CERN Seminar: “LHC, Week 1”
A first public seminar at CERN – standing room only – was held today (Thanksgiving Day, 26-Nov-2009). The slides from the talks are available here: INDICO web page.
Steve Myers (CERN) kicked off the meeting with a pithy contrast of photos of a severely damaged set of magnets with a beautiful machine monitor trace showing manifestly stable beam. The audience responded with enthusiastic applause. The progress leading to the first collisions was amazing, as he told it. For example, they were able to obtain the “beta beat” of the machine on the first try – it took five years to obtain this with LEP. The bottom line: they circulated both beams 2-1/2 days after starting to circulate the first beam, and all four experiments recorded collisions some hours after that (p.40 of his talk). As Myers points out, thorough preparations pay off. The cryo system – the largest in the world by far – has worked flawlessly since Oct 8. The new magnet system which failed catastrophically due a splice resistance of 220 nano-Ohms last year, has no magnet with a resistance above 1 nano-Ohm today. And they are all protected by the new quench protection system.
Frederico Antinori (INFN Padova) presented the results from the ALICE Collaboration. The first event display showing a collision appeared seconds after colliding beams were present, and they recorded some hundreds of such events with typically 20 charged tracks. He showed good timing of their beam scintillators, and a beautiful vertex distribution obtained online from their higher-level trigger monitor some minutes into the run! The vertex distribution reflecting the luminous region, is 475 um in transverse direction and 4.2 cm in the longitudinal direction. This is quite impressive, showing that the tracker works well, that it is aligned, and that their software is ready. Jet analyses should follow soon. (He finished with a clip of the goings on in the ALICE control room when the first collisions were observed, but for some reason his slides are not linked at the INDICO web site.)
Andreas Hoecker (CERN) presented the results from the ATLAS Collaboration. All subsystems were operational, but a few were off for safety reasons. Interestingly, their muon toroid field was on although their solenoid field was necessarily off. They were able to record all beam splash events, and their forward tagging performed well. They could check the accuracy of their timing with the splash events. There is a beautiful event display showing the bending of beam halo events, and of course the first collision event (45 tracks!). They have good confidence again based on timing measurements – from their liquid argon calorimeter, good to 1.5 ns, among other measures. The “cogging” of the beam gives a shift of the impact parameters from good events exactly as expected. They see about 9 GeV of calorimeter energy, consistent with zero missing energy, well reproduced by their simulation (rms 1.2 GeV on the projection). It is exciting to see a di-jet candidate, with the jets in the forward direction and transverse energy of roughly 10 GeV. From 197 golden candidate events, they derive a very rough estimate of 4.9 mb-1 integrated luminosity.
Ivan Mikulec (HEPHY – Vienna) presented the results from the CMS Collaboration (of which I am a member). Beam splash events verified that the timing of the detector is greatly improved compared to last year, for example in the HCAL. Beam halo events left clear tracks in the cathode strip chambers (as I described a few days ago). Ivan showed trigger rates during the now famous “Monday afternoon” fill. Reconstruction of primary vertices shows a narrow peak with and rms of 4.6 cm. Energy losses (dE/dX) is consistent with min-ionizing tracks. Hits in the ECAL and HCAL calorimeter give information on the timing, consistent with collisions. The grand finale was a reconstructed pi-zero peak. The best peak has a width of 10 MeV, and a peak position a little below the nominal pi-zero mass, due to the fact that the magnetic field is not on (the effect is predicted by the simulation).
Olivier Callot (LAL-Orsay), a former ALEPH colleague, reported on behalf of LHCb. On page 6 he showed a beautiful display indicating the time-evolution of the beam splash events (animation on page 7). LHCb also observed beam-gas events, which are rather important for them right now, and confirmed them on the basis of the beam crossing number. Very pretty track were reconstructed in their “velo” (bicycle) tracker. LHCb also sees a very nice pi-zero peak, with the correct mass. Their Ring-Imaging Cerenkov Detector recorded some beautiful rings from beam-induced interactions. Collision events give a higher sum of transverse energy than beam gas events, and nice vertices can be reconstructed. The Z distribution of the vertices shows a beautiful peak a the correct position, with a width of 10cm.
Olivier’s closing remarks are the perfect summary:
This machine is fantastic!
and, of course, all collaborations are ready for more…
A final observation: Tommaso Dorigo, a famous physics blogger, was present in the front of the auditorium:
Entry filed under: Particle Physics.