LHC Lumi Days

ALFAResults 2011 & Plans 2012

K.Hiller on behalf of the ALFA group

Fiber detector seen from the beam side(illuminated for mapping checks)29/02/2012 1All photos by courtesy of S.Jakobsen

LHC Lumi Days 2

Principle of ALFAExploiting 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.

29/02/2012

Nuclear scattering

Coulomb scattering Program:

1) Get via optics angles at IP: θY = Y / Leff_Y

Leff_Y = Leff_Y(β*,β, Ψ)

2) Momentum transfer t = - θ2 p2 with θ2= θX

2+ θY2

3) Fit t-spectrum for σtot, luminosity, B-slope …

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

29/02/2012

Data taking at beta* 90m: overview

29/02/2012 LHC Lumi Days 4• 4

Massimiliano Ferro-Luzzi

scraping optics measurement 2 fills lost VdM failed data taking

LHC Lumi Days 529/02/2012

Data taking at beta* 90m: conditions & statistics

Date Run Triggers Position Purpose

Oct 18 191291-191305 17 million Various, 7mm Scraping

Oct 18 191312-191313 0.5 million 7mm OD only for distance

Oct 18 191321 2 million 5.5 sigma MD&OD standalone

Oct 18 191322 10 million 8.0 sigma MD&OD standalone

Oct 18 191323 0.5 million 8.0 sigma OD only for distance

Oct 20 191366 10 million 6.5 sigma MD&OD standalone

Oct 20 191367 1.2 million 6.5 sigma OD only for distance

Oct 20 191373 55 million+ 7million

6.5 sigma ATLAS combined runPEB + full streams

Oct 20 191377-191383 4 million 6.5 sigma OD only for distance

Oct 20 191386-191388 10 million 6.2 sigma MD&OD standalone

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 629/02/2012

Ingredients towards physicsBefore 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 μmSigma reflects detector resolutionand angular divergence at IP

LHC Lumi Days 729/02/2012

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 829/02/2012

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 929/02/2012

Luminosity: debuggingGeneral: All AND algorithms are very sensitive to the low statistics related to the low beam intensity OR algorithms seems to be more robustLUCID: 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 01/05/2011BCM: BCM_AND impact of background subtraction has to be clarified

+- 1 %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

LHC Lumi Days 1029/02/2012

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 1129/02/2012

Now about activities & plans for 2012

LHC Lumi Days 1229/02/2012

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 Pt100on Titanium trigger plate

3 hours

LHC Lumi Days 1329/02/2012

Improved temperature monitoring

30 sensors per station and beam pipe

4 add. Sensors in ALFA2

LHC Lumi Days 1429/02/2012

500m

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:1) beta*2) emmittance3) distance of detectors to the beam

Plot for 3 scenarios gives the impression: Assuming we can run as 2011 with 2μm emmittanceat 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 is50% acceptance.

LHC Lumi Days 1529/02/2012

For 50% acceptance cut an additional detectoroffset of 2 sigma is assumed.

Observation:For 2011 emmittance and positions (2μm, 6.5 sigma)beta* ~ 900m is needed to reach Coulomb-Nuclear interference region.

If beta* of 900m is not achievable the interestingt-range is accessible by:1) smaller emmittance and/or2) Detector positions closer to the beam For beta* of 500m a useful combination of emmitanceand positions seems to in the range:• emmittance 1.0 – 2.0 μm • detectors at 4 – 5 sigma

900m

Measurements at higher β* (2)

LHC Lumi Days 1629/02/2012

Summary2011 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 feasible2012 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