From wayne@undhep.hep.nd.edu Wed Jul 16 12:59:35 1997 Received: from nd.edu (nd.edu [129.74.250.101]) by budoe.bu.edu (8.8.3/8.7.1) with SMTP id MAA19462 for ; Wed, 16 Jul 1997 12:59:32 -0400 (EDT) From: wayne@undhep.hep.nd.edu Received: from undhd0 (actually undhd0.hep.nd.edu) by nd.edu with ND-SMTP (PP); Wed, 16 Jul 1997 11:58:12 -0500 Date: Wed, 16 Jul 1997 11:57:53 -0500 Message-Id: <97071611575309@undhep.hep.nd.edu> To: jmbutler@budoe.bu.edu Subject: note on fibers for TEV33 X-VMS-To: SMTP%"jmbutler@budoe.bu.edu" Status: RO Tracking Issues/Strategies for TeV33 R. Ruchti and M. Wayne University of Notre Dame 10 July 1997 ------------------------------------ The concept of TeV33 is a real "poser" for central tracking because of several concerns: 1. How much money is available for developments for D0. 2. How much time is available to get the "new" tracker installed. 3. The current onset of Run 2 is actually like TeV33, in that a 396ns crossing interval (and associated n-bar) lead to occupancy problems in the fibers. 4. TeV33 might be a gradual, but continuously increasing intensity rise in the machine, rather than low luminosity running (Run2) followed by a break and then high luminosity running (TeV33). 5. Manpower is a serious concern for getting Run 2 going; TeV33 manpower may well be even more serious. ____________________________________ Make/break items. Silicon. Here it is radiation damage effects over time - principally in the inner layers. Occupancy is never really an issue. Silicon is never used in any level-1 decisions. Scenarios: 1. Running until bias values become untenable - then replace layers. 2. Alternative cooling to lower temperature. 3. Remove inner layers to larger radii, and add outer layers to preserve in ratio of inner to outer radii. 4. Replace inner layers with pixels, and leave the outer radii as is. Fibers. While there are radiation damage effects that occur over time, these are not the pivotal problem for fibers. Rather the issue is occupancy in the fibers, particulary as n-bar rises above 5. This impacts directly the level-1 trigger capability. Recall that the fibers are important for both electron and muon triggering at level-1. Scenarios: 1. Remove the inner fiber layers to larger radii. 2. Reduce the diameter of the fibers from 830um to 500um. 3. Split the fibers in the middle for selected layers (presumably those at inner radii) 4. Add more fiber layers at larger intermediate radii. -------------- Tactics The above scenarios may or may not be tenable, depending upon what TeV33 actually is (or means) for D0. Note that TeV33 for D0 may be very different strategically than for CDF - in terms of potential physics strategy and running strategy. What if: Run 2 is at 396ns for a while, and things gradually increase in luminosity over time so the TeV33 problems actually occur earlier than originally forseen. Here - silicon is "ok". limps along. bias is raised. we run as cold as we can. ulimately, we might want to replace the inner two layers of silicon with two layers of pixels. fibers are in tough shape at t-zero. occupancy makes stresses the level-1 triggering, forcing high pt thresholds to keep rates under control. it might be desirable to add additional layers of fibers between 30cm and 50cm at the outset to strengthen pattern recognition and triggering as funds are available to instrument them with VLPC channels. It would be even better if we could use them in the trigger also, but this would require additional electronics modifications. besides the inner two layers of fibers at 20,25cm, there could be up to 9 additional axial layers and 8 stereo layers in the radial region of 30 to 52 cm. The fiber layers and waveguides would be built into the run2 tracker and then instrumented as resources are available (presumably as soon as possible). Downside. Impedes run2 plans underway now. Can leave dead material (uninstrumented layers) in place until electronics readout channels are available. Imposes a solution, which later experience in Run2 may show to be not the right direction because of the physics goals which may change with time. Upside. The tracker can be upgraded adiabatically right during run2 (when in fact we may need it). New layers will strengthen pattern recognition, and if new trigger boards can be developed, the trigger can be strengthened with a large number of available layers of fibers. A shutdown for TeV33 changeover would then involve basically reworking the tracker inside a radius of 30cm, rather than gutting the full 52cm radius and starting over from scratch. Both a time and manpower advantage should result from such an approach. Run 2 is actually at 132ns as advertised with a low nbar (1.5 or so) and then a shutdown of sensible length (2years) and of sensible funding (>~30M?) is available for the work. Here - the inner layers of the silicon are replaced with pixel layers. the outer tracker can be configured in several ways. Only fibers, with reduced diameters and split in the center. More VLPC channels and (presumably) smaller pixel VLPCs must be developed. the radial region covered is 20-52cm as at present. the other option is to develop silicon trigger towers for radii 20-30cm and place scintillating fibers in the radial region outside 30cm. here the fibers would be presumably of 830micron diameter. since the trigger burden is now on the silicon towers. the fiber layers are for pattern recognition and 3D tracking and momentum measurement. upside - you get a more robust tracker, and potentially stronger trigger ing capability. downside - tracker really gets gutted and everything with 52 cm radius is reworked. it will require substantial manpower and resoures (and a collaboration commitment) to do this. What if: The situation is somewhere in between - there are many alternatives. ------------------ Commentary: We have discussed and argued the issues at great length. The strain to try to do something right away and anticipate a "nasty" phase into TeV33 conditions early is driven by uncertainty that the accelerator will perform as originally advertised for run 2. This drives the thinking of the first "what if". Is there a "right way" and a "wrong way". Yes.... The wrong-way is to start redesigning everything now to accomodate new pixel layers and new fiber layers. The right-way is to build what we have set out to do so far. The building and then the operating of these systems will teach us much about performance, which will be invaluable to coming to a design for TeV33. These may lead "naturally" to a set of choices which may or may not be obvious now. Note that this "right way" may well lead to an expensive and time consuming upgrade imposed at the "end" of run 2.