University of AmsterdamFaculty of Science

Nikhef National Institute for Subatomic Physics

SupersymmetryDistinguishing SUSY signals from Standard Model background

Author: Roel Sluijter (0477699)

Bachelorthesis Physics and Astronomy, size 12 EC, research conducted betweenMay 3, 2010 and July 15, 2010

Supervisor: Robin van der Leeuw

Second reader: Marcel Vreeswijk

Handed in on: August 3, 2010

Abstract

In particle physics, the 20th century has evolved around the discoveryof the standard model, but the time has come to look beyond. One theorythat goes beyond is Supersymmetry (SUSY). In this thesis analysis will bedone on searching for Supersymmetric particles, more specificly searchingfor the SU4 benchmark point in minimal SUper GRAvity (mSUGRA) andthe GMSB1 point in Gauge Mediated Supersymmetry Breaking (GMSB).The SU4 point will be examined in the 1-lepton channel. To distinguish theSUSY signals, specific cuts are made on the Standard Model background,and the missing transverse energy and effective mass are analysed.

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Summary

In particle physics, the basis of all fundamental particles is described in theStandard Model of particles (SM). It’s power to describe all observable matter isremarkable, but it still has trouble explaining certain phenomena.

Supersymmetry (SUSY) forms a possible explaination to these phenomenaand might even form a so called Grand Unified Theory (GUT), which combinesall four fundamental forces to one. Supersymmetric theories come in all sortsand kinds, but they all share the same fundamental idea that each elementaryparticle of the SM has a so called Super Partner (SP). A SM particle and it’sSP are identical except for their spin number differing by a half. This meansthat a boson’s SP is a fermion and a fermion’s SP is a boson. The differencebetween bosons and fermions essentially is that they follow different statistics inQuantumdynamic theories.

To prove the existence of Supersymmetry, the Large Hadron Collider (LHC)was built to create SP particles amongst others. One of the detectors that willtry to detect these particles, is the ATLAS detector. The real data from ATLASis not available yet, so the research is done on computer simulations.

The main focus of this thesis is to research the possibility of finding twodistinct SUSY models. They are the SU4 point in minimal SUper GRAvity(mSUGRA) and the GMSB1 point in Gauge Mediated Supersymmetry Breaking(GMSB). The real challenge in this quest, is to filter out collisions in which onlySM particles were created and leave the collisions in which SPs were created. Thisis done by putting demands on certain characteristics of the collisions, which iscalled a ’cut’. One of these demands is to ask for a large missing energy, which iscaused by the fact that a SP is expected to have no interaction with the detector.After these cuts, the SP particles are looked for in a histogram plot of the missingenergy and the effective mass, which are quantities that are characteristic forSUSY events. This eventually leads to the conclusion that, if these SUSY modelstruly exist, the possibility of finding the SU4 point is reasonable. The possibilityof finding the GMSB1 point is more difficult.

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Contents

1 Introduction 4

2 The Standard Model 52.1 Standard Model of particles . . . . . . . . . . . . . . . . . . . . . 52.2 Shortcomings of the Standard Model . . . . . . . . . . . . . . . . 5

3 Supersymmetry 63.1 What is Supersymmetry ? . . . . . . . . . . . . . . . . . . . . . . 63.2 MSSM, R-parity and the lightest SUSY particle . . . . . . . . . . 73.3 Minimal Supergravity and SU4 . . . . . . . . . . . . . . . . . . . 83.4 Gauge Mediated Supersymmetry Breaking and GMSB1 . . . . . . 9

4 The Large Hadron Collider 94.1 Detectors of LHC . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 The ATLAS detector . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Method 125.1 Discriminating signal from background . . . . . . . . . . . . . . . 125.2 Standard Model background . . . . . . . . . . . . . . . . . . . . . 135.3 Monte Carlo simulations . . . . . . . . . . . . . . . . . . . . . . . 14

6 Analysis and results 146.1 Used cuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156.2 SU4 analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166.3 GMSB1 analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

7 Conclusion 21

Acknowledgements 21

Bibliography 22

A Datasets used in this analysis 23

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1 Introduction

The 20th century has been an exciting century for the discovery of new parti-cles. It started with the discovery of the electron by J.J.Thomson in 1897 andeventually lead to the proposition of the quark model by Gell-Mann and Zweigin 1964. The aftermath would lead to a new theory describing all elementaryparticles called ‘The Standard Model of particles’ (SM).

Though the standard model is quite comprehensive, it still has it’s flaws andlacks explanation in certain areas. It doesn’t include gravity and it doesn’t allowneutrino’s to have a nonzero mass, even though experiments suggest they do. Twoother unexplainable phenomena are the ‘hierarchy problem’ and ‘dark matter’.

The hierarchy problem in general lies in the fact that fundamental values,like couplings and masses, don’t have the values scientists would expect. Forexample, the question ‘why is the weak force 1034 times stronger than gravity?’ is one of the questions originating from this problem. A solution needs to befound in order to unify all forces to a single one.

Dark matter is, like the name suggests, matter scientists can’t ‘see’. As-tronomers know it excists, because of it’s gravitational effect on visible matter.Calculations tell us dark matter constitutes ∼90% of the mass in the universe,but what it actually consists of is yet to be discovered.

To explain dark matter and solve the hierarchy problem a variety of theorieshave been proposed, some of which seem quite plausible. They vary from modifiedlaws of gravity to string theory extensions, but the theory many physicists areexcited about is called ‘Supersymmetry’ (SUSY). It forms a solution to darkmatter as well as the hierarchy problem and it leaves an entire new set of particlesyet to be discovered.

In this research analysis I will elaborate on two distinct SUSY models (mSUGRAand GMSB) and investigate their detection possibilities with the ATLAS detectorat the Large Hadron Collider (LHC) in Geneva.

This introduction is followed by an overview of the Standard Model of particlesin the second chapter. The third chapter explains the basics of Supersymmetryand introduces the two supersymmetric models that are the focus of this research.In the fourth chapter, information is given about the LHC and the ATLAS detec-tor. This is followed by an overview of the Standard Model background and themethods used to discriminate it from the SUSY signals in the fifth chapter. Itwill also explain Monte Carlo simulations and give information about the samplesthat were used. The sixth chapter contains the actual research analysis and it’sresults. This is all finalized by the conclusion and a short discussion in the lastchapter.

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2 The Standard Model

2.1 Standard Model of particles

The Standard Model of Particles is a theory that describes elementary particles.It includes three of the four known fundamental interactions: strong interaction,weak interaction and electromagnetism, combining the latter two into the elec-troweak force. Their mediating particles are respectively the gluon (g), the W-and Z-bosons (W±, Z), and the photon (γ). Besides these particles, it describeselementary particles which are subject to these fundamental forces. They are thequarks (up (u), down (d), charm (c), strange (s), top (t) and bottom (b)) andleptons (electron (e), muon (µ), tau (τ), electron-neutrino (νe), muon-neutrino(νµ) and tau-neutrino (ντ )), all catagorized into three generations sorted by mass.In addition, each particle has its own anti-particle with the same mass and oppo-site charge. To make the model complete, a yet to be discovered particle calledthe ‘Higgs-boson’ is included. This particle is the mediater of the Higgs fieldand is needed to explain why particles have mass. These are the particles thatmake up all ‘observable’ matter in the universe. Experiments done from the mid20th century up untill now have proved to be quite consistent with this ‘standardmodel’ [1].

Figure 1: Particles of the Standard Model: showing quarks in green, leptons inblue and mediating bosons in red. The first three columns seperatethe leptons and quarks in three generations by mass. [Picture taken from:

http://www.daviddarling.info/encyclopedia/S/standard model.html].

2.2 Shortcomings of the Standard Model

As mentioned in the introduction, the standard model has it’s shortcomings.They range from the hierarchy problem to massles neutrino’s. To solve them,many physicists strive to find a Grand Unified Theory (GUT) or better yet, a

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Theory Of Everything (TOE). Such a theory should at least be able to unifythe strong force with the electroweak force at high energy scales and includegravity. This is impossible to achieve with the Standard Model, because thegauge couplings of the strong interaction and the electroweak interaction, whichdetermine the strength of the forces, do not converge to a single point at highenergy (see figure 2). To establish unification, the focus should therefore be ontheories ‘Beyond the Standard Model’.

Figure 2: The inverse strenght of the strong force, weak force and electromagneticforce at different energies, as suggested by the SM. The three forces do notintersect eachother in one point and can therefore not be unified. [Picture taken

from: http://www.clab.edc.uoc.gr/materials/pc/proj/running alphas.html]

3 Supersymmetry

3.1 What is Supersymmetry ?

One of the more popular extensions to the Standard Model and a possible GUTis Supersymmetry. There are many varieties in supersymmetric theories, butthey all have the same basic concept. Idealy, supersymmetry says that everyparticle of the SM has its own supersymmetric partner (SP), being exactly thesame except for their spin number differing by a half, making them fermionicand bosonic twins. When Supersymmetry was introduced in the 1970s, it wasimmediately clear that no SM boson could be the superpartner of a SM fermionand the other way around, because they differ to much. Each of them needs tohave his own superpartner.

There is another difficulty, for if the SP-particles are exactly the same astheir SM-partners, except for their spin, how come we haven’t detected any yet?

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Clearly we have not yet been able to reach enough energy in our particle accel-erators to produce them. This means the SP-particles have a larger mass thantheir SM-partners and therefore supersymmetry has to be a broken symmetry.

With the introduction of supersymmetry, a new set of terminology comesalong. The SP’s of fermions are called sfermions and the SP’s of charged bosonsare called bosino’s. In general the fermion SP’s always start with an ‘s’ (e.g.squark) and the boson SP’s end with ‘ino’ (e.g. neutralino). In addition a newset of abbreviations is used. Neutralino’s for example are denoted by χ0

1,..,χ04,

from lightest to heaviest respectively. Table 1 gives an overview of SM particlesand their superpartners.

SM particle: Superpartner:

neutral boson neutralino (χ01,..,χ

04)

charged boson bosino (χ±1 , χ±2 )

W- and Z-boson wino (W±), zino (Z)

Higgs boson Higgsino (H)gluon gluino (g)

graviton gravitino (G)lepton slepton (e, µ, τ , νe, νµ, ντ )

quark squark (u, d, c, s, t, b)

Table 1: Names and abbreviations of SP particles.

3.2 MSSM, R-parity and the lightest SUSY particle

A theory that realizes supersymmetry in the most minimalistic way was originallyproposed in 1981 and is called the ‘Minimal Supersymmetric Standard Model’(MSSM). Even in this minimalistic form, it constitutes a parameter space of morethan 100 entries, leading to a large amount of possible models. [2]

Just like the Baryon number (B), Lepton number (L) and Spin number (S) ofparticles in the Standard Model, a new discrete quantum number R, known asR-parity, can be defined in SUSY as:

R ≡ (−1)3(B−L)+s

In short it means all SM particles have R = +1 and all SP particles have R= -1 [3]. In a supersymmetric model that conserves R-parity, extra constraintsare put on decay products. When a SP particle decays, it needs to produce atleast another SP particle to conserve R-parity. The end of the decay chain willtherefore show a stable Lightest Supersymmetric Particle (LSP).

It is expected that in the proton-proton collisions at LHC initially gluinosand squarks will be created. Their decay products will result in stable LSP’s.

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In general LSP’s are difficult to detect, just like SM neutrinos. This fact, incombination with their relatively large mass, makes them perfect candidates fordark matter.

Figure 3: Typical decay chains in SP particle events. A gluino (g) and a down squark(d) decay into a neutralino (χ0

1,χ02) or a bosino (χ±1 ), which will eventually

decay to a LSP [4].

The focus of this research will be on two specific SUSY models: Minimal SU-per GRAvity (mSUGRA) and Gauge Mediated Supersymmetry Breaking (GMSB).

3.3 Minimal Supergravity and SU4

mSUGRA is a theory that combines the principles of MSSM with General rela-tivity. This way the theory is constrained even more and only 5 undeterminedparamaters are left to choose from. The parameters are:

• m0: the common scalar mass. It is the mass of all scalar particles beforeSUSY breaking.

• m 12: the common gaugino mass. It is the mass of all gauge particles before

SUSY breaking.

• A0: the common trilinear coupling. It determines the scalar couplings ofthe squarks and sleptons.

• tan(β): the ratio of the vacuum expectation values of the Higgs doublets.

• sign(µ): the sign of SUSY conserving Higgs mass.

For every point in this parameter space, the expected mass of the SP particlescan be calculated. The benchmark point SU4 (m0 = 200 GeV, m 1

2= 160 GeV,

A0 = -400, tan(β) = 10 and sign(µ) = +) is especially interesting, because itexpects relatively low mass SP particles, e.g. the gravitino is expected to have a

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mass of ∼400 GeV and the LSP a mass of ∼60 GeV. In mSUGRA the LSP is thelightest neutralino (χ0

1) [5]. At a center-of-mass energy of 7 TeV, SU4 will have across-section (σ) of 41.52 pb. This cross-section is a measure for the probabilitythat an event occurs in which a SU4 particle was created. SU4 will form the mainpoint of focus in this research thesis.

3.4 Gauge Mediated Supersymmetry Breaking and GMSB1

Another SUSY theory is Gauge mediated supersymmetry breaking (GMSB).GMSB is a theory that realises Supersymmetry breaking through standard modelgauge interactions. In this theory the LSP is the Graviton (G). It leaves 6 un-specified parameters, which are: Λ = 90 TeV, Mm = 500 TeV, N5 = 1, tan(β) =5, sign(µ) and Cgrav = 1 for the benchmark point GMSB1. At a center-of-massenergy of 7 TeV, GMSB1 has a cross-section of 0.6186 pb. GMSB1 will be anotherpoint of focus.

Figure 4: Typical GMSB decay chain of a gluino (g) or squark (q) decaying into neu-tralinos (χ0

1,χ02) and a slepton (l), which eventually decays to the Graviton

(G) [6].

4 The Large Hadron Collider

The apparent way of proving the existence of supersymmetry, would be by pro-ducing a SP particle and detecting it. This is one of the reasons why the LargeHadron Collider (LHC) was built. The LHC is a proton-proton collider locatedat the French-Swiss border close to Geneva. It was constructed by the EuropeanOrganization for Nuclear Research (CERN) inside the already existing tunnelformerly used by the Large Electron Positron Collider (LEP), and has a circum-ference of 27 kilometers. It’s original purpose is to accelerate protons in oppositedirections and make them collide with a maximum center-of-mass energy of 14TeV. This energy hasn’t been reached before and makes the potential of finding

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new particles and ‘new physics’ very high. Another feature of the LHC thatmakes it a high potential particle finder, is it’s high Luminosity (L ), which is ameasure of the rate at which particles collide.

On September 10th 2008, LHC started its first ‘testflight’ and a proton-beamwas succesfully circulated through the tunnel. After several drawbacks and overa year of damage repairments, LHC is currently producing collisions at 7 TeV.

4.1 Detectors of LHC

Of course the production of new particles is useless if they can’t be detected.For this task a total of seven different experiments have been constructed. Theyinclude two multi purpose detectors, ATLAS and CMS, two large specializeddetectors, LHCb and ALICE, and three smaller specialized detectors, TOTEM,LHCf and MoEDAL. They are located in and around the tunnel.

Figure 5: A schematic drawing of LHC and the positions of it’s four largestdetectors: ATLAS, CMS, ALICE and LHCb. [Picture taken from:

http://www.phy.duke.edu/ seog/lhcv3/].

LHCb is designed to study CP-violation in the B meson system. ALICE isoptimized to study heavy ion collisions in a later stage, when lead ions are accel-erated instead of protons. It is expected to reach a large enough temperature andenergy density to generate a quark-muon plasma. TOTEM is specially designedto study the total proton-proton cross section and is located at the same inter-section point as CMS. LHCf is used to study aspects of particle emissions alongthe beampipe and is located at the same intersection point as ATLAS. Finallythe MoEDAL is constructed to find massive stable particles, like the magneticmonopole, and it is located near LHCb. In figure 5 you can see an overview ofthe LHC and it’s four largest detectors: ATLAS, CMS, ALICE and LHCb.

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4.2 The ATLAS detector

ATLAS is the biggest LHC detector, with a length of 45 meters, a diameter of 25meters and a weight of 7000 tonnes. The detector components consist of multipleconcentric cylindrical layers situated around the interaction point as can be seenin figure 6.

Figure 6: The ATLAS detector stripped in layers.

The innermost layer is called the inner detector and it serves to track chargedparticles. When a particle crosses the inner detector, it will interact with it’smaterial (sillicon in the inner part and gas in the outer part), causing smallionizations that leave a detectable trail.

Immediately around the inner detector, a solenoid magnet system is used toproduce a magnetic field of 2 Tesla inside the inner detector. It will cause chargedparticles to have a bend track, which can be used to determine their momentum.

When particles leave the inner detector, they will encounter calorimeters usedto measure the particles energy by absorbing them.

The outer part of ATLAS consists of muon detectors. It functions similarlyto the inner detector, but specializes in tracking muons [4].

Even though the ATLAS detector is not solely designed to find Supersymme-try, it’s components should be well suited for the task.

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5 Method

When LHC is running at high luminosity, a huge amount of particles will becreated. If SUSY truly exists, only a minuscule fraction of these particles willactually be SP particles. When an event occurs in the ATLAS detector, all kindsof details will be measured, like the amount of particles detected, their momen-tum, their charge, etc. This way many particles can be identified immediately,but for a SP particle it isn’t that simple, because it doesn’t interact with thedetector and can’t be measured directly. However, events in which a SP particleis created have certain special characteristics that form a so called ‘signature’ ofa SUSY event. By looking at events that satisfy this signature, the SP particlecan be measured indirectly. The difficulty is that certain events, in which onlySM particles are created (SM background), resemble this SUSY signature. Thismakes measurement more difficult. The main goal of this research is to exam-ine ways to dispose of the SM background events in favour of the SUSY signal.Criteria for which an event is kept or discarded is called a ‘cut’.

5.1 Discriminating signal from background

To filter the SM background from the SUSY signal, certain quantities and vari-ables can be used to discriminate. The most significant variable in the SUSYsignature is the Missing Transverse Energy (Emiss

T ). When protons are accel-erated inside the LHC, they only gain momentum in the direction along thebeam-axis. Because of conservation of momentum, the total momentum in thetransverse direction (direction perpendicular to the beam-axis) should be zero be-fore and after the collision. If the detector measures otherwise, you have missingtransverse energy.

This EmissT can be classified into fake and real Emiss

T . Fake EmissT could be

caused by mis-measurements in the detector or by particles not produced inthe collision, such as cosmic rays. The real Emiss

T however, is caused by anundetectable particle, usually a neutrino. A LSP is also not detectable by ATLASand due to it’s big mass it will contribute significantly to the Emiss

T and is thereforean important distinguishing variable.

Another feature of the SUSY signature, is the high amount of jets. A jet isa narrow cone of hadrons and other particles, formed when a parton (gluon orsquark) is created in a collision. Because of QCD-confinement, a parton can notexist in free form and it therefore immediately fragments into hadrons resultingin a trail of particles. Gluinos and squarks are heavy particles and will usuallyhave long decay chains resulting in events with many high momentum jets andleptons.

To benefit from this, a quantity called the Effective Mass is used. It is definedas:

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Meff =4∑i=1

pjetTi + EmissT

In this equation,∑4i=1 p

jetTi

is the sum of the transverse momenta of the fourmost energetic jets. For the SU4 signal, one of our demands is that an eventhas exactly 1 lepton (1-lepton channel). In this case, the leptons transversemomentum pleptonT is added to the Meff .

Another relevant quantity is the Transverse Mass, defined as:

MT =√

2(EmissT · pleptonT − Emiss

x · pleptonx − Emissy · pleptony )

This is the mass calculated for the lepton and the missing energy in thetransverse plane. For SM background events, mis-measurements of jet or leptonenergies can cause an imbalance in the Emiss

T . In this case the lepton energyis more alligned with Emiss

T and will result in a low MT . SUSY events on theother hand are characterized by a large MT , so a cut on the minimum MT canbe rewarding [7].

5.2 Standard Model background

The most important SM background for SUSY-signals can be devided into fourcatagories: TTbar, W+jets, Z+jets and QCD-jets.

TTbar includes all events in which a top-antitop pair is produced (pp→ tt).It is the most important background due to the large mass of the top quark,which will lead to high energetic jets and leptons in the decay chain.

Figure 7: Possible decay chain of a top quark (t). A top quark decays to a bottomquark and a W+ boson. The W+ boson then decays to a lepton and aneutrino. This leads to high energetic jets, leptons and missing transverseenergy [4].

W+jets and Z+jets include all events in which initially a W or Z boson isproduced respectively. These kind of events also form a big part of the back-ground, but will typically be less energetic than TTbar, making them easier todistinguish.

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Finally, QCD-jets are events in which a number of high energetic jets arecreated. This category has the highest cross-section and will initially show themost events.

The general reason why these are considered important backgounds, is thateach of them is likely to produce events with high energetic leptons, jets andEmissT .

5.3 Monte Carlo simulations

At this moment the LHC is running at√s = 7 TeV and ATLAS is doing its

job collecting data, but not enough data is ready for analysis yet. Therefore thisanalysis will be done by using Monte Carlo simulations. These are computergenerated simulations made according to theoretical input.

Analyzing simulated events is important for the optimization of reconstructingalgorithms and other analyses. This can later be used for analyses on the realdata, once it’s ready.

To make the simulations, special event generators are used. Some genera-tors are more specialized in certain events, so choosing the right one could beimportant. In this research, the used generators are: Pythia for TTbar andQCD-jets, Alpgen/Jimmy for W + jets and Z + jets, Herwig for SU4 and Jimmyfor GMSB1. This simulated data was turned into flat ntuples by D3PDmaker forSUSY research.

The samples used in this research were generated for a center-of-mass energy√s = 7 TeV. To make the samples comparable to each other, they have all been

individually scaled to an integrated luminosity (L ) of 1 fb−1. This is done bymultiplying the amount of events by a scaling factor (α) defined by:

α =σ ·LN

Here N is the amount of events in the specific data sample and σ ·L is theamount of events expected for 1 fb−1.

Information about the original datasets can be found in appendix A.

6 Analysis and results

In order to analyse the large amount of data from the LHC, a software frameworkcalled Root was developed by CERN. Root works by running a so called ‘macro’,which contains information about the operations that need to be processed. Thesemacro’s can be written and customized by the researcher in C++. It was used inthe now following section to make cuts on the data and make graphs, histogramsand tables needed for analysis.

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6.1 Used cuts

To filter SM background from a SUSY signal, a number of cuts can be made. Thecut criteria are based on the amount of SUSY signal loss compared to the SMbackground loss. If the loss in SUSY events is kept to a minimum, the cut canbe justified. Using this principal, the following cuts were found to be effective:

• At least four jets with pT > 40 GeV for the most energetic jet and pT > 30GeV for the other three jets.

• A EmissT of at least 30 GeV .

• At least a 20% contribution of EmissT to the Meff .

• A MT of at least 100 GeV.

For the SU4 signal analysis, an additional demand for exactly one lepton withpleptonT > 20 GeV is made. This is not a cut, but merely a channel choice thatinfluences the other cuts. To get an idea of the influence of such a choice, theseperate contributions in each lepton channel can be seen in figure 8.

Number of leptons0 1 2 3 4 5

Eve

nts

310

410

510

610

710

810

910

1010

1110

1210SU4

TTbar

W+Jets

Z+JetsQCD-Jets

Figure 8: Contributions of SM background and SUSY signal seperated according tolepton channel on a logarithmic scale. In the 1-lepton channel, a relativelysmaller amount of QCD-jets is present than in the 0-lepton channel.

In certain events, the measurement of particles could become unreliable. Thishappens for example when two jets are too close to each other or when particlesmove too close to the beampipe. These are only a few events and are removedby Overlap Removal (OR) and Object PreSelection (OPS). Additional cuts aremade on the transverse sphericity (TS) (because SUSY events tend to be relativelyspherical) and the angle between the leading jets and the Emiss

T (dRPhiMin) [5].

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6.2 SU4 analysis

First we take a look at SU4. Before any cuts are made, the SU4 signal is burriedunder the gigantic amount of SM background. In figures 9(a) and 9(b) can beseen what the seperate contributions are to the Meff and Emiss

T . It shows theamount of events left after OPS, OR and a demand for at least 4 jets and alepton, which is needed to calculate the Meff .

Effective Mass [GeV]0 200 400 600 800 1000 1200 1400 1600 1800 2000

Eve

nts

/40

GeV

210

310

410

510

610

710

810SU4

TTbarW+JetsZ+JetsQCD-JetsStandard Model

(a) Meff

Missing transverse energy [GeV]0 200 400 600 800 1000

Eve

nts

/40

GeV

210

310

410

510

610

710

810SU4

TTbarW+JetsZ+JetsQCD-JetsStandard Model

(b) EmissT

Figure 9: The amount of SU4 signal and SM background events sorted by EmissT andMeff on a logarithmic scale (after OPS and OR). Each event has at least4 jets and a lepton. As expected QCD-jet dominates the background.

A distinction can be seen between the seperate distributions. The peak ofSU4 clearly lies in a higher Emiss

T and Meff region and the general course of thespectra are different. When looking at real data however, these contributionswould all be stacked and SU4 would not be noticeable.

When applying the ‘regular’ cuts mentioned in the above section, the SU4signal becomes clearer. This can be seen in figures 10(a) and 10(b). The SU4signal now dominates in the higher regions.

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Effective mass [GeV]0 200 400 600 800 1000 1200 1400

Eve

nts

/40

GeV

1

10

210

SU4TTbarW+JetsZ+JetsQCD-Jets

Standard Model

(a) Meff

Missing transverse energy [GeV]0 100 200 300 400 500 600 700

Eve

nts

/40

GeV

1

10

210SU4TTbarW+JetsZ+JetsQCD-Jets

Standard Model

(b) EmissT

Figure 10: SU4 and SM background after cuts. In the higher regions (Meff > 600GeV and EmissT > 200 GeV), the SUSY signal dominates.

To get an idea of each individual cut’s efficiency, table 2 shows the amount ofevents left after each cut, which is called the cutflow.

Kind of cut: SU4 TTbar W+jets Z+jets QCD-jets

No cut 47733 145810 8.9405e+06 958493 7.21377e+11OPS & OR 47238 142609 8.84474e+06 897629 7.21098e+11

#leptons = 1, P leptonT > 20 GeV 7734 65710 1.27891e+06 261166 1.15824e+08

At least 4 jets 5109 39628 24004 3064 710416Jet PT > 40, 30, 30, 30 GeV 3885 23025 6832 941 105444EmissT > 30 GeV 3695 18026 5008 282 5416

EmissT > 0.2 · Meff 2063 4688 1109 44 64

TS cut 1653 4108 861 35 62dRPhiMin cut 1533 3845 801 27 0MT > 100 GeV 665 333 27 2 0

Table 2: Cutflow of SU4. The left collumn shows the kind of cut, followed by theamount of events left of the SU4 signal and the seperate SM backgroundcontributions.

As expected, the largest background after the cuts is TTbar. The demandfor at least 4 high energetic jets, cuts away many events for W+jets, Z+jets andQCD-jets, but is slightly disappointing for TTbar. Nevertheless the differencebetween SU4 and background could be called significant.

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6.3 GMSB1 analysis

Finding GMSB1 is a completely different challenge. The cross-section of GMSB1at 7 TeV is only 0.61861 pb and as can be seen in figures 11(a) and 11(b) itresults in alot fewer events than SU4.

Effective mass [GeV]0 200 400 600 800 1000 1200 1400 1600 1800 2000

Eve

nts

/40

GeV

-110

1

10

210

310

410

510

610

710

810GMSB1

TTbarW+JetsZ+JetsQCD-JetsStandard Model

(a) Meff

Missing transverse energy [GeV]0 200 400 600 800 1000

Eve

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/40

GeV

-110

1

10

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410

510

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910

GMSB1

TTbarW+JetsZ+JetsQCD-JetsStandard Model

(b) EmissT

Figure 11: GMSB1 and SM background after OPS, OR and a demand for 4 jets.The GMSB1 signal shows very few events compared to the SU4 signaldiscussed in the previous section.

This asks for a slightly different approach. To keep enough signal events, thefocus is no longer on the 1-lepton channel. However, the 0-lepton channel eventswill still be discarded, because of the cut on the MT . When the regular cuts aremade, the GMSB1 signal is still not significantly distinguishable. This can beseen in figures 12(a) and 12(b).

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Effective mass [GeV]0 200 400 600 800 1000 1200 1400 1600 1800 2000

Eve

nts

/100

GeV

-110

1

10

210 GMSB1

TTbar

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(a) Meff

Missing transverse energy [GeV]0 200 400 600 800 1000

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GeV

1

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GMSB1

TTbar

W+Jets

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(b) EmissT

Figure 12: GMSB1 and SM background after regular cuts. The GMSB1 signal is ofthe same order as the SM background.

According to GMSB theory, signal events are likely to produce 2 or morephotons with PT > 15 GeV [6]. This invites us to look at an additional cut onthe amount of photons. Figure 13 shows the amount of events in each photonchannel.

Number of photons0 1 2 3 4 5 6 7 8 9

Eve

nts

-110

1

10

210

310

410

510

610

GMSB1

TTbar

W+Jets

Z+JetsQCD-Jets

Figure 13: Contributions of SM background and SUSY signal in each photon channel.In the region with one or less photons, the amount of SM background ishuge, whereas the amount of SUSY signal is relatively small.

Most GMSB1 signal events indeed show 2 or more photons with pT > 15 GeVas opposed to the SM background which dominates in the 0-photon channel. Ademand for 2 or more photons with pT > 15 GeV is a justified cut.

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Effective mass [GeV]0 200 400 600 800 1000 1200 1400 1600 1800 2000

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(a) Meff

Missing transverse energy [GeV]0 100 200 300 400 500 600 700 800 900

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(b) EmissT

Figure 14: GMSB1 and SM background after a demand of two or more photons inaddition to the regular cuts. Only TTbar background is left. The GMSB1signal is barely distinguishable and it’s amount of events is very small.

Figures 14(a) and 14(b) show the Meff and EmissT after the extra photon cut.

The cutflow can be seen in table 3.Just like the SU4 signal analysis, the most dominant background is TTBar.

The GSMB1 signal looks slightly distinguishable, but shows very few events at 1fb−1. If the integrated luminosity could be increased, the difference between SU4signal and SM background could become significant.

Cut# GMSB1 TTbar W+jets Z+jets QCD-jets

No cut 618 145810 8.9405e+06 958493 7.21377e+11OPS & OR 594 142609 8.84474e+06 897629 7.21098e+11

#photons > 1, P photonT > 15 GeV 514 13939 28868 16072 2.12407e+08

At least 4 jets 387 11181 5825 2153 3.07005e+07Jet PT > 40, 30, 30, 30 GeV 326 8744 2788 1130 1.27679e+07EmissT > 30 GeV 307 7189 2112 651 705899

EmissT > 0.2 · Meff 111 2242 597 263 4667

TS cut 94 1859 442 197 3354dRPhiMin cut 85 1736 400 175 335MT > 100 GeV 20 58 0 0 0

Table 3: Cutflow of GMSB1. Behind each cut is shown how many events are left ofthe GMSB1 signal and the seperate SM background contributions.

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7 Conclusion

According to the analysis on the simulated events presented in this thesis, makingspecific cuts on SM background will increase the possibility of finding the SU4mSUGRA point signifcantly. The discovery potential of SU4, if it exists, with theATLAS detector at 7 TeV and 1 fb−1 of data will therefore be reasonable. Findingthe benchmark point GMSB1 at 7 TeV and 1 fb−1 is less likely, partly becauseof it’s small cross-section. A solution would be getting a higher center-of-massenergy or higher integrated luminosity.

Discussion

The goal of this research was to investigate methods to distinguish SUSY signalsfrom SM background. An important question however, is how reliable the out-come is when only simulated data is used. Simulated data for SM backgroundcould become unreliable when there are too many uncertainties. Of course forthe SUSY signal there is no alternative to simulated data. To get a more reli-able analysis, further studies should be complemented by real data of the SMbackgrounds.

In general, the biggest problem of finding SUSY particles in LHC, is the vastamount of possible SUSY models. In this thesis only two such models wereinvestigated, which were chosen for their high discovery potentials. Doing thesame research for all other models would become a tiresome task. A bettersolution would be to find new techniques that involve a bigger part of the SUSYparameter space. This way, discovery probabilities could be increased.

Acknowledgements

This thesis would not have been possible without the help of my supervisor Robinvan der Leeuw, who guided me during the entire course of this bachelor projectand helped me whenever he could. I would also like to thank Marcel Vreeswijkfor coordinating the project and his overall support and advice.

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References

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[2] J. S. Gainer. MSSM Dark Matter Without Prejudice. AIP Conf. Proc.,1200:1015–1018 (2010). doi:10.1063/1.3327526. 0910.1375.

[3] M. Gozdz, W. A. Kaminski and A. Wodecki. Minimal supersymmet-ric standard model with gauge mediated supersymmetry breaking andneutrinoless double beta decay. Phys. Rev., C69:025501 (2004). doi:10.1103/PhysRevC.69.025501. hep-ph/0305123.

[4] S. van Til. Scalar top pair production ATLAS. Master’s thesis, Universityof Amsterdam, the Netherlands (2007).

[5] G. Aad et al. (The ATLAS). Expected Performance of the ATLAS Experi-ment - Detector, Trigger and Physics (2009). 0901.0512.

[6] M. Terwort. Searches for gmsb at the lhc. Technical Report ATL-PHYS-CONF-2008-017. ATL-COM-PHYS-2008-075, CERN, Geneva (2008).

[7] V. M. Abazov et al. (D0). Search for supersymmetry via associated pro-duction of charginos and neutralinos in final states with three leptons.Phys. Rev. Lett., 95:151805 (2005). doi:10.1103/PhysRevLett.95.151805.hep-ex/0504032.

[8] R. Barbier et al. R-parity violating supersymmetry. Phys. Rept., 420:1–202(2005). doi:10.1016/j.physrep.2005.08.006. hep-ph/0406039.

[9] N. Ozturk. Search for supersymmetry signatures at the lhc. Technical ReportarXiv:0910.2964 (2009). Comments: To be published in the proceedings ofDPF-2009, Detroit, MI, July 2009, eConf C090726.

[10] S. Dawson. SUSY and such. NATO Adv. Study Inst. Ser. B Phys., 365:33–80(1997). hep-ph/9612229.

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A Datasets used in this analysis

Table 4Process DatasetID Generator Dataset Name σ [pb] L [fb−1]

SU4 106484 HERWIG mc09 7TeV.106484.SU4 herwigpp susy.merge.AOD.e542 s765 s767 r1250 r1260.V 1.1 41.52 1.046GMSB1 105410 JIMMY mc09 7TeV.105410.GMSB1 jimmy susy.merge.AOD.e530 s765 s767 r1250 r1260.V 1.1 6.1861E-01 16.153Top 105861 PYTHIA mc09 7TeV.105861.TTbar PowHeg Pythia.merge.AOD.e521 s765 s767 r1250 r1260.000402.V 1 1.4581E+02 1.3705Weν 107681 ALPGEN+JIMMYmc 09 7TeV.107681.AlpgenJimmyWenuNp1 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 1293 0.19212Weν 107682 ALPGEN+JIMMYmc mc09 7TeV.107682.AlpgenJimmyWenuNp2 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 377.1 0.50091Weν 107683 ALPGEN+JIMMYmc mc09 7TeV.107683.AlpgenJimmyWenuNp3 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 100.9 0.50026Weν 107684 ALPGEN+JIMMYmc mc09 7TeV.107684.AlpgenJimmyWenuNp4 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 25.3 0.51347Weν 107685 ALPGEN+JIMMYmc mc09 7TeV.107685.AlpgenJimmyWenuNp5 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 6.9 0.49985Wµν 107693 ALPGEN+JIMMYmc mc09 7TeV.107693.AlpgenJimmyWmunuNp3 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 101.1 0.50333Wµν 107694 ALPGEN+JIMMYmc mc09 7TeV.107694.AlpgenJimmyWmunuNp4 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 25.7 0.49978Wµν 107695 ALPGEN+JIMMYmc mc09 7TeV.107695.AlpgenJimmyWmunuNp5 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 7.13 0.49060Wτν 107700 ALPGEN+JIMMYmc mc09 7TeV.107700.AlpgenJimmyWtaunuNp0 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 6835.8 0.19683Wτν 107703 ALPGEN+JIMMYmc mc09 7TeV.107703.AlpgenJimmyWtaunuNp3 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 100.8 0.50071Wτν 107704 ALPGEN+JIMMYmc mc09 7TeV.107704.AlpgenJimmyWtaunuNp4 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 25.7 0.50568Wτν 107705 ALPGEN+JIMMYmc mc09 7TeV.107705.AlpgenJimmyWtaunuNp5 pt20.merge.AOD.e511 s765 s767 r1250 r1260.V 1.1 7.00 0.57114QCD J1 105010 PYTHIA mc09 7TeV.105010.J1 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 1 6.7803E+08 2.0579e-06QCD J2 105011 PYTHIA mc09 7TeV.105011.J2 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 1 4.0979E+07 3.4116e-05QCD J3 105012 PYTHIA mc09 7TeV.105012.J3 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 2 2.1960E+06 0.00061358QCD J4 105013 PYTHIA mc09 7TeV.105013.J4 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 1 8.7701E+04 0.015933QCD J5 105014 PYTHIA mc09 7TeV.105014.J5 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 2 2.3483E+03 0.59175QCD J6 105015 PYTHIA mc09 7TeV.105015.J6 pythia jetjet.merge.AOD.e468 s766 s767 r1251 r1260.000402.V 3 3.3608E+1 11.495Z→ νν 107713 ALPGEN+JIMMYmc mc09 7TeV.107713.AlpgenJimmyZnunuNp3 pt20 filt1jet.recon.AOD.e530 s765 s767 r1250.000402.V 1 60.1581 0.19939Z→ νν 107714 ALPGEN+JIMMYmc mc09 7TeV.107714.AlpgenJimmyZnunuNp4 pt20 filt1jet.recon.AOD.e530 s765 s767 r1250.000402.V 1 15.866 0.50378Z→ νν 107715 ALPGEN+JIMMYmc mc09 7TeV.107715.AlpgenJimmyZnunuNp5 pt20 filt1jet.recon.AOD.e530 s765 s767 r1250.000402.V 1 4.3781 0.57102Z→ee 107650 ALPGEN+JIMMYmc mc09 7TeV.107650.AlpgenJimmyZeeNp0 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.14 659.583 0.46122Z→ee 107651 ALPGEN+JIMMYmc mc09 7TeV.107651.AlpgenJimmyZeeNp1 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.14 132.462 0.47892Z→ee 107652 ALPGEN+JIMMYmc mc09 7TeV.107652.AlpgenJimmyZeeNp2 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.5 41.3555 0.47144Z→ee 107653 ALPGEN+JIMMYmc mc09 7TeV.107653.AlpgenJimmyZeeNp3 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.2 10.7899 0.50964Z→ee 107654 ALPGEN+JIMMYmc mc09 7TeV.107654.AlpgenJimmyZeeNp4 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.2 3.12818 0.47919Z→ee 107655 ALPGEN+JIMMYmc mc09 7TeV.107655.AlpgenJimmyZeeNp5 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.3 0.753425 0.66363Z→ µµ 107663 ALPGEN+JIMMYmc mc09 7TeV.107663.AlpgenJimmyZmumuNp3 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.2 11.2173 0.49004Z→ µµ 107664 ALPGEN+JIMMYmc mc09 7TeV.107664.AlpgenJimmyZmumuNp4 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.1 2.83234 0.52924Z→ µµ 107665 ALPGEN+JIMMYmc mc09 7TeV.107665.AlpgenJimmyZmumuNp5 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.9 0.756621 0.65951Z→ ττ 107673 ALPGEN+JIMMYmc mc09 7TeV.107673.AlpgenJimmyZtautauNp3 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.9 11.5552 0.47571Z→ ττ 107674 ALPGEN+JIMMYmc mc097TeV.107674.AlpgenJimmyZtautauNp4 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.1 2.87886 0.52069Z→ ττ 107675 ALPGEN+JIMMYmc mc09 7TeV.107675.AlpgenJimmyZtautauNp5 pt20.merge.AOD.e529 s765 s767 r1250 r1260.V 1.1 0.780538 0.63930

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