On Future High-Energy Colliders

An outline of the physics reasons to pursue a future programme in high-energy colliders is presented.Comment: 9 page

Largest temperature of the radiation era and its cosmological implications

The thermal history of the universe before the epoch of nucleosynthesis is unknown. The maximum temperature in the radiation-dominated era, which we will refer to as the reheat temperature, may have been as low as 0.7 MeV. In this paper we show that a low reheat temperature has important implications for many topics in cosmology. We show that weakly interacting massive particles (WIMPs) may be produced even if the reheat temperature is much smaller than the freeze-out temperature of the WIMP, and that the dependence of the present abundance on the mass and the annihilation cross section of the WIMP differs drastically from familiar results. We revisit predictions of the relic abundance and resulting model constraints of supersymmetric dark matter, axions, massive neutrinos, and other dark matter candidates, nucleosynthesis constraints on decaying particles, and leptogenesis by decay of superheavy particles. We find that the allowed parameter space of supersymmetric models is altered, removing the usual bounds on the mass spectrum; the cosmological bound on massive neutrinos is drastically changed, ruling out Dirac (Majorana) neutrino masses $m_\nu$ only in the range 33 keV \simlt m_\nu\simlt 6 (5) MeV, which is significantly smaller from the the standard disallowed range 94 eV \simlt m_\nu\simlt 2 GeV (this implies that massive neutrinos may still play the role of either warm or cold dark matter); the cosmological upper bound on the Peccei-Quinn scale may be significantly increased to $10^{16}$GeV from the usually cited limit of about $10^{12}$GeV; and that efficient out-of-equilibrium GUT baryogenesis and/or leptogenesis can take place even if the reheat temperature is much smaller than the mass of the decaying superheavy particle.Comment: 27 pages. LaTeX (using revtex) with 10 eps figures embedded using epsf Revised version, with changes mostly in the section on the relic neutrino density in low-reheat model

Constraints on extra-dimensional theories from virtual-graviton exchange

We study the effective interactions induced by loops of extra-dimensional gravitons and show the special role of a specific dimension-6 four-fermion operator, product of two flavour-universal axial currents. By introducing an ultraviolet cut-off, we compare the present constraints on low-scale quantum gravity from various processes involving real-graviton emission and virtual-graviton exchange. The LEP2 limits on dimension-6 four-fermion interactions give one of the strongest constraint on the theory, in particular excluding the case of strongly-interacting gravity at the weak scale

Counting dark matter particles in LHC events

We suggest trying to count the number of invisible particles produced in missing energy events at the LHC, arguing that multiple production of such particles provides evidence that they constitute stable Dark Matter and that counting them could yield further insights into the nature of Dark Matter. We propose a method to count invisible particles, based on fitting the shapes of certain transverse- or invariant-mass distributions, discuss various effects that may affect the measurement, and simulate the use of the method to count neutrinos in Standard Model processes and Dark Matter candidates in new physics processes.Comment: 18 pages, 13 figures, revtex4 forma

Big Science and the Large Hadron Collider

The Large Hadron Collider (LHC), the particle accelerator operating at CERN, is probably the most complex and ambitious scientific project ever accomplished by humanity. The sheer size of the enterprise, in terms of financial and human resources, naturally raises the question whether society should support such costly basic-research programs. I address this question here by first reviewing the process that led to the emergence of Big Science and the role of large projects in the development of science and technology. I then compare the methodologies of Small and Big Science, emphasizing their mutual linkage. Finally, after examining the cost of Big Science projects, I highlight several general aspects of their beneficial implications for society.Comment: 17 pages; final version to appear in Physics in Perspectiv

LHC bounds on large extra dimensions

We derive new dominant bounds on the coefficient of the effective operator generated by tree-level graviton exchange in large extra dimensions from pp \rightarrow jj data at LHC: M_T > 2.1TeV (ATLAS after 3.1/pb of integrated luminosity), M_T > 3.4 TeV (CMS after 36/pb), MT > 3.2 TeV (ATLAS after 36/pb). We clarify the role of on-shell graviton exchange and compare the full graviton amplitude to ATLAS data, setting bounds on the fundamental quantum-gravity scale.Comment: 16 pages, 6 figures. v2: updated with CMS data. v3: updated with ATLAS data at 36/pb; final published versio

What if Charged Current Events at Large Q2Q^2 are Observed at HERA?

An excess of events at large Q2 with a positron in the final state has been observed at HERA which, if confirmed, would be a signal of new physics. It is not clear at present if a signal of comparable rate is also seen in the charged current channel (with an antineutrino in the final state). In this note we analyse the implications of the presence of such a signal in models of new physics based on contact terms, leptoquarks and squarks with R-violating decays. We find that in all cases the most likely possibility is that the charged current signal is absent. As a consequence if this signal is present the resulting indications are very selective. In particular for squarks only charged current events with multi-quark final states are possible with quite definite predictions on the spectrum of supersymmetric particles

Phenomenological implications of neutrinos in extra dimensions

Standard Model singlet neutrinos propagating in extra dimensions induce small Dirac neutrino masses. While it seems rather unlikely that their Kaluza-Klein excitations directly participate in the observed neutrino oscillations, their virtual exchange may lead to detectable signatures in future neutrino experiments and in rare charged lepton processes. We show how these effects can be described by specific dimension-six effective operators and discuss their experimental signals