ALFA Results 2011 & Plans 2012 K.Hiller on behalf of the ALFA group

1. ALFA Results 2011 & Plans 2012 K.Hiller on behalf of the ALFA group Fiber detector seen from the beam side (illuminated for mapping checks) LHC Lumi Days All photos by courtesy of S.Jakobsen

2. Principle of ALFA Exploiting small angle elastic scattering for measurements of luminosity and total cross section. Data taking is performed in special runs with high beta* and low beam intensity. Program: 1) Get via optics angles at IP: θY = Y / Leff_Y Leff_Y = Leff_Y(β*,β, Ψ) 2) Momentum transfer t = - θ2 p2with θ2= θX2+ θY2 3) Fit t-spectrum for σtot, luminosity, B-slope … Coulomb scattering Nuclear scattering LHC Lumi Days

3. Installation & Commissioning 2011 • 2 x 2 stations ~ 240m from ATLAS IP • fiber detectors in Roman Pots • movable in the vertical coordinate • September 2011: integration in ATLAS LHC Lumi Days

4. Data taking at beta* 90m: overview scraping optics measurement 2 fills lost VdM failed data taking • 4 LHC Lumi Days Massimiliano Ferro-Luzzi

5. Data taking at beta* 90m: conditions & statistics • Bunch structure: • 1 colliding BC of 7E10 for physics , 1 non-colliding of 7E10 for background studies, • 13 colliding bunches of 1E10 • Detector positions: • mainly 6.5*) sigma (~ 5mm) after scraping at about 4 sigma • *) nominal sigma for 3.5 μm emmittance • Trigger: • standalone runs: minimum bias = any detector triggers • combined run PEB stream: minimum bias • combined run physics stream: trigger menu for elastics, diffractive, systematics • Data runs: • many standalone runs for • fast cross checks and • distance measurements • one run combined with • ATLAS, apart from μ-detector • for physics and calibration LHC Lumi Days

6. Ingredients towards physics • Before physics analysis an alignment of the detectors in respect to the beam positions is needed . • The link between track positions in the detectors and kinematics is given by the beam line optics. • Optics: • design optics based on MADX transport program • re-tuned by measured magnet currents see talks of H.Burkhardt, S.Cavalier • agreement with design optics in Leff looks very good • Alignment: • 3D measurements in lab and laser survey in tunnel give positions to nominal beam • beam-based alignment (scraping) : detectors centered better than 100μm to beam • distance measurements with overlap detectors as constraint for track-based alignment • track-based alignment assuming gives detector positions better then 10μm Example: Residuals of global track fits in station A7R1 All offests < 5 μm Sigma reflects detector resolution and angular divergence at IP LHC Lumi Days

7. Angular plots of elastic scattering • Physics stream in combined run: • total events: 6620953 • events BCID 1: 1898901 • elastics after cuts: 785091 • background contribution: ~ 0.1% • angular correlation indicates • elastic back-to-back topology • rotation-symmetric scattering • as expected for elastics • input for t-spectrum LHC Lumi Days

8. Luminosity: first glance *** all results and plots from the LMTF group, many thanks ***) • Remind: • at presently measured values of t > -0.01 GeV2 luminosity from ATLAS is needed as input to obtain • the total cross section. • Detectors for luminosity: • LUCID, BCM with standard algorithms of event counting using visible cross section plus corrections • not usable: • MBTS not calibrated • FCAL & TILE beam intensity too low • First outcome: • due to high beta* and low beam intensity μ = 0.03 - 0.04 • instantaneous luminosity is very small: ~ 6-7 10E27 cm-2sec-1 • various algorithms differ more than in standard physics runs, offset of ratios are: • BCMV_OR / BCMH_OR: -2% • LUCID_OR / BCMH_OR: -8% • LUCID_AND / BCMH_OR: -15% • in addition to the offsets in most ratios a variation over the μ-range is observed which exceeds the • typical 1% range visible at higher μ-values LHC Lumi Days

9. Luminosity: debugging • General: • All AND algorithms are very sensitive to the low statistics related to the low beam intensity •  OR algorithms seems to be more robust • LUCID: • to cope better with 2011 calibration LUCID was re-filled with gas • however due to higher beam intensities increasing PMT currents caused PMT aging and new • calibrations needed • this effect clearly visible in deviation of LUCID based-luminosity compared to values from other • detectors in period with gas 01/05/2011 • BCM: • BCM_AND impact of background subtraction has to be clarified • Observations in the μ-scan • at “standard” μ-values > 2 all algorithms agree • within +- 1% • at smallest μ large deviations up to 5% can be • observed, mostly for ANDs • Conclusion: • for the very small μ-values of ~ 0.035 in the • ALFA run the corrections are very sensitive for • details eventually hidden at higher μ +- 1 % LHC Lumi Days

10. Luminosity: outlook • the analysis of the ALFA luminosity using the traditional detectors indicates some inconsistencies • since the ATLAS inner detector was switched ON there one can use the vertex-based luminosity • as referee method, essential here is the high statistics via the luminosity calibration stream • on the last luminosity meeting there was a break trough concerning vertex-based luminosity • the vertex-based values are in rather good agreement with the presently most reliable BCM_OR • values • a few more iteration of this topic in the luminosity group are needed before the final can be given • an uncertainty ΔL / L in the range of 3 – 5% seems to be feasible combining BCM and vertex • methods LHC Lumi Days

11. Now about activities & plans for 2012 LHC Lumi Days

12. Detector heating and prospects • Observations & investigations: • 2011 typical temperature increase during • high intensity runs of 10 - 15° (max 18) • no danger for scintillating fibers – acceptable • light yield of ~ 5% expected, mostly reversible • more critical part due to different thermal • expansion is the fiber structure glued onTitanium • detector ALFA2 taken out indicates no changes • of structure from heating • results from cycling of test plates in an oven: •  45° are save •  60° have significant impact on the structure • cycling in steps of 5° continues to find the • critical temperature when detector are in danger • simulations going on about additional increase of • temperature by higher beam intensities • (~ 10° assuming heat proportional to RF losses) • simple measures to improve cooling on the way: •  removed back out isolation from bellows •  install an external fan (under test) • Combining all known facts, it is presently very likely • that detectors can be kept all 2012 in the tunnel • (All details ATL-COM-LUM-2012-001) 2 temperature sensor Pt100 on Titanium trigger plate 3 hours LHC Lumi Days

13. Improved temperature monitoring 30 sensors per station and beam pipe 4 add. Sensors in ALFA2 LHC Lumi Days

14. Measurements at higher β* (1) What are the conditions to measure elastics in the Coulomb-Nuclear interference region ? Smallest t defined by a mixture of 3 variables: beta* emmittance distance of detectors to the beam Plot for 3 scenarios gives the impression: Assuming we can run as 2011 with 2μm emmittance at 6.5 sigma than t_min is accessible at beta* 500m But this estimate ignores good acceptance range. A typical value which limits the useful t-range is 50% acceptance. 500m LHC Lumi Days

15. Measurements at higher β* (2) • For 50% acceptance cut an additional detector • offset of 2 sigma is assumed. • Observation: • For 2011 emmittanceand positions (2μm, 6.5 sigma) • beta* ~ 900m is needed to reach Coulomb-Nuclear • interference region. • If beta* of 900m is not achievable the interesting • t-range is accessible by: • 1) smaller emmittance and/or • 2) Detector positions closer to the beam • For beta* of 500m a useful combination of emmitance • and positions seems to in the range: • emmittance 1.0 – 2.0 μm • detectors at 4 – 5 sigma 900m LHC Lumi Days

16. Summary • 2011 was a busy and successful year for ALFA: • installation • commissioning • data taking at beta* 90m • physics results in preparation • with Δ L/L ~ 3% - 5% from ATLAS a total cross section error of 3% seems to be feasible • 2012 base line scenario: • follow temperature profile at higher intensities to prevent premature damage of the detectors • emergency case if detectors are dying due to heat: remove detectors from tunnel • (some degradation is acceptable, but not a total loss – we want to continue after LS1) • finish ALFA upgrades and re-commissioning of all systems. For this purpose some data runs • are needed to check triggers and readout similar as 2011: •  LEDs and shower particles in garage position •  trigger test and latency checks at 7mm in runs with low intensity and collision optics (20 sigma) •  for additional test data taking at 25mm at standard optics, preferably at lower beam intensity •  loss maps •  parasitic runs during fills for development & qualification of the next beta* optics •  data taking at highest beta* optics, including beam-based alignment (scraping) • in respect to the tests which has to be done before data taking tentatively in August / October • Physics goal 2012: • Total cross section and luminosity based on data in the Coulomb-Nuclear interference region LHC Lumi Days