Elementary Particle Physics with natural and artificial accelerators

JISCRISSTokyo Sept 19th 2009

R. Santonico Physics Department and INFNUniversity of Roma “Tor Vergata”

IntroductionThe purpose of this talk is to review at a very

simple level the Elementary Particle Physics in his historical development and to show the achievements that were obtained by means of naturally and artificially accelerated beams.

This presentation is oversimplified, qualitative and by far incomplete. A more complete and satisfactory one would require much more time than that reserved for this talk

Why do we need to accelerate particles?

The best way to study the behavior of elementary particles is to make collisions between them. This is a very peculiar aspect of the elementary particles.

This can be explained as follows: an elementary particle is surrounded by a field of force that can be studied by penetrating it with another particle working as a probe

Why high energies are needed?- Scattering and transferred momentum- Creation of new particles

Scattering and transferred momentum The “beam” particle is scattered by the “target” particle at an

angle θ The momentum interchanged in the interaction Δp = p sinθ is

related to the interaction distance: Δr = h / p sinθ The larger are the beam momentum and the scattering angle the deeper is the penetration of probe inside the field

Creation of new particles There is also another reason why high energies are needed: a

case of special interst occurs when in the collision a newparticle is created. In order to make this possible a sufficientenergy must be available

A photon penetrating the electric field of an atomic nucleus can produce an electro-positron pair provided: Ephoton= 2mec2

+ Ekinetic > 2mec2

The four fundamental forces

Force strength:assuming =1 the emforce between 2 protonsin a nucleus

the four interactions strength would be:

- Gravitational force 10-36

- e.m. force 1- Week Nuclear Force 10-7

- Strong Nuclear force 20

Forces

Gravity

falling objects

planetorbits

starsgalaxies

inversesquare law

inversesquare law

shortrange

±

Electro-magnetic

atomsmoleculesopticselectronicstelecom.

Weak

betadecay

solarfusion

Strong

nuclei

particles

shortrange

Natural Accelerators: Cosmic Radiation

Cosmic radiation is a wonderful natural source of very high energy particles: mainly protons, deuterons and CNO nuclei

The CR energy spectrum extends up to incredibly high energies such as 1021 eVfollowing a power law

dN/dE = E-2.7 Fig

Cosmic Showers in the atmosphere The “primary” CR interact with the highest

atmospheric layer and produce Extensive Air Showers through multiple interactions with

The CR particles reaching the Earth surface are the latest products of the EAS

The born of Elementary Particle Physics At the beginning of the decade of 1930 the

neutron discovery completed the series of the particles constituting the ordinary matter atoms: electron, proton and neutron

A fourth particle, the photon, was understood to be the carrier of the e.m. interaction

In the following years an impressive series of new particles were discovered in the cosmic radiation

Development of the Particle PhysicsThe positron discovery (1933)

Muon decay in a cloud chamber

3 bodies decayThe lifetime τ=2.2 10-6 s can be measured (Rasetti 1942) from the delay in the electron emission

The Yukawa particle and the π/µ puzzle Hideki Yukawa proposed in 1935 that the nuclear interaction was

mediated by a new particle, the π meson, of mass about 200 electron masses

The cosmic muon of mass 105 MeV, was an ideal candidate for the Yukawa particle

An experiment to definitely establish if the muon was the Yukawa particle was carried out by Conversi, Pancini and Piccioni

The experiment was based in the different behavior of positive and negative muons stopped in the matter:µ- should be captured by nuclei and interact with the nuclear matter in

10-23 sµ+ should decay normally

The conclusion of the experiment was completely unexpected: the cosmic muon was not the Yukawa particle. It is indeed a Lepton

The Conversi, Pancini and Piccioni experiment(Roma 1945)

The true Yukawa particle The pion meson π (mass 139.57 MeV/c2; spin = 0;

τ = 2.6×10-8s) was discovered (Lattes, Muirehad, Occhialini and Powell) using emulsions exposed tocosmic rays at high altitude

The π + decay at restπ + µ+ + ν produces amonocromatic µ+

The π- was capturedby the nucleus as expected

The Artificial Accelerators Era

Starting from the 50’s the artificial accelerators became the a central instrument of the particle physics

The study of the acceleration mechanisms is beyond the purposes of this talk

Two kinematical concepts of accelerator can be distinguished:- Accelerators for “fixed target experiments”: the accelerated beam

collides with a target at rest with respect to the detector (like for the collisions of primary CR with the atmosphere)

- “Colliders”: two accelerated beams of equal and opposite momentacollide between them. There are

“electron colliders” with e+ and e- interacting beams“hadron colliders” with proton-proton interacting beams

Antiproton discovery at the Bevatron- A proton beam collides with a copper target- The momentom transfer of proton beam in the field of the target proton was sufficient to create a proton-antiproton pair- The beam line selected negative particles with momentum p = 1.19 GeV/c (mostly pions)- Pions (β=0.79) rejected by Cerenkov counters- Time of flight measurement between S1 and S2 counters on a 12 m base covered in 40 ns pionsand in 51 ns by antiprotonsIf we measure the momentum and the velocity of a particle we can reconstruct its mass and identify the particle

LEP and LHC colliders at Cern

Accelerators and LHC experiments at CERN

Energies:

Linac 50 MeV

PSB 1.4 GeV

PS 28 GeV

SPS 450 GeV

LHC 7 TeV

The quark model of the nuclear matter The “highly inelastic” electron-proton scattering observed at SLAC

shown that the proton itself is not an “elementary particle” but a complex system of point-like components that we call now “quarks”

The six quarks

According to the quark model “hadrons” (= strongly interacting particles are made of quarks

the nucleons (barions) are 3-quark systems the mesons are quark-antiquark systems Quarks are fermions of spin 1/2

barions are fermions mesons are bosons

The quark model of the nuclear matter (3)

The “Charm” quark observation J/Ψ is a c-cbar state

that can decay in e+e-pairs

The figure shows the J/Ψ resonance observedat e+e-colliders (Spearand Adone)

The “beauty” quark The observation of a beauty

particle in the process proton +Nucleus µ+ µ- + anything

The beauty is identified by the peak in the invariant mass plot

The top quark observed by CDF at the Tevatron

The Electro-Week unification and W/Z0discovery at the Cern p-pbar Collider

The neutrino physics The neutrino physics even in the accelerator era re-

evaluated the natural radiation sources which appeared as an ideal instrument to study the neutrino

Two main natural neutrino sources were used- The solar neutrinos produced by the fusion processes responsible for the energy irradiated by the sun- The atmospheric neutrinos produced in the Extensive Air Cosmic Showers

There are 3 kinds of neutrino νe , νµ , ντ

The measurement of the solar neutrino flux is a crucial test to understand

- how the sun does work

- how the neutrino does work

Atmospheric neutrinos

Results of the solar and atmospheric neutrino experiments

The neutrinos are not massless particles The 3 kinds of neutrino, νe , νµ , ντ , are mixed

together and “oscillate” switching from onetype to another

The Standard Model

Is there any new physics beyond the Standard Model ? The theoretical physicists have supposed that “Super

Symmetrical” SUSY particles, never seen so far, might exist

According to this exiting hypothesis a “parallel” undetected universe could exist

SUSY particles, being very massive, would also candidates to explain the dark matter

If they exist how natural and artificial accelerators could contribute the their discovery?

How a SUSY particle would appear in one of the LHC experiments? SUSY particles always produced in pairs get decay chains as below Final state with 2 undetected stable Large Missing ET + jets Lightest SUSY particle (LSP) stable, hence potential WIMP Dark Matter

candidate

15 May 2009 Anna Di Ciaccio --RICAP09--- slide 35lqq

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g~ qL~

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g~qR~

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Two sparticles initially :cascade decays down to LSP: jets, leptons

LSP escapes undetected: large ETmiss

Characterization of SUSY particle From the experimental point of view SUSY

particles would be characterized by a large “missing energy” and “missing momentun”

But the main problem is: how can we be sure that what is “missing” is not due to the inefficiency of our detector?

An alternative approach to the search of SUSY particles

In a large ground based shower detector a SUSY particle might appear as a big hole in the “core” of an high energy shower