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Goals of the LHC: Finding the Higgs boson
:: 01 January, 2002
The two main objectives are achieving new accelerator is the discovery of the
Higgs boson and the discovery of particles predicted in models of supersymmetry.
To do this enter the LHC will collide a beam of protons and antiprotons with a
total energy of 14 TeV. When run at full capacity the total brightness of the
detector will
.
Energy collisions enable a better understanding of physical phenomena in high
energy physics top quarks and the physics of heavy ions.
The Higgs boson
All models renormalisable electrofaibles interactions are based on a broken
gauge symmetry. The consequence of this break and the renormalisation of the
theory is the presence of a neutral scalar particle, called Higgs boson. When
the energy scales are too small for the weak and electromagnetic forces are
unified, there is spontaneous symmetry breaking. We can compare this phenomenon
to a ferromagnetic metal, which when cooled after being melted, acquires a
non-zero magnetization. This symmetry breaking, through the Higgs mechanism to
give a non-zero mass to the bosons of the weak force and all fermions. This
boson has the same quantum numbers except that the vacuum mass. The vacuum could
be filled with virtual particles and Higgs would be the amplitude of interaction
of the particle with the Higgs boson that gives mass to the particle. Different
accelerators around the world have tried to find out without success so far.
With Energy Accelerator in games, the minimum mass of the hypothetical Higgs
boson is in the vicinity of 80 GeV and in the standard model maximum limit for
its mass is about 1 TeV. Moreover, it is the only Standard Model particle not to
have been discovered. Meanwhile mass predicted by the standard model, there are
99% chance that we can see the Higgs boson in the LHC between 2007 and 2009. His
absence would give a hard blow to the standard model and open the door to new
theories such as supersymmetry.
In the new LHC, 5 experiments are currently under construction. It is of ATLAS,
CMS, ALICE, LHCb and TOTEM. Each of his experiments will consist of a series of
detectors that will be built to study collisions of particles from several
different viewpoints. We will focus mainly on two experiments, ATLAS and CMS.
The operation experiences will be peeled and the purpose and expected results of
these detectors.
One of the main detectors of the new LHC is ATLAS. The sensor will 25 meters
long and 45m wide and weigh 7000 tonnes. The purpose of this experiment is to
detect high-mass particles that have not yet been detected by the accelerator to
lower energies. It will check the standard model to higher energies. In the
proton-antiproton collisions, a wide variety of particle will be produced. ATLAS
is designed to detect the widest range of signal. The complexity of this
detector will be larger than the energy and the amount of collisions that the
detector will see are enormous.
Collisions between hadrons
The nature of the LHC leads us to consider the nature of collisions between
protons there. The collision between the beams of protons at high energy can be
represented as a quark soup, the antiquark and gluons. For collisions, the most
important parameter is the cross section. The starting model is a way to
calculate this cross section. In the model proposed by partons RP Feynman, the
nucleon is composed of particles without mass, timely and free. These elements
are called partons. Using the quantum chromodynamics, and the model of partons,
it is possible to calculate the cross sections of processes with gluons and
quarks. Suppose the reaction.:
(1)
where a and b are hadrons, c is a parton and X is the remainder of the
reaction. With the model of partons, it is only one parton of the hadron i A,
which interacts with a parton j in the hadron B to give the product v. We can
then written as the interaction:
(2)
When the partons emerging from the hadron to interact
together, they take with them a quantity of the
momentum of the hadron.
X is the amount of
pulse i and x b that
of j. The section of the collision between parton is
obtained with the following equation:
(3)
The problem with this model is that it does not take into account the strong
interactions between particles. Indeed, we know that hadrons are composed of
quarks and gluons. With the theory of asymptotic freedom, it is impossible to
have free quarks. To reflect this, QCD corrections must be made to the template.
In the limit of high energies, it is possible to use the theory of perturbative
QCD.
Creation of the Higgs boson
The main source of the Higgs boson at the collision between a proton and an
antiproton at CERN will be the fusion of gluons or merger of bosons of the weak
force. According to the standard model as well as some more exotic models like
the minimal supersymmetric standard model, the channel for the production of the
Higgs will be the fusion of two gluons with the mediation of top quarks. The
following figure shows the cross sections of different modes to create the Higgs
boson mass by the boson.
Figure 11)-section for the production of Higgs in the
light of the mass of the Higgs boson.
The Feynman diagram on the left illustrates the production of the Higgs to the
lowest order contribution to QCD, and the right diagram, the production of Higgs
the theory complete.
Figure 12) Feynman diagrams for the production of a Higgs boson at da merging
gluon.
Note that the radiative corrections of orders higher than the cross section of
this reaction are currently calculated. These corrections are very important and
requires a very good fit parameters.
The second process which has the largest section is the merger of two weak
vector bosons that is to say WW or ZZ. The Feynman diagram below illustrates
this phenomenon.
Figure 13) Merger of weak bosons given for a Higgs boson.
Although this reaction has a cross section less than the gluon fusion, it offers
the advantage of leaving a signature easily detectable.
Disintegrations of Higgs
The LHC detectors can not directly detect the Higgs boson, but rather the
product of its disintegration. Indeed, the Higgs should disintegrate or pair of
photons, a pair of massive particles or gluons. The following figure shows the
ratio of branches to the disintegration of the Higgs by its mass.
Figure 14) Report of sidings to the disintegration of the
Higgs by its mass.
The report of the different branches for the disintegration of the boson depend
on the coupling of these Higgs particles and their mass. As we can see from
Figure 3), with a weak boson mass, the main channel of disintegration will be
the creation of a pair of bottom quarks, while greater mass, the creation of
pairs of weak bosons WW and ZZ or a pair of top quarks will be privileged. At
low energies so the disintegration fermionique is advantaged. The width of
disintegration in this case is given by the following formula:
(4)
x f
is
, N
c color factor for fermions which is 1 for leptons and 3 for
quarks and G F at constant Fermi.
Bosons decay into the low will be possible if the Higgs mass is larger than the
mass of a pair ZZ or WW.
In this case, with x =
w
et x Z =
and x =
Z
, The width of decays
are:
(5)
In addition, the coupling of
the Higgs boson with zero mass bosons like the photon or gluons is possible. In
particular, its decay into two photons, of which has a low cross section, with
the advantage of leaving a trail easy to detect and connect to the Higgs. This
channel will be particularly monitoring for m H less than 130 GeV. The width of
this reaction is expressed as:
(6)
If the mass of the Higgs boson is between 110 and 130 GeV, the branch most
likely to the disintegration of the Higgs will be the production of a quark and
an antiquark bottom. Unfortunately, this method can not be observed in the LHC
detectors, as it will be drowned in background noise QCD. For this mass scale is
the decay into two photons which will be easier to detect. With this in mind,
the electromagnetic calorimeters have been calibrated. They are specially
optimized to observe the best possible resolution of such decays.
For a mass between 130 GeV and twice the mass of the boson
Z
0, chains of transformations that can be observed with greater
ease will be the disintegration of a two-Higgs Z, then 4 leptons and the
disintegration of un Higgs into two W, two leptons and two neutrinos. These
leptons are mostly electrons and muons. The CMS will be particularly suited to
detect these by-products.
With a mass of more than twice the mass of the Z boson, the Higgs fall apart
only in pairs of weak bosons WW and ZZ. As in the case of the intermediate boson
mass, these bosons disintegrate into leptons, allowing clear detection of the
Higgs. In the case of a mass of more than 350 GeV, it is possible decays into
two quarks, top and antitop. The observation of this reaction will be like in
the case of bottom quarks, with the difficult background QCD.
The discovery of the Higgs boson at the LHC will not be an end in itself. After
its discovery, if any, will determine its mass accurately, its quantum numbers,
parity and its coupling with other particles. This is thanks to the
determination of coupling constants that could link the Higgs boson with
standard model predictions and other theoretical models.
Tags: LHC , Higgs boson , accelerator , particles , Energy collision , physics of heavy ions , quarks , high energy physics , Higgs mechanism , virtual particles , ,
