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Fonseca, R. M. (2015). On the chirality of the SM and the fermion content of GUTs. Nucl. Phys. B, 897, 757–780.
Abstract: The Standard Model (SM) is a chiral theory, where right- and left-handed fermion fields transform differently under the gauge group. Extra fermions, if they do exist, need to be heavy otherwise they would have already been observed. With no complex mechanisms at work, such as confining interactions or extra-dimensions, this can only be achieved if every extra right-handed fermion comes paired with a left-handed one transforming in the same way under the Standard Model gauge group, otherwise the new states would only get a mass after electroweak symmetry breaking, which would necessarily be small (similar to 100 GeV). Such a simple requirement severely constrains the fermion content of Grand Unified Theories (GUTs). It is known for example that three copies of the representations (5) over bar + 10 of SU(5) or three copies of the 16 of SO(10) can reproduce the Standard Model's chirality, but how unique are these arrangements? In a systematic way, this paper looks at the possibility of having non-standard mixtures of fermion GUT representations yielding the correct Standard Model chirality. Family unification is possible with large special unitary groups for example, the 171 representation of SU(19) may decompose as 3(16) + 120 + 3(1) under SO(10).
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LHCb Collaboration(Aaij, R. et al), Martinez-Vidal, F., Oyanguren, A., Ruiz Valls, P., & Sanchez Mayordomo, C. (2015). Observation of Two New Xi(-)(b) Baryon Resonances. Phys. Rev. Lett., 114(6), 062004–9pp.
Abstract: Two structures are observed close to the kinematic threshold in the Xi(0)(b)pi(-) mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb(-1), recorded by the LEICb experiment. In the quark model, two baryonic resonances with quark content bds are expected in this mass region: the spin-parity J(P) = (1/2)(+) and J(P) = (3/2)(+) states, denoted Xi'(-)(b) and Xi*(-)(b). Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be m(Xi'(-)(b)) – m(Xi(0)(b)) – m(pi(-))=3.653 +/- 0.018 +/- 0.006 MeV/c(2), m(Xi*(-)(b)) – m(Xi(0)(b)) – m(pi(-)) = 23.96 +/- 0.12 +/- 0.06 MeV/e(2), Gamma(Xi*(-)(b)) = 1.65 +/- 0.31 +/- 0.10 MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of Gamma(Xi'(-)(b)) < 0.08 MeV at 95% confidence level. Relative production rates of these states are also reported.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Observation of top-quark pair production in association with a photon and measurement of the t(t)over-bar gamma production cross section in pp collisions at root s=7 TeV using the ATLAS detector. Phys. Rev. D, 91(7), 072007–28pp.
Abstract: A search is performed for top-quark pairs (t (t) over bar) produced together with a photon (gamma) with transverse energy greater than 20 GeV using a sample of t (t) over bar candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The data set used corresponds to an integrated luminosity of 4.59 fb(-1) of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. In total, 140 and 222 t (t) over bar gamma candidate events are observed in the electron and muon channels, to be compared to the expectation of 79 +/- 26 and 120 +/- 39 non-t (t) over bar gamma background events, respectively. The production of t (t) over bar gamma events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The t (t) over bar gamma production cross section times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the ATLAS acceptance. The measured value is sigma(fid)(t (t) over bar gamma) x BR = 63 +/- 8(stat)(-13)(+17) (syst) +/- 1(lumi) fb per lepton flavor, in good agreement with the leading-order theoretical calculation normalized to the next-to-leading-order theoretical prediction of 48 +/- 10 fb.
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CMS and LHCb Collaborations(Khachatryan, V. et al), Martinez-Vidal, F., Oyanguren, A., Ruiz Valls, P., & Sanchez Mayordomo, C. (2015). Observation of the rare B-s(0)->mu(+)mu(-) decay from the combined analysis of CMS and LHCb data. Nature, 522(7554), 68–72.
Abstract: The standard model of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The probabilities, or branching fractions, of the strange B meson (B-s(0)) and the B-0 meson decaying into two oppositely charged muons (mu(+) and mu(-)) are especially interesting because of their sensitivity to theories that extend the standard model. The standard model predicts that the B-s(0)->mu(+)mu(-) and B-0 ->mu(+)mu(-) decays are very rare, with about four of the former occurring for every billion B-s(0) mesons produced, and one of the latter occurring for every ten billion B-0 mesons(1). A difference in the observed branching fractions with respect to the predictions of the standard model would provide a direction in which the standard model should be extended. Before the Large Hadron Collider (LHC) at CERN2 started operating, no evidence for either decay mode had been found. Upper limits on the branching fractions were an order of magnitude above the standard model predictions. The CMS (Compact Muon Solenoid) and LHCb(Large Hadron Collider beauty) collaborations have performed a joint analysis of the data from proton-proton collisions that they collected in 2011 at a centre-of-mass energy of seven teraelectronvolts and in 2012 at eight teraelectronvolts. Here we report the first observation of the B-s(0)->mu(+)mu(-) decay, with a statistical significance exceeding six standard deviations, and the best measurement so far of its branching fraction. Furthermore, we obtained evidence for the B-0 ->mu(+)mu(-) decay with a statistical significance of three standard deviations. Both measurements are statistically compatible with standard model predictions and allow stringent constraints to be placed on theories beyond the standard model. The LHC experiments will resume taking data in 2015, recording proton-proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of B-s(0) and B-0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.
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LHCb Collaboration(Aaij, R. et al), Martinez-Vidal, F., Oyanguren, A., Ruiz Valls, P., & Sanchez Mayordomo, C. (2015). Observation of the decay (B)over-bar(s)(0) -> psi(2S)K+pi(-). Phys. Lett. B, 747, 484–494.
Abstract: The decay (B) over bar (0)(s) -> psi(2S)K+pi(-) is observed using a data set corresponding to an integrated luminosity of 3.0 fb(-1) collected by the LHCb experiment in pp collisions at centre-of-mass energies of 7 and 8 TeV. The branching fraction relative to the B-0 -> psi(2S)K+pi(-) decay mode is measured to be B((B) over bar (0)(s) -> psi(2S)K+pi(-))/B(B-0 -> psi(2S)K+pi(-)) = 5.38 +/- 0.36 (stat) +/- 0.22 (syst) +/- 0.31 (f(s)/f(d)) %, where f(s)/f(d) indicates the uncertainty due to the ratio of probabilities for a b quark to hadronise into a B-s(0) or B-0 meson. Using an amplitude analysis, the fraction of decays proceeding via an intermediate K*(892)(0) meson is measured to be 0.645 +/- 0.049 (stat) +/- 0.049 (syst) and its longitudinal polarisation fraction is 0.524 +/- 0.056 (stat) +/- 0.029 (syst). The relative branching fraction for this component is determined to be B((B) over bar (0)(s) -> psi(2S)K*(892)(0))/B(B-0 -> psi(2S)K*(892)(0)) = 5.58 +/- 0.57 (stat) +/- 0.40 (syst) +/- 0.32 (f(s)/f(d)) %. In addition, the mass splitting between the B-s(0) and B-0 mesons is measured as M(B-s(0)) – M(B-0) = 87.45 +/- 0.44 (stat) +/- 0.09 (syst) MeV/c(2).
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