ATLAS Plans for Elastic CrossSection and Luminosity Measurement

1
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
2
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

3
Introduction (2/3)
Roman Pots _at_ 240 m from IP1
4
Introduction (3/3)
ATLAS
Calorimetry
Tracking
R
?-chambers
Barrel
Diffraction/Proton Tagging Region
EndCap
RP
Tracking
ZDC/TAN
FCAL
TAS
LUCID
y
5
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
6
Elastic scattering at the CNI region (1/3)
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

7
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

Systematic error dominated by the luminosity
measurement (ATLAS-TDR-15, May 1999)
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Physics interest (2/3)

• Total cross-section
• 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
COMPETE coll.
9
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?

S.Bultmann et.al. - RIHIC
10
Elastic scattering at the CNI region (2/3)

• Experimental conditions
• t-value reach for CNI _at_ 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

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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 ns ? 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

12
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
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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!

Endorsed by LHC Technical Committee Compatible
with TOTEM optics (see LEMIC minutes 9/12/2003)
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Experimental setup (3/4)

• Boosting the LHC performance
• Emittance of 1106 m?rad is needed to reach
  Coulomb region
• nominal LHC emittance 3.75106 m?rad
• 1106 m?rad is the designed commissioning
  emittance for LHC !!
• Encouraging results from SPS MDs
• V 1.1106 m?rad H 0.9106 m?rad for 7x1010
  ppb
• and also 0.6-0.7106 m?rad obtained for
  0.51010 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 eN 1.5106 m for Np 1010, ns,coll 6

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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 ns
  dmin/sbeam
• Working scenario 43 bunches, 1010ppb, eN 1.0
  µm rad, at ns10
• Expected halo rate of about 6 kHz

(RA LHC MAC 13/3/03)
Working point ??
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Roman Pot Detector RD (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

17
Roman Pot Detector RD (2/3)

• Pot assembly

Detector plane and the overlap detectors
Detector plane prototype assembly
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Roman Pot Detector RD (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

Down detector in the garage position
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Detector performance (1/1)

• Simulation results
• Reconstruct ?
• 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

t -0.0010 GeV2
ns 15
t -0.0007 GeV2
ns 10
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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 (_at_ 140m from IP1)
• Rates of well-calculable physics processes
• QED pp ? (p?)(p?)?p(????)p
• small rate 1pb (0.01 Hz at L1034) clean
  signal
• QCD W/Z ? leptons
• high rate W?l? 60 Hz at L1034 (e 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 ? coverage - ATLAS coverage
  in the forward direction is limited
• Two alternatives (if CNI cannot be reached)

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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 !!!

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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 , ?
  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

23

• Backup Slides

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ZDC instrumentation at the TAN
IP1IP5 absorbers
TAN_at_140m
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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 ? 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

26
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,

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Luminosity Monitor Detector - LUCID
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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