arXiv subjects
Publications and source records attributed to Jure Zupan.
Both CDF and DO report a forward-backward asymmetry in t-tbar production that is above the standard model prediction. We review new physics models that can give a large forward backward asymmetry in t-tbar production at the Tevatron and the constraints these models face from searches for dijet resonances and contact interactions, from flavor physics and the t-tbar cross section. Expected signals at the LHC are also reviewed.
We show that the discovery channel for dark matter (DM) production at colliders can be through flavor violating interactions resulting in a novel signature of a single top and large missing transverse energy. We discuss several examples where the production of DM is dominated by flavor violating couplings: minimal flavor violating models with a large bottom Yukawa, models with horizontal symmetries, and DM in nontrivial flavor group representations. Discovery at the 7 TeV LHC with a few fb^-1 may already be possible.
Recently, the CoGeNT collaboration presented a positive signal for an annual modulation in their data set. In light of the long standing annual modulation signal in DAMA/LIBRA, we analyze the compatibility of both of these signal within the hypothesis of dark matter (DM) scattering on nuclei, taking into account existing experimental constraints. We consider the cases of elastic and inelastic scattering with either spin-dependent or spin-independent coupling to nucleons. We allow for isospin violating interactions as well as for light mediators. We find that there is some tension between the size of the modulation signal and the time-integrated event excess in CoGeNT, making it difficult to explain both simultaneously. Moreover, within the wide range of DM interaction models considered, we do not find a simultaneous explanation of CoGeNT and DAMA/LIBRA compatible with constraints from other experiments. However, in certain cases part of the data can be made consistent. For example, the modulation signal from CoGeNT becomes consistent with the total rate and with limits from other DM searches at 90% CL (but not with the DAMA/LIBRA signal) if DM scattering is inelastic spin-independent with just the right couplings to protons and neutrons to reduce the scattering rate on xenon. Conversely the DAMA/LIBRA signal (but not CoGeNT) can be explained by spin-dependent inelastic DM scattering.
We show that the forward-backward asymmetry in top quark pair production can be enhanced by fields that transform nontrivially under the flavour group and satisfy Minimal Flavour Violation, while at the same time the constraints from associated effects on the d σ(t anti-t)/d M_{t anti-t} distribution, dijet resonance searches, same sign top pair production and other phenomenology are satisfied. We work out two examples in detail, one where a scalar colour anti-sextet field, that is also an anti-sextet of SU(3)_U, enhances the forward-backward asymmetry, and one where the enhancement arises from a vector colour octet field that is also an octet of SU(3)_U.
In this time of transition to a new generation of quark flavor experiments we review both the theoretical and the experimental progress on the determination of unitarity triangle angles from penguin-dominated B decays. This summarizes the activities of the Working Group VI at the CKM2010 workshop.
We first review the methods for determining gamma from B->D K decays that appeared after CKM 2008. We then discuss the theoretical errors in gamma extraction. The errors due to neglected D-Dbar and B_{d,s}-Bbar_{d,s} mixing can be avoided by including their effects in the fits. The ultimate theoretical error is then given by electroweak corrections that we estimate to give a shift delta gamma/gamma O(10^-6).
Dark matter interacting predominantly with leptons instead of nuclear matter has received a lot of interest recently. In this talk, we investigate the signals expected from such 'leptophilic Dark Matter' in direct detection experiments and in experiments looking for Dark Matter annihilation into neutrinos in the Sun. In a model-independent framework, we calculate the expected interaction rates for different scattering processes, including elastic and inelastic scattering off atomic electron shells, as well as loop-induced scattering off atomic nuclei. In those cases where the last effect dominates, leptophilic Dark Matter cannot be distinguished from conventional WIMPs. On the other hand, if inelastic scattering off the electron shell dominates, the expected event spectrum in direct detection experiments is different and would provide a distinct signal. However, we find that the signals in DAMA and/or CoGeNT cannot be explained by invoking leptophilic DM because the predicted and observed energy spectra do not match, and because of neutrino bounds from the Sun.
The D0 Collaboration reported a 3.2sigma deviation from the standard model prediction in the like-sign dimuon asymmetry. Assuming that new physics contributes only to B_{d,s} mixing, we show that the data can be analyzed without using the theoretical calculation of ΔΓ_s, allowing for robust interpretations. We find that this framework gives a good fit to all measurements, including the recent CDF S_{ψϕ} result. The data allow universal new physics with similar contributions relative to the SM in the B_d and B_s systems, but favors a larger deviation in B_s than in B_d mixing. The general minimal flavor violation framework with flavor diagonal CP violating phases can account for the former and remarkably even for the latter case. This observation makes it simpler to speculate about which extensions with general flavor structure may also fit the data.
The Higgs boson may decay predominantly into a hidden sector, producing lepton jets instead of the standard Higgs signatures. We propose a search strategy for such a signal at hadron colliders. A promising channel is the associated production of the Higgs with a Z or W, where the dominant background is Z or W plus QCD jets. The lepton jets can be discriminated from QCD jets by cutting on the electromagnetic fraction and charge ratio. The former is the fraction of jet energy deposited in the electromagnetic calorimeter and the latter is the ratio of energy carried by charged particles to the electromagnetic energy. We use a Monte Carlo description of detector response to estimate QCD rejection efficiencies of order 10^-3 per jet. The expected Higgs mass reach is about 155 GeV at the Tevatron with 10 fb^-1 of data and about 135 GeV at the 7 TeV LHC with 1 fb^-1.
We show that ratios of tree and penguin amplitudes in B-> K*pi and B->rho K are two to three times larger than in B-> K pi. This allows for considerably larger CP asymmetries in the former processes than the 10% asymmetry measured in B0 -> K+pi-. We study isospin sum rules for rate asymmetries in B-> K pi, K*pi, rho K, estimating small violation from interference of tree and electroweak penguin amplitudes. The breaking of the K pi asymmetry sum rule is estimated to be one to two percent and negative. Violation of K*pi and rho K sum rules can be estimated from B->rho pi amplitudes using flavor SU(3), while breaking of a sum rule combining K*pi and rho K asymmetries can be measured directly in a Dalitz analysis of B0-> K+pi-pi0. The three sum rules can be tested at the LHCb and at a future Super Flavor Factory, providing precision searches for new Delta S =Delta I=1 operators in the low energy effective Hamiltonian.
The Higgs and some of the Standard Model superpartners may have been copiously produced at LEP and the Tevatron without being detected. We study a novel scenario of this type in which the Higgs decays predominantly into a light hidden sector either directly or through light SUSY states. Subsequent cascades increase the multiplicity of hidden sector particles which, after decaying back into the Standard Model, appear in the detector as clusters of collimated leptons known as lepton jets. We identify the relevant collider observables that characterize this scenario, and study a wide range of LEP and Tevatron searches to recover the viable regions in the space of observables. We find that the Higgs decaying to lepton jets can be hidden when the event topology mimics that of hadronic backgrounds. Thus, as many as 10^4 leptonic Higgs and SUSY decays may be hiding in the LEP and Tevatron data. We present benchmark models with a 100 GeV Higgs that are consistent with all available collider constraints. We end with a short discussion of strategies for dedicated searches at LEP, the Tevatron and the LHC, that allow for a discovery of the Higgs or SUSY particles decaying to lepton jets.
We perform a global fit to data from Dark Matter (DM) direct detection experiments, including the recent CDMS-II results. We discuss possible interpretations of the DAMA annual modulation signal in terms of spin-independent and spin-dependent DM-nucleus interactions, both for elastic and inelastic scattering. We find that in the spin-dependent inelastic scattering off protons a good fit to all data is obtained. We present a simple toy model realizing such a scenario. In all the remaining cases the DAMA allowed regions are disfavored by other experiments or suffer from severe fine tuning of DM parameters with respect to the galactic escape velocity. Finally, we also entertain the possibility that the two events observed in CDMS-II are an actual signal of elastic DM scattering, and we compare the resulting CDMS-II allowed regions to the exclusion limits from other experiments. In this arXiv version of the manuscript we also provide in appendix A the updated fits including recent CoGeNT results.
We consider the hypothesis that Dark Matter (DM) has tree-level interactions only with leptons. Such a framework, where DM recoils against electrons bound in atoms, has been proposed as an explanation for the annually modulated scintillation signal in DAMA/LIBRA versus the absence of a signal for nuclear recoils in experiments like CDMS or XENON10. However, even in such a leptophilic DM scenario there are loop induced DM-hadron interactions, where the photons emitted from virtual leptons couple to the charge of a nucleus. Using a general effective field theory approach we show that, if such an interaction is induced at one or two-loop level, then DM-nucleus scattering dominates over DM-electron scattering. This is because the latter is suppressed by the bound state wave function. One obtains a situation similar to standard DM-nucleus scattering analyses with considerable tension between the results of DAMA and CDMS/XENON10. This conclusion does not apply in the case of pseudoscalar or axial vector coupling between DM and leptons, where the loop diagrams vanish. In this case the explanation of the DAMA signal in terms of DM-electron scattering is strongly disfavored by the spectral shape of the signal. Furthermore, if DM can annihilate into neutrinos or tau leptons, the required cross sections are excluded by many orders of magnitude using the Super-Kamiokande bound on neutrinos from DM annihilations in the Sun.
The b -> ssdbar and b -> ddsbar decays are highly suppressed in the SM, and are thus good probes of new physics (NP) effects. We discuss in detail the structure of the relevant SM effective Hamiltonian pointing out the presence of nonlocal contributions which can be about λ^{-4} (m_c^2/m_t^2) ~ 30% of the local operators (λ= 0.21 is the Cabibbo angle). The matrix elements of the local operators are computed with little hadronic uncertainty by relating them through flavor SU(3) to the observed ΔS = 0 decays. We identify a general NP mechanism which can lead to the branching fractions of the b\to ss\bar d modes at or just below the present experimental bounds, while satisfying the bounds from K-Kbar and B_{(s)}-Bbar_{(s)} mixing. It involves the exchange of a NP field carrying a conserved charge, broken only by its flavor couplings. The size of branching fractions within MFV, NMFV and general flavor violating NP are also predicted. We show that in the future energy scales higher than 10^3 TeV could be probed without hadronic uncertainties even for b-> s and b-> d transitions, if enough statistics becomes available.
We propose a scenario where Dark Matter (DM) annihilates into an intermediate state which travels a distance $λ\equiv v/Γ$ on the order of galactic scales and then decays to Standard Model (SM) particles. The long lifetime disperses the production zone of the SM particles away from the galactic center and hence, relaxes constraints from gamma ray observations on canonical annihilation scenarios. We utilize this set up to explain the electron and positron excesses observed recently by PAMELA, ATIC, and FERMI. While an explanation in terms of usual DM annihilations seems to conflict with gamma ray observations, we show that within the proposed scenario, the PAMELA/ATIC/FERMI results are consistent with the gamma ray data. The distinction from decay scenarios is discsussed and we comment on the prospects for DM production at LHC. The typical decay length $λ\gtrsim 10$ kpc of the intermediate state can have its origin from a dimension six operator suppressed by a scale $Λ\sim 10^{13}$ GeV, which is roughly the seesaw scale for neutrino masses.
We explain the PAMELA positron excess and the PPB-BETS/ATIC e+ + e- data using a simple two component dark matter model (2DM). The two particle species in the dark matter sector are assumed to be in thermal equilibrium in the early universe. While one particle is stable and is the present day dark matter, the second one is metastable and decays after the universe is 10^-8 s old. In this model it is simple to accommodate the large boost factors required to explain the PAMELA positron excess without the need for large spikes in the local dark matter density. We provide the constraints on the parameters of the model and comment on possible signals at future colliders.
A model independent study of the minimal flavor violation (MFV) framework is presented, where the only sources of flavor breaking at low energy are the up and down Yukawa matrices. Two limits are identified for the Yukawa coupling expansion: linear MFV, where it is truncated at the leading terms, and nonlinear MFV, where such a truncation is not possible due to large third generation Yukawa couplings. These are then resummed to all orders using non-linear sigma-model techniques familiar from models of collective breaking. Generically, flavor diagonal CP violating (CPV) sources in the UV can induce O(1) CPV in processes involving third generation quarks. Due to a residual U(2) symmetry, the extra CPV in B_d-\bar B_d mixing is bounded by CPV in B_s-\bar B_s mixing. If operators with right-handed light quarks are subdominant, the extra CPV is equal in the two systems, and is negligible in processes involving only the first two generations. We find large enhancements in the up type sector, both in CPV in D-\bar D mixing and in top flavor violation.
Using isospin relations, we predict the Standard-Model correlation between S_{pi^0 K_S}=(sin 2beta)_{pi^0 K_S} and A_{pi^0 K_S}, the mixing-induced and direct CP asymmetries of B^0-> pi^0 K_S. The calculation uses flavour SU(3) only to fix the isospin-3/2 amplitude through the B^\pm->pi^\pm pi^0 branching ratio, and thus has a small irreducible theoretical error. It can reach percent level precision thanks to expected future lattice-QCD progress for the calculation of the relevant SU(3)-breaking form-factor ratio, and serves as a benchmark for new-physics searches. We obtain an interesting picture in the A_{pi^0 K_S} - S_{pi^0 K_S} plane, where the current experimental data show a discrepancy with the Standard Model, and comment on the direct CP asymmetries of B^0->pi^-K^+ and B^+ -> pi^0 K^+. A modified electroweak penguin with a large new CP-violating phase can explain the discrepancy and allows us to accommodate also the corresponding data for other b->s penguin-dominated decays.