LHC News – Page 15 – Of Particular Significance

Winds are Rising

UPDATE: LHCb reports precise measurement of B_s meson decaying to J/Psi meson + Phi meson. Agrees beautifully with the Standard Model. Fantastic measurement!! But disappointing — no sign of any new phenomena.

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Well, the hurricane now arriving on the east coast of the United States has forced significant changes in many people’s plans, mine included. Certain big chores simply had to get done in preparation for the storm’s arrival. Consequently the post that I had hoped to write analyzing the results from Mumbai on the searches for the Higgs particle, and for more speculative phenomena, simply hasn’t been composed yet. But since Irene the Giant is expected to impact the New York area for about 24 hours, starting late tomorrow afternoon, I should have plenty of time to catch up while it’s on its way in. Whether I’ll be able to finish the post, or post it when it’s finished, will depend on when the power goes out, and for how long.

[p.s. apologies to those who have asked questions that have gone unanswered; I’ll get to those during the storm as well.]

Meanwhile, there are some “flavor physics” results being presented in Mumbai right now. Flavor specifically refers here to the behavior of bottom quarks and of charm quarks, as they behave inside of hadrons, and as they decay to other quarks lighter than themselves. These behaviors are predicted in considerable detail by the Standard Model of particle physics, and it is rather easy for some new phenomena, involving some new heavy particles, to come along and screw up the Standard Model prediction. In fact there are several measurements, from various experiments including the Tevatron and also the so-called B-factories [which make lots and lots of bottom quarks and anti-quarks] that don’t currently agree with the Standard Model very well. (more…)

POSTED BY Matt Strassler

ON August 27, 2011

Searches for New Phenomena: Results from Mumbai Conference Today

Real Time Updates at end of post:

(all times Mumbai)

12:15: update 1 (top quarks seem to behave mostly as expected)
13:00: update 2 (more ways in which top quarks behave as expected)
13:15 update 3 (general thoughts on the lack of new top-quark surprises)
14:15 one huge summary of everything new at LHC begins — oof!
14:45-15:0o a summary of the huge summary — for what it is worth
15:30 Tevatron: only a few updates from Grenoble, nothing too striking.
20:00 GRRR: still waiting for the organizers to post today’s slides so I can review them and understand the details!!!

No blockbusters today! But way too much, way too fast. Will have to look at the slides carefully once they finally post them, and then probably will still have to dig into the details of the new results to learn anything really interesting.

—————

Ok, getting ready for an interesting day. On the morning agenda: measurements of things we think we know — W and Z particles, photons, and top quarks — looking for surprises. In the case of the top quark, we have a very good reason to be looking closely — the “Forward-Backward Asymmetry” for top-quark production, which should be small in the Standard Model, is measured at the Tevatron, by both CDF and D0, to be moderately large. This suggests a possibility that new physics is affecting top quarks. So although the LHC experiments can’t make exactly the same measurement as the Tevatron to check for the same discrepancy, it is very natural for them to look for something else odd about the top quark in their data. So far nothing strange has shown up, but … perhaps today?

After that, in the afternoon, we’ll have results on searches for various speculative ideas, including supersymmetry, extra dimensions, and presumably a few other things. Won’t know the details until the talks are given… but I expect we’ll see quite a few new results. With so many results, the chances are that at least one of them will get people talking… although there is a tendency for exciting results to not be shown for quite a while, for reasons detailed in this article. If there’s anything really exciting in the data, we might not see if for a few more months, until the data set doubles again.

——-

12:15 — Albert De Roeck from CMS reviews lots of top quark measurements from Tevatron and LHC — but focuses on precision measurements of production rates. Flies through everything involving possible new physics. Advances on many fronts, but plots zipped by, no time to really look at them and understand whether there’s anything funny in them. Only a few new results for this conference. Most interesting technically, at first glance, is search for heavy particle with 1 – 1.5 TeV mass decaying to top quark and top anti-quark, using new “boosted-particle” techniques. Searches for various types of fourth-generation quarks (usually called top-prime or bottom-prime) putting lower limits on their masses in the 400-450 GeV range. Nothing new on the top quark forward-backward asymmetry, the one discrepancy from the Standard Model. Nothing obviously to get excited about since the Grenoble conference… but need to look at the actual slides (not yet posted.)

13:00 — Yuji Takeuchi reviews other types of measurements from Tevatron and LHC involving the top quark (mass, charge, details of decays, spin effects) — nothing violating Standard Model expectations. ATLAS has a nice new measurement of W helicity in top quark decay, and another on the correlation of the spins of the top quark and top anti-quark when they are produced together.

13:15 — I should say that today’s results on top quarks, while not surprising after the Grenoble conference last month showed almost nothing new with the top quark, are in general somewhat disappointing. The Tevatron measurement of the top-quark forward-backward asymmetry, which appears even now to be in conflict with the Standard Model, has been our best hope so far for something indicating a new phenomenon involving the top quark. And we have many reasons to expect the top quark might not behave as predicted — it is by far the heaviest quark, and the reason for its very large mass is not known. Equivalently, it is the particle which feels the Higgs field more strongly than any other particle known. So there are many speculative theories of new physics in which the top quark plays a special role, which tends to be reflected in some novel effect on the top quark’s properties. Yet except for the forward-backward asymmetry, nothing else odd about the top quark has shown up yet to provide more evidence that something is amiss with the Standard Model. Granted, these are still early days at the LHC, and the various measurements from ATLAS and CMS will improve and proliferate with more data and more time… so there will be plenty more to watch for over the next 18 months…

14:15 Here we go — one talk on everything the LHC has done on new physics. Henri Bachacou presents.

14:45 Ok, this was only a moderate update of what we saw in more detail in Grenoble. There were a few new results, but not that many we hadn’t seen before. And it all went by far too fast, with far too few details, for any thoughtful analysis. But with only 30 minutes, what could the speaker do? He should have had double the time. (Why weren’t the LHC speakers more numerous and given more time at this conference??!!)

The talk itself was very nice, the results presented were spectacular, and the speaker was very careful to make precise statements, which I always appreciate. Basically: Standard Supersymmetry with a large jets-plus-missing-energy signal is excluded for squarks and gluinos up to the 1 TeV range — putting such models in serious trouble. The same is true for the version of supersymmetry known as “minimal gauge mediation” with that produce 2 photons in every event (and some new and interesting limits on the case where often there is only one photon per event.) Limits on heavy Z-like and W-like particles are in the 1.8 and 2.3 TeV range, as are particles decaying to two photons. Technicolor particles decaying to WZ are excluded up to 380 GeV. No sign of any excess in events with two charged leptons or two charged anti-leptons. Nothing in the classic searches for invisible sectors and/or extra dimensions; no deviations at high energy in the distribution with energy of charged lepton-antilepton or two-photon events. No huge signals of lots of quarks or leptons produced at once, as expected from black holes. And certain powerful searches for long-lived particles and slowly-moving particles came up empty. So — the experiments have done a fantastic job! Unfortunately, nothing obvious has popped out.

BUT — as the speaker emphasized, there are many classes of searches that have not been done. Many variants of supersymmetry would not have shown up in current searches (my perspective on that is here) and will have to be looked for in future analyses with different strategies. Also, he emphasized, with the large data sets expected by fall, entirely new methods of analysis will become possible. So for the next round of conferences, expect results involving ultra-high-energy leptons (with 1 TeV or so of energy), “boosted” objects (a technique for finding high-energy W and Z particles and top quarks, and perhaps Higgs and other new particles), and searches for a number of less obvious signatures of new physics.

I’m going to have to look at all of this again once they post the slides, and frankly the slides were almost useless anyway, given how much material had to be squeezed in such a short talk. Really, if one wants to see anything more subtle than the bottom line, what one has to do is read the corresponding papers and notes — and that will be my goal for the next couple of weeks.

15:30 In contrast to the LHC speaker, the Tevatron speaker Alberto Annovi got to spend most of his time on a relatively small number of results. A significant fraction of the time was spent on the widely reported (one might say widely over-reported) W-plus-two-jets excess that CDF sees and D0 does not. I remain unconvinced. Nothing else too dramatic that we didn’t see in Grenoble.

8/23/11

POSTED BY Matt Strassler

ON August 22, 2011

The Latest Word on the Higgs from the Mumbai Conference

Restructured the post: My preliminary discussion is first, the updates from the talks are now at the end. The take-away message from the LHC talks:

The most interesting point here is that ATLAS and CMS [crudely] taken together exclude all regions for the Standard Model Higgs particle except

below 145 GeV,
the range 288-296 GeV, and
above 464 GeV.

Since precision measurements of other quantities in nature indirectly make the region above 464 GeV very unlikely, and even that small window around 290 is probably excluded when the two experimental results are combined properly, we are probably down to the region 115 — 145 GeV for the Standard Model Higgs.

Not much change in the Higgs hint at 140… some details have shifted, but they went by too fast. It needs a closer look. But don’t expect any real news. [Update: the significance of the hint has gone down a bit — but is this due just to the new data or have the analyses changed? Also, from the past month’s data, CMS saw no additional two-lepton/two-antilepton events in that mass range (at Grenoble they reported two events at 142 GeV.)]

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8/22/11 10:30 Mumbai time

The search for the Higgs particle — a ripple in the Higgs field, the field which is on average non-zero and provides the mass for the known elementary particles — continues at the Tevatron and Large Hadron Collider. We will probably get some new information within an hour or so.

Right now, the focus is on the search for the simplest possible Higgs particle — the one that goes by the name of Standard Model Higgs and arises from the simplest possible type of Higgs field. There are searches also going on for somewhat more complicated possibilities — Higgs particles that decay in unusual ways, or several types of Higgs particles, etc. But the first task of the experiments is to find the Standard Model Higgs particle, if it is there in nature, or rule it out, forcing us to look for something more complex.

Last month, we heard (a) that the Standard Model Higgs is probably not heavy (its mass-energy [E=m c-squared] probably does not lie in the range above 150 GeV and below about 450, though there are still some gaps in the coverage), and (b) there is a hint of a Standard Model Higgs particle around 140 GeV, although this could also be a more complicated non-standard Higgs particle at masses somewhat lower (perhaps 115-130 GeV.)

But as I emphasized in this detailed article, there are two types of searches for the Standard Model Higgs particle: some simple ones, easy to interpret, that take a while, and difficult ones that require less data but are subject to more subtleties. That hint at 140 relies on a very difficult technique, and so there is some question as to whether it might be an artifact of some type.

We will be watching today at the conference in Mumbai to see the updates on both the easy and the difficult searches. If the experiments really do update their results from the Grenoble conference by including more data, we should see (1) a firmer exclusion of a heavy Standard Model Higgs, except — if it is actually there! or if there is a statistical fluke — at one or two points where there will be a slight hint of a Higgs; (2) an update of the easier search strategies, which could be quite interesting indeed, as there may be enough data now for them to be useful; and (3) probably very little new on the more difficult search strategy, because there are probably internal studies being done to check the methodology, and they may not be complete. The hints seen in the more difficult strategies will still be there, but I suspect they will continue for now to rest on somewhat uncertain ground.

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8/22/11 10:45 Mumbai time

[Right now we have an introductory theory talk by Abdelhak Djouadi, from France. Why the theorist gets 35 minutes and the ATLAS and CMS experimental talks — the real data! — get only 20 minutes each is beyond me. The ATLAS talk will be given by Aleandro Nisati from Italy (with whom I had the pleasure to work briefly), and the CMS talk by Vivek Sharma from the U.S.A. And the Tevatron talk, combining CDF and D0 data, will be given by Marco Verzocchi, also from the U.S.A.]

Update 1: 11:39 ATLAS talk — Excellent! the full data set is being used for the two-lepton/two-antilepton search! [Nothing special in two-photon signature.] 11:53 EXACTLY as was predicted above… nice improvement in exclusion in heavy mass region, except for a few gaps; no real change in the Higgs hint below 150. I think at 95%: excluded 146-232, 256-282, 296-464 GeV. Striking: 99% exclusion of Standard Model Higgs between 160-220 and 300-420 GeV.

Update 2: 12:07 CMS talk (not quite all the recorded data I think) [in two-photon signature, some excess at 140 GeV, but I think ATLAS did not see one there; presumably just statistical fluctuation] 12:20 And AGAIN, exactly as predicted from CMS.

The most interesting point here is that ATLAS and CMS [crudely] taken together exclude all regions except below 145 GeV, 288-296 GeV, and above 464 GeV.

12:51 The Tevatron (CDF and D0 experiments) reports exclusion of the Standard Model Higgs in these ranges

Observed: 100-109 and 156-177 GeV
Expected: 100-108 and 148-181 GeV

Since observed is weaker than expected at the low end of the upper range, that means that there is a tiny hint of a Higgs below 150 or so, so this result isn’t in conflict with the LHC hint at around 140 GeV. But we wouldn’t expect Tevatron to be as sensitive as the LHC experiments to a 140 GeV Higgs, if there is one.

Most interesting result from Tevatron: both CDF and D0 see an excess of events consistent with a supersymmetric Higgs particle in the mass range 120-150 GeV, in a production mechanism that is absent in the Standard Model. However this is a tough measurement and it is hard to know whether to trust this… and the excesses are not that large. (Experts: this is b-quark plus Higgs, where Higgs decays to b-quark and b-antiquark. Note b-quark-plus-Higgs where Higgs decays to tau-lepton and tau-antilepton is not observed at the corresponding expected rate, so this Higgs would have to be exotic in at least one other way if this were a real signal.)

POSTED BY Matt Strassler

ON August 22, 2011

Current LHC Data and Supersymmetry; Is Supersymmetry in Trouble?

Ok, today I’ve posted the article you’ve been waiting for: What does the LHC have to say, so far, about supersymmetry? [Here’s an article about supersymmetry and what it predicts, and another about standard ways to look for signs of it at the Large Hadron Collider, under certain assumptions; if you want to review the known particles and forces first, you can read about them here.]

If you’ve been reading the press, you may have seen statements such as “the air is getting thin for supersymmetry” or “we’re painting supersymmetry into a corner”. And recently on Cosmic Variance some broad statements about supersymmetry being in serious trouble were made by experimentalist John Conway (before being somewhat watered down after I raised an objection.)

[Update: in response to a comment posted below, I reread this post, and I see that indeed I did not, in the original version, give enough words attesting to the tremendous progress that has been made. So let me state clearly: what the LHC and the ATLAS and CMS experiments have achieved, in the supersymmetry-aimed searches made public so far, is fantastic. They have wiped not only many variants of supersymmetry, but also variants of many other speculative ideas, off the map. And I don’t mean in any way to downplay this… just to try to put it in proper perspective. To do so better, I also added a sentence after the list of assumptions below.]

A number of experimentalists seem to have (and, in their public statements, give) the impression that most supersymmetry theorists, faced with an apparent disaster, are rushing around in circles desperately trying to think of new ways that supersymmetry might have escaped notice. This view also seems to be showing up on various blogs. This is a profound misconception, one that history clearly contradicts. Rather than a picture of universally panicked theorists trying to figure out how to save their favorite theory, the image should be of a number of theorists sitting calmly in their chairs, saying, “we knew this situation was a reasonable possibility — and we’ve been trying to alert you to it for a very long time.” The statements have been in the scientific literature for years.

Many theorists, including myself [and let me emphasize that I am not a huge proponent of supersymmetry, though I have worked on it and do view it as a serious contender as a theory of nature] have been saying for years that the main search strategies planned at the LHC would be insufficient to cover large regions of the supersymmetric continent. Important papers on this subject go back into the 1990s and a good chunk of the last decade. Here is the point. The bulk of the search strategy used at the LHC to find supersymmetry rests upon three assumptions.

in any process, the number of superpartners can only change by an even number;
the lightest superpartner [which is stable, by assumption 1] is a superpartner of a particle we know (and therefore, to avoid conflict with other data, an undetectable neutralino or sneutrino);
the superpartners that are affected by the strong nuclear force are significantly heavier than the other superpartners of known particles.

These assumptions are not unreasonable, and there are strong arguments in their favor, especially for assumption 1 and to a degree assumption 3 (described here). But we must keep in mind that if we relax any one of those assumptions, the limits on supersymmetry from current LHC data become much weaker. If you want to read more details about how this works, you can click here, though you will find it useful to read today’s article first unless you already know a lot about the subject. Or click on the figure below (taken from this post, which also explains why these assumptions are often made) to understand how these assumptions lead to the prediction of “jets and missing energy” on which the most powerful search strategies — including the ones we’ll hear about at this week’s conference in Mumbai — are all based.

The Logical Chain that Leads to ``Jets and Missing Energy'' — The logical chain that leads particle physicists to look for supersymmetry in the standard way. SP and LSP stand for "superpartner'' and "lightest superpartner".

So until you hear a consensus building, you should be cautious in making too much of statements that supersymmetry, as a part of nature and a solution to the hierarchy problem, is in serious trouble. The truth is that certain variants of supersymmetry — ones based on certain assumptions that might be wrong — are indeed strongly constrained by current data. Others, simply put, are not. In short, we have a long way to go.

8/19/11

POSTED BY Matt Strassler

ON August 19, 2011

About the NY Times article from 8/02/11

On Tuesday, the New York Times had an article on the Higgs particle search. Not bad, and does quote relevant people, but just a little bit thin on content. If you want some actual content, try my article on the hints of the Higgs particle.

Also, the article falls into the common mistake of not distinguishing “The Higgs particle” from “The Standard Model Higgs particle.” One paragraph says

The Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory. If the Higgs boson does not exist, theorists will have to go back to their blackboards.

If you conflate the general with the specific, you will at some point down the line end up very confused about what the LHC is trying to do and what its results do and don’t mean. It would be good for the public if the NY Times reporter would read the Higgs FAQ before phrasing a statement like this. A more correct statement is:

The Standard Model Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory. If the Standard Model Higgs boson does not exist, this will be very exciting for particle physicists, as it will imply (as has been known for decades) that there must be other particles — perhaps multiple Higgs bosons, and/or perhaps other types of particles — that physicists have not yet discovered, but should be able to discover with the LHC.

The spin is pretty different.

And then there’s the generic paragraph about supersymmetry — more or less right, but again with some big missing points. Well, stay tuned at this website for an article or two on supersymmetry, and what we have and haven’t learned about it from the LHC so far.

POSTED BY Matt Strassler

ON August 4, 2011

New Post on the Higgs Hints

Just finished my new article on the hints of a Higgs particle. I hope you find it useful! I have tried to explain, in largely non-technical terms,

how experimentalists at the Large Hadron Collider are looking for the Higgs, using various methods;
what makes methods of this type easy or difficult, with analogies;
that the current hints of a Higgs particle rely on the more difficult techniques, making it unclear whether we should trust them;
that more data over the coming year will bring the simpler techniques into their own, eliminating this problem over time.

I welcome your comments, questions and advice as to how to make this article more transparent to the non-technical reader!

POSTED BY Matt Strassler

ON July 31, 2011

Two interesting posts recommended

Nice post went up yesterday on the ATLAS experiment’s blog [ATLAS is one of the detectors at the Large Hadron Collider] from Kyle Cranmer (Professor at New York University) ; as I posted on this blog, Kyle gave Friday’s presentation summarizing ATLAS’s results on the search for the Higgs particle. Kyle is a young star with a special gift for the statistical reasoning that is essential for combining many different measurements together so as to squeeze the maximum knowledge out. His post has some technical parts, but even if you can’t follow the details I still recommend skimming it for the non-technical insights.

Another interesting post went up on the Cosmic Variance blog, by John Conway (Professor at UC Davis [and not to be confused with the mathematician John Conway]). John mentions that ATLAS and CMS (the other big LHC experiment) have observed no sign of quarks being composite (made of other yet smaller things) and have seen no sign of supersymmetry. [I’ll explain supersymmetry on this site soon.] He then goes on to discuss the Higgs search situation (see also here for my initial take.) But I disagree quite strongly with what John has to say about the search for supersymmetry; look at comment #9 in reply to his post for my opinion. John states as a near-fact that we pretty much know now that the super-partners (heavy new particles that are predicted by supersymmetry) of the quarks are heavier than 1 TeV, but I just put a paper out yesterday with Mariangela Lisanti, Philip Schuster and Natalia Toro that contradicts that statement, arguing that the searches performed so far do not by any means cover all the territory, and suggesting that other search strategies need to be added. And I know that there is more unexplored territory that we did not even discuss in our current paper. [I’ll also give a more detailed discussion of our paper’s argument soon.] This is a key job of particle theorists; make sure all the ground gets covered by the experimentalists before they give up and move on!

POSTED BY Matt Strassler

ON July 28, 2011

Category: LHC News

Winds are Rising

POSTED BY Matt Strassler

Searches for New Phenomena: Results from Mumbai Conference Today

POSTED BY Matt Strassler

The Latest Word on the Higgs from the Mumbai Conference

POSTED BY Matt Strassler

Current LHC Data and Supersymmetry; Is Supersymmetry in Trouble?

POSTED BY Matt Strassler

About the NY Times article from 8/02/11

POSTED BY Matt Strassler

New Post on the Higgs Hints

POSTED BY Matt Strassler

Two interesting posts recommended

POSTED BY Matt Strassler

More Striking Results From Grenoble

POSTED BY Matt Strassler

Taking Stock of Today’s Events

POSTED BY Matt Strassler

ATLAS and CMS report on Higgs decaying to W particles

POSTED BY Matt Strassler

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