Física de Partículas Experimental

5a clase

Luis Manuel Montaño Zetina

Departamento de Física Cinvestav

Departamento de Física USON Hermosillo Sonora 5-9 agosto 2013

ALICE @ LHC (i)Quarks and gluons in ordinary matter are confined in hadrons. Theory predicts that in extreme conditions of temperature and energy density a phase transition from ordinary nuclear matter to Quark Gluon Plasma (QGP) should occur. In this new state, probably existed 10-6 s after the Big Bang, quark and gluons are not anymore confined.

Experiments at LHC

energy density of nuclei

0.13 GeV/fm 3

energy density in the proton

0.44 GeV/fm3

Las 4 fuerzas de la naturalezaDébil• Decaimiento Beta• Fusión pp

Fuerte• quarks

Electromagne-tismo• TV, etc • Imanes• creación ee

GravedadSólo atractiva

Carga eléctrica

masa

Carga débil

Cargafuerte

Fuerza Electromagnética

e- e-

Fotón

La fuerza repulsiva que dos electrones aproximándose “sienten”

El fotón es la partícula asociada a la fuerza electromagnética

Weak force: W-,W+,Z0

Decaimiento βn→peνe

La carga eléctrica se conserva en Cada vértice.

W-

Interacción Electrodébil

En el modelo estandar las interacciones electromagnética y débil se combinaron en una teoría unificada llamada electrodébil.

A distancias pequeñas (10-18 m) la intensidad de la interacción débil es comparable a la electromagnética. Sin embargo, a 30 veces esa distancia (3x10-17 m) esa intensidad de interacción es 1/10000 veces la intensidad electromagnética. A distancias típicas del protón (10-15 m) la fuerza es aún menor.

La diferencia observada entre estas dos fuerzas es debida a la gran diferencia de las masas de W y Z con respecto al fotón..

Fuerza fuerte: gluones

Gluones interaccionan con quarks Gluones interaccionan con gluones

Strong interactions

The strong force holds the nuclei together to form hadrons. The theory of strong interactions is called Quantum Chromodynamics (QCD). This name is due to the fact that quarks, besides the electric charge, have a different kind of charge called “color charge”, which is responsible of the strong force.

The force carrier particles are called “gluons”, since they so tightly “glue” quarks together

Gluons have color charge, quarks have color charge but hadrons have no net color charge (“color neutral”). For this reason, the strong force only takes place on the small level of quark interactions.

Color charge

Color charged particle interact by exchanging gluons. Quarks constantly change their color charges as they exchange gluons with other quarks.

There are 3 color charges and 3 corresponding anti-color charges. Each quark has one of the color charges and each antiquark has one of the anticolor charges.

In a baryon a combination of red, green and blue is color neutral. Mesons are color neutral because they carry combinations as red and antired.

Because gluon emission and absorption always changes color, gluons can be thought of as carrying a color and an anticolor charge. QCD calculations predict 8 different kinds of gluons.

Quark confinement

Color-charged particles cannot be found individually. They are confined in hadrons.

Quarks can combine only in 3-quarks objects (baryons) and quark-antiquark objects (mesons) which are color-neutral, particle as ud or uddd cannot exist.

If one of the quarks in a given hadron is pulled away from its neighbours, the color force field stretches between that quark and its neighbours. More and more energy is added to the color-force field as the quark are pulled apart.

At some point it’s energetically cheaper to snap into a new quark-antiquark pair. In so doing energy is conserved because the energy of the color-force field is converted in the mass of the new quarks.

Gravity

Gravity is one of the fundamental interactions, but the Standard Model cannot satisfactorily explain it.

This is one of the major unanswered problems in physics today

The particle force carrier for gravity, the graviton, has not been found

Fortunately, the effects of gravity are extremely tiny in most particle physics situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Thus, the Standard Model works without explaining gravity.

I. Newton

El Modelo Estandar

Materia• 6 quarks• 6 leptonesAgrupados en 3 generaciones

Fuerzas• Electrodébil:

− γ (fotón) - Z0, W±

• Fuerte - g (gluon)

Teoría exitosa para describir el mundo subatómico

Incluye:

H= Lo que faltaba, el bosón de Higgs

Higgs bosonThe Standard Model cannot explain why a particle has a certain mass.

Physicists have theorized the existence of the so-called Higgs field, which in theory interacts with other particles to give them mass. The Higgs field requires a particle, the Higgs boson. The Higgs boson has not been observed.

The Higgs mechanism was postulated by British physicist Peter Higgs in the 1960s. The theory hypothesizes that a sort of lattice, referred to as the Higgs field, fills the universe. This is something like an e.m. field, which affects particles moving in it. It is known that when an electron passes through a positively charged crystal lattice, its mass can increase as much as 40 times. The same may be true in the Higgs field: a particle moving through it creates a little bit of distortion and lends mass to the particle.

Higgs mechanism (i)

Higgs mechanism (ii)

To understand the Higgs mechanism, imagine that a room full of physicists chattering quietly is like space filled with the Higgs field ...

... a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step ...

... this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field...

Higgs mechanism (iii)

... if a rumor crosses the room, ...

... it creates the same kind of clustering, but this time among the scientists themselves. In this analogy, these clusters are the Higgs particles.

Grand Unified Theory

Physicists hope that a Grand Unified Theory will unify the strong, weak, and electromagnetic interactions. If a Grand Unification of all the interactions is possible, then all the interactions we observe are all different aspects of the same, unified interaction.

However, how can this be the case if strong and weak and electromagnetic interactions are so different in strength and effect?

Current data and theory suggests that these varied forces merge into one force when the particles being affected are at a high enough energy.

Grand Unified Theory

Más allá del modelo estandar: unificación de fuerzas

DÉBIL

FUERTE GRAVEDAD

ELECTRO-MAGNÉTICA

FUERZA UNIFICADA

¿Será posible, es necesario?

Beyond the Standard Model

The SM explains the structure and stability of matter, but there are many unanswered questions:

Why do we observe matter and almost no antimatter?Why can’t the SM predict a particle’s mass?Are quarks and leptons actually fundamental?Why are there 3 generations of quarks and leptons?How does gravity fit into all of this?

Is the SM wrong?No, we need to extend the SM with something totally new in order to explain mass, gravity and other phenomena.

Supersymmetry

Some physicists attempting to unify gravity with the other fundamental forces have come to a startling prediction: every fundamental matter particle should have a massive "shadow" force carrier particle, and every force carrier should have a massive "shadow" matter particle. This relationship between matter particles and force carriers is called supersymmetry. For example, for every type of quark there may be a type of particle called a "squark." No supersymmetric particle has yet been found, but experiments are underway at CERN and Fermilab to detect supersymmetric partner particles.

AMS measures antimatter excess in space

• Alpha Magnetic Spectrometer, antimatter by dark matter.

• Positrons’ origin in the anihilation of dark matter particle in space

• An excess of antimatter within the cosmic ray flux was seen by PAMELA.

• Supersymmetry says that positrons are produced when two dark matter particles collide and anihilate.

• Maybe from pulsars around the galactic plane.

ATRAP makes world’s most precise measurement of antiproton

magnetic moment• CERN antiproton decelerator reported new result of

antiproton magnetic moment 680 times more precise than previous measurements.

• Thanks to ability of trapping antiprotons and use large magnetic gradient. Used Penning trap (suspended at the center of an iron ring electrode sandwiched between cooper electrodes.

• Useful result to understand the matter-antimatter imbalance of the universe. This tests the SM CPT theorem, magnetic moment of proton and antiproton are exactly but opposite: equal in strength but opposite in direction.

Producción de antihidrógeno (CERN)