The ALFA project in ATLASThe ALFA project in ATLAS

Antwerpen 25/10/07Per Grafstrom

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ATLAS FORWARD DETECTORSATLAS FORWARD DETECTORS

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Purpose of ALFAPurpose 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? 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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Relative precision on the measurement of HBR for various channels, as function of mH, at Ldt = 300 fb–1. The dominant uncertainty is from Luminosity: 10% (open symbols), 5% (solid symbols).

(ATLAS-TDR-15, May 1999)

Higgs couplingtan measurement

ExamplesExamples

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 |η| 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 measurementAbsolute 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). |t|~0.00065 GeV2 or Θ ~ 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 (< 100 micron)

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Elastic scatteringElastic 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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tot vs sand fit to (lns) =1.0

) =2.2(best fit)

The total cross sectionThe total cross section

AlanValery Mishka

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The The ρρ parameter parameter

ρ = Re F(0)/Im F(0) linked to the total cross section via dispersion relations ρ is sensitive to the total cross section beyond the energy at which ρ 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 peakThe b-parameter or the forward peak The b-parameter for lt l< .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 DiffractionSingle Diffraction

RP

IP

240m 240m

RPRP RP

RP RP RP RP

RP

IP

240m 240m

RPRP RP

RP RP RP RPZDC

ZDC

140m

LUCID

LUCID

ZDC

ZDC

140m

LUCID

LUCID

ATLAS

ATLAS

17m 17m

elastic scattering

single diffraction

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Forward detectorsForward detectors

80% acceptance

GeV 2.7E

6.1|η|5.4

90% acceptance

GeV 60 n, TeV 1E

8.3|η|

(elastic) 67% acceptance

99%efficiency

14|η|10

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Trigger conditionsTrigger conditions

For the special run (~100 hrs, L=10For the special run (~100 hrs, L=102727cmcm-2-2ss-1-1))

1. ALFA trigger1. ALFA trigger coincidence signal left-right arm (elastic trigger) coincidence signal left-right arm (elastic trigger) each arm must have a coincidence between 2 stations each arm must have a coincidence between 2 stations rate about 30 Hzrate about 30 Hz

2. LUCID trigger2. LUCID trigger coincidence left-right arm (luminosity monitoring)coincidence left-right arm (luminosity monitoring) single arm signal: one track in one tube single arm signal: one track in one tube

3. ZDC trigger3. ZDC trigger single arm signal: energy deposit > 1 TeV (neutrons)single arm signal: energy deposit > 1 TeV (neutrons)

4. Single diffraction trigger4. Single diffraction trigger ALFA.AND.(LUCID.OR.ZDC) ALFA.AND.(LUCID.OR.ZDC) central ATLAS detector not considered for now (MBTS good central ATLAS detector not considered for now (MBTS good

candidate)candidate)

For the special run (~100 hrs, L=10For the special run (~100 hrs, L=102727cmcm-2-2ss-1-1))

1. ALFA trigger1. ALFA trigger coincidence signal left-right arm (elastic trigger) coincidence signal left-right arm (elastic trigger) each arm must have a coincidence between 2 stations each arm must have a coincidence between 2 stations rate about 30 Hzrate about 30 Hz

2. LUCID trigger2. LUCID trigger coincidence left-right arm (luminosity monitoring)coincidence left-right arm (luminosity monitoring) single arm signal: one track in one tube single arm signal: one track in one tube

3. ZDC trigger3. ZDC trigger single arm signal: energy deposit > 1 TeV (neutrons)single arm signal: energy deposit > 1 TeV (neutrons)

4. Single diffraction trigger4. Single diffraction trigger ALFA.AND.(LUCID.OR.ZDC) ALFA.AND.(LUCID.OR.ZDC) central ATLAS detector not considered for now (MBTS good central ATLAS detector not considered for now (MBTS good

candidate)candidate)

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Event generation and simulationEvent generation and simulation

PYTHIA6.4modified elastic

with coulomb- and ρ-termsingle diffraction

PHOJET1.1 elastic & single diffraction

beam propertiesat IP1

size of the beam spot σx,y

beam divergence σ’x,y

momentum dispersion

beam transportMadX

tracking IP1RP high β* optics V6.5

including apertures

ALFA simulationtrack reconstruction

t-spectrumξ-spectrum

luminosity determination

single diffractionL1 filter

LUCID & ZDCpre-selection

elastic scattering

(Work of Hasko Stenzel-Giessen)

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Single diffraction: trigger conditionsSingle diffraction: trigger conditions

Efficiency [%] Pythia Phojet

Preselection

ξ<0.2 97.1 94.8

ZDC [E>1 TeV] 51.5 38.7

LUCID [1 track] 45.1 57.3

[Central ATLAS E> 100 GeV] 24.9 38.7

Total preselection 75 74

RP selection

ALFA(Relative to preselection)

60.1 54.2

Total acceptance 44.9 40.1

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Hit pattern in ALFAHit 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 acceptance for t and ξξ

global acceptance: PYTHIA 45 % PHOJET 40.1 %

global acceptance: PYTHIA 45 % PHOJET 40.1 %

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Feedthrough for trigger photodetectors

Kapton flat cable

motherboard

MAPMT + VD + RO cards

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The fiber trackerThe fiber tracker

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ALFA 2007: a full scale detection module 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-throughNew design for the

fiber plates support

10-2-64 fiber plates:

New substrates design

3 overlaps fiber plates:

New substrates design

Trigger scintillators:

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Roman Pot Concept

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FE electronicsFE electronics

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Test Beam campaigns at DESY and at Test Beam campaigns at DESY and at CERNCERN

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DESY test beam resultsDESY test beam results

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The test beam at DESYThe test beam at DESY

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.

Conclusions from DESY test beam

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Test beam at CERNTest beam at CERN

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Test Beam at CERNTest Beam at CERN

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Time lineTime 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-upSimulation of the LHC set-up

elastic generatorPYTHIA6.4

with coulomb- and ρ-termSD+DD non-elastic

background, no DPE

beam propertiesat IP1

size of the beam spot σx,y

beam divergence σ’x,y

momentum dispersion

beam transportMadX

tracking IP1RP high β* optics V6.5

including apertures

ALFA simulationtrack reconstruction

t-spectrumluminosity determinationlater: GEANT4 simulation

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Acceptance Acceptance

Global acceptance = 67%at yd=1.5 mm, including losses in the LHC aperture.Require tracks 2(R)+2(L) RP’s.

distance of closest approach to the beam

radGeVTOT

EMa

t

Nf

Cf

5.324106

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|||| :Region Coulomb

Detectors have to be operated as close as possible to the beam in order to reach the coulombregion!

-t=6·10-4 GeV2

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L from a fit to the t-spectrumL from a fit to the t-spectrum

2

222/

2

22

2

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c

e

t

e

t

cL

FFLdt

dN

tBtot

tBtot

NC

input fit errorcorrelation

L 8.10 1026 8.151 1026 1.77 %

σtot 101.5 mb 101.14 mb 0.9% -99%

B 18 Gev-2 17.93 Gev-20.3%

57%

ρ 0.15 0.143 4.3% 89%

Simulating 10 M events,running 100 hrsfit range 0.00055-0.055

large stat.correlation between L and other parameters

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Simulation of elastic scatteringSimulation of elastic scattering

2

,

2

,

2

2222*

yeffxeff

yx

L

y

L

xp

ppt

t reconstruction:

hit pattern for 10 M elastic events simulated with PYTHIA + MADX for the beam transport

2

sin

effL

special optics parallel-to-point focusing high β*

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t- and t- and ξξ-resolution: PYTHIA vs PHOJET -resolution: PYTHIA vs PHOJET

Good agreement between PYTHIA and PHOJET for the reolutions Good agreement between PYTHIA and PHOJET for the reolutions

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reconstruction biasreconstruction bias

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

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 Introduction – physics case

single diffraction ppX+p:

complements the elastic scattering program measurement of cross section and differential distributions fundamental measurement, tuning of models, background

determination special detectors ALFA+LUCID+ZDC high β* optics same special run as for luminosity calibration

single diffraction ppX+p:

complements the elastic scattering program measurement of cross section and differential distributions fundamental measurement, tuning of models, background

determination special detectors ALFA+LUCID+ZDC high β* optics same special run as for luminosity calibration

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resolution for t and resolution for t and ξξ

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

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

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Systematic uncertainties 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

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 & outlookConclusion & outlook

A measurement of single diffraction with ATLAS appears to be possible,

however it won’t be a precision measurement in contrast to elastic

scattering.

Combination ALFA, LUCID and ZDC Special running conditions measurement of cross section and t-, ξ-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.

A measurement of single diffraction with ATLAS appears to be possible,

however it won’t be a precision measurement in contrast to elastic

scattering.

Combination ALFA, LUCID and ZDC Special running conditions measurement of cross section and t-, ξ-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 errorsSystematic errors Background subtraction ~ 1 %

Background subtraction ~ 1%

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Luminosity transfer 10Luminosity transfer 102727-10-1034 34 cmcm-2-2 sec sec-1-1

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 < 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-resolutiont-resolution

The t-resolution is dominated by the divergence of the incoming beams.

σ’=0.23 µrad

ideal case

real world

2*231

ˆ pppt

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