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Universidad de Granada CAFPE


A Fortran code for probing indirect effects of new physics


Package description


NPhytter is a Fortran code for the evaluation of generic new physics contributions to LHC and electroweak precision observables.


The current version of the code (not public yet) allows to perform  analyses using the experimental inputs from the electroweak precision data (EWPD) taken at LEP and SLC, properties of the W boson, low-energy measurements, fermion pair production at LEP 2 and LHC energies, and Higgs signal strengths. LHC triple gauge boson processes and other observables measured at the LHC are currently being implemented.


The computation of the Standard Model (SM) predictions for several electroweak observables (mainly Z-pole and LEP 2 observables) relies to a large extent on the use of the ZFITTER package, updated with new implementations for many observables, to include the state-of-the-art of radiative corrections.


New physics predictions to observables are computed within a model-independent approach, using a dimension-six effective Lagrangian. New physics contributions to observables are parameterized in terms of the Wilson coefficients of the dimension-six operators. This also allows for an easy interepretation within specific scenarios, upon matching with the high-energy theory.


Note: The NPhytter package is still under development and it is not available for download yet.


Physics Results


In this section we briefly present some physics results obtained using the NPhytter package. We include both, SM and some new physics analysis of interest. Most of these results have been already published (see Publications section below), and are updated here including the latest experimental data and theoretical calculations. (Last update: Nov 14th, 2015)


The Standard Model fit


The measurement of the Higgs mass at the LHC provides a direct determination of the last of the SM input parameters. Including this measurement the SM fit to EWPD is overconstrained. At the minimum:



The consistency between the experimental data and the SM predictions is noteworthy, with the indirect determination of all input parameters around or below 1 sigma. 


The good agreement is also apparent by directly comparing the SM predictions for electroweak precision observables with the data. The following picture shows the pulls for many of the most relevant observables in the fit:



Model-independent bounds on new physics


In absence of any hint about the nature of any possible physics beyond the SM, deviations from the SM predictions can be most conveniently parameterize by the use of an effective Lagrangian. The following illustrates the stringent limits provided by EWPD and LEP2 data in the effective interactions at dimension six, from a fit considering only one new operator at a time:



Combining LHC and electroweak precision data constraints


With about 20 1/fb of data collected at the LHC at 8 TeV we still do not have any significant deviation from the SM. The absence of new signals imposes  constraints on new physics, in some cases comparable to those coming from EWPD or LEP 2 data. In particular, the LHC can be directly sensitive to interactions that do not contribute to EWPD (and viceversa). Such is the case of four-fermion operators built exclusively from quarks fields. These can be constrained using, e.g. the LHC dijet data, producing the following bounds:


(Results corresponding to 7 TeV dijet data only)



On the other hand, for those cases where both electroweak and LHC observables are sensitive to the new interactions, the combined limits would in general improve the individual bounds obtained from each experiment. For instance, combining the electroweak constraints on lepton-quark four-fermion interactions with those coming from the LHC dilepton searches, we often find a nice complementary between both results, with each data set (EWPD or LHC) improving the worst limit derived from the other. The combined bounds are given by:



for lepton universal interactions, or interactions only with electrons or muons, respectively.


It is therefore clear that limits from both experiments are complementary, and should be combined in order to obtain the most stringent bounds over the most general class of new physics scenarios.


Using this effective Lagrangian approach, one can easily study some other popular scenarios. For instance, from the list of operators included above, certain combinations of interactions can be put in one-to-one correspondence with the so called (extended) oblique parameters S, T, W, Y and Z (U,V,X=0 at dimension six), which parameterize new physics contributing only to gauge boson vacuum polarizations. The simultaneous fit to all these 5 parameters yield the following allowed regions at the 68% and 95% C.L.




The dimension six effective Lagrangian at NLO


All the previous results have been derived considering only the leading contributions from the dimension six interactions at the tree level. Higher-order corrections are also included in NPhytter, using the calculations of the full renormalization group equations (RGE) for the entire dimension six effective Lagrangian. 


Combined with the high precision of the EWPD, the results from the RGE allow to put significant constraints on a much larger set of dimension six interactions. This includes several operators to which we have little or no direct experimental access. In particular, the strongest EWPD bounds can be set in the top sector, for the largest RGE effects on EWPD are those proportional to the top Yukawa coupling. Using a leading-logarithmic approximation we find the following limits on several types of new top interactions:


(See [1] for details and a larger set of bounds)


The bounds on that table include limits on electron-top contact interactions, which could be tested at future lepton colliders. While less precise, the EWPD limits on four-top contact interactions are comparable or better than the current LHC bounds. Finally, EWPD can also set bounds on new physics in the electroweak top couplings, at the 10% level or below. For new physics at the TeV we find at 95% C.L.,



which are also beyond the LHC reach. These limits are strongly correlated, as can be seen in the following figure (left),




while the figure on the right shows how the bounds evolve as we increase the new physics scale.





This section contains a list of publications where the NPhytter package was used to generate physics results.


[1]  Renormalization Group Constraints on New Top Interactions from

     Electroweak Precision Data

     J. de Blas, M. Chala and J. Santiago

     Published in JHEP 1509 (2015) 189


[2]  Observable Effects of General New Scalar Particles

     J. de Blas, M. Chala, M. Pérez-Victoria and J. Santiago

     Published in JHEP 1504 (2015) 078


[3]  Electroweak limits on physics beyond the Standard Model

     J. de Blas.

     Published in EPJ Web Conf. 60 (2013) 19008 


[4]  Global Constraints on Lepton-Quark Contact Interactions

     J. de Blas, M. Chala and J. Santiago.

     Published in Phys.Rev. D88 (2013) 095011


[5]  Combining searches of Z' and W' bosons

     J. de Blas, J.M. Lizana and M. Perez-Victoria.

     Published in JHEP 1301 (2013) 166


[6]  Electroweak constraints on new physics

     F. del Aguila and J. de Blas.

     Published in Fortsch.Phys. 59 (2011) 1036-1040


[7]  Impact of extra particles on indirect Z' limits

     F. del Aguila, J. de Blas, P. Langacker and M. Perez-Victoria.

     Published in Phys.Rev. D84 (2011) 015015


[8]  Electroweak Limits on General New Vector Bosons

     F. del Aguila, J. de Blas and M. Perez-Victoria.

     Published in JHEP 1009 (2010) 033


[9]  Evidence for right-handed neutrinos at a neutrino factory

     F. del Aguila, J. de Blas, R. Szafron, J. Wudka and M. Zralek.

     Published in Phys.Lett. B683 (2010) 282-288 


[10] Effects of new leptons in Electroweak Precision Data

     F. del Aguila, J. de Blas and M. Perez-Victoria.

     Published in Phys.Rev. D78 (2008) 013010