Comment on the Updated CDF "Ghost" Events

In 2008 the CDF Collaboration announced the discovery of an excess of events with two or more muons, dubbed "ghost" events for their unusual properties. In a recent update, CDF finds that the azimuthal angle distribution between the primary (trigger) muons is significantly more back-to-back than that of all known sources of di-muon backgrounds. Here we show that this angular distribution cannot be reproduced in models where the muons are produced in the decays of relatively light X-particles: all models of this kind also predict a much broader distribution than that found by CDF. We conclude that the CDF measurement cannot be described via the annihilation of strongly interacting partons, and thus seems to be in conflict with basic tenets of QCD.Comment: 5 pages, 3 figure

Determination of the CMSSM Parameters using Neural Networks

In most (weakly interacting) extensions of the Standard Model the relation mapping the parameter values onto experimentally measurable quantities can be computed (with some uncertainties), but the inverse relation is usually not known. In this paper we demonstrate the ability of artificial neural networks to find this unknown relation, by determining the unknown parameters of the constrained minimal supersymmetric extension of the Standard Model (CMSSM) from quantities that can be measured at the LHC. We expect that the method works also for many other new physics models. We compare its performance with the results of a straightforward \chi^2 minimization. We simulate LHC signals at a center of mass energy of 14 TeV at the hadron level. In this proof-of-concept study we do not explicitly simulate Standard Model backgrounds, but apply cuts that have been shown to enhance the signal-to-background ratio. We analyze four different benchmark points that lie just beyond current lower limits on superparticle masses, each of which leads to around 1000 events after cuts for an integrated luminosity of 10 fb^{-1}. We use up to 84 observables, most of which are counting observables; we do not attempt to directly reconstruct (differences of) masses from kinematic edges or kinks of distributions. We nevertheless find that m_0 and m_{1/2} can be determined reliably, with errors as small as 1% in some cases. With 500 fb^{-1} of data tan\beta as well as A_0 can also be determined quite accurately. For comparable computational effort the \chi^2 minimization yielded much worse results.Comment: 46 pages, 10 figures, 4 tables; added short paragraph in Section 5 about the goodness of the fit, version to appear in Phys. Rev.

Mitigation of the LHC Inverse Problem

The LHC inverse problem refers to the difficulties in determining the parameters of an underlying theory from data (to be) taken by the LHC experiments: if they find signals of new physics, and an underlying theory is assumed, could its parameters be determined uniquely, or do different parameter choices give indistinguishable experimental signatures? This inverse problem was studied before for a supersymmetric Standard Model with 15 free parameters. This earlier study found 283 indistinguishable pairs of parameter choices, called degenerate pairs, even if backgrounds are ignored. We can resolve all but 23 of those pairs by constructing a true \chi^2 distribution using mostly counting observables. The elimination of systematic errors would even allow separating the residual degeneracies. Taking the Standard Model background into account we still can resolve 237 of the 283 "degenerate" pairs. This indicates that (some of) our observables should also be useful for the purpose of determining the values of SUSY parameters.Comment: 32 pages, 13 figures, typo in (3.6) corrected, version to appear in Phys. Rev.

Hunting for CDF Multi-Muon "Ghost" Events at Collider and Fixed-Target Experiments

In 2008 the CDF collaboration discovered a large excess of events containing two or more muons, at least one of which seemed to have been produced outside the beam pipe. We investigate whether similar "ghost" events could (and should) have been seen in already completed experiments. The CDF di-muon data can be reproduced by a simple model where a relatively light X particle undergoes four-body decay. This model predicts a large number of ghost events in Fermilab fixed-target experiments E772, E789 and E866, applying the cuts optimized for analyses of Drell-Yan events. A correct description of events with more than two muons requires a more complicated model, where two X particles are produced from a very broad resonance Y. This model can be tested in fixed-target experiments only if the cut on the angles, or rapidities, of the muons can be relaxed. Either way, the UA1 experiment at the CERN ppbar collider should have observed O(100) ghost events.Comment: 15 pages, 9 figure