Oddelek za fiziko

Seminar Ia - 1.letnik, II. stopnja

Spectrometer ATLAS at LHC

Author: Anže Medved

Supervisor: prof. dr. Peter Križan

Ljubljana, March 2014

Abstract

ATLAS experiment is one of seven experiments at the Large Hadron Collider(LHC), which use detectors to analyse the myriad of particles produced bycollisions in the accelerator. The biggest of these experiments, ATLAS andCMS, use general-purpose detectors to investigate the largest range of physicspossible. ALICE and LHCb have detectors specialized for focussing on specificphenomena. These four detectors are underground in huge caverns on the LHCring. The smallest experiments on the LHC are TOTEM, LHCf and MoEDAL.We are going to focus on ATLAS experiment.

CONTENTS 1 INTRODUCTION

Contents1 Introduction 1

2 Higgs boson decay 2

3 Particle Detection Principles 2

4 ATLAS detector layout 4

5 Inner Detector 45.1 The Pixel Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55.2 The SemiConductor Tracker (SCT) . . . . . . . . . . . . . . . . . . . . . . . 55.3 The Transition Radiation Tracker (TRT) . . . . . . . . . . . . . . . . . . . 6

6 Central solenoid 6

7 Calorimetry 77.1 Electromagnetic calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.2 Hadronic calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

8 Muon system 9

9 Magnet system 109.1 Barrel toroid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

10 Conclusion 10

1 IntroductionThe ATLAS collaboration, the group of physicists who built and now run the detector, wasformed in 1992 [1]. At first, let us see what abbreviation ATLAS means. It stands for AToroidal LHC ApparatuS. The design was a combination of the two previous experiments,EAGLE1 and ASCOT2. The ATLAS experiment was proposed in 1994, and officially fundedby the CERN member countries in 1995. Construction was completed in 2008 and theexperiment detected its first single beam events on 10 September of that year.

At LHC they don’t collide just protons (p), but they are also colliding heavy ions (A).Those heavy ions are particular lead nuclei. Inside the LHC, bunches of up to 1011 protons(p) will collide 40 million times per second to provide 14 TeV proton-proton collisions at a

1Experiment for Accurate Gamma, Lepton and Energy Measurements2Apparatus with Super Conducting Toroids

1

3 PARTICLE DETECTION PRINCIPLES

design luminosity3 of 1034 cm−2s−1. Heavy ions will collide at 5.5 TeV per nucleon pair, ata design luminosity of 1027 cm−2s−1. Two general purpose detectors, ATLAS and CMS4

have been built for probing p-p and A-A collisions [2].ATLAS is designed to be a general purpose detector [1]. When the proton beams

produced by the LHC interact in the center of the detector, a variety of different particleswith a broad range of energies are produced. It is designed to measure the broadest possiblerange of signals. This is intended to ensure that whatever form any new physical processesor particles might take, ATLAS will be able to detect them and measure their properties.

One of the most important goals of ATLAS is to investigate a missing piece of theStandard Model, the Higgs boson. On July 4, 2012, ATLAS and CMS reported an evidencefor the existence of a particle consistent with the Higgs boson [1].

2 Higgs boson decayBefore we start describing about the ATLAS detector, let us see how can we detect Higgsboson. We know that Higgs decay in two photons

H → γγ, (1)

and in 4 leptonsH → ll̄ll̄. (2)

Our goal, to investigate Higgs boson, is to detect photons and leptons. But this is just

Figure 1: Higgs decay to photons (left), Higgs decay to leptons (right). [2]

for Higgs boson. Many other particles, which are also produced in detector, are hadrons.So we don’t want detector, which is capable to detect just photons and leptons, but alsohadrons.

3 Particle Detection PrinciplesIn Figure 2, we can see the particles path throught detector. Particles, produced in colli-sions, normally travel in straight lines, but in the presence of a magnetic field their pathsbecome curved. But curved paticles will be only particles with electric charge. They willexperience Lorentz force

3Luminosity is defined as L = φAρB , where φA is incident flux on target, and ρB is area density ofparticles in target able to participate in reaction.

4One of the detectors at the LHC.

2

3 PARTICLE DETECTION PRINCIPLES

F = q(E + v×B). (3)

Now it is seen, why we need detector in strong magnetic field. This can be seen on theright figure. Physicists can calculate the momentum of a particle from the curvature of itspath. This is a clue to its identity.

Modern particle detectors consist of layers of subdetectors, which we are gooing todiscuss in this seminar. Each of these subdetectors are designed to look for particularproperties or specific types of particle. Tracking devices reveal the path of a particle.Calorimeters stop, absorb and measure the energy of particles. Particle-identification de-tectors use a range of techniques to pin down the identity of particles.

We can also see, when the particles decay in detector, they produce jets. From thesejets, they can figure out, which particle have had decayed. Dimensions of the detector we

Figure 2: Particle paths throught detector, and its cross section. [2]

will discuss later in seminar. Now we will focus just on one particle at time:Photon: Photon interacts electromagnetically. That means, photon will be detected

in the electromagnetic calorimeter, where it decays into jets. We will see it just in EMcalorimeter, because photon doesn’t have electric charge and we won’t see it in InnerDetector.

Electron/Positron: They both have electric charge, so we will detect them in Innerdetector. Like photon, they both interact electromagnetically, so that will also be detectedin electromagnetic calorimeter, where they decay into jets. Difference between electronand positron is, that one will bend in one direction, and other in the opposite one.

Muon: It has electric charge, so it will bend. We can detect it in all detectors. Mouninteraction with calorimeter is small, so it will go throught all of the detector.

Proton: It has electric charge, so it will bend. It is built up of quarks (uud). Particles,which are built up of quarks are hadrons and they are strongly interacting particles. Allparticles, which are strongly interacting, are detected in the Hadronic calorimeter.

Neutron: It doesn’t have electric charge, so it won’t be detected in Inner detector.Like proton, neutron is built up of quarks (udd), so it is a hadron, and it will be detectedin the Hadronic calorimeter.

Neutrino: It is the only particle that can’t be detected in the ATLAS detector.

3

5 INNER DETECTOR

4 ATLAS detector layoutThe ATLAS detector is nominally forward-backward symmetric with respect to the inter-action point. The magnet configuration comprises a thin superconducting solenoid sur-rounding the inner-detector cavity, and three large superconducting toroids arranged withan eight-fold azimuthal symmetry around the calorimeters. This fundamental choice hasdriven the design of the rest of the detector. [2]

Figure 3: The dimensions of the detector are 25 m in height and 44 m in length. The overall weight ofthe detector is approximately 7000 tonnes. [3]

Approximately 1000 particles will emerge from the collision point every 25 ns, creatinga very large track density in the detector. To achieve the momentum and vertex resolution,high-precision measurements must be made with fine detector granularity.

The nominal interaction point is defined as the origin of the coordinate system, whilethe beam direction defines the z-axis and the x-y plane is transverse to the beam direction.The positive x-axis is defined as pointing from the interaction point to the centre of theLHC ring and the positive y-axis is defined as pointing upwards. The azimuthal angle φis measured as usual around the beam axis, and the polar angle θ is the angle from thebeam axis. The side-A of the detector is defined as that with positive z and side-C is thatwith negative z [2]. The entire detector is built to withstand for several years of operating.Particle won’t destroy segments so much, that they will have to be raplaced every event.

5 Inner DetectorThe Inner Detector is immersed in a 2 T magnetic field generated by the central solenoid.The Inner Detector comprises three complementary sub-detectors: the Pixel Detector, theSemiConductor Tracker and the Transition Radiation Tracker [5]. In the barrel region

4

5.1 The Pixel Detector 5 INNER DETECTOR

they are arranged on concentric cylinders around the beam axis, while in the end-capregions they are located on disks perpendicular to the beam axis. The highest granularityis achieved around the vertex region using silicon pixel detectors [2]. The combinationof the two silicon and TRT techniques gives a very robust pattern recognition and highprecision in both φ and z coordinates [7].

The trackers detect particles with electric charge. They measure their positions atdifferent times. Since the trackers are permeated by a homogeneous magnetic field, chargedparticles are deflected. With the help of the curvature they can calculate the momentumand determine the electric charge. The interaction between the particles produced duringthe collision and the detector material of the trackers is very small. Thus particles onlydeposit a small amount of energy there [8].

Figure 4: The Inner Detector. The outer radius is 1.15 m, and the total length 7 m. [2]

5.1 The Pixel Detector

Sensitive elements cover radial distances between 50.5 mm and 150 mm. The detectorconsists of ∼1700 silicon pixel modules arranged in three concentric barrel layers and twoendcaps of three disks each. It provides typically three measurement points for particlesoriginating in the beam-interaction region. Each module covers an active area of 16.4mm×60.8 mm and contains ∼47 000 pixels, most of size 50µm× 400µm [5].

Every module is covered with thin layer of silicon. When the charged particle goesthrough silicon, it liberate electrons. These electrons move to the bottom, where theycreate electric signal in one of pixel. In the end we can see, in which module (moreprecisely in which pixel) was the signal. That mean we have first tracking point. The totalnumber of readout channels is ∼80.4 million. This signal is then readout from the detector,to the powerful computers in the control room.

5.2 The SemiConductor Tracker (SCT)

The detector consists of∼4000 modules of silicon-strip detectors arranged in four concentricbarrels and two endcaps of nine disks each. It provides four precision measurements per

5

5.3 The Transition Radiation Tracker (TRT) 6 CENTRAL SOLENOID

track in the intermediate radial range. The strips in the barrel are approximately parallelto the solenoid field and beam axis, and have a constant pitch of 80 µm [5].

It is based on a classic single-sided p-in-n technology [2]. When the charged particlegoes through silicon, it produce electron-hole state. That means electric potencial andthis is equal to electric signal. The total number of readout channels is ∼6.3 million [5].Difference between SCT and Pixel is that, that Pixel is more precisely.

5.3 The Transition Radiation Tracker (TRT)

The detector consists of ∼300 000 proportional drift tubes (straws), 4 mm in diameter,read out by ∼350 000 channels of electronics. The TRT provides a large number of trackingpoints (about 36 per track) [7].

Every tube is filled with gas (in our case Xe-based gas mixture [2]). In the center, thereis a wire (anode). When the charged particle goes through tube, it will ionise the gas.After ionisation, electron will drift to anode, and we will get electric signal.

This detector is made of gas because the silicon detecor is hard to do and it costs a lotof money. And in the outher part of Inner detector we don’t need to measured position soprecisly.

6 Central solenoidThe central solenoid is designed to provide a 2 T magnetic field surrounding the InnerDetector [2]. This high magnetic field allows even very energetic particles to curve enoughfor their momentum to be determined, and it’s nearly uniform direction and strength allowmeasurements to be made very precisely.

Figure 5: Central solenoid: the inner and outer diameters of the solenoid are 2.46 m and 2.56 m andits axial length is 5.8 m. The coil mass is 5.4 tonnes and the stored energy is 40 MJ. [1, 2]

Particles with momenta below roughly 400 MeV will be curved so strongly that theywill loop repeatedly in the field and most likely not be measured; however, this energy isvery small compared to the several TeV of energy released in each proton collision [1]. Thethin layer of coil won’t have effect on so energetic particles.

6

7 CALORIMETRY

7 CalorimetryAtlas calorimetry consit of two parts: Elecromagnetic calorimetry and Hadronic calorime-try.

The fine granularity of the EM5 calorimeter is ideally suited for precision measurementsof the energy and position of electrons and photons [6]. They can reconstruct the positionof particles from electrodes in which they leave some energy. The position is not so crucial,more important are measuremenst of energy.

Figure 6: ATLAS EM and hadronic calorimetry. [6]

The ATLAS calorimeters consist of a number of sampling detectors with full φ-symmetryand coverage around the beam axis. The calorimeters closest to the beam-line are housedin three cryostats6, one barrel and two end-caps. All these calorimeters use liquid argon asthe active detector medium; liquid argon has been chosen for it’s intrinsic linear behaviour.

When high enery particle (e−, e+ and γ) penetrates matter it produce an electromag-netic cascade mainly via two processes: bremsstrahlung for the charged particles and pairproduction for the photons [7]. This is called electromagnetic showers. We also knowthe hadronic showers. When the incident hadron strongly interacts with the medium itis producing a number of hadrons, and so on. The charged hadrons ionise the mediumon the path. As a result of electromagnetic and strong interactions in hadronic showers alarge number of different particle (p,n,γ, e−, e+...) may be produced during the showerdevelopment [7].

7.1 Electromagnetic calorimetry

The main part of ATLAS EM calorimeter is a lead-liquid argon (LAr) sampling detectorwith accordion-shaped electrodes and lead absorber plates over its full coverage. Theabsorber lead thickness is constant over large areas. The argon gap thickness is constantin the Barrel but changing with the radius in the End-Cap. [6]

Such a geometry provides naturally a full coverage in φ without any cracks. The readoutelectrodes are located in the gaps between the absorbers and consist of three conductive

5electromagnetic6A cryostat is a device used to maintain low cryogenic temperatures of samples or devices mounted

within the cryostat. Low temperatures may be maintained within a cryostat by using various refrigerationmethods, most commonly using cryogenic fluid bath such as liquid helium.

7

7.2 Hadronic calorimetry 7 CALORIMETRY

copper layers. The two outer layers are at the high-voltage potential and the inner one isused for reading out the signal via capacitive coupling. [2]

How does LAr sampling detector works? Scheme of such detector in shown in Figure7. When e− or γ penetrates absorber it produce EM showers. These shower then ioniseliquid argon. Signal is given from collection of released electrons as shown in Figure 7 [10].

Figure 7: Scheme of a LAr sampling detector. [10]

In the forward region, the EM calorimetry is done by another type of LAr calorimeter.The Forward Calorimeter (FCAL) consists of copper rods parallel to the beam axis insidean outer tube with 250 µm liquid argon gap in between. The main objective for the FCALis to provide an acceptance region for the forward jets which can be signatures of heavyHiggs production in the vector boson fusion process [7].

In the electromagnetic calorimeter (LAr electromagnetic barrel), particles and theirantiparticles, which interact electromagnetically, are detected. These are mainly electronsand photons. The whole energy of a particle flying through the electromagnetic detectoris absorbed and transformed into an electronical signal. The strength of the signal is ameasure for the energy of the particle [8].

7.2 Hadronic calorimetry

ATLAS hadronic calorimeter is an iron-scintillating tiles calorimeter. Those are locatedbehind the solenoid coil and the EM calorimeter. It is a sampling calorimeter using iron asabsorber material and scintillating tiles (3 mm thick) as active material [2]. The tiles areplaced in planes perpendicular to the colliding beams and are staggered in depth. Theyare read out by wavelenght shifter fibers (WLS) oriented in the same direction as the tiles.These fibers are connected to Photomultiplier Modules (PMTs) [7]. Those are not classicalPMTs but special ones, because they are in the presence of magnetic field.

The Hadronic End-Cap calorimeter (HEC) is an LAr sampling calorimeter. It hasparallel Cu plate absorbers orthogonal to the beam axis.

8

8 MUON SYSTEM

In the hadronic calorimeter (Tile barrel), strongly interacting particles, that are builtup of quarks and/or antiquarks, which are so-called hadrons, are detected. Those are, forexample, protons or neutrons [8]. Here we mention only, that in the hadronic calorimeterwe find more dameges. The main reason for these dameges are neutrons, but those aredetails.

8 Muon systemMuons deposit only a small amount of their energy in the calorimeters, and are the only"visible" particles which pass through every layer of the ATLAS detector. Why they depositso small amount of energy? Bacause they are heavier than other particles, and they simply"fly" through the meterial. You can imagine that in the case of billiard ball(muon) andpinkponk balls(electrons and hadrons). Those small balls can’t stop the big and heavierball. Therefore, there are muon chambers located at the outermost part of ATLAS toidentify muons. The muon chambers are situated in an additional magnetic field to measurethe momentum more precisely than with the trackers. This magnetic field is produced byhuge toroidal coils. The muon chambers are made up of thousands of long tubes filled withgas. There is a wire in the middle of each tube. Incident muons create free charge carriersin the gas via ionisation. These carriers move towards the outer wall or towards the wirebecause of a large voltage difference between the tube and the wire, thereby creating anelectronically readable signal [8].

Muon system is based on the magnetic deflection of muon tracks in the large super-conducting air-core toroid magnets, instrumented with separate trigger and high-precisiontracking chambers [11]. In the barrel region megnetic field is produced by Barrel toroid.This magnet configuration provides a field which is mostly orthogonal to the muon trajec-tories, while minimising the degradation of resolution due to multiple scattering.

Figure 8: Muon system. [2]

Precision-tracking chambers in the barrel region are located between and on the eightcoils of the superconducting barrel toroid magnet, while the end-cap chambers are in front

9

10 CONCLUSION

and behind the two end-cap toroid magnets. The φ symmetry of the toroids is reflectedin the symmetric structure of the muon chamber system, consisting of eight octants. Eachoctant is subdivided in the azimuthal direction in two sectors with slightly different lateralextensions, a large and a small sector, leading to a region of overlap in φ. This overlap ofthe chamber boundaries minimises gaps in detector coverage.

9 Magnet systemATLAS features a unique hybrid system of four large superconducting magnets. Thosefour magnets consists of one solenoid and three toroids. This magnetic system is 22 min diameter and 26 m in length, with a stored energy of 1.6 GJ. They are providing themagnetic field over a volume of approximately 12,000 m3.

Figure 9: a) Barrel toroid: the overall size of the barrel toroid system as installed is 25.3 m in length,with inner and outer diameters of 9.4 m and 20.1 m, respectively. [1, 2]

9.1 Barrel toroid

The cylindrical volume surrounding the calorimeters and both end-cap toroids is filled bythe magnetic field of the barrel toroid. Cool down and testing of the barrel toroid in thecavern took place in 2006. The cool down of the 360-tonne cold mass to 4.6 K takes fiveweeks. The conductor and coil-winding technology is essentially the same in the barrel andend-cap toroids. It provide up to 3 T magnetic field. [2]

10 ConclusionATLAS is intended to investigate many different types of physics that might become de-tectable in the energetic collisions of the LHC. Some of these are confirmations or improvedmeasurements of the Standard Model, while many others are possible clues for new physicaltheories [1].

ATLAS brings experimental physics into new territory. Most exciting is the completelyunknown - new processes and particles that would change our understanding of energy andmatter [9]. It is designed to investigate many different processes of physics, like Higgs bosonproduction and decay. It will learn about the basic forces that have shaped our Universe

10

REFERENCES REFERENCES

since the beginning of time and will determine its fate. Among the possible unknowns areextra dimensions of space, microscopic black holes and the string theory.

The LHC recreates, on a small scale, the conditions of the Universe just after the BigBang in order to learn why the Universe is like it is today. ATLAS will investigate whythe matter of the Universe is dominated by an unknown type of matter called dark matter.If the constituents of dark matter are new particles, ATLAS should discover them andelucidate the mystery of dark matter. It might be that this new particle is the lightestparticle postulated in theories of supersymmetry [9].

It is the biggest experiment at the LHC and it is different from the other smallerexperiments. Those smaller ones investigate only specific type of physics.

References[1] Wikipedia (2013) ATLAS experiment, [online], Available:

http://en.wikipedia.org/wiki/ATLAS_experiment [15 Nov 2013]

[2] Aad, G., Abat, E., Abdallah, J. (2008) The ATLAS Experiment at the CERN LargeHadron Collider, August, pp. 1-205.

[3] ATLAS experiment (2013) ATLAS photos, [online], Available:http://www.atlas.ch/photos/full-detector-cgi.html [21 Nov 2013]

[4] ATLAS (2013) Magnet System, [online], Available: http://www.fsp101-atlas.de/e197881/e200214/ [12 Dec 2013]

[5] Aad, G., Abbott, B., Abdallah, J. (2010) The ATLAS Inner Detector commissioningand calibration, June, pp. 11-13.

[6] Puzo, P. (2002) ATLAS calorimetry, Nuclear Instruments and Methods in PhysicsResearch, pp. 340-345.

[7] Weber, P. (2008) ATLAS Calorimetry: Trigger, Simulation and Jet Calibration,Heilderberg, February, pp. 23-37.

[8] LHC International Masterclasses (2013) Structure and Function of the ATLAS De-tector, [online], Available: http://kjende.web.cern.ch/kjende/en/zpath_atlas.htm [15Dec 2013]

[9] ATLAS experiment (2013) The unknown, [online], Available:http://www.atlas.ch/the-unknown.html [20 Dec 2013]

[10] Indico CERN (2013) Calorimetry (for pedestrians), [online], Available:http://indico.cern.ch/event/115059/material/slides/0 [26 Feb 2014]

[11] Snuverink, J. (2009) THE ATLAS MUON SPECTROMETER: COMMISSIONINGAND TRACKING Twente, October, pp. 19-34.

11