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1 Luminosity measurement in the ATLAS experiment at the LHC 26 November 2008 Per Grafstrom CERN

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3 Electroweak symmetry breaking The Higgs Super symmetry CP-violation Extra dimensions, black holes ATLAS

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4 15-April-2008 ATLAS RRB 4 ATLAS Collaboration (Status April 2008) 37 Countries 167 Institutions 2200 Scientific Authors total (1750 with a PhD, for M&O share) Albany, Alberta, NIKHEF Amsterdam, Ankara, LAPP Annecy, Argonne NL, Arizona, UT Arlington, Athens, NTU Athens, Baku, IFAE Barcelona, Belgrade, Bergen, Berkeley LBL and UC, HU Berlin, Bern, Birmingham, UAN Bogota, Bologna, Bonn, Boston, Brandeis, Bratislava/SAS Kosice, Brookhaven NL, Buenos Aires, Bucharest, Cambridge, Carleton, Casablanca/Rabat, CERN, Chinese Cluster, Chicago, Chile, Clermont-Ferrand, Columbia, NBI Copenhagen, Cosenza, AGH UST Cracow, IFJ PAN Cracow, DESY, Dortmund, TU Dresden, JINR Dubna, Duke, Frascati, Freiburg, Geneva, Genoa, Giessen, Glasgow, Göttingen, LPSC Grenoble, Technion Haifa, Hampton, Harvard, Heidelberg, Hiroshima, Hiroshima IT, Indiana, Innsbruck, Iowa SU, Irvine UC, Istanbul Bogazici, KEK, Kobe, Kyoto, Kyoto UE, Lancaster, UN La Plata, Lecce, Lisbon LIP, Liverpool, Ljubljana, QMW London, RHBNC London, UC London, Lund, UA Madrid, Mainz, Manchester, Mannheim, CPPM Marseille, Massachusetts, MIT, Melbourne, Michigan, Michigan SU, Milano, Minsk NAS, Minsk NCPHEP, Montreal, McGill Montreal, FIAN Moscow, ITEP Moscow, MEPhI Moscow, MSU Moscow, Munich LMU, MPI Munich, Nagasaki IAS, Nagoya, Naples, New Mexico, New York, Nijmegen, BINP Novosibirsk, Ohio SU, Okayama, Oklahoma, Oklahoma SU, Oregon, LAL Orsay, Osaka, Oslo, Oxford, Paris VI and VII, Pavia, Pennsylvania, Pisa, Pittsburgh, CAS Prague, CU Prague, TU Prague, IHEP Protvino, Regina, Ritsumeikan, UFRJ Rio de Janeiro, Rome I, Rome II, Rome III, Rutherford Appleton Laboratory, DAPNIA Saclay, Santa Cruz UC, Sheffield, Shinshu, Siegen, Simon Fraser Burnaby, SLAC, Southern Methodist Dallas, NPI Petersburg, Stockholm, KTH Stockholm, Stony Brook, Sydney, AS Taipei, Tbilisi, Tel Aviv, Thessaloniki, Tokyo ICEPP, Tokyo MU, Toronto, TRIUMF, Tsukuba, Tufts, Udine/ICTP, Uppsala, Urbana UI, Valencia, UBC Vancouver, Victoria, Washington, Weizmann Rehovot, FH Wiener Neustadt, Wisconsin, Wuppertal, Yale, Yerevan

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5 The very first Event in ATLAS

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6 Motivation-why we need to measure the luminosity Measure the cross sections for Standard processes Top pair production Jet production …… New physics manifesting in deviationof x BR relative to the Standard Model predictions. Precision measurement becomes more important if new physics not directly seen. (characteristic scale too high!) Important precision measurements Higgs production x BR tan measurement for MSSM Higgs ……. Relative precision on the measurement of H BR for various channels, as function of m H, at L dt = 300 fb –1. The dominant uncertainty is from Luminosity: 10% (open symbols), 5% (solid symbols). (ATLAS Physics TDR, May 1999) Theoretically known to ~ 10 % Higgs coupling Instantaneous Luminosity or Ldt Integrated Luminosity

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7 Absolute versus Relative measurement Relative measurements or Luminosity Monitoring Using suitable observables in existing detectors Beam condition monitor Current in Tile calorimeter PMs Minimum bias scintillators Using dedicated luminosity monitor LUCID Absolute measurements Several different methods-next slide Strategy: 1. Measure the absolute luminosity with a precise method at optimal conditions 2. Calibrate luminosity monitor with this precise measurement and then use the calibrated monitor at all conditions

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8 Absolute Luminosity Measurements Goal: Measure L with 3% accuracy (long term goal) How? Three major approaches LHC Machine parameters Rates of well-calculable processes: e.g. QED (like LEP), EW and QCD Elastic scattering Optical theorem: forward elastic rate + total inelastic rate: Luminosity from Coulomb Scattering Hybrids Use tot measured by others Combine machine luminosity with optical theorem We better pursue all options

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9 Outline Methods for Absolute Measurement of Luminosity Processes with known cross sections Machine Parameters Elastic scattering Methods for Relative Measurement of Luminosity LUCID

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10 Two photon production of muon pairs-QED p p Pure QED Theoretically well understood No strong interaction involving the muons Proton-proton re-scattering can be controlled Cross section known to better than 1 % Muon pairs

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11 Two photon production of muon pairs + - P t 3 GeV to reach the muon chambers P t 6 GeV to maintain trigger efficiency and reasonable rates Centrally produced 2.5 P t ( ) MeV Close to back to back in (background suppression) Muon pairs

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12 Backgrounds Strong interaction of a single proton Strong interaction between colliding proton Di-muons from Drell-Yan production Muons from hadron decay Muon pairs

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13 Event selection-two kind of cuts Kinematic cuts P t of muons are equal within 2.5 σ of the measurement uncertainty Good Vertex fit and no other charged track Suppress Drell-Yan background and hadron decays Suppresses efficiently proton excitations and proton-proton re-scattering Muon pairs

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14 What are the difficulties ? The resolution The p t resolution has to be very good in order to use the P t ( ) MeV cut. The rate The kinematical constraints σ 1 pb A typical /cm 2 /sec year 6 fb -1 and 150 fills 40 events fill Luminosity MONITORING excluded What about LUMINOSITY calibration? 1 % statistical error more than a year of running Efficiencies Both trigger efficiency and detector efficiency must be known very precisely. Non trivial. Pile-up Running at /cm 2 /sec vertex cut and no other charged track cut will eliminate many good events CDF result First exclusive two-photon observed in e + e -. …. but…. 16 events for 530 pb -1 for a σ of 1.7 pb overall efficiency 1.6 % Summary – Muon Pairs Cross sections well known and thus a potentially precise method. However it seems that statistics will always be a problem. Muon pairs

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15 W and Z counting W and Z

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16 W and Z counting Constantly increasing precision of QCD calculations makes counting of leptonic decays of W and Z bosons a possible way of measuring luminosity. In addition there is a very clean experimental signature through the leptonic decay channel. The Basic formula Ldt is the integrated luminosity N is the number of W or Z candidates B is the number of back ground events is the efficiency for detecting W or Z decay products a is the acceptance th is the theoretical inclusive cross section W and Z

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17 Uncertainties on th th is the convolution of the Parton Distribution Functions (PDF) and of the partonic cross section The uncertainty of the partonic cross section is available to NNLO in differential form with estimated scale uncertainty below 1 % (Anastasiou et al PRD 69, ) PDFs more controversial and complex W and Z

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18 NNLO Calculations Bands indicate the uncertainty from varying the renormalization ( R ) and factorization ( F ) scales in the range: M Z /2 < ( R = F ) < 2M Z At LO: ~ % x-s error At NLO: ~ 6 % x-s error At NNLO: < 1 % x-s error Anastasiou et al., Phys.Rev. D69:094008, 2004 Perturbative expansion is stabilizing and renormalization and factorization scales reduces to level of 1 % W and Z

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19 Sensistive to x values > x > x10 -4 Sea quarks and antiquark dominates g qqbar Gluon distribution at low x HERA result important x and Q 2 range of PDFs at LHC W and Z

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20 Sea(xS) and gluon (xg) PDFs PDF uncertainties reduced enormously with HERA. Most PDF sets quote uncertainties implying error in the W/Z cross section 5 % However central values for different sets differs sometimes more ! W and Z

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21 Uncertainties in the acceptance a The acceptance uncertainty depends on QCD theoretical error. Generator needed to study the acceptance The acceptance uncertainty depends on PDF,s, Initial State Radiation, intrinsic k t ….. Uncertainty estimated to about 2 -3 % Uncertainties on Uncertainty on trigger efficiency for isolated leptons Uncertainty on offline reconstruction efficiencies Estimation using tag and probe method 50 pb -1 of data 3 % 1 fb -1 < 1 % W and Z

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22 Summary – W and Z W and Z production has a high cross section and clean experimental signature making it a good candidate for luminosity measurements. The biggest uncertainties in the W/Z cross section comes from the PDFs. This contribution is sometimes quoted as big as 8 % taking into account different PDFs sets. Adding the experimental uncertainties we end up in the 10 % range. The precision might improve considerably if the LHC data themselves can help the understanding of the differences between different parameterizations The PDFs will hopefully get more constrained from early LHC data. The experimental errors will also diminish with time Aiming at 3-5 % error in the error on the Luminosity from W/Z cross section after a couple of years of data taking. W and Z

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23 Luminosity from Machine parameters Luminosity depends exclusively on beam parameters: Depends on f rev revolution frequency n b number of bunches N number of particles/bunch * beam size or rather overlap integral at IP Machine parameters The luminosity is reduced if there is a crossing angle ( 300 µrad ) 1 % for * = 11 m and 20% for * = 0.5 m

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24 Luminosity accuracy limited by extrapolation of x, y (or, x *, y *) from measurements of beam profiles elsewhere to IP; knowledge of optics Precision in the measurement of the bunch current beam-beam effects at IP, effect of crossing angle at IP, … Machine parameters

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25 What means special effort? Calibration runs with simplified LHC conditions Reduced intensity Fewer bunches No crossing angle Larger beam size …. Simplified conditions that will optimize the condition for an accurate determination of both the beam sizes (overlap integral) and the bunch current. Calibrate the relative beam monitors of the experiments during those dedicated calibration runs. Machine parameters

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26 Determination of the overlap integral (pioneered by Van der Machine parameters

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27 Example LEP Machine parameters

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28 Summary – Machine parameters Special calibration runs will improve the precision in the determination of the overlap integral. In addition it is also possible to improve on the measurement of N (number of particles per bunch). Parasitic particles in between bunches complicate accurate measurements. Calibration runs with large gaps will allow to kick out parasitic particles. Calibration run with special care and controlled condition has a good potential for accurate luminosity determination. Less than ~5 % might be in reach at the LHC (will take some time !) Ph.D student in the machine department is working on this (supervisor Helmut Burkhardt) Machine parameters

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29 Elastic scattering and luminosity Elastic scattering has traditionally provided a handle on luminosity at colliders. Optical theorem σ tot = 4π Im f el (0) The basis for this is the Optical Theorem which relates the total cross-section to the forward elastic scattering amplitude The optical theorem is a general law of wave scattering theory derived from conservation of probability using quantum mechanics. It is a reflection of the fact that the very existence of scattering requires scattering in the forward direction in order to interfere with the incident wave, and thereby reduce the probability current in this direction.

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30 The Optical theorem can be used in several ways for luminosity determination. Method 1 Extrapolate elastic scattering to t=0 (the optical point) and in addition measure the total rate. Method 2 Extrapolate elastic scattering to t=0 and use a measurement of total from another experiment. Method 3 Measure elastic scattering as such small angles that the cross section is also sensitive to the Coulomb part. Optical theorem

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31 Optical theorem σ tot = 4π Im f el (0) N tot = tot · L where = Re f el (t=0)/Im f el (t=0) Thus we need to Extrapolate the elastic cross section to t=o Measure the total rate Use best estimate of ( ~ % in L/L ) Method 1: Extrapolate elastic scattering to t=0 (the optical point) and in addition measure the total rate.

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32 Extrapolate to t=0 Exponential region Optical theorem

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33 Measure the total rate N total =N el +N inel N inel = N nd +N sd +N dd Trigger efficiencies with MBTS ATLAS covers an range up to 5 Optical theorem ND 99 % DD 54% SD 45 %

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34 Measure the total rate (cont.) PHOJET 7 % of the total rate not triggered PHYTIA 12.5 % of the total rate not triggered Taking this as extremes we would have a 5 % error in estimates of the total rate and this dominates and gives us about 10 % error in Luminosity Optical theorem Tuning PHOJET or PYTHIA with real data (here ALFA will be important) will improve this considerably….however difficult to be quantitative without simulations. Theses studies have started. If the diffractive rate could be estimated at the 10% level the total rate estimate would be at the level of 2% and the luminosity number could be better than 5 %

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35 * uncertainty % in L *extrapolation to t =0 1 % in L * tot from Totem 1-2 % 2-4 % in L 5 % in luminosity possible Method 2: Extrapolate to t=0 and use the optical theorem in combination with and independent measurement of tot Optical theorem

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36 Method 3: Optical theorem and measuring of elastic scattering at very small angles Measure elastic scattering at such small t-values that the cross section becomes sensitive to the Coulomb amplitude Effectively a normalization of the luminosity to the exactly calculable Coulomb amplitude No total rate measurement needed UA4 used this method to determine the luminosity to 2-3 % Coulomb

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37 Elastic scattering at very small angles Coulomb

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38 ATLAS Roman Pots Absolute Luminosity For ATLAS Coulomb

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39 What is needed for the small angle elastic scattering measurements? Special beam conditions Edgeless Detector Compact electronics Precision Mechanics in the form of Roman Pots to approach the beam Coulomb

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40 Mechanics - the pot itself ATLAS specific Roman Pot concept Coulomb Beam

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41 Mechanics - the Roman Pot unit Profiting from AB collimation group Reproducible and precise movements required Coulomb

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42 The detectors-fiber tracker Choice of technology: minimum dead space no sensitivity to EM induction from beam resolution ~ 30 m Concept 2x10 U planes 2x10 V planes Scintillating fibers 0.5 mm 2 squared 64 fibers per plane Staggered planes MAPMT readout Coulomb

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43 Test beam campaigns-DESY and CERN ALFA A number of prototypes with limited amount of fibers were tested Main results: Light yield ~ 4 p.e. resolution ~ 25 m non-active edge 100 m Coulomb

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44 The ALFA electronics ATLAS TDAQ Coulomb

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45 The ALFA electronics – front end - PMF Adjustable amplifier-MAPMT Adjustable threshold High speed Small cross talk Compact Requirements Coulomb

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46 The beam conditions Nominal divergence of LHC is 32 rad We are interested in angles ~ x 10 smaller high beta optics and small emittance (divergence / β* ) y*y* y * parallel-to-point focusing y det IP L eff Zero crossing angle fewer bunches Insensitive to vertex smearing large effective lever arm L eff To reach the Coulomb interference region we will use an optics with β* ~ 2.6 km and N ~ 1 m rad β [m] D [m] Compatible with TOTEM High β* and few bunches low luminosity Coulomb Amount of beam halo very important

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47 Summary - Coulomb Getting the Luminosity through Coulomb normalization will be extremely challenging due to the small angles and the required closeness to the beam. Main challenge is not in the detectors but rather in the required beam properties Will the optics properties of the beam be know to the required precision? Will it be possible to decrease the emittance as much as we need? Will the beam halo allow approaches in the mm range? No definite answers before LHC start up UA4 achieved a precision using this method at the level of 2-3 % but at the LHC it will be harder..... Coulomb

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48 Measure the LHC luminosity. Count the number of charged particles per BX, pointing to the primary pp interactions. LUCID in ATLAS Monte Carlo simulation Two symmetrical arms at 17 m from the pp interaction region. Relative measurement

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49 Relative measurement

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50 LUCID location Expected dose: 7 higest luminosity (10 34 cm -2 s -1 ) Relative measurement

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51 LUCID detector Array of mechanically polished Aluminum tubes in a Cherenkov gas (C 4 F 10 ). C 4 F 10 pressure mantained at 1.25/1.5 bar (Leak <10 mbar/day). coverage: 5.6, 6.0 Relative measurement

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52 Optical fibers (PUV700) via Winston Cone to multi-anode PMT (Hamamatsu H7546B). Better for high luminosity runs (MAPMT not exposed to high radiation doses). Direct coupling to Photo-Multiplier Tubes (PMT, Hamamatsu R762). PMT must be radiation hard. Cherenkov Tube PMT PMT quartz window (1 mm) Cherenkov Tube MAPMT Winston cone Fiber bundle 15 mm Relative measurement Read-Out scheme

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53 Overall conclusions To start with we will work with LHC Machine parameters In the beginning % precision. Special calibration runs with simplified conditions will improve : maybe 10 % after some time and then gradually towards 5 % Rates of well-calculable processes will be next step: Two photon production of muon pairs have a low cross section EW processes like W/Z production are good candidates: high cross section and clean signature QCD NNLO corrections to the partonic cross section have been calculated and the scale error is less than 1 % PDFs more problematic 5-8 % Taking into account experimental error a precision of 10 % in L might be reached quite early Aiming at 3- 5 % after some time - LHC data itself might constrain the PDF Ultimate goal in ATLAS: Measure L with ~ 3 % accuracy How do we get there?

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54 Overall conclusion (cont.) Third step : Elastic scattering Elastic scattering without reaching Coulomb region Measurements of the total rate in combination with the t-dependence of the elastic cross section is a well established and potentially powerful method for luminosity calibration and measurement of tot 10 % rather straight forward – 5% after some time Optical theorem in combination with independent measurement of tot Depends on the error on tot that will be provided but total error on Luminosity of 5 % seems reasonable Coulomb Interference measurement Getting the Luminosity through Coulomb normalization will be extremely challenging due to the small angles and the required closeness to the beam.However there is a potential for 3 % precision in luminosity

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55 Back up

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56 Overall conclusion (cont.) ATLAS proposes to use scintillating fibers in Roman Pots to measure Elastic Scattering at small angles. The ultimate goal is to reach the Coulomb interference region. This will be extremely challenging due to the small angles and the required closeness to the beam. Main challenge is not so much in the detectors but rather in the required beam properties. Especially the level of beam halo at small distances will be important. The ALFA project will be an important part of the luminosity determination in ATLAS even if the Coulomb Interference region is not completely reached. In addition, ALFA opens up the door for a Forward Physics program in ATLAS. Experience with working close to the beam will prepare us for expanding towards a Forward Physics program as a future upgrade.

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57 Hit pattern and acceptance Coulomb

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58 Error evaluation as in csc notes and gueillemin article Error evaluation for pdfs Tords comment

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59 Performance simulation Coulomb

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60 Systematic errors Coulomb

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61 Statistical results from fit 10 M events corresponding to 100 hours at L= Edge 1.5 mm from beam B (Gev -2 ) tot ( mb ) L (10 26 cm -2 s -1 ) Error (%)Lin.fitInput Coulomb

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62 Machine induced background Multiple Sources Beam gas scattering in the arcs Local beam gas Inefficiencies of collimation system Summary: 2kHz integrated above 10 Important and difficult determines how close we can come backgrounds estimate often wrong Coulomb

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63 Machine induced background (cont.) Main handles: Elastic signature: left –right coincidence acollinearity cut Vertex cut Background reduced to 2 % of the elastic signal Single diffractive background (generated with PYTHIA ) negligible : 1 permille Coulomb

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64 Vertex reconstruction for background rejection Coulomb

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65 The overlap detector concept Coulomb

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66 MAROC2 chip bonded at CERN

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67 Test beam-this summer Complete detector for one Roman Pot i.e channels Coulomb

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68 The ATLAS Detector Calorimetry: TAS R Barrel FCAL LUCID Tracking EndCap RP ZDC/TAN Diffraction/Proton Tagging Region y 109 -chambers Tracking:

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69 Luminosity Measurement (cont.) Relative precision on the measurement of H BR for various channels, as function of m H, at L dt = 300 fb –1. The dominant uncertainty is from Luminosity: 10% (open symbols), 5% (solid symbols). (ATLAS-TDR-15, May 1999) Higgs coupling tan measurement Examples Systematic error dominated by luminosity (ATLAS Physics TDR )

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