Proofread my QCHS7 proceedings paper - part 1 October 30, 2006
Posted by dorigo in language, personal, physics, science.8 comments
For you only, here is the first part of a proceedings article I am writing for the QCHS7 conference (”Quarks confinement and the hadron structure”), which I attended last September at the Azores islands.
The article discusses standard model tests at the Tevatron, and is aply titled “Standard Model tests at the Tevatron”.
If you have a chance, proofread it and send me any errors, typos, or bad sentences…
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Abstract
The CDF and DO collaborations at the Tevatron have been producing exquisite precision measurements on high-P_T physics with their large datasets of p-antip collisions.
The Higgs boson is being sought in all available channels, and the Tevatron experiments will have a chance to discover it before LHC starts operating. The top quark is being studied in great detail, and a precision of 1.2\% in the measurement of its mass has been achieved. In this brief report an overview of the most interesting topics will be given.
The facilities
The Tevatron accelerator has been subjected at the turn of the millennium to a massive upgrade, with the construction of an entirely new ring, the Main Injector, and several improvements in the facility producing and storing antiprotons — the most challenging part of the whole project. The collider has recently surpassed the peak luminosity of 2.3 x 10^32 /(cm^2 s). An integrated luminosity of 2.0 /fb has been delivered to CDF and DO so far, and 5 to 8 inverse femtobarns are expected by end of 2009. Up-to-date information on the performance of the machine can be found in [1].
An overview of the CDF and D\O\ detectors for Run II at the Tevatron can be found in [2]. In what follows their most important features for high-P_T physics are briefly mentioned.
Both detectors are all-purpose, near-hermetic devices consisting of a tracker immersed in a solenoidal field and an outer shell of calorimeters and muon chambers.
In DO an excellent system of silicon microstrip detectors has been installed in Run II. Six barrels of silicon sensors organized in four concentrical layers provide coverage for central tracks, while a total of sixteen silicon disks allow reconstruction of large rapidity tracks. A similar set of seven barrels of silicon strips is organized in the core of CDF.
Outside of the silicon barrels CDF features a large gas tracking chamber, and DO has a compact scintillating fiber tracker. Both allow measurement of track momenta with better than percent accuracy for P_T<100 GeV.
Associated hits in the silicon strips allow the determination of track
impact parameter with accuracy sufficient to reconstruct B-hadron decay and enable 45% efficient tagging of b-quark originated jets with fake rates well below 1%.
Calorimeters are divided in a inner electromagnetic and an outer hadronic section. Electrons within the pseudorapidity interval |\eta|<2.0 are identified with high purity and efficiency, with a resolution of 14%/E^0.5 in CDF and 17%/E^0.5 in DO.
Hadronic jets are reconstructed with resolutions better than 100%/E^0.5.
Muon chambers cover the rapidity region |\eta|<1.5 in CDF and |\eta|<2.0 in DO.
Both detectors have a sophisticated trigger system that reduces the 2.5MHz collision rate to about 100 Hz of events written to tape. Of particular relevance for the present review is the Silicon Vertex Tracker (SVT), a device designed and built in CDF to achieve precise online tracking.
The SVT identifies track candidates by comparing hit patterns to a predetermined array of possible roads stored in associative memory banks. A linearized R-phi fit of track hits in the silicon layers provides track momentum and impact parameter with precision close to that attainable offline in less than 10 us, thanks to a highly parallelized architecture. This allows the collection of datasets based on the presence of b-quarks in the final state, enhancing the B-physics program of CDF but also providing higher efficiency for several Higgs boson signatures.
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Ok that is it for now, a second piece is coming soon…
Four trillion antiprotons in the waste bin October 30, 2006
Posted by dorigo in news, physics, science.2 comments
Yesterday the day shift at the CDF control room looked promising at the beginning, when a huge stack of antiprotons was waiting to be injected in the Tevatron ring. The more antiprotons we can get to collide, the more data we collect, and the happier everybody is…
Actually, things are not so straightforward: no matter how high is the number of particles in the beam, and the resulting collision rate, we always write to tape about 100 Hz of events. But those events we write are the more interesting the more they were selected by our trigger system.
Still confused ? Ok, imagine we have a trillion antiprotons and ten trillion protons colliding. That will cause of the order of 5 MHz of collisions in our detector. We select 100 Hz of those with a trigger system that sorts out the most promising events for data analysis. The particles in the beam go on colliding for hours, and their number decrease as they collide or interact with the residual gas in the vacuum beam pipe. As the number decreases, the collision rate will go down accordingly, say to 2.5 MHz at some point. But our data acquisition system will adjust automatically the trigger selection cuts to keep the 100 Hz output constant.
Antiprotons do not come for free. They do not exist in nature, and have to be produced by colliding a beam of protons with a thin target. One such interaction every 50,000 will produce an antiproton, which is then collected in a dedicated facility, the Antiproton Accumulator ring.
It takes hours to build a stack of antiprotons large enough to make a meaningful beam for tevatron collisions. Yesterday morning we had a pretty large stack, a total of 5 trillion antiprotons. But we wasted 4 of them when, after injecting them in the tevatron tunnel, one of the magnets of the accelerator had a quench.
Magnets are used to focus the beam and to get particles trajectories to bend in the circular tunnel. The ones at the Tevatron are superconducting ones, so they produce intense fields in a compact size and with less current expense. To keep them in the superconducting state, liquid helium is flowed through them. But sometimes, the liquid helium will have not enough pressure to take away the heat from the magnet. The temperature will rise, and the field will decrease sharply. This is what is called a quench.
A quench can be destructive for the magnet, but most of the times things are not that bad. However, at the very least a fast quench causes the beam circulating in the accelerator to be lost.
Too bad for the nice stack of antiprotons, whose life had been designed to produce exotic new particles after a few hours spent orbiting and then smashing against a proton. They did not live long enough to make a fancy Higgs boson… They ended their life annihilating against a dump.
And today, we had another quench, again just before starting collisions… Too bad! These two stores might have allowed us to collect a couple of Higgs bosons and maybe 30 top quark events…
