Stuff

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* Are the laws of physics different for matter and antimatter?
* Why is there an abundance of matter in our universe, instead of equal amounts of matter and antimatter?

##### What is CP?

CP is a possible *symmetry* of nature. If the laws of nature were symmetric under CP, then matter and antimatter would be governed by the same rules. This means that if we communicated with aliens from a distant galaxy, there would be no way to find out if they are made from matter or antimatter: No experiment they could perform would allow us to deduce if they lived in a matter or antimatter world. On the other hand, if there was a fundamental difference between matter and antimatter, such an experiment would be possible. It turns out that this is the case in our universe!

##### How was CP violation discovered?

In 1964, a team lead by Val Fitch and Jim Cronin performed experiments with *neutral Kaons*, particles formed by a strange and an anti-down quark. These neutral Kaons have the amazing property that they can spontaneously transform into their own antiparticle. Fitch and Cronin discovered that the rate at which these Kaons changed from matter to antimatter and vice versa was different, clear evidence for CP violation! This discovery came as a total surprise to physicists (it was assumed that nature was symmetric under CP) and earned Cronin and Fitch the Nobel price in 1980.

##### How is CP violation currently understood?

In 1973, two Japanese physicists, Makoto Kobayashi and Toshihide Maskawa, found a very simple and elegant way to explain the occurence of CP violation in our universe. The only problem: The explanation required a third generation of quarks (the *top* and *bottom* quarks) for which there was zero evidence at the time.

This turned out to be an incredible prediction, when both of these quarks were discovered decades later. So far, the idea of Kobayashi and Maskawa, called the CKM mechanism, has been able to explain every single occurence of CP violation that physicists managed to detect in the lab. They were awarded the Nobel price in 2008.

##### What's next for CP violation?

CP violation is one of the necessary ingredients for explaining the abundance of matter over antimatter in our universe. But there is one problem: The CKM mechanism predicts too little of it. The amount of matter in our universe suggests that a correction or even a complete revolution in our understanding of CP violation is necessary.

CP violation remains a hot topic in Physics research. Specialized experiments like the LHCb detector at CERN in Switzerland are currently searching for hints of New Physics that could explain how our universe came to be the way it is.

##### Resources for CP violation

* Wikipedia on CP violation
* The original publication by Cronin, Fitch et al.
* Neutral kaon mixing on Wikipedia

#### The J/ψ meson

The J/ψ is a meson consisting of a charm quark and its antiquark. It is the first excited state of the charmonium (a bound charm-anticharm state), and was discovered independently by two research groups in 1974: one at the Stanford Linear Accelerator Center, led by Burton Richter, and one at the Brookhaven National Laboratory, led by Samuel Ting of MIT. Richter and Ting were awarded the 1976 Nobel Prize in Physics for their shared discovery.

##### History of the name

The J/ψ is the only particle with a two-letter name, as a result of its nearly simultaneous discovery by two independent groups. Ting wanted to name the particle “J”, while Richter called it “SP” (after the SPEAR accelerator used at SLAC), a name none of his colleagues liked. Richter finally settled on the Greek letter “ψ” (pronounced “psi”).

Since the scientific community considered it unjust to give one of the two discoverers priority, most subsequent publications have referred to the particle as the “J/ψ”.

##### Resources for The J/ψ meson

* The original presentation of J. J. Aubert et al.
* The original presentation of J.-E Augustin et al.
* J/ψ meson on Wikipedia

#### The τ lepton

The τ (tau) is an elementary particle that can be thought of as a much heavier cousin of the electron, with a spin of ½. It belongs to the family of leptons, along with the electron, the muon, and the three neutrinos. Despite the origin of the word lepton (meaning fine, small, thin) the τ is very massive at 1776.82 MeV/c2, which is nearly 3500 times the mass of the electron and around twice the mass of the proton.

##### The discovery of the τ

The τ was detected in a series of experiments between 1974 and 1977 by Martin Lewis Perl and his colleagues at the SLAC-LBL group. Their equipment consisted of SLAC’s then-new e+e− colliding ring, called SPEAR, and the LBL magnetic detector. They could detect and distinguish between leptons, hadrons and photons.

Martin Perl shared the 1995 Nobel Prize in Physics with Frederick Reines. The latter was awarded his share of the prize for experimental discovery of the neutrino.

##### Resources of The τ lepton

* The original publication by M. L. Perl et al.
* The τ lepton on Wikipedia.

#### The bottom (or beauty) quark

The bottom (or beauty) quark is a third-generation quark with a charge of −⅓ times the electron charge. It has a large mass (around 4.2 GeV/c2 — more that four times the mass of a proton!). The bottom quark is notable because it is a product in almost all decays of the top quark and is a frequent decay product for the Higgs boson.

##### History of the discovery

The bottom quark was predicted in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa as part of their explanation for CP violation. The name “bottom” was introduced in 1975 by Haim Harari. The bottom quark was discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonia (mesons with a bottom quark and its antiquark). Kobayashi and Maskawa won the 2008 Nobel Prize in Physics for their explanation of CP violation. Upon its discovery, there were efforts to name the bottom quark “beauty”, but “bottom” became the predominant name.

##### Resources the bottom (or beauty) quark

* The original presentation of S. W. Herb et al.
* The bottom quark on Wikipedia

#### Quarks and Gluons

For a long time, it was believed that the Proton and the Neutron, which make up the atomic nucleus, were fundamental particles. During the 1950s and 1960, an immense number of new, seemingly fundamental particles was discovered. This "particle zoo" confused physicists greatly, until a radical idea was proposed in 1964: What if these new particles were not fundamental, but instead made up of other particles, called *quarks*. These quarks would have a new three-fold charge called "color charge" (which has nothing to do with visible colors). Color charge would be transmitted via a new (eight-fold) fundamental particle, called the *gluon*.

Spectacularly, this model could explain all of the newly discovered composite *hadrons*, and even predict a few that had not been discovered! Shortly afterwards, it was confirmed through experiments with deep inelastic scattering that the Proton and Neutron were not fundamental. They, too, are made up of quarks!

#### **The weak force**

The weak interaction is a nuclear process that is responsible, among other things, for β (beta) decay the transformation of neutrons into protons. The weak force is mediated by two bosons called the W and the Z. The W comes in two types: W and W-. The Z is neutral and is sometimes represented as Z0.

##### **Discovery of the W and Z bosons**

The W and Z bosons are quite massive and so require powerful accelerators in order to be produced and studied. The Super Proton Synchrotron at CERN was the first machine capable of this, and the UA1 collaboration lead by Carlo Rubbia discovered both particles in 1983. Rubbia along with Simon Van der Meer, whose developments on the accelerator allowed such a machine to be built, were jointly awarded the Nobel Prize in Physics in 1984.

##### Resources of W and Z bosons

* CERN Press Release announcing the discovery of the W boson
* The weak interaction on Wikipedia
* The W and Z bosons on Wikipedia

#### **The top quark**

At 174.2 GeV/c2, the top quark is the heaviest particle we know of. It belongs to the third generation of quarks and has a charge of ⅔ times the electron charge. As a result of its large mass, it decays (mostly into bottom quarks) almost instantly after it is produced. This behemoth does not form bound states with other quarks or antiquarks.

It was discovered by the DØ and CDF collaborations at Fermilab in the US. Nowadays, top quarks and their properties are studied intensively by ATLAS and CMS at CERN.

#### **The Oscillation of Neutral B mesons**

Like neutral Kaons (with which CP violation was discovered for the first time), neutral B mesons can also spontaneously turn into their own antiparticle. Although suspected for a long time, this was first discovered in 1987 by the ARGUS collaboration at DESY in Germany.

With neutral B mesons, CP violation can now be studied very effectively. Important experiments making use of B oscillation have been the so-called B factories BaBar (in the US) and Belle (in Japan), as well as LHCb at CERN. These experiments have studied CP violation on a massive scale, looking at a large number of different decays of B mesons, as well as (more recently) Bs mesons and D mesons. So far, no deviation from the CKM mechanism by Kobayashi and Maskawa has been observed.

#### The Higgs field

The Higgs-Englert field has a central role in our current understanding of the universe. Through a process called *Spontaneous Symmetry Breaking*, it is responsible for the masses of all massive fundamental particles that we know of.

##### What is Spontaneous Symmetry Breaking?

At every point in space, the Higgs field has a certain *strength*, a number that tells you how active the field is. This is quite similar to temperature: You can assign a temperature to every point in a room, and the temperatures might be different for different points in the room and even change with time.

The Higgs-Englert field is the most special fundamental field that we know of: It interacts with nearly all of the other fields (like the electron field or the quark fields). This means that the Higgs field can greatly influence the other fields: If it is active somewhere, then electrons, quarks and other particles in that region will be slowed down by it. This is equivalent to them gaining mass!

But if the Higgs field would have an average strength of zero (as is usual for a field), then we would not be able to observe this slowdown (meaning no mass for other particles).

So how come this is not the case? It turns out that the Higgs field's *potential*, which governs how much energy is needed to increase its strength, has a very special form (see below). If the energy density in the universe is low enough, the field will drop down into the valley in the potential. This means it will be *locked to a non-zero strength*, and other particles gain mass everywhere in the universe!

##### Higgs' contribution

The mechanism of spontaneous symmetry breaking was discovered and explored by various different researchers. But it was Peter Higgs who first proposed, in 1964, that we could find evidence of it by searching for a new fundamental particle, now called the *Higgs boson*.

##### Discovery at the LHC

After decades of work, the discovery of the Higgs boson was announced in 2012 by the ATLAS and CMS collaborations at CERN. In 2013, Englert and Higgs received a Nobel Price for their contributions to the Higgs mechanism and the prediction of the Higgs particle.

##### The future of Higgs physics

You might think that we now know everything there is to know about the Higgs field, but it turns out that we actually know very little! Questions like

* What are the coupling strengths of the Higgs boson to itself?
* Is there just one Higgs particle or could there more?
* What is the role of the Higgs field in the early, mysterious *inflationary* phase of the universe?are sure to have physicists on the edge of their seats for many years to come!

##### Resources of the Higgs-boson

* Higgs boson on Wikipedia
* Quantum Diaries article on Spontaneous Symmetry Breaking

#### D\*sJ(2860)- meson.

The first ever heavy flavored spin-3 particle.

##### Resources of D\*sJ(2860)- meson

* Original Presentation from the LHCb Experiment
* Article in the CERN Courier

#### Ξb'- and the Ξb\*- baryons

Recently discovered strange beauty baryons. Both differ only in their spin configurations.

##### Resources of the Ξb'- and the Ξb\*- baryons

* The official LHCb publication on ArΧiv
* Official CERN press release
* Article on physicsworld.com