Thursday, 21 May 2015

How long until it's interesting?

Last night, for the first time, the LHC  collided particles at the center-of-mass energy of 13 TeV. Routine collisions should follow early in June. The plan is to collect 5-10 inverse femtobarn (fb-1) of data before winter comes, adding to the 25 fb-1 from Run-1. It's high time dust off your Madgraph and tool up for what may be the most exciting time in particle physics in this century. But when exactly should we start getting excited? When should we start friending LHC experimentalists on facebook? When is the time to look over their shoulders for a glimpse of of gluinos popping out of the detectors. One simple way to estimate the answer is to calculate what is the luminosity when the number of particles produced  at 13 TeV will exceed that produced during the whole Run-1. This depends on the ratio of the production cross sections at 13 and 8 TeV which is of course strongly dependent on the particle's mass and production mechanism. Moreover, the LHC discovery potential will also depend on how the background processes change, and on a host of other experimental issues.  Nevertheless, let us forget for a moment about  the fine-print, and  calculate the ratio of 13 and 8 TeV cross sections for a few particles popular among the general public. This will give us a rough estimate of the threshold luminosity when things should get interesting.

  • Higgs boson: Ratio≈2.3; Luminosity≈10 fb-1.
    Higgs physics will not be terribly exciting this year, with only a modest improvement of the couplings measurements expected. 
  • tth: Ratio≈4; Luminosity≈6 fb-1.
    Nevertheless, for certain processes involving the Higgs boson the improvement may be a bit  faster. In particular, the theoretically very important process of Higgs production in association with top quarks (tth) was on the verge of being detected in Run-1. If we're lucky, this year's data may tip the scale and provide an evidence for a non-zero top Yukawa couplings. 
  • 300 GeV Higgs partner:  Ratio≈2.7 Luminosity≈9 fb-1.
    Not much hope for new scalars in the Higgs family this year.  
  • 800 GeV stops: Ratio≈10; Luminosity≈2 fb-1.
    800 GeV is close to the current lower limit on the mass of a scalar top partner decaying to a top quark and a massless neutralino. In this case, one should remember that backgrounds also increase at 13 TeV, so the progress will be a bit slower than what the above number suggests. Nevertheless,  this year we will certainly explore new parameter space and make the naturalness problem even more severe. Similar conclusions hold for a fermionic top partner. 
  • 3 TeV Z' boson: Ratio≈18; Luminosity≈1.2 fb-1.
    Getting interesting! Limits on Z' bosons decaying to leptons will be improved very soon; moreover, in this case background is not an issue.  
  • 1.4 TeV gluino: Ratio≈30; Luminosity≈0.7 fb-1.
    If all goes well, better limits on gluinos can be delivered by the end of the summer! 

In summary, the progress will be very fast for new heavy particles. In particular, for gluon-initiated production of TeV-scale particles  already the first inverse femtobarn may bring us into a new territory. For lighter particles the progress will be slower, especially when backgrounds are difficult.  On the other hand, precision physics, such as the Higgs couplings measurements, is unlikely to be in the spotlight this year.

29 comments:

Theo Nieuwenhuizen said...

Can't wait until hearing that the SM is confirmed.

Anonymous said...

Dear Jester,

You have said this before, but with all the updated limits, what do you think will happen at LHC? I am obviously asking for your personal intuitive feeling and obviously Nature does not have to follow that ;)

StevieB said...

Dear Jester,

What do we learn by applying the Jester Exclusion Principle in conjunction with the Minimum BS Conjecture?

Jester said...

Anon, most likely nothing but the SM. Assuming there's something, my bet is on more scalars in the Higgs sector.
Stevie, the principle makes a very strong prediction: it's either the SM, or the MSSM, or the composite Higgs, or the twin Higgs, or something we don't yet know ;)

Unknown said...

I'm so looking forward to them finding the bouffon.

StevieB said...

So "there are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know." When a quote from Donald Rumsfeld is relevant, that's a sad state of affairs! :)

Luboš Motl said...

Let me remind you that your main, $10,000 bet, is actually on the absence of superpartners of any spin, Jester. ;-)

Dilaton said...

Oh Lubos! I'm you're biggest fan! Will you marry me and have babies with me?

Gordon said...

No, I'M his biggest fan! I've been reading his blog posts a lot longer than YOU, Dilaton!

Dilaton said...

Yeah, well I COMPLIMENT him on his posts a lot more! Who do you think he will like more, huh?

So Lubos, I get so giddy whenever I see a new post on your blog. Then, I light a candle, turn out all the lights, and read your post with pleasure.

How do you feel about me, Lubos?

Anonymous said...

I didnt know 12 year olds liked particle physics.

Anonymous said...

Dear Jester,

When you say that you expect "more scalars in the Higgs sector" then are you thinking of SUSY? Is there any scenario in SUSY where the extra Higgs' will show up before gluinos?

Alex said...

The 13 TeV run will provide us with hard proof of SUSY at 14 TeV, which upcoming theory papers will remind us is a longstanding prediction of SUSY.

Anonymous said...

Ah, so hard proof of SUSY is a longstanding prediction of SUSY. Got it.

Alex said...

I hope the funding situation improves to the point where we can get a 100 TeV collider, which will validate the long-standing prediction of SUSY at 101 TeV.

phxmarker mark said...

Aren't you the least bit concerned that the same math/theory that cannot handle the proton radius puzzle is being to analyze the Higgs?

Jester said...

Anon, susy requires at least 2 Higgs doublets, but a priori more Higgses has nothing to do with susy.
Mark, muonic hydrogen is a little bit disturbing, but the same math works impeccably in 1000 other places. So in this case the best bet is on some underestimated systematics in experiment and/or theory.

Anonymous said...

It is already a lot more interesting than a minor deviation that will be subject to debates for a few weeks and put it the "underestimated systematics in experiment and/or theory" drawer for decades under the pretext that "the same calculus works in 1000 other places".
In the muonic hydrogen case, recall that the muon magnetic moment anomaly is also wrong... with the same calculus... and at 2.8 sigma, and then nobody cares because muons physics is already perfect. There is no doubt, no uncertainty with respect to the method or the framework; this is not science, just well hidden integrism and plain procrastination.
K.

RBS said...

"a small cloud on the horizon"?

StevieB said...

Dear Jester,

Slightly off topic, perhaps, but do any of the known unknowns you mentioned (MSSM, or the composite Higgs, or the twin Higgs, or something we don't yet know ;) jive with the LHCb anomaly you recently tweeted? Or is that a whole other can of worms?

It's been almost 20 years since I did anything in particle physics, so thanks for indulging my questions and my sense of humor (or lack thereof).

Mitchell said...

K., you inspired me to look for work on the muon g-2 anomaly, and I found this review... Do you have any preferred direction of investigation? Maybe ideas for relating the anomaly to other unexplained phenomena like neutrino masses?

Anonymous said...

Mitchell, my favorite is no more particles to find and BS < 1. Maybe I am wrong but if right it solves the real problems. I sent you an email about that, I hope the address works. (@hotmail)
K.

RBS said...

Jester, only for a layperson, what'd be the schedule - or deadline for Motl to visit his bank to commission the departure of hard earned and cherished 100 bucks? Do you foresee any more bunkers for him after this summer subject to you know what? Or first year of this run? Or is it first twenty?

mfb said...

@Anonymous 26 May: It is a relevant deviation, and there are tons of doubts and possible effects discussed.
"some underestimated systematics" is a good guess as there were many similar effects that turned out to be underestimated systematics later. Making hundreds of correct predictions without error (which often need hundreds of steps each) is hard, a few errors are possible.

Jester said...

StevieB, there's no straightforward connection between the natural models and the LHCb anomalies. The existing solutions do not rely on any particles required by naturalness ( though it is certainly possible to embed them in SUSY or other models).

Jester said...

RBS, I cash in if supersymmetric particles are not discovered before 30fb-1 at the LHC. It is by no means certain that i will win... there can always be an asteroid strike before ;)

RBS said...

Jester, the solution of supersymmetry for naturalness problem was supposed to be cancellation of mass corrections for heavy particles and their partners. How does it work then that we need a non-zero (and significant) contribution of top with its stops in this plot?

Cédric Bardot said...

Jester can you please share with science enthusiasts your expertise to estimate the ratio of 13 and 8 TeV cross sections for the hypothetical 2TeV heavy W gauge bosons (W' or WR) that CMS and ATLAS might have glimpsed recently if I understand correctly arxiv.org/abs/1407.3683 and and arxiv.org/abs/1506.00962)? When can LHC2 could bring us new information about these anomalies?

Svik Vik said...

You can't compete with a guurl.