There were many interesting results presented yesterday at the HCP conference in Kyoto, and they were both too numerous and too detailed for me to completely absorb as yet — a follow-up will clearly be needed. But a few are obviously so important that I want to point them out now.
First, both ATLAS and CMS, the two general purpose experiments at the Large Hadron Collider [LHC], produced important new results on “multileptons”. Based on a significant fraction of their 2012 data, they looked for signs of new phenomena that would appear as proton-proton collisions that produce at least three leptons or anti-leptons, or even (in unusual combinations and/or along with other unusual things) two leptons or anti-leptons. (I’ll just summarize this class of studies as “multileptons” for the purpose of this brief post and be more specific at a later date.) ATLAS used about 50% more data than CMS, but CMS had a more intricate analysis of their data, so I believe the results were similar where they can be compared. [By the way, the CMS result was approved to be shown at this conference under extreme conditions; at least two of the major players in the analysis had no power or internet for over a week following Hurricane Sandy!]
The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model. And this now kills off another bunch of variants of many different speculative ideas. The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing
- two or more top quark/anti-quark pairs
- multiple W and Z particles
- two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
- new moderately heavy particles that decay to many tau leptons
and probably a few others I’m forgetting. While multilepton searches (especially those for 3 or more leptons) are often touted as a great way to look for supersymmetry in particular, that description vastly understates their power — they are a great way to look for many different types of phenomena not predicted in the Standard Model. (This is something that a number of scientists at Rutgers University have been emphasizing in talks and papers.) And both experiments have demonstrated this with various interpretations of their results; CMS has over a dozen of them!
Each interpretation involves looking to see what specific variants of a particular subclass of theories is now excluded by the new data. Many of these subclasses of are called “variants of supersymmetry”, but in fact the phenomena considered are so stripped-down, typically involving two or three new particles and one or two new physical processes, that they often have nothing specifically to do with supersymmetry at all; the interpretation given applies just as well, or almost as well, to variants of certain non-supersymmetric speculative theories. [The use of these "simplified models", as they are now termed (I used to call them "theory fragments", see the last of my lectures from 2006; the Harvard group termed them "OSETs") allows for results from data to be interpreted in a broad and simple fashion that applies to a wide variety of speculative ideas. This comes at the cost of not being able to see transparently which variants of a particular speculative idea are excluded.]
ATLAS also produced another powerful result, using a significant fraction of their 2012 data, where they looked for an excess of events which contain multiple jets (at least 4), of which 3 or more appear to be from bottom quarks, along with signs of undetected particles (which appear through an imbalance of the momentum of the detected articles; this imbalance is called “missing energy”, a misnomer that has stuck for historical reasons.) This is a very powerful way of ruling out any theory variant that produces both invisible particles and large number of bottom quarks. No excess of events was found, and this allows excludes variants of theories that abundantly produce invisible particles along with two top-quark/anti-quark pairs, two bottom-quark/anti-quark pairs, or two Higgs particles. (Precise limits were only presented for a particular supersymmetric subclass, in which pairs of gluinos are produced, each gluino then decaying to a top quark/anti-quark pair and an invisible particle.)
CMS has presented the first measurement of the rate for something called Vector Boson Fusion production of a Z particle. Vector Boson Fusion is one of the ways to produce Higgs particles, and both ATLAS and CMS will be studying this process in great detail and attempting to make precise measurements of it. But to make sure we understand this process well, it’s important to measure similar processes and make sure that the Standard Model prediction matches the data. So this measurement, where the Higgs is replaced by a Z, is going to be an important test case to give us confidence that theory and experiment are in good agreement where we strongly expect them to be.
Finally (for now) ATLAS also presented a very interesting measurement — very interesting to me, anyway, since my collaborators (Mariangela Lisanti, Philip Schuster and Natalia Toro) and I have been pushing the experiments to make something like it — where they look for an excess of collisions with one lepton (electron or muon) or anti-lepton, and seven or more jets. (I am not sure why they chose seven and not six, which I would have thought would have been enough; I have some reading to do.) This is a very important search because it has the chance to fill a gap in the search strategies which has been around for a while — most (non-Higgs) searches have either been for
- big signals that produce undetectable particles seen as “missing energy”) along with energetic quarks, gluons or leptons
- big signals that produce small numbers of particles in special configurations or combinations
- signals with an exceptional rate that produce tremendous amounts of energy and numerous quarks and gluons
There’s a gap there: signals that aren’t exceptional in rate, don’t produce tremendous amounts of energy or substantial amounts of missing energy, but produce numerous particles. We pointed this out early last year, and suggest methods to fill the gap, but in my view CMS and ATLAS have been a bit slow to address the issue. And unfortunately, in this latest search, ATLAS yet again insisted on large amounts of missing energy — greater than 180 GeV. I don’t know why they felt they had to do this, but they did. So they still haven’t filled the gap. But nevertheless, this measurement is a step in the right direction.
And guess what? There’s a small excess in the number of events! 14 where
4 TYPO! 6 (sorry!!) are expected [that's a crude statement; look at the talk for details]. But the ATLAS speaker warned: don’t get excited. If there were a real excess from a real signal in 1 lepton plus 7 jets, one would at least expect a slight excess in 1 lepton plus 6 jets. Instead, there’s a big deficit. So the observed excess is likely just a statistical fluctuation. Something to keep an eye on, of course, especially since this search was done with last year’s data, and this year we’ll have about 5 times as much, at higher energy.
Yet maybe ATLAS is looking in the wrong place? Maybe they should go back and look at whether there’s an excess of events when they lower or eliminate the requirement on the missing energy? Because as far as I know no one has looked carefully at such events to see whether they agree with the Standard Model prediction.
[NOTE ADDED: Not So! in another talk at HCP, the following search by CMS was briefly mentioned: http://arxiv.org/pdf/1210.7471v1.pdf . See in particular Figure 1; this is almost exactly the method that we recommended. This search needs to be more widely studied and interpreted; it rules out many more variants of many more theories than the ones analyzed in the paper.]
This certainly doesn’t cover everything new that was presented… but it’s all I have time for today. There’s more to come!