LHC Physics

SeongchanParkYonsei&KIAS

Colloquium for 2016 Winter School on Collider PhysicsHigh-1 Resort, 24-30 Jan, 2016

from ‘God particle’

LHC is here

We are at the very front of human intelligence

What we are doing is something GREAT.

Be proud of what you are doing!

Plan

• A theorist’s introduction to LHC physics

• Physics learned from the LHC so far

• Some speculations about BSM

• Conclusion

LHC: Large Hadron Collider

Q. Why large?

Actually proton.Q. Why not electron?

LHC in Seoul

google “LHC MAP”

Tevatron in Seoul

FCC in Seoul

FCC in Seoul

available Energy/momentum

distance one can probe

The uncertainty principle

GeV

★~1970 reached strong scale 10-13cm≈(1/M) e-2pi/as b0 ★~1900 reached atomic scale 10-8cm≈1/(α2me)

eV

★~2011 reach weak scale 10-17cm ⇐LHC here!

TeV

Why large?

Why hadron?

lighter, more efficient!

Q. Why proton rather than electron?

Power of radiation:[Larmor’s formula]

(see Jackson)

Proton• proton is NOT an

elementary particle but a composite state of many colored particles

• p={u,d,s,g,ubar, dbar…}

• It is highly dynamical!

• Q. Is this good or bad?F. Bissey et.al. Phys. Rev. D 76, 114512 (2007)

Only a fraction of the proton energy is actually participating the scattering processes ..

Parton Distribution Function (PDF)

• First introduced by Feynman (1969) in Parton model.• i=u,d,s,c,b,t,g, and anti-quarks (Parton id =[-6,6])• Q=scale of the interactions • scale dependence determined by Alterelli-Parisi equation

from Lattice calculations, fits to the data (and AdS/QCD)

LO, NLO, NNLO…

(more in SJLee, MHPark’s lectures)

PDFs in MarketCTEQ fortran code and grids

CTEQ-Jefferson Lab (CJ) the CJ12 PDF sets

GRV/GJR fortran code and grids

MRST fortran code and grids, C++ code

MSTW fortran, C++ and Mathematica codes + grids etc.

ALEKHIN fortran,C++,Mathematica code, and grids

ZEUS ZEUS 2002 PDFs, ZEUS 2005 jet fit PDFs

HERAPDF Combined H1/ZEUS page, HERAPDF1.0 paper

H1 H1 2000

BBG BBG06_NS

NNPDF Non Singlet PDF code - hep-ph/0701127

CTEQ6, Q=100 GeV

gluon

up

http://hepdata.cedar.ac.uk/pdf/pdf3.htmlOn-line PDF plotter

Hadron Collider=Infinitely many colliders with different CM energies

Useful when you want to search for something unknown!For precision measurements, you’d better have e-e+

machine.

but complicated…..

Q. Assuming that ~20% energy of proton energy is contributing to a hard collision, how small distance scale can be probed at LHC14?

Answer:

Q. why can we use hbar=c=1?

LHC~TeV ~10-17 cm

What’s Physics of 10-17 cm?

• The distance scale for electroweak symmetry breaking… (Q. what is EWSB?)

• In the SM, the Higgs mechanism is responsible for EWSB …(Q. any other idea?)

• => excitation of Higgs field (=Higgs particle) can be seen.

• The LHC discovered a Higgs boson!

누가 “힉스 입자”라는 이름을 붙였나?

P. Higgs “My life as a boson” (2010)

누가 “힉스 입자”라는 이름을 붙였나?

S. Weinberg (2012) The New York Book Review

Steven Weinberg 曰

Refs in S. Weinberg [Model of Leptons] 1967

잘못 된 순서

…

힉스입자 발견의 숨은 의미(1)

• The first discovered spin-0 elementary particle The mechanism of Electroweak symmetry breaking

but theoretically problematic (Q. what’s the problem?)

Note) there were many spin-0 particles discovered but they are all composite states of quarks. (mesons: q-qbar)

힉스 입자 발견의 숨은 의미(2)

• Completion of the SM All elementary particles are now discovered! (Q. what are they?)

All free parameters in the SM are now measured! (Q. how many?)

but it does not mean that particle physics ends.. (later more)

표준모형(1)• 양자 역학 + 상대성 이론 =

• 시공간 차원 수 =

• 시공간 대칭성 =

• 게이지 대칭성 =

• 기본 입자 =

• +게이지 대칭성의 붕괴 메커니즘

표준모형(2) • QFT with Lorentz invariance, local gauge invariance

• spacetime = 4D Minkowski (gravity ignored. why?)

• gauge symmetries: G=SU(3)CXSU(2)LXU(1)Y, Y=T3-Qe

• Matter s=½: Weyl spinors (not Dirac? Why?) (Where are neutrinos?)

• [Q~(3,2,yQ), u~(3,1,yu), d~(3,1,yd), L~(1,2,yL), e~(1,1,ye) ]X3 gen

• Complex scalar s=0: H~(1,2,yH) with negative mass term for EWSB (why not fermion for EWSB?)

from wikipedia

Quiz: 표준모형의 자유계수는 몇 개인가?

자유 계수(free parameter) = ??

Free parameter

=parameter in theory determined only by experiments not by theory

자유 계수: 소금 한 큰 술 …

125 GeV

Free parameters in the SM

NEW!

(NOTE) Flavor problem

• Most of free parameters in the SM are about Fermion masses and mixings.

• How can we understand those parameters? Any underlying principle?

• More in Prof. Y. J. Kwon’s talk in this school.

CMS PAS HIG-14-009

ATLAS arXiv:1406.3827

PDG new

Current status of Higgs mass measurement

CMS PAS HIG-14-009

Decay pattern is consistent with the SM!

• The Higgs in the SM plays two main roles: EWSB (or gauge boson masses) and fermion masses. Both have been experimentally checked!

The Higgs potential

The most generalrenormalizablepotential with

Tachyonic mass

NOTE: 2 free parameters in the entire Higgs sector.

VEV of the Higgs

known since 1970s

Higgs quartic coupling

(2012)

4제곱항의 계수

Higgs potential

Now, all the parameters in the Higgs sector are experimentally measured!

Ultimate confirmation of the SM!

h-h-h h-h-h-h

Indeed, these are main motivations for future colliders

At large field limit

Quantum effective coupling

RG running of lambda

Top quark Yukawa coupling

self gauge

Top massCMS+ATLAS arXiv:1512.02244

2-loop effective coupling

criticality

arXiv:1405.4781

[Hamada, Kawai, Oda, SCP, PRL 2014]

Higgs potential with Quantum corrections

Higgs potential near criticality

Hamada, Kawai, Oda, SCP (2014, 2015)

% of woman science major

Intriguing implication

• The Higgs potential at high scale becomes flat.

• That may contribute to the accelerated expansion of the Universe in early time…

sol: Higgs inflation

A big beautiful hierarchical

structure

SM:

what’s wrong with this?

A big hierarchical

structuretends to collapse

unless there is a mechanism protecting the

structure

But keep in mind hierarchy exists in nature

Solar Eclipse: angular size of the sun is the same as the angular size of the moon within 2.5% (pure coincidence!)

Politics: Florida recount, 136/2,913,144 =1.000061 (!!)

Numerology: 987654321/123456789 =8.000000073 (!!!)

(Food for thought: is it really numerology?)

The hierarchy problem

• We don’t understand why the Higgs mass is so much lighter than the UV scale of the physics

★ We DO understand why the gauge boson’s are much lighter than the UV scale

★ We DO understand why the electron is much lighter than the UV scale

The 1-loop Higgs mass

bare parameter (in Lagrangian) cutoffphysical

~17 digits tuned

~natural!

The 1-loop Higgs mass in dimensional regularization

only sees “log divergence” not “power divergence”

Q. where is the quadratic divergence?

Criticality

scattering at E*:

for scalar mass:

Why so small?

BSM at TeV

• Many BSM models have been proposed …Low scale SUSY, extra dimensions (= new strong dynamics, technicolor), no-Higgs models…

• Most of them predicted new particles at around TeV

SM

Exotica

Higgs

CMS publications

SUSYTop

Bottom

http://cms-results.web.cern.ch/cms-results/public-results/publications-vs-time/

but no new physics seen so far @ LHC

• Impressive progress in NP searches..

• but no NP is seen yet (diphoton @ 750 GeV???)

Interpretation-1• New physics is just around the corner, once again… LHC13

and LHC14 will see new particles! Q. why LHC7/LHC8 didn’t see anything other than the Higgs boson?

• Maybe the new physics scale is still too high to be seen in LHC8…Q. why the new scale is at least O(10-100) bigger than the EW scale if the new physics is responsible for the hierarchy problem? (little hierarchy problem)

• Maybe the new physics has so degenerate spectrum so that the LHC detector could not identify the soft signals.. (e.g. stop/top degeneracy) Q. Why so degenerate? Is it simply due to that GOD is cruel to us?

Interpretation-2• “The hierarchy problem” may be just an artifact generated by

trouble-making theorists?

• If one takes the Higgs mass problem serious, what about the cosmological constant problem, which is a way more serious …

• (Mp/Mw)2 vs (Mp/cc)4 ~(Mp/Mν)4

• Can anthropic argument be the (only possible) solution to the CC problem ?

• Maybe we have not reached to the proper level of understanding these problems?

clues for BSM• Dark matter : no candidate for CDM exist in the

SM => indicating ~TeV physics

• Baryon asymmetry: CKM picture does not provide enough CP violation

• Neutrino oscillations : may need RH neutrinos?

• …many more…(Q. what’s yours?)

Interplay with astrophysics

• New generation of astrophysical observations based on cosmic-ray detection and CMBR measurement started to give us new data about our universe..

• IceCube (neutrino), Fermi-Lat(photon), Planck (CMBR) etc..

• Very interestingly, newly obtained data often suggests that what we know is only a small fraction of the whole story

• (Energy of unknown source)/(Energy of known source)~ (95%)/(5%) …the SM is only responsible for this 5%!

• New source for cosmic ray may call for our attention..

http://xkcd.com

So

• There are many more new discoveries waiting for us!

• Let’s be optimistic

• Let’s be prepared!

9th planet could have seen only by who were looking for

Hurt (IPAC) [Diagram was created using WorldWide Telescope.] See more at: http://m.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523#sthash.2XiwckxI.dpuf