## Saturday, 15 August 2015

### Weekend plot: ATLAS weighs in on Higgs to Tau Mu

After a long summer hiatus, here is a simple warm-up plot:

It displays the results of ATLAS and CMS searches for h→τμ decays, together with their naive combination. The LHC collaborations have already observed Higgs boson decays into two 2 τ leptons, and should be able to pinpoint h→μμ in Run-2. However,  h→τμ decays (and lepton flavor violation in general) are forbidden in  the Standard Model, therefore a detection would be an evidence for exciting new physics around the corner.  Last summer, CMS came up with their 8 TeV result showing a 2.4 sigma hint of the signal. Most likely, this is just another entry in the long list of statistical fluctuations in the LHC run-1 data. Nevertheless, the CMS result is quite intriguing, especially in connection with the LHCb hints of lepton flavor violation in B-meson decays.  Therefore, we have been waiting impatiently for a word from ATLAS. ATLAS is taking his time, but finally they published the first chunk of the result based on hadronic tau decays. Unfortunately, it is very inconclusive.  It shows a small 1 sigma upward fluctuation, hence it does not kill the CMS hint.  At the same time, the combined significance of the h→τμ signal increases only marginally, up to 2.6 sigma.

So, we are still in a limbo.  In the near future, ATLAS should reveal the 8 TeV h→τμ measurement with leptonic tau decays. This may clarify the situation, as the fully leptonic channel is more sensitive (at least, this is the case in the CMS analysis).  But it is possible that for the final clarification we'll have to wait 2 more years, once enough 13 TeV data is analyzed.

Anonymous said...

If the BR gets established at 1% level then would it also mean TeV scale new particles that LHC should find? Or would it be 10TeV scale new particles out of LHC's reach? Would New Physics (NP) contributions to this BR be generally order ~ (Weak scale/NP scale) or (Weak/NP)^2? Thanks!

Jester said...

It scales as BR~(Weak/NP)^2, but with a large coefficient in front because the SM Higgs width is very suppressed. To explain BR~1% you need NP~multi TeV, so there is no strict guarantee of new particles at the LHC. However, most realistic models give additional suppression of BR by the small SM lepton masses, and then one needs NP~weak scale.

Anonymous said...

The Higgs-mu-e-coupling has strong indirect constraints from muon precision measurements (e. g. mu -> 3 e). How does new physics look like that gives H->mu tau without H->mu e?

Jester said...

Yes, this is a challenge for model building (also, the h tau e coupling cannot be large if h tau mu is observable). Models that address it are not pretty, but they do exist, e.g. 1503.03477

Anonymous said...

An interesting suggestion is 1504.01125, even though the model seems a bit contrived, but there are quite a few points on which it can be falsified.

Anonymous said...

Xezlec said...

The central values of those measurements are amusingly close together, considering the size of the uncertainties.

Anonymous said...

why would flavour non-conservation be such a big deal? Doesn't it happen constantly in neutrino propagation ? is it then not expected to find hints of the same in other leptons sooner or later ?

Anonymous said...

Yes, I strongly endorse this pledge!

andrew said...

How easy is it to distinguish H-->TT-->T-mu from H-->T-mu directly at the LHC?

The tau mean lifetime isn't really all that long (2.9*10^-13 seconds) and a certain percentage of T in any set of 2 are going to take a second decay step reasonable quickly (certainly in less than the mean lifetime on average, an even faster for the soonest conversion infrequent enough to match the observed T-mu decay rate).

RBS said...

Hi Jester, I saw something like this on Motl blog but I'd just feel a bit more comfy getting it from a cold headed pro than a SUSY enthusiast (it's just me no offence Josef). In any case would it take a lot to compile a list of still standing SM anomalies from LHC with their current status? Just curious, thanks!

Jester said...

Anon, lepton flavor violation in neutrino oscillations is a tiny effect, which can be due to new physics at a very high scale, even as high as 10^15 GeV. That effect indeed does induce h > tau mu decays, but the branching fraction is so small that it can never be observed. On the other hand, observing lepton flavor violation in Higgs decays would point to new physics around 1 TeV or below.

Jester said...

Andrew, h>tau tau is indeed one of the backgrounds, but not the worst one (e.g. Z > tau tau is larger). There are some angular variables that help to isolate the signal, but in general the separation is difficult. At the end of the day, they just look for an excess on top of known backgrounds around the tau-mu invariant mass of 125 GeV.

Jester said...

RBS, I think Lubos' list is more or less complete. However, one should remember that the number of deviations in ATLAS and CMS data what one expects just from background fluctuations, given the number of searches performed. So any single anomaly in this list is most likely just a fluctuation of the SM background (including the ones I wrote about in this blog).

chris said...

"h>tau tau is indeed one of the backgrounds, but not the worst one"

yesterdays discovery is todays measurement is tomorrows background. so it's already tomorrow :-)

Anonymous said...

Coupling measurements from ATLAS and CMS combined, probably the best we get until late 2016: https://indico.cern.ch/event/389531/session/31/contribution/51

Looks like CMS H->e mu and H->e tau will be shown soon: https://indico.cern.ch/event/374792/contribution/51