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LIGO'S First Science Run - A Special Report

A successful Science Run requires plenty of trained hands and experienced eyes. It requires a pool of qualified personnel willing and able to keep a close watch as the experiment unfolds. For the LIGO Science Runs, this pool is comprised in great part by members of the LIGO Scientific Collaboration (LSC). The LSC is a broad gathering of scientists from institutions around the world who are all eager to participate in the quest for gravity-waves. In addition to LIGO's own run overseers, it was members of the LSC who helmed the control panels, monitored the equipment, converged on trouble flare-ups, and spent many an hour coaxing the sometimes contrary apparatus into sweet-tempered compliance. Keith Riles, chair of the LSC's detector characterization working group, reports on the invaluable contribution of the LSC in S1, and shows that this first run really is just the beginning.

The S1 Role of the LSC

- Contributed by Keith Riles

The recent LIGO First Science Run included active participation not only from Laboratory scientists and staff, but also in many ways from the broader community of scientists known as the LIGO Scientific Collaboration. These scientists contributed to the real-time monitoring of interferometer data for detector diagnostics, and continue to carry out analysis of that data in the search for gravitational waves.

Keeping the LIGO interferometers running smoothly and continuously (seismic noise permitting!) requires a cadre of skilled operators at each site, working in teams on rotating shifts. The operators must bring the interferometers into lock, tune the alignments and gains to optimize sensitivity, and try to preserve those optimum conditions. They must watch for signs of impending trouble, and if necessary, adjust any of a multitude of interferometer control parameters to avoid degradation of detector sensitivity. Traditionally, the key figures of merit have been the buildup of light intensity in the interferometer arms, and the buildup of "sideband" power in the central recycling cavity at the vertex of the interferometer. (The sideband refers to a component of the laser light that is deliberately offset from the laser's nominal frequency; it is used to determine and control the lengths of the various optical cavities of the system.)

Keeping Watch with the Data Monitoring Tool

More recently, new and more probing measures of interferometer sensitivity have become available in the control room. The S1 Run provided a showcase for a variety of new or improved "DMT monitors" for providing real-time feedback. DMT stands for Data Monitoring Tool, an elaborate and useful suite of programs that look continuously at the data streaming from the interferometers. More than 15 scientists in the LSC have contributed to this suite. The author of the DMT program environment (and author of several programs that run in it) is Caltech physicist John Zweizig. This environment supports real-time graphical displays, summary web pages, control room alarms, database record insertion, and writing of summary data files (trends). Major components of this "DMT World" were also provided by physicists Daniel Sigg (Hanford) and Szabi Marka (Caltech).

During S1 a large number of monitors ran continuously in the DMT world to watch for an assortment of potential problems, and to record summary information for later use on the states of the three interferometers and their environments. For example, there were monitors of lock status, seismic noise, "glitchiness" in the gravitational wave channel and in auxiliary channels, noise levels in various frequency bands (narrow and wide), noise correlations among channels, and expected sensitivity (in kilo-parsecs!) to standard astrophysical sources of gravitational waves. The DMT world also provides support for interactive exploration of the data, exploited by graphics-intensive DMT monitors of time-frequency behavior in the gravitational wave channel and of non-linear noise processes.

This array of S1 monitors was written primarily by the following LSC scientists: Dave Chin (Michigan), Nelson Christensen (Carleton), Ed Daw (LSU), Masahiro Ito (Oregon), Sergey Klimenko (Florida), Szabi Marka (Caltech), Benoit Mours (Annecy), Adrian Ottewill (Dublin), Steve Penn (Hobart-Smith), Rauha Rahkola (Oregon), Kevin Schlaufman (Pennsylvania State), Daniel Sigg (Hanford), Patrick Sutton (Pennsylvania State), Julien Sylvestre (MIT), Natalia Zotov (La. Tech), and John Zweizig (Caltech). Most of these scientists had participated in a special "E8" Engineering run at Hanford in early June during which the monitors underwent a "shakedown," one that paid off in S1 with monitoring programs that proved useful from day one.

Not that there weren't some difficulties! We learned how not to do a number of monitoring tasks, and we learned the importance of reliably tracking drifts in calibration (due mainly to alignment drifts). The lessons learned in S1 will prepare us better for the S2 run, tentatively scheduled for February 2003.

Send In the Scimons

Aside from the control room operators, the DMT monitors have another audience during data runs: the Scimons. The Scimons are LSC scientists staffing the scientific monitoring shifts run in parallel with the operator shifts. These scientists are focused on ensuring that the interferometer data is of the highest quality. They pay close attention to the various DMT monitors outputs, both in real-time and in retrospect, looking at long-term trends. They also use other software tools in the control room, such as the online Data Viewer and the Diagnostic Test Tool, to investigate particular problems that arise.

Since the LSC is a collaboration of scientists with many diverse specialties and backgrounds, a formal training system has been instituted for bringing the inexperienced up to speed. For most shifts staffed during S1, an "expert" Scimon was paired with a "trainee," a pattern that was begun back in the November 2000 E2 engineering run. As a result, the pool of Scimon experts has steadily increased, an essential development as we look ahead to periods of steady-state, multi-month data runs, starting in 2003. During S1, more than 180 8-hour Scimon shifts were staffed by scientists from Caltech, Carleton College, U. Florida, Hanford Observatory, Livingston Observatory, Louisiana Tech. U., Southeast Louisiana U., Loyola U., Louisiana State U., U. Michigan, MIT, U. Oregon, Pennsylvania State U., U. Rochester, Syracuse U., U. Texas - Brownsville, Washington State U. - Pullman, and U. Wisconsin - Milwaukee.

Setting the Upper Limits

Most of these scientists have another affiliation: they are members of one or more of the four "Upper Limits" analysis groups. Formed in Summer 2000, these groups are pioneering the analysis of LIGO data in the search for gravitational waves. The groups search for different sources of these waves:

The Upper Limits groups were so named in recognition that in the first few LIGO data runs, the interferometer sensitivities make it unlikely that a source will be seen. These groups expect to set upper limits on the strengths of hypothetical sources rather than to measure significant non-zero values.

Although the bulk of the analysis occurs after the data run, two of the groups (Inspirals and Bursts) also carried out a real-time search of the data using computers at the observatories. The goal was not so much to get the "first jump" on a real source, as it was to provide rapid feedback to the control room on any instrumental pathologies that might mimic a true gravitational wave source. In some sense these online searches produced the "bottom line" on acceptableness of such pathologies--the ultimate DMT monitors!

Several of the Upper Limits groups also took the opportunity the night before S1 began to verify that an emulated gravitational wave signal could be detected in the data. By sending currents through actuation coils placed near magnets mounted on the interferometer mirrors, they simulated the response of the mirrors to a variety of gravitational waveforms, allowing downstream confirmation that the signals did indeed appear as expected.

The Upper Limits groups are now busily poring over the S1 data to look for gravitational wave signals. Publishable results are expected in early 2003.

    Onwards! to LIGO Hanford to LIGO Livingston to LIGO MIT to LIGO Caltech