ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for New Phenomena in t(t)overbar Events with Large Missing Transverse Momentum in Proton-Proton Collisions at root s=7 TeV with the ATLAS Detector. Phys. Rev. Lett., 108(4), 041805–18pp.
Abstract: A search for new phenomena in t (t) over bar events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on 1: 04 fb(-1) of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeVand a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, toppartner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.
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LHCb Collaboration(Aaij, R. et al), Jaimes Elles, S. J., Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Rebollo De Miguel, M., et al. (2023). First Observation of a Doubly Charged Tetraquark and Its Neutral Partner. Phys. Rev. Lett., 131(4), 041902–12pp.
Abstract: A combined amplitude analysis is performed for the decays B0-+ D0D thorn s & pi;- and B thorn -+ D-D thorn s & pi; thorn , which are related by isospin symmetry. The analysis is based on data collected by the LHCb detector in proton -proton collisions at center-of-mass energies of 7, 8, and 13 TeV. The full data sample corresponds to an integrated luminosity of 9 fb-1. Two new resonant states with masses of 2.908 ⠂ 0.011 ⠂ 0.020 GeV and widths of 0.136 ⠂ 0.023 ⠂ 0.013 GeV are observed, which decay to D thorn s & pi; thorn and D thorn s & pi;- respectively. The former state indicates the first observation of a doubly charged open-charm tetraquark state with minimal quark content 1/2cs over bar ud over bar ⠃, and the latter state is a neutral tetraquark composed of 1/2cs over bar u over bar d ⠃ quarks. Both states are found to have spin-parity of 0 thorn , and their resonant parameters are consistent with each other, which suggests that they belong to an isospin triplet.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS. Phys. Rev. Lett., 123(4), 042001–21pp.
Abstract: Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb(-1) of pp and 0.49 nb(-1) of Pb + Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb + Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb + Pb events.
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CDF Collaboration(Aaltonen, T. et al), & Cabrera, S. (2010). Measurement of W-Boson Polarization in Top-Quark Decay in p(p)over-bar Collisions at root s=1.96 TeV. Phys. Rev. Lett., 105(4), 042002–8pp.
Abstract: We report measurements of the polarization of W bosons from top-quark decays using 2.7 fb(-1) of p (p) over bar collisions collected by the CDF II detector. Assuming a top-quark mass of 175 GeV/c(2), three measurements are performed. A simultaneous measurement of the fraction of longitudinal (f(0)) and right-handed (f(0)) W bosons yields the model- independent results f(0) =0. 88 +/- 0.11(stat) +/- 0.06(syst) and f(+) = 0.15 +/- 0.07(stat) +/- 0.06(syst) with a correlation coefficient of -0.59. A measurement of f(0) [f(+)] constraining f(+) [f(0)] to its standard model value of 0.0 [0.7] yields f(0) 0.70 + 0.07(stat) +/- 0.04(syst) [f(+) – 0.01 +/- 0.02(stat) +/- 0.05(syst)]. All these results are consistent with standard model expectations. We achieve the single most precise measurements of f(0) for both the model- independent and modeldependent determinations.
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LHCb Collaboration(Aaij, R. et al), Jashal, B. K., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., Ruiz Vidal, J., et al. (2023). Nuclear Modification Factor of Neutral Pions in the Forward and Backward Regions in p-Pb Collisions. Phys. Rev. Lett., 131(4), 042302–12pp.
Abstract: The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of 8.16 TeV with the LHCb detector. The p(0) production cross section is measured differentially in transverse momentum (p(T)) for 1.5 < p(T) < 10.0 GeV and in center-of-mass pseudorapidity (?(c.m.)) regions 2.5 < ?(c.m.) < 3.5 (forward) and -4.0 < ?(c.m.) < -3.0 (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of p(0) production, while the backward measurement shows the first evidence of p(0) enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.
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