Contents

# Contents

## Idea

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 flavour anomaly, which would be a sign of “New Physics” (NP) beyond the standard model.

graphics grabbed from Cartelle 18

## Statistical significance

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, Bouchard-Cao-Owen 19). 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

graphics grabbed from Zupan 19

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, Zupan 19, 4.6) and hence conventionally count as detection of flavour anomaly (see here), which would make it the first established “new physics” seen at the LHC.

This situation was confirmed, if not further improved yet, with the data presented at Moriond 2019 (Straub 19, Allanach 19). On the other hand, Caria 19, slide 9 sees a previous 3.8$\sigma$ decrease to $3.1$, but see Descotes-Genon 19 for a comprehensive assessment.

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.

## Possible models

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:

### Leptoquarks

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.

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.

### Other

Other possible models besides leptoquarks (above) which have been proposed as possible explanations of the apparently observed flavour anomalies include the following:

## References

### General

Early suggestion to look for relevant channels are due to

• Patrick Koppenburg, Selection of $B_u \to \ell \ell K$ at LHCb and Sensitivity to $R_K$, 2007 (cern:1027442, pdf)

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)

• LHCb collaboration, Measurement of the ratio of the $B^0 \to D^{\ast -} \tau^+ \vu_\tau$ and $B^0 \to D^{\ast - \mu^+ \nu_\mu}$ branching fractions using three-prong τ-lepton decays (arXiv:1708.08856)

• Belle collaboration, Measurement of the $\tau$ lepton polarization and $R(D^\ast)$ in the decay $\bar B \to D^\ast \tau^- \bar \nu_\tau$ with one-prong hadronic τ decays at Belle (arXiv:1709.00129)

• Belle collaboration, Measurement of the $D^{\ast -}$ polarization in the decay $B^0 \to D^{\ast -} \tau^+ \nu_\tau$ (arXiv:1903.03102)

• LHCb collaboration, Search for lepton-universality violation in $B^+ \to K^+ \ell^+ \ell^-$ decays (cern:2668514)

• Belle Collaboration, Test of lepton flavor universality in $B \to K^\ast \ell^+ \ell^-$ decays at Belle (arXiv:1904.02440)

Review includes

Outlook:

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:

• Andreas Crivellin, Christoph Greub, Dario Müller, Francesco Saturnino, Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark, Phys. Rev. Lett. 122, 011805 (2019) (arXiv:1807.02068)

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

### Possible explanations/models

#### General EFT parameterization

• 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)

• Marcel Algueró, Bernat Capdevila, Andreas Crivellin, Sébastien Descotes-Genon, Pere Masjuan, Joaquim Matias, Javier Virto, Addendum: “Patterns of New Physics in $b \to s \ell^+ \ell^-$ transitions in the light of recent data” (arXiv:1903.09578)

• Ashutosh Kumar Alok, Amol Dighe, Shireen Gangal, Dinesh Kumar, Continuing search for new physics in $b \to \mu s s$ decays: two operators at a time (arXiv:1903.09617)

• Marco Ciuchini, António M. Coutinho, Marco Fedele, Enrico Franco, Ayan Paul, Luca Silvestrini, Mauro Valli, New Physics in $b \to s \ell^+ \ell^-$ confronts new data on Lepton Universality (arXiv:1903.09632)

• Jason Aebischer, Wolfgang Altmannshofer, Diego Guadagnoli, Meril Reboud, Peter Stangl, David M. Straub, B-decay discrepancies after Moriond 2019 (arXiv:1903.10434)

• Alakabha Datta, Jacky Kumar, David London, The $B$ Anomalies and New Physics in $b \to s e^+ e^-$ (arXiv:1903.10086)

• Ashutosh Kumar Alok, Dinesh Kumar, Suman Kumbhakar, S Uma Sankar, Impact of $D^\ast$ polarization measurement on solutions to $R_D$-$R_{D^\ast}$ anomalies (arXiv:1903.10486)

• Pere Arnan, Andreas Crivellin, Marco Fedele, Federico Mescia, Generic Loop Effects of New Scalars and Fermions in $b \to s \ell^+ \ell^-$ and a Vector-like 4th Generation (arXiv:1904.05890)

#### Leptoquarks

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)

• Ufuk Aydemir, Tanumoy Mandal, Subhadip Mitra, A single TeV-scale scalar leptoquark in SO(10) grand unification and B-decay anomalies (arXiv:1902.08108)

• Ivo de Medeiros Varzielas, Stephen F. King, Origin of Yukawa couplings for Higgs and leptoquarks (arXiv:1902.09266)

• Oscar Cata, Thomas Mannel, Linking lepton number violation with $B$ anomalies (arXiv:1903.01799)

• Bhubanjyoti Bhattacharya, Alakabha Datta, Saeed Kamali, David London, CP Violation in $\bar B^0 \to D^{\ast +} \mu^- \bar \nu_\mu$ (arXiv:1903.02567)

leptoquarks within a Randall-Sundrum model:

and as possible explanation also of the anomalies seen in the ANITA experiment

• Bhavesh Chauhan, Subhendra Mohanty, A common leptoquark solution of flavor and ANITA anomalies (arXiv:1812.00919)

#### Other

• David Marzocca, Addressing the B-physics anomalies in a fundamental Composite Higgs Model, JHEP07(2018)121 (arXiv:1803.10972)

• Andreas Crivellin, Dario Müller, Christoph Wiegand, $b \to s \ell^+ \ell^-$ Transitions in Two-Higgs-Doublet Models (arXiv:1903.10440)

• 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)

• Anirban Biswas, Avirup Shaw, Reconciling dark matter, $R_{K^{(\ast)}}$ anomalies and $(g-2)_\mu$ in an $L_\mu-L_\tau$ scenario (arXiv:1903.08745)

• Carlo Marzo, Luca Marzola, Martti Raidal, Common explanation to the $R_{K^{(\ast)}}$, $R_{K^{(\ast)}}$ and $\epsilon'/\epsilon$ anomalies in a 3HDM+$\nu_R$ and connections to neutrino physics (arXiv:1901.08290)

• Luigi Delle Rose, Shaaban Khalil, Simon J.D. King, Stefano Moretti, $R_K$ and $R_{K^\ast}$ in an Aligned 2HDM with Right-Handed Neutrinos (arXiv:1903.11146)

• 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)

• P. Ko, Takaaki Nomura, Chaehyun Yu, $b \to s \mu^+ \mu^-$ anomalies and related phenomenology in $U(1)_{B_{3-x_\mu L_\mu - x_\tau L_\tau}}$ flavor gauge models (arXiv:1902.06107)

Last revised on April 23, 2019 at 08:02:24. See the history of this page for a list of all contributions to it.