Hints of physics behind standard model?

The LHCb collaboration is studying the decay of mesons $B$ in order to find some violations in standard model rules. In particular LHCb has measured a particular ratio, named $R (D^*)$, between two decay modes of $\overline{B}^0$ and they find a violation from the standard model prediction that is compatible with other similar measures:

In the SM all charged leptons, such as taus ($\tau$) or muons ($\mu$), interact in an identical fashion (or, in physicists' language, have the same "couplings"). This property is called "lepton universality". However, differences in mass between the leptons must be accounted for, and affect decays involving these particles. The $\tau$ lepton is much heavier than the $\mu$ lepton and therefore the SM prediction for the ratio $R(D^*)$ is substantially smaller than 1. This ratio is considered to be precisely calculable thanks to the cancellation of uncertainties associated with the $B$ to $D^*$ meson transition.

from press release | read also LHC signal hints at cracks in physics' standard model

But there is another hint of new physics. At the end of July Nature Physics published a new paper from the LHCb collaboration about the possible existence of a new particle:

The LHCb collaboration published in Nature Physics a paper based on run 1 data which reports the determination of the parameter $|V_{ub}|$ describing the transition of a $b$ quark to a $u$ quark. This measurement was made by studying a particular decay of the $\Lambda_b^0$ baryon. Other measurements of $|V_{ub}|$ by previous experiments had returned two sets of inconsistent results, depending on which method was used to determine the parameter. Theorists had suggested that this discrepancy could be explained by the presence a new particle contributing to the decay process, which affected the result differently, depending on the measurement method. Today's result from LHCb removes the need for this new particle, while the puzzle of why the original sets of measurements do not agree persists.

from press release | read also: From generation to generation

where $|V_{ub}|$ is connected to the Cabibbo-Kobayashi-Maskawa matrix.

Quarks of power

about @LHCbExperiment #pentaquark discovery

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Once upon a time, there was a controversy in particle physics. There were some physicists who denied the existence of structures more elementary than hadrons, and searched for a self-consistent interpretation wherein all hadron states, stable or resonant, were equally elementary. Others, appalled by the teeming democracy of hadrons, insisted on the existence of a small number of fundamental constituents and a simple underlying force law. In terms of these more fundamental things, hadron spectroscopy should be qualitatively described and essentially understood just as are atomic and nuclear physics.⁽¹¹⁾

The need of the partons

From FermiLab Today

When we descrive the collisions between particles, we calculate the cross section, the area of the distribution of the collisions' products. The mathematical object used to calculate the cross section are the structure functions, that mathematically describes the inner structure of the particle. In 1969 studying the deep inelastic scattering J. D. Bjorken^{(4, 18)}, in order to explain the experimental results, proposed a particular property for the hadronic structure function in the cross section called scaling. In the same year Richard Feynman^{(5, 18)} suggested the necessity to adopt a new description of hadrons: they had to be made by smaller components, more elementary than the hadrons themselves. These components are called partons.
The Feynman's thesys was immediatly verified by Bjorken and Paschos^{(6, 7, 18)}, in this way starting a great discussion about the parton models, described in the paper by De Rújula, Georgi and Glashow quoted at the beginning of the post⁽¹¹⁾ (an interesting review of the parton model and its story is in Greenberg⁽¹⁸⁾).
Probably the most strong motivation to adopt the parton model to describe hadrons is the great production of particles in the ring particles accelerators⁽⁵⁾. So, theoretical physicists produced a lot of model, but the most succesfull is the quarks model, developed by Murray Gell-Mann⁽¹⁾ and Georg Zweig^{(2, 3)}, that introduced a new quantum number, the flavor. The first formulation involved three type of quarks (and so three flavors): up, down and strange. To this first set of elementary particles in 1970 the quark charm was added by Glashow, Iliopulos and Maiani⁽⁸⁾ and finally in 1973 Kobayashi and Maskawa⁽⁹⁾ completed the family with the two last quark, top and bottom, named by Harari⁽¹⁰⁾ in 1975.

Three quarks for Muster Mark!
Sure he has not got much of a bark
And sure any he has it's all beside the mark.

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from Finnegan's Wake by James Joyce

The B mesons and the new physics

posted by @ulaulaman via @LHCbExperiment #newphysics #Bmesons #LHC #CERN #particlephysics

The search about B mesons decays has a great importance in physics for the possible clues of new physics that could be discovered. So theoretical phisicists have developed some new observables in order to test this possibility. LHCb produced new results about these new parameters, in particular the so called $P_5'$.

According to Joaquim Matias from Universitat Autonoma de Barcelona and colleagues the deviation in $P_5'$ and small discrepancies in the other angular observables for this decay, follow a pattern. In a recent paper the authors claim that a global analysis of the LHCb data, together with previous measurements, show a deviation of $4.5 \sigma$ with respect to Standard Model expectations, which can be explained with the same mechanism. This demands further investigation, in particular to re-evaluate all the sources of theoretical uncertainty, and to understand the effects of correlations between the experimental measurements.

(via LHCb)

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The image shows the distribution of the $P_5'$ observable as a function of the $\mu^+ \mu^-$ invariant mass squared $q^2$. The black data points are compared with the Standard Model prediction.

First evidence of photon polarisation in a quark transition

posted by @ulaulaman via @LHCbExperiment #newPhysics #StandardModel #LHC #CERN

There are a lot of model about physics beyond standard model, and the experimental work is concentrate to search signals to select the new models for the future. LHCb has recently released a press release about the transition of a b-quark to an s-quark with the emission of a photon. This transition

is considered a very important process to investigate possible manifestation of new physics. This decay process is forbidden in the first approximation in the Standard Model (SM) of particle physics and moreover in the second-order processes that govern the process in the SM the emitted photon is expected to be strongly polarised. Therefore it is very sensitive to new physics effects arising from the exchange of new heavy particles in electroweak penguin diagrams (see 14 June 2013 news). Indeed, several models of new physics predict that the emitted photon should be less polarised than in the SM. Up to now different experiments have measured the decay rate of this process, ruling out significant deviations of the rate from the SM prediction and strongly reducing the allowed parameter space of new physics models. The photon polarisation was, however, never previously observed.

I think that this is a really intriguing news for a particle physics point of view.

In search of violations

This week high-energy physics published some interesting results for our fundamental knowledge of the universe. On 14th November, LHCb published the preliminar analysis about a possible CP-violation in charm decays (see also the CERN's Bulletin and Mat Charles' presentation).
The researches about CP-violations are very important because in this way we can argue the differences between matter and anti-matter. If a physics law is CP-invariant, we must write that the beahvior of matter and anti-matter is the same. But our universe is constituted by matter and we don't know why it is so. The answer could be in CP-violation studies, like the preliminary data analyzed by LHCb team. The main goal of the experiment is the search of the properties of quark b, but he could also measure the properties of quark c. And studying the preliminary data about c decays the team find a clue of a CP-violation in a non expected channel. Following Tommaso Dorigo and Marco Delmastro (english translation by Google), if the result will be confirmed by further analysis, this could be the first sign of physics beyond Standard Model.

The other possible violation is the wall of the speed of light: indeed, OPERA experiment confirm their previous data. Yesterday, in the updated version of their famous preprint, OPERA's researchers described a new serie of measures realized with CNGS using a short-bunch wide-spacing beam.