Monday, 3 December 2012

WW puzzle


 The mood of this blog usually oscillates between  depressive and funereal, due to the lack of any serious hints of new physics near the electroweak scale. Today, for a change, I'm going to strike an over-optimistic tone. There is one, not very significant, but potentially interesting excess sitting in the LHC data. Given the dearth of anomalies these days, it's a bit surprising that the excess receives so little attention: I could find only 1 paper addressing it.

The LHC routinely measures cross sections of processes predicted by the standard model. Unlike the Higgs or new physics searches, these analyses are not in the spotlight, are completed at a more leisurely pace, and are forgotten minutes after publication. One such observable is the WW pair production cross section. Both CMS and ATLAS measured that cross section in the 7 TeV data using the dilepton decay channel, both obtaining the result slightly above the standard model prediction. The situation got more interesting last summer after CMS put out a measurement based on a small chunk of 8 TeV data. The CMS  result stands out more significantly, 2 sigma above the standard model, and the rumor is that in 8 TeV ATLAS it is also too high.

It is conceivable that new physics leads to an increase of the WW cross section at the LHC. This paper proposes SUSY chargino pair production as an explanation. If chargino decays dominantly  to a W boson and an invisible  particle - neutralino or gravitino,  the final state is almost the same as the one searched by the LHC. Moreover, if charginos are light the additional missing energy from the invisible SUSY particles is small, and would not significantly distort the WW cross section measurement.  A ~110 GeV wino would be pair-produced at the LHC with the cross section of a few pb - in the right ballpark to explain the excess.

Such light charginos are still marginally allowed. In the old days, the LEP experiments excluded new charged particles only up to ~100 GeV, LEP's kinematic reach for pair production. At the LHC, the kinematic reach is higher, however small production cross section of uncolored particles compared to the QCD junk the makes chargino searches challenging.  In some cases, charginos and neutralinos have been recently excluded up to several hundred GeV (see e.g. here), but these strong limits are not bullet proof as they  rely on trilepton signatures. If one can fiddle with the SUSY spectrum  so as to avoid decays leading to trilepton signatures (in particular, the decay χ1→ LSP Z* must be avoided in the 2nd diagram) then 100 GeV charginos can be safe.
 
Of course, the odds for the WW excess not being new physics are much higher. The excess at the LHC could simply be an upward fluctuation of the signal, or higher-order corrections to the WW cross section in the standard model may have been underestimated.  Still, it will be interesting to observe where the cross section will end up after the full 8 TeV dataset is analyzed. So, if you have a cool model that overproduces WW (but not WZ)  pairs, now may be the right moment to step out.

23 comments:

Anonymous said...

Oh, come on! You just commented in your previous post that Bs \to mumu is another handful of earth upon the coffin, now you recommend us a paper with charginos.

Jester said...

After weekends I'm always more optimistic ;-) Besides, charginos are just an example, there's nothing SUSY-like in the excess, many different models could explain it.

muon said...

Note that the ZZ cross section measured by CMS and ATLAS agree very well with the standard model. So we don't tend to think this "excess" reflects missing higher-order diagrams or PDFs.

Michael

Jester said...

Good point, although ZZ in 8 TeV ATLAS (as opposed to CMS) is also a bit high.

Luboš Motl said...

Dear Jester,

pretty interesting although the excesses would probably be below the TRF thresholds, unless you were hiding some big numbers. I would sort of like to hear about another, non-SUSY model that is compatible with the other data but would produce this excess.

Also, a question about the things you wrote: the second diagram produces WWW right? Is the WW search inclusive or what's done with the extra W?

All the best
Lubos

Luboš Motl said...

Oh, I see the answer: the third W is virtual and probably decays to too soft, far off-shell final products so it's not seen.

Jester said...

Correct, most of the times the W* products will be too soft to be detected. I fully agree it's not a significant excess... one can consider this post a sign of how desperate the situation has become ;-)

Luboš Motl said...

It's not desperate yet at all. Have you already considered an algorithm to inject interesting collisions into the LHC datastream? ;-)

Incidentally, you have a silver lining, too. I have readied $100 for you because if there's no SUSY after the final evaluation of the 2011+2012 data, which will be by the summer, I hope, you will get this money. Just for the tiny risk that you will pay $10,000, perhaps as soon as during Moriond 2013 in March. ;-)

Luboš Motl said...

Holy crap, I just ran into the same diagrams ;-) that are used as motivation to do some "immediate" computational job. Do you also follow the developments at S*v*nn*h?

Anonymous said...

Shut up, Lubos.

Anonymous said...

Ha ha Lumo,

you and your bets always make me LOL :-D

I'm wondering if the following equation holds (setting AT:= Anonymous Troll) ?

AT(directly above me) = AT(in the first comment) = AT (in the first comment of the previous post)

Cheers

Dilaton

Younghun Park said...

Jester
I want to listen to your opinion.
If that anomaly is correct, not systematic error, Higgs signal to be captured is real?
We really find the Higgs?
Higgs-like boson is really equal to Higgs?
I don't know what is true really.

Jester said...

If we were seeing the Higgs only in the WW channel there would be reasons to worry. But we're seeing it first of all in the diphoton and ZZ>4l channels, which are cleaner. So the Higgs is there, no doubt about it.

Anonymous said...

Dilaton, go back to TRF for further instructions from Lubos. Has he authorized your comment here?

Anonymous said...

Jester, why do you allow spam of anonymous trolls in the comments ? It is distracting and maybe they are not even human but some spambots.

I know the server that generates most of this scornful spam: It is called NEW, located at Columbia, the owner is known too. So why is it not taken down? I thought mass spamming the internet is not allowed ...

Dilaton

Anonymous said...

Dilating, are you Lubos' apprentice. Shall I call you Lubos Jr.? Mini Lubos?

Anonymous said...

I mean Dilaton.

Anonymous said...

Hahaha fascinating,

since the owner of the spamserver is a system administrator, he is obviously a very skilled programmer too ...
He has managed to implement quite interesting characteristics into his spambots, such as an astonishingly high amount of agressivity and agility.
And the new improved spambots are able to interactively communicate with the environment, very cool :-D

I generally think physics blogs should have an appropriate spam filter to keep them out.

But this now is really funny and interesting!

Dilaton

Jester said...

Omg, I haven't been looking for a few hours and there's immediately a food fight in the comment section :-| From now on only comments about WW (or insulting me personally) will go through.

Anonymous said...

In case you didn't noticed:
Measurement of ZZ production in pp collisions at sqrt(s)=7 TeV and limits on anomalous ZZZ and ZZgamma couplings with the ATLAS detector

http://arxiv.org/abs/1211.6096

Anonymous said...

Very interesting! If higgs has a universal larger coupling to W and Z comparing to SM, then it should raise up all the following channels, h->WW/ZZ and 2gamma. Is that right? The numbers may not quite work out for the central values, but the errors bars are large.

YZ

Jester said...

Well, no... then a larger excess would show up in dedicated Higgs searches, and a smaller one in continuum WW searches where Higgs makes only an O(10%) contribution. At the moment, Higgs searches in the WW and ZZ channels do not show any sign of excess.

Kyle Cranmer said...

FYI: the notes are public now:
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults
combination is here:
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2012...

Kyle