The Journal Club on Particle Phenomenology takes place usually on Thursdays at 2:00pm (in lecture room #3). Twice a month we have a joint JC with the group at the Physics Institute at the University of São Paulo (IFUSP). One of these joint sessions is hosted at IFT and the other at IFUSP; in the latter case the JC happens at 5:00pm at "Jayme Tiomno" room.
In general it is divided in two parts:
1) The “what got me interested this week” part, in which people comment on papers they have found recently but not necessarily read. The idea is to have a quick pass at the level of the abstract of the paper, so we are up to date and can see if anyone is interested in reading some of the papers in more depth (which leads us to the second part)
2) Someone leads a deeper discussion of a single paper. The idea is to have someone on the blackboard presenting the paper, but the audience is also supposed to have read it. These papers will be selected with as much antecedence as possible, possibly selected from those mentioned in the first part of the JCs.
(click individual titles for details)
Abstract (of the paper): We study the prospects for constraining the Higgs boson's couplings to up and down quarks using kinematic distributions in Higgs production at the CERN Large Hadron Collider. We find that the Higgs pT distribution can be used to constrain these couplings with precision competitive to other proposed techniques. With 3000 fb−1 of data at 13 TeV in the four-lepton decay channel, we find ***, where +++ is a scaling factor that modifies the q quark Yukawa coupling relative to the Standard Model bottom quark Yukawa coupling. The sensitivity may be improved by including additional Higgs decay channels.
Abstract (of the paper): Precision studies of scattering processes at colliders provide powerful indirect constraints on new physics. We study the helicity structure of scattering amplitudes in the SM and in the context of an effective Lagrangian description of BSM dynamics. Our analysis reveals a novel set of helicity selection rules according to which, in the majority of 2 → 2 scattering processes at high energy, the SM and the leading BSM effects do not interfere. In such situations, the naive expectation that dimension-6 operators represent the leading BSM contribution is compromised, as corrections from dimension-8 operators can become equally (if not more) important well within the validity of the effective field theory approach
Abstract (of the paper): Nonrelativistic scalar field theories can exhibit a natural cascading hierarchy of scales, protected by a hierarchy of polynomial shift symmetries. Using a simple model, we argue that a high-energy cross-over to such nonrelativistic behavior naturally leads to light scalars, and thus represents a useful ingredient for technically natural resolutions of scalar mass hierarchies, perhaps even the Higgs mass hierarchy puzzle.
Abstract (of the paper): We consider limits on the local (z=0) density (n0) of extragalactic neutrino sources set by the nondetection of steady high-energy neutrino sources producing >30 TeV muon multiplets in the present IceCube data, taking into account the redshift evolution, luminosity function and neutrino spectrum of the sources. We show that the lower limit depends weakly on source spectra and strongly on redshift evolution. We find n0>10−7 Mpc−3 for standard candle sources evolving rapidly, ns∝(1+z)3, and n0≳10−5 Mpc−3 for nonevolving sources. The corresponding upper limits on their neutrino luminosity are *** and ***, respectively. Applying these results to a wide range of classes of potential sources, we show that powerful blazar jets associated with active galactic nuclei are unlikely to be the dominant sources. For almost all other steady candidate source classes (including starbursts, radio galaxies, and galaxy clusters and groups), an order of magnitude increase in the detector sensitivity at ∼0.1−1 PeV will enable a detection (as point sources) of the few brightest objects. Such an increase, which may be provided by next-generation detectors like IceCube-Gen2 and an upgraded KM3NET, can improve the limit on n0 by more than two orders of magnitude. Future gamma-ray observations (by Fermi, HAWC and CTA) will play a key role in confirming the association of the neutrinos with their sources.
Abstract (of the paper): A hidden sector with a mass gap undergoes an epoch of cannibalism if number changing interactions are active when the temperature drops below the mass of the lightest hidden particle. During cannibalism, the hidden sector temperature decreases only logarithmically with the scale factor. We consider the possibility that dark matter resides in a hidden sector that underwent cannibalism, and has relic density set by the freeze-out of two-to-two annihilations. We identify three novel phases, depending on the behavior of the hidden sector when dark matter freezes out. During the cannibal phase, dark matter annihilations decouple while the hidden sector is cannibalizing. During the chemical phase, only two-to-two interactions are active and the total number of hidden particles is conserved. During the one way phase, the dark matter annihilation products decay out of equilibrium, suppressing the production of dark matter from inverse annihilations. We map out the distinct phenomenology of each phase, which includes a boosted dark matter annihilation rate, new relativistic degrees of freedom, warm dark matter, and observable distortions to the spectrum of the cosmic microwave background.
Abstract: We consider scenarios of Higgs compositeness where the Higgs doublet arises as a pseudo-Nambu Goldstone boson. Our focus is the physical scalar ("radial") excitation associated with the global symmetry breaking vacuum, which we call the "global Higgs". We derive the various couplings of this particle to the SM fields and show that in a wide class of Composite Higgs models studied in the literature the coupling of the global Higgs to gluons can be sizeable. We then investigate in detail some of the LHC signatures of the global Higgs.
Abstract (of the paper): We show that high energy measurements of Drell-Yan at the LHC can serve as electroweak precision tests. Dimension-6 operators, from the Standard Model Effective Field Theory, modify the high energy behavior of electroweak gauge boson propagators. Existing measurements of the dilepton invariant mass spectrum, from neutral current Drell-Yan at 8 TeV, have comparable sensitivity to LEP. We propose measuring the transverse mass spectrum of charged current Drell-Yan, which can surpass LEP already with 8 TeV data. The 13 TeV LHC will elevate electroweak tests to a new precision frontier.
Abstract: The determination of the nature of Dark Matter (DM) is one of the most fundamental problems of particle physics and cosmology. If DM is light enough and interacts with Standard Model particles directly or via some mediators with a strength beyond the gravitational one, it can be directly produced at the Large Hadron Collider or future particle accelerators. The typical signature from DM produced in particles collisions is missing transverse energy, MET, due to the fact that they escape undetected from the experimental apparatus. We present study for the complete set of dimension 5 and dimension 6 effective operators involving scalar, fermion and vector DM to explore the possibility to distinguish these operators and characterise the spin of DM. We have found that, depending on the spin of the DM, the DM parts of the effective operators lead to a different energy dependence of the cross-sections and to different distributions of the invariant mass of the DM pair, and consequently to different MET distributions. Using statistical analysis, we have found that at the LHC with high luminosity, certain classes of EFT operators can be distinguished from each other and, through this, it is possible to characterise the spin of DM in some cases. We have also observed a drastic difference in the efficiencies (up to two orders of magnitude) for large MET cuts scenarios with different DM spin, thus indicating that the DM discovery potential strongly depends on it. The study we perform here can be applied more generally than within the EFT paradigm, where the DM mediator is not produced on-the-mass-shell, where the invariant mass of the DM pair is not fixed.