On the metastability of the Standard Model vacuum

If the Higgs mass m_H is as low as suggested by present experimental information, the Standard Model ground state might not be absolutely stable. We present a detailed analysis of the lower bounds on m_H imposed by the requirement that the electroweak vacuum be sufficiently long-lived. We perform a complete one-loop calculation of the tunnelling probability at zero temperature, and we improve it by means of two-loop renormalization-group equations. We find that, for m_H=115 GeV, the Higgs potential develops an instability below the Planck scale for m_t>(166\pm 2) GeV, but the electroweak vacuum is sufficiently long-lived for m_t < (175\pm 2) \GeV.Comment: LaTex 23 pages, 4 eps figures. Misprint in the abstract corrected, reference adde

Long-term timing of pulsars in globular clusters

Pulsars are fast rotating, magnetized neutron stars that result from the supernova explosion of massive stars. Thanks to their coherent radiation, emitted in the form of collimated beams from the two magnetic poles, pulsars can be exploited as outstanding natural laboratories for fundamental physics. Some pulsars can be spun-up (or "recycled") up rotation periods as low as a few milliseconds, by accreting matter and angular momentum from a companion star. Globular clusters (GCs), spherical groups of stars that are gravitationally bound, are very efficient "factories" of recycled pulsars, thanks to their very high stellar densities, which favour two- or three-body gravitational interactions and the formation of exotic binary systems. This thesis is about the study of the pulsars in 47 Tuc and M15, which are two among the richest GCs for the number of pulsars hosted. I first present the results of the timing analysis of about two decades of data of 47 Tuc, taken with the Parkes radio telescope. The timing results of all the 25 pulsars of 47 Tuc are also used to study the dynamics and other important properties of the cluster. The much more precise measurements of the pulsar proper motions are used to infer the proper motion of the cluster as a whole. The measured higher-order spin frequency derivatives, instead, are used to derive the cluster distance, which results to be no smaller than 4.69 kpc. All the observed properties of the pulsars can be accounted for without invoking the presence of an intermediate-mass black hole at the core of 47 Tuc, although this hypothesis cannot be ruled out yet. Since almost all the pulsars are located very close to the cluster core, the population of neutron stars in 47 Tuc is likely to have reached a dynamical equilibrium with the stellar population. The only exception is 47 Tuc X, a very peculiar binary system that is much farther away than any other known pulsar of the cluster and that is therefore discussed in detail. I also present a study of the seven "black widow"/"redback" pulsars of 47 Tuc. These pulsars are known to show orbital variability due to a companion star that is losing mass. I find that, while some of these pulsars show a strong orbital variability, a few others appear remarkably stable. The pulsars in the other globular cluster, M15, are studied primarily through polarimetry. I use recent data taken with the 305-m Arecibo radio telescope to derive the polarimetric properties of five pulsars of the cluster for the first time. One of the pulsars, called M15C, is a binary millisecond pulsar in a double neutron star system and its peculiarity is that it is showing evidence of relativistic spin precession (RSP) occurring, an effect predicted by Einstein's General Relativity. Because of RSP the pulsar spin axis is precessing about the total angular momentum of the binary system, with a full cycle every 275 years. This in turn causes the pulsar radiation beam to change orientation with respect to the distance observer. The variations of the polarimetric properties over time are thus used to model RSP in M15C, and derive constraints on the geometry of the system. I find a large misalignment angle between the pulsar spin axis and the orbital angular momentum, which is not surprising given that the binary has probably formed in a chaotic three-body exchange interaction. The pulsar's visible beam is slowly moving away from our line of sight and it might become undetectable by as early as 2018. On the other hand, the secondary beam (from the other magnetic pole) is approaching our line of sight and could become detectable from around 2041

An algorithm for determining the rotation count of pulsars

We present here a simple, systematic method for determining the correct global rotation count of a radio pulsar; an essential step for the derivation of an accurate phase-coherent ephemeris. We then build on this method by developing a new algorithm for determining the global rotational count for pulsars with sparse timing data sets. This makes it possible to obtain phase-coherent ephemerides for pulsars for which this has been impossible until now. As an example, we do this for PSR J0024-7205aa, an extremely faint MSP recently discovered in the globular cluster 47 Tucanae. This algorithm has the potential to significantly reduce the number of observations and the amount of telescope time needed to follow up on new pulsar discoveries.Comment: 13 pages in MNRAS emulation format, 7 figures. Accepted for publication in MNRA