The ALFA project in ATLAS

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The ALFA project in ATLAS
Antwerpen 25/10/07 Per Grafstrom
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ATLAS FORWARD DETECTORS
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Purpose of ALFA

• Additional handle on the luminosity
• ALFA Absolute Luminosity For ATLAS
• Measurement of ? tot and elastic scattering
  parameters
• Tag proton for single diffraction

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Luminosity measurements-why?

• Cross sections for Standard processes
• t-tbar production
• W/Z production
• .
• Theoretically known to better than 10 will
  improve in the future
• New physics manifesting in deviation of ? x BR
  relative the Standard Model predictions
• Important precision measurements
• Higgs production ? x BR
• tan? measurement for MSSM Higgs
• .

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Examples
Higgs coupling
tan? measurement
Systematic error dominated by luminosity (ATLAS
Physics TDR )
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• Elastic scattering as a handle on luminosity
• optical theorem forward elastic rate total
  inelastic rate
• needs large ? coverage to get a good
  measurement of the inelastic rate- otherwise rely
  on MC in unmeasured regions
• Use ?tot measured by others (TOTEM)
• Combine machine luminosity with optical theorem
• luminosity from Coulomb Scattering
• ATLAS pursuing all options

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Absolute vs relative measurement

• STRATEGY
• 1. Measure the luminosity with most precise
  method at optimal conditions
• 2. Calibrate luminosity monitor with this
  measurement, which can then be used at different
  conditions
• Relative Methods
• LUCID (dedicated luminosity monitor)
• BCM
• Min. Bias Scintillators
• Tile/LAr Calorimeters

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Elastic scattering at small angles

• Measure elastic rate dN/dt down to the Coulomb
  interference region
• (à la UA4). t0.00065 GeV2 or T 3.5
  microrad.

• This requires (apart from special beam optics)
• to place detectors 1.5 mm from LHC beam axis
• to operate detectors in the secondary vacuum of a
  Roman Pot
• spatial resolution sx sy well below 100 micron
  (goal 30 micron)
• no significant inactive edge (lt 100 micron)

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Elastic scattering

• All very simplified we need
• Electromagnetic form factor
• Proper treatment of the Coloumb-hadron
  interference phase
• t- dependence of rho and phase
• non-exponential behaviour -t dependence of the
  slope
• Saturation effects

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The total cross section
? Alan Valery Mishka
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The ? parameter

• ? Re F(0)/Im F(0) linked to the total cross
  section via dispersion relations
• ? is sensitive to the total cross section beyond
  the energy at which ? is measured ? predictions
  of ?tot beyond LHC energies is possible
• Inversely Are dispersion relations still valid
  at LHC energies?

(Figures from Compete collaboration)
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The b-parameter or the forward peak

• The b-parameter for lt llt .1 GeV2
• Old language shrinkage of the forward peak
• b(s) ? 2 ? log s ? the slope of the
  Pomeron trajectory ? ? 0.25 GeV2
• Not simple exponential dependence of local
  slope
• Structure of small oscillations?

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Single Diffraction
elastic scattering
RP
RP
RP
RP
240m
240m
IP
RP
RP
RP
RP
single diffraction
ATLAS
RP
RP
RP
RP
LUCID
LUCID
ZDC
ZDC
IP
RP
RP
RP
LUCID
LUCID
ATLAS
RP
ZDC
ZDC
240m
240m
140m
140m
17m
17m
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Forward detectors
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Trigger conditions

• For the special run (100 hrs, L1027cm-2s-1)
• 1. ALFA trigger
• coincidence signal left-right arm (elastic
  trigger)
• each arm must have a coincidence between 2
  stations
• rate about 30 Hz
• 2. LUCID trigger
• coincidence left-right arm (luminosity
  monitoring)
• single arm signal one track in one tube
• 3. ZDC trigger
• single arm signal energy deposit gt 1 TeV
  (neutrons)
• 4. Single diffraction trigger
• ALFA.AND.(LUCID.OR.ZDC)
• central ATLAS detector not considered for now
  (MBTS good candidate)

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Event generation and simulation
PYTHIA6.4 modified elastic with coulomb- and
?-term single diffraction PHOJET1.1 elastic
single diffraction
beam properties at IP1 size of the beam spot
sx,y beam divergence sx,y momentum dispersion
single diffraction L1 filter LUCID
ZDC pre-selection
elastic scattering
ALFA simulation track reconstruction
t-spectrum ?-spectrum luminosity determination
beam transport MadX tracking IP1?RP high ß
optics V6.5 including apertures
(Work of Hasko Stenzel-Giessen)
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Single diffraction trigger conditions
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Hit pattern in ALFA
hit pattern for 10 M SD events simulated with
PYTHIA MADX for the beam transport
Dispersion
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acceptance for t and ?

• global acceptance
• PYTHIA 45
• PHOJET 40.1

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MAPMT VD RO cards
Kapton flat cable
motherboard
Feedthrough for trigger photodetectors
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The fiber tracker
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ALFA 2007 a full scale detection module
23 MAPMTs 10x2 for fiber detector 3x1 for overlap
detector Frame from the 2006 TB
Base plate similar to the 2006 version, but with
central fixation for fiber plates and 1 free slot
for triggers feed-through
New design for the fiber plates support
3 overlaps fiber plates New substrates design
10-2-64 fiber plates New substrates design
Trigger scintillators
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Roman Pot Concept
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FE electronics
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Test Beam campaigns at DESY and at CERN
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DESY test beam results
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The test beam at DESY
Conclusions from DESY test beam

• the validity of the chosen detector concept with
  MAPMT readout
• the baseline fibre Kuraray SCSF-78 0.5 mm2 square
• expected photoelectric yield 4
• low optical cross-talk
• good spatial resolution
• high track reconstruction efficiency
• No or small inactive edge
• Technology appears fully appropriate for the
• proposed measurement.

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Test beam at CERN
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Test Beam at CERN
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Time line

• Mechanics
• Prototype tested
• Full production launched
• Delivery end February 2008
• Detector
• A number of small prototypes tested
• Construction of one full detector started (1/8 of
  total system)
• Production start after validation spring 2008.
• Full detector in 2009
• Electronics
• Prototypes tested
• Electronics corresponding to one full detector by
  end 2007
• All electronics by end 2008

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• Back up

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Simulation of the LHC set-up
elastic generator PYTHIA6.4 with coulomb- and
?-term SDDD non-elastic background, no DPE
beam properties at IP1 size of the beam spot
sx,y beam divergence sx,y momentum dispersion
ALFA simulation track reconstruction
t-spectrum luminosity determination later
GEANT4 simulation
beam transport MadX tracking IP1?RP high ß
optics V6.5 including apertures
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Acceptance
distance of closest approach to the beam
Global acceptance 67 at yd1.5 mm, including
losses in the LHC aperture. Require tracks
2(R)2(L) RPs.
Detectors have to be operated as close as
possible to the beam in order to reach the
coulomb region!
-t610-4 GeV2
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L from a fit to the t-spectrum
Simulating 10 M events, running 100 hrs fit range
0.00055-0.055
large stat.correlation between L and other
parameters
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Simulation of elastic scattering
hit pattern for 10 M elastic events simulated
with PYTHIA MADX for the beam transport
t reconstruction

• special optics
• parallel-to-point focusing
• high ß

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t- and ?-resolution PYTHIA vs PHOJET

• Good agreement between PYTHIA and PHOJET for the
  reolutions

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reconstruction bias

• True and reconstructed values are in average
  slightly shifted
• ? needs to be corrected
• some differences observed at small t

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Introduction physics case

• single diffraction pp?Xp
• complements the elastic scattering program
• measurement of cross section and differential
  distributions
• fundamental measurement, tuning of models,
  background determination
• special detectors ALFALUCIDZDC
• high ß optics
• same special run as for luminosity calibration

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resolution for t and ?

• main contribution to the resolution
• t vertex smearing, beam divergence (small t),
  det. resolution (large t)
• ? vertex smearing and detector resolution

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Systematic uncertainties

• generator difference, model dependence
• ? acceptance, detector corrections 5-10
• beam conditions, optical functions, alignment
• ? 2 (based on results for elastic scattering)
• background (being estimated)
• double diffraction
• minimum bias
• beam halo
• DD 2 , MB 0.5 , beam halo DD/MB 1-2
• luminosity
• ? 3, very best possible luminosity
  determination, at calibration point!
• statistical uncertainty small, expect 1.6-2.3 M
  accepted events

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Conclusion outlook

• A measurement of single diffraction with ATLAS
  appears to be possible,
• however it wont be a precision measurement in
  contrast to elastic
• scattering.
• Combination ALFA, LUCID and ZDC
• Special running conditions
• measurement of cross section and t-,
  ?-distribution
• not a precision measurement, 10 systematic
  uncertainty achievable?
• goal improve model predictions and background
  estimates for central diffraction
• This first pilot study must be pursued and
  confirmed by full simulation and
• systematic studies involving the LUCID and ZDC
  communities. The option of
• including the MBTS for tagging the diffractive
  system should be investigated.

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Systematic errors

• Background subtraction 1

Background subtraction 1
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Luminosity transfer 1027-1034 cm-2 sec-1

• 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 ? lt 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

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t-resolution
The t-resolution is dominated by the
divergence of the incoming beams. s0.23
µrad
ideal case
real world
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