LHC: a quick tourLHC: a quick tour

Kajari MazumdarExperimental High Energy Physics Group, TIFR

Particle Physics Course, Lecture1, August 3, 2009

What principles govern energy, matter, space and time at the most elementary level?

• What is the world made up of? What is the world made up of? • How does it work?How does it work?High Energy Physics tries to answer them all!High Energy Physics tries to answer them all!

Eternal Questions

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!

• Observe and interprete.• Beauty is in size, symmetry and tranquility.

• 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

Cosmic Recipe: How we understand the universe

Energy =kT, Length scale = hc/E

92 Elements Atoms

Nuclei (and Electrons)

protons & neutrons

quarks ?What lies within…?What lies within…?

?

The probe wavelength should be smaller than the distance scale to be probed: << h/p = hc/E

xE

41 mm 10 eV

20 1310 m 10 eV10 TeV

ToolTool

21 nm 10 eV (1 TeV = 1012 electronVolt = 1.6 * 10 -7 Joule)

8.6 Km

Price Tag: US $ 12 billion

No of scientists: 8000+

LHC is the Biggest and most Expensive Science Experiment ever

attempted .

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

• 4 types of basic forces : Gravitational, Weak, Electromagnetic, Strong.

• 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!

Standard Model (SM) of Particle Physics as of today

• 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!

Relative strengths: 10 -40: 10 -5: 10-2: 1

Presently SM cannot explain this mass spectrum.

• One of the corner stone of SM, the Higgs mechanism of EWSB is yet to be verifiedHiggs 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)

Problems with Standard Model

Motivation for LHC: We got to unravel the mystery of mass!

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!

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.

LHC is an exploratory, high energy, high intensity machine which can produce heavy particles of mass upto few TeV.

Need accelerators for high energy in a controlled fashion in the laboratory.

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 theHiggs 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.

Typical LHC Event

Most sophisticated and complex detectors with size of big buildings.Unprecedented computing challenge to be met by GRID technology.

TIFR, Mumbai is preparing to host one of the GRID computing centres for regional scientists

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!

• 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 !

Interesting facts about 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.

Science motivation of LHC:

•Study of Quark-Gluon Plasma.•…………

•But We may find something completelyunexpected 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 toHawking 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.

Like asking what a new continent is going to be like when we can just glimpse the shore….

What do we expect the LHC to find?Many good reasons to discover Higgs boson and more

For the rest…

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