Hadron colliders can be an abundant source of heavy flavor quarks, but pose a
challenge to isolate the physics signals from the high backgrounds. The
upgraded CDF II detector, with its precise tracking capabilities and powerful
trigger system, is well equipped for this task. The detector is described with
an emphasis on actual performance and on techniques to maximize the heavy
flavor yield. Some first heavy flavor results are summarized.Comment: Invited talk at the Workshop on the CKM Unitarity Triangle, IPPP
  Durham, April 2003 (eConf C0304052). 4 pages LaTeX, 2 eps figure

Oldeman, R. G. C.

English

arXiv

Hadron colliders can be an abundant source of heavy flavor, but pose a challenge to isolate the physics signals from the high backgrounds. The upgraded CDFII detector, with its precise tracking capabilities and powerful trigger system, is well equipped for this task. The detector is described with an emphasis on actual performance and on techniques to maximize the heavy flavor yield. Some first heavy flavor results are summarized

Oldeman, R.

UNT Digital Library

Fermilab FERMILAB-Conf-03/156-E June 2003Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003CKM03Performance of CDF for B physicsR.G.C. Oldemanfor the CDF collaborationUniversity of Pennsylvania, PhiladelphiaHadron colliders can be an abundant source of heavy avor, but pose a challenge to isolate the physics signals from thehigh backgrounds. The upgraded CDF II detector, with its precise tracking capabilities and powerful trigger system, iswell equipped for this task. The detector is described with an emphasis on actual performance and on techniques tomaximize the heavy avor yield. Some rst heavy avor results are summarized.1 IntroductionIn Run I of the Tevatron, CDF made essential contri-butions to B physics, providing some of the best mea-surements of masses, lifetimes, mixing and branchingratios. The B factories are now setting new standardswith a wealth of precision measurements, posing strin-gent tests of the standard model. CDF has made sig-nicant improvements of the detector and the triggerto expand its capabilities beyond simply proting fromthe higher luminosities of the Tevatron in Run II.Both B and D mesons are amply produced at theTevatron: in Run I the B+ cross section with trans-verse momentum pT  6GeV/c and rapidity jyj  1was measured in the B+ ! J= K+ mode to be3:6 0:6b [1]. A new preliminary Run II measure-ment of the charm meson cross sections for the samekinematic region yields even larger values: 4:3 0:7bfor D+ and 9:3 1:1b for D0, both measured usingfully reconstructed hadronic decays.Heavy avor cross sections at hadron colliders arethree orders of magnitude larger than at e+e  ma-chines running at the (4S), and include the heavierBs and Bc mesons and the weakly decaying baryonsb and b. The challenge at a hadron collider is toselect in real time the heavy avor events from theoverwhelming QCD background. Therefore the em-phasis in this talk is on the trigger system, which is aparticular strength of CDF II.2 Performance of the TevatronThe center-of-mass energy of the Tevatron has in-creased from 1.8TeV in Run I to 1.96TeV in Run II,and the number of proton and anti-proton buncheshas increased from 6 to 36, with a bunch separation of396ns. We anticipate the number of interactions perbunch crossing to rise from 1.2 at present peak lumi-nosities of 4 1031 cm 2s 1 to 6 at future luminositiesof 2 1032 cm 2s 1. This number can be reduced byincreasing the number of bunches, but the upgrade to132ns bunch spacing has been indenitely postponed.At the time of this workshop, the Tevatron has deliv-ered 190pb 1, of which CDF has recorded 145pb 1 ontape. Present B physics analyses use up to 70pb 1 ofthis data, where the selection criterion with the largestrejection came from requiring stable operation of thesilicon detector.3 The CDF II detectorCDF II [2] has the typical structure of a 4 detectorat a symmetric collision point. A superconductingsolenoid provides a uniform 1.4T eld over a track-ing volume of 34m3. Closest to the beam is a siliconsystem to provide an accurate measurement of the im-pact parameter of charged tracks. A large-radius, low-material drift chamber gives accurate measurementsof tens of charged particles per interaction. New inRun II is a Time-of-Flight (TOF) detector for particleidentication at low momenta. Outside the magnetare calorimeter systems followed by muon identica-tion systems. The components most relevant for Bphysics are described in more detail below.3.1 The Silicon systemCDF II has three silicon systems [3]. Layer 00 (L00)is a single-sided low-mass radiation-hard silicon detec-tor, mounted on the beryllium beam pipe at a radius of1.6 cm. The SVX II is a ve-layer double-sided siliconstrip detector. It consists of 3 barrels, each 30 cm inlength and covers 90% of the extended interaction re-gion (RMS  30 cm). The Intermediate Silicon Layer(ISL) provides one additional double-sided layer in thecentral region and two in the forward and backwardregions, to improve track reconstruction using silicononly. A light-weight carbon-ber space frame keepsthe 3.5m2 of ISL sensors stably aligned at the 10m2 Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003level. All silicon sensors use the same 128 channelSVX III readout chip that uses an analog pipeline fordeadtimeless operation. The silicon system has 750kchannels in total and can be readout in 20s.3.2 Commissioning the silicon detectorIn the rst year of data-taking, CDF has faced manystartup problems with the silicon system that havenow mostly been overcome. The SVX II has sueredfrom several beam incidents where a relatively smallamount of radiation was incident on the silicon sensorsand chips within a few nanoseconds, resulting in thepermanent loss of several sensors. The Tevatron hasnow installed interlocks to prevent this class of beamincidents. Other damage occurred when the silicondetector was read out at xed frequencies of severalkHz during anomalous trigger conditions. This hasbeen traced down to resonant oscillations of wirebondsperpendicular to the 1.4T magnetic eld. CDF hasnow implemented hardware protection against xed-frequency readout. The ISL suered from insuÆcientcooling as 11 of the 12 cooling lines were internallyblocked with epoxy at some 90Æ elbow connections.Using boroscopes to nd the blockages and a pulsed40W laser to destroy them, 10 of the 11 blocked lineshave now been cleared. L00 is aected by pickup fromthe digital readout lines on the analog signal cables.Methods have been developed to correct for this insoftware, but they require non-sparsied readout, pro-hibiting the use of L00 in the trigger. The CAENpower supplies that supply the L00 sensors with thebias voltage suered from beam related failures. Thishas been solved by replacing the MOSFET voltageregulator by a BJT circuit. At the time of this work-shop more than 90% of the silicon detector is opera-tional and used in physics analyses. One disadvantageof the present silicon detector is the large amount ofpassive material inside the tracking region: a tracktraverses on average 15% of a radiation length.3.3 The Central Outer Tracker (COT)The Central Outer Tracker [4] is a cylindrical Ar/Etdrift chamber based on the highly successful CTCused in Run I, modied to cope with the shorter timebetween interactions. The COT has 96 layers, or-ganized in 8 super layers, alternating between axialand 2Æ stereo readout. The single-wire resolutionis 200m, resulting in a momentum resolution ofpTpT (0:7 0:1pT )%. The measurement of the spe-cic energy deposition dE=dx gives a =K separa-tion of 1.2 in the region of the relativistic rise(pT  2GeV). The COT has proven to be a very reli-able detector, with fewer than 1% dead channels. Thetracking eÆciency for high-pT isolated tracks has beenmeasured from data using W+ ! e+e events to bebetter than 99.5%.3.4 The Time-of-Flight detectorFor the rst time at a hadron collider experiment,CDF II has installed a Time of Flight (TOF) detectorfor particle identication. It consists of 216 scintillatorbars, 3m long and 4 4 cm in cross section, mountedbetween the COT and the superconducting solenoid.The bars are read out on both sides by compact ne-mesh 19-stage photomultipliers, that allow operationinside the full 1.4T magnetic eld. The achieved tim-ing resolution is 120ps, close to the 100ps design, giv-ing 2 separation between pions and kaons up to mo-menta of 1.5GeV/c. The TOF has been designed pri-marily to improve B avor tagging, in particular forthe Bs, which is often produced together with a kaon,correlated in charge to the b quark produced. TheTOF has also found use for proton identication, cos-mic ray rejection and monopole searches. The TOFperforms reliably and all 432 channels are fully func-tional. However, the occupancy of 20% is about twotimes higher than expected, which has aected the re-construction eÆciency.4 The three-level triggerCDF uses a three-level trigger system to reduce the1.7MHz bunch crossing rate to 75Hz written on tape.The rst trigger level is deadtimeless, with a 42 buerdeep pipelined system, allowing 5s to form a triggerdecision. The eXtremely Fast Tracker (XFT) [5] re-constructs COT tracks in the axial projection downto transverse momenta of 1.5GeV/c, using hits withcoarse timing information. Muon and electron signalscan also be matched to tracks. Information about en-ergy deposition in the hadron calorimeter are availableat Level 1, but are not used for B triggers. The maxi-mum input rate for Level 2 is presently about 20 kHz.We expect this to reach 25kHz soon, but the orig-inal design rate of 50kHz will be diÆcult to reach.At Level 2, the Silicon Vertex Tracker (SVT) [6] pro-vides a list of tracks using axial hits of 4 silicon layers.This allows to select displaced tracks, which greatlyenhances the purity of heavy avor signals. Typicalrate reductions are O(10 1) when requiring one trackwith a displacement of 100m, or O(10 2) whenrequiring two displaced tracks. Further trigger ratereductions are achieved when selecting on the open-ing angle between two tracks, requiring positive life-time or pointing of the two-track momentum vectorto the primary vertex. The readout time of the COTcurrently limits the rate into the third trigger levelto 250Hz. At Level 3, the complete events are pro-cessed on a farm of 300 PCs, which perform full eventWorkshop on the CKM Unitarity Triangle, IPPP Durham, April 2003 3reconstruction and write up to 20MB/s to tape. Theevent size of almost 300 kB limits the Level 3 outputrate to 75Hz, but data compression techniques arebeing developed to increase this to more than 100Hz.5 Trigger strategiesThe vast majority of B physics at CDF is basedon three sets of triggers. The dimuon trigger re-quires two muons with pT  1:5GeV/c. It collectsvast samples of J= ! +  decays, which are 15%from B decay. At present peak luminosities this trig-ger uses 40Hz at Level 2 and new hardware is beingbuilt to select on the transverse mass of a muon pair,which should reduce this rate signicantly. The lep-ton+displaced track trigger uses muons and electronswith pT  4GeV/c combined with a displaced trackfound by the SVT. The specic yield of semileptonicB decays with this trigger is 5 times higher than in theRun I equivalent, which was based on an 8GeV/c lep-ton. These signals are used for lifetime measurementsand for optimizing avor tagging algorithms. The two-track trigger requires two displaced tracks. One ver-sion requires a large opening angle and pointing tothe primary vertex. It is designed to collect 2-bodyB decays, such as Bd ! + . The other requiressmaller opening angles and has stricter requirementson the track displacement (120m instead of 100m).This second version is designed for multi-body B de-cays, such as Bs ! D s +; it has also proven usefulfor collecting huge samples of hadronic D meson de-cays. The two-track trigger uses almost all of the avail-able Level 1 bandwidth and a signicant fraction ofthe bandwidth at Level 3. We are constantly trying toreduce the trigger rates while keeping the maximumamount of real heavy avor.6 Techniques for optimizing heavy a-vor yields6.1 Dynamic prescalingA typical \store" at the Tevatron lasts about 20 hours,during which time the luminosity has decreased by afactor 2.5 with respect to the initial luminosity. Sincethe trigger bandwidth has been designed for peak lu-minosity, signicant bandwidth is available in the laterpart of a store. We have developed techniques toll this available bandwidth with B physics by im-plementing triggers with several dierent pT thresh-olds and increasing the acceptance fraction (prescale)of the triggers with lower thresholds as the luminos-ity decreases. The two-track trigger comes in threeversions. The high-pT trigger has a low rate dueto high thresholds (two oppositely charged tracks of1.78 1.8 1.82 1.84 1.86 1.88 1.9 1.92 1.94 1.96020004000600080001000012000140001600018000200002200024000CDF RUN II Preliminary -1L=42.6 pb]2 [GeV/cππKM2Candidates per 2 MeV/c ππ K→ ±D = 214K±DNDPS = + 28 %DPS triggerunprescaled triggerFigure 1. Event yield of D+ ! K+   since the in-troduction of dynamic prescaling. Blue curve is the D+yield of the nominal trigger, the green curve is the D+ to-tal yield including the low-pT trigger that was eectivelyprescaled by a factor 0.22.outsidepT  2:5GeV/c with pT  6:5GeV/c) and is neverprescaled. The rate is low enough that it can besafely run up to luminosities of 8 1031 cm 2s 1. Thenominal trigger requires two oppositely charged tracksof pT  2:0GeV/c with pT  5:5GeV/c. This trig-ger is prescaled when the Level 1 accept rate exceeds20 kHz. A low-pT trigger that requires two tracks ofpT  2:0GeV/c and no pT or opposite charge re-quirement is enabled when the Level 1 accept ratedrops below 16kHz. In the six months that dynamicprescaling has been enabled, the low-pT trigger hasgiven a yield increase of 28% in the D+ ! K+  channel, as shown in Figure 1, while it was prescaledon average by a factor 4.5.6.2 XFT 1-miss congurationUntil October 2002, the XFT required 10 out of 12 hitson the four axial superlayers of the COT to reconstructa track. This resulted in an eÆciency of >95%, butwith a purity that was considered insuÆcient. Studiesshowed that the COT single hit eÆciency was highenough that requiring 11 out of 12 hits resulted in aneÆciency still better than 90%, while reducing the rateof the two-track trigger by almost a factor two at highluminosity.4 Workshop on the CKM Unitarity Triangle, IPPP Durham, April 20030.30.40.50.60.70.80.91SVT efficiency0 10 20 30 40 50 60 70 80 90t (days since Nov 7)Figure 2. EÆciency of the SVT in days since Nov 7,2002. The rise in eÆciency from 60% to 70% comes fromimproved SVX coverage. The high point at t = 26 corre-sponds to a test run with 4/5 SVT tracking.6.3 4/5 SVT trackingThe Silicon Vertex Tracker requires the presence offour strip clusters in four layers, resulting in an overalleÆciency that scales as the fourth power of the singlehit eÆciency. It is possible to program the SVT touse all ve SVX layers, and allow a missing hit in onelayer. In a test-run, we found a 15% increase of theSVT single-track eÆciency, as shown in Figure 2. Thiswill result in a signicant increase of the heavy avoryields from the two-track trigger. However, the triggerrate at Level 2 rises by 70% and we are studying howto improve the purity.6.4 Silicon tracking at Level 3While the core of the impact parameter resolutionof the SVT is similar to the oine resolution (both50m, including a contribution of 35m from thebeamspot), the SVT impact parameter measurementhas signicant non-Gaussian tails, which can be at-tributed to the handling of large clusters of overlap-ping hits in the silicon detector. The reconstructionsoftware at Level 3 can reduce the non-Gaussain reso-lution tails because more time and more informationare available for complex pattern recognition and thehandling of overlapping hits. In May 2003, silicontracking at Level 3 was implemented and resulted ina factor 2 reduction of the rate to tape of the two-track trigger with a small (5%) loss of heavy avorsignal.7 Some recent resultsThe low pT threshold of the dimuon trig-ger has made it possible to collect J= de-cays down to zero transverse momentum.We measured a cross section (J= )jyj0:6 B(J= ! ) = 240 1(stat)+35 28(syst) nb. Usingfully reconstructed B ! J= X decays we mea-sure: (B+) = 1:57 0:07(stat) 0:02(syst) ps,(B0) = 1:42 0:09(stat) 0:02(syst) ps, and(Bs) = 1:25 0:20(stat) 0:02(syst) ps.In the two-track trigger, we have reconstructed450k D0 ! K + decays. We looked for D0 !+  decays, found zero events on a backgroundof 1.7, and set a preliminary limit of 2:7 10 6at 90% CL. We also looked for direct CP violationin D-tagged D0 ! +  and D0 ! K+K  decaysand nd ACP;KK = 2:0 1:7(stat) 0:6(syst)% andACP; = 3:0 1:9(stat) 0:6(syst)%.References1. D. Acosta et al. [CDF Collaboration], Phys. Rev.D 65, 052005 (2002).2. R. Blair et al. [CDF-II Collaboration],FERMILAB-PUB-96-390-E.3. A. Sill [CDF Collaboration], Nucl. Instrum. Meth.A 447, 1 (2000).4. K. T. Pitts [CDF Collaboration], Nucl. Phys.Proc. Suppl. 61B, 230 (1998).5. E. J. Thomson et al., IEEE Trans. Nucl. Sci. 49,1063 (2002).6. W. Ashmanskas et al. [CDF Collaboration], Nucl.Instrum. Meth. A 447, 218 (2000).

Performance of CDF for B physics

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Performance of CDF for B physics

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