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In the current standard model of particle physics the fundamental particles in the three generations of fermions have identical properties from one generation to the next, except for their mass, a state of affairs referred to as lepton universality. A possible violation of lepton universality is called a flavor anomaly, which would be a sign of “New Physics” (NP) beyond the standard model.
graphics grabbed from Cartelle 18
Indications of flavour anomalies have been observed consistently and with increasing statistical significance around $3$ σ in B meson processes in various experiments such as at the LHC, specifically at the LHCb, the Belle experiment and the BaBar experiment (see references below).
The global tension with the standard model by end of 2018 has statistical significance around $4.1$ σ (Strumia et al. 17, Cartelle 18, slide 22/25, Dey 18, slide 16/23, HAMN 18, p. 4). Various authors see the global tension (i.e. of all decay channels jointly) at over $5 \sigma$ already (CCDGMV 17, CFMVV 17 CGMS 18, Dordei 18, slide 12), which traditionally qualifies as detection (see here), some authors already quote $5.3 \sigma - 5.8 \sigma$ (ACDGMM 19) or $6 \sigma$ (Kumar-London 19).
graphics grabbed from Dey 18
If the ongoing evaluation of the data of LHC‘s Run 2 confirms the measurements of Run 1, then the statistical significance of the effect in each decay channel separately should have reached 5 σ (Crivellin et. al 18, p. 12) and hence conventionally count as detection of flavour anomaly (see here), which would make it the first established “new physics” seen at the LHC.
Moreover, it had been argued (Lyons 13b, Dorigo 15) that due to the specific nature of the experiment, the statistical significance-threshold for detection of anomalies in B meson-decays should not be taken to be $5 \sigma$, but just $3 \sigma$, hence well exceeded already by the experimental significance:
table taken from Lyons 13b, p. 4
table taken from Dorigo 15, p. 16
In conclusion, with currently available data, the observed flavour anomalies are possible signs of New Physics beyond the current standard model of particle physics.
graphics taken from Dordei 18, slide 22
In any case, further and more sensitive experiments are needed to confirm and explore the effect, such as possibly the “HL-LHC” or “HE-LHC” experiment. General outlook, prospects and suggestions for future collider design in this respect are discussed in detail in Allanach-Gripaios-You 17, Crivellin et al. 18.
Candidate models of “New Physics” beyond the standard model of particle physics that could possibly explain the flavour anomalies (if indeed they are real) includes the following:
One apparently promising model that could potentially explain the apparently observed flavour anomalies are leptoquarks, which naturally arise in, and hence potentially point to, models of $SU(5)$-grand unified theory.
(see Bauer-Neubert 15, CCDM 16, Crivellin 17, Falkowski17, Mueller 18, Matsuzaki-Nishiwaki 18, Monteux-Rajaraman 18, AMST 18, BDFKFS 18, Crivellin 18, MMR 18, Kumar-London 19, section 2.2.1, MVT 19)
From Crivellin 18, p. 2:
the global fit $[$ to flavour anomalies $]$ even shows compelling evidence for New Physics $[$… $]$ The vector leptoquark (LQ) $SU(2)_L$ singlet with hypercharge $-4/3$ arising in the famous Pati-Salam model is capable of explaining all the $[$flavour $]$ anomalies and therefore several attempts to construct a UV completion for this LQ to address the anomalies have been made. It can give a sizeable effect in $b \to c(u)\tau \nu$ data without violating bounds from $b \to s(d)\nu \bar \nu$ and/or direct searches, provides (at tree level) a $C_9 = - C_{10}$ solution to $b \to s \ell^+ \ell^-$ data and does not lead to proton decay at any order in perturbation theory.
Incidentally, leptoquarks are also a candidate explanation of the anomaly seen at over 4$\sigma$ statistical significance in the anomalous magnetic moment of the muon, see there.
(Blanke Crivellin 18, Crivellin 18)
Other possible models besides leptoquarks (above) which have been proposed as possible explanations of the apparently observed flavour anomalies include the following:
Measurements include
LHCb collaboration, Differential branching fractions and isospin asymmetries of $B \to K^{(\ast)}\mu^+ \mu^-$ decays, JHEP 06 (2014) 133 (arXiv:1403.8044)
LHCb collaboration, Test of lepton universality using $B^+\to K^+ \ell^+ \ell^-$ decays, Phys. Rev. Lett. 113, 151601 (2014) (arXiv:1406.6482)
LHCb collaboration, Angular analysis and differential branching fraction of the decay $B^0_s \to \phi \mu^+ \mu^-$, JHEP09(2015)179 (arXivL1506.08777)
LHCb collaboration, Differential branching fraction and angular analysis of $\Lambda^0_b \to \Lambda \mu^+ \mu^-$ decays, JHEP 06 (2015) 115; JHEP 09 (2018) 145 (arXiv:1503.07138)
LHCb collaboration, Test of lepton universality using $B^0\to K^{\ast 0} \ell^+ \ell^-$ decays, JHEP 08 (2017) 055 (arXiv:1705.05802)
Review includes
Elena Graverini, Flavour anomalies: a review, in Proceedings of the 13th International Conference on Beauty, Charm and Hyperon hadrons (BEACH 2018) (arXiv:1807.11373)
Diego Guadagnoli, Flavour anomalies on the eve of the Run-2 verdict, Modern Physics Letters AVol. 32, No. 07, 1730006 (2017) (arXiv:1703.02804)
CernCourier, Beauty quarks test lepton universality, March 2018
Paula Alvarez Cartelle on behalf of the LHCb collaboration, New results on flavor anomalies at LHCb, April 2018 (pdf)
Biplab Dey on behalf of the LHCb collaboration, Flavor anomalies at LHCb, May 2018 (pdf)
Vera Lüth, A Challenge to Lepton Universality in B Meson Decays, Invited tall at 7th Symposium on Symmetries in Subatomic Physics (SSP 2018) at RWTH Aachen University (Germany), June 11-15, 2018 (arXiv:1808.02587)
Francesca Dordei, Lepton flavour universality at LHCb, August 2018 (cern:2634813, pdf)
Karol Adamczyk (Belle collaboration), Semitauonic B decays at Belle/Belle II, Proceedings of the 10th International Workshop on the CKM Unitarity Triangle (CKM 2018), Heidelberg, Germany, September 17-21, 2018 (arXiv:1901.06380)
Outlook:
B.C. Allanach, Ben Gripaios, Tevong You, The Case for Future Hadron Colliders From $B \to K^{(\ast)}\mu^+ \mu^-$ Decays, JHEP03(2018)021 (arXiv:1710.06363)
A. Cerri, Andreas Crivellin et. al. Opportunities in Flavour Physics at the HL-LHC and HE-LHC, Report from Working Group 4 on the Physics of the HL-LHC, and Perspectives at the HE-LHC (arXiv:1812.07638)
Emphasis of non-perturbative effects:
Ulrich Nierste, Flavour Anomalies: Phenomenology and BSM Interpretations, talk at Planck 2018, Bonn 2018 (pdf)
Saeed Kamali, New physics in inclusive semileptonic $B$ decays including nonperturbative corrections (arXiv:1811.07393)
Emphasis of higher loop order-effects:
See also
CERN, LHCb flavour anomalies continue to intrigue, June 2017
Vasudevan Mukunth, A Similar Anomaly Has Showed up in Three Physics Experiments Since 2009. What’s Happening?, 2017
Diego Guadagnoli, On the status of flavor anomalies, 2016 (web)
Original articles include:
Guido D’Amico, Marco Nardecchia, Paolo Panci, Francesco Sannino, Alessandro Strumia, Riccardo Torre, Alfredo Urbano,
Flavour anomalies after the $R_{K^\ast}$ measurement,
J. High Energ. Phys. (2017) 2017 (arXiv:1704.05438)
Andrea Mauri, Nicola Serra, Rafael Silva Coutinho, Towards establishing Lepton Flavour Universality violation in $\bar B \to \bar K^\ast \ell^+ \ell^-$ decays (arXiv:1805.06401)
Bernat Capdevila, Andreas Crivellin, Sébastien Descotes-Genon, Joaquim Matias, Javier Virto, Patterns of New Physics in $b \to s \ell^+ \ell^-$ transitions in the light of recent data, JHEP 1801 (2018) 093 (arXiv:1704.05340)
Alejandro Celis, Javier Fuentes-Martin, Avelino Vicente, Javier Virto, Gauge-invariant implications of the LHCb measurements on Lepton-Flavour Non-Universality, Phys. Rev. D 96, 035026 (2017) (arXiv:1704.05672)
Monika Blanke, Andreas Crivellin, Stefan de Boer, Teppei Kitahara, Marta Moscati, Ulrich Nierste, Ivan Nišandžić, Impact of polarization observables and $B_c \to \tau \nu$ on new physics explanations of the $b \to c \tau \nu$ anomaly (arXiv:1811.09603)
Jacky Kumar, David London, New physics in $b \to s e^+ e^-$? (arXiv:1901.04516)
Domagoj Leljak, Blazenka Melic, Monalisa Patra, On lepton flavour universality in semileptonic $B_c \to \eta_c, J/\psi$ decays (arXiv:1901.08368)
Cautionary remarks include
In contrast, an argument that the threshold statistical significance for flavour anomalies should be taken to be $3\sigma$ instead of $5 \sigma$ is made in
Louis Lyons, Discovering the Significance of 5 sigma (arXiv:1310.1284)
Tommaso Dorigo, Extraordinary claims: the $0.000029\%$ solution, EPJ Web of Conferences 95, 02003 (2015) (doi:10.1051/epjconf/20159502003)
T. Hurth, A. Arbey, F. Mahmoudi, S. Neshatpour, New global fits to $b \to s$ data with all relevant parameters, Proceedings of the Seventh Workshop on Theory, Phenomenology and Experiments in Flavour Physics, Capri, 8-10 June 2018 (arXiv:1812.07602)
Srimoy Bhattacharya, Aritra Biswas, Zaineb Calcuttawala, Sunando Kumar Patra, An in-depth analysis of $b \to c(s)$ semileptonic observables with possible $\mu \to \tau$ mixing (arXiv:1902.02796)
Marcel Algueró, Bernat Capdevila, Sébastien Descotes-Genon, Pere Masjuan, Joaquim Matias, What $R_K$ and $Q_5$ can tell us about New Physics in $b \to s \ell \ell$ transitions? (arXiv:1902.04900)
Attempts to explain the anomaly include
Martin Bauer, Matthias Neubert, One Leptoquark to Rule Them All: A Minimal Explanation for $R_{D^{(\ast)}}$, $R_K$ and $(g-2)_\mu$, Phys. Rev. Lett. 116, 141802 (2016) (arXiv:1511.01900)
Yi Cai, John Gargalionis, Michael A. Schmidt, Raymond R. Volkas, Reconsidering the One Leptoquark solution: flavor anomalies and neutrino mass (arXiv:1704.05849)
Estefania Coluccio Leskow, Andreas Crivellin, Giancarlo D’Ambrosio, Dario Müller, $(g-2)_\mu$, Lepton Flavour Violation and Z Decays with Leptoquarks: Correlations and Future Prospects, Phys. Rev. D 95, 055018 (2017) (arXiv:1612.06858)
Andreas Crivellin, New Physics in Flavour Observables (arXiv:1706.00929)
Adam Falkowski, Leptoquarks strike back, November 2017
Dario Müller, Leptoquarks in Flavour Physics, EPJ Web of Conferences 179, 01015 (2018) (arXiv:1801.03380)
Shinya Matsuzaki, Kenji Nishiwaki, Kei Yamamoto, Simultaneous interpretation of K and B anomalies in terms of chiral-flavorful vectors (arXiv:1806.02312)
Angelo Monteux, Arvind Rajaraman, B Anomalies and Leptoquarks at the LHC: Beyond the Lepton-Quark Final State, Phys. Rev. D 98, 115032 (2018) (arXiv:1803.05962)
Ufuk Aydemir, Djordje Minic, Chen Sun, Tatsu Takeuchi, $B$-decay anomalies and scalar leptoquarks in unified Pati-Salam models from noncommutative geometry, JHEP 09 (2018) 117 (arXiv:1804.05844)
Damir Bečirević, Ilja Doršner, Svjetlana Fajfer, Nejc Košnik, Darius A. Faroughy, Olcyr Sumensari, Scalar leptoquarks from GUT to accommodate the $B$-physics anomalies, Phys. Rev. D 98, 055003 (2018) (arXiv:1806.05689)
Tanumoy Mandal, Subhadip Mitra, Swapnil Raz, $R_{D^{(\ast)}}$ in minimal leptoquark scenarios: impact of interference on the exclusion limits from LHC data (arXiv:1811.03561)
Jason Aebischer, Andreas Crivellin, Christoph Greub, QCD Improved Matching for Semi-Leptonic B Decays with Leptoquarks (arXiv:1811.08907)
Michael J. Baker, Javier Fuentes-Martin, Gino Isidori, Matthias König, High-pT Signatures in Vector-Leptoquark Models (arXiv:1901.10480)
Ivo de Medeiros Varzielas, Jim Talbert, Simplified Models of Flavourful Leptoquarks (arXiv:1901.10484)
Natascia Vignaroli, Seeking leptoquarks in the $t \bar t$ plus missing energy channel at the high-luminosity LHC (arXiv:1808.10309)
leptoquarks within a Randall-Sundrum model:
Andreas Crivellin, Explaining the Flavour Anomalies with the Pati-Salam Vector Leptoquark, PoS LHCP2018 (2018) 269 (spire:1713260, doi:10.22323/1.321.0269)
Monika Blanke, Andreas Crivellin, $B$ Meson Anomalies in a Pati-Salam Model within the Randall-Sundrum Background, Phys. Rev. Lett. 121, 011801 (2018) (arXiv:1801.07256)
and as possible explanation also of the anomalies seen in the ANITA experiment
Seungwon Baek, Scalar dark matter behind $b \to s \mu \mu$ anomaly (arXiv:1901.04761)
D.G. Cerdeno, A. Cheek, P. Martin-Ramiro, J.M. Moreno, B anomalies and dark matter: a complex connection (arXiv:1902.01789)
Rhorry Gauld, Florian Goertz, Ulrich Haisch, An explicit Z’-boson explanation of the $B \to K^\ast \mu^+ \mu^-$ anomaly, JHEP01(2014)069 (arXiv:1310.1082)
G. D’Ambrosio, A. M. Iyer, F. Piccinini, A.D. Polosa, Confronting $B$ anomalies with atomic physics (arXiv:1902.00893)
Last revised on February 14, 2019 at 02:01:52. See the history of this page for a list of all contributions to it.