## X-Pipeline Review Meeting Monday 10 March 2008 09:00 Pacific / 12:00 Eastern

### Minutes: Monday 10 March 2008 09:00 Pacific / 12:00 Eastern

- We finished the technical "Notes on Coherent Analysis Techniques in GWB Searches" [ PDF ].
- E_NULL, E_TOT, E_SL. Suppose just have H1 and L1 (two detector,
not aligned case). What happens then?
*A. E_TOT is the same as E_SL. There is no NULL stream.* - LIGO network: "trivial" to create a coherent combination of H1
and H2, so have effective two detector network with LHO and LLO.
*Yes, then NULL is h_H1-h_H2, and E_TOT is the same as E_SL with the two effective detectors.* - So, for LIGO network, E_SL and E_NULL are not really sky-position
dependent (apart from time shift issues)? Why problem of searching
over sky positions?
*Yes, but X-Pipeline doesn't use E_SL ... it uses E_+ discussed later. Also, really want this to be a method that uses larger networks (though the NULL stream is of significant benefit even within the LIGO-only network).* -
**Patrick will send maps of |F+| and |Fx| for the LIGO network to illustrate the dominant polarization frame.** - Q. When there are lots of instruments with similar sensitivity
and various orientations, then presumably |F+|~|Fx| and so the
dominant polarization frame is only weakly dominated.
*A. Yes.* - Discussion on dominant polarization frame.
- Q. Why choose E_+ rather than E_SL? Isn't E_SL, the max
likelihood method, the "optimal" method?
*A. Actually, the Bayesian optimal would involve the integral over possible sky positions.*But these are weak integrals, and the exponential of the likelihood is strongly peaked, so shouldn't the max likelihood point dominate?*Regions where the X-polarization matters are isolated near detector nulls, where the sensitivity is poor and these regions are not favored in integral. There is an AMALDI proceedings discussing this.***Patrick will send AMALDI proceedings.** - Criticism: even if E_+ is empirically better than E_SL now,
will it still be so when other sensitive detectors are added to
the network? At what point does one "flip-the-switch" and go from
E_+ to E_SL? Should one use soft-constraint likelihood which
smoothly dials from one to the other?
*Soft constraint had problems in all-sky search, but might work well for GRB search. (Problem was that the regions near the detector nulls always dominated statistic -- not a problem for fixed sky location problems.)*

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