Our Universe is Yours! The experiments at the LHC.

Kajari Mazumdar Tata Institute of Fundamental Research

Mumbai.

Kajari.Mazumdar@tifr.res.in http://www.tifr.res.in/~mazumdar

Panjab University, Chandigarh 25 September, 2013

• What is the universe, how did it begin, what is it made up of? • How will it evolve in the future?

• What are the fundamental building blocks and forces of the universe?

– Cosmology and Particle Physics are the areas of science that address

these questions now.

• These questions are as old as human civilization (philosophers in India, Greece,..)

We are searching for the genetic code of the universe

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• In the history of science many scientists had trouble initially to understand the breakthroughs which occurred during their time.

• But great minds faced the facts and interpreted them correctly.

Example : • Experiments showed that light traveled on nothing. This was unexpected turn of events. • Einstein: there is no absolute reference frame for motion in the

universe the speed of light must be the same in all reference frames: Relativity!

The questioning mind

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How old is the universe? 13.7 billion years!

What did the Big Bang produce during its first few minutes? All elementary particles.

What is elementary?

• Nuclei are formed t = 3 minutes. • What was the situation before? • We know the evolution from ~10 -12 s till today.

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The definition keeps changing over the time.

u

u d

Are there more inside?

In the Universe there are about 1080 quarks formed in the earliest moments of life of the Universe.

Modern idea of Nature’s ingredients

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De Broglie relation the probe wave length/energy should be smaller than the distance scale to be probed.

To probe 10 -20 m need 10 TeV = 1012 eV. LHC provides such energies!

1 TeV = 1.6 * 10 -7 Joule

• Our unit for energy is the electron-volt (eV) – Motion of air atom, room temp: 0.04 eV – Chemical reactions/atom, visible photons of light: ~ few eV – Nuclear reactions, per atom: 1 MeV – Rest energy mc2 of proton: 1 GeV – Each proton in each LHC beam 4 TeV (this phase is over) – From 2015, proton beam energy at LHC: ~ 7 TeV Centre of mass energy 13 TeV

Energy scales

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• To remove an electron from an atom we need ~ 10 eV. • To remove a proton from a nucleus we need ~10 MeV.

Energy =kT Length scale = hc/E

Atom Proton

Big Bang

Radius of Earth

Radius of Galaxies

Earth to Sun

Universe

Super-Microscope

LHC

Hubble ALMA

VLT AMS

Dimensions in our Universe and LHC

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Cartoon of our Universe

Today 13.7 Billion Years

1028 cm

1. Quantum gravity era: t≈ 10-43 s 1032 K (1019 GeV, 10-34m)

3. Protons and neutrons formed: t ≈ 10-4 s, 1013 K (1 GeV, 10-16 m)

•4. Nuclei are formed t = 3 minutes, • 109 K (0.1 MeV, 10-12 m)

Big Bang

(30 K)

2. LHC 10-12 s, 1016 K (104 GeV, 10-20 m)

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10 thousand km Connection with our Universe

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We belong to Milkyway galaxy

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A 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!

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All behaviour of matter particles (fermions) can be explained in terms of few forces carried by exchange or carrier particles (bosons).

LHC

Present Wisdom

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Relative strength of gravitation, weak, electromagnetic, strong: ~10 -40 : 10 -5 : 10 -2 : 1

Fundamental Forces

BUT MATTER IN THE UNIVERSE IS NEUTRAL, because positive and negative charges cancel each other precisely. THEREFORE: Gravitation is the dominant force in the Universe. 9/25/2013 13

Elementary particles and their masses

• Everyday matter

Matter:

the missing piece till now

Though mass is a fundamental property of particles, we have lack of wisdom: we do not know the origin of the masses!

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How do we put everything together?

Five fold symmetry Radial symmetry Reflection/Bilateral symmetry

Dogma of Symmetry

Symmetry has practical uses too. (a) position of pivot in load balance (b) 2 eyes correct judgment of distance.

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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: F = m a • Covariant under rotations F, a changes in the same way under rotation. • Invariant under Galilean transformations F, a do not change.

Mathematics Physics • Calculus • Complex numbers/functions • Differential geometry • Group theory • Hermitian operators, Hilbert space •….

Any global symmetry leads to some conservation law (Noether, 1915). eg., invariance under space translation momentum conservation.

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Symmetry could be a physical or mathematical feature of the system (observed or intrinsic) that is "preserved" under some change. Ex.1: The temperature of a room translational symmetry. Ex.2: Symmetry of mathematical functions: a2c + 3ab + b2c

Symmetry considerations

Electromagnetism Maxwell’s equations: • First attempt to combine electricity and magnetism. • Invariant under Lorentz transformation. • Also invariant under gauge transformation.

• Define scalar (φ) and vector (A) potentials related to electric & magnetic fields : E = −∇φ − ∂A/∂t, and B = ∇ × A • E and B remains unchanged even if we change the potentials as : φ′ = φ − ∂Λ/∂t and A′ = A + ∇Λ, where Λ is a function of (x, t).

Gauge transformation

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• Quantum Mechanics is invariant under a change in the phase of the wave function: Ψ’ = e iθ Ψ

• The electromagnetic interaction can naturally be introduced into QM by demanding that the Lagrangian should be invariant under local (space-time dependent) phase transformations: Ψ’ = e iΛ (x,t) Ψ • Invariance under local phase transformation is called GAUGE symmetry. • However, when the derivative terms in the Lagrangian act on Λ(x,t), the invariance is spoilt due to extra terms. • To cancel these extra terms and restore the invariance, one has to introduce a vector field into the theory. • This vector field transforms under gauge transformations, exactly as the electromagnetic field. φ′ = φ − ∂Λ/∂t and A′ = A + ∇Λ.

Identify this vector field with the electromagnetic field which mediates interaction among charged particles, photon (γ).

Gauge symmetries

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•The GAUGE principle: The requirement of invariance under gauge symmetry creates the interaction. • Invariance under the symmetry requires the photon to be massless. • This matches well with experimental observation of infinite range of EM interaction.

• Quantum electrodynamics is the most successful theory: predicts correctly the magnetic moment of the electron to 12 significant digits! • This success paves the way to apply similar ideas for weak and strong interactions. • The gauge symmetries involved here are more complicated, but the mediators of interactions could be invoked naturally from the principle of invariance.

• Now carriers of weak interaction are charged (eg. in Beta decay: n p e+ νe W+, W- should couple to photon Electromagnetic and weak interactions are combined together in a single Electroweak theory Carriers (boson) : photon (γ) , W+, W- and Z 0 . Standard Model encompasses electroweak and strong interactions.

Standard Model of Particle Physics

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Spontaneous Symmetry Breaking

In nature the symmetry does get broken up spontaneously sometimes! • A ferromagnet below Curie temperature has all the spins aligned to the direction of the magnetisation. there is a preferred direction in the system • But above the Curie temperature the spins are randomly oriented all directions are equally preferred.

• Invariance under the symmetry requires all the carrier particles to be massless. However weak interaction is short-ranged! • Question: How do W and Z particles become massive?

Symmetry disappears at low energy, reappears at high energy. 9/25/2013 20

1. Francois Englert and Robert Brout at Brussels 2. Peter Higgs in Edinburgh 3. Gerald Guralnik, Carl Hagen and Tom Kibble at London

The mechanism of symmetry breaking in physical systems studied in early 1960s (Nobel prize to Nambu, 2008). During 1964, several groups utilized this idea to explain the phenomenon of mass generation of carrier particles in electroweak theory.

• When the universe was much hotter the particles were massless. • In the process of cooling down some of the symmetries are lost. Electroweak symmetry breaking caused photon to remain massless while W± and Z0 particles became massive. Short range of weak interaction incorporated at low energies.

Origin of mass

Plausible phenomenon:

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The price of symmetry breaking : list of the fundamental particles got extended by atleast one new particle of spin 0 The HIGSS BOSON • Simply put, there is an all pervading Higgs field. • All particles necessarily interact with it. •The amount of interaction of a particle in the Higgs field causes its mass. • The Higgs field also interacts with itself as well resulting in its own mass!

The simplest test of this hypothesis is to observe the Higgs boson experimentally. But we did not know its mass, which could be anything between 0 to 1000 GeV! Pre-LHC experiments and other theoretical considerations restrained the range (114 – 750) GeV

LHC is built to find the Higgs boson OR, resolve the issue of mass generation of the carriers of weak interaction.

Predicament!

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1998 Construction of surface buildings for CMS begins.

2000 LEP shut down, construction of cavern begins.

2004 Cavern completed. 10 September 2008 First beam in CMS.

23 November 2009

First collisions in CMS.

30 March 2010 First 7 TeV collisions in CMS.

4 July 2012

Announcement: evidence for a particle at about 125 GeV.

Timeline for CMS experiment

Roman coins excavated from site

9/25/2013 23 Largest ever human endeavour!

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

2 major multipurpose experiments at LHC • A Toroidal LHC Apparatus (ATLAS) • Compact Muon Solenoid (CMS)

• 80 Million electronic channels per experiment, ready for data/25 ns. 20 years to build, 20 years to operate.

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Mandate of the experiments :1. Discover Higgs particle or rule out its existence. 2. Elucidate on physics at the new energy range of TeV 3. Search for the candidate of the dark matter in the universe. ………

• Temperature generated at LHC due to proton-proton collision ~1016 0c, compare with sun: 5506 0c, a matchstick: 250 0c

• LHC machine to be maintained at -271 0c vs. Home freezer: -8 0c, Boomerang nebula: -272 0c, Antarctica: -89.2 0c

LHC: The Giant Marvel of Technology

• 100-150 m under the surface and 27 km at 1.9 K (super-fluid Helium) • Vacuum ~ 10-13 Atm. • SuperConducting coils: 12000 tonnes/7600 km , cooled with 100 tons of He. • energy stored in each beam ~ 15 Kg of chocolate!

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Cartoon of till recently working CMS Detector Hadron Outer calorimeter TIFR, U.Panjab

Silicon preshower of Electromagnetic Cal BARC, U.Delhi

• Designed meticulously for hard collisions • Serving well even for softer ones

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• Maintain excellent detector performance in spite of harsh situation.

Event rate for a given physical process depends on instantaneous luminosity R= σ L Highest value reached till now: L = 7.54 *1033 /cm 2 /s = 7.54 Hz/nb

Crucial factor: LHC machine operation

7 TeV

8 TeV 8 TeV 2012

7 TeV 2011

Total data accumulated : ~ 1017 p-p collisions

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Large amount of data volume distributed across the globe, including 2 centres in India, using high speed network connections.

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Grid computing behind the success.

Higgs production at LHC

gluon-gluon fusion Vector boson fusion Associated productions with W, Z, top

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•At the start of LHC, since the Higgs mass was not known in broad sense, experiments needed to consider all the production and decay modes at different mass values. • There were already experimental

lower bound ~ 114 GeV • Theoretical upper bound ~ 700 GeV • Preferred value < 200 GeV

Strategy for experimental Search

Higgs decay ~ 10 -24 s! Available decay modes depend on the mass. Divide the mass region according to the decay mode which is easy to identify and equip the detector accordingly. 1) For Higgs mass below 140 GeV look for H 2γ, H WW(*), H ZZ(*) 2) For mass 140 -180 GeV, H WW, H ZZ(*) 3) Above 180 GeV, H ZZ

High demands on the detectors at lower values of Higgs mass, the detector resolution must be excellent.

Branching ratios (%) H WW* : 23 H ZZ* :2.9 H bb : 56 H cc: 2.8 H ττ : 6.2 H µµ: 0.021 H gg : 8.5 H γγ : 0.23 H γ Z : 0.16 9/25/2013 30

Higgs decaying to a pair of photons

• Simple and clean signature: final state with 2 energetic photons. • Narrow peak to be identified on top of huge continuously falling background in the invariant mass distribution.

• Crucial for mass resolution: individual energy measurements and angle between 2 photons. Achieved mass resolution ~ 2GeV, natural width ~ 5 MeV!

The calorimeter material chosen to have compact detector with good energy, position, and angular resolutions Excellent mass resolution of about 1%

m2γγ= 2 E1 E2 (1-cosα)

simulation

signal

Backgrounds

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CMS result for 2 photon final state

• Discovery corresponded to observed significance 4.1 σ • Probability for a background fluctuation to be at least as large as the observed maximum excess at 125 GeV : 20 in a Million.

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Easier situation for H ZZ* 4 leptons Mass resolution ~ 1 % For discovery,~ 24 signal, 1 background

With complete data collected by CMS

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Continuum production of ZZ pair

This channel also gives the best measurement of spin, parity of the resonance: 0+

Summary of the results from of the main channels

3.4 σ combined!

Given mH ,standard model can predict the cross section and Branching ratio of decay modes

mH = 125.7 ± 0.3(stat) ± 0.3(syst) GeV = 125.7 ± 0.4 GeV

To check with the consistency of SM Test production modes in various decay modes wrt SM Couplings to vector boson vs. to fermions Data in good agreement with SM.

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Beyond standard model

Main results till now: • Coloured SUSY particles of first generation ruled out upto mass 1 TeV • Natural SUSY probed upto a few hundred GeV of 3rd generation spartners • Exotica: heavy objects probed upto masses 2-3 TeV Note: • Probe into TeV scale has been limited by machine energy. • LHC probes physics at TeV energy scale, typical rate: 10 – 0.01 pb

• Future operation of LHC probes well into TeV energy scale.

No observation of physics beyond standard model as yet! Searches are focused on finding OR, excluding SUSY and non-SUSY models (extra dimensions, …).

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LHC Roadmap

Under discussion

New particle discovery potential

SuperSymmetric particles: Gluinos upto 1.7 TeV Stop: 750 -950 GeV Sbottom: 600-700 GeV Electroweakinos: 500 -600 GeV

• New Vector Bosons: • Exclude Z’ upto ~ 6.5 TeV • Discover W’ @ 5s level upto 6 TeV • Massive vector-like top quark upto • 1.5 TeV.

At 5σ, with 300 fb-1

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At 5σ, with 3000 fb-1

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Scattering of longitudinal vector bosons

Each diagram ~ s2

Unitarity restored by scalar Higgs

• Could be managed by other means : other resonances, complex objects, multi-Higgs models, noHiggs, .. Experiment will tell us • If the restoration is delayed at scale M, the scattering cross section will rise for range mW << √ s << M

ΛSB > 1TeV SB sector strongly coupled dσ/dM(VV)

ΛSB < 1TeV SB sector weakly coupled

WW self interaction diagram also to be taken into account for WW scattering.

Black hole

• How is a black hole formed? -- In the collapse of a massive star. • What happens to the scientist

when he falls into a Black hole? -- He becomes flat as a dosa! -- Gravity rips him apart! -- Actually he never arrives there!

LHC will also answer many more questions

• Does it exists? Yes, by looking through big Telescopes we learnt about it. • What is it? We do not know as yet!

Dark Matter

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• What is the Universe made of? Mostly by a mysterious type of energy

• How do we know about “dark energy” ? 1. By observing distant exploding stars 2. By observing cosmic background radiation.

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Conclusion

• Finding a resonance at LHC, has been a giant leap for science. • Most likely it is the Higgs boson of standard model. • The discovery has established the main stream idea about the

mechanism behind the mass generation of fundamental particles.

• Exploration of the high energy frontier provided by LHC has just started. • LHC upgrades present several experimental challenges which must be addressed to. • Coming years are going to be very promising stay tuned!

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“What we know is a droplet, what we don’t know is an Ocean”

Sir Isaac Newton (1643-1727)

Last word

Thank you!

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Backup

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LHC Future: from here to there Phase 1 upgrade

Long shutdown 1: 2013-14 Data: 2015-17 Data: 2019-21

Long shutdown 2: 2018 Long shutdown 3: 2022-23

L=1*10 34 /cm2/s L=2*10 34 /cm2/s L=5*10 34 /cm2/s

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The missing piece we have been after

A slice of CMS detector

• The detector can only “see” γ, e±, µ ±, π±, n, p, K ! • Measure the position and energy-momentum with high resolution.

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Measurement of Higgs Couplings

simplifies the problems to event yield only. 46 9/25/2013