Archive for November, 2009
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:
Inside CMS, people are busy learning what they can from the handful of collision events recorded yesterday. Our magnet was not on at the time because it would have interfered with the beams, at this early stage, so we cannot understand anything about charged track momenta. But those straight-line tracks still provide a testing ground for several key aspects of event reconstruction.
Since I am a loyal member of the CMS Collaboration, I won’t divulge here any details about what my colleagues are doing, but I will say that I am very impressed by what I see. Stay tuned – public presentations are scheduled in a matter of days…
CMS and the other LHC experiments have seen the first LHC collisions ever!
You can clearly see a slew of green tracks coming from the interaction point. They are all straight because the magnetic field is off today. (It will be turned on later – remember that the experimental solenoids affect the beam so the LHC operators have do carry out careful measurements to compensate for these effects – to be done asap.) You can also see energy registered in the calorimetry (red is the hadronic calorimeter, and blue is the electromagnetic calorimeter). There are no muons in this event, and muons are not expected in this kind of minimum bias event. (They may appear as a consequence of the decays of pions and kaons – more about that later.)
I need to board my flight now (I am en route back to my home institution) – see Darin Acosta’s log for good, real-time reporting of what his happing in the CMS experiment.
This is a wonderful, wonderful day!
If you want to turn on a light, or start your car, you rarely pause to think about possible damage that might result. But when beam is coursing through the CMS muon end caps, we think about it very carefully. In fact, we discuss in all seriousness when to turn on the high voltage, when there is beam in the machine.
Images of the CMS muon end cap detector (EMU for short) have been shown hundreds of times, including on the cover of Newsweek. Lots of copper surfaces, thick steel disks – what could be delicate about that?
The chambers inside are very well made by experts in Russia, China and the United States. They are relatively robust in a mechanical sense, though we would not want any of them to fall from their mounting on those large steel disks.
The danger is in the gas and the wires. The detecting layers consist of thin gas layers sandwiched between cathode strips, with anode wires stretched in planes between the cathode planes. A large voltage (several thousand volts) are applied between the cathodes and the anode; this is part of the gas amplification mechanism which allows us to detector the wispy muon tracks as they pass through the chamber. In order to detect the muons, we need to have this high voltage turned on.
This is a delicate instrument, and quite sensitive to minute amounts of ionization (on the order of 100 electron-ion pairs or fewer). You can’t spray it with intense concentrated charged particle fluxes or you risk damaging the instrument. Putting the beam through one of these chambers would be like igniting an old-fashioned camera flash in front of night vision goggles.
You might object that we splashed millions of muons through the detector in the famous beam splash events, which I wrote about a couple of days ago. It is true that this is an immense amount of ionization compared to a single muon or a muon pair. But it was spread around a hundred squared-meters of area, not concentrated in a narrow area close to the beam. (Nonetheless, we put the high voltage at low values just to be very safe, as mentioned in my posting.)
So, when the LHC operators are circulating beam, looking for the rough spots and making systematic adjustments to machine controls and monitors, we have to protect our delicate detector. At this point in time, when an eager physicist asks:
Can we turn on the high voltage?
the answer will often be:
be patient, not quite yet…
Yesterday we enjoyed our second set of beam splash events, generated with beam one in contrast to earlier splash events generated with beam two.
Today we are thrilled to see beautiful beam halo events in CMS, like this one:
You can clearly see a trajectory (in red) extending across the CMS detector based on short track segments (light blue dashes) reconstructed in the muon endcap systems. (The grey cylinder in the middle indicates the tracking volume.) This picture was made by my graduate student as part of our effort to understand and validate the performance of the CMS cathode strip chambers (which are traced here in dark blue).
Muons in beam halo events run parallel to the beam and typically have a few hundred GeV, according to simulations. (We would love to test that with the real data…) They are generated when protons from the beam pass out of the beam pipe and strike some object near by, leading to an energetic hadronic shower out of which emerge one or more muons. This shower occurs many tens or hundreds of meters away from the experiment, so any muons that reach the apparatus have hardly any angle with respect to the beam.
These beam halo muons may be a nuisance one day, but right now they are a novelty. In fact, they are quite useful for the end cap muon systems, since they provide straight lines through the muon chambers which can be used to refine the chamber alignment, and their arrival time is well-defined thanks to the bunched nature of the LHC proton beams. (For some nice illustrations, see the postings by Darin Acosta.) This strobe-like signal helps us refine the synchronization of the chamber signals, which is important for recording them properly and for defining an accurate and efficiency trigger.
Two months ago we were busy analyzing cosmic rays. Two weeks ago, and two days ago, we were extracting information from beam splash events. Today, and for the next few days, we are looking at beam halo tracks. What comes after that? collisions!.
Excitement returns to CMS this month, as the LHC begins to circulate beam. There are many good sources of information, for example, the online commentary by Darin Acosta, among others.
My team from Northwestern University is busy providing prompt feedback on the response of the cathode strip chambers (CSCs) from the CMS experiment. On 9-November, we observed the beam splash events produced when Beam 2 struck collimators and a wall of muons passed from the -Z to the +Z side of CMS. Here is a depiction of the charge measured on the radial strips of the CSCs:
The arrow indicates the direction of Beam 2, and one sees clearly more charge on the strips on the upstream side compared to the downstream side. The red fans show the inner set of chambers, while the blue fans show the outer. (There is one pair of green fans, but they are too small and to faint to make out in this picture.)
Here is a new event from this evening, 20-November, in which Beam 1 produces a splash in the CSCs:
Comparing to the picture above, it is clear that the two muon endcaps have exchanged rolls (and indeed, Andy reversed the direction of the arrow).
It is worth noting that the HV is set to stand-by values. The flux of muons is so great, on the order of 5 muons per cm2, that we nonetheless see a tremendous about of charge compared to what we expect for a normal single muon, such as a cosmic ray or one coming from a pp collision.
A more conventional, and colorful, view of these kinds of events is given by the official iSpy event display program. Here is an example:
The purple parts in the end caps are the CSCs, obviously registering lots of charge while many other subdetector systems are off.
As I write this post, the LHC operators are `capturing’ the beam, which means that the protons’ orbit is determined by the RF cavities that are turned on. This is a major milestone on the way to collisions.