Search

Ultraviolet completions for axionlike particles (ALPs) lighter than the neutral pion generically induce ALP-neutral pion mixing and are therefore sensitive to direct constraints on the mixing angle. For ALPs below the pion mass, we demonstrate that strong and novel bounds on the ALP-pion mixing angle can be extracted from existing rare pion decay data, measured by the PIENU and PIBETA experiments.

The determination of |Vcb| from exclusive semileptonic B→D∗ν decays is sensitive to the choice of form factor parametrization. Larger |Vcb| values are obtained by fitting the Boyd-Grinstein-Lebed (BGL) versus the Caprini-Lellouch-Neubert (CLN) parametrization to recent Belle measurements. For the BGL parametrization, published fits use different numbers of parameters. We propose a method based on nested hypothesis tests to determine the optimal number of BGL parameters to fit the data, and find that six parameters are optimal to fit the Belle tagged and unfolded measurement [1]. We further explore the differences between fits that use different numbers of parameters. The fits which yield |Vcb| values in better agreement with determinations from inclusive semileptonic decays tend to exhibit tensions with heavy quark symmetry expectations. These have to be resolved before the determinations of |Vcb| from exclusive and inclusive decays can be considered understood.

We calculate the Λb→Λc?ν form factors and decay rates for all possible b→c?ν four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order αsΛQCD/mc,b and ΛQCD2/mc2 in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic Λb→Λc decays in terms of only two unknown sub-subleading Isgur-Wise functions, which can be determined from fitting LHCb and lattice QCD data. We thus obtain model independent results for Λb→Λc?ν decays, including predictions for the ratio R(Λc)=B(Λb→Λcτν)/B(Λb→Λcμν) in the presence of new physics, that are more precise than prior results in the literature, and systematically improvable with better data on the decays with μ (or e) in the final state. We also explore tests of factorization in Λb→Λcπ decays, and we emphasize the importance of measuring at LHCb the double differential rate d2Γ(Λb→Λc?ν)/(dq2dcosθ), in addition to the q2 spectrum.

We explore semileptonic B decays to the four lightest excited charm mesons, D∗∗={D0∗,D1∗,D1,D2∗}, for nonzero charged lepton mass and for all b→cν four-Fermi interactions, including calculation of the O(ΛQCD/mc,b) and O(αs) corrections to the heavy quark limit for all form factors. In the heavy quark limit, some form factors are suppressed at zero recoil; therefore, the O(ΛQCD/mc,b) corrections can be very important. The D∗∗ rates exhibit sensitivities to new physics in b→cτν mediated decays complementary to the D and D∗ modes. Since they are also important backgrounds to B→D(∗)τν, the correct interpretation of future semitauonic B→D(∗) rate measurements requires consistent treatment of both the D∗∗ backgrounds and the signals. Our results allow more precise and more reliable calculations of these B→D∗∗ν decays and are systematically improvable by better data on the e and μ modes. As an example, we show that the D∗∗ rates are more sensitive to a new cσμνb tensor interaction than the D(∗) rates.

We advocate for the construction of a new detector element at the LHCb experiment, designed to search for displaced decays of beyond Standard Model long-lived particles, taking advantage of a large shielded space in the LHCb cavern that is expected to soon become available. We discuss the general features and putative capabilities of such an experiment, as well as its various advantages and complementarities with respect to the existing LHC experiments and proposals such as SHiP and MATHUSLA. For two well-motivated beyond Standard Model benchmark scenarios - Higgs decay to dark photons and B meson decays via a Higgs mixing portal - the reach either complements or exceeds that predicted for other LHC experiments.

The neutral component of a real pseudoscalar electroweak (EW) triplet can produce a diphoton excess at 750 GeV, if it is somewhat mixed with an EW singlet pseudoscalar. This triplet-singlet mixing allows for greater freedom in the diboson branching ratios than the singlet-only case, but it is still possible to probe the parameter space extensively with 300 fb-1. The charged component of the triplet is pair produced at the LHC, which results in a striking signal in the form of a pair of Wγ resonances with an irreducible rate of 0.27 fb. Other signatures include multiboson final states from cascade decays of the triplet-singlet neutral states. A large class of composite models feature both EW singlet and triplet pseudo-Nambu-Goldstone bosons in their spectrum, with the diboson couplings generated by axial anomalies.