Post on 14-Aug-2020
LHC: the unbelievable pursuit for the unimaginable
Kajari Mazumdar
Department of High Energy Physics
Tata Institute of Fundamental Research
Mumbai
http://www.tifr.res.in/~mazumdar
mazumdar@tifr.res.in
Evening lecture, IWSA, Vashi, Mumbai. 20 December, 2014
• Ancient civilizations constituents of the universe:
Earth, Air, Fire, Water
• By 1900, nearly 100 elements!
• 1936 three basic particles: proton, neutron, electron
The eternal question of mankind
• Which principles govern energy, matter, space and time at the most elementary level?
• What is the world made up of? • How does it work? High Energy Physics tries to answer these. It also brings in technological spin-offs.
Today
What lies within…? 12/20/2014 2
Essential tool: Microscope
The probe wavelength should be smaller than the distance scale to be probed higher energy needed:
xE
41 mm 10 eV21 nm 10 eV
20 1310 m 10 eV
10 TeV
(1 eV = 1.6 * 10 -19 Joule)
Typical energy scales:
• Motion of air atom, room temp: 0.04 eV
• Chemical reactions/atom, visible photons
of light:: 1 to a few eV
• Nuclear reactions, per atom: 1 MeV
• Rest energy mc2 of proton
~1 Billion eV (1 GeV)
• Each proton in each LHC beam:
4 Trillion eV (4 TeV) during 2010-2012,
6.5 TeV from 2015
• We have already probed matter up to a distance
scale of ~ 10-18 m with probes of energy ~ 100 GeV.
• LHC probes length scales ~ 10-19 to 10-20 m
12/20/2014 3
Atom Proton
Big Bang
Radius of Earth
Radius of Galaxies
Earth to Sun
Universe
Study physics laws of first moments after Big Bang
increasing symbiosis between Particle Physics,
Astrophysics and Cosmology.
Super-Microscope
LHC
Hubble ALMA
VLT AMS
Dimensions in Physics
12/20/2014 4
Technological advancements make it
possible to stretch the limits of our
knowledge for the smallest and the largest
tera
sc
ale
Superstrings ?
Unified
Forces
Inflationary
Expansion
Separation
of Forces Nucleon
Formation Formation
of Atoms Formation
of Stars Today
Big Bang
Time 10-43 s 10-35 s 10-10 s 10-5 s 300 000 years 109 years 14∙109 years
Energy 1017 TeV 1013 TeV 1 TeV 150 MeV 1 eV 4 meV 0.7 meV
Travelling back in time
30 Kelvin
12/20/2014 5
10 thousand km
Connection with our Universe
6 12/20/2014
We belong to Milkyway galaxy
7 12/20/2014
A typical galaxy: 100 million (10 8) larger than the earth!
Contains a million million of stars!
Our universe contains hundred thousand million (10 11) galaxies 10 23 stars: all made up of same elements of matter!
8 12/20/2014
20
Institute of Physics Peter Kalmus Particles and the Universe
Forces
Gravity
falling
objects
planet
orbits
stars
galaxies
inverse
square law
graviton
inverse
square law
photon
short
range
W±, Z0
Electro-
magnetic
atoms
molecules
optics
electronics
telecom.
Weak
beta
decay
solar
fusion
Strong
nuclei
particles
short
range
gluon
BUT MATTER IN THE UNIVERSE IS NEUTRAL, because positive and negative charges cancel each other precisely. Gravitation is the dominant force in the Universe
Relative strength of gravitation, weak, electromagnetic, strong interactions ~10-40 : 10-5 : 10-2 : 1
Fundamental Forces
12/20/2014 9
The messengers of the forces: 1) g for electromagnetic interaction 2) W ±, Z for weak .. 3) 8 gluons for strong ..
Fundamental particles
• Experiments measure the masses of all the elementary particles which are basic inputs to theory. • Once the mass, electric charge etc. are known, theory can predict the results of experiments.
Matter particles interact with various forces via the carrier particles.
constituents of everyday matter
Matter particles
Newton: F = ma
Einstein: E = mc2 12/20/2014 10
LHC
Present Wisdom
11
• All behaviour of the matter particles (fermions) can be explained in terms
of few forces carried by the exchange / carrier particles (bosons):
simplistic nature of basic rules.
• All interactions
behaved as a single
one when the universe
was much younger
and hotter
idea of unification!
How did we achieve this picture?
Connects the largest with the smallest entities in the universe.
12/20/2014
Five fold symmetry Radial symmetry Reflection/Bilateral symmetry
Dogma of Symmetry
• Beauty in symmetry has been appreciated by mankind.
• Symmetry considerations have practical applications too!
eg., position of eyes, weighing balance.etc 12/20/2014 12
Highlights of 20th century physics
• Special relativity
• General Relativity
• Quantum Mechanics
• Quantum Field Theory
• Standard Model of elementary
particles and their interactions
First example of embedded symmetry in physical law: Newton’s law:
Covariant under rotations F, a changes same way under rotation. Invariant under Galilean transformations F, a does not change in a boosted reference frame.
F = m a
Mathematics Physics • Calculus • Complex numbers/functions • Differential geometry • Group theory • Hermitian operators, Hilbert space •….
Symmetry considerations play big role in physics
and in mathematical formulations.
Symmetry of a physical system is a physical or mathematical
feature of the system (observed or intrinsic)
that is "preserved" under some change. 12/20/2014 13
Ex.1: The temperature of a room translational symmetry.
2: Symmetry of a mathematical function: a2c + 3ab + b2c
Manifestation of symmetry
Electromagnetism: Maxwell’s equations
• First attempt to combine electricity and magnetism.
• Invariant under Lorentz transformation.
• Also invariant under gauge transformation.
• Define scalar (f) and vector (A) potentials related to electric & magnetic fields : E = −∇f − ∂A/∂t, and B = ∇ × A • E and B remains unchanged even if we change the potentials as : f′ = f − ∂L/∂t and A′ = A + ∇L, where L is a function of (x, t).
Gauge transformation
14 12/20/2014
Quantum theory is invariant under constant phase
transformations of wave function This symmetry leads to charge conservation. If the phase is a function of space-time, the phase invariance is lost.
Gauge symmetry demands quantum theory should be invariant under
space-time dependent phase transformations.
• To ensure the invariance, one has to introduce a vector field into theory.
• This vector field corresponds to the photon (g), carrier of EM interaction.
The role of photon as carrier particle logically defined.
Gauge Symmetry
Gauge Invariance, proposed by Hermann Weyl, is
the cornerstone of modern day particle physics.
12/20/2014 15
Electromagnetic and weak interactions are similar in nature.
Electroweak symmetry : basic idea
16
Enrico Fermi was the first to write down a theory of beta
decay (1934), with the name neutrino coined by Pauli.
Improved theory (1956): Intermediate Vector Bosons:
proposed by Sudarshan and Marshak,
and, independently, by Feynman and Gell-Mann.
Neutrino
electromagnetic weak
n p e- n
d -1/3 u +2/3 W-1 ( e-1 n) d
u
W
Weak Interaction; • Responsible for some kinds of radioactivity (b decay)
• Only force neutrinos (n) can feel, makes sun burn.
• Typically very slow process: C14 decay takes 6,000 years!
• Carried by weak bosons : charged ( W±) or neutral (Z0)
12/20/2014
• Gauge symmetry is required to correctly describe the interaction of matter
• Quantum electrodynamics is the most complete and successful theory.
• The theoretical predictions match very well with experimental results.
Eg. , value of fine structure constant matches upto 9 digits after decimal!
• Carrier of EM interaction, Photon, is massless.
• Carriers of weak interaction, W+, W- should couple to photon as
electron does : couplings are universal unification of two forces
Electroweak theory 4 carriers particles (spin-1 boson): g, W+, W- , Z0 .
• Carriers of weak interaction needed to be massless to respect symmetry
with electromagnetic interaction infinite range expected like in EM.
• Experimentally observed: weak interaction has short range
the carriers must be massive!
Unification of two forces?
However there is a big problem
Several scientists, in independent groups, worked the way out for the mass
generation of carrier particles of weak interaction during early 1960s. 12/20/2014 17
Nambu-san, in 1960, suggested how
it is possible to have our cake and eat it.
• The equations describing the system
are invariant under some symmetry.
• The ground state of a system breaks
the symmetry in the dynamics. phase transition
Spontaneous Symmetry Breaking
In nature, spontaneous symmetry breaking
is not very uncommon. eg., a ferromagnet
below and above Curie temperature
Symmetry disappears at low energy, reappears at high energy.
Electromagnetic and weak interactions are combined together in a single
Electroweak theory where symmetry is spontaneously broken at lower energy
between EM mediator g and mediators of weak interaction W+, W-, Z0 .
Nobel prize in 2008
12/20/2014 18
Nambu’s idea was taken up in earnest in 1964 by three groups to propose thereotical argument for mass generation in weak interaction:
1. Francois Englert and Robert Brout at Brussels
2. Peter Higgs in Edinburgh
3. Gerald Guralnik, Carl Hagen and Tom Kibble at London
The spontaneous symmetry breaking leads to the existence of an
additional, spin-0, elementary particle called the Higgs boson.
Electroweak symmetry breaking and its consequences
Symmetric position is not the ground state!
• At high energy W boson appears massless, ie., at
short distance scale of less than ~1 fermi (10-13 cm).
• Close to the ground state, at low energy, symmetry
disappears and, the effect corresponds to massive
W, Z bosons.
Experimental verification of this hypothesis was essential. 12/20/2014 19
The drag in movement
• Simply put, there is an all pervading Higgs field.
• All particles necessarily interact with this field and thereby acquire mass.
• Coupling of a particle with Higgs boson depends on its mass.
• The Higgs field also interacts with itself as well resulting in its own mass!
20 12/20/2014
Mass spectrum of elementary particles
The missing piece till now
Mass of the Higgs boson could have
been anything up to ~ 1 TeV
equivalent energy density is needed
for Higgs boson to be created in the
laboratory.
• LHC has been built to discover the
Higgs particle and study EWSB
• LHC experiments discovered the
Higgs boson in 2012,
hypothesized ~ 50 years back!
Great triumph of human intellect
• Hunt for the particle started
seriously during 1980s.
LHC project: largest human endeavour
ever taken up. 12/20/2014 21
22
The recognized giants of the standard model
• 1967: Steven Weinberg and Abdus Salam used their ideas to build a model of electron mass and weak boson mass.
• Also earlier ideas by Sheldon Glashow.
Nobel prize in 1979
Nobel prize in 1999
• Not much attention paid to 1967 work until 1972 paper by Gerard ’t Hooft and Martinus Veltman,
which relied even more explicitly on Higgs’ idea.
It was indeed time for more attention towards Higgs sector!
12/20/2014
• Results of all the past experiments matched well to the theoretical prediction
of Standard Model corresponding theoretical description is correct.
23
Why was the Higgs discovery so important
• Success of standard model crucially depended on the experimental
confirmation of the “Higgs mechanism” the existence of the Higgs boson.
• We have now all the essential members of the particle zoo in SM.
Need to appreciate the elegance of the idea of unification
and other hall marks achieved in the field during the last century.
Gigantic efforts in many fronts led to the success at
the LHC, in first phase of its operation (2010-13).
-- 2012: “a Higgs like new particle”
-- 2013: “a Higgs boson”
Physics Nobel prize to P. Higgs and F. Englert
After the discovery: Is it the Higgs boson of standard model ?
• Is there any other Higgs particle?
• Is the resonance a window to new physics?
12/20/2014
• LHC is the world’s most powerful microscope doing nanonano physics.
• Collide these protons to accumulate sufficient energy so that heavy
particles could be produced in the lab E = mc2
Perfect role of Large Hadron Collider
Accelerates charged particles (protons or lead ions) using electric field and
bending them in circular arc using magnetic field, repeatedly.
• Mass of particles comes from energy of the short-distance reaction.
• The larger the energy the greater the variety of particles produced.
• Otherwise, equal amount of matter and antimatter can also be
produced when energy is converted to matter.
LHC: the giant marvel of technology
• 100-150 m under the surface
• 27 km at 1.9 K (superfluid He)
• Vaccuum ~ 10-13 Atmosphere
• SuperConducting coils: 12000 tonnes/7600
• Temperature generated at LHC due to proton-proton collision ~1016 0c, compare with sun: 5506 0c, a matchstick: 250 0c
• LHC machine maintained at -271 0c vs. home freezer is at -8 0c Boomerang nebula: -272 0c, antarctica: -89.2 0c,
Indian contributions in LHC:
• Main accelerator magnet components
• CMS and ALICE experiments
• Grid computing
• One of the fastest race tracks: protons zipping past with 99.999999% of velocity of light around 27 km of LHC ring 11000 times/sec.
2 major, multipurpose experiments which discovered the Higgs boson: ATLAS and CMS
12/20/2014 25
What happens in LHC experiment
Proton-Proton 1380 bunch/beam Protons/bunch 2. 1011
Beam energy 4 TeV Luminosity 7.5*1033 /cm 2/s Crossing rate 20 MHz Total event rate 5.4*108 Hz Higgs production <1 Hz
Summer, 2012
• 80 Million electronic channels per experiment, ready for data/25 ns.
20 years to build, 30 years to operate (in phases)
26 12/20/2014
Cartoon of ATLAS detector
12/20/2014 27
CMS Collaboration: 1740 Ph.D.s + 1535 students (845 for Ph.D.) + 790 engineers from 179 institutes in 41 countries.
Only a small fraction of ~4500 people who made CMS possible
~ 120 Indians 12/20/2014 28
C CMS experiment
Cartoon of current CMS detector
HO TIFR, U.Panjab
Silicon preshower BARC, U.Delhi
RPC detector BARC, U.Panjab 12/20/2014 29
The missing piece
we have been after
Slice of CMS detector
Measure the position and momentum of photons and leptons (electron, muon, tau) with high accuracy and reliability. Measure hadronic shower (jets of particles, like pion, kaon etc.) and missing energy. 12/20/2014 30
Event rate of a physical process: R = s L = cross section*instantaneous luminosity
Cross-section of Higgs production for m H = 125 GeV at LHC energy of 8TeV 22 pb = 22* 10-36 cm2
• 0.5 Million Higgs particles produced till now in CMS/ATLAS expt.
Each constituent of proton (quark/gluon) carries only a fraction of the parent energy. Effective energy in a violent collision varies on event-by-event basis possibility of producing particles of different masses Higgs of any mass within allowed range could be produced at LHC
LHC motivations: explore, search, measure
Background rate is ~1012 times higher
efficient and diligent strategy needed. Any event 109/s Higgs event: 0.2 /s 12/20/2014 31
• Higgs boson decays within ~ 10 -24 s. • Decay modes of an unstable, heavy particle X X A : a% of total decay events X B : b% of total decay events Not all decay channels are experimentally suitable. Discovery mode H gg , Branching ratio= 0.23% • Experimental signature is simple and easy to identify final state with 2 energetic, isolated photons. But there are many process which can produce similar signature in the detector. Search for resonance structure in diphoton mass distribution
Crucial for mass resolution: • individual energy measurement for each photon • angle between 2 photons huge investments in every sense paid very well.
m2γγ= 2 E1 E2 (1-cosα)
Higgs decaying to a pair of photons
12/20/2014 32
Golden decay channel H ZZ* 4 leptons
• Signal: 4 energetic, isolated leptons (electron or muon)
(2 pairs of opposite sign, same flavour)
Use kinematic properties of final state leptons to discriminate
signal vs. background on event-by-event basis.
Used for discovery & determination of properties
mass, width, spin, parity, couplings. Z 4l
H 4l
4l continuum
Backgrounds:
Interference of diagrams for
off-shell resonance and
continuum background must
be taken into account.
12/20/2014 34
Lot of investments in measuring decay
leptons accurately
extract multiple information about the
properties. of Higgs boson to identify
Its exact nature.
Work of a detective
• Measurement of mass Fundamental property, not predicted by theory
Once measured, SM predictions are completely determined
Use resonance structure in high resolution channels H gg, H 4leptons
CMS : 125.03 ± 0.26 (stat.) ± 0.14 (syst.) GeV ATLAS: 125.36 ± 0.37 (stat.) ± 0.18 (syst.) GeV
GH < 22 MeV ~ 5GH SM
Compatible within errors
• Total width of Higgs boson
• Spin-Parity JP = 0+
• there is no other particle of different spin or similar mass
• Couplings of Higgs to various particles are similar to as in standard model 12/20/2014 35
• Background distribution mostly Gaussian
stability of result expressed in terms of width s of the Gaussian.
• Characterization of excess using test statistic
Significance where
• Greater the significance (s) minor the p-value lower is the chance
that the observed excess is due to background fluctuation.
Statistical analysis
Signal strength for H gg
12/20/2014 36
Individual analyses
Significance of observation of 125 GeV Higgs boson: CMS summary
sobs /sSM = m
measure of signal strength compared to SM expectation for Higgs mass at the fitted value.
Illustration
12/20/2014 37 CMS: m = 1.0 ± 0.09(stat. )+.08-.07 (theo.)± 0.07(syst) compatible with SM
38
Scattering of longitudinal vector bosons
Each diagram ~ s2
s(ppWW) > s(pp anything)!
Unitarity restored by scalar Higgs
Cancellation also requires Higgs < 800 GeV
• Taming the rate could be managed
by alternative EWSB mechanism
Search for possible resonances
LSB > 1TeV
SB sector
strongly coupled d
s/d
M(V
V)
LSB < 1TeV
SB sector
weakly coupled
12/20/2014
VV Scattering spectrum, σ(VVVV) vs M(VV)
Fundamental probe to test the nature of Higgs boson and its role in EWSB
Energy
Data rates @ CMS as foreseen for design parameters
data collection and archiving rate ~ few hundred Hz 12/20/2014 39
LHC collides 6-8 hundred million proton
on-proton /second for several years.
Only 1 in 20 thousand collisions has an
important tale to tell, but we do not
know which one!
need to search through all of them!
15 PBytes (1015 bytes) of data/year
Analysis requires ~100,000 computers
to get results in reasonable time.
Distributed computing is essential
Science without borders
LHC computing in hard numbers
World wide LHC computing GRID was the
natural evolution of internet technology.
12/20/2014 40
1. Share more than information Data, computing power, applications in dynamic, multi-institutional, virtual
organizations.
2. Efficient use of resources at many institutes.
People from many institutions working to solve a common problem.
3. Join local communities. Need comparatively large hardware resources with
high speed connectivity
4. Interactions with the underneath layers transparent & seemless to the user.
From Web to Grid Computing
CMS and ALICE Tier2 GRID computing
centers in TIFR (Mumbai) and VECC (Kolkata).
• Today ~ 200 sites • ~40k CPU cores • ~100 PB disk
WWW was born in early 1990s to satisfy the
needs of previous generation HEP experiments.
12/20/2014 41
mu = 0.003 mt = 184 mb = 5.0 me = 0.0005446 mm = 0.1126
For example, it does not explain this bizarre set of numbers for mass (in GeV)
Is our job over?
By no chance! Higgs boson fixes a crucial problem, and accounts for the origin of mass, but it leaves a lot unexplained
There are many reasons to believe that there is lot of unknown,
new physics at higher energy densities.
We are able explain the evolution up to an epoch of ~ 10 -11 s after big bang.
Future operations of LHC will take us more backward in time.
• 2015 – 2030: LHC operates with intermittent stops for ~2 -3 years
• centre of mass energy 13 TeV, but gradually increasing luminosity
helps in exploring physics beyond standard model upto energy ~ few TeV
12/20/2014 42
Seeing the dark!
Passage of 2 galaxies 100 M years back
Rotation curve of a galaxy
• With increasing distance the gravitational
pull should decrease.
• Observations suggested gravitational pull
from additional heavy objects which are
not yet detected (through EM interaction).
• Identification of the dark matter is one of
the most intriguing problem at present.
LHC can shed light on the nature of the dark matter
Constituents of the universe
12/20/2014 43
LHC experiments have discovered Higgs particle of mass 125 GeV
• Current measurements are in agreement with minimal Higgs mechanism.
• No exotic discovery as yet
• Established : Origin of mass (scalar field BEH mechanism) of particles in a
quantum field theory with local (point-like) gauge interaction.
• Starting from a reductionism strategy: question of structure of matter
evolved into the question of origin of interactions (local gauge symmetries)
and matter (interaction with Higgs field)
• The rise in centre of mass energy at LHC in next run, gives access to new
territory for the search of the unexpected lot of potential!
Miles to go before we sleep!
• However , we shall always manage to know only a drop of the ocean!
Summing it up
44 Stay tuned!
12/20/2014
backup
12/20/2014 45
1. At the core is a device called the inner tracker detects and analyzes the momentum of particles passing through the detector.
2. Surrounding the inner tracker is a calorimeter measure the energy of particles by absorbing them. 3. The outermost subdetector is muon spectrometer measures muon position and momentum. Scientists look at the path the particles took and extrapolate information about them. Reconstruct 20K charged tracks in a single event (lead-lead collisions at LHC)
Essential components of a detector
Data collected by an experiment in a year ~ Peta Byte how to handle? 12/20/2014 46