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:

Audience in the CERN Auditorium while Steve Myers speaks.

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.

A close-up of straight-line tracks from the earliest LHC collisions

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!

One of the very first CMS Collision Events at 900 GeV

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?

One of the CMS muon end cap disks being lowered into the experimental hall

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.

Diagram of the gas gap in a cathode strip chamber

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:

Event display of a nice beam halo event from CMS

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:

Beam 2 splash event in the CSCs

Here is a new event from this evening, 20-November, in which Beam 1 produces a splash in the CSCs:

Beam 1 splash event in the CSCs

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:

Beam Splash event as depicted by iSpy

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.

The D0 Collaboration recently posted a brief paper describing a search for Higgs bosons in the NMSSM (arXiv:0905.3381). The model supposes that the scalar Higgs boson, h, decays predominantly to a pair of very light pseudo-scalar bosons, a. If the a bosons are very light, then they may be expected to decay predominantly to a pair of muons or tau leptons. So the signature would include events with four muons (very distinctive!) or two muons and the decay products of two tau leptons (also distinctive).

I like the idea of finding a light particle produced in high-energy collisions through its decay to muons (or photons or electrons) since it helps underscore the fact that hadron collider experiments are also a good place to look for very rare processes, not just heavy particles. I wrote earlier this year about looking for a bosons at a hadron collider and it is very nice to see this analysis by D0.

The physicists who conducted this analysis were faced with at least one interesting challenge: reconstructing two high-energy muons which come close together in space. (The muons are close together because they come, hypothetically, from a fairly light particle which itself comes from the decay of a fairly heavy particle. So the muons have a large Lorentz boost in the laboratory frame.) Most muon detectors are designed to register a single muon well, or perhaps two muons that land far apart. Their granularity is poor, compared to tracking devices or even the calorimetry. There can be problems with producing a valid muon trigger, and also with reconstructing the muon tracks themselves, even offline. Finally, one has to be careful when demanding that the muons be isolated, since they are not isolated, strictly speaking.

For the four-muon channel, the D0 physicists approach this challenge by asking only that one muon out of each pair be reconstructed, and then they pair each of two reconstructed muons with a “companion” track, meant to be the muons that were not reconstructed successfully. This may sound like it should lead to a large background, but remarkably, it does not, thanks to the isolation criteria they applied. Only two events are selected in over 4 fb^-1, consistent with expected backgrounds, and neither of these has more than two muons.

The reconstruction of nearly-collinear tau decays is much harder. In fact, the D0 group did not try to reconstruct the tau’s directly, but rather leaned on the fairly large missing energy coming from the neutrinos emitted in the tau decays. (For a nice simulated event, see the analysis web page.) With two muons, significant missing energy, and then evidence of tau decays and vetos against jets, again the expected background is quite small. The two muons from the a decay (remember, for this channel one a decays to a muon pair, and the other to a tau pair) can be used to reconstruct the mass of the a boson. So the natural strategy is to look for a bump in the di-muon spectrum for this sample of events.

Here is the D0 result:

di-muon spectrum for candidate events with two muons and two tau leptons

The D0 paper contains some limits placed on this production channel, which require a few not unreasonable assumptions. I’m not so interested in those limits, which generally are higher than the theory predicts. What pleases me about this piece of work is the ability of the D0 detector and event reconstruction to look for such events.

Earlier this year I made a brief post drawing attention to the International Year of Astronomy, i.e., this year. Risa Wechsler at Cosmic Variance also made a post on 29-March, and I am sure there are many others.

The idea is to build a sense of community among amateur astronomers and to promote an appreciation of science by ordinary citizens.

Northwestern University, and in particular, the Department of Physics and Astronomy, is hosting a special Skygazing Event at Dearborn Observatory located on the sometimes verdant campus in Evanston, Illinois.

On Saturday evening, April 4th, visitors will be able to gaze through the telescope (pictured above) at the moon and Saturn, as well as connect in live webcast to similar activities around the world. Astronomers from NWU will be on hand for discussions and to answer questions. Details are given in a press release.

One of my favorite papers of the last year is a very well-done scan of the phenomenological-MSSM (pMSSM) by Berger, Gainer, Hewett and Rizzo: arXiv:0812.0980 which has the wonderful title, Supersymmetry Without Prejudice. The authors applied an absolute minimum of theoretical constraints, and a maximum of experimental ones, and they took real care to do it right. (OK, you do have to accept the premise that the MSSM is the right theory of nature, but many people are prepared to do that for the sake of discussion, i.e., as an hypothetical.)

So what are the consequences?

A follow-up paper just appeared, arXiv:0903.4409, which elucidates the consequences of the first paper for the properties of dark matter particles. The authors (R.C. Cotta, J.S. Gainer, J.L. Hewett and T.G. Rizzo) find that the lightest neutralino and chargino states should be rather light, in the range 100 to 400 GeV, and this conclusion does not depend crucially on the way they sample parameter space. That’s the good news. The bad news is that the mass difference typically is small – less than 10 GeV in 80% of the cases generated by the scan. Interestingly, searches for quasi-stable, heavy charged particles at the Tevatron have had an important impact here – something that the LHC experimenters should (and will) notice.

M(NLSP) - M(LSP) versus M(LSP)

Another interesting point is that the majority of the models surviving current experimental bounds are inconsistent with mSUGRA.

The lightest neutralino provides the best candidate for dark matter, but as it turns out, only a small fraction of the models predict a relic density high enough for this one particle to explain what is observed. Typical models predict a relic density ten times smaller than what is observed. So while it is true that Supersymmetry provides a good candidate for dark matter particles, obtaining a high enough relic density does not happen naturally. It might have been interesting to see whether the models which do match the WMAP relic density are different from the bulk of the models (sorry for the pun), but the authors do not furnish that information, unfortunately.

The authors checked predictions for direct detection of dark matter and found a huge range of predicted cross sections extending some eight orders of magnitude below the current bounds from XENON10 and CDMS. Most of the models, however, could be tested if the sensitivity of these experiments were improved by a factor of a thousand. (Again the question: is there anything distinctive about the models which are near the current XENON and CDMS sensitivities?) The authors also compared predictions to the PAMELA data; large boost factors are needed and the shape does not, to my eye, agree with the published data.

Bottom line: SUGRA-lovers and tri-lepton devotees, beware!

illuminated building in Torino, Italy

This week CMS held a three-day workshop dedicated to studies of detector performance using real cosmic ray data (so-called CRAFT = “cosmic rays at four Tesla”). For me, the workshop was a real pleasure. First, it was great to get away from rainy, chilly Geneva and walk around the warm, sunny streets of Torino.

Secondly, and more importantly, the presentations were excellent, and the content was impressive. We talked about tracking, muon reconstruction, triggering and calorimetry, as well as certain “physics analyses” associated with cosmic rays. These topics are predictable, but the level of the work was much higher than anyone would have expected. It is clear that CMS physicists are in the mind set to do top-level physics analysis. The attitude of individuals talking about calibration, efficiency, alignment was very serious. These people are aiming for the highest levels of detector performance, and they will get to the bottom of existing detector problems. And of course such problems exist – the point is to find them now and solve them before collision data arrive.

Kudos to the organizers of the workshop, members of the CMS group at Torino. They did a great job setting up and running the conference – everything was smooth and well-organized. There were no technical problems, and the refreshments were excellent!