Measuring smuon-selectron mass splitting at the CERN LHC and patterns of supersymmetry breaking

With sufficient data, CERN LHC experiments can constrain the smuon-selectron mass splitting through differences in the dielectron and dimuon edges from supersymmetry (SUSY) cascade decays. We study the sensitivity of the LHC to this mass splitting, which within minimal supergravity may be constrained down to $\mathcal{O}({10}^{-4})$ for $30{\mathrm{fb}}^{-1}$ of integrated luminosity. Over substantial regions of SUSY breaking parameter space the fractional edge splitting can be significantly enhanced over the fractional mass splitting. Within models where the selectron and smuon are constrained to be universal at a high scale, edge splittings up to a few percent may be induced by renormalization group effects and may be significantly discriminated from zero. The edge splitting provides important information about high-scale SUSY breaking terms and should be included in any fit of LHC data to high-scale models.

Figure 1

The enhancement factor $(\Delta m_{ll}/m_{ll})/(\Delta m_{\tilde{l}}/m_{\tilde{l}})$ as a function of $\Delta m_1/\Delta m_2\equiv (m_{\tilde{l}}-m_{{\chi }_1^0})/(m_{{\chi }_2^0}-m_{{\chi }_1^0})$ for three different values of $n\equiv m_{{\chi }_2^0}/m_{{\chi }_1^0}$.

Figure 2

Log-likelihood fit to dielectron edge in ACERDET as described in the text. $m_{ee}$ has been placed in 0.25 GeV bins on abscissa. The ordinate shows events per bin per $16{\mathrm{fb}}^{-1}$.

Figure 3

Log-likelihood fit to dimuon edge in ACERDET as described in the text. $m_{\mu \mu }$ has been placed in 0.25 GeV bins on abscissa. The ordinate shows events per bin per $16{\mathrm{fb}}^{-1}$.

Figure 4

Expected $30{\mathrm{fb}}^{-1}$ 1-sigma sensitivity, $E$, to smuon-selectron mass splitting in perturbed mSUGRA around SPS1a. The region to the right-hand side of the almost-vertical line has $m_{{\tilde{\tau }}_1}<m_{{\chi }_1^0}$. The region underneath the mostly horizontal line has $m_{{\chi }_2^0}-m_{\tilde{l}}<10\mathrm{GeV}$. The lighter lines show contours of ${log}_{10}E=-2,-2.5,-3$ (top to bottom).

Figure 5

Probability distribution difference in ${\mathrm{GeV}}^{-1}$ of SUSY cascade chain electrons and muons as a function of their invariant mass $m_{ll}$ (in GeV) for an end point of 70 GeV and a relative splitting of 2%. The solid line shows the distribution without any energy resolution taken into account, whereas the dashed line displays the effect of smearing due to energy resolution. The dotted line shows a smeared distribution with no mass splitting.