Machine induced background in ALFA

• The ALFA detector• elastic scattering and luminosity • background generation, rejection and subtraction• impact on luminosity determination• Conclusion & open issues

Hasko Stenzel

Background WG meeting

ALFA background H.Stenzel, 16.03.07 2

Forward Roman Pots for ATLAS

240 mATLAS ALFA

ALFA background H.Stenzel, 16.03.07 3

The ALFA detector

RP

IP

240m 240m

RPRP RP

RP RP RP RP

PMT baseplate

optical connectors

scintillating fibre detectors glued on ceramic supports

10 U/V planesoverlap&trigger

Roman Pot

MAPMTsFE electronics

& shield

Roman Pot Unit

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elastic scattering

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Transversal displacement of particles in the ring away from the IP:

Special optics with high * and parallel-to-point focusing:

independent of the vertex position

properties at the roman pot (240m)

y*

y*

parallel-to-point focusingydet

IP Leff

advance phase 90 ,point -to-parallel

; o

2**,

**det ptLy yyeffy

special optics: high ß*

1227

*y

10

09

0

m 119

m 2625

scmL

*****

sinsincos yyy

m rad1

GeV 098.0

GeV 0004.0

rad 7.44

rad 7.2

mm 12.0

2max

2min

max

min

d

N

t

t

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Simulation 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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Simulation 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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luminosity determination

input fitStat.

error

L 8.10 1026 8.151 1026 1.77 %

σtot 101.5 mb 101.14 mb 0.9%

B 18 Gev-2 17.93 Gev-20.3%

ρ 0.15 0.143 4.3%

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

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Performance estimation: systematic uncertainties

Recent work obtained for the ALFA TDR (in review)

Backgroundcontribution

ALFA background H.Stenzel, 16.03.07 10

background considerations

● physics background: single diffraction

• can be rejected by means of vertex and acollinearity cuts• is reduced to a negligible level

● machine background

• beam halo originating from cleaning inefficiencies and distant quasi-elastic beam gas interactions, calculations were provided by Igor Bayshev, IHEP

• local inelastic beam-gas interactions (showers), calculations were provided by Igor Azhgirey, IHEP

● physics background: single diffraction

• can be rejected by means of vertex and acollinearity cuts• is reduced to a negligible level

● machine background

• beam halo originating from cleaning inefficiencies and distant quasi-elastic beam gas interactions, calculations were provided by Igor Bayshev, IHEP

• local inelastic beam-gas interactions (showers), calculations were provided by Igor Azhgirey, IHEP

ALFA background H.Stenzel, 16.03.07 11

beam halo

Calculations are carried out for the high β*-optics with

εN =1μrad m and at L=1027cm-2s-1

● beam halo from collimation inefficiencies

• betatron cleaning• momentum cleaning

● halo beam-gas interactions

• elastic and quasi-elastic p-N interactions

Calculations are carried out for the high β*-optics with

εN =1μrad m and at L=1027cm-2s-1

● beam halo from collimation inefficiencies

• betatron cleaning• momentum cleaning

● halo beam-gas interactions

• elastic and quasi-elastic p-N interactions

ALFA background H.Stenzel, 16.03.07 12

beam halo background

● distributions of halo impacts in the transversal plane at the detector

● normalized per proton hitting a collimator/interacting with beam gas

● This can be turned into single and accidental coincidence rates by

● main question: what is the lifetime contribution for beam gas?

• 100 hrs for MC & BC• 1000 hrs for beam gas

● distributions of halo impacts in the transversal plane at the detector

● normalized per proton hitting a collimator/interacting with beam gas

● This can be turned into single and accidental coincidence rates by

● main question: what is the lifetime contribution for beam gas?

• 100 hrs for MC & BC• 1000 hrs for beam gas

bunchbsacc

partbunchbbs

tRR

kkNR

2

● accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz)

● potentially dangerous since all at small t

● accidental coincidence rate inside detector acceptance of about 9 Hz (elastic: 27 Hz)

● potentially dangerous since all at small t

2-1-27

10

cms10L

s021.2

10

43

bunch

part

bunch

t

k

k

single rates

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beam halo rejection cuts

Exploit back-to-back signature of elastic events and vertex reconstruction

after vertex and acollinearity cuts still 140 k events survive!

(compared to 6.6 M elastic signal)

irreducible background at small t in the luminosity region!

must be subtracted

ALFA background H.Stenzel, 16.03.07 14

background calculation

RP

IP

signal & background in asymmetric configuration

240mRPRP RP

RP RP RP RP

240m

pure background

● signal and irreducible background appear in asymmetric configurations: +/- and -/+

● pure background is also present in symmetric configurations +/+ and -/-

● from this the irreducible background can be calculated by inverting randomly (left/right) the vertical sign of the hits

● halo asymmetries can be corrected for using data

● free of MC, good systematics

● signal and irreducible background appear in asymmetric configurations: +/- and -/+

● pure background is also present in symmetric configurations +/+ and -/-

● from this the irreducible background can be calculated by inverting randomly (left/right) the vertical sign of the hits

● halo asymmetries can be corrected for using data

● free of MC, good systematics

ALFA background H.Stenzel, 16.03.07 15

systematic uncertainty of background

● In principle the method is free of syst. uncertainties, since all is determined from the data itself

● However, the calculated background sample is subject to statistical fluctuations, i.e. the subtraction not exact.

● this effect is estimated by generating a large number of background sample with equal statistics and applying the subtraction procedure. In the end the RMS of the fitted luminosity results is quoted as syst. error.

● Result: ΔL/L = 1.1-1.5 %

● Total systematic error: 2.2-2.6 %

● Total error : 2.8-3.2 %

● In principle the method is free of syst. uncertainties, since all is determined from the data itself

● However, the calculated background sample is subject to statistical fluctuations, i.e. the subtraction not exact.

● this effect is estimated by generating a large number of background sample with equal statistics and applying the subtraction procedure. In the end the RMS of the fitted luminosity results is quoted as syst. error.

● Result: ΔL/L = 1.1-1.5 %

● Total systematic error: 2.2-2.6 %

● Total error : 2.8-3.2 %

ALFA background H.Stenzel, 16.03.07 16

local inelastic beam-gas background

The comparison of the rate of distant and local beam-gas background shows thatthe latter contribution can be neglected.

The comparison of the rate of distant and local beam-gas background shows thatthe latter contribution can be neglected.

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conclusion

● ATLAS proposes to determine the absolute luminosity using elastic scattering in the Coulomb-Nuclear interference region measured with the ALFA subdetector

● The success of this measurement depend crucially on the beam conditions

● The background calculations provided by IHEP Protvino constitute an essential element in the performance estimation

● A precision of about 3% for the luminosity is within reach

● Other methods for the luminosity determination (W/Z counting, optical theorem, ..) are in parallel pursued

● Open issues : beam-gas background for LUCID ...

● ATLAS proposes to determine the absolute luminosity using elastic scattering in the Coulomb-Nuclear interference region measured with the ALFA subdetector

● The success of this measurement depend crucially on the beam conditions

● The background calculations provided by IHEP Protvino constitute an essential element in the performance estimation

● A precision of about 3% for the luminosity is within reach

● Other methods for the luminosity determination (W/Z counting, optical theorem, ..) are in parallel pursued

● Open issues : beam-gas background for LUCID ...

ALFA background H.Stenzel, 16.03.07 18

from Vincent Hedberg

ALFA background H.Stenzel, 16.03.07 19

open issue: beam-gas background for LUCID

● The beam-gas background entering LUCID from the back has been estimated to be at a small level

● The beam gas entering LUCID from the front is presumably rather small (length ratio) but could be dangerous, since it is pointing to LUCID

● Can we get a background calculation for this contribution at a scoring plane of the LUCID front face (~17m)?

● The beam-gas background entering LUCID from the back has been estimated to be at a small level

● The beam gas entering LUCID from the front is presumably rather small (length ratio) but could be dangerous, since it is pointing to LUCID

● Can we get a background calculation for this contribution at a scoring plane of the LUCID front face (~17m)?