Saturday, 26 September 2015

Weekend Plot: celebration of a femtobarn

The LHC run-2 has reached the psychologically important point where the amount the integrated luminosity exceeds one inverse femtobarn. To celebrate this event, here is a plot showing the ratio of the number of hypothetical resonances produced so far in run-2 and in run-1 collisions as a function of the resonance mass:
In the run-1 at 8 TeV, ATLAS and CMS collected around 20 fb-1. For 13 TeV collisions the amount of data is currently 1/20 of that, however the hypothetical cross section for producing hypothetical TeV scale particles is much larger. For heavy enough particles the gain in cross section is larger than 1/20, which means that run-2 now probes a previously unexplored parameter space (this simplistic argument ignores the fact that backgrounds are also larger at 13 TeV, but it's approximately correct at very high masses where backgrounds are small). Currently, the turning point is about 2.7 TeV for resonances produced, at the fundamental level, in quark-antiquark collisions, and even below that for those produced in gluon-gluon collisions. The current plan is to continue the physics run till early November which, at this pace, should give us around 3 fb-1 to brood upon during the winter break. This means that the 2015 run will stop short before sorting out the existence of the 2 TeV di-boson resonance indicated by run-1 data. Unless, of course, the physics run is extended at the expense of heavy-ion collisions scheduled for November ;)

12 comments:

Elbi Gilgen said...

As the heavy ion people are the only ones who have found anything unexpected in the last several decades, I suggest that the "physics" (snicker) run be terminated immediately and the ions be injected right away.

Anonymous said...

The last sentence souns like heavy ion run isn't physics...

Anonymous said...

Concerning the diboson excess: if we get 3/fb and there is nothing visible at 2 TeV, I don't think it will get that much more attention in 2016. If there are ~2 sigma again, then things get really interesting in 2016.
Either way, the 2016 dataset should be so large that Run 1 data becomes irrelevant below 1 TeV. And hey, not enough data in 2015 is one more year of hope to find something ;).

Brian Dolan said...

This shows how amazingly efficient the LHC is. An inverse femtobarn is 10^43 m^(-2). In contrast my car does 20 kms/litre, which is about 10^7 m^(-2). The LHC is therefore about 10^36 times more efficient than my car! If the LHC costs 10 billion euro and a car costs 10,000 euro, a factor of 10^36 in efficiency for a factor of 10^6 in cost is pretty impressive.

Anonymous said...

Hi Brian,

following your calculus I would suggest that LHC asks Audi for a SW update; the luminosity should improve dramatically.

K :-)

Anonymous said...

Ask VW, they are experienced with such "updates".

Jack Collins said...

Any idea how the QCD dijet background scales for an invariant mass of 2 TeV? I think that the dominant background was QCD, but I can't see anything definitive about that in the paper.

Would a good estimate be that the QCD background will scale like your gg -> V0 line? In that case, it looks like at Christmas time the background will be twice as high as at 8 TeV while the signal will have the same number of events (probably a little less if the combined results with the semileptonic and leptonic channels are taken into account), leading to probably somewhat less than 2-sigma bump this Winter whether it is real or not.

Anonymous said...

Roads would have a higher luminosity if the two opposing beams wouldn't be separated everywhere?
The car-car cross-section is quite large at a few m^2 (a few 10^28 barn, there is no SI prefix large enough for that).

Anonymous said...

2.2/fb and counting. ATLAS quotes 2.24, CMS 2.11. Is there a reason for the difference? Just different calibration of the measurements? I can imagine 5% are within the uncertainties, but I don't know how correlated they are between the experiments.

Anonymous said...

Look at that : "The ATLAS detector: status and performance in Run-II" , October 15, 2015.
https://indico.ibs.re.kr/event/21/session/3/contribution/60/material/slides/0.pdf

Obviously they found a high energy dijet events (5 TeV). A somewhat unexpected.

Raise up your voice said...

Anonymous Anonymous said...

Concerning the diboson excess: if we get 3/fb and there is nothing visible at 2 TeV, I don't think it will get that much more attention in 2016. If there are ~2 sigma again, then things get really interesting in 2016.
Either way, the 2016 dataset should be so large that Run 1 data becomes irrelevant below 1 TeV. And hey, not enough data in 2015 is one more year of hope to find something.
Is that ok
Tonmoy

Anonymous said...

Concerning the 5 TeV dijet event: that was with 80/pb, or 2% of the total dataset this year. If it was more than a statistical fluctuation, it should be extremely visible in the full dataset and I guess we would have heard of it.