WDS'12 Proceedings of Contributed Papers, Part III, 142 146, 212. ISBN 978-8-7378-226-9 MATFYZPRESS Non-collision Background Monitoring Using the Semi-Conductor Tracker of ATLAS at LHC I. Chalupková, Z. Doležal Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. The SemiConductor Tracker (SCT) is a silicon strip detector and one of the key precision tracking devices in the Inner Detector of the ATLAS experiment at Large Hadron Collider at CERN. It was installed inside the ATLAS experimental cavern in 27 and has been operational since then. We present the current status of the detector and its outstanding performance. We focus our study on the non-collision beam background in the SCT. We use trigger selection of non-colliding isolated bunches, analyse these events and correlate their rates and asymmetries with high flux of beam halo or worsening vacuum conditions in the beam-pipe around the interaction point. We propose algorithms and variables which will be used for on-line monitoring of background during data taking. The same algorithms can be after small modifications used as a part of the off-line data quality monitoring to estimate the rate and type of background and suppress it in physics analyses. Introduction Non-collision backgrounds in the ATLAS detector can be beam-induced or caused by cosmic particles. The beam-induced background is able to deposit significant energy especially in calorimeters and muon detectors resulting in production of fake jets and missing transverse momentum which can have similar signature to new-physics signals. Therefore it is important to study properties of these events, monitor their evolution in time and implement algorithms to suppress their influence on physics data. One of techniques to study the beam background is to analyse specific events when bunch from only one beam passes through the detector. ATLAS detector and SCT The ATLAS detector [ATLAS Coll., 28] at the LHC [L. Evans, P. Bryant (eds.), 1988] covers nearly the entire solid angle around the collision point. The coordinate system is righthanded, the azimuthal angle φ is measured with respect to the x-axis pointing towards the centre of the LHC ring. Side A of ATLAS is defined as the side of the incoming clockwise LHC beam 1, while the opposite side is labelled C. The z-axis points from C to A, i.e. along LHC beam 2 direction. The detector consists of an inner tracker detector embedded in a 2 T super-conducting solenoid which is surrounded by electromagnetic and hadronic calorimetry, and an external muon spectrometer built around three large superconducting toroid magnets. Each magnet consists of eight coils assembled radially and symmetrically around the beam axis. The Inner detector [ATLAS Coll., 1997] is responsible for an accurate measurement of charged particles momentum and vertex positions as well as particle identification. It consists of Pixel detector closest to the interaction point, Semi-Conductor Tracker (SCT) and a Transition Radiation Tracker (TRT). The overall inner detector is 5.6 m long and 2.1 m in diameter. The SemiConductor Tracker (SCT) is a silicon strip detector built of 488 modules arranged in barrel with 4 concentric layers and 9 end-cap discs on both sides. The layout ensures that particles pass through at least 4 silicon layers and it is illustrated in Figure 1. Each module consists of two pairs of back-to-back sensors rotated by 4 mrad to form a stereo angle in order to enable a resolution measurement in the direction parallel to the strips. In total, SCT integrates area of 61 m 2 of silicon micro-strip sensors with 6.3 million readout channels. 142
Figure 1. Geometrical layout of ATLAS Inner Detector [ATLAS Coll., 1997]. The readout of the SCT modules is performed by six 128-channel chips on each side of the module. The data signals processed by the chips are pre-amplified, shaped, discriminated (compared to a nominal threshold of 1 fc) and finally digitized. The binary output is produced in three bins, each with 25 ns duration (using 4 MHz LHC clock). The signals from particles originating from collisions should be in an ideal case in the middle bin, i.e. producing 1X occupancy pattern. The readout mode is usually set to any-hit mode (XXX) for low-luminosity and cosmic data-taking, while for high luminosity and high occupancy periods it is in compressed mode (X1X or 1X). While recording data, part of Data Acquisition system is reconstructing small fraction of data and results are used for monitoring of actual detector conditions (so called on-line monitoring) but also as an input for data preparation to flag possible problems which could have impact on data quality, e.g. various detector-related and infrastructure problems but also high background rates or unusual beam conditions. SCT performance LHC delivered 5.6 fb 1 of proton-proton collisions in 21 and 211, as well as two periods with heavy ion collisions, approximately 5 weeks each. The SCT has been fully operational during these periods, recording high quality data for 99.9% and 99.6% of delivered luminosity in 21 and 211 respectively [ATLAS Coll., 212]. More than 99% of the SCT strips have been fully functional and available for tracking, the missing part is mainly due to typically 3 modules (.7% out of 488) disabled due to various readout, connection or cooling problems. High tracking performance of SCT can be measured by parameters such as intrinsic hit efficiency. For data in 211 was the efficiency 99.8%, well above design requirement of 99%. The alignment of Inner Detector is performed usually once or twice a year using the collision data and it is already close to the ideal one as demonstrated for SCT barrel in Figure 2 left, where the distributions of track residual (distance of the measured hit position and space point on extrapolated track) are shown for 7-TeV collision data compared to Monte-Carlo simulation. High tracking precision is important for invariant mass reconstruction, the distribution for J/ψ µµ candidates is shown in Figure 2 right as an example. Non-collision background In this study, the term non-collision backgrounds refers to signals seen in the ATLAS detector which are not produced in collisions of the LHC beams. The main components are 143
Hits on tracks / 4 µm 1 1 Autumn 21 Alignment FWHM/2.35=25 µm Pythia Dijet Monte Carlo 8 FWHM/2.35=24 µm 6 4 2 3 ATLAS Preliminary SCT barrel s = 7 TeV Track p > 15 GeV T -.2 -.1.1.2 Local x residual [mm] J/ψ Candidates / (.4 GeV) 24 22 ATLAS Preliminary Data: 21 MC: Prompt J/ψ 2 s = 7 TeV Fit projection -1 Fit projection of bkg. 18 L dt = 78 nb 16 14 N J/ψ = 535 ± 9 12 m J/ψ = 3.95 ±.1 GeV 1 σ m = 71 ± 1 MeV 8 6 4 2 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 m µµ [GeV] Figure 2. Performance of ATLAS Inner Detector: SCT barrel alignment in autumn 21 local track residuals compared with simulations (left) [ATLAS Coll., 211] and the invariant mass distribution of reconstructed J/ψ µµ candidates from data using tracks in Inner Detector and muon spectrometer (right) [ATLAS Collaboration, 21]. cosmic-ray showers and beam-induced background. We will focus on the beam-induced background, which is caused by proton energy losses before interaction point. High-energy protons produce in interactions with limiting apertures(typically collimators) secondary particles which can reach the ATLAS detector. Another sources of background are inelastic collisions of protons with residual gas inside beam-pipe and elastic beam-gas scattering (the later can result in small angular deflections of protons which after some time hit collimators and produce particles). The rate of beam-background is proportional to beam intensity and depends on operational parameters of LHC such as collimator settings, quality of vacuum in beam-pipes and filling scheme. The composition of background halo is also varying along the accelerator depending on position and actual conditions. Generally we can say that the shielding, which is hermetically closing tunnel around ATLAS detector, is absorbing high fraction of particles and most of beam halo is coming through the beam-pipe. However, high-energy muons are rather insensitive to this material and as their typical energy is of the order of 1 GeV they may cross the calorimeters few meters from the beam pipe and cause fake jet signals. LHC beam structure and ATLAS trigger The LHC beam structure is given by the frequency of accelerating electromagnetic field and the machine parameters. Radio-frequency operates at 4 MHz and therefore provides buckets every 2.5 ns. Nominally every tenth bucket can be filled with particles, these allowed positions we refer to as Bunch Crossing Identifiers (BCID), of which there are 3564 in total. In 211 every second BCID was filled resulting in bunch spacing of 5 ns. For this analysis we use unpaired isolated bunches of LHC. They are defined as bunches in only one beam with at least 3 empty BCIDs in the other beam, for these events has ATLAS implemented a special random trigger. Background analysis with SCT Data selection We use ATLAS datasets from 211 which contain mainly events selected by background triggers (stream physics Background). Some of the runs were reported by other ATLAS subdetectors (mostly by muon spectrometer) to have higher rate of beam background and were correlated with known operational issues such as high pressure of residual gas or filling scheme with increased number of muons in beam halo. 144
entries/bin 18 16 14 ATLAS work in progress h_z_asymmetry Entries 414 Mean -.3238 RMS.8563 entries/lb 5 4 ATLAS work in progress h_nzasym_lb Entries 438 Mean 424.6 RMS 3.21 12 1 3 8 2 6 4 1 2-1 -.8 -.6 -.4 -.2.2.4.6.8 1 A_z 42 422 424 426 428 43 LB Figure 3. Asymmetry A z distribution for 11 lumi-blocks of run 187219 (left) and number of flagged events with A z >.9 per LB (right). Events were saved in so called lumi-blocks (LB), an average duration of 1 LB in 211 was 58 s. Due to high computational complexity we use raw data just from few LBs, typically 1 per run. For analysis of full run we use centrally reconstructed data sets containing only basic variables (particles, tracks, hits etc.). Analysis The strategy is to count early hits (i.e. 11 and 111 hits in X1X read-out mode, in XXX including also 1 and 11) in the SCT outermost regions. We can identify from which beam the background hits are based on the analysis of the early signals higher number of early hits on A side implies that they are from particles in Beam 1 halo and the same for side C and Beam 2. It is than possible to cross-check this identification with list of unpaired bunches for given filling scheme. Because one quarter of the last wheel of end-cap C is not operational due to cooling failure, we take hits in next two rings. We can compute z-asymmetry of each event defined as A z = (z + z )/(z + +z ), where z + and z are the numbers of hits with positive and negative z respectively. We also require at least 45 hits in these regions to reject events with low number of hits and high uncertainty of computed asymmetry. An example of A z distribution is shown in Figure 3 left where we plot a histogram of asymmetries for events selected with above criteria for run known to have high background rate (run number 187219 from August 211). Clearly this data sample contains many events with high asymmetry. If we flag events with A z >.9 we can use their multiplicity per LB as one of possible variables for monitoring the time evolution of background rates as shown in Figure 3 right. Results Results of analysis of selected runs are in Table 1. For each run we compute the fraction of events flagged as asymmetric and ratio of events with positive and negative asymmetry (proportional to ratio of background from Beam 1 and Beam 2). We list both on-line (from analysis of few LBs of raw data) and off-line values (whole run) and try to correlate these values with possible sources of high background. The table shows an agreement between on-line and off-line values, small differences are caused by used tracking algorithm in the second case. B1/B2 ratio shows that, as expected from pressure profile and LHC design, beam background is asymmetric and more particles originate from halo of Beam 2 (C side). 145
Run number Flagged events B1/B2 ratio Correlation online offline online offline 185649.27.36.81.74 pressure 18649.12.15.64.77 pressure 187219.21.22.37.48 large halo 191676.2.4 1.7.67 small halo Table 1. Results of selected runs: fraction of flagged events with A z >.9 and ratio of events with high background from Beam 1 and Beam 2, which can indicate a possible sources of background (pressure denotes high pressure difference between measurements at 22 and 58 m from interaction point). Conclusion We presented a preliminary study of possible beam-background monitoring using ATLAS SCT based on asymmetry of events from unpaired isolated bunches of LHC. The algorithms were implemented both for on-line monitoring of conditions and off-line analysis. The next step is to include into this analysis other sub-detectors (muon chambers, Pixel) and design a background-suppression tool to be used in future. Acknowledgments. This work was supported in part by grants MSM2162859 and LA832 of Ministry of Education, Youth and Sports of the Czech Republic. References ATLAS Collaboration, ATLAS Inner Detector Technical Design Report, Volume 2, CERN/LHCC/97 17, ISBN. 92-983-13-, 1997. ATLAS Collaboration, The ATLAS Experiment at the CERN Large Hadron Collider, JINST 3 S83, 28. ATLAS Collaboration, J/ψ Performance of the ATLAS Inner Detector, ATLAS-CONF-21-78, CERN, Geneve, 21. ATLAS Collaboration, Alignment of the ATLAS Inner Detector Tracking System with 21 LHC protonproton collisions at s = 7 TeV, ATLAS-CONF-211-12, CERN, Geneve, 211. ATLAS Collaboration, ATLAS EXPERIMENT Public Results, http://twiki.cern.ch/twiki/bin/view/atlaspublic/runstatspublicresults21, 212. L. Evans and P. Bryant (eds.), LHC machine, JINST 3 S81, 28. 146