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Monte-Carlo Simulation and Data Analysis using the ATLAS Detector at the CERN-LHC (Large Hadron Collider) Drew Miles, Cody Haycraft, Jovan Andjelich, Sean Dockery, Benjamin Draper, Dr. Akhtar Mahmood, and Dr. Syed Ahmad, Dept. of Physics, Bellarmine University

ABSTRACTThe international high profile LHC (Large Hadron Collider) experiment at CERN (now known as EuropeanCenter for Particle Research), located in Geneva, Switzerland is searching for new subatomic particles todecipher the cosmic ingredients that created the Universe some 13.7 billion years ago (see Fig.1). The energyin these collisions is similar to what was present in the universe just after the Big Bang, when the universe wasless than a billionth of a second old. ATLAS (A Torridal LHC ApparatuS) experiment at the LHC records theresults of these intense collisions and measures the properties of these subatomic particles with micronprecision. At Bellarmine University, using CERN‟s Atlantis and Hypatia software package, we have carried outMonte-Carlo simulation studies and data analyses tasks using the ATLAS simulation and collision data to studythe particle tracks and decay patterns of the Higgs boson, including the identification of the W and Z bosonsand the top quark. We are focusing on filtering methodologies and pattern recognition techniques for the Higgssearch in a number of decay modes. Using BU‟s ATLAS Tier3 Supercomputer, we have conducted eventreconstruction on selected sets of ATLAS monte-carlo data to study the events via the reconstruction of decayvertices of various particles, including leptons and their interaction with the Weak force carriers, the W and Zbosons and studying very energetic quark and gluon jets. Since these quarks and gluons combine to formother exotic particles, the ATLAS detector is able to observe the final state particles - electrons, muons,photons, pions, charged kaons, and protons (see Fig. 18 -19). We carried out the monte-carlo simulationtasks by studying the various detector parameters - focusing on data reduction and filtering methodologies andpattern recognition techniques hoping to find “rare signals” of the Higgs boson in specific channels from theoverwhelming backgrounds.

INTRODUCTION AND METHODOLOGYIt‟s the dawn of an exciting age of new discovery in physics! The LHC is embarking on a new round ofexploration at the frontier of high energies. The 27 km LHC accelerator, which lie 100m below ground (see Fig.4) was built at a cost of $10B, and took over 15 years to complete. At the LHC (see Fig 2), four giant particledetectors measure and record the properties of subatomic particles from the intense head-on collision of protonat 7 TeV. ATLAS, the largest of the LHC detectors (see Fig. 16) weighs 7000 tons and is 44 m long, 25 m highand 25 m wide. Figures 8-15 shows pictures of the ATLAS detector taken during the construction phase.Among other particles, the LHC has been designed and built to find evidence of the Higgs boson. Just like theelectromagnetic and gravitational fields, the Higgs field permeates all space. Photons are massless becausethey do not interact with the Higgs field, while W and Z do interact with the Higgs field and thereby acquire theirlarge masses (see Fig 20).

The neutral Z boson, and the electrically charged W+ and W- bosons are all mediators of the weak force, likefor instance the photon is the mediator of the electromagnetic force. So what is the Z boson good for? Well, weknow that neutrinos interact among themselves, and without the Z boson this would be impossible! Sinceneutrinos do not have electric charge they cannot self-interact via a photon, which would be the only otheroption. In fact, the Z boson is closely related to the photon. It turns out that all electromagnetic interactionsproceed through photons. Because the photon has no mass, it can travel infinite distances and two electriccharges can feel each other even at very large distances. The Z boson, on the other hand, is very heavy andhas a very short lifetime and, therefore, travels only a very tiny distance. This is the reason why, contrary to thecommon light (made of photons), you don‟t see a “light” of Z bosons. Although we do not notice the Z boson inour everyday-life, in extreme conditions of the early Universe and of supernovae explosions in galaxies, the Zboson is a “daily” particle, just like photon s for us. Well, at high enough energi es, the photon and the Z bosonsare closely connected. Hadn‟t the Z boson had the mass it does, the photon would probably not be masslessand free to travel were it would want and light would not be! The W  bosons, on the other hand are working hardconstantly inside the Sun and other stars, by allowing the conversion of Hydrogen to Helium, a process knownas fusion. As a by-product of this fusion process that can only happen due the presence of the W+ and W- bosons, the carrier of the weak force, do we get sunlight, that‟s all photons. 

Although 30 years of precision tests of the Standard Model (with enormous comparisons between data andtheory) have resulted in a large number of successful discoveries and more than a dozen Nobel Prizes inphysics, current theories in particle physics indicate that answers to some of the most profound questions maylie in energies at the TeV region at the LHC. Our current understanding of the Standard Model that provides thebest description of the subatomic (quantum) world is incomplete. An essential ingredient of the StandardModel, the Higgs boson – the key to the origin of particle mass, has yet to be found. The “electroweakunification” requires that the electroweak force -carrying particles (W and Z0 bosons) be massless. Butexperiments show that W and Z0 bosons have mass and are heavy. So the quest ion is why are W and Zbosons, the carriers of the weak f orce, so heavy, while photons, the carrier of the electromagnetic force,massless? In the 1960s, theorists Higgs, Brout and Englert devised a clever solution to solve this conundrum.They suggested that all particles had no mass just after the Big Bang. As the Universe cooled and thetemperature fell below a critical value, a force-field called the “Higgs field ” was formed together with theassociated “Higgs boson .” The field prevailed throughout the cosmos: any particles that interacted with itacquired a mass via the Higgs boson. The more they interacted, the heavier they became, whereas particlesthat never interacted remained massless. Although this idea provided a satisfactory solution, no one has everobserved the Higgs boson in an experiment to confirm the theory. Therefore our understanding of the origin ofthe electroweak symmetry breaking is still incomplete. Finding the Higgs would give an insight into why eachparticle has a certain mass. The Standard Model cannot explain why any particle has a certain mass. Theexistence of a Higgs boson is a crucial prediction of the Standard model and so its identification is currently ahigh priority in physics. If the Higgs does exist, it must be observable at the TeV scale energy at the LHC. It isanticipated that LHC will make the next generation of breakthrough discoveries at the TeV energy scale.

At the LHC, the individual quarks that make up the proton only have a fraction of this energy. Most of theenergy comes from relativistic kinetic energy since these protons travel near the speed of light at the LHC (seeFig 6). The collisions create new particles that promptly decay into lighter particles within a fraction of asecond (sometimes less than a trillion-trillionth of a second). The original decay can then be reconstructed bythe ATLAS detector using sophisticated electronic trigger systems that precisely measure the passage time of

a particle to accuracies with very high precision, and the location of the particles to less than a micron. Whenprotons collide with such large energies as at the LHC, the collision results in a sea of all types of particles,ordinary matter that we are made of, and others that only existed just after the Big Bang. The new particles areusually much heavier than the original colliding particles, thanks to the relation E=mc2. To say it simply: All theenergy we put into the collision can come out as mass instead! In a proton-proton collision “anything” canhappen, provided some important principles are obeyed, such as energy conservation. In the middle of ATLASdetector, two proton bunches (each with 100 billion protons) collide with each other after they have beenaccelerated in opposite directions in the LHC. Each proton bunch has a total energy of 3.5 TeV. Therefore thecenter-of-mass energy is 7.0 TeV. It is neither possible to predict which parts of one proton will collide withwhich parts of another nor which protons collide at all. I n the collisions, scattering (deviation of protons) andcollisions may occur. In the latter case, new particles are formed. From the data, we are able to say whichphysical processes took place during the collisions.

At the LHC, there can be several different event topologies for the Higgs boson. For the Higgs boson in themass range 130 GeV < MH < 1000 GeV, the two dominant decay channels are H Z0Z0 and HW-W+. TheZ and the W bosons would then decay into their respective channel as shown in the flowchart in Figure 29. Asshown in Figure 23, the “golden”decay modes we are targeting to search for the Higgs using the ATLASmonte-carlo simulation data are - H 0Z0Z0e-e+e-e+, H0 Z0Z0e-e+µ-µ+, and H0 Z0Z0µ-µ+µ-µ+. Asshown in figure 25, the process of data analyses is a bit of detective work because at a proton-proton collision

 – a so-called "event" – a lot of particles are generated. The key task is to search for specific decay particlesand to assign them to physical processes, by recognizing and distinguishing different elementary particles asshown in Figure 18-19. Conservation laws allow us to see patterns in these decays. Often, quarks arescattered in collisions. As they separate, the binding energy between them converts to sprays of new particlescalled jets. As shown in Fig 26.a, in proton-proton collisions, electrons and muons can emerge out of hadron jets. These are not what we are looking for. As shown in 26.b we are looking for isolated Z bosons producedfrom proton-proton collisions, where the Z boson decays directly into (a muon and an anti-muon) or (anelectron and a positron). We are also looking for the Z bosons produced among hadronic background tracks(see Fig. 22 and Fig. 26.c); these are the “dimuon” or “dielectron” events. After filtering out low -momentumhadron tracks, we can isolate for example the Z boson decaying into an electron and a positron (see Fig 16.d).Our next goal is to reconstruct the Z boson mass, by using the mass, energy and momentum of the Z boson'sdecay products and hence determine the mass of the Higgs boson.

DATA ANALYSES - RESULTS  – CONCLUSIONAfter undergoing a thorough and rigorous filtration process by BU Tier3 Supercomputer, a totalof 422 Monte-Carlo events were carefully analyzed using both the Atlantis and Hypatia software. We found 160 Wbosons, 24 Z bosons and 3 Higgs boson in our monte-carlo data. W and Z boson decay mode flowchart is shown inFigure 6. The results are shown in Table 1 and sample signal decay events of the W, Z and the Higgs boson are shownin Figure 1 – 4. The results are also displayed in pie chart as shown in Figures 2 – 4. We measured the branching ratioof Weν / Wµν to be 1.3 which is very close to the theoretical predicted value of 1.0. We also measured thebranching ratio of Zee / Zµµ to be 1.2 which is very close to the theoretical predicted value of 1.0. We alsomeasured W/Z branching ration to be 6.7, which is lower than the theoretically predicted value of 10.2. Our next goal isto analyze over 3000 ATLAS events from the 2010 LHC experimental run and compare the monte-carlo simulationresults using the 2010 ATLAS experimental datasets.

Figure 1. History of the Universe after the Big Bang

REFERENCES. 

1. S.L. Glashow, Nucl. Phys. 20, 579 (1961); S. Weinberg, Phys. Rev. Lett. 19, 1264 (1967) A. Salam, ElementaryParticle Theory, eds.: Svartholm, Almquist, and Wiksells, Stockholm, 1968; S. Glashow, J. Iliopoulos, andL. Maiani, Phys. Rev. D2,1285 (1970).

2. P.W. Higgs, Phys. Rev. Lett. 12, 132 (1964); Phys. Rev. 145, 1156 (1966); F. Englert and R.Brout, Phys. Rev.Lett. 13, 321 (1964); G.S. Guralnik, C.R. Hagen, and T.W. Kibble, Phys. Rev. Lett. 13, 585 (1964).

3. Weinberg, S. Phys. Rev. 118 838-849 (1960); Weinberg, S. Phys. Rev. 127 965-970 (1962).

ACKNOWLEGDEMENTWe are grateful to the National Science Foundation (NSF) for funding this research project. We would like to thankour ATLAS collaborators and the management of the ATLAS collaboration at the Large Hadron Collider (LHC) at

CERN for allowing us to participate in the ATLAS experiment.

Figure 2. The 27 km Circumference LHC Particle Accelerator Figure 4. Arial photo of the CERN„s ATLAS Experiment Complex. 

H→eeee 1

H→µµµµ 0

H→eeµµ 2

Z→ee 13

Z→µµ 11

W→ee 92

W→

µµ 68

Event Type Number of Events

W Boson 160

Z Boson 24

H Boson 3

Background 235

W/Z Ratio 6.7

Theory 10.2

Table 1. (a) Monte-Carlo Event Distribution of W boson decay modes, (b) Monte-

Carlo Event Distribution of Z boson decay modes, (c) Monte-Carlo EventDistribution of W boson decay modes, (d) Monte-Carlo Event Distribution of W, Zand Higgs Events and Ratio of W to Z Bosons.

Figure 27. Screenshot of the Atlantis Data Analyses Package

Figure 28. W and Z Boson Decay Mode Flowchart

Figure 29. Data Analysis Workstation in Pasteur 209

Figure 30. Screenshot of W Boson Decaying into eν (Weν) Figure 31. Screenshot of W Boson Decaying into µν (Wµν)

Figure 32. Screenshot of Z Boson Decaying into ee (Z0e-e+) Figure 33. Screenshot of Z Boson Decaying into µµ (Z0µ-µ+)

Figure 37. (a) W Boson Decay Mode Distribution using ATLAS MC data; (b) Z Boson Decay Mode Distribution using ATLAS MC data;(c) W, Z, and Higgs Event Distribution using ATLAS MC Data.

(a)

(c)(b)

Figure 34. Screenshot of Higgs Decaying into eeee (H0e-e+e-e+) Figure 35. Screenshot of Higgs Decaying into eeµµ (H0e-e+µ-µ+)

Figure 36. ATLAS Data: Boson Decaying intoµµ (Z0µ-µ+)

(a)

(b)

(c)

(d)

Figure 5. Sketch of the LHC Tunnel 100 m below ground Figure 6. Path to Discovery: ASketch Showing How LHC Works Figure 7. The LHC Control Room at CERN

Figure 8. LHC Accelerator Tunnel Figure 9. Picture of ATLAS Calorimeter Figure 10. Another Picture of ATLAS Calorimeter Figure 11. Picture of ATLAS Muon Chamber (Front End)

Figure 12. Front View of the ATLAS Detector Figure 14. ATLAS Inner Detector Under Construction Figure 15. ATLAS Inner Detector InstalledFigure 13. ATLAS Detector Under Construction

Figure 16. ASketch of the 7000 ton ATLASDetector (44 m long, 25 m high and 25 m wide)

Figure 17. Cross-Sectional View of ATLAS Detector

Figure 18. Identification of ParticleTracks in the ATLAS Detector (FrontView)

Figure 19. Identification of Particle Tracksin the ATLAS Detector (Side View)

Figure 20. Higgs Mechanismand Interaction with the Quarks,Leptons and the W and Z Bosons. Thequestion is - Why is the Photon is Massless

while the W and Z Bosons are Massive?

Figure 24. Production of Higgs Boson fromProton-Proton Collisions.

Figure 25. Searching for the Higgs: It‟s Like

Looking for a Needle in the Haystack.

Figure 21. The Standard ModelShowing the Quarks, Leptons,Higgs, Photons, Gluons, Wand Z Bosons Figure 22. Rare production of Z Bosons from

p-p Collisions

Figure 23. Three “Golden” Decay Modes of the Higgs via the Z Boson Channel (These Higgs Decays

are Extremely Rare! 1 in 10 Trillion proton- proton Collisions at the LHC could produce such “clean”

Higgs Events.)

Figure 26. As shown - (a) in proton-proton collisions, electrons and muons can emerge out of hadron jets.These are not what we are looking for. As shown in (b) we are looking for isolated Z bosons produced from

proton-proton collisions, where the Z boson decays directly into (a muon and an antimuon) or (an electronand a positron). (c) We are also looking for the Z bosons produced among hadronic background tracks (see

Fig. 22); these are the “dimuon” or “dielectron” events. After filtering out low -momentum hadron tracks, wecan isolate for example the Z boson decaying into an electron and a positron.

(a)

(b)(c)

(d)