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At least 1,675 records · Page 93

Probe nuclear structure using the anisotropic flow at the Large Hadron Collider

Abstract Recent studies have shown that the shape and radial profile of the colliding nuclei have strong influences on the initial condition of the heavy ion collisions and the subsequent development of the anisotropic flow. Using A Multi-Phase Transport model (AMPT) model, we investigated the impact of nuclear quadrupole deformation $$\beta _2$$ β 2 and nuclear diffuseness $$a_0$$ a 0 of $$^{129}$$ 129 Xe on various of flow observables in Xe–Xe collisions at $$\sqrt{s_{\textrm{NN}}}=$$ s NN = 5.44 TeV. We found that $$\beta _2$$ β 2 has a strong influence on central collisions while $$a_0$$ a 0 mostly influences the mid-central collisions. The relative change of flow observables induced by a change in $$\beta _2$$ β 2 and $$a_0$$ a 0 are also found to be insensitive to the values of parameters controlling the strength of the interaction among final state particles. Our study demonstrates the potential for constraining the initial condition of heavy ion collisions using future system scans at the LHC.

Physics↗

Search for supersymmetry in hadronic final states with missing transverse energy using the variables α T and b-quark multiplicity in pp collisions at $\sqrt{s} = 8\ \mathrm{TeV}$

An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 fb -1 collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, α T , is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950–1125 GeV are excluded depending on the assumed model. Finally, for the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600 GeV for bottom squarks.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Erratum to: Hadron transverse momentum distributions in muon deep inelastic scattering at 160 GeV/ $$c$$ c

This paper is an erratum to a previous paper (EPJC 73 (2013) 2531) published by C. Adolph et al. (The COMPASS Collaboration) using a data sample taken in 2004. In the course of the ongoing analysis of the more recent 2006 data sample, and following some additional hints we identified a problem in the analysis of the 2004 data presented in EPJC 73 (2013) 2531. The acceptance correction for the 2004 data sample did not properly account for the use of semi-inclusive triggers on top of the inclusive triggers.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Hadronic origin of the TeV flare of M87 in April 2010

M87 is a giant radio galaxy with FR-I morphology. It underwent three episodes of TeV flaring in recent years with the strongest one in April 2010 which was jointly monitored by MAGIC, VERITAS and H.E.S.S. We explain its spectral energy distribution in the energy range 0.3–5 TeV by assuming that the flaring occurs in the innermost region of the jet. In this region the low energy SSC photons serve as the target for the Fermi-accelerated high energy protons of energy ≤30 TeV to form a delta resonance. The TeV photons are produced from the subsequent decay of the delta resonance to neutral pions. In this scenario the observed TeV flux of April 2010 flare is fitted very well.

79 ASTRONOMY AND ASTROPHYSICS↗

Search for direct pair production of supersymmetric top quarks decaying to all-hadronic final states in pp collisions at √s = 8 TeV

Here, results are reported from a search for the pair production of top squarks, the supersymmetric partners of top quarks, in final states with jets and missing transverse momentum. The data sample used in this search was collected by the CMS detector and corresponds to an integrated luminosity of 18.9 fb –1 of proton-proton collisions at a centre-of-mass energy of 8 TeV produced by the LHC. The search features novel background suppression and prediction methods, including a dedicated top quark pair reconstruction algorithm. The data are found to be in agreement with the predicted backgrounds. Exclusion limits are set in simplified supersymmetry models with the top squark decaying to jets and an undetected neutralino, either via a top quark or through a bottom quark and chargino. Models with the top squark decaying via a top quark are excluded for top squark masses up to 755 GeV in the case of neutralino masses below 200 GeV. For decays via a chargino, top squark masses up to 620 GeV are excluded, depending on the masses of the chargino and neutralino.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Search for new phenomena with the $M_{\mathrm {T2}}$ variable in the all-hadronic final state produced in proton–proton collisions at $\sqrt{s} = 13$ $\,\text {TeV}$

A search for new phenomena is performed using events with jets and significant transverse momentum imbalance, as inferred through the $M_{\mathrm {T2}}$ variable. The results are based on a sample of proton–proton collisions collected in 2016 at a center-of-mass energy of 13 $\,\text {TeV}$ with the CMS detector and corresponding to an integrated luminosity of 35.9 $\,\text {fb}^\text {-1}$ . No excess event yield is observed above the predicted standard model background, and the results are interpreted as exclusion limits at 95% confidence level on the masses of predicted particles in a variety of simplified models of R-parity conserving supersymmetry. Depending on the details of the model, 95% confidence level lower limits on the gluino (light-flavor squark) masses are placed up to 2025 (1550) $\,\text {GeV}$ . Mass limits as high as 1070 (1175) $\,\text {GeV}$ are set on the masses of top (bottom) squarks. Information is provided to enable re-interpretation of these results, including model-independent limits on the number of non-standard model events for a set of simplified, inclusive search regions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Exploring the universality of hadronic jet classification

The modeling of jet substructure significantly differs between Parton Shower Monte Carlo (PSMC) programs. Despite this, we observe that machine learning classifiers trained on different PSMCs learn nearly the same function. This means that when these classifiers are applied to the same PSMC for testing, they result in nearly the same performance. This classifier universality indicates that a machine learning model trained on one simulation and tested on another simulation (or data) will likely be optimal. Our observations are based on detailed studies of shallow and deep neural networks applied to simulated Lorentz boosted Higgs jet tagging at the LHC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Charged-hadron identification at Belle II

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $426\,\text {fb}^{-1}$ of data collected at the energy-asymmetric $e^{+}e^{-}$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $\Upsilon$ (4S).

Adachi, I. (ORCID:0000000322870173)↗

Top quark physics at hadron colliders

The top quark is the heaviest fundamental particle known so far. As such, it is expected to play a crucial role in the study of the electroweak symmetry breaking mechanism and the generation of mass, as well as to serve as an ideal window into new physics. The discovery of a Higgs boson provides us additional experimental opportunities to test our current understanding of top quarks physics. In this contribution I will discuss the status of top quark physics as of 2014, and present a few recent highlights.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗