Selected puzzles in particle physics

20 Dec 2016, 10:00 → 22 Dec 2016, 13:00 Europe/Rome

High Energy Building, Seminar Room (Laboratori Nazionali di Frascati)

Barbara Mele (ROMA1), Enrico Nardi (LNF), Gennaro Corcella (LNF), Giuseppe Degrassi (ROMA3)

Aim of the workshop:

In spite of its consolidated experimental success, the standard model of particle physics falls short of describing all observed phenomena. For instance, the cosmological matter/antimatter asymmetry, dark matter, neutrino masses, all require some extension of the standard model. Well motivated theoretical considerations also support the expectation that physics beyond the standard model should sooner or later show up. Presently, it is far from clear which directions should be taken to extend the theory, and while the high energy frontiers keep being explored at the LHC, it is important to maintain under attentive scrutiny any discrepancy between theory predictions and experimental results.
In this 3-day workshop we plan to review the status of a few particle physics puzzles of different typologies: long-standing unresolved issues awaiting for further experimental clarification (muon g-2, neutrino physics anomalies), recent hints of anomalies in specific measurements where signals of new physics might preferentially show up (bottom and top physics), and discrepancies particularly robust, which cannot be ascribed to statistical fluctuations (internal e+e- production in ⁸Be nuclear transitions, proton charge radius). We plan to have only two or three talks each day, and plenty of time to jointly reanalyse the status of the various issues, confront and discuss strategies and ideas. The general approach and format of the talks (on invitation only) will be pedagogical, and should favor extended interaction among participants.

Scientific Program and Speakers:

 Javier Virto  (Bern)
"Flavor anomalies in B physics"

Andrea Giammanco (Louvain, CP3)
"Who ordered that? Investigations of the top-Higgs connection"

Mátyás Hunyadi  (MTA Atomki, Debrecen)
"Anomalous e⁺e^-  production in ⁸Be"

Carlo Giunti (INFN Torino)
"Short baseline neutrino anomalies"

Manfred Lindner (Heidelberg, Max Planck)
"Reactor antineutrinos: anomalies, interpretations and new experiments"

Krzysztof Pachucki (Warsaw)
"The proton charge radius conundrum"

Alberto Lusiani     (SNS & INFN Pisa)
"Muon g-2 experiments"

Massimo Passera (INFN Padova) 
"Muon g-2: theoretical interpretations"

Tuesday, 20 December

10:00 → 11:00 Flavor anomalies in B physics

In the last few years, several interesting anomalies have been observed in B meson decays. First, some branching fractions and angular distributions in b --> s mu mu seem to disagree with current Standard Model predictions. Second, LHCb has reported a measurement of an observable sensitive to lepton-flavor non-universality which is different from zero at the 2.6 sigma level. This observable is a b-->s transition and the deviation is consistent with the anomalies in b --> s mu mu; thus this is very persuasive. Third, there is another hint of lepton-flavor non-universality in charged b --> c semileptonic exclusive transitions, which might be related to the one in b --> s. I will review these B-physics anomalies, paying attention to the theoretical uncertainties in the SM predictions, and to the new physics implications.

Speaker: Javier Virto (Bern)

Slides VIRTO.pdf

11:30 → 12:30 Who ordered that? Investigations of the top-Higgs connection

The top quark is the heaviest known elementary particle, and it features a tantalizing numerical coincidence: from its measured mass, the Standard Model predicts a value of its Yukawa coupling to the Higgs boson strikingly close to 1. This stimulated a flourishing theoretical literature entertaining the possibility of a deep connection between the top quark and the actual mechanism of Electro-Weak Symmetry Breaking. This talk reviews the state of the art and the future prospects for the following, complementary, experimental efforts at the LHC: precisely measuring the top quark mass; constraining the modulus of the Higgs-top Yukawa coupling via the search for the ttH process; and constraining the phase of this coupling (relative to the Higgs coupling to W bosons) by exploiting a subtle interference effect.

Speaker: Andrea Giammanco (Louvain, CP3)

Slides frascati_dec2016.pdf

15:00 → 16:00 Anomalous e+e- production in 8Be

Predictions on the identity of the gauge boson of light dark matter suggest vector bosons in the mass range of 10 MeV - 10 GeV. Several attempts were made to find such particles by using data from running facilities. Since no evidence was found, limits were set on their mass and their coupling strength to ordinary matter. Researchers thus turn their attention to search for less massive candidates in the MeV scale and with a short life time, which decay mostly into electron-positron (e+-e-) pairs. In our recent work, we searched for such e+-e- pairs in nuclear transitions. We measured their angular correlation in internal pair creation for the M1 transition depopulating the 18.15 MeV state in 8Be. A significant, peak-like deviation was observed with respect to the predicted angular correlation for the internal pair creation. To our best knowledge, present nuclear physics theories cannot account for such deviation, however, assuming the creation and subsequent decay of a Jπ=1+ particle with a mass m0c2 = 16.70(61) MeV gives a satisfactory fit to our observations.

Speaker: Matyas Hunyadi (MTA Atomik, Debrecen)

Slides 8Be_anomaly.pdf

Wednesday, 21 December

10:00 → 11:00 Short baseline neutrino anomalies

I review the experimental indications in favor of short-baseline neutrino oscillations. I discuss their interpretation in the framework of neutrino mixing schemes with one or more sterile neutrinos which have masses around the eV scale. I present arguments in favor of effective 3+1 neutrino mixing with one sterile neutrino at the eV scale. I discuss the implications for future neutrino oscillations experiments and for the experiments sensitive to the absolute values of neutrino masses (beta decay, neutrinoless double-beta decay and cosmology).

Speaker: Dr Carlo Giunti (TO)

Slides giunti-161221-frascati.pdf

11:30 → 12:30 Reactor antineutrinos: anomalies interpretations, and new experiments

Reactor anti-neutrinos have contributed important insights into neutrino properties. Precision measurements led, however, also to certain discrepancies between simulated neutrino spectra an measurements. The talk will cover aspects of the so-called bump and the flux anomaly, possible explanations and prospects for experimental tests. The talk will in addition cover the potential of certain new reactor experiments.

Speaker: Manfred Lindner (MAX-PLANCK-INSTITUT)

Slides lindner.pdf

15:00 → 16:00 The proton charge radius conundrum

The 2010 measurement of the muonic hydrogen Lamb shift by Pohl and collaborators has questioned our understanding of hydrogenic systems. A significant disagreement with theoretical predictions, which can be interpreted as a discrepancy in the proton charge radius between electronic and muonic measurements has not been resolved till now. A single muonic measurement stands against dozens of electronic hydrogen and electron- proton scattering ones and no simple extension of the Standard Model can fix this. We will argue that the only solution which does not violate the lepton universality is the underestimated uncertainty of all the electronic measurements and it is the muonic hydrogen value which is the correct one. As a consequence this hypothesis will cause a significant changes in fundamental physical constants and I will present all possible means of verifying the new proton charge radius value.

Speaker: Krzysztof Pachucki (Warsaw)

Slides pachucki.pdf

Thursday, 22 December

10:00 → 11:00 Muon g-2: theoretical interpretations

I will present recent developments in the Standard Model prediction of the muon g-2 and the long-standing discrepancy with its measured value

Speaker: Massimo Passera (PD)

Slides Passera_LNF.pdf

11:30 → 12:30 Muon g-2 experiments

We review the status and the prospects of the experimental measurement of the muon magnetic moment anomaly, a_mu = (g_mu-2)/2. The most precise measurement has been done by the BNL E821 experiment and has an uncertainty of 0.54 ppm. The Muon g-2 Fermilab experiment is approaching data-taking and has the goal to reduce the uncertainty by a factor 4. As second experimental effort is on-going at J-PARC with the aim to measure a_mu with an uncertainty of abour the same size of the BNL experiment in a first stage, with a subsequent upgrade to reduce the uncertainty at the same level as the Fermilab experiment.

Speaker: Dr Alberto Lusiani (Scuola Normale Superiore and INFN, Pisa)

Slides lusiani-puzzles16.pdf