Archive for March, 2010
It is an exciting day, with thousands of 7 TeV collision events flowing in. I watched the frustrating first failures from my laptop at home, in the middle of the night. Now that the sun is up, the scenario is much brighter – CERN just held a press conference in which the Steve Myers and the leaders of the four LHC experiments proudly and joyfully showed that they are taking data.
Here is a pretty event from my own experiment, CMS, in which a clean muon track is reconstructed well:
The muon is the long red track the curves down to the lower right-hand corner. It has been detected and reconstructed in the Cathode Strip Chambers (CSCs), and my group at Northwestern belongs to the CSC group within CMS. So I am pleased and proud to see this nice event!
Edgar Carrera posted a different CMS event at US LHC Blog, where you can also see events from ATLAS.
The official CMS Press Release, in several world languages, is available here.
Mike Lamont delivered a report on the commissioning of the LHC. The slides are here. Below I offer some notes from his talk.
Problem: as discovered at the final stages of the hardware commissioning, the quench protection system (QPS) can be triggered erroneously due to a converter switch being turned off at the same time as a fast discharge. The workaround involves new thresholds, and the implication is that ramping must be slower than planned – it will take 3/4 of an hour. A real solution will come in a few weeks.
Success: the availability of the technical systems was good. For week 10 of 2010, the LHC was available 66% of the time, with 17% planned downtime (technical stop) and only 17% unavailable for unplanned reasons.
Success: lifetimes of 450 GeV beams were quite high, on the order of 100 hours.
Success: the aperture was measured by kicking the beam and seeing at what transverse distance and where along the beam losses occur. The optics show apertures that are more than ten times the size (σ) of the beam.
Success: the magnet model turns out to be remarkably accurate. The largest deviations are at the couple 10-4 level.
Success: the beam dump system, which is highly nontrivial and may be crucial to the survival of the experiments, works beautifully. See this illustration of 10 bunches dumped into the target precisely where they are meant to go (red line).
Critical Path: the machine protection system will dump the beam if “anything out there decides it’s had enough.” There are many inputs to the decision to dump the beam, and subtle interplay among them. Careful testing has proceeded well so far.
Mystery: “the hump” drives beam excitations which are bad because the emittance blows up leading to lower luminosities and higher backgrounds. The experts are open to suggestions…
Success: the ramp Friday morning with two pilot beams was a complete success – on the first try. The orbit looks stable and reproducible.
Puzzle/Problem: apparently the bunch length is not as short as expected after the ramp is completed, and it grows over time. This is not understood and may be a concern if fills last for many hours – see the plot below.
The ramp ends just before 60 minutes. The lower green cross shows what the bunch length should be, while the upper cross shows that it is significantly larger (don’t miss the suppressed zero!). The purple crosses show a small increase from 60 to 130 minutes.
Success: beta beating relates to the stability of the orbit and is a measure of success of the commissioning of the machine. For the LHC, this is already at the 20% level – it took weeks/months for earlier machines to achieve this.
In short, the LHC is in good shape, and the main area of concern is the machine and quench protection systems. According to Lamont, there are no show stoppers, and all of the hard work and thorough preparations over the last months and years puts the LHC in an excellent position for physics this year. :)
This weekend I read a superb paper by the CDF Collaboration (arXiv:1003.3146):
Studying the Underlying Event in Drell-Yan and High Transverse Momentum Jet Production at the Tevatron
This paper is written so very clearly that a very thorny and confusing phenomenon can be grasped with little effort. Even better, new results are presented which elucidate the underlying event in ways that help a lot to understand what is going on.
First, what is the “underlying event” and why do we care about it? The UE is all that you see in a hadron collider event which is not coming from the primary hard scattering process. So, aside from a q-qbar pair annihilating to give you a pair of leptons for example, there are tracks and calorimeter energy coming from the non-scattering fragments of the two beam particles, potentially from other partons in the same beam particles which interact along with the “primary” ones, and other beam particles which happen to interact. All three pieces are hard to calculate because these processes are soft and so non-perturbative interactions play a dominant role. We must use models (HERWIG or PHOJET or PYTHIA, with several “tunes” of model parameters) to simulate these events. Since the UE has several components, adjusting these models to mimic the data is challenging.
The need to tune and test these models is urgent now, since they tend to give wildly different predictions for the UE at the LHC. Why? The three main components of the UE (beam remnant, multiple parton interaction or MPI, and pile-up of different beam-beam interactions) do not vary the same way with c.m. energy, so a match at 1.96 TeV does not guarantee a match at 7 TeV.
Let’s build up the picture as follows. Consider a typical hard 2→2 scatter, producing two jets which are back-to-back in the transverse plane. Inevitably, one of them will have a higher transverse energy, ET, than the other. The higher-ET jet is the “tag” for the event, and we’ll put it at 12 o’clock. The other jet, called the “away” jet, would normally be at 6 o’clock. The authors call the 12 o’clock direction the “toward” side, which matches better with the word “away.”
In an e+e– collider, that would be the end of the story, for two-jet events. In a hadron collider, however, the “transverse” regions at 3 o’clock and 9 o’clock are interesting because they will not be empty – the UE contributes there as well as anywhere else. The CDF analysis method consists of examining the transverse regions, characterizing the level of activity by the particle density and by the mean pT there.
An important innovation is to substitute a Z-boson for the tag jet. The Z boson is color-neutral, so the particle density in the toward region should be low, similar to the transverse regions (after excluding the two leptons from the Z decay). Thus the intense particle flow from the tag jet does not obscure the UE in the toward region.
The authors compare to a selection of models. PYTHIA is the work-horse of event generators, and the UE model in PYTHIA has been tuned multiple times. The interesting cases are called “A” (good for di-jet events), “AW” (good for Z+jet) and “ATLAS” (meant to be accurate at the LHC). In addition, there is HERWIG augmented by an MPI model – this is expected to be less accurate that PYTHIA and it is.
The first interesting results are shown below:
The horizontal axis is a measure of the “hardness” of the primary interaction – the pT of the jet [top plot] or Z-boson [bottom plot]. In both cases, the particle density opposite the tag object (jet or Z) increases rapidly with pT – see the blue dots . This is clear – the recoil jet in the away side must balance this pT. In contrast, the particle density in the transverse region (green dots) does not care about pT, which is also intuitive – this is the UE. Notice the dramatic contrast in particle density for the toward (or tag) area: it matches the recoil jet when the tag is a jet, and it matches the transverse density when the tag is a pair of leptons. This is very pretty, and very well reproduced by PYTHIA.
There is plenty of evidence that HERWIG does not reproduce the data as well as a tuned version of PYTHIA. Here is one particularly clear example:
This plot shows the particle density in the transverse regions, for Z-boson events, as a function of the pT of the Z-boson. Clearly, HERWIG (“HW”) is too low while PYTHIA (“pyAW”) gets it right. The main cause for the difference is the lack of MPI in HERWIG. So the difference between the two curves can be seen as a measure of MPI (multiple-parton interactions). It is not negligible, and should increase rapidly with c.m. energy.
if the pT of the Z boson is large, then there is a lot of energy available for gluon radiation in the initial state. If there are more than one ISR jet, the particle density in the transverse region will rise – basically, the second and third jets will tend to leak into the transverse region and will not be confined to the away region. The toward region, however, where the Z-boson went, will still be clear of ISR jets. Here is a comparison of the particle density in the transverse and toward regions, illustrating this effect:
The authors differentiate the two transverse regions (at 3 o’clock and 9 o’clock) according to their energies – TransMIN is the lesser of the two, and should be more sensitive to the UE; TransMAX will have the contamination of any extra ISR. This is an excellent place to check models (HERWIG vs PYTHIA) and tunes of models. Here is the plot of the charged particle density as a function of the pT of the Z-boson, in the TransMIN region:
PYTHIA matches the data very well, both with tune “AW” and tune “DW”. The ATLAS tune of PYTHIA is also reasonably good. A more detailed look (available in the paper), however, indicates that the ATLAS tune produces somewhat too many particles which are, however, too soft. HERWIG alone (“HW”) is too low, as already noted. Trying to fix HERWIG by adding in a model for MPI called JIMMY (“JIM” curve) does not work – it is too high. Perhaps a tune of the combination HERWIG+JIMMY would yield better results.
The point of showing so many models at Tevatron energies is to prepare for an extrapolation to LHC energies (14 TeV). Here is a comparison of two successful PYTHIA tunes as well as HERWIG:
The two tunes (DW and DWT) show a large increase in the particle density, but the increase is significantly different and could be easily verified with the first LHC data. CDF data taken at 630 GeV favor the DW tune over the DWT tune. HERWIG, which lacks MPI, shows a much smaller increase. Thus, much of the increase comes from MPI (in which there are two parton-parton scatters from the same beam particles).
The final study in this excellent paper involves soft collisions logged through a minimum-bias trigger. Such collisions are related to the UE, but are not the same as the UE. Despite this fact, collider physicists use min-bias events as a model of the UE. Thus it is important to study min-bias models to unravel what is truly happening in the UE and how well or how poorly it is represented by min-bias events.
The main observable is the increase of the average transverse momentum, <pT>, with the number of charged particles, Nch. The soft and hard components of min-bias events play varying roles as a function of Nch. This also gives a handle in the MPI. A comparison of the models to CDF min-bias data shows that PYTHIA with tune AW gets it right, while the ATLAS tune is too low. (Again, too many particles with too low momentum.)
One can also compare this observable for min-bias events and Z-boson events with a low pT (so that there is little ISR). The behavior is very similar even though the theoretical framework is, at first glance, very different.
The PYTHIA tunes match the data in both cases, indicating the MPI plays an important role in both cases.
This blog post gives only a cursory treatment of this fine paper. The analysis is done in a very careful way – there are many interesting technical details which I did not even mention. Also, details of the various PYTHIA tunes are spelled out. Of course, there are many plots and discussion which I cannot reproduce here.
All students of collider physics should read this paper. The very murky issues in understanding the underlying event are clearly explained, and there are new results which elucidate the role of multiple-parton interactions.
I wrote already about double-parton scattering some months ago, and I’ll bet that this phenomenon crops up again and again, since its role increases rapidly with c.m. energy.
FYI: tomorrow (Friday 26-March) there will be a webcast of a report by Mike Lamont on the current status of the LHC commissioning and of the plans for the next steps.
The time is 15:15 at CERN, or 9:15am in Chicago (for example). For more information, see http://indico.cern.ch/conferenceDisplay.py?confId=88711 and http://www.cern.ch/webcast.
This should be very interesting, in light of the 7 TeV Collisions scheduled for Tuesday, 30-March. (More information here.)
The next presentation will be on 9-April.
Rolf Heuer, Director General of CERN, announced today that the LHC is on schedule to provide collisions at 7 TeV on 30-March-2010, Tuesday a week from now.
A quote from Steve Myers: “With two beams at 3.5 TeV, we’re on the verge of launching the LHC physics programme. But we’ve still got a lot of work to do before collisions. Just lining the beams up is a challenge in itself: it’s a bit like firing needles across the Atlantic and getting them to collide half way.
And a quote from Heuer: “The LHC is not a turnkey machine. The machine is working well, but we’re still very much in a commissioning phase and we have to recognize that the first attempt to collide is precisely that. It may take hours or even days to get collisions.”
The CERN press release points out that three days were required to bring the electron and positron beams at LEP into collision. There will be a live webcast and, I presume, lots of press at the LHC control room as well as at CMS, ATLAS, LHCb and ALICE.
I’ll bet that all experiments will perform splendidly, and I hope that the LHC performs as wonderfully as it has these past few weeks…
We were all rather down when the LHC magnet blew up in December 2008. Enough has been written about that. Quickly enough, the CMS Collaboration made the best of the situation and launched a serious campaign to commission the detector as much as possible using cosmic rays. The result is twenty-three scientific papers appearing as a special volume in the Journal of Instrumentation published by IOP Science. The link is 2010 J. Inst. 5. The editors of this journal have been very helpful and the CMS Collaboration is grateful for their cooperation.
The papers cover nearly every aspect of the detector. There is an overview paper, which describes how the data were logged and how the data acquisition and event processing were carried out. There are papers on the alignment of the tracking devices and of the muon system – the result is equivalent to tens of pb-1 of collision data. In a related effort, the magnetic field map in the muon chambers, with highly non-trivial spatial variations, was verified at the percent level. The energy deposited in the electromagnetic and hadronic calorimeters as a function of muon momentum was measured and compared to simulations. Anomalous signals (“noise”) in the calorimeters were also studied extensively, pointing the way to filters to remove them in collision data.
The event sample amounts to approximately 300 million cosmic ray muon triggers (remember that the CMS detector is installed in an underground cavern) collected over a four-week period. Essentially all of the CMS subdetectors delivered high-quality data, good enough for physics analysis. The operational efficiency was rather high, above 80%, and sustained over that four-week period. The superconducting solenoid was on for most of that period, delivering a field of 3.8 T.
The HCAL group at my home institution, Northwestern University, contributed in a major way to all three HCAL papers. For example, they studied the per-tower calibration as quantified with cosmic ray muons, deriving corrections that clearly improve the uniformity of response:
Here is the measured response as a function of muon energy. One can see clearly the relativistic rise for high-momentum muons, which evidently is reproduced well by the CMS detector simulation:
The muon group at Northwestern contributed to several papers as well. In fact, I was the main author of the paper on the cathode strip chambers (CSCs) and my group produced more than half of the content of this paper. (Yes I am proud of that!) We checked the detector simulation. It is good but far from perfect. We performed serious measurements of the efficiency, and found some problems which have been fixed since then. Here is an example of the efficiency of the local charged track triggers – basically, the trigger primitives generated in the on-board CSC electronics:
The plot on the left shows the efficiency for the anode LCT, or ALCT, as a function of the angle. The trigger is designed to be efficient when the muons point to the interaction point, and in that region, they are well above 99% efficient. On the right, the corresponding plot for the cathode LCT, or CLCT. Again, for the region where the efficiency should be high, we measured above 99%.
We also studied the chamber resolution. Due to the nature of cathode strip chambers, the resolution is better with more charge measured than with small charge. The 1/Q trend can be seen in the plot below, up to about 300 fC (the mean charge left by a muon). The plot shows a worsening in the resolution at very high charges, due to the interference of delta ray electrons with the charge measurement.
We also studied the resolution as a function of the muon impact within a strip, muon incident angle, and magnetic field.
I could write pages and pages about all the nice results obtained for all the CMS detectors – after all, there is a lot of material in twenty-three articles accepted by the referees of JINST. If you are interested (and I am sure the experts of you have specific interests), please go take a look using the link above.
The work done for these papers has been of immense value for the CMS Collaboration, placing the detectors in an unprecedented degree of preparedness – the impact of the understanding of the tracking devices for the first CMS physics paper has been tremendous.
Finally, let me point out that there is some interest in cosmic ray characteristics and one can hope that CMS will use their cosmic ray data to perform some physics measurements, too.
Here is a summary of accelerator operations last night:
- 16:30 Started ramp without beam at 2 A/s - 17:38 Trip of Sector 78 during dry ramp to 6 kA. To be understood. - Dump during the ramp due to BLM (problem with optical link for BLM in pt1, fixed). - 20:45 Try a second ramp without beam - 22:04 Ramp to 3.5 TeV without beam completed. LBDS/BETS OK. - 00:20 Ready at injection - 01:45 Injecction started - Found large chromaticity (negative) and important orbit distortion => regenerated sextupole functions (SF SD), much better - Performed checks: * beam1: o Q' Trim : H = -25.0 & V = 0.0, now at H= 5, V= 5 o Q trim : H only by +0.02. Tunes at 0.28 and 0.31 o Orbit corrected against "golden" * beam2: o Q' trim : H by -20.0 & V by +8. Now at H= 10, V= 9 o Small tune trims to get back to nominal o Orbit corrected - 04:00: Ramp started with buckets 1 (beam1) and 1001 (beam2) with tune feedback on, LHCPROBE, ~5e9. - 05:23: both bams at 3.5 TeV ! 100 hrs lifetime - At 5:25: RQTF.A81B2, RQTF.A45B2, RQTF.A34B2, RQTF.A23B2 tripped. PM indicates that QPS triggered - Beams dumped properly - At flat top, aad time to send 50% of an orbit correction on both beams, before RQTFs tripped. - Tune feedback was on * Final Q-FB trims before beam loss: o beam1: dQH = -0.008 , dQV = -0.054 o beam2: dQH = -0.019 , dQV = -0.066 * Tunes at nominal settings after the end of the ramp. * Trims requested to RQT circuits therefore increase and probably led to trip of QPS. To be understood. - Transverse emittances * BSRT vs WS Vertical excellent up to ~ 2.5 TeV, then BSRTa calibration with D3 light to be studied. WS profiles with small sigmas to be looked in detail. BSRT vs WS HOR with already seen systematic difference, to be understood .
Note the innocuous mention at 05:23 hours: both beams at 3.5 TeV!. As has been pointed out in Cosmic Variance and Not Even Wrong, this is a new world record, far surpassing the beam energies of the Tevatron and the earlier running of the LHC itself. You can also read the CERN Press Release, and a nice upbeat message from Director General Rolf Heuer. Details from the LHC operations center are available at http://cern.ch/lpc.
Bravo! Now let’s see the beams held for a significant period of time, and then… collisions!