Discovery of The Higgs Bosonskipper.physics.sunysb.edu/~drees/PHY599/DongwonHan.pdf · Discovery of The Higgs Boson. What is the Higgs Boson?A Historical Account of the Higgs BosonThe

What is the Higgs Boson? A Historical Account of the Higgs Boson The ATLAS experiment Production/ Decay Channels at the LHC Result : The ATLAS Experiment Ongoing Research Summary

Discovery of The Higgs Boson

Dongwon ”DW” Han 1

Stony Brook University, USA

October 12th, 2015

1Local Expert: Professor John HobbsDongwon ”DW” Han Stony Brook University, USA



Outline

1 What is the Higgs Boson?

2 A Historical Account of the Higgs Boson

3 The ATLAS experiment

4 Production/ Decay Channels at the LHCH → ZZ ∗ → 4l ChannelH → γγ ChannelH →WW ∗ → lνlν Channel

5 Result : The ATLAS Experiment

6 Ongoing Research

7 Summary

Dongwon ”DW” Han Stony Brook University, USA



What is the Higgs Boson?

Within the Standard Model (SM), electroweak symmetrybreaking is described by Brout-Englert-Higgs mechanism

BEH mechanism predicts the existence of a spin-0 scalarboson, known as the Higgs Boson, H.

Unfortunately, the SM does not predict the mass of the HiggsBoson, mH

Fortunately, given mH , we can calculate the production crosssections and the decay branching ratio




A Historical Account of the Higgs Boson I

In 1964, The Standard Model Higgs Boson was firstintroduced

Spontaneous breaking of the electroweak gauge symmetry.Proposed by three independent groups

Englert and Brout

Higgs

Guralnik, Hagen and Kibble

In 1967, the Higgs Boson got incorporated into the SM




A Historical Account of the Higgs Boson II

In the early 70s, its existence was only hypothetical.

Constraint on the Higgs mass: mH > 15MeV

”We should perhaps finish our paper with an apology and acaution. We apologize to experimentalists for having noidea what is the mass of the Higgs boson.”[Ellis et al., 1976]

Eventually, due to the stunning succes of the SM, a serioussearch for the Higgs Boson began




A Historical Account of the Higgs Boson III

Searches for the Higgs Boson at the LEP

ee+ collision experiment(LEP1) ee+ → Z → H + f f̄(LEP2) ee+ → Z + HIn 2000, Higgs-like events were observed aroundmH = 115GeV .The LEP was shut down at the end of 2000, leavingmH > 114.4GeV .

CDF and D0 experiments at Tevatron Collider

pp̄ collision experimentStudied H → bb and H →W+W−

Excluded the mass range 100GeV < mH < 103GeV and147GeV < mH < 180GeV




A Historical Account of the Higgs Boson IV

The LHC started operating in late 2009

pp collision experiment

(2011) 5fb−1 of data at 7 TeV in the center of mass

(2012) 7fb−1 of data at 8 TeV in the center of mass

On July 4th, 2012, CERN announced discovery of a newparticle consistent with the Higgs Boson predicted by theStandard Model at a mass of 125 GeV




A Historical Account of the Higgs Boson V

In 2013, Nobel prize in physics was awarded to Englert andHiggs ”for the theoretical discovery of a mechanism thatcontributes to our understanding of the origin of mass ofsubatomic particles, and which recently was confirmedthrough the discovery of the predicted fundamentalparticle, by the ATLAS and CMS experiments atCERN’s Large Hadron Collider.” [The Nobel Foundation, ]




The ATLAS experiment

ATLAS stands for AToroidal LHC Appartus

pp collision experiment

46 meters long, 25 meters ina diameter

3000 physicists from aroundthe globe are involved.

http://atlas.ch/



http://atlas.ch/


Production/ Decay Channels at the LHC I

Note: the table is for mH = 125.36GeV [Aad et al., 2015]




Production/ Decay Channels at the LHC II

https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections#Higgs_cross_sections_and_decay_b



https://twiki.cern.ch/twiki/bin/view/LHCPhysics/CrossSections# Higgs_cross_sections_and_decay_b


Production/ Decay Channels at the LHC III

http://lepewwg.web.cern.ch/LEPEWWG/



http://lepewwg.web.cern.ch/LEPEWWG/


H → ZZ∗ → 4l Channel

H → ZZ ∗ → 4l Channel I

ZZ ∗ decays into two pairs of leptons with the same flavor andopposite charge.

Candidates: two pairs of isolated leptons

Leading pair: a pair with invariant mass closest to the Zboson mass.

Background: dominated by tt̄ and Z+ jets

Provide good sensitivity over a wide mass range (110-600GeV), because of the high momentum resolution

Leptons are very well measured at the LHC detector, so youcan reconstruct mH




H → ZZ∗ → 4l Channel

H → ZZ ∗ → 4l Channel II

[Aad et al., 2012]

[Aad et al., 2012]




H → γγ Channel

H → γγ Channel I

Mass Range between 110GeV and 150 GeV

Triggers: Energetic photonpairs

Et > 20GeV for eachcluster (7 TeV data)Et > 35GeV on theleading cluster and 25GeV on the subleadingcluster(8 TeV data)

Dominant background: theSM γγ production

mH can be reconstructed [Aad et al., 2012]




H → WW∗ → lνlν Channel

H → WW ∗ → lνlν Channel I

Signature: two oppositecharge leptons with largetransverse momentum and alarge momentum imbalance

Because the outgoingneutrinos can’t be observedat the LHC detector, mH

can’t be reconstructed inthis case.

The background and itscomposition depends on thejet multiplicity

[Aad et al., 2012]




H → WW∗ → lνlν Channel

H → WW ∗ → lνlν Channel II

[Aad et al., 2012]




Result : The ATLAS Experiment I

[Aad et al., 2012]




Result : The ATLAS Experiment II

[Aad et al., 2012]




Result : The ATLAS Experiment III

[Aad et al., 2012][Aad et al., 2012]




Result : The ATLAS Experiment IV

[Aad et al., 2012]




Ongoing Research

Is it a spin 0 particle?

Spin 1 is ruled out. We still need to check spin n > 2 cases

Is it a scalar or pseudoscalar?

In the SM, there is only one scalar Higgs Boson. However,other theories, such as SUSY, predicts the existence of a familyof Higgs Boson, which at least one of them is pseduscalar

Does this new particle couple to other particles in proportionto their masses?

If it is the case, it strongly suggests that this new particle giverises to particle massesIn order to test this, we need to observe other decay channelswhich haven’t been observed yet.




Summary

A Higgs-like particle was discovered in 2012.

This new particle is the SM Higgs Boson beyond anyreasonable doubt.

So far, the properties of the Higgs Boson is consistent withthe prediction of the Standard Model.

Further studies will tell us whether there is any deviation fromthe SM.




Bibliography I

Aad, G. et al. (2012).

Observation of a new particle in the search for the Standard Model Higgsboson with the ATLAS detector at the LHC.

Phys. Lett., B716:1–29.

Aad, G. et al. (2015).

Measurements of the Higgs boson production and decay rates andcoupling strengths using pp collision data at

√s = 7 and 8 TeV in the

ATLAS experiment.

ATLAS Experiment.

What is ATLAS?

Accessed October 10, 2015.




Bibliography II

Ellis, J., Gaillard, M. K., and Nanopoulos, D. (1976).

A Phenomenological Profile of the Higgs Boson.

Nuclear Physics B, 106:292 – 340.

Schott, M.

Standard Model at the LHC.


TANEDO, F.

Why do we expect a Higgs boson? Part I: Electroweak SymmetryBreaking.


The CDF Collaboration, the D0 Collaboration, the Tevatron New Physics,Higgs Working Group (2012).

Updated Combination of CDF and D0 Searches for Standard Model HiggsBoson Production with up to 10.0 fb−1 of Data.




Bibliography III

The Nobel Foundation.

The Nobel Prize in Physics 2013.




Documents

Discovery of The Higgs Bosonskipper.physics.sunysb.edu/~drees/PHY599/DongwonHan.pdf · Discovery of The Higgs Boson. What is the Higgs Boson?A Historical Account of the Higgs BosonThe