The Forward Detectors of CDF and DØ

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1 The Forward Detectors of CDF and DØ Konstantin Goulianos The Rockefeller University, 123 York Avenue, New York, NY , USA DOI: will be assigned The forward detectors of CDF II are resented with emhasis on design asects that roved crucial for carrying out a successful rogram on diffraction at the Tevatron. Alignment, calibrations and backgrounds are discussed, ointing out their relevance to the diffractive and central exclusive roduction hysics rograms lanned at the LHC. The DØ forward detectors, which with forward sectrometer on both the and sides offer the oortunity for a rogram comlementarity to that of CDF are briefly resented for comleteness. 1 Introduction Figure 1: The CDF and D detectors in Run II The Collider Detector at Fermilab (CDF) and the DØ collaborations have been conducting studies in diffraction since the start of Tevatron oerations in A lethora of results have been obtained on forward, central, and multi-ga diffraction rocesses, as well as on central exclusive roduction which is of secial interest as it serves to calibrate theoretical models for exclusive Higgs boson roduction st the Large Hadron Collider (LHC). In this aer, we resent the CDF II (CDF in Run II) and DØ forward detector configurations in Run II of the Tevetron collider and discuss issues of alignment, calibrations, backgrounds and hysics reach. EDS 9 1

2 The CDF II diffractive hysics data were collected with an ugraded CDF detector, which included the following forward comonents [1] (see Figs. 1, 2): Roman Pot Sectrometer (RPS) to detect leading antirotons, MiniPlug (MP) forward calorimeters covering the region 3.5 < η < 5.5, Beam Shower Counters (BSC) around the beam ie at 5.5 < η < 7.5, Cerenkov Luminosity Counters (CLC) covering the range 3.7 < η < 4.7. The Roman Pot Sectrometer was the same one that was used in Run Ic [2]. It consists of X-Y scintillation fiber detectors 1 laced in three Roman Pot (RP) stations located at an average distance of 57 m downstream in the direction. The detectors have a osition resolution of ±1 µm, which makes ossible a.1% measurement of the momentum. In Run Ic, the -beam was behind the roton beam as viewed from the RPS side. An inverted olarity (with resect to Run I) of the electrostatic beam searators enabled moving the RPS detectors closer to the -beam and thereby gain accetance for small t down to 1 x F ( ) =.3. For larger t, the values of that can be reached are lower. The MiniPlug calorimeters were installed within the inner holes of the muon toroids. They consist of layers of lead lates immersed in liquid scintillator. The scintillation light is collected by wavelength shifting fibers strung through holes in the lead lates and read out by multi-channel hotomultilier tubes (MCPMT s). The calorimeter tower structure is defined by arranging fibers in grous to be read out by individual MCPMT ixels. There are 84 towers in each MiniPlug measuring energy and osition for both electromagnetic (EM) and hadron showers [3]. The Beam Shower counters are rings of scintillation counters hugging the beam ie. The BSC-1 rings are segmented into four quadrants, while the other BSC s are segmented into two halves. These counters are used to rovide: (a) raidity gas triggers, (b) exclusivity constraints in studies of exclusive roduction, and (c) beam losses, by gating the BSC2 (BSC2 ) signal to the assage of the ( ) beam. The Cerenkov Luminosity Counters comrise a finely segmented system of gas Cerenkov counters ointing to the interaction oint (I) and are normally used by CDF to measure the number of inelastic collisions er beam-beam bunch crossing and thereby the luminosity. In the diffractive rogram, they are used in the raidity ga definition by detecting charged articles that might enetrate the MP calorimeters roducing a signal too small to be detected above the MP tower thresholds used. Their high efficiency for detecting charged articles rovides a valuable comlementary inut to that of the MP calorimeters. The DØ Run II forward detectors include two state of the art Forward Proton Detector (FPD) sectrometers on the outgoing and directions equied with X-Y -V silicon based trackers caable of resolving ambiguities in events with multile tracks at high luminosities. The beam-line configuration on the -side is similar to that of CDF; however, the -side sectrometer has no diole magnets between the IP and the sectrometer, which restricts access to low-t recoil rotons. In contrast, the MiniPlug based system of CDF allows access to lowt events but is not caable of measuring angular φ-correlations between the outgoing and, which are imortant for certain hysics studies. The CDF II [4] and DØ [5] detector layouts are schematically shown and comared in Fig We use a coordinate system with origin at the center of the CDF detector, Z along the roton beam direction and Y ointing u; the X coordinate oints away from the center of the accelerator ring. 2 EDS 9

3 2 Alignment A recise alignment of the RPS detectors is crucial for an accurate determination of the and t values of the recoil antiroton. The standard alignment method based on surveying the detectors relative to the centerline of the beam ie and using beam osition monitors (BPM) to determine the beam osition relative to the center of the ie was hamered in Run II by several difficulties, including: there was no BPM in the vicinity of the RPS detectors; the beam osition and angle could change with beam store as a result of tuning to increase the luminosity, and also during the course of a store; the osition of the RP relative to the beam ie could also change relative to the surveyed osition due to sliage of the ositioning mechanism. The RPS alignment was addressed in CDF by develoing a dynamic alignment method in which the actual detector osition during data taking is determined from the recorded data. The method is based on the exectation that the t-distribution of the recoil antiroton be maximal at t =. However, the measured distribution will dislay such a maximum only if the detectors are correctly aligned. This is illustrated in Figs. 2, 3, 4. In ractice, offsets in the X and Y coordinates of the RPS detectors with resect to the beam line are introduced in the off-line analysis and iteratively adjusted until a maximum for dσ/dt is obtained at t =. Using the resulting values for X offset and Y offset corresonds to having aligned detectors. This method is very recise and only limited by the statistics of the event samle, the size of the beam, and the jitter in the beam osition during data taking of the articular data samle used. Using a secial data samle collected during a relatively short dedicated run an accuracy of ±3 µm in beam osition was obtained [6]. The dynamic alignment method is quite general and can equally well be used to accurately calibrate the osition of RPS detectors lanned for the LHC. In rincile, the offsets could be determined from small data samles and alied on-line during data taking to enrich the recorded data samle with useful events. 3 Calibrations and backgrounds 3.1 Missing forward momentum and W mass from diffractive events A well known and frequently used data analysis tool is missing transverse energy. This tool is articularly useful in cases involving neutrinos in the final state. A good examle is the determination of the mass of the W -boson through the W eν / µν decay modes. The usual technique is to use the transverse W mass, which results is a skewed distribution that requires Monte Carlo and detector simulations that affect the accuracy of the measurement. The detection in the RPS of the forward in diffractive W roduction makes ossible the determination of the full kinematics of the W eν/µν decay. Figure 5: Diffracive W mass and Gaussian fit. EDS 9 3

4 DIPOLEs QUADs CDF II QUADs DIPOLEs 57m to CDF 2m TRACKING SYSTEM CCAL PCAL MPCAL CLC BSC RPS Figure 2: Schematic drawing of the CDF II detector (not to scale) Figure 3: Schematic reresentation of a track detected by the RPS. dσ/dt [arbitrary units] CDF Run II Preliminary 1 4 X offset at nominal X offset +.2 cm X offset +.4 cm t [GeV ] sloe at t = [arbitrary units] sloe at t = [arbitrary units] CDF Run II Preliminary Y offset [cm] CDF Run II Preliminary X offset [cm] Figure 4: Left: t-distribution of reconstructed RPS tracks for ositive X offset shifts; Right: b sloe versus Y (to) and X (bottom) offsets. 4 EDS 9

5 Events RPS + Jet5 RPS (w/track)+jet5 - rescaled seed Jet5 (E >5 GeV) - rescaled T CAL SD BG CAL RPS Events χ / ndf 17.4 / 13 Constant 25.6 ± 2.45 Mean.586 ±.114 Sigma.151 ±.12 CAL χ / ndf 8.59 / ± ± CAL RPS Figure 6: CAL RP S vs. RP S (see text): (t-l) CAL ; (b-l) CAL for a slice of the 2D-dist. of.55 < RP S distributions; (t-r) 2D-distribution of CAL <.6; (b-r) CAL vs. RP S. vs. The neutrino transverse energy ET ν is obtained from the missing E T, as usual, and the seudoraidity η ν from the formula RPS - cal = (E T / s) ex[ η ν ], where cal is calculated from the calorimeter towers: cal = Σ i towers (ET i / s) ex[ η i ]. This method has been alied to CDF data yielding the reliminary result shown in Fig. 5. Using a data samle of 3 diffractive W events, CDF obtained M ex W = 8.9 ±.7 GeV [7] in good agreement with the world average W -mass of MW PDG = 8.43 ±.29 GeV [8]. 3.2 Overla background and CAL calibration The main event samle of diffractive events in Run II was collected without RPS tracking information. For these events, was evaluated from calorimeter information using the formula: CAL = Σ i towers (E i T / s) ex[ η i ]. The calorimeter based CAL was calibrated against RP S using a data sub-samle for which tracking was available. In imlementing the calibration rocedure, secial care had to be taken to handle the background from overla events at the high instantaneus luminosity (L) of the run. Figure 6 illustrates the overla background handling and the CAL vs. RPS calibration. EDS 9 5

6 3.2.1 Overla background The CAL distribution is shown in Fig. 6 for three diffractive dijet data samles defined in the insert. A single-vertex requirement was alied to all samles. The region indicated as SD contains events which are mostly due to single-diffractive (SD) dijet roduction, while the events in the ND region are mainly overlas of a non-diffractive (ND) dijet and a soft diffractive interaction that triggered the RPS but yielded no reconstructed vertex. The majority of the diffractive events are reresented by the excess of events of the RPS Jet5 over the rescaled Jet5 distribution in the SD region. The overla background in the region.3 < CAL <.9 is of O(1%). Figure 6 (t-r) shows a two-dimensional scatter lot of CAL vs. RPS for the events with a reconstructed RPS track. The mountain-like eak in the region of CAL.1 is attributed to diffractive events. The events with CAL >.1 are mostly due to ND dijets with a suerimosed soft SD overla event CAL calibration The calibration of of CAL and fit the CAL is erformed by dividing the data into bins of width RPS =.5 values in each bin with a Gaussian distribution excluding the now well searated <.6. The ratio of the half-width / CAL.3. Figure 6 (b-r) shows the results for the region <.9. A linear relationshi is observed between CAL and RPS in this region. background, as shown in Fig. 6 (b-l) for.55 < CAL average of the fitted curve is δ CAL.4 < RPS 4 Summary The CDF and DØ forward detectors designed and used for studies of diffraction in Run II at the Ferrmilab Tevatron collider were resented with a focus on issues of alignment, calibrations, backgrounds and hysics reach. The rocedures develoed are quite general can be directly adated to the exeriments lanning diffractive hysics studies at the LHC. References [1] A. Aaltonen et al. (CDF Collaboration), Observation of Exclusive Dijet Production at the Fermilab Tevatron Collider, Phys. Rev. D77:524, 28. DOI : 1.113/PhysRevD Reort number: FERMILAB-PUB E. [2] H. Nakada, Ph.D. thesis, University of Tsukuba, January 21. [3] K. Goulianos, M. Gallinaro, K. Hatakeyama, S. Lami, C. Mesroian, A. Solodsky, The CDF Minilug Calorimeters (and references therein), Nucl. Instrum. Methods A 496, 333 (23 ). [4] D. Acosta et al. (CDF Collaboration, Phys. Rev. D 71, 2321 (25). [5] DØ Collacoration, The Ugraded D Detector, arxiv:hysics/57191v2. [6] Michele Gallinaro, DIS-26, arxiv:he-ex/6624. [7] K. Goulianos, Diffractive W and Z roduction at Tevatron arxiv: v2 [he-ex]; aer resented for the CDF collaboration at XLIV th Rencontrès de Moriond, QCD and High Energy Interactions, La Thuile, Aosta Valley, Italy, March 14-21, 29. [8] W.-M. Yao et al., Journ. Phys. G 33, 1 (26) and 27 artial udate for 28, htt://dg.lbl.gov. 6 EDS 9