Download The LHC Experiment at CERN

LHC: a quick tour
Kajari Mazumdar
Experimental High Energy Physics Group, TIFR
Particle Physics Course, Lecture1, August 3, 2009
Eternal Questions
What 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 them all!
• Observe
and interprete.
• Beauty is in size, symmetry and tranquility.
Present wisdom: Behaviour of matter particles can be explained in
terms of very few fundamental interactions, which might have
evolved over time as the universe cooled down from a single unified
one.
GRAND UNIFIED THEORY!
Cosmic Recipe: How we understand the universe
• LHC takes us back in time towards the beginning of the universe!
at an epoch of 10-12 sec. after the big bang.
•Earlier experiments have probed the prevailing situation upto a time
t ≈ 10-10 s, when the world was as hot as 1015 K
Protons and neutrons formed around: t ≈ 10-4 s, 1013 K
•Nuclei are formed after t = 3 minutes, 109 K
(equivalent energy density ~0.1 MeV, distance scale ~10-12 m)
•Today: t = 14 Billion years since the beginning, 3 K
Energy =kT, Length scale = hc/E
92 Elements
 Atoms
 Nuclei (and Electrons)
 protons &
neutrons
 quarks
?
What lies within…?
Tool
?
The probe wavelength should be smaller than the distance scale to
be probed: l<< h/p = hc/E
4
x 
E
(1 TeV = 1012 electronVolt
= 1.6 * 10 -7 Joule)
1 mm  10 eV
2
1 nm  10 eV
10
20
13
m  10 eV
 10 TeV
LHC is the Biggest and most
Expensive Science Experiment
ever attempted .
Price Tag:
US $ 12 billion
No of scientists: 8000+
8.6 Km
Technological progress
pushes frontiers of basic
science research and there
are important spin offs.
Eg. World Wide Web!
What will happen in LHC
Mammoth detectors will register signals
Standard Model (SM) of Particle Physics as of today
• 4 types of basic forces :
Gravitational, Weak, Electromagnetic, Strong.
Relative strengths:
10 -40: 10 -5: 10-2: 1
• And 2 types of fundamental particles :
(i) fermions (matter particles like electron, quarks)
(ii) bosons (carrier particles, like photon)
• Almost all the predictions of SM match very well, till date, with
experimental observations.SM is still not a satisfactory Theory!
• One of the most disturbing fault of our understanding as of today, is,
we can’t explain the origin and the mass patterns of particles.
• Theoretically, in an unified description, massless particles can be
described very well.
• But, in nature, we do encounter mass of elementary particles.
Introduction of mass in the theory causes complications!
Presently SM cannot explain this mass spectrum.
Problems with Standard Model
• One of the corner stone of SM, the Higgs mechanism of
EWSB is yet to be verifiedHiggs particle is not yet seen!
EWSB endows masses to various fundamental particles.
All are experimental inputs!
• Mass of the Higgs boson itself is not known from theory.
Higgs particle has to be hunted out!  experimentalists’ job!
SM is NOT the complete description of physics at very high
energies.
e.g., corrections to Higgs mass grows beyond control unless
some New Physics, which is at work at TeV energy scale, is
postulated.
 Particles of interesting properties should show up if
enough energy is gathered to produce them. (E=mc2)
Motivation for LHC: We got to unravel the mystery of mass!
Most plausible: all fundamental particles acquire mass by interacting
with an all pervading field, as a consequence, this idea also evokes
another fundamental particle, the Higgs Boson!
Higgs particle not yet seen have to hunt it out in experiment. e
f==a
Strategy: Heavy particles show up if enough energy is gathered to
produce them. (E=mc2).
They existed when universe was hot.
Need accelerators for high energy in a controlled fashion in the
laboratory.
LHC is an exploratory, high energy, high intensity machine which can
produce heavy particles of mass upto few TeV.
The primary goal of the LHC is to find the Higgs boson…
… if it isn’t found, to find out why it isn’t there!
More on Higgs boson
• Quantum Electrodynamics is the most successful theory to
date which has been tested to accuracy of 1 in 100000.
Calculations give finite results.
Not the case of unified Electroweak theory until we take into
account interaction of particles with Higgs.
In vacuum, photon has zero mass and velocity = c.
But in glass velocity < c  photon has an effective mass!
This is the effect of photon interacting with EM field of matter.
Higgs is a quantum field permeating the universe.
In analogy, particles acquire masses by interacting with the
Higgs field.
A room filled with scientists
chattering  like space filled with
Higgs field
A well known sciientist walks
in creating a disturbance and
attracting a cluster of
admirers at each step.
He feels resistance for
movement  acquires mass
through Higgs field.
Now consider a rumour crossing the room
 there is clustering among the scientists themselves
 these clusters are the Higgs particles.
This explanation of Higgs mechanism convinced British science minister to
fund LHC liberally.
Symmetry Breaking and Higgs Mechanism
Nature has various symmetries (translational, rotational, ..) and related
conservation laws: guiding principles in theoretical formulations.
Some of the symmetries are also broken, sometimes spontaneously.
Eg. behaviour of ferromagnet wrt temperature: above Curie point, the spin
alignments are all random. Below critical temperature, the alignment
direction is degenerate.
Below C point
Above C point
In high energy physics assume all elementary particles to be massless to
start with.
As the universe cooled, particles acquired masses through breaking of
symmetry.
Most sophisticated and
complex detectors with size of
big buildings.
Unprecedented computing
challenge to be met by GRID
technology.
Typical LHC Event
TIFR, Mumbai is preparing to
host one of the GRID
computing centres for regional
scientists
Interesting facts about LHC
• LHC vacuum is 100 times more tenuous then the medium in which typical
communications satellites move
• LHC magnetic fields of 8.4 Tesla are 100,000 times that of the Earth’s.
• LHC magnets will use 700,000 ltr. of liquid Helium & 12,000,000 ltr. of liquid N2.
• LHC protons will have energies comparable to that of a flying mosquito !
LHC Timeline
 First LHC studies were done in 1982, final decision in 1996
 Construction started in 2002. Several Indian groups working for LHC
accelerator and experiments for last 10-12 years.
 First beams sent around the storage ring: September 10, 2008
 Helium leak caused by electric short-circuit delayed start
 First collisions will start around end 2009
 Some idea about Higgs boson, SUSY,.. by 2012.

Stay Tuned!
Science motivation of LHC:
• Unravel the mystery of ElectroWeak Symmetry Breaking.
• Probe the physics at TeV energy scale.
(Is SuperSymmetry or Extra Dimension relavant for EWSB?).
• Direct Search for production of dark matter candidate.
• Study the asymmetry between matter and anti-matter.
•Study of Quark-Gluon Plasma.
•…………
•But We may find something completely
unexpected and really exotic.
We have to be ready for everything.
Extra Dimensions: String theory with Radical ideas!
• Different particles and forces are just different oscillation modes
of tiny strings, of length ~ 10-33 cm.
• Instead of usual 3 spatial dimensions, there may be additional
ones, which are curled up too small to be observed!
• Gravity may appear weak only because its force is being shared
with (or leaks into) other spatial dimensions.
Or, LHC may produce almost 10 Million completely harmless Micro
Quantum Black Holes per year!
The Black Hole will disappear within 10 -25 sec after creation due to
Hawking Radiation!
No threat whatsoever of any kind.
Black Hole production
In large extra dimensions Schwarzchild
radius of proton increases from 10-33 to 10-17 cm
If the impact parameter
of two colliding protons
is smaller than this
distance, they coalesce
into a micro blackhole .
It evaporates, via
Hawking radiation,
within 10-25 s spewing
out many particles
isotropically in the
detector.
What do we expect the LHC to find?
Many good reasons to discover Higgs boson and more
For the rest…
Like asking what a new continent is going to be like
when we can just glimpse the shore….
Backup, Some other basics etc.
Fundamental Building Blocks in Standard Model
Fermion sector:
• 3 generations of quark & lepton doublets
Gauge sector mediators:
• EM: photon, massless.
• Weak: W+, W-, Z0: massive (~ 100 proton mass)
• Strong: 8 gluons in total.
Properties of 4 fundamental interactions & mediators in SM