Supersymmetry and proton decay

Using supergraph techniques it is shown that all one-loop contributions to the proton decay amplitude, which are suppressed by only one power of the unification mass and are not of fourth order in Higgs couplings, vanish in a general supersymmetric grand unification setting. Corrections to this statement due to soft supersymmetry breaking are analysed.     

Fast proton decay

We consider proton decay in the testable flipped SU(5)×UX₍₁₎$\mathit{SU}(5)\times U{(1)}_X$ models with TeV-scale vector-like particles which can be realized in free fermionic string constructions and F-theory model building. We significantly improve upon the determination of light threshold effects from prior studies, and perform a fresh calculation of the second loop for the process p→e⁺π⁰$p\to e^{+}{\pi }^0$ from the heavy gauge boson exchange. The cumulative result is comparatively fast proton decay, with a majority of the most plausible parameter space within reach of the future Hyper-Kamiokande and DUSEL experiments. Because the TeV-scale vector-like particles can be produced at the LHC, we predict a strong correlation between the most exciting particle physics experiments of the coming decade.     

终于看到希格斯玻色子衰变成底夸克

50年前,Steven Weinberg和Abdus Salam分别提出了弱相互作用理论。传递弱相互作用的W和Z玻色子必须具有大质量才能解释弱力的短程性。为引入这些质量,Weinberg和Salam 假定W和Z玻色子通过与一种场相互作用而获得质量。这种场是Peter Higgs (希格斯)等早些时候提出的。“希格斯场”的存在意味着希格斯玻色子的存在,经过几十年的寻找,最终在强子对撞机LHC上工作的ATLAS和CMS合作组于 2012年在质子—质子对撞产生的碎片中发现了这种粒子。     

Toponium decay to bottom quarks: QCD corrections

The first-order radiative QCD corrections to the width of toponium decays to bottom quarks are presented.     

Chiral phase transition in lattice QCD with Wilson quarks

The nature of the chiral phase transition in lattice QCD is studied for the cases of 2, 3 and 6 flavors with degenerate Wilson quarks, mainly on a lattice with the temporal direction extensionN _t=4. We find that the chiral phase transition is continuous for the case of 2 flavors, while it is of first order for 3 and 6 flavors.     

The role of valence quarks in proton fragmentation

Following earlier theoretical and experimental work (Kalinowski et al. [6], Bourquin et al. [7]), baryon production by fragmentation of incident protons in high energy hadron collisions is re-investigated in terms of a fragmentation-recombination mechanism similar to a model proposed by Fukuda and Iso [4]. The multiplicitiesn _f (B) for the various types of baryonsB produced in proton fragmentation are expressed in terms of the probabilitiesa _i fori=0, ..., 3 valence quarks of the incident proton to emerge in the baryonB and 3?i to emerge in mesons (following [7] we suppose that antibaryon production is mostly due to baryon-antibaryon pair production by a mechanism of non-fragmentation type, and this is taken into account in deducingn _f (B) from baryon and antibaryon multiplicity data). The positivity of thea _i is found to impose remarkably narrow constraints on then _f (B) for (meta)stableB, and we find the data to satisfy these constraints. We show furthermore that the data are compatible with uncorrelated behaviour of the valence quarks of the incident proton, each of them having a probability ? 0.6 to emerge in the fragmentation baryonB, and a probability ? 0.4 to emerge in a meson. We also briefly discuss the relation of our analysis to previous work on the recombination model of proton fragmentation and its possible extension to meson fragmentation.     

Chiral suppression of scalar-glueball decay

Since glueballs are SU(3)Flavor singlets, they should couple equally to u, d, and s quarks, so that equal coupling strengths to pi+ pi- and K+ K- are expected. However, we show that chiral symmetry implies the scalar-glueball amplitude for G0 --> qq is proportional to the quark mass, so that mixing with ss mesons is enhanced and decays to K+ K- are favored over pi+ pi-. Together with evidence from lattice calculations and experiment, this supports the hypothesis that f0(1710) is the ground state scalar glueball.     

Jet-like distributions from the weak decay of heavy quarks

We consider the production and weak decay of a pair of heavy quarks (mesons) ine ⁺ e ^? experiments and study their effect on the various jet distributions. The relative magnitudes of the two-quark-jet and three-quark-jet final states, in the decay of a heavy quark are estimated in the framework of an SU(2) _L ×U(1) model. We find that the three quark configuration dominates over the two quark configuration. For the quark jets resulting from the weak decay of the heavy quarks, we calculate the jet distributions in Sphericity, Spherocity and Thrust for the process \(e^ + e^ - \to Q\bar Q \to 6\) quarks. These distributions are compared with the corresponding quantities from the non-perturbative process \(e^ + e^ - \to q\bar q\) and the QCD process \(e^ + e^ - \to q\bar qg\) . We find that the weak decay of heavy quarks is the dominant mechanism for jet broadening ine ⁺ e ^? experiments, in the intermediate energy region relevant for PETRA and PEP.     

Chiral phase transition in lattice QCD with Wilson quarks

The nature of the chiral phase transition in lattice QCD is studied for the cases of 2, 3 and 6 flavors with degenerate Wilson quarks, mainly on a lattice with the temporal direction extensionN _t=4. We find that the chiral phase transition is continuous for the case of 2 flavors, while it is of first order for 3 and 6 flavors.     

Chiral phase transition in lattice QCD with dynamical quarks

The effect of virtual light quark loops on the chiral phase transition in lattice QCD with staggered fermions is investigated by employing the pseudo-fermion Monte Carlo method on a 6³ × 2 lattice. The variation in the order parameter $\text{〈}\text{ψ}\text{ψ〉}$ is found to become less sharp than the quenched case, indicating a second order chiral phase transitioon in the full theory.     

Chiral quarks,chiral limit,nonanalytic terms and baryon spectroscopy

It is shown that the principal pattern in baryon spectroscopy, which is associated with the flavor-spin hyperfine interactions, is due to the spontaneous breaking of chiral symmetry in QCD and persists in the chiral limit. All corrections, which are associated with a finite quark (Goldstone boson) mass are suppressed by the factor (μ/Λ_χ)² and higher.     

Measuring the transverse polarization of quarks in the proton

The transverse spin polarization distribution of quarks in a proton contributes at leading twist to the angular distribution of lepton pair production in doubly polarized proton-proton collisions. Although this distribution is straightforward to define in the QCD-improved parton model and represents a fundamental quark degree of freedom, it has never been measured. Moreover, deeply inelastic electroproduction experiments are only sensitive to this distribution at the higher twist level, and then mix this contribution with others, including the one coming from multiparton initial states. Reviewing this, we conclude that the Drell-Yan process is the best way to measure the quark transverse spin polarization distribution.Unité propre du CNRS     

Nuclear effects on proton decay

It is shown that the nuclear field affects the probability of creation of a meson inside a nucleus, and not only the survival probability when travelling through the nuclear medium. Decay into heavy (m_M ≳ 500 MeV) mesons can be strongly affected by the interaction with the nuclear medium.     

Two-loop virtual top-quark effect on Higgs-boson decay to bottom quarks

In most of the mass range encompassed by the limits from the direct search and the electroweak precision tests, the Higgs boson of the standard model preferably decays to bottom quarks. We present, in analytic form, the dominant two-loop electroweak correction, of O(GF2mt4), to the partial width of this decay. It amplifies the familiar enhancement due to the O(GFmt2) one-loop correction by about +16% and thus more than compensates the screening by about -8% through strong-interaction effects of order O(alphasGFmt2).     

From current to constituent quarks and the spin of the proton

We show in the context of an effective Lagrangian for baryons and pseudoscalar mesons how derivative couplings to Goldstone bosons modify the axial charges. Applying these considerations to constituent quarks we relate a modification of their axialU(1) charges to their internal structure which is represented here in term of a flavour singletφ ⁰ field attached to quarks. We end up with aSU (6)-type constituent quark model which describes the experimental results very well.