Archive for March 5, 2010

ALEPH Closes the 4-tau Gap

Supersymmetry should have been discovered already. (If it is the correct theory, that is.) Since there is no positive signal for supersymmetric particles, people talk about a “tension” between SUSY phenomenology and the negative outcome of countless searches. In particular, the Higgs boson should be lighter than about 135 GeV, and since it has not been observed below 114 GeV, supersymmetric models are constrained.

One way out is to extend the Higgs sector by including an extra field. This small extension of the minimal supersymmetric model is called the Next-to-Minimal Supersymmetric Model, or NMSSM for short. An extra scalar appears in such models, to which the standard model-like Higgs boson could decay. If the searches at LEP do not have the right selection criteria, a Higgs boson of, say 90 GeV or even 30 GeV might have gone undiscovered.

A particular scenario people like has the Higgs boson (h) decaying to a pair of light pseudoscalar particles, a, which then decay to a pair of light fermions, such as b-quarks or τ-leptons. The decay a → τ+τ would dominate if half the mass of the a boson lies between the τ and b masses.

ALEPH have analyzed LEP II data looking for the production of h in association with a Z boson, reconstructed through its decays to e+e–, μ+μ and νν, and their report came out on the archive yesterday (arXiv:1003.0705):

Search for neutral Higgs bosons decaying into four taus at LEP2

The topology of the decay h → aa → 4τ is special in this kinematic regime: one τ+τ pair flies opposite the other τ+τ pair. This is true, at least, when the Higgs is relatively heavy and has little boost in the lab frame.

ALEPH feynman diagram for 4-tau Higgs final state

Feynman diagram for the ZH channel, with h→3τ

The authors select Zs in the usual way, and find the dual pairs of τ+τ leptons using the JADE jet algorithm. Since the pairs are back-to-back, or at least in opposite hemispheres, each “jet” contains one pair. Given the way that τ leptons decay, these jets usually have two or four charged tracks, which provides a powerful handle on backgrounds. The other powerful handle is the missing energy produced by the non-detection of the neutrinos from the τ decays.

When the Z decays to charged leptons, the backgrounds are truly negligible, far less than one event. Z decays to neutrinos are less clean due mainly to continuum four-fermion production, and two-photon interactions. (The absence of energy is less robust than the presence of two charged tracks whose invariant mass is about 90 GeV!) Two events were selected in the latter channel, which is significantly fewer than the expectation of 5.8 events. A signal for a Higgs mass of 100 GeV, however, would have produced an additional eleven events, so the search is not ill-conceived.

The authors derived upper limits on the production of a Higgs boson decaying in this manner: their parameter ξ2 < 1 means the model is excluded at 95% C.L. Here are the plots:
ALEPH 4tau Limit

ALPEH 4tau exclusion curves

Contours in the (mh,ma) plane of upper limits on ξ2

It can be seen that the exclusion is a bit better than expected. There is little chance that there is a Higgs decaying in this manner, produced in the LEP2 data. Note this result follows on an earlier search by the OPAL Collaboration (arXiv:hep-ex/0209068) which excluded the Higgs in the mass range 45 to 86 GeV.

As far as I know, this represents a significant challenge to this version of the NMSSM, geared toward releasing the “tension” of the MSSM.


March 5, 2010 at 2:41 pm Leave a comment

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