What is 5 sigma?

Before the announce of the discover of a new boson in LHC by the two experiments ATLAS and CMS, I try to explain the concepts of sigma, $\sigma$. Today I would propose you a video from Fermilab about the statistics used in physics and in particular in the high energy field:

In alternative you can read 5 Sigma - What's that? by Evelyn Lamb.

Higgs at the Tevatron

posted by @ulaulaman about #higgs #physics #tevatron

This is the week of the Higgs. Indeed, wednesday, at CERN, ATLAS and CMS announced the results of the elaboration of the data collected in the first part of 2012... and a lot of journalists write about the probable discover of the Higgs boson. Indeed the two collaborations are disegned in order to discover the boson related to the mechanism that provides the mass to the other particles. Waiting for the conference, today CDF and DZero, the two collaborations of Tevatron, publicize in two conferences the first elaboration of the complete set of data about Higgs research. Their result was summarize by the following plot:

In the image there is the combination of the final results from the two collaborations. The two experiments combined detecte an excess in signals around 125 GeV with a 2.5 sigma. It is not the discover of the Higgs boson, but it could be a good clue for the existence of the boson. So I don't know if ATLAS and CMS will confirm or update this result in their next conferences, but in every case I must remmber to the readers that with a mass of 125 GeV we have need of physics beyond Standard Model, because the only SM is not sufficient to explain our universe. In order to explain better, I reprint here some considerations that I just published for the previous Higgs day:

Refining the mass of W

In our standard model of elementary particles we have four fundamental interactions: gravity, electromagnetism, strong nuclear force and weak interaction. In particular the last force is responsible for the radioactive decay and for the hydrogen fusion in stars. The bosons of the interaction (the particle exchanged between two fermions) are $W^\pm$ and $Z$ bosons. An example of weak interaction is $\pi^+$ decay:

The weak bosons are predicted in 1968 by Glashow, Weinberg and Salam⁽¹⁾ and discovered at CERN in 1983 in a series of experiments conducted by Carlo Rubbia and Simon van der Meer⁽²⁾. Now, from one of the last analysis from Tevatron, we have the last measure of W bosons. Indeed CDF's researchers propose the following preliminary value for $W$: \[M_W = (80.387 \pm 0.019) GeV\] and combining it with previous measures, the new preliminary world average is

I must remember that $(80.390 \pm 0.016) GeV$ will became the new $W$ mass only after the publication of the CDF's preprint (pdf) in a peer review journal and after the publication of the further calculation on the Particle Data Group. Indeed Wired (and en.wiki, following Wired), following Tommaso Dorigo, who simply described the experimental process that carries to the measure and to the new proposal, has just setted the new mass, forgetting the reviewing scientific process. So, until then, the average mass is $(80.399 \pm 0.023) GeV$⁽³⁾.

The Top of the Tevatron

Today is the last day of Tevatron. Tevatron is a particle accelerator, and it started the physics measure in 1985, on the night of 13rd october. The story of Tevatron is reach of great events, and Tommaso Dorigo write a great summary of Tevatron's physics, in particular the first great physics result: the discover of the top quark!
In Standard Model we have 6 quarks, and they are the elementary particles that constitue barionic matter. They was introduced in physics with a parton model indipendetly developed by Murray Gell-Mann⁽¹⁾ and George Zweig^{(2, 3)} in 1964. The original theory is constituted by three partons (up, down, strange), but in the subsequent years were provided also the others three quark. In particular in 1972 Makoto Kobayashi and Toshihide Maskawa⁽⁶⁾ proposed the existence of a new quark, the well known top quark: they introduce in weak interaction theory, discovered by Weinberg in 1967⁽⁴⁾ and 1971⁽⁵⁾, the CP-violation. In particular they write the hadronic parts of the lagrangian in four terms: kinetics, massive, strong and $L'$. Following the Higgs mechanism⁽⁹⁾, they supposed that the CP-violation it could be in massive term, because the spontaneous breaking of gauge symmetry.
Their calculations are group theory calculations: we can imagine the group that Kobayashi and Maskawa used like a space generated by two 4-dimensional spaces (the space of $SU (4)$ group). They pictured three possible partitions for every vector space:

1. two 2-dimensional subspaces;
2. one 2-dimensional subspace, and two mono-dimensional subspaces;
3. four mono-dimensional subspaces.

They studied only the combination with a physical sense, and a consequence of the breaking of the symmetry is the existens of a new parton, quark top that was discovered in 1995 at Tevatron:

(from D0 paper)

(from CDF paper)

I conclude with the following video by Maria Scileppi with Rob Snihur, a Tevatron's researcher:

Observation of a new neutral baryon

In the origin (late 1960s) the particle zoo⁽¹⁾ is the colloquially word used to describe the extensive list of known elementary particles. Indeed, before Standard Model becames the more accepted theory in particle physics, physicists discovered a lot of particles in their accelerators, but we know today that they are simply a combination of a little numbers of particles classified in three fundamental families: leptons, quarks (that they constitute fermions, particles with half-integer spin) and bosons (particles with integer spin).

(Commons)

We can classify also particles in a lot of sub-families, like baryons, the heavy particles constituted by three quarks: for example proton and neutron are barions, with the following composition: uud and udd respectively, where u is the up quark and d the down quark.
We know six types of quarks: up (u) and down (d), that explain protons and neutrons, charm (c), strange (s), top (t) and bottom (b) that explain a lot of other heavy particles. Standard Model predicts a series of combination of this quarks that they are summirized in a picture like this:

(source)

In the up there is with angular momentum $J =1/2$ and down with angular momentum $J=3/2$. Today we examine $J=1/2$ group, in particular to the last discover by CDF, one collaboration at Tevatron in Fermilab. Indeed, not all particles predicted by SM are found, and the hunt to them is open. On the 20th July, Pat Lukens announced the first observation of $\Xi_b^0$, a baryon with the structure usb:

In orther to detect the new baryon, researchers at Tevatron must reconstruct the following decay chain: