AION/Magis

Abstract:

The azimuthal anisotropy of charged particles in Pb-Pb collisions at √s NN=2.76TeV is measured with the CMS detector at the LHC over an extended transverse momentum (p T) range up to approximately 60GeV/c. The data cover both the low-p T region associated with hydrodynamic flow phenomena and the high-p T region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter (v 2) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0-60% most-central events, the observed v 2 values are found to first increase with p T, reaching a maximum around p T=3GeV/c, and then to gradually decrease to almost zero, with the decline persisting up to at least p T=40GeV/c over the full centrality range measured. © 2012 CERN. Published by the American Physical Society.

Abstract:

It is challenging to explain the tentative 125 GeV Higgs signal in the Minimal Supersymmetric Standard Model (MSSM) without introducing excessive fine-tuning, and this motivates the study of non-minimal implementations of low energy supersymmetry (SUSY). A term \lambda SH_uH_d involving a Standard Model (SM) singlet state S leads to an additional source for the quartic interaction raising the mass of the lightest SM-like Higgs. However, in order to achieve m_h \approx 125 GeV with light stops and small stop mixing, it is necessary for \lambda \gtrsim 0.7 and consequently \lambda may become non-perturbative before the unification scale. Moreover, as argued by Barbieri, Hall, et al. low fine-tuning prefers the region \lambda~1-2, leading to new or non-perturbative physics involving S below the GUT scale (`\lambda SUSY' models). This raises the concern that precision gauge coupling unification, the prime piece of indirect experimental evidence for low energy SUSY, may be upset. Using the NSVZ exact \beta-function along with well motivated assumptions on the strong coupling dynamics we show that this is not necessarily the case, but rather there exist classes of UV completions where the strong-coupling effects can naturally correct for the present ~3% discrepancy in the two-loop MSSM unification prediction for \alpha_s. Moreover, we argue that in certain scenarios a period of strong coupling can also be beneficial for t-b unification, while maintaining the small to moderate values of tan\beta preferred by the Higgs mass.