Tuesday, 31 January 2012

New Year, Same Old Song

Today the CMS collaboration revealed several new analyses based on the full dataset the LHC collected in 2011. As usually, the recurrent theme was "no-significant-excess-was-found". Here is a selection of the most interesting searches and limits.

t' searches
In this analysis, filed under "4th generation", one looks for a heavier copy of the top quark: a fermionic particle with charge 2/3 produced in pairs and decaying to one b-quark and one W boson. We sort of know by now there is no 4th generation of quarks and leptons in nature, nevertheless this search is relevant to more interesting new physics models. For example, in a large class of little Higgs and composite Higgs models the fermionic partner of the top quark decays as t' → b W about half of the time. The current limit on the t' mass assuming 100% branching fraction for the t' → b W decay is 525 GeV. For little Higgs et al. the limit is slightly weaker, slightly above 400 GeV (due to the smaller branching fraction) but that is also beginning to feel uncomfortable from the point of view of naturalness of these models.

W' searches
This time the target is a heavy cousin of the W boson, decaying to one lepton and one neutrino. Unlike in the former case, there is no compelling theoretical reasons for such a creature to exist. However they represent a characteristic and clean signature that is fairly straightforward to look for: an energetic electron or muon accompanied by missing energy from a neutrino. To tell W' from the ordinary W boson one looks for events with a large transverse mass (for an on-shell particle whose decay products include a neutrino the transverse mass is less than the particle mass). Intriguingly, in the muon channel an outlier event with a very large transverse mass of 2.4 TeV is observed in the data. Of course, most likely it's just a fluke, but in any case it'll be interesting to see what ATLAS has in store.

t-tbar resonance searches
This search targets heavy (more than 1 TeV) particles decaying to a pair of top quarks, a signature very common in models with a new strongly interacting sector, like composite Higgs or the Randall-Sundrum model. Such a particle would produce a bump in the invariant mass spectrum of t-tbar pairs, which are otherwise copiously produced at the LHC. Top quark decays most often to 3 hadronic jets, but for a heavy mother resonance the daughter top quarks move so quickly that their decay products merge into one fat jet. Therefore this search relies on fancy modern techniques of studying substructure of jets, in order to identify closely packed jets that could originate from a fast moving top quark. No resonance is observed in the t-tbar spectrum. What is interesting is that the LHC sensitivity now reaches the cross sections predicted by popular versions of the Randall-Sundrum model, excluding Kaluza-Klein gluons lighter than about 1.5 TeV. My guess is that the explanation of the Tevatron anomalous top forward-backward asymmetry in terms of heavy KK gluon is now dead and gone.

SUSY searches
The only vanilla SUSY search updated with the full 2011 dataset is the one in the Z+jets+missing energy channel. This is not the first place you'd look for supersymmetry (that would be jets+MET); this search is relevant to a subset of models where a cascade of neutralinos and gravitons produces, often enough, an shell Z boson. Therefore the limits on the gluino mass are not stunning: 600-900 GeV depending on how squeezed is the SUSY spectrum. More spectacular SUSY limits are probably saved for the Moriond conference in about 1 month from now.

For more details, more models, more limits, and more disappointment have a look at the slides or the original summary notes on the CMS wiki page.


Anonymous said...

What is this figure about:


was posted at http://blog.vixra.org/2012/01/31/hscp/ but the post has now been deleted.

Anonymous said...

fat fingers, image url:


Jester said...

The figure is from the search for heavy metastable charged particles also updated today by CMS, see slide 33

Anonymous said...

What about the CMS search for multilepton and multijet signatures of susy ?

Lubos wrote about it recently.

Jester said...

The multilepton results that CMS showed today are not new, Matt Strassler discussed them last October, see e.g. http://profmattstrassler.com/2011/10/21/quick-summary-of-the-multi-lepton-situation-at-the-lhc/
Not sure which multijet searches you mention.

Chris Austin said...

With regard to the t' limits, is there any good reason why searches for a t' tend to assume a 100% branching ratio for t' -> b W?

According to 1108.1218, the only directly measured element of the third row of the CKM matrix is V_{tb} = 0.88 \pm 0.07, so V_{t'd} and V_{t's} could be as large as the best values of |V_{td}| and |V_{ts}| currently inferred assuming 3 x 3 unitarity, i.e. 0.0086 and 0.041 respectively. It would be very interesting to know how far the lower limit on m_t' could be relaxed by less restrictive assumptions on the CKM' matrix.

With regard to the W' search, would I be correct in thinking that if the outlier at 2.4 TeV is not a fluke, the only well-motivated theoretical interpretation would be a KK recurrence of W, with mass > 2.4 TeV?

Jester said...

There is no reason for the branching fraction to be 100%, but the result on slide 24 is given in a form that allows you to easily rescale the limit to the branching fraction in your favorite model.
If the outlier is new physics it would be a leptonically decaying W' with mass above 2.4 TeV, but not necessarily a KK particle; there are many models predicting W primes.

Anonymous said...

"What is interesting is that the LHC sensitivity now reaches the cross sections predicted by popular version of the Randall-Sundrum model, excluding Kaluza-Klein gluons ligher than about 1.5 TeV."

I never understand statements like this. the second clause should read "excluding Randall-Sundrum gluons lighter than about 1.5 TeV." There are other viable Kaluza-Klein models that haven't been tested at all against LHC data (outside of half-assed analyses).