sign in sign up
<<
Home , List of particles, Massless particle, Matter, Standard Model

The and Beyond Maxim Perelstein, LEPP/Cornell U. NYSS APS/AAPT Conference, April 19, 2008 The basic question of : What is the world made of? What is the smallest indivisible building block of ? Is there such a thing?

In the 20th century, we made tremendous progress in observing smaller and smaller objects

Today’s accelerators allow us to study matter on length scales as short as 10^(-18) m The world’s largest /: the (located at in suburban Chicago) 4 miles long, accelerates and to 99.9999% of of and collides them head-on, 2 The CDF million collisions/sec. detector

The control room Particle Collider is a Giant Microscope!

• : diffraction limit, ∆min ≈ λ • mechanics: , λ ≈ h¯/p • Higher shorter distances: ∆ ∼ 10−13 cm M c2 ∼ 1 GeV • Nucleus: p • today: E ∼ 100 GeV ∆ ∼ 10−16 cm • Colliders in near future: E ∼ 1000 GeV ∼ 1 TeV ∆ ∼ 10−17 cm Particle Colliders Can Create New Particles! • All naturally occuring matter consists of particles of just a few types: protons, , , , • Most other known particles are highly unstable (lifetimes << 1 sec) do not occur naturally In Special Relativity, and are conserved, • 2 but mass is not: energy- is possible! E = mc • So, a collision of 2 protons moving relativistically can result in production of particles that are much heavier than the protons, “made out of” their kinetic energy • This is how most elementary particles are discovered! Another basic question: how did the begin?

High-energy particle collisions, today seen only in accelerators, were quite common in the high- , high- universe within the first second after the ! All our knowledge about subatomic physics is summarized in the Standard Model - the most successful Physics theory ever!

[from: particleadventure.org] (The - just like , but 16 different elementary particles much simpler and way cooler!) have been observed in collider experiments: 12 “matter particles” and 4 “ particles”

Matter particles are further divided into and

There are 6 leptons and 6 quarks: 3 “generations”, 2 leptons and 2 quarks in each

Particles in each row (e.g. u, c and t quarks) are identical except for their : t is heavier than c, which is heavier than u Some Basic Properties of Matter Particles: • Each matter particle has an , with exactly the same mass but opposite electric • Quarks do not exist as free objects, but only as constituents of “” (a of 3 quarks) and “” (a bound state of a and an antiquark) ¯ + ¯ • Examples: p = uud, n = udd, p¯ = u¯u¯d, π = ud, . . . • 100’s of baryons and mesons have been observed, all can be understood as bound states of the known quarks • Most particles are unstable (decay into other particles, with lifetimes <<1 sec) • Exceptions: electron, 3 neutrinos, 2 baryons: proton (uud) and (udd), and their , are STABLE FOUR FUNDAMENTAL :

Gravitational force: of planets, rockets, apples, ...

Electromagnetic force: , radiowaves, light, ...

Weak force: origin of radioactivity

Strong force: binds protons and neutrons in the nucleus

Gravity is very weak – a small can balance the gravitational effect of the entire , BUT: Only one type of gravitational charge (always attractive), forces add up – very relevant over long distances (while E&M charges cancel) Modern Picture of Forces: forces between “matter particles” are due to exchange of “force particles”

For example: Electromagnetic force between electrons is due to a exchange “” - combines Maxwell’s theory of , and Electric and magnetic forces are described by emission and absorption of photon – a particle of zero mass, the of electromagnetism

Feynman

“”

[Nobel prize 1965, with Schwinger, Tomonaga] Weak Interactions

Weak force is described by a theory just like electrodynamics, but instead of photon, mediating particles are the W and Z

● Weak force is short-range, with range about 1 0 − 1 5 cm: e−r/r0 V ∝ weak r ● This implies that the are massive: ¯h M ∼ ∼ 100 GeV cr0 Feynman diagram for ● Discovery of W and Z: CERN, 1983 the neutron beta-decay [Nobel prize 1984: Rubbia, van der Meer] EM-Weak Unification

HERA I high Q2 e-p EM and Weak forces )

2 • e-p NC - 10 ZEUS e p NC 98-99 become equally strong SM e-p NC (CTEQ5D) (pb/GeV 2 1 at short distances of /dQ

! −15 d -1 10 10 order cm

-2 10

-3 10 Same theory describes - • H1 e p CC -4 - 10 ZEUS e p CC 98-99 both forces in a unified SM e-p CC (CTEQ5D) -5 10 framework

-6 10 y < 0.9 -7 10 3 4 10 10 Q2 (GeV2) [blue=EM, red=weak] [Nobel prize 1979: Glashow, Salam, Weinberg] Strong Interactions Strong force is also described by a theory very similar to electrodynamics, the force particle is the Due to peculiar of the gluon, the strong force grows with distance between charges: V ∝ r

Only quarks experience the strong force, leptons are immune to it (neutral). This explains why quarks are confined and leptons are not!

[Nobel prize 2004: Gross, Politzer, Wilczek]

At short distances, the strong force gets weaker - the closer together you bring the quarks, the more freedom they feel! (””) Gravitational Interactions Gravitational force is supposedly described by a theory very similar to electrodynamics, the force particle is the Just like photon is a quantum of electromagnetic , graviton would be a quantum of Gravitaional waves are predicted by , and their indirect effects have been seen, but NO direct observation so far!

LIGO gravitational wave detectors in Hanford, Washington and Livingston, Louisiana This concludes our brief tour of matter particles...

and their interactions/force particles! Predictive Power of the Standard Model • The Standard Model is not just a and a classification - it is a theory that makes detailed, precise quantitative ! • Consider a head-on collision of a 100 GeV and a 100 GeV antielectron (””). Possible outcomes: e+e−, µ+µ−, τ +τ −, pp¯, W +W −, e+e+e−e−, . . . • Quantum mechanics: there is no way to know for sure which outcome will occur in a given collision, but the SM predicts probabilities (”cross sections”) of each outcome, plus details like directions of the produced particles, etc. • Works spectacularly well! (some predictions experimentally verified to 0.1% accuracy) Anything Left to Discover?

• The Standard Model has been the accepted theory of since ~1974 • No statistically significant deviations from the Standard Model predictions have been experimentally observed to date, in spite of years of effort • Most aspects of the model have already been experimentally verified • ~10 Nobel prizes have already been awarded for theoretical and experimental work related to the Standard Model • So, is particle physics over? NO! Truffle 1: The Higgs • The : a hugely important aspect of the Standard Model still awaits experimental verification! • SM postulates that the Universe is filled with a uniform “Higgs field” (somewhat similar to electric/magnetic fields) • A moving in a Higgs field is equivalent to a Higgs field “gives mass to all particles” • Different particles have different masses due to different strength of their interaction with the Higgs field (”charge”)

Where Is The Higgs Boson? • Relativity + Quantum Mechanics guarantee that any field must have a particle associated with it: e.g. electric/magnetic field photon, gravitational field graviton, etc. • Higgs field Higgs particle (or Higgs boson) • Experiments at the Tevatron and other colliders have hunted for the Higgs for years and have not found it! • Maybe this aspect of the SM is wrong! (Alternative theories of exist, although none is as appealing...) • Maybe the Higgs is just too heavy - theory only predicts a range for its mass, with upper bound well above the current sensitivity... • Need a more powerful accelerator/collider to test this! Large Collider (LHC) at CERN (Geneva, Switzerland) will turn on in Summer 2008!

The LHC: 7 TeV protons (7 more powerful than the Tevatron!), 17 miles long, few GEuros

If Higgs exists, the LHC will find it! LHC cooldown status (this morning)

[from http://lhc.web.cern.ch/lhc/] Truffle 2: Beyond the Higgs • It is highly unlikely that the Higgs boson will be the only new particle discovered at the LHC • It’s obvious: in a generic quantum field theory, any field mass term receives quadratically divergent radiative corrections, and severe fine-tuning is required to maintain a between the scalar mass and the cutoff of the theory (Ken Wilson, 1970s) • In other words, if the Higgs is there, something must “stabilize” it! (a.k.a. SUSY) • Supersymmetry is the idea that there is a “” for each : selectron, smuon, stau, squarks, , wino, , etc. • This would stabilize the Higgs (trust me!) • Many could be discovered at the LHC! However, Higgs+SUSY is not the only possible to generate masses...

Alternatives include: , extended technicolor, walking technicolor, composite Higgs, large , warped extra dimensions, universal extra dimensions, gauge-Higgs unification, Little Higgs models, Higgsless models, twin Higgs, ...

Good news: all these theories will be tested (and most of them ruled out) by the LHC experiments! What is the Universe REALLY Made Of?

• Particle physicist’s answer: stable particles - protons, neutrons, electrons, neutrinos • (Why not antiprotons, , etc.? another puzzle - maybe next ?) • But astronomical observations indicate that the known particles make only about 4% of the stuff in the Universe!!! Truffle 4: Rotation velocities of satellites of galaxies imply that galaxies have large halos of non-luminous matter

What Is Dark Matter Made Of? • The simplest explanation would be that dark matter consists of ordinary (protons, neutrons, electrons) that, for some reason, did not collapse into stars and remained cold • However, this simple hypothesis is ruled out by measurements of the number of baryons (protons+neutrons) in the universe - there is not enough baryons to account for the observed dark matter • The most sensitive measurement is provided by observing inhomogeneities of cosmic microwave background - a snapshot of the Universe at a tender age of 300000 years (vs 14 Gyr now!)

• Conclusion: there MUST be a new stable particle (not in the Standard Model!) that makes up dark matter • Dark matter has been observed only indirectly, through its gravitational effects

• Why hasn’t the dark matter particle been seen in colliders? It’s either too heavy, or interacts too weakly with ordinary matter...

• The most attractive theoretical idea is a WIMP - Weakly Interacting Massive Particle

• Many extensions of the Standard Model (e.g. supersymmetry) contain WIMPs

• WIMPs could be produced directly at the LHC and studied in the lab - direct connection between collider physics and ! Conclusions

• The Standard Model of particle physics is a simple theory describing a very wide range of observations

• SM allows for precise quantitative predictions, many of which have been verified with better than 1% accuracy

• Remaining piece of the SM - the Higgs boson - is about to be tested at the at CERN

• Strong theoretical reasons to expect physics beyond the SM to show up in the same experiments, for the first time

• Dark Matter cannot be explained in the SM, possibly will be directly produced at the LHC