Machine induced background in ALFA The ALFA detector elastic scattering and luminosity

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 StenzelBackground WG meeting

ALFA background H.Stenzel, 16.03.07 2

Forward Roman Pots for ATLAS

240 mATLAS ALFA


The ALFA detector

RP

IP240m 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


elastic scattering


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

1009

0m 119

m 2625

scmL

***** sinsincos yyy

m rad1GeV 098.0

GeV 0004.0rad 7.44

rad 7.2mm 12.0

2max

2min

max

min

d

N

tt


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


Simulation of elastic scattering

2

,

2

,

2

2222*

yeffxeff

yx

Ly

Lxp

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*


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


Performance estimation: systematic uncertaintiesRecent work obtained for the ALFA TDR (in review)

Backgroundcontribution


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


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


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

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

2-1-27

10

cms10Ls021.2

1043

bunch

part

bunch

tkk

single rates


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


background calculation

RP

IP

signal & background in asymmetric configuration

240m RPRP 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


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 %


local inelastic beam-gas background

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


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 ...


from Vincent Hedberg


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)?

Date post:	19-Mar-2016
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Machine induced background in ALFA The ALFA detector elastic scattering and luminosity

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