Accelerator Neutrinos

Abstract:

The current laws of physics do not explain the observed imbalance of matter and antimatter in the universe. Sakharov proposed that an explanation would require the violation of CP symmetry between matter and antimatter. The only CP violation observed so far is in the weak interactions of quarks, and it is too small to explain the matter-antimatter imbalance of the universe. It has been shown that CP violation in the lepton sector could generate the matter-antimatter disparity through the process called leptogenesis. The quantum mixing of neutrinos, the neutral leptons in the Standard Model, provides a potential source of CP violation through a complex phase dCP, which may have consequences for theoretical models of leptogenesis. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible with accelerator-produced beams as established by the T2K experiment. Until now, the value of dCP has not been significantly constrained by neutrino oscillation experiments. Here the T2K collaboration reports a measurement that favors large enhancement of the neutrino oscillation probability, excluding values of dCP which result in a large enhancement of the observed anti-neutrino oscillation probability at three standard deviations (3 sigma). The 3 sigma confidence level interval for dCP, which is cyclic and repeats every 2pi, is [-3.41,-0.03] for the so-called normal mass ordering, and [-2.54,-0.32] for the inverted mass ordering. Our results show an indication of CP violation in the lepton sector. Herein we establish methods for sensitive searches for matter-antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger data samples will determine whether the leptonic CP violation is larger than the quark sector CP violation.

Abstract:

Abstract This paper reports the measurement of Higgs boson production in association with a $$t\bar{t}$$ t t ¯ pair in the $$H\rightarrow b\bar{b}$$ H → b b ¯ decay channel. The analysis uses 140 fb $$^{-1}$$ - 1 of 13 $$\text {TeV}$$ TeV proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider. The final states with one or two electrons or muons are employed. An excess of events over the expected background is found with an observed (expected) significance of 4.6 (5.4) standard deviations. The $$t\bar{t}H$$ t t ¯ H cross-section is $$\sigma _{t\bar{t}H} = 411~^{+101}_{-92}~\text {fb} = 411 \pm 54(\text {stat.})~^{+85}_{-75}(\text {syst.})~\text {fb}$$ σ t t ¯ H = 411 - 92 + 101 fb = 411 ± 54 ( stat. ) - 75 + 85 ( syst. ) fb for a Higgs boson mass of 125.09 $$\text {GeV}$$ GeV , consistent with the prediction of the Standard Model of $$507^{+35}_{-50}$$ 507 - 50 + 35 fb. The cross-section is also measured differentially in bins of the Higgs boson transverse momentum within the simplified template cross-section framework.

Abstract:

<jats:p>A search for the production of three Higgs bosons (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>H</a:mi><a:mi>H</a:mi><a:mi>H</a:mi></a:math>) in the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>b</c:mi><c:mover accent="true"><c:mi>b</c:mi><c:mo stretchy="false">¯</c:mo></c:mover><c:mi>b</c:mi><c:mover accent="true"><c:mi>b</c:mi><c:mo stretchy="false">¯</c:mo></c:mover><c:mi>b</c:mi><c:mover accent="true"><c:mi>b</c:mi><c:mo stretchy="false">¯</c:mo></c:mover></c:math> final state is presented. The search uses <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mn>126</k:mn><k:mtext> </k:mtext><k:mtext> </k:mtext><k:msup><k:mi>fb</k:mi><k:mrow><k:mo>−</k:mo><k:mn>1</k:mn></k:mrow></k:msup></k:math> of proton-proton collision data at <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msqrt><m:mi>s</m:mi></m:msqrt><m:mo>=</m:mo><m:mn>13</m:mn><m:mtext> </m:mtext><m:mtext> </m:mtext><m:mi>TeV</m:mi></m:math> collected with the ATLAS detector at the Large Hadron Collider. The analysis targets both nonresonant and resonant production of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>H</o:mi><o:mi>H</o:mi><o:mi>H</o:mi></o:math>. The resonant interpretations primarily consider a cascade decay topology of <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>X</q:mi><q:mo stretchy="false">→</q:mo><q:mi>S</q:mi><q:mi>H</q:mi><q:mo stretchy="false">→</q:mo><q:mi>H</q:mi><q:mi>H</q:mi><q:mi>H</q:mi></q:math> with masses of the new scalars <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>X</u:mi></u:math> and <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mi>S</w:mi></w:math> up to 1.5 and 1 TeV, respectively. In addition to scenarios where <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mi>S</y:mi></y:math> is off-shell, the nonresonant interpretation includes a search for Standard Model <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"><ab:mi>H</ab:mi><ab:mi>H</ab:mi><ab:mi>H</ab:mi></ab:math> production, with limits on the trilinear and quartic Higgs self-coupling set. No evidence for <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"><cb:mi>H</cb:mi><cb:mi>H</cb:mi><cb:mi>H</cb:mi></cb:math> production is observed. An upper limit of 59 fb is set, at the 95% confidence level, on the cross section for Standard Model <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"><eb:mi>H</eb:mi><eb:mi>H</eb:mi><eb:mi>H</eb:mi></eb:math> production.</jats:p> <jats:sec> <jats:title/> <jats:supplementary-material> <jats:permissions> <jats:copyright-statement>© 2025 CERN, for the ATLAS Collaboration</jats:copyright-statement> <jats:copyright-year>2025</jats:copyright-year> <jats:copyright-holder>CERN</jats:copyright-holder> </jats:permissions> </jats:supplementary-material> </jats:sec>