AION/Magis

Abstract:

A search for Higgs boson pair production in the b b ¯ γ γ final state is performed. The proton–proton collision dataset corresponds to an integrated luminosity of 308 fb − 1 , consisting of two samples, 140 fb − 1 at a centre-of-mass energy of s = 13 TeV and 168 fb − 1 at s = 13.6 TeV , recorded between 2015 and 2024 by the ATLAS detector at the CERN Large Hadron Collider. In addition to a larger dataset, this analysis improves upon the previous search in the same final state through several methodological and technical developments. The Higgs boson pair production cross section divided by the Standard Model prediction is found to be μ H H = 0 . 9 − 1.1 + 1.4 ( μ H H = 1 − 1.0 + 1.3 expected), which translates into a 95% confidence-level upper limit of μHH  < 3.7. At the same confidence level the Higgs self-coupling modifier is constrained to be in the range − 1.6 < κ λ < 6.6 ( − 1.8 < κ λ < 6.9 expected).

Abstract:

The thermal confinement phase transition in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mrow> <mi>SU</mi> </mrow> <mo stretchy="false">(</mo> <mi>N</mi> <mo stretchy="false">)</mo> </mrow> </math> Yang-Mills theory is first order for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mi>N</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math> , with bounce action scaling as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <msup> <mrow> <mi>N</mi> </mrow> <mrow> <mn>2</mn> </mrow> </msup> </mrow> </math> . Remarkably, lattice data for the action include a small coefficient whose presence likely strongly alters the phase transition dynamics. We give evidence, utilizing insights from softly broken supersymmetric Yang-Mills models, that the small coefficient originates from a deconfined phase instability just below the critical temperature. We predict the maximum achievable supercooling in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mrow> <mi>SU</mi> </mrow> <mo stretchy="false">(</mo> <mi>N</mi> <mo stretchy="false">)</mo> </math> theories to be a few percent, which can be tested on the lattice. We briefly discuss the potentially significant suppression of the associated cosmological gravitational wave signals.

Abstract:

Anisotropic flow and radial flow are two key probes of the expansion dynamics and properties of the quark-gluon plasma (QGP). While anisotropic flow has been extensively studied, radial flow, which governs the system’s radial expansion, has received less attention. Notably, direct experimental evidence for the global and collective nature of radial flow fluctuations has been lacking. This Letter presents the first measurement of transverse momentum ( $p_{\mathrm{T}}$ ) dependence of radial flow fluctuations ( $v_0(p_{\mathrm{T}})$ ) over $0.5<p_{\mathrm{T}}<10\mathrm{GeV}$ and demonstrates its collective nature using a two-particle correlation method in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ . The data reveal three key features supporting the collective nature of radial flow: long-range correlation in pseudorapidity, factorization in $p_{\mathrm{T}}$ , and centrality-independent shape in $p_{\mathrm{T}}$ . The comparison with a hydrodynamic model demonstrates the sensitivity of $v_0(p_{\mathrm{T}})$ to bulk viscosity, a crucial transport property of the QGP. These findings establish a new, powerful tool for probing collective dynamics and properties of the QGP.