Alessandro Strumia

Minimal Flavour Violation: an effective field theory approach

We present a general analysis of extensions of the Standard Model which satisfy the criterion of Minimal Flavour Violation (MFV). We define this general framework by constructing a low-energy effective theory containing the Standard Model fields, with one or two Higgs doublets and, as the only source of SU(3) 5 flavour symmetry breaking, the background values of fields transforming under the flavour group as the ordinary Yukawa couplings. We analyse present bounds on the effective scale of dimension-six operators, which range between 1 and 10 TeV, with the most stringent constraints imposed by B → Xsγ. In this class of theories, it is possible to relate predictions for FCNC processes in B physics to those in K physics. We compare the sensitivity of various experimental searches in probing the hypothesis of MFV. Within the two-Higgs-doublet scenario, we develop a general procedure to obtain all tan β-enhanced Higgs-mediated FCNC amplitudes, discussing in particular their impact in B → l + l − , �MB and B → Xsγ. As a byproduct, we derive some two-loop tan β-enhanced supersymmetric contributions to B → Xsγ previously unknown.

Investigating the near-criticality of the Higgs boson

A bstractWe extract from data the parameters of the Higgs potential, the top Yukawa coupling and the electroweak gauge couplings with full 2-loop NNLO precision, and we extrapolate the SM parameters up to large energies with full 3-loop NNLO RGE precision. Then we study the phase diagram of the Standard Model in terms of high-energy parameters, finding that the measured Higgs mass roughly corresponds to the minimum values of the Higgs quartic and top Yukawa and the maximum value of the gauge couplings allowed by vacuum metastability. We discuss various theoretical interpretations of the near-criticality of the Higgs mass.

Minimal Dark Matter

Abstract A few multiplets that can be added to the SM contain a lightest neutral component which is automatically stable and provides allowed DM candidates with a non-standard phenomenology. Thanks to coannihilations, a successful thermal abundance is obtained for well defined DM masses. The best candidate seems to be a SU ( 2 ) L fermion quintuplet with mass 4.4 TeV, accompanied by a charged partner 166 MeV heavier with life-time 1.8 cm , that manifests at colliders as charged tracks disappearing in π ± with 97.7% branching ratio. The cross section for usual NC direct DM detection is σ SI = f 2 1.0 × 10 −43 cm 2 where f ∼ 1 is a nucleon matrix element. We study prospects for CC direct detection and for indirect detection.

PPPC 4 DM ID: A Poor Particle Physicist Cookbook for Dark Matter Indirect Detection

We correct a few mistakes of the original version of this work (notably related to the computations of extragalactic gamma rays), while at the same time improving and upgrading other aspects (notably as a consequence of the discovery of the higgs boson at the LHC). A brief list of the main changes is: - We include a higgs boson channel hh with mass mh = 125 GeV. All previous channels hmhm are removed. - We correct the formulae for the computation of extragalactic gamma rays (fixing in particular the redshift dependence) as well as the numerical computations (also including a corrected impact of absorption). - We provide a new version of the Optical Depth function, employing updated models of Extragalactic Background Light (EBL) and fixing the redshift dependence. All these corrections and updates are reflected on the numerical ingredients provided on the website; they correspond to Release 2.0.

Model-independent implications of the e±, p¯ cosmic ray spectra on properties of Dark Matter

Abstract Taking into account spins, we classify all two-body non-relativistic Dark Matter annihilation channels to the allowed polarization states of Standard Model particles, computing the energy spectra of the stable final-state particles relevant for indirect DM detection. We study the DM masses, annihilation channels and cross sections that can reproduce the PAMELA indications of an e + excess consistently with the PAMELA p ¯ data and the ATIC/PPB-BETS e + + e − data. From the PAMELA data alone, two solutions emerge: (i) either the DM particles that annihilate into W , Z , h must be heavier than about 10 TeV or (ii) the DM must annihilate only into leptons. Thus in both cases a DM particle compatible with the PAMELA excess seems to have quite unexpected properties. The solution (ii) implies a peak in the e + + e − energy spectrum, which, indeed, seems to appear in the ATIC/PPB-BETS data around 700 GeV. If upcoming data from ATIC-4 and GLAST confirm this feature, this would point to a O ( 1 ) TeV DM annihilating only into leptons. Otherwise the solution (i) would be favored. We comment on the implications of these results for DM models, direct DM detection and colliders as well as on the possibility of an astrophysical origin of the excess.

Electroweak symmetry breaking after LEP1 and LEP2

In a generic “universal” theory of electroweak symmetry breaking, simple symmetry considerations and absence of tuning imply that heavy new physics affects the lowenergy data through four parameters. These include and properly extend the generally insufficientS and T. Only by adding the LEP2 data to the global electroweak fit, can all these four form factors be determined and deviations from the SM be strongly constrained. Several of the recently proposed models (little Higgs, gauge bosons in extra dimensions or Higgsless models in 5D) are recognized to be “universal” in a straightforward way after a proper definition of the effective vector boson fields. Among various applications, we show that proposed Higgsless models in 5D, when calculable, do not provide a viable description of electroweak symmetry breaking in their full range of parameters.

Cosmology and Astrophysics of Minimal Dark Matter

Abstract We consider DM that only couples to SM gauge bosons and fills one gauge multiplet, e.g., a fermion 5-plet (which is automatically stable), or a wino-like 3-plet. We revisit the computation of the cosmological relic abundance including non-perturbative corrections. The predicted mass of, e.g., the 5-plet increases from 4.4 to 10 TeV, and indirect detection rates are enhanced by 2 orders of magnitude. Next, we show that due to the quasi-degeneracy among neutral and charged components of the DM multiplet, a significant fraction of DM with energy E ≳ 10 17 eV (possibly present among ultra-high energy cosmic rays) can cross the Earth exiting in the charged state and may in principle be detected in neutrino telescopes.

Baryogenesis through leptogenesis

Baryogenesis by heavy-neutrino decay and sphaleron reprocessing of both baryon and lepton number is reconsidered, paying special attention to the flavour structure of the general evolution equations and developing an approximate but sufficiently accurate analytic solution to the prototype evolution equation. Two different models of neutrino masses are examined, based on an Abelian U(1) or a non-Abelian U(2) family symmetry. We show that a consistent picture of baryogenesis can emerge in both cases, although with significant differences.

Higgs mass implications on the stability of the electroweak vacuum

Abstract We update instability and metastability bounds of the Standard Model electroweak vacuum in view of the recent ATLAS and CMS Higgs results. For a Higgs mass in the range 124–126 GeV, and for the current central values of the top mass and strong coupling constant, the Higgs potential develops an instability around 10 11 GeV, with a lifetime much longer than the age of the Universe. However, taking into account theoretical and experimental errors, stability up to the Planck scale cannot be excluded. Stability at finite temperature implies an upper bound on the reheat temperature after inflation, which depends critically on the precise values of the Higgs and top masses. A Higgs mass in the range 124–126 GeV is compatible with very high values of the reheating temperature, without conflict with mechanisms of baryogenesis such as leptogenesis. We derive an upper bound on the mass of heavy right-handed neutrinos by requiring that their Yukawa couplings do not destabilize the Higgs potential.

Violations of lepton flavour and CP in supersymmetric unified theories

Abstract As a consequence of the large top quark Yukawa coupling, supersymmetric unified theories with soft supersymmetry breaking terms generated at the Planck scale predict lepton flavour and CP violating processes with significant rates. The flavour violating parameters of the low energy theory are derived in both SU(5) and SO(10) theories, and are used to calculate the rate for μ → eγ. The sensitivity of the search for μ → eγ is compared with that for μ → e conversion in atoms, τ → μγ and the electric dipole moment of the electron. The experimental search for these processes is shown to provide a very significant test of supersymmetric unification, especially in SO(10) but also in SU(5).