Search for the Standard Model Higgs boson decaying in four leptons using Calorimeter Tagged muons with the ATLAS experiment

CHAPTER
1
The ATLAS experiment at LHC
1.1 The LHC
The Large Hadron Collider (LHC) is the world’s largest and most powerful
particle accelerator. Built by the European Organization for Nuclear Research
(CERN)from1998to2008,itliesinacirculartunnel27kilometerslongand175
meters underground at the Franco-Swiss border near Geneva, the same hosting
the Large Electron-Positron Collider (LEP). It is designed to collide proton-
proton beams up to 14 TeV in the center of mass, or lead nuclei at an energy
of 2.76 TeV per nucleon.
The collider tunnel hosts two adjacent circular beam pipes where two beams
travelinoppositedirectionsonewithrespecttotheother. Thebeamsarekeptin
their circular paths by 1232 superconducting dipole magnets, while quadrupole
magnets provide the focusing of the beam. The two beam-lines intersect at four
points in which four main experiments are installed: ATLAS, CMS, ALICE,
LHCb.
7
1. The ATLAS experiment at LHC 1.1. The LHC
1.1.1 Layout and performances
The number of events per second generated in the LHC collisions is given by
N =L  (1.1)
where  is the cross section for the process under study (see gure 1.1) and
L the accelerator luminosity, which depends only on the beam parameters and
can be written as
L=
N
2
b
n
b
f
rev
 r
4 n
  F: (1.2)
N
b
is the number of particles per bunch, n
b
the number of bunches per beam,
f
rev
the revolution frequency,  r
the relativistic gamma factor,  n
the normal-
ized transverse beam emittance,   the beta function at the collision point ad
F the geometric luminosity reduction factor due to the crossing angle at the
Interaction Point (or IP for short) [1].
The LHC machine performance is limited by several eects such as the non
linear beam-beam interaction that each particle experiences when the bunches
collide with each other, the maximum dipole eld (that limits  r
) and the col-
lective beam instabilities due to the electromagnetic mutual interactions of the
particles [1]. Thus the luminosity of LHC is not constants over a physics run
butdecaysduetothedegradationofintensitiesandemittanceoveracirculating
beams. The main cause of luminosity decay are the collisions themselves. The
decay time due to this eect is
 nuclear
=
N
tot;0
L
tot
k
(1.3)
where N
tot;0
is the initial beam intensity, L the initial luminosity and  tot
the
total cross section. The luminosity as a function of time is given by
L(t)=
L
0
(1+t=
nuclear
)
2
(1.4)
The design luminosity of LHC is 10
34
cm
 2
s
 1
providing a bunch collision rate
of 40 MHz. Each beam is distributed in  2800 bunches of  10
11
proton per
bunch. The resulting time between two bunch crossing is  25 ns at L= 10
34
cm
 2
s
 1
. Given an inelastic cross section of  100 mb
(1)
, this will result in
 20 interactions per crossing [2].
1
1 b = 1 barn = 10
 28
m
2
= 10
 24
cm
2
8
1. The ATLAS experiment at LHC 1.1. The LHC
Figure 1.1: Cross Sections for specic physics processes. The dotted lines
show the energies of two hadron collider (The Tevatron at 1.96 TeV, and
The LHC at 14 TeV).
Day in 2011
28/02 30/04 30/06 30/08 31/10
]
-1
Total Integrated Luminosity [fb
0
1
2
3
4
5
6
7 = 7 TeV s     ATLAS Online Luminosity
LHC Delivered
ATLAS Recorded
-1
Total Delivered: 5.61 fb
-1
Total Recorded: 5.25 fb
(a) Total integrated luminosity delivered in 2011
Day in 2011
28/02 02/0405/05 08/06 11/07 14/08 16/0920/10 22/11
]
-1
 s
-2
 cm
33
Peak Luminosity per Fill [10
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
 = 7 TeV s     ATLAS Online Luminosity
LHC Stable Beams
-1
 s
-2
 cm
33
 10 ´ Peak Lumi: 3.65 
(b) The maximum instantaneous luminosity versus day de-
livered to ATLAS
Figure 1.2
9
1. The ATLAS experiment at LHC 1.1. The LHC
In 2011 LHC delivered 5.61 fb
 1
of total integrated luminosity, of which
the ATLAS experiment recorded 5.25 fb
 1
, at a peak luminosity of 3.65 10
33
cm
 2
s
 1
as depicted in gure 1.2(a) and (b).
Figure 1.3: The CERN accelerator complex
The LHC is supplied by protons from the injector chain in gure 1.3. Ini-
tially,theprotonsareextractedfromanionizedhydrogensourceandareacceler-
atedto750KeVusingRFcavities. Subsequently,alinearaccelerator(LINAC2)
increases their energy to 50 MeV and passes them to the Proton Synchrotron
Booster (PSB) which further increases their energy at 1.4 GeV. Afterwards,
the beams enter subsequently into the Proton Synchrotron (PS) and the Super
Proton Synchrotron (SPS) exiting with energies 26 GeV and 450 GeV respec-
tively. Withthe energy of 450GeV theprotonsare injectedintotheLHC rings.
The beams follow circular trajectories produced by the 8.33 T 1232 supercon-
ducting dipole magnets operating at 1.9 K. In table 1.1 the designed operation
parameters of LHC in pp (proton-proton) mode are summarized.
1.1.2 Other experiment
Besides ATLAS, which will be described in detail in section 1.2, the other
three experiment that take place at LHC are:
ALICE (acronym of A Large Ion Collider Experiment) is optimized to study
heavy ion (Pb-Pb) collisions at a center of mass energy of 2.76 TeV per
10
1. The ATLAS experiment at LHC 1.2. The LHC
Parameter Value
Energy/Beam 7 TeV
Dipole Field 8.33 T
Dipole Current 11.850 kA
Bunches/Beam 2808
Protons/Bunch 1.15 10
11
Bunch Spacing 24.95 ns
Events/BeamCrossing 19.02 ns
Total cross section 100 mb
Beam current 0.58 A
Luminosity 10
34
cm
 2
s
 1
Luminosity lifetime 14.9 h
Stored energy 6.93 MJ
Energy Loss/Proton/Turn 6.61 keV
Table 1.1: Operation parameters of the Large Hadron Collider in pp mode
[3].
nucleon. Theresultingtemperatureandenergydensityareexpectedtobe
large enough to generate a quark-gluon plasma, a state of matter wherein
quarks and gluons are deconned.
CMS (acronymofCompact Muon Solenoid)experimentusesageneral-purpose
detector to investigate a wide range of physics, including the search for
the Higgs boson, extra dimensions, and particles that could make up dark
matter. The CMS detector is built around a huge solenoid magnet that
generates a magnetic eld of 4 T. The magnetic eld is conned by a steel
\yoke" that forms the bulk of the detector’s weight of 12 500 tonnes.
LHCb (standing for Large Hadron Collider beauty) is a specialized B-physics
experiment, thatismeasuringtheparametersofCPviolationintheinter-
actions of b-hadrons (heavy particles containing a bottom quark). Such
studies can help to explain the Matter-Antimatter asymmetry of the Uni-
verse.
11
1. The ATLAS experiment at LHC 1.2. The ATLAS detector
1.2 The ATLAS detector
1.2.1 Detector overview
Figure 1.4: ATLAS detector overview
ATLAS(AToroidalLHCApparatuS)isageneralpurposedetectoroperating
at LHC. It has an approximate cylindrical shape and it consists of a tracking
systemina2Tsolenoidalmagneticeld,samplingelectromagneticandhadronic
calorimeters and muon chambers in a toroidal magnetic eld.
The detector is described by the following system of coordinates:
 the origin of the ATLAS coordinate system is dened as the nominal
interaction point (IP);
 the z axis is oriented along the counter clockwise beam direction;
 the xy plane, perpendicular to z axis, denes the transverse variable such
as the transverse momentump
T
, the transverse energy,E
T
, and the miss-
ing transverse energyE
miss
T
. The positive x-axis direction points towards
the center of the LHC ring and the positive y-axis points upward.
  is the azimuthal angle measured on the plane orthogonal to z;
  is the angle formed by the direction of the emitted particle with the z
axis. Instead of this angle, the variable = ln
 tan
 2
 (called pseudora-
12
1. The ATLAS experiment at LHC 1.2. The ATLAS detector
pidity
2
) is often used because it transforms additively under a Lorentz
transformation making the   =  2
  1
between two particles rela-
tivistically invariant. The distance  R in  space is dened as  R =
p
  2
+  2
.
For completeness, two more parameters are needed in order to describe track
trajectories inside the experimental volume:
 d
0
isthedistanceofthetracktothez-axisinthetransverseplane, dened
positive when the particle trajectory is clockwise;
 z
0
is the z coordinate at perigee.
1.2.2 Design criteria
The basic design criteria include:
 very good electromagnetic calorimetry for electron and photon identica-
tionandmeasurements,complementedbyfull-coveragehadroniccalorime-
try for accurate jet and missing transverse energy (E
miss
T
) measurements;
 high-precision muon momentum measurements, with the capability to
guarantee accurate measurements at the highest luminosity using the ex-
ternal muon spectrometer alone;
 ecient tracking at high luminosity for high- p
T
lepton-momentum mea-
surements, electron and photon identication,  -lepton and heavy-avour
identication,andfulleventreconstructioncapabilityatlowerluminosity;
 large acceptance in pseudorapidity ( ) with almost full azimuthal angle
( ) coverage everywhere;
 Triggering and measurements of particles at low-p
T
thresholds, providing
high eciencies for most physics processes of interest at LHC.
The ATLAS detector consists of four major components:
 inner detector
 calorimeters (in which an electromagnetic calorimeter and a hadronic
calorimeter can be distinguished by shape, material, and operative mode)
 magnet system
 muon spectrometer
These components (see gure 1.4) will be described in details in the following.
2
Psuedorapidity is an ultra-relativistic approximation of the rapidity, y, dened as y =
1
2
ln
 E+p
Z
E p
Z
 13
1. The ATLAS experiment at LHC 1.2. The ATLAS detector
Figure 1.5: Plan view of a quarter-section of the ATLAS inner detector
showing each of the major elements with its active dimensions
1.2.3 Inner Detector
The task of the inner detector is to track charged particles and to deter-
mine their charge, momentum, direction and vertex location. It combines a
high resolution in its inner part with continuous tracking in the outer and is
immersed in a 2 T magnetic eld provided by the central solenoid that will
be described in section 1.2.5. The whole ID system begins a few centimeters
from the proton beam axis, extends to a radius of 1.15 and is 7 m in length,
limited respectively by the EM calorimeter and the end-cap calorimeters. It
provides full tracking coverage over j j < 2:5 and impact parameter measure-
ment. Thetrackeciencyasafunctionoftransversemomentum,averagedover
all pseudorapidities, raises from about 10% at 100 MeV to about 86% at high
momentum [4]. The inner detector consists of three major components, whose
characteristics are summarized in table 1.2:
 The Pixel Detector;
 The SemiConductor Tracker, or SCT;
 The Transition Radiation Tracker, or TRT.
1.2.3.1 Pixel Detector
The ATLAS Pixel Detector is situated as close as possible to the interaction
point. Its pixel are based on a silicon semiconductor technology and combine
a very high granularity with a good time resolution. The coverage extends up
to j j < 2:5. It consists of three concentric barrels, of average radii of 50.5,
88.5 and 122.5 mm respectively, oriented along the z-direction and of three
disks on each side of these barrels at distances of 495, 580 and 650 mm from
the interaction point. Silicon modules are segmented into small rectangles, the
14
1. The ATLAS experiment at LHC 1.2. The ATLAS detector
Figure 1.6: Drawing showing the sensors and the structural elements tra-
versed by a charged particle in the barrel inner detector. The track crosses
successively the beryllium beam-pipe, 3 pixel layers, 4 SCT layers and ap-
proximately 36 TRT straws.
pixels. The thickness of each layer is only 2.5% of a radiation length at normal
incidence so the three pixel layers are typically crossed by each track. The
overall precision of the detector is of 10  m in the R  plane and 115  m in
the longitudinal plane, both in the barrel and the endcap. Because of the high
radiation damage, the innermost layer of pixel detector (the so called B-layer)
is designed to be replaced every few years. The high exposure to radiation and
the need for noise suppression require the pixel detector to be constantly cooled
down to a temperature between 5 and 10 .
1.2.3.2 SCT
The SCT system is formed by four barrel layer and nine endcap disks. Each
module is formed by two silicon \stereo" strips attached back to back with
a relative angle of 40 mrad, thus allowing three-dimensional reconstruction of
the trajectory space points. In the barrel region modules are rectangular and
parallelto beam direction, whileendcap modules arewedgeshapedareoriented
perpendicularly to the z axis. The accuracy of measurement in SCT is of 17
 m in theR  plane and 580 m in the longitudinal plane both in the barrel
and in the endcaps.
15