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1 Introduction
In the Standard Model of particle physics quarks and leptons, which are the so-called fermions, are described as fundamental, elementary particles, with their interactions described as mediated by means of the exchange of another set of elementary particles, more precisely the bosons. In the case of the electromagnetic interaction the force carrying particles are photons, in the weak interaction case they are W and Z bosons, and finally it is the gluons in the case of the strong interaction. Ever since the experimental discovery of the W and Z bosons, uncovering the mechanism by which they and the fermions acquire mass became one of the primary goals for particle physics. The Standard Model dictates that the W and Z bosons acquire their masses through the Brout-Englert-Higgs symmetry breaking mechanism, giving rise to a massive scalar particle, the Standard Model Higgs boson.
2 The LHC
Constructed at CERN in a 27 km long tunnel, The Large Hadron Collider aims to probe the TeV energy scale as the worlds largest particle collider. One of the main scientific goals of the LHC was demystifying the electroweak symmetry breaking mechanism by means of searching for in the Standard Model postulated Higgs boson.

The collider has been constructed to accelerate and collide protons at centre-of-mass energies of approximately 14 TeV, and to achieve an instantaneous luminosity of more than

1

0

34

c

m

-2

s

-1

. The counter-rotating proton bunches are separated by a mere 25 ns, giving rise to a bunch crossing rate equalling 40 MHz.

The LHC began operating as of 2010 at

s
=7

TeV

. Following this, in 2011 the number of bunches making up the beams was raised to 1380, changing the separation between the bunches to 50 ns, and giving rise to a significant increase in the luminosity. The centre-of-mass energy was then increased to 8 TeV in 2012, and during that time period the luminosity was also further raised, reaching maximum luminosities of near

7 ? 1

0

33

c

m

-2

s

-1

.

Based on 2011 data, a SM Higgs boson could be excluded in the high mass range up about 600 GeV, leaving only a small mass window below 127 GeV open.

In 2011 and during the first months of data taking in 2012, the data accumulated by the CMS experiment, corresponding to slightly more than

5

f

b

-1

per year, was analyzed, which resulted in the first observation of the Higgs boson.

An important experimental variable at CMS or the LHC in general is the pseudorapidity,
?
, is defined as

? =- ln (

tan (

?

2

)

)

, where
?
is the polar angle measured from the anticlockwise beam direction. Indicated by
?
is the azimuthal angle, measured relative to the axis defined by the beam directions. The component of momentum perpendicular to the beam axis is called the transverse momentum,

p

T

.

There is also the so-called pileup effect. A detected event often contains signals from multiple proton-proton collision and from more that one bunch crossings. Due to the high proton beam intensity, multiple proton-proton collisions can take place for each bunch crossing. The mean number of p-p collisions per bunch crossing was approximately 10 in 2011 and was raised to roughly 20 by 2012.
3 The CMS experiment
Having been designed as a general-purpose detector, CMS can identify and reconstruct photons, muons, electrons, hadronic jets, and the missing of transverse momentum, carried away by weakly interacting particles, very precisely. CMS consists of multiple sub-detectors, each making use of different technologies, calibration and reconstruction methods.

The backbone of CMS is a superconducting solenoid, giving rise to an axial magnetic field of

3.8

Tesla

. Both the central tracker and the calorimeters are positioned inside the the bore of the solenoid. The steel flux return yoke outside the solenoid is filled with ionized gas detectors which is used to detect and reconstruct muons. Trajectories of electrically charged particles are measured by a silicon pixel and strip tracker. This instrument has full coverage within a pseudo-rapidity range of

|
?
|
2

m

W

or

m

H

>2m

Z

). Given that

m

H

>2

m

t

, decays to

t

t
?

can reach up to a 20% branching ratio. Decays to fermions other than

t

t
?

, such as

H ? b

b
?

, only becomes relevant for Higgs masses less than

2

m

W

.

For a Higgs boson with

m

H

=125GeV

the branching fractions for

H ? b

b
?

and

H ? ? ?

reach about 56.9% and 6.2%, respectively. For decays into vector bosons, at least of of the two has to be off-shell, as a lower mass virtual

W*

and

Z*

bosons, which then go on to decay promptly. Calculations for the branching ratios into

WW*

and

ZZ*

result in 22.3% and 2.8%, respectively. Finally, at a branching branching ratio of 0.23%, a possibility are decays into a pair of photons (diphoton decay) via loop processes involving heavier charged particles.
6 Search strategies
Higgs production cross section is not as large as the ones of most other processes at the LHC. Large momentum transfer quark on quark, quark on gluon, and gluon on gluon scattering processes (respectively qq, qg and gg) give rise to jets with large values for

p

T

. W, Z and top pair production dominate by orders of magnitude over Higgs boson production, unfortunately causing a strong background in the search for Higgs bosons.

Higgs boson production with

m

H

=125GeV

at

s
=8TeV

is nearly a factor of

1

0

7

smaller than the production of

b

b
?

, which in turn only makes up a small percentage of the inclusive pair production of jets resulting from qq, qg and gg scattering. In the data sample collected until June 2012 the production of close to

2 ? 1

0

5

Higgs bosons at

m

H

=125GeV

were expected, which is tiny in relation to the number of inelastic proton-proton collisions that took, of which there were about

1

0

15

.

To avoid these backgrounds, the most viable strategy is to only look at non-hadronic final states, focusing on final state leptons, photons and missing transverse momentum.

The

channel, in which the Higgs decays into a

pair, which in turn each decay into an oppositely charged pair of leptons ( denoted as
l
= electron or muon) and the

H ? ? ?

channel, in which the Higgs decays into a pair of photons are the experimentally the most sensitive decay channels. Another interesting decay channel is given by

, in which the Higgs decaying into two W bosons, each decaying into an electron or a muon and neutrinos. While the two charged leptons can be detected, the neutrinos will be noticed through its

E

T

miss

signature.

In the

H ? ? ?

and

channels the Higgs boson invariant mass can be reconstructed as a sharp resonance on top of continuous backgrounds. These backgrounds are predominantly caused by prompt

? ?

and

pair production. Due to the missing transverse momentum, no mass peak can be reconstructed in the

channel, which shows itself as a wide peak in the dilepton mass.

Lastly, the fermionic

H ? b

b
?

and

H ? ? ?

decays were used, which did not contribute significantly to the discovery. Identification of the

H ? b

b
?

decay when the Higgs boson is produced by gluon-fusion is not possible because it is drowned out by the jet production background. To achieve this nevertheless, additional distinctive signatures have been used, such as the decays of the vectors or tops produced together with a Higgs bosons (

WH

,

ZH

or

t

t
?
H

). These processes have a lower production cross sections, making them less sensitive for a given integrated luminosity.

No statistically significant excess over the expected background was observed in either

H ? b

b
?

and

H ? ? ?

.
7 Discovery and decay modes
On 4th July 2012 CMS claimed the discovery of a new particle with a mass of about 125 GeV and the properties of the SM Higgs boson. The

,

H ? ? ?

, and

bosonic decay modes were used as the main evidence for this claim.
7.1

H ? Z

Z

*

? llll

The

H ? Z

Z

?

? llll

decay channel gives a relatively background free signature, but has a rather total small branching ratio. The Higgs is looked for through the selection of two pairs of isolated leptons, each consisting of two leptons with the same flavor and opposite charge. More precisely these are:

?

+

?

?

+

?

(

4 ?

)

,

e

+

e

?

+

?

(

2e2 ?

)

,

?

+

?

e

+

e

(

2 ? 2e

)

and

e

+

e

e

+

e

(

4e

)

.

The largest background comes from continuum

production. Another background is caused by Z+jets production, such as

Zbb

, and

t

t
?

pairs. This may give rise to isolated leptons through

Z ? ll

decay or through

t ? Wb ? l ? b

. The decays of the fragmentation products of the heavy b-quarks could bring forth two more leptons. Misidentification of jets as leptons is another problem. These leptons are less isolated and, when originating from b-quarks, don’t point back to the Higgs and Z bosons decay vertices.

Calorimeter and track-isolation requirements together with impact-parameter requirements were used to suppress these background to a minimum. The Z+jets and

t

t
?

backgrounds have been estimated from data control regions.

The expected and observed invariant mass distributions can be seen in Figure fig:HtoZZ. An excess of events is present near 125 GeV.

The probabilities for the background-only hypotheses are given by the minimum local p-values in the data, which occurs at 125.6 GeV, and has a significance of 3.2
?
and an expected significance of 3.8
?
.
7.2

H ? ? ?

This channel involves measuring a narrow peak on top of a diphoton invariant mass spectrum background. The background originates due to processes involving two promptly produced photons, or due to

pp ? ? +jet

and dijet processes where one of the reconstructed photon objects originated in a jet.

Diphoton triggers are used to detect diphoton events. Further selections are applied in an attempt to suppress the

pp ? ? +jet

and dijet backgrounds. Cuts based on the shapes and locations of the photon showers in the calorimeter, and on the isolation of the photons in the detectors are used to suppress backgrounds.

For the statistical analysis, the sum of a signal mass peak and a background distribution is fitted to the diphoton invariant mass distribution, the shape of which was obtained from simulations. The fits are performed for a range of Higgs boson mass hypotheses.

A statistically significant excess of events was found near

m ? ?

= 125 GeV as can be seen in Figure fig:Htogammagamma.
7.3

H ? W

W

*

? l ? l ?

This channel is challenging in around the mass range of 125 GeV because the expected signal rates are small due to small

H ? W

W

*

branching fraction.

The reconstruction of a narrow mass peak is not possible due to the presence of neutrinos. Instead, one must get the necessary information out of a potential excess of events on top of the expected backgrounds. The so-called WW transverse mass, dependent on the leptons and the missing transverse momentum is used to distinguish background from signal. The main background is due to WW,

t

t
?

and
t
production processes.

The distributions of the transverse mass and the invariant mass of the two leptons, after application of all relevant cuts, is visible in Figure fig:HtoWW.

Once again, an excesses of events above the no-Higgs-boson hypothesis is visible.

Near 125 GeV, the p-value is minimal with a local observed significance of 1.6
?
and expected significance of 2.4
?
.
7.4 Combined significance of all channels
In the summer of 2012, CMS observed an excess of events near

m

H

=125GeV

in the

H ? ZZ* ? 4l

,

H ? WW* ? ? l ? l

and

H ? ? ?

channels. The observed local significance is 5.0
?
, compared to the expected 5.8
?
at a mass of approximately 125 GeV.

Calculating the best-fit value of the ratio between the fitted cross section and the expected SM Higgs cross section,

? =

?

?

SM

, is a way to test the compatibility of the observed particle with the SM Higgs. The result is shown in Figure fig:combined. The observed value for the excess at

m

H

=125.5GeV

is

0.87 ± 0.23

, meaning that the observed signal lies within 1 standard deviation from the SM Higgs expectation.
8 Conclusion
The measured particle was found to be consistent with the one expected from the SM Higgs boson.

Evidence of the decays of the Higgs to pairs of vector bosons summing up to a net electric charge of zero means the new particle must be a neutral boson. The existence of a significant excess in the diphoton channel means the observed particle can not be of spin 1. Tests of the angular distributions indicated that the newly discovered Higgs-like particle is indeed a scalar.

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