seminar atlas 1103 by grisha kirilin

8/6/2019 Seminar ATLAS 1103 by Grisha Kirilin

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Ldt=35pb-1 were collected between March and November

Search for an excess of events with an identicalflavour leptonpairand signicantmissing transverse

momentum

* ATLAS collaboration

*ArXiv:1103.6208* Submitted to EPJC*s=7TeV

>|| || |2010 20112009 t


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a search for the supersymmetric (SUSY) particles in events with

exactly two leptons of identical flavour (e or) and oppositecharge, and signicant missing transverse momentum

2~0

l~

l+/-

l-/+

1~0

2~0

l+

l-

1~0

ETmiss >

>

lepton pair invariant massdistribution{ }

>

>

>

Aims


3/21

2~0

l~

l+

l-

1~0

>

>

>

>

>

>

>

>

>

>

>

>

q

g

q

q

q

q-

q-

q

1~

1~0

W

q

g

q

q

~

~ ~-

Decay chains


4/21

End-points in mll distribution

(ml l2

)edge

=

(m

2

0

2 mlR

2 )(mlR

2m

1

0

2 )

m lR2

OSOF subtraction

In signal region

* Gjelsten, Hisano, Kawagoe, Lytken, Miller, Nojiri, Osland & Polesello(in LHC/IC study group) 04

* SUSY Parameter and Mass Determination at the LHC, C. Sander, Cambridge PhenomenologySeminar, 2010

Model:SPS1a


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*one of the best routes to model-independent measurements of themasses of SUSY particles via end-points in the lepton pair invariantmass distribution

*Standard Model background is (almost) equal for lepton pairs ofidentical and different flavour (in the signal region):Bg(e+e-) = Bg(+-) = Bg(e+-)= Bg(+e-)

* SM bg. can be removed with a flavour subtraction procedure:Signal(e+e-V+-)=Data(e+e-)+Data(+-)-Data(e+-V+e-)

Features


6/21

is used to develop analysis procedure and estimate residual SM bg.

Monte-Carlo

* QCD jets* Drell-Yan*top quark pairs

*single top* W and Z/* production* Diboson production (WW, WZ, ZZ)* Fragmentation and hadronization

* Underlying event* Parameter tune* Detector simulation

}} PYTHIA, LO-PDF: MRST2007LO*

MC@NLO, NLO-PDF: CTEQ6.6

ALPGEN

HERWIG

JIMMY

ATLAS MC09

GEANT4

}

>

>

>

>

>

>

>


7/21

Electron identification (general)ATLAS, JHEP12(2010)060

starts in the high-granularity liquid-argon sampling electromagnetic (EM)calorimeters. Further, there are three reference sets of requirements:

Loose: uses EM shower shape information and discriminant variables from

hadronic calorimeters

Medium: full information from EM + some from the inner tracking detector (ID)(track quality variables + cluster-track matching variable)

Tight: exploits the full electron identication potential of the ATLAS detector (fullinformation from ID and EM)


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Electron identification (ArXiv:1103.6208)

* pass tight electron selection criteria

* have pT>20GeVand ||


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Muon identification* have pT>20GeVand ||


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Jet identification

* have pT>20GeVand ||


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Common criteria

* identified medium electrons or muons are only considered if they satisfyR>0.4 with respect to the closest remaining jet

* if a jet and a medium electron are both identified within a distance R20GeVbut over the full calorimeter coverage ||


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Signal region

*events that contain a lepton pair of identical or different flavour

*signs of the leptons are opposite

*invariant mass mll>5GeV

*missing ET>100GeVin order to reject SM Z+jets events whilst maintainingeffciency for a range of SUSY models.

*events must also possess at least one reconstructed primary vertex with at leastfive associated tracks (?)


13/21

Flavour subtractionUsing the quantity S defined as follows:

S =N(e+e-)

(1-(1-e)2)

N(+-)

(1-(1-)2)

N(e+-Ve-+)

(1-(1-e)(1-))+ -

*electron plateau trigger efficiency e=(98.51.1)%

*muon plateau trigger efficiency =(83.71.9)%

*the ratio of electron to muon effciency times acceptance =0.690.03

*the value ofS obtained from selected identical-flavour and different-flavourlepton SM events is expected to be small but non-zero, due primarily to Z/*boson production


14/21

Data vs MC

N(e+e-)

(1-(1-e)2)

N(+-)

1-(1-)2

N(e+-Ve-+)

1-(1-e)(1-)+ and

*weighted invariant mass distribution ofe+e- or +- pairs prior to applying themissing ETrequirement

*the distribution for different flavourpairs

*in the region with mll < 100 GeV, thedominant contributions to the differentflavour data events are expected to comefrom tt, QCD and Z/*+jets events-


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e+e- e+- or e-+ -+

Data 4 13 12

Z/*+jets 0.400.46 0.360.20 0.910.67

Diboson 0.300.11 0.360.10 0.610.10

tt 2.501.02 6.612.68 4.711.91Single top 0.130.09 0.760.25 0.670.33

Fakes 0.310.21 -0.150.08 0.010.01

Total SM 3.641.24 8.082.78 6.912.20

Data vs SM background

>

dominated, but

cancel out inS

,but signal-free RMSdominated by stat.fluctuations in numberof tt events

Actually dominated in S

>

> negative is an artifact

Sobs=1.910.15()0.02(e)0.06() SSM=2.060.79(stat.)0.78(sys.)


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Contibution to S from SM*from single top and diboson events are estimated using the MC samples described

above, scaled to the luminosity of the data sample*from Z/*+jets, ttand events containing fake leptons (from QCD jets and W+jetsevents) are estimated using MC samples normalised to data in an appropriatecontrol region

Z/*

*(ET)miss


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Model-independent constrains on Ssignal* adding signal event contributions to the input mean numbers of

background events in each channel

* assumption about the relative branching ratio of new physics events intoidentical flavour and different flavour channels

* new set of signal-plus-background pseudo-experiments

* If Brnew physics(e)=0, thenSsignal


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* mean numbers of signal events added to each channel are sampled according to

the expectations from each point in the parameter space of the model togetherwith the uncertainties in these expectations

* 24 parameter MSSM model: mA=1TeV, =1.5minP(mq,mg), tan=4,At=/tan,Ab=Al=tan. The masses of the 3d generation sfermions are set to 2 TeV, and

common squark mass and slepton mass parameters are assumed for the first two

generations* Two grids in the (mq,mg) plane are considered (MSSM PhenoGrid2):

Model-dependent constrains

compressed spectrum: light neutralino:

m20

=M50GeVm

1

0=M150GeV

mlL=M100GeV

M=min(mq, mg)

m20=M50GeVm

1

0=100GeV

mlL=M/2

M=min(mq,mg)


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For compressed spectrum (light neutralino) models andmg = mq + 10 GeV, the 95% confidence lower limit on mq is 503 (558) GeV


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Conclusion

* a flavour subtraction technique has been used to search for an excessbeyond SM expectations of high missing transverse momentum eventscontaining opposite charge identical flavour lepton pairs

* no signicant excess has been observed, allowing limits to be set on themodel-independent quantity Ssignal, which measures the mean excess from

new physics taking into account flavour-dependent acceptances and

effciencies.

seminar atlas 1103 by grisha kirilin

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