Test of lepton universality using B⁰→K_S⁰l⁺l^- and B⁺→K*⁺l⁺l^- decays.

Today at the CERN seminar and at the Rencontres de Blois the LHCb Collaboration presented new tests of lepton universality, one of the basic principles of the Standard Model (SM) of particle physics. This principle states that the SM treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. The new measurements, while of limited statistical sensitivity, exhibit the same coherent pattern of deviations from lepton universality as seen in previous LHCb results.

In March 2021, the collaboration reported an evidence at 3.1 standard deviations (σ) for the breaking of lepton universality in a measurement of the ratio R_K. The ratio R_K describes how often a B⁺ meson decays to a charged kaon and either a positive and a negatively charge muon (K⁺μ⁺μ^-) or a positron-electron pair (K⁺e⁺e^-). These decays are extremely rare, occurring at a rate of only one in two million B⁺ meson decays. The decays involve the transformation of a beauty quark into a strange quark (b→s), a process that is highly suppressed in the Standard Model and can be affected by the existence of new particles, which could have masses too high to be produced directly at the Large Hadron Collider. The left graph shows Standard Model contributions involving γ, W⁺, and Z⁰ bosons. The right one shows a possible new physics contribution to the decay with a hypothetical leptoquark (LQ) which, unlike the Standard Model bosons, could have different interaction strengths with the different types of leptons.

Deviations at the level of 2.2-2.5 σ in a measurement of the ratio R_K*⁰ were also reported in April 2017.

Both ratios exhibit a deficit of μ⁺μ^- pairs with respect to e⁺e^- pairs. Today’s results also report tests of lepton universality, using ratios R_K and R_K*. However, in this measurement, K and K* mesons with different electric charges have been used. Physicists call these newly measured decays the“isospin partners” of the previously measured decays. The new ratios are labelled R_KS⁰ and R_K*⁺.

LHCb has studied a number of other such ratios comparing decays with different leptons (l=e, μ, or τ leptons) in beauty particle decays (see R(D^*), R(J/ψ) and R(pK)). These results revealed hints of deviations from lepton universality, none of which is significant enough to constitute an observation of new physics on its own. However, according to theorists who study possible extensions of the SM, these deviations combined suggest an interesting and coherent pattern, which could be evidence for physics beyond the SM.

The values of the RKS0 and RK*+ ratios are presented at the top of this article. These ratios correspond to the transitions B⁰→K_S⁰l⁺l^- and B⁺→K*⁺l⁺l^-.

Figure 1. Quark diagrams for a B⁰→K_S⁰l⁺l^- decay.

The ratios are also presented in figure 2 for different regions of the dilepton invariant mass squared, q². They are individually consistent with the SM at the 1.4 and 1.5σ level, their combination at the 2σ level (for experts: using a fit for a single Wilson coefficient). It is interesting to note that the central values exhibit the same coherent pattern of deviation as the other lepton universality tests performed by LHCb – the muon pairs are observed less frequently than the electron pairs. The image to the left lists the new measurements by LHCb, the most precise to date, of R_KS0 and R_K*+ ratios, and previous measurments by Belle.

Figure 2. R_KS0 and R_K*+ Ratios for different regions of the dilepton invariant mass squared, q²

The bottom left image above shows the K_S⁰μ⁺μ^- mass distribution and the bottom right image the K_S⁰e⁺e^- mass used to determine the R_KS⁰ ratio. The top left image shows the K_S⁰π⁺ μ⁺μ^- mass distribution and the top right the K_S⁰π⁺e⁺e^- mass distribution used to determine the R_K*⁺ ratio. Note that a K*⁺ meson decays into a K_S⁰ and a π⁺ meson and note also a clear accumulation of events at the B⁰ or B⁺ mass.

The Institute of Cosmos Sciences of the University of Barcelona participates in this experiment with the researchers Carla Marín, Eugeni Graugés, Lluís Garrido, Ricardo Vázquez who have contributed to the development and optimisation of the electron reconstruction process, which is a crucial ingredient in the obtention of these measurements. We should also mention that the ICC-LHCb group counts with a new addition to their team, PhD Student Albert López, who will work on a similar Lepton Universality measurements as described in this news item with baryons. He will analyse the transition Λ_b⁰→pK^-l⁺l^-

Read more in the LHCb presentations at CERN and at Blois, and in the LHCb paper. You can also stay on the loop of the announcements of new results by following the experiment’s twitter account @LHCbExperiment.

The ICCUB postdoctoral researcher Ryotaku Suzuki has been recognized by the XVI edition of the prestigious Seitaro Nakamura award for his work on the paper “Topology-changing horizons at large D as Ricci flows" which was published in 2019 alongside ICCUB’s senior researcher Roberto Emparan as co-author.

Dr. Suzuki joined the Institute of Cosmos Sciences of the University of Barcelona in 2018 as a member of the gravitation and cosmology research group and has since worked in several areas including general relativity in the large D limit.

The award, which is named after Dr. Seitaro Nakamura (1913-2007) who had produced substantial research on theoretical particle physics particularly on two-meson and beta decay, was established in 2006 in order to encourage young researchers for their future endeavors in theoretical physics and the related fields.

The award is not only given to scientific areas but also to cultural creators in fields of art or technology thus becoming a transversal support motor.

You can read more about this on the following link.

The ceremony took place last Tuesday, June 8, in the Palau de la Generalitat and it was chaired by the President of the Comunitat Valenciana Ximo Puig.

Licia Verde joined the ICCUB as an ICREA researcher on 2009. She has carried her research in several fields in all of which she has become a key referent: from theoretical Cosmology and Cosmic Microwave radiation to Large Scale Structure, Galaxy Clusters or Statistical Applications to precision Cosmology. Throughout her career, she has been involved in the main cosmology projects such as 2dF Galaxy Redshift Survey (2dFGRS), Wilkinson Microwave Anisotropy Probe (WMAP), Sloan Digital Sky Survey (SDSS) and the Dark Energy Spectroscopic Instrument (DESI).

She stands on a long list of awards and distinctions: the Gruber Prize for Cosmology (2012), the Group Achievement NASA award (for her contribution on the Wilkinson Microwave Anisotropy Probe space mission, the National Research Award of the Generalitat de Catalunya (2018) and the Narcís Monturiol Medal (2019), all of which are a testimony to her scientific career.

You can learn more about her research in the piece: «Licia Verde: una carrera reconeguda en cosmologia de precisió»

The master's degree in Quantum Science and Technologies, coordinated by the University of Barcelona, is the result of common effort of the University of Barcelona, the Autonomous University of Barcelona, the Polytechnic University of Catalonia, the Institute of Photonic Sciences, the Barcelona Supercomputing Center, the Institute of High Energy Physics and the Catalan Institute of Nanoscience and Nanotechnology. It arises in parallel to the establishment of the emerging community in Quantum Technologies, QuantumCat.

The program is aimed at graduates in Physics, Physical Engineering or equivalent degrees who want to continue specialization studies in Quantum Science and Technologies and its fundamental objectives are on the one hand to provide a solid education in theoretical physics and quantum information theory and to offer a variety of elective subjects to form a forward-looking study plan oriented to the development of quantum science and technology.

The program is taught by 30+ expert professors, and covers the core concepts of quantum science and technology, as well as practical tools and techniques.
The master gives access to cutting-edge theoretical and experimental research in the fields of quantum sensing, communication, computing and simulation.

Direct connections to industry through the QuantumCAT community, and the participation of high-tech companies in the teaching program and an annual careers symposium, will facilitate the future integration of students in both academic and industrial sectors.

Prof Bruno Julia, member of the Institute of Cosmos Sciences and coordinator of the Master in the Quantum Science and Technology program notes, “Quantum physics will play a major role in the next future developments in our capacity to compute, communicate, and measure with precision. A solid education in Quantum sciences will be a crucial asset to both push the boundaries of our knowledge and also to develop new industrial products profiting from the intricate quantum properties. This master is the seed for a quantum future.”

Prof. Morgan Mitchell, Coordinator QuantumCAT, the Quantum Technologies Hub of Catalonia, adds, “Quantum technology is growing rapidly in Catalonia, with universities, research centres, startup companies and established industrials all working to bring about the second quantum revolution. QuantumCAT, strongly supports the creation of the Master in QST, as a way to boost the skilled workforce in this dynamic industry. We look forward to working with MQST students and graduates.”

• Master in Quantum Science and Technology
• Enrollment Information
• QuantumCAT
• Informative Flier

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The thesis "Radiación no térmica asociada a estrellas de gran massa", defended by Dr. Santiago del Palacio in 2018 under the direction of the researcher of the Institute of Cosmos Sciences, Prof. Valentí Bosch-Ramón, has been selected by the jury appointed by the Associació Argentina de Astronomía, the organizing body.

The Carlos M. Varsavsky Prize for the best doctoral thesis has a biennial character and has been awarded since 2006. Its aim is to promote and recognize scientific research and academic activity in Argentine astronomy and astrophysics.

The jury highlights the impact that the thesis has had in its area of ??research with the publication of articles in international journals and scientific meetings. In the thesis of Dr. del Palacio is investigated the nonthermal emission produced in systems with stars of great mass. The aim is to understand the acceleration efficiency of cosmic rays in these systems, under what conditions they are capable of being non-thermal emitters and what are the characteristics of their magnetic fields, managing to combine the observation and theory behind the issuance of these objects.