on behalf of the atlas collaboration

Damien Prieur - Moriond QCD 2005 1

A T L A S________________________________________________________________________________________________

GMSB SUSY models with non-pointing photons GMSB SUSY models with non-pointing photons signaturessignatures

On behalf of the ATLAS collaborationOn behalf of the ATLAS collaboration


NLSP :NLSP : • neutralinoneutralino : 01 + G

• sleptonslepton : lR l + G~~

~ ~ NLSP NLSP typical decay lengthtypical decay length may be macroscopic: may be macroscopic:

54

0NLSP

GeV 100

TeV 100 B100

1 )cm(cτ

NLSPm

F54

0NLSP

GeV 100

TeV 100 B100

1 )cm(cτ

NLSPm

F

Gauge Mediated Supersymmetry Breaking (GMSB)Gauge Mediated Supersymmetry Breaking (GMSB)

gravitino G get its mass only through gravitational interaction

G is the LSP, 10-2 < mG < 104 eV

if R-parity is assumed, G is the only stable SUSY particle

all SUSY decay chains lead to the production of gravitinos

~

~

~

• Minimal GMSB model phenomenology is driven by 6 parameters

• SUSY breaking is mediated to the visible sector via gauge interactions

F, MF, Mmessmess, N, N55, , , tan, tan, sign(, sign(μ)μ)


• We consider here cases where LSP lighest neutralino

• Measuring ccττ is a fair way to access to FF00, a free parameter of the model

• To reconstruct decay vertex :• Impact position• Photon direction • Time of arrival ttγγ

• From these informations it is possible to determine gravitino’s directiongravitino’s direction ψψ :

2

2

1

1cos

sin

)cos1(

L

Lct with

χ 01

~

Gauge Mediated Supersymmetry Breaking (GMSB)Gauge Mediated Supersymmetry Breaking (GMSB)

• missing ET

• non pointingnon pointing photons photons

• With an intermediateintermediate lifetime, we get the following event topologies :

χ 01

~

01 + G~ ~


OutlineOutline

How to reconstruct photon direction with EM calorimeter

How to reconstruct SUSY GMSB signal and determine m and m in ATLAS

original method developed by Kawagoe, Kobayashi, Nojiri, OchiKawagoe, Kobayashi, Nojiri, Ochi (hep-ph/0309031)

using a fast simulationfast simulation of ATLAS detector

more realistic performancesrealistic performances parametrisation for non pointing photonsnon pointing photons

Estimate ATLAS sensitivity to ccττ and FF00

χ 01

~l~

• In the following:


CalorimetersCalorimeters Electromagnetic

Hadronic

Solenoïd 2T

ATLAS detectorATLAS detector


Electromagnetic calorimeterElectromagnetic calorimeter

Electromagnetic calorimeter - EndCapElectromagnetic calorimeter - EndCap

CryostatCryostat

Electromagnetic calorimeter - BarrelElectromagnetic calorimeter - Barrel


MiddleMiddle

FrontFront

Z

r

0

gen

EM showerEM shower 60 GeV60 GeV

True direction: gengen

Reconstructing photon directionReconstructing photon direction

LayerLayer GranularityGranularity (x)

Pre-sampler

Front

Middle

Back

0.025 x 0.10.025 x 0.1

0.003 x 0.10.003 x 0.1

0.025 x 0.0250.025 x 0.025

0.05 x 0.0250.05 x 0.025


MiddleMiddle

FrontFront

Z

r

0

Reconstruct barycenter :

EE

i

ii

_

for each layer : 11 , 22

11

22

Reconstructed shower direction : recrec

rec

True direction: gengen

Parametrisation of shower depth: R1(11) , R2(22)

R2(22)

R1(11)

Reconstructing photon directionReconstructing photon direction


EM Calorimeter angular resolutionEM Calorimeter angular resolution

• EM calorimeter designed for pointing photonsdesigned for pointing photons (coming from the IP - σz=5.6 cm) resolution on polar angle polar angle θθ : σθ 6060 mrad/E(GeV) (barrel, ||<1.4) resolution direction direction : not possible because of coarse granularity

& not necessary for normal processes (σr=15 μm)

• Performance for non pointing photons studied from detailled simulationdetailled simulation (Geant3)

barrelbarrel

resolution parameterized and resolution parameterized and implemented in the fast simulation of implemented in the fast simulation of ATLAS detector ATLAS detector

used in the following studyused in the following study

systematic biassystematic bias with standard reconstruction algorithms

need specific correctionsspecific corrections to improve resolution


Reconstruction of m and mReconstruction of m and m

• We consider the following decay chain :

χ01

~l~

• Gravitino’s energy EEGG is unknown, but we can determine its direction (coscosψψ)

• EEll , EEγγ , cosψcosψ , cosθcosθGlGl , and cosθcosθlγlγ being all measured, we can compute aa and bb parameters for each event.

• Some kinematics…

ammbl

2~

2~ 0

1 ammb

l

2~

2~ 0

1• In the (a,b)(a,b) plane, points should group along the line

)cos1(2)( ~2

~2~0

1

GG

EEppm

bam

EEmE

E

pppm

lllGl

lGl

2~

2~

~

2~

2~

01

01

)cos1(2)cos1(

)cos1(1

)(

lχ01

~ll

~γ G

~


• GMSB point G1G1

Λ M N5 Tanβ μ

90 TeV 500 TeV 1 5 +1

• 105 SUSY events generated with HERWIGHERWIG (pp pp sparticles sparticles 8 pb) one year of LHC at low luminosity (13.9 fb-1)

• Fast simulation for all particles

χ01

~

162

324

~

~

R

L

l

l

m

m

218

117

02

01

~

~

m

m

Sparticules Masses (GeV)

456

436

04

03

~

~

m

m

Event generation and simulationEvent generation and simulation

• energy, position, direction and time of non pointing photons are smeared according to realistic resolutions from test-beam datatest-beam data or detailed simulationdetailed simulation:

• Energy : EMCAL

• Timing : EMCAL

• Position : EMCAL

• Direction : • along : EMCAL previously parametrized resolutionpreviously parametrized resolution• along φ: TRT

%7.0 MeV245%10

EEE

E

ps 100t

)GeV(

004.0

E )GeV(

mrad 5

E

mrad 1

barr

el +

end

cap

barr

el +

end

cap

Will require converted photons !!!Will require converted photons !!!


4,3,2,1, TTTTmissTeff ppppEM

• Pre-selection cuts to limit background contribution from standard modelFull background analysis not performed

Use cuts from I. Hinchliffe/ F.E. PaigeI. Hinchliffe/ F.E. Paige (hep-ph/9812233 )

GeV 400effM

effmissT ME 1.0

Pre-selection cutsPre-selection cuts

- require 4 jets- build an effective masseffective mass :

We require :We require :

+ 2 leptons

+ 2 photons

71%71%

22%22%

efficiencyefficiency

Further require non pointing photonnon pointing photon

•We choose lifetime ccτ = τ = 100100 cm cm χ01

~


Apply cuts to select non pointing photons candidates

Resolution on Resolution on ψψ

EEγγ > 30 GeV > 30 GeVEEγγ > 30 GeV > 30 GeV αα > 0.2 rad > 0.2 radαα > 0.2 rad > 0.2 rad

ΔΔttγγ > 1ns > 1nsΔΔttγγ > 1ns > 1nsσσψψ= 60 mrad= 60 mrad

Selection of non pointing photonsSelection of non pointing photons

ψrec-ψtrue (mrad)t (ns)

94%94%

24%24%

74%74%

(conservative)(conservative)


• For each NP photon, we select if possible an isolated lepton with ppTT>20 GeV>20 GeV to form lγ pairs

• If several leptons available, we choose the one which minimizes mlγ

At this point, we end up with 300300 lγ pairs

ammbl

2~

2~ 0

1 ammb

l

2~

2~ 0

1

p0: 26111p1: 13635

m = 116.8116.8 GeV

m = 161.6161.6 GeV

χ01

~

l~

Reconstruction of m and mReconstruction of m and mχ01

~l~

Scatter plot - (a,b) plane

linear fit :

Need more work for improving pairing

InputInput

161.4161.4 GeV GeV

117.1 GeV


Resolution on reconstructed masses is about 2%2% for ccτ = τ = 100100 cm cm

Input mass : 161.4161.4 GeV

Reconstructed : 159.9159.9 GeV

σ σ :: 2.1 2.1 GeVGeV

Input mass : 117.1117.1 GeV

Reconstructed : 115.5115.5 GeV

σ σ :: 1.7 1.7 GeVGeV

Reconstruction of m and mReconstruction of m and mχ01

~l~

Simulation is repeated 100100 times to estimate resolution on reconstructed sparticle masses

preliminary


Reconstruction of NLSP decay vertexReconstruction of NLSP decay vertex

Reconstruct decay time of NLSP in lab. frame σσttDD 2 ns 2 ns

• same cuts as previously, except photon conversion not not required required

• for each NP photon, we select if possible an isolated lepton with ppTT>20 GeV>20 GeV to form lγ pairs

B

BppEE

A

AppEE ll

01

01

01

01

~~~~• We minimize

• Once we have m and m No longer need toNo longer need to : - know photon’s direction along direction along

- require a photon conversionphoton conversion

χ01

~l~

Αmmmpp lll 22~

2~~ 0

101 2

1~~

Bm

pp 2

~~2~

~

01

01

once NLSP decay position is known, we can reconstruct all the decay chain


Reconstruction of NLSP lifetimeReconstruction of NLSP lifetime

make lifetime cc vary from 10 up to 200 cm fit of proper timeproper time ttDD// by an exponential function simulation reproduced 100 times to estimate sensitivity on fitted values in this case we need lot of statistics 10 fb-1 is not enough, use 100 fb-1 instead (1 year @ high lumi)

Systematic biais for cc > 100 cm (due to increasing amount of NLSP escaping detection) Need to know momentum distribution to correct for bias at high c cc/c/c vary from 33 to 88%

χ 01

~

8%8%

3%3%

cc

/c/c


• A realistic angular resolutionrealistic angular resolution parameterization for non pointing photonsnon pointing photons implemented in a fast simulationfast simulation of ATLAS detector

• Resolution on reconstructed masses for NLSP and slepton masses about 2%2%

• Resolution on NLSP lifetime below 8%8% for cc between 10 and 200 cm

SummarySummary

SystematicsSystematics and acceptanceacceptance need to be carefully studied with a detailed simulationdetailed simulation of ATLAS :

- effects on direction reconstruction of pile-up, underlying events- description of EM showers- contributions from background

PR

EL

IMIN

AR

Y

%2

m

m%5

c

c%4

0

0

F

F%8

~

~

G

G

m

m

Can extend accessible cc rangerange using statistical method (but need careful studies - under investigation)

Can determine SUSY fundamental breaking scale FF00 with 4%4% precision

for cc = 100 cm


Readout electrode (barrel)Readout electrode (barrel)Front layerMiddle layer

Back layer=0 =0.8

=1.4

Pre samplerr

EM calorimeter granularityEM calorimeter granularity

LayerLayer GranularityGranularity (x)

Pre-sampler

Front

Middle

Back

0.025 x 0.10.025 x 0.1

0.003 x 0.10.003 x 0.1

0.025 x 0.0250.025 x 0.025

0.05 x 0.0250.05 x 0.025


Reconstruction of NLSP lifetimeReconstruction of NLSP lifetime

• Having m and m , we are able to reconstruct the slepton decay chain

No longer need toNo longer need to : - know photon’s direction along direction along - require a photon conversionphoton conversion inside the inner detector

χ01

~l~

lχ01

~ll

~γ G

~

Αmmmpp lll 22~

2~~ 0

101 2

1~~

Bm

pp 2

~~2~

~

01

01

c

AM

c

OMt

22~

~~~ll

ppp

2~2 ~~~

pppG

- photon impact positionimpact position- photon polar anglepolar angle

we measure :

few relations :

B

BppEE

A

AppEE ll

01

01

01

01

~~~~

we minimize

CalorimeterCalorimeter


Production of sleptonsProduction of sleptons

q

χ02

~l~ χ0

1

~

ll

pp

ppg~

G~

q

q~

χ02

~l~

l

pp

ppq~

q

χ02

~ l~

l

pp

pp

on behalf of the atlas collaboration

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