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
I. Efthymiopoulos

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

4. 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

5. ATLAS Assembly Status - UX15 cavern
Barrel em & hadronic calorimeters wheels
4th toroid magnet being installed
I. Efthymiopoulos

6. 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
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

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
Ldt = 300fb–1
Systematic error dominated by the luminosity measurement
(ATLAS-TDR-15, May 1999)
I. Efthymiopoulos

8. 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

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?
LHC
S.Bultmann et.al. - RIHIC
I. Efthymiopoulos

10. Elastic scattering at the CNI region (2/3)
Experimental conditions
t-value reach for 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

11. 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

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 m from IP1
I. Efthymiopoulos

13. 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 = 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

14. 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: ×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

15. 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

16. 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

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

18. 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

19. 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  GeV2
no systematic errors
t = GeV2
ns = 15
t = GeV2
ns = 10
I. Efthymiopoulos

20. 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

21. 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: 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

22. 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

23. Backup Slides
I. Efthymiopoulos

24. ZDC instrumentation at the TAN
IP1&IP5 absorbers
I. Efthymiopoulos

25. 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

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, …
I. Efthymiopoulos

27. 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

28. 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