we mustadd another term to the variation of the gauge field; the theory is invariantnot under (6.356), but underˆδ i = ∂ i λ + iλ ∗ Âi − iÂi ∗ λ. (6.358)This is the gauge invariance of noncommutative YM theory, and in recognitionof that fact we henceforth denote the gauge field in the theory definedwith point splitting regularization as Â. A sigma model expansionwith Pauli–Villars regularization would have preserved the standard gaugeinvariance of open string gauge field, so whether we get ordinary or noncommutativegauge fields depends on the choice of regulator.We have made this derivation to lowest order in Â, but it is straightforwardto go to higher orders. At the nth order in Â, the variation isi n+1 ∫Â(x(t 1 )) . . .n!Â(x(t n))∂ t λ(x(t)) (6.359)∫()+ in+1Â(x(t 1 )) . . .(n − 1)!Â(x(t n−1)) λ ∗ Â(x(t n)) −  ∗ λ(x(t n)) ,where the integration region excludes points where some t’s coincide. Thefirst term in (6.359) arises by using the naive gauge transformation (6.356),and expanding the action to nth order in  and to first order in λ. Thesecond term arises from using the correction to the gauge transformationin (6.358) and expanding the action to the same order in  and λ. Thefirst term can be written asi n+1 ∑∫Â(x(t 1 )) . . .n!Â(x(tj−1))Â(x(t j+1)) . . .j). . . Â(x(t n))( ∗ λ(x(tj )) − λ ∗ Â(x(t j))∫)= in+1Â(x(t 1 )) . . .(n − 1)!Â(x(t n−1))( ∗ λ(x(tn )) − λ ∗ Â(x(t n)) ,making it clear that (6.359) vanishes. Therefore, there is no need to modifythe gauge transformation law (6.358) at higher orders in Â. For furthertechnical details, see [Witten (1986b); Seiberg and Witten (1999)].6.8.2 K−Theory Classification of StringsIn this subsection, following [Witten (1998c); Witten (2000)], we will revisitthe relation between strings, D−brane and K−theory, which we started in

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!