Sunday, 19 October 2014

Weekend Plot: Bs mixing phase update

Today's featured plot was released last week by the LHCb collaboration:

It shows the CP violating phase in Bs meson mixing, denoted as φs,  versus the difference of the decay widths between the two Bs meson eigenstates. The interest in φs comes from the fact that it's  one of the precious observables that 1) is allowed by the symmetries of the Standard Model, 2) is severely suppressed due to the CKM structure of flavor violation in the Standard Model. Such observables are a great place to look for new physics (other observables in this family include Bs/Bd→μμ, K→πνν, ...). New particles, even too heavy to be produced directly at the LHC, could produce measurable contributions to φs as long as they don't respect the Standard Model flavor structure. For example, a new force carrier with a mass as large as 100-1000 TeV and order 1 flavor- and CP-violating coupling to b and s quarks would be visible given the current experimental precision. Similarly, loops of supersymmetric particles with 10 TeV masses could show up, again if the flavor structure in the superpartner sector is not aligned with that in the  Standard Model.

The phase φs can be measured in certain decays of neutral Bs mesons where the process involves an interference of direct decays and decays through oscillation into the anti-Bs meson. Several years ago measurements at Tevatron's D0 and CDF experiments suggested a large new physics contribution. The mild excess has gone away since, like many other such hints.  The latest value quoted by LHCb is φs = - 0.010 ± 0.040, which combines earlier measurements of the Bs → J/ψ π+ π- and  Bs → Ds+ Ds- decays with  the brand new measurement of the Bs → J/ψ K+ K- decay. The experimental precision is already comparable to the Standard Model prediction of φs = - 0.036. Further progress is still possible, as the Standard Model prediction can be computed to a few percent accuracy.  But the room for new physics here is getting tighter and tighter.

Saturday, 4 October 2014

Weekend Plot: Stealth stops exposed

This weekend we admire the new ATLAS limits on stops - hypothetical supersymmetric partners of the top quark:

For a stop promptly decaying to a top quark and an invisible neutralino, the new search excludes the mass range between m_top and 191 GeV. These numbers do not seem impressive at first sight, but let me explain why it's interesting.

No sign of SUSY at the LHC could mean that she is dead, or that she is resting hiding. Indeed, the current experimental coverage has several blind spots where supersymmetric particles, in spite of being produced in large numbers, induce too subtle signals in a detector to be easily spotted. For example, based on the observed distribution of events with a top-antitop quark pair accompanied by large missing momentum, ATLAS and CMS put the lower limit on the stop mass at around 750 GeV. However, these searches are inefficient if the stop mass is close to that of the top quark, 175-200 GeV (more generally, for m_top+m_neutralino ≈ m_stop). In this so-called stealth stop region,  the momentum carried away by the neutralino is too small to distinguish stop production from the standard model process of top quark production. We need another trick to smoke out light stops. The ATLAS collaboration followed theorist's suggestion to use spin correlations. In the standard model, gluons couple  either to 2 left-handed or to 2 right-handed quarks. This leads to a certain amount of correlation between  the spins of the top and the antitop quark, which can be seen by looking at angular distributions of the decay products of  the top quarks. If, on the other hand, a pair of top quarks originates from a decay of spin-0 stops, the spins of the pair are not correlated. ATLAS measured spin correlation in top pair production; in practice, they measured the distribution of the azimuthal angle between the two charged leptons in the events where both top quarks decay leptonically. As usual, they found it in a good agreement with the standard model prediction. This allows them to deduce that there cannot be too many stops polluting the top quark sample, and place the limit of 20 picobarns on the stop production cross section at the LHC, see the black line on the plot. Given the theoretical uncertainties, that cross section corresponds to the stop mass somewhere between 191 GeV and 202 GeV.

So, the stealth stop window is not completely closed yet, but we're getting there.