ATLAS Plans for Elastic Cross-Section and Luminosity Measurement Ilias Efthymiopoulos – CERN ( for the ATLAS collaboration ) Many thanks to the colleagues who contributed for the material of this talk Some of them are present here – contact them directly for further information or questions XVIIth International Conference on Elastic and Diffractive Scattering Towards the High Energy Frontiers Blois – France May 15-20, 2005

Introduction (1/3) ATLAS submitted a Letter of Intent complement the experiment with a set of forward detectors for luminosity measurement and monitoring It can be considered as part of a two stage scenario: Short time scale Forward detectors in Roman Pots at 240 m from IP1 Probe the elastic scattering in the Coulomb interference region Dedicated detector for luminosity monitoring – LUCID Used also to transfer the calibration form 1027 1034 Gain experience in working close to the beam Longer time scale Study opportunities for diffractive physics with ATLAS “critical mass” within the collaboration is under formation Propose a diffractive physics program using additional detectors I. Efthymiopoulos

Introduction (2/3) Roman Pots @ 240 m from IP1 I. Efthymiopoulos

Diffraction/Proton Tagging Region Introduction (3/3) ATLAS Calorimetry: Tracking: R -chambers Barrel Diffraction/Proton Tagging Region EndCap RP Tracking ZDC/TAN FCAL LUCID TAS 1 2 3 4 5 6 7 8 9 10 y I. Efthymiopoulos

ATLAS Assembly Status - UX15 cavern Barrel em & hadronic calorimeters wheels 4th toroid magnet being installed http://atlaseye-webpub.web.cern.ch/atlaseye-webpub/web-sites/pages/UX15_webcams.htm I. Efthymiopoulos

Elastic scattering at the CNI region (1/3) Nuclear slope: ebt Coulomb region: 1/t2 C–N interference Sensitivity to r “structure” PQCD: 1/t8 BSW - 2003 Using the optical theorem, the measured elastic rate at small t values can be expressed as: which can be fitted to obtain: stot, r, b, and L I. Efthymiopoulos

Physics interest (1/3) Luminosity Measurement – Why? Important for (precision) comparison with theory: e.g. bb, tt, W/Z, n-jet, … cross-section deviations from SM could be a signal for new physics Goals for ATLAS: Measure luminosity with ~2% accuracy Ldt = 300fb–1 Systematic error dominated by the luminosity measurement (ATLAS-TDR-15, May 1999) I. Efthymiopoulos

Physics interest (2/3) Total cross-section The r parameter Understand the asymptotic behavior of stot new (precise) data to constraint the fit: stot vs (ln s)g 1% error  ~1mb The r parameter linked to stot via dispersion relations sensitive to stot beyond the energy at which is measured predictions of stot beyond LHC energies Or, are dispersion relations still valid at LHC energies? C.Augier et.al., 1993 LHC COMPETE coll. I. Efthymiopoulos

Physics interest (3/3) The nuclear slope parameter b t-region of 10-2  10-1 GeV2 The b parameter is sensitive to the exchange process Its measurement will allow to understand the QCD based models of hadronic interactions “Old” language : shrinkage of the forward peak b(s)  2 ’ log s ; where ’ is the slope of the Pomeron trajectory  0.25 GeV2 Not simple exponential - t-dependence of local slope Structure of small oscillations? LHC S.Bultmann et.al. - RIHIC I. Efthymiopoulos

Elastic scattering at the CNI region (2/3) Experimental conditions t-value reach for CNI @ LHC Beam optics requirements small intrinsic beam angular spread at IP insensitive to transverse vertex smearing large effective lever arm Leff detectors close to the beam, at large distance from IP Parallel-to-point focusing ydet independent of the vertex position y* y* parallel-to-point focusing ydet IP Leff I. Efthymiopoulos

Elastic scattering at the CNI region (3/3) How low in the t-value can we go? Thus, to reach the smallest possible t-value: Leff,y large  detectors must be far away form the IP  potential interference with machine hardware small tmin implies: * large  special optics small emittance small nσ  halo under control and the detector must be close to the beam Reaching the Coulomb Region is very challenging Good knowledge of LHC machine and its backgrounds is required, combined with edge-less detectors and precise mechanical construction Most likely not a first-day measurement when LHC turns ON I. Efthymiopoulos

Experimental setup (1/4) Roman Pot Locations One Roman Pot Station per side on left and right from IP1 Each RP station consists of two Roman Pot Units separated by 3.4 m, centered at 240.0 m from IP1 I. Efthymiopoulos

Experimental setup (2/4) High b Optics Solution At the IP  = 2625 m * = 610 m * = 0.23 rad At the detector y,d = 119 m, y,d = 126 m x,d = 88 m, x,d = 109 m (for N =1 m rad) Detector at 1.5 mm or 12 tmin = 0.0004 GeV2 Smooth path to injection optics exists All Quads are within limits Q4 is inverted w.r.t. standard optics! β [m] D [m] Endorsed by LHC Technical Committee Compatible with TOTEM optics (see LEMIC minutes 9/12/2003) I. Efthymiopoulos

Experimental setup (3/4) Boosting the LHC performance Emittance of ~1×10–6 mrad is needed to reach Coulomb region nominal LHC emittance: 3.75×10–6 mrad 1×10–6 mrad is the designed commissioning emittance for LHC !! Encouraging results from SPS MD’s: V: 1.1×10–6 mrad ; H: 0.9×10–6 mrad for 7x1010 ppb and also: 0.6-0.7×10–6 mrad obtained for 0.5×1010 ppb However Preserve the emittance into LHC requires that injection errors must be controlled Synchrotron radiation damping might help us at LHC energy Have to understand the instability limits at the collimators Resistive collimator wall instability criterion: thus: εN ≥ 1.5×10–6 m for Np = 1010, nσ,coll= 6 The best parameter space will be found during beam tuning sessions I. Efthymiopoulos

Experimental setup (4/4) LHC operation conditions Beam halo is a serious concern for the Roman Pot operation it determines the distance of closest approach dmin of the sensitive part of the detector: nσ = dmin/σbeam Working scenario: 43 bunches, 1010ppb, εN = 1.0 μm rad, at nσ=10 Expected halo rate of about 6 kHz (RA LHC MAC 13/3/03) Working point ?? I. Efthymiopoulos

Roman Pot Detector R&D (1/3) Scintillating fiber tracker Kuraray 0.5 mm × 0.5 mm fibers 10 layers per coordinate 50 μm offset between layers Detector simulations: Npe/hit ~ 34.9 20 mm resolution with 95% efficiency Large scintillator plane for trigger 2-3 mm thick double fiber readout from the edges 30mm I. Efthymiopoulos

Roman Pot Detector R&D (2/3) Pot assembly Detector plane and the overlap detectors Detector plane prototype assembly I. Efthymiopoulos

Roman Pot Detector R&D (3/3) Detector implementation in the Roman Pot Up detector in beam-in position Preliminary studies using the RP prototype developed by TOTEM (many thanks !) Collaboration continues to develop the final RP device that will serve both experiments with as much as possible of common parts UV detector planes Overlap detector Trigger scintillator Beam Down detector in the garage position I. Efthymiopoulos

Detector performance (1/1) Simulation results Reconstruct θ*: Full t range : 4  10-4 0.1 GeV2 Simulated dNel/dt distribution Event generation: 5 M events generated ~90 hr at L  1027 cm-2 s-1 NO systematics on beam optics! Only 1 Roman Put unit/arm ~4 M events “measured” for dN/dt Simple fit t-range: 5.6  10-4  0.030 GeV2 no systematic errors t = -0.0010 GeV2 ns = 15 t = -0.0007 GeV2 ns = 10 I. Efthymiopoulos

ATLAS plans (1/2) Other possibilities for absolute luminosity measurement Luminosity from LHC machine parameters Could reach ~5% accuracy, limited by: extrapolation of beam spot sizes from profile measurements beam-beam effects and x-sing angle precision at IP, beam current, etc. Use ZDC in heavy ion runs to calibrate and understand the machine optics proposal to instrument the TAN (@ 140m from IP1) Rates of well-calculable physics processes QED: pp  (p+*)+(p+*)p+()+p small rate ~1pb (~0.01 Hz at L=1034) ; clean signal QCD: W/Z  leptons high rate: Wℓν : ~60 Hz at L=1034 (ε = 20%) ; systematics ~4% from PDF and parton x-sections; detector systematics? Using the Roman Pot detectors + Optical Theorem Measure Nel + Ninel ; luminosity independent method requires complete |η| coverage - ATLAS coverage in the forward direction is limited Two alternatives (if CNI cannot be reached) use the measured tot by others (e.g. TOTEM), the forward elastic rate to a medium –t value and the optical theorem to determine the luminosity use the machine luminosity, the forward elastic rate to a medium –t value and the optical theorem to determine tot with half the error of the luminosity I. Efthymiopoulos

ATLAS plans (2/2) The measurement of the absolute luminosity at the Coulomb interference region remains the primary goal And future prospects? It is interesting to extend the measurement of the elastic rate to the maximum possible t-values Medium t-values: 0.1-1.0 GeV2 elastic scattering needs medium b* optics, low luminosity, short runs Large t-values: 1-10 GeV2 elastic scattering needs high luminosity, standard optics, and continuous runs. Proton tagging to identify a diffractive interaction must be possible at some level with the proposed RP detectors. t and  acceptance and resolution need to be understood Simulation and optics investigations required to understand the physics potential for single and central diffraction using proton tagging. Signal and background rates have to be studied, trigger set up? Many open questions; more studies are required to address these issues in detail but a very interesting program ahead !!! I. Efthymiopoulos

Summary ATLAS pursues a number of options for Absolute Luminosity Measurement at LHC, with the primary goal to reach the Coulomb interference region using Roman Pot detectors at 240m from IP1 Optimized optics is available; detector development has started The measurement is very challenging, seems within reach but “no guarantees can be given” Small angle elastic scattering will provide valuable input to the physics models for tot , ρ and b This experience of working close to the beam will open the door for a Forward Physics Program with ATLAS in a possible future upgrade I. Efthymiopoulos

Backup Slides I. Efthymiopoulos

ZDC instrumentation at the TAN IP1&IP5 absorbers TAN@140m I. Efthymiopoulos

Luminosity calibration transfer: 1027  1034 Bunch to bunch resolution  we can consider luminosity / bunch  ~ 2 x10-4 interactions per bunch to 20 interactions/bunch  Required dynamic range of the detector ~ 20 Required background  < 2 x10-4 interactions per bunch main background from beam-gas interactions Dynamic vacuum difficult to estimate but at low luminosity we will be close to the static vacuum. Assume static vacuum  beam gas ~ 10-7 interactions /bunch/m We are in the process to perform MC calculation to see how much of this will affect LUCID I. Efthymiopoulos

Absolute luminosity from machine parameters Luminosity depends exclusively on beam parameters: 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 the bunch current beam-beam effects at IP, effect of crossing angle at IP, … I. Efthymiopoulos

Luminosity Monitor Detector - LUCID Services In/Out To UXA PMTs Inner radius of LUCID ~8 cm, outer radius ~16cm ~17< |z| <~18.5 m 5.4< || <6.1 LUCID I. Efthymiopoulos

LUCID Detector Performance Simulation of a 20 GeV muon incident along the axis of a LUCID Cerenkov tube gives ~320 photons and ~230 photons are collected at the Winston cone exit. PYTHIA-6 events generated with increasing numbers of pileup Perfect linearity, with little sensitivity for secondaries I. Efthymiopoulos