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Ambulance-chasing Large Hadron Collider collisions

Guest blog by Ben Allanach on the impure fun of rapid-response physics

B.A. is a professor of theoretical physics at the University of Cambridge. He is a supersymmetry enthusiast, and is always looking for ways to interpret data using it. You can watch his TEDx talk giving some background to the LHC, supersymmetry and dark matter, or (for experts) look at the paper that this blog refers to.
“Ambulance chasing” refers to the morally dubious practice of lawyers chasing down accident victims in order to help them sue. In a physics context, when some recent data disagrees with the Standard Model of particle physics and researchers come up with an interpretation in terms of new physics, they are called ambulance chasers too. This is probably because some view the practice as a little glory-grabbing and somehow impure: you’re not solving problems purely using your mind (you’re using data as well), and even worse that that, you’ve had to be quick or other researchers might have been able produce something similar before you. It’s not that the complainers get really upset, more that they can be a bit sniffy (and others are just taking the piss in a fun way). I’ve been ambulance chasing some data just recently with collaborators, and we’ve been having a great time. These projects are short, snappy and intense. You work long hours for a short period, playing ping-pong with the draft in the final stages while you quickly write the work up as a short scientific paper.

A couple of weeks ago, the CMS experiment released an analysis of some data (TRF) that piqued our interest because it had a small disagreement with Standard Model predictions. In order to look for interesting effects, CMS sieved the data in the following way: they required either an electron and an anti-electron or a muon and an anti-muon. Electrons and muons are called `leptons’ collectively. They also required two jets (sprays of strongly interacting particles) and some apparent missing energy. We’ve known for years that maybe you could find supersymmetry with this kind of sieving. The jets and leptons could come from the production of supersymmetric particles which decay into them and a supersymmetric dark matter particle. So if you find too many of these type of collisions compared to Standard Model predictions, it could be due to supersymmetric particle production.

The ‘missing energy’ under the supersymmetry hypothesis would be due to a supersymmetric dark matter particle that does not interact with the detector, and steals off momentum and energy from the collision. Some ordinary Standard Model type physics can produce collisions that pass through the sieve: for example top, anti-top production. But top anti-top production will give electron anti-muon production with the same probability as electron anti-electron production. So, to account for the dominant background (i.e. ordinary collisions that we are less interested in but that get through the sieve still), the experiment does something clever: they subtract off the electron anti-muon collisions from the electron anti-electron collisions.

The picture below shows the number of collisions that passed through the sieve depending upon the invariant mass of the lepton pair. The big peak is expected and is due to production of a Z-boson. But toward the left-hand side of the plot, you can see that there are a few too many observed events with low invariant masses, compared to the “background” prediction. We’ve interpreted this excess with our supersymmetric particle production hypothesis.

Plot of the dilepton mass distribution from CMS

For those in the know, this is a “2.6 sigma” discrepancy in the rate of production of the type of collisions that CMS had sieved. The number of sigma tells you how unlikely the data is to have come from your model (in this case, the Standard Model). The greater the number of sigma, the more unlikely. 2.6 sigma means that, if you had performed a hundred LHC experiments with identical conditions, the measurement would only have such a large discrepancy once, on average, assuming that the Standard Model is the correct theory of nature. At this point, it’s easy to make it sound like the signal is definitely a discovery. The trouble is, though, that the experiments look at hundreds upon hundreds of measurements, so some of them will come up as discrepant as 2.6 sigma and of course those are the ones you notice. So no one can claim that this is a discovery. Perhaps it will just disappear in new data, because it was one of those chance fluctuations (we’ve seen several like this disappear before). But perhaps it will stick around and get stronger, and that’s the possibility that we are excited about.

When you do this kind of project, the first thing is to check and see if your hypothesis is ruled out by other data, in which case it’s dead in the water before it can get swimming. After that, the question is: does your hypothesis make any other predictions that can be tested? For instance, we’ve been suggesting how the experiment can take another look at their own data to check our hypothesis (there should also be an obvious excess in the events if you plot them against another variable: `jet di-lepton invariant mass’). And we’ve been making predictions of our hypothesis for the prospects of detecting supersymmetry in Run II next year.

You can be sniffy about our kind of ambulance chasing for a variety of reasons - one of them is that it might be a waste of time because it’s “only a 2.6 sigma effect”. There is an obvious response to this: it’s better to work on a 2.6 sigma signal than a 0.0 sigma one.

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reader andrew said...

Dear Ben,

IIRC, there are four interesting invariant mass combinations from llq (lq high, lq low, llq, and ll) and a contrived observable, llq threshold.

One can measure them all and try to invert the formulae for the endpoints to give values for all four sparticle masses in the decay chain. This isn't always possible, however. Because of ambiguities in inverted formulae, there might be multiple solutions for the sparticle masses.

What do you expect with your explanation? Will we be able to unambiguously recover all four sparticle masses? If so, with what sort of (correlated) errors?

PS. In the arXiv Fig. 1, the x-axis is missing from the plot. You probably know this already - sorry if that is the case!

reader Leo Vuyk said...

Perhaps the missing mass could be related to strange dustparticles inside the machine called UFO dust particles.see:

Dark Matter UFO Dust Particles or Quantum Knots in the LHC at CERN ?

reader Ben Allanach said...

Dear Andrew,

Those edge measurements constrain mass differences of sparticles in the chain,

and then the overall mass is largely unconstrained - so that's the

correlation. See to see in detail.

The rate constrains the overall mass times a branching ratio

(and we don't know the latter without additional assumptions).

There are caveats to this: there are combinatoric issues associated with the

jets that can muddy the edges, but one can do tricks and work on these.

Thanks for the PS - I was aware of it, and we shall fix the x-axis in a

revised version.

reader Shannon said...

Well, Gene, I was in NY this summer and my favourite moment was when I landed back in Dublin.

reader Shannon said...

Thanks Cyril. Not my cup of tea this music... I didn't watch the video to the end: is somebody doing hara-kiri at some point? If so I might watch it.

reader Cyril said...

They are chinese, unfortunately. Therefore there will be no hara-kiri.

It seems that you like mathematics doesn't you?

reader Swine flu said...

"You cannot make the case that the Ukraine’s future is of vital interest to the US; it just isn’t."

In some sense, we artificially made it of interest to the US when NATO was expanded eastward. If Putin sees no response to his Ukraine actions, he might be tempted to do something in the Baltics, and as no one wants to find out what happens when Article 5 kicks in there, we are almost forced to take some actions now as a warning/dterrent. Which may not work anyway.

The smart thing would have been to stay out of ALL the places that are not of vital interest to us to begin with. This probably includes Poland.

reader Swine flu said...

I generally avoid arguing religious matters, but I do have a question about your comment that, "the vast majority of Muslims take the attitude that religion is just
a relation between the person and god. Meaning, it has nothing to do
with others and the degree of conformity to rules is nobody's business
but the person."

There are a number of countries that adhere to Islamic law to a greater or lesser degree, some to a very significant degree. Is this done against the wishes of the majority of the population in all of them?

reader Swine flu said...

Wouldn't keeping Assad in power actualy be in Israel's interest?

reader Swine flu said...

Judging the US by New York City is like judging a person by his rectum.

reader Shannon said...

Swine flu, the answer is yes and you know it.

FYI the Islamic State in Irak are blowing up what they call "heretical" mosques.

reader Shannon said...

Swine flu, the rest of the US is what comes out of this rectum ;¬). The countryside is ok but the people spoil it with their ugly consumerist temples that you find identically in every states. As I've always said: even the most beautiful diamond found in the US loses its beauty as soon as a Unitedstatian touches it.
Sorry :-(

reader Shannon said...

I've learned something: Chinese can have the same complexion as Japanese. ;-)

reader Ariful Hossain Tuhin said...

WOW Totally different perspective. I'm no US fan. As an avid reader of Chomsky/zinn , i do not have any fetishes towards US. But as i always follow US/UK journals for analysis about world politics(Mostly new york review of books, new yorker, london review of books), i was under the impression that :

[1] Russia is acting like an imperialist power(No less of US)

[2] US+EU is using this opportunity to siege some lost ground against Russia.

So i was totally convinced of the satellite photos. I never knew they were fabricated. Can i see the proof behind this claim?

reader Gene Day said...


reader Gene Day said...

You would be warmly welcomed here in California, Shannon. My wife and I would love to show you beautiful area where we live and you could enjoy local wine, bread and cheese, which are fully the equals of those in France.

reader Swine flu said...

Are you saying that the majority of the population in the countries whose legal systems are based to a greater or lesser degree on Islamic law are opposed to it? I am talking about countries with internationally reconginzed governments, Saudi Arabia, Pakistan, Iran, etc.

My disbelief was about QsaTheory's assertion that most Muslims view religion as a private matter. Maybe it's true worldwide, but there seem to be a number of countries where that is not the case.

reader Swine flu said...

"As I've always said: even the most beautiful diamond found in the US loses its beauty as soon as a Unitedstatian touches it."

Oh, well. If you insist on being a one-trick pony, so be it.

reader Luboš Motl said...

I also think that there is no justification for animosity as California is almost equivalent to the French Riviera. Some Mexicans and burritos instead of

reader QsaTheory said...

In theory all Muslim countries constitution have some clause that says we shall follow the Islamic law. However, when it comes to practice all these countries have a regular common law.That is because there is NO standard Islamic laws in the majority of situation since there are many sects and it is not practical to pick any. Only in limited situation like marriage and inheritance the sect laws are tried to be applied but mostly unsuccessful because of cross marriages and other reasons.

In the Arab world only Sudan and KSA governments portray themselves as Islamic states. Sudan is ruled by a weak government which has the Muslim Brotherhood Ideology similar to Turkey, so the rules are relatively mild( Iran is similar). KSA has a especial status as the original country of Islam and has the Sunni Salafi background so it has more rules.

In all situations many of the "Islamic rules" which are accepted by the individual societies are nothing but some social norms with "Islam" clothing them. As you are aware in any society these social norms are broken( most with no price to pay) by many people and they change with time and place. So these norms sometime become laws, of course, with resentment, outrage and indifference of many others in these conservative societies.

In short, in most Muslim countries and especially in the urban areas you do not feel the Islamic "rules" to any significant value. You might find some in the more rural areas but most are really nothing but some social norms. The governments ( they are only three) that use "Islam" as the controlling ideology mostly for political reasons and in the end many don't care for them one way or another and many resent them as I said before.

Of course there is more to it than what I have summarized. BUT, it is no where near as the media tries to portray. People live ordinary chaotic life,concentrating on making a living worrying about their childern ...etc, like anywhere else, that is the bottom line.

reader QsaTheory said...

I might add, the sentence "it is between him and god" is a daily cliche when people talk about another person who has done a bad thing.

reader Swine flu said...

This is but a phrase. I am talking about the legal systems and practices in whole countries. Saudi Arabia has "morality police", which enforces various rules. Either the majority of the population there disagree with such practice or they don't, and if they don't, one cannot possibly say that religion is just a private matter in that country.

reader Gene Day said...

There is little point in responding to John Archer’s inane xenophobia, QsaTheory.

reader QsaTheory said...

First, we are talking about the Muslim/Arabic World (1.3 billion) as the majority , KSA is only 25 million. like I said it is a special case. But it is not black and white, even in KSA there are many sects and many differences due to geographic areas, also there is a very severe contention between their rural/desert and the city people. I am sure they are proud of their country being the origin of Islam which they want to preserve but I am sure they don't agree on exactly what that means. Including what rules and how much to enforce, it is a daily battle for that government.

My main point was that in almost all Muslim countries common law rules and "Islamic" laws is just a sentence or a paragraph in the constitution. Some "Islamic rules" are nothing but norms. And in the end it is how people behave in reality despite the propaganda of the state in the form of untranslatable "laws" here and there.

In actual daily practice people don't give a damn about how others live, their own troubles are enough for them. And the states have enough problems on their hand to go to any length to police "morality", they are more interested in policing any threat to their existence.

reader Gene Day said...

Putin has said that he could sink an entire American fleet in the Black Sea within 30 minutes. This is true but he would not use the 50 year-old technology discussed in your link.

I was already working on the foundation of more advanced technology in 1965 but there is no defense against a Mach 7 missile with 20-30 G evasive capability coming down vertically from overhead. Ultimately, physics alone makes the attacker the winner.

reader Gene Day said...

The last war that was “won” ended in 1945.