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June 27, 2013

I lost

It’s been 20 years since I had the surreal experience of turning on C-Span late at night to see my future boss, Steve Weinberg, testify before Congress on behalf of the SSC.

Steve, alas, was unsuccessful; the SSC was cancelled, and the High Energy Physics community threw our collective eggs in the basket of the LHC. The SSC, at s=40\sqrt{s}=40TeV, was designed as a discovery machine for TeV-scale physics. The LHC, with a design energy of s=14\sqrt{s}=14TeV, is the best one could do, using the existing LEP tunnel. It was guaranteed to discover the Higgs. But for new physics, one would have to be somewhat lucky.

14 TeV sounds like more than enough energy, to hunt for new particles with masses of a few TeV. But that appearance is deceptive. The protons circulating in a hadron collider are like sacks of marbles, and each marble (“parton”, if you want to sound sophisticated) carries only a fraction of the total kinetic energy of the proton. At the energies we are talking about, the collisions are actually parton-parton collisions. So it’s the energy of the pair of partons undergoing the actual collision that matters. And that energy is typically far less than the nominal s\sqrt{s}. In fact, things are slightly worse than the metaphor implies. Each sack contains a variable number of marbles, and the mean number of marbles (sharing, between them, the total kinetic energy of the proton) increases with increasing s\sqrt{s}.

The upshot is that, at a hadron collider, the “interesting” collisions — the ones where, by chance, the colliding partons happen to carry a large-enough fraction of the proton’s total energy — are few and far between. To some extent, you can compensate for their rarity by increasing the total number of collisions (running the machine at higher luminosity). That introduces its own difficulties, but it’s the tradeoff that the designers of the LHC needed to make.

Still, there are (or were) lots of scenarios with new physics, accessible to the LHC. And theorists, being perennial optimists, put a lot of effort into exploring those scenarios. Moreover, I think we’d have to go back to Isabelle to find an example of an accelerator which opened up a new range of energies and didn’t find anything new. So, back in 2006, when Tommaso Dorigo proposed a bet, I was willing to take the position that the LHC would discover new physics.

I didn’t, however, like Tommaso’s original terms (a new particle discovery, announced before the end of 2010).

Experience with previous machines, like the Tevatron, is that startup dates tend to slip, and that it can often take years to ramp up to the full design luminosity. As it turns out, the LHC had barely begun to collect data by then, and the very first trickle of physics results started coming out in October of 2010. So I had wisely insisted that, rather than fixing a date, we agree on a fixed amount of data collected (10fb 110\,\text{fb}^{-1}), plus a suitable period (12 months) for the analyses to be done.

Moreover (for reasons that I will recall, below), I thought the “new particle” criterion too narrow, and substituted “a 5σ5\sigma discrepancy with the Standard Model.”

Those terms seemed pretty solid to me, and I agreed to put $750 behind them.

One thing which I didn’t count on was the 2008 quench incident, which led to the aforementioned delay in starting up the LHC and (more important for the bet, at hand), to its operation at about half of the design energy (s=7\sqrt{s}=788 TeV) up through 2013.

Historically, the ramp-up in energy tends to be much easier and (since it drastically improves the “reach” for new physics) tends to be accomplished much more quickly than the ramp-up in luminosity. So I fully expected that most of that first 10fb 110\,\text{fb}^{-1} would be collected at s=14\sqrt{s}=14TeV. Alas, none of it was (and, foolish me for not insisting on a provision about s\sqrt{s} of the data).

What about the “new particle” criterion?

There are lots of scenarios where you would see a stark deviation from SM expectations at the LHC, but still be unable to ascribe that deviation to a new particle of a particular mass, etc. For example, much excitement was generated by the initial measurements of the HγγH\to\gamma\gamma branching ratio, which were higher than the SM prediction by 223σ3\sigma. With more data, that discrepancy seems to have gone away, but imagine if it had persisted. We would now find ourselves with a 5σ5\sigma deviation from the SM — clear indication of the existence of new heavy charged particle(s) which couple strongly to the Higgs. But, since they only contribute to HγγH\to\gamma\gamma via a loop, we would have almost no handle on their mass or other quantum numbers.


Well, it’s been a little over a year since we reached the 10fb 110\,\text{fb}^{-1} mark. The Lepton-Photon Conference seemed like a natural end-point for the wager. If there had been a discovery to announce, that would have been the natural venue.

Needless to say, there were no big announcements at the Lepton-Photon Conference. And, since the LHC is shut down for an upgrade until 2015, there won’t be any forthcoming. So Tommaso is $750 richer.

Would the outcome (aside from being delayed for another ~3years) have been any different had I been smart enough to add a stipulation about s\sqrt{s}? Put differently, would I be willing to bet on the 2015 LHC run uncovering new BSM physics?

The answer, I think, is: not unless you were willing to give me some substantial odds (at least 5–1; if I think about it, maybe even higher).

Knowing the mass of the Higgs (125\sim 125GeV) rules out huge swaths of BSM ideas. Seeing absolutely nothing in the 7 and 8 TeV data (not even the sort of 2–3σ\sigma deviations that, while not sufficient to claim a “discovery,” might at least serve as tantalizing hints of things to come) disfavours even more.

The probability (in my Bayesian estimation) that the LHC will discover BSM physics has gone from fairly likely (as witnessed by my previous willingness to take even-odds) to rather unlikely. N.B.: that’s not quite the same thing as saying that there’s no BSM physics at these energies; rather that, if it’s there, the LHC won’t be able to see it (at least, not without accumulating many years worth of data).

Ironically, a better bet for discovering new physics in this energy range might be on an ILC, running as a precision Higgs factory. I’ll leave it to you to calculate the odds that such a machine gets built.

Rereading the comments on Tommaso’s post (and other things he’s written), you might well think this discussion is a proxy for a narrower one, about the status of supersymmetry. The 7– and 8–TeV runs at the LHC have, indeed, been very unkind to the MSSM. But they have been even more unkind to other BSM ideas. So

  • While the probability that the LHC will see any BSM physics (supersymmetric or not) has plunged dramatically,
  • the conditional probability that if the LHC were to see BSM physics, then said new physics would turn out to be supersymmetry, has gone up.

That may be of little immediate consolation (and not an obviously-exploitable vehicle for making back some of the money I lost), but it is motivation for my experimental colleagues to spend the next couple of years thinking about how to optimize their searches to tease the maximum amount of information out of the post-upgrade LHC data.

Update (2013/6/28)

A couple of people wrote me privately, expressing … concern … at the apparently pessimistic tone of the post. Evidently, I was insufficiently clear. So let me reassure everyone that I am simply being a good Bayesian. The original bet was that

  • Some sort of BSM physics, which was pretty much invisible in 4fb 14\,\text{fb}^{-1} of Tevatron data1 (at s=2\sqrt{s}=2 TeV), would produce a 5σ5\sigma signal in 10fb 110\,\text{fb}^{-1} of LHC data (at s=14\sqrt{s}=14 TeV).

I was willing to accept even odds for that proposition.

The above (hypothetical) bet about the 2015 run says, in effect

  • Some sort of BSM physics, which was pretty much invisible in 20fb 120\,\text{fb}^{-1} of LHC data (at s=7\sqrt{s}=788 TeV), would produce a 5σ5\sigma signal in 20fb 120\,\text{fb}^{-1} of LHC data (at s=14\sqrt{s}=14 TeV).

Purely knowing that nothing was seen, to date, significantly lowers (for any reasonable set of priors) the probability of producing a 5σ5\sigma signal in the 2015 data. And that’s before taking account of the other thing that we learned in this run, namely the mass of the Higgs. Models of BSM physics which had either no Higgs, or one much heavier than 125125 GeV, were perfectly viable candidates for the first bet, but are now excluded for the purposes of the second.

So, even hypothetically, I would be a fool to bet on the second proposition without significant odds in my favour.


1Run II of the Tevatron eventually produced 12fb 112\,\text{fb}^{-1} of integrated luminosity. But, when I made the bet, it had only produced about 4fb 14\,\text{fb}^{-1}.

Posted by distler at June 27, 2013 11:59 AM

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23 Comments & 1 Trackback

Re: I lost

So with hindsight, should Steve have swallowed his pride and answered the (in)famous question of a congressman with “Yes, absolutely, the SSC will help us find God!” ?

Posted by: wolfgang on June 27, 2013 2:01 PM | Permalink | Reply to this

Re: I lost

The total price tag of the SSC (of which $2 billion had already been spent, when the project was cancelled) was 0.3% of the cost of the Iraq and Afganistan wars or, alternatively, about twice what the London Whale lost JP Morgan-Chase in a single ill-considered trade.

In other words … no, we didn’t stand a chance.

Posted by: Jacques Distler on June 27, 2013 2:27 PM | Permalink | PGP Sig | Reply to this
Read the post Guest Post: Jacques Distler, Why I Lost $750 On New Physics At The LHC
Weblog: Science 2.0
Excerpt: Jacques Distler is a Professor of Physics at the University of Texas at Austin, and a distinguished theorist, as well as a physics blogger. Along with experimentalist Gordon Watts (who covered $250) he took my $1000 bet that the LHC would not discover new
Tracked: June 27, 2013 5:01 PM

Re: I lost

One thing in this blog entry stands out for me: you remain a believer in Bayesian statistics. A lot of string theorists, at least in their communications to the general public, exhibit anti-Bayesian philosophy, e.g. “SUSY has to be right, so it is just a matter of waiting for LHC to get to higher energy levels.” This makes you a scientist in the Popper sense, a rarity these days.

Posted by: M.Wang on June 27, 2013 9:21 PM | Permalink | Reply to this

Re: I lost

One thing in this blog entry stands out for me: you remain a believer in Bayesian statistics.

I don’t know what meaning you could possibly ascribe to a phrase like “the probability that the 2015 run of the LHC will discover new physics” other than a Bayesian one.

A lot of string theorists … exhibit anti-Bayesian philosophy …

I don’t think that’s what’s going on at all. I just think that you and they have different priors. And, evidently, you don’t like theirs.

This makes you a scientist in the Popper sense…

I don’t know how you got from Bayes to Popper, but the latter has a pretty naïve view of how Science works. So, while I may be guilty of many things, that’s probably not one of them.

Posted by: Jacques Distler on June 27, 2013 11:29 PM | Permalink | PGP Sig | Reply to this

Re: I lost

IMHO Tommaso was lucky to win that bet, because there’s oodles of “new physics” just waiting to be “discovered”, but it’s within the standard model, not beyond it. I’m amazed it hasn’t come out yet. You mentioned partons, and Dan Freed mentions collaborating with you, so take a look at the tricoloured trefoil here. Then starting from the bottom left, go round anticlockwise calling out the crossing-over directions: up down up. Then take a look at this TQFT web page. Look at the top. Here’s another TQFT web page. The blue torus isn’t a proton. That’s an electron. And it’s a standing wave. It might be an idea to have another bet with Tommaso.

Posted by: John Duffield on June 28, 2013 2:41 AM | Permalink | Reply to this

Re: I lost

I’m curious what you meant by this statement:

“Moreover, I think we’d have to go back to Isabelle to find an example of an accelerator which opened up a new range of energies and didn’t find anything new.”

Since Isabelle got cancelled before it could carry out any collisions, I wouldn’t say it opened up a new range of energies. Perhaps it was designed to do so, but in the mid-70s SPS and Tevatron were working with beams of similar energy, although with fixed-target rather than collider experiments in mind. I am unsure of the history, but my reading is that it was Rubbia’s bold suggestion to retool SPS as a proton-antiproton collider, made possible by van der Meer’s stochastic cooling technique for antiproton beams, which led to the discovery of the W and Z at CERN in 1983. Meanwhile Isabelle had just emerged from a long struggle to get its superconducting magnets to work properly, and by that time motivation for continuing work on a proton-proton collider of similar energy was greatly reduced.

Posted by: Will Orrick on June 28, 2013 11:02 AM | Permalink | Reply to this

Re: I lost

Since Isabelle got cancelled before it could carry out any collisions, I wouldn’t say it opened up a new range of energies.

You’re right. My bad.

Perhaps it was designed to do so, but in the mid-70s SPS and Tevatron were working with beams of similar energy, although with fixed-target rather than collider experiments in mind.

The physically-relevant quantity is center-of-mass energy (s\sqrt{s}), not the beam energy of a fixed target experiment.

Had it been completed, Isabelle would have held that title (for a while). And would not have discovered anything. But that’s retrospective gedankenhistory, which is not what I meant to engage in.

Posted by: Jacques Distler on June 28, 2013 11:35 AM | Permalink | PGP Sig | Reply to this

Re: I lost

“The physically-relevant quantity is center-of-mass energy…”

True. The ability to use antiprotons was crucial since it allowed CERN to achieve counterrotating beams without the substantial infrastructure investment that a second proton beam would have entailed. Without that development, Isabelle might have been the first collider capable of detecting the W and Z. Fermilab might also have been competitive had they decided to use the antiproton idea in the main ring rather than waiting for the completion of the Tevatron.

Posted by: Will Orrick on June 28, 2013 1:08 PM | Permalink | Reply to this

Re: I lost

Seems to me that both you and Dorigo have an overly pessimistic view on the prospects for BSM physics being discovered in the coming years.
As you rightly point out, `new physics’ doesn’t necessarily mean directly discovering new particles (or extra dimensions or whatnot) at the LHC, but simply discovering a discrepancy (at the 5sigma level) between experiment and SM theory predictions. But for the latter you seem to be only thinking about Higgs-related stuff (e.g. the branching ratio for Higgs decay into 2 photons). There are other places where tension between experiment and SM theory has already been existing for some time. I’m thinking in particular of the “unitary triangle” for the CKM matrix elements, where 3sigma discrepancy between SM theory and experiments has been reported; see e.g. arXiv:1204.0791.

One thing I think needs to be emphasized when discussing the possibility of discrepancies between SM theory and experiment is that many of the SM theory calculations, and also the experimental measurements, still have significant error bars. So when people talk about “no disagreement between SM theory and experiment” it doesn’t mean that they really agree; it could also be that the theory calculations and experimental measurements have not yet been done precisely enough to reveal disagreements.

On the theory side, the SM is not like perturbative QED where everything can be calculated to great precision. There are also low-energy QCD contributions to the physical quantities of interest, and these need to be computed nonperturbatively via lattice QCD. It is only in recent years that high precision calculations have become possible in lattice QCD (thanks to a combination of advances in computing power and lattice QCD theory and algorithms). With the way things are going, it looks like we can expect ever-increasing precision in SM theory calculations in the coming years, and when this is combined with the greater precision of experimental measurements at the LHC and elsewhere, the prospects for pushing the SM to breaking point look not bad I reckon. In particular it will be interesting to see how things develop for the CKM unitary triangle, and more generally in the quark flavor sector of the SM, which is a promising arena for discovering BSM physics regardless of how things turn out with the Higgs.

IMHO the “serious work” in high energy physics in the last many years has been the push to compute physical quantities of the SM to ever-increasing precision, both on the theory and experimental side, with the goal being to push the SM to breaking point (which would then point, in a bottom-up way, to the new physics beyond the SM). (Well, ok, discovering the Higgs was quite a big deal too on the experimental side.) This is never highlighted to the public though, in contrast to the grandiose BSM models that the LHC was supposedly going to check out. I guess it’s partly because it is unglamorous work, with incremental advances rather than dramatic breakthroughs, and partly (mostly :-)) because it doesn’t add to the greater glory of the dominant figures in hep theory.

Posted by: amused on June 29, 2013 2:13 AM | Permalink | Reply to this

Re: I lost

As you rightly point out, `new physics’ doesn’t necessarily mean directly discovering new particles (or extra dimensions or whatnot) at the LHC, but simply discovering a discrepancy (at the 5sigma level) between experiment and SM theory predictions. But for the latter you seem to be only thinking about Higgs-related stuff (e.g. the branching ratio for Higgs decay into 2 photons).

No.

That was just the simplest-to-explain of a wide class of potential discoveries at the LHC that could not (at least not initially) be ascribed to a new particle of some definite mass (and other quantum numbers).

To pick another example, one of the main search channels at the LHC is to look for events with a large amount of missing transverse energy (ME TE_T). These are events where a putative new particle decays into stuff we can see plus one (or more) neutral particles that escape the detector.

Now, if you know something about the neutral particle(s) carrying off the ME TE_T, you can often do something. For instance, the experiments easily reconstruct leptonically-decaying W’s, because we know that the neutral particle is a massless neutrino.

In the case at hand, however, you don’t know squat about the mass(es) of the neutral particle(s) carrying the ME TE_T. So all you see is a broad excess over SM predictions.

IMHO the “serious work” in high energy physics in the last many years has been the push to compute physical quantities of the SM to ever-increasing precision, both on the theory and experimental side …

The LHC is not the place where such high-precision measurements can be made. That’s just not what it was designed for.

I did mention the ILC, running as a Higgs factory. That’s one place where such high-precision measurements can be made.

This is never highlighted to the public though, in contrast to the grandiose BSM models that the LHC was supposedly going to check out. I guess it’s partly because it is unglamorous work, with incremental advances rather than dramatic breakthroughs, and partly (mostly :-)) because it doesn’t add to the greater glory of the dominant figures in hep theory.

Sorry, whatever personal bitterness you may feel about your own work being neglected, I think you have misread the situation.

LEP II, the B-factories, maybe a future ILC, … those are machines for precision experiments. The LHC (or the Tevatron, before it) is not.

And it’s kinda natural that, when talking about the LHC, people will focus on the kind of measurements that can be made there, rather than on the kind that cannot.

Posted by: Jacques Distler on June 29, 2013 3:19 AM | Permalink | PGP Sig | Reply to this

Re: I lost

“In the case at hand, however, you don’t know squat about the mass(es) of the neutral particle(s) carrying the MET. So all you see is a broad excess over SM predictions.”

Sure, that’s also a way that new physics could show up.

“The LHC is not the place where such high-precision measurements can be made. That’s just not what it was designed for.”

I’m no expert, but it seems that is one of the main purposes of the LHCb detector.
And I didn’t just have in mind the LHC but also the other relevant experiments over the last many years, in particular the “b factories”, which have been producing increasingly precise experimental constraints on various CKM matrix elements.

“Sorry, whatever personal bitterness you may feel about your own work being neglected, I think you have misread the situation.”

Nothing personal for me since I never worked on that stuff (and am not very well-informed about it either). But I will humbly disagree that it is a misreading of the situation. Anyway, more interesting to stick to physics…

Any thoughts about prospects for new physics emerging in coming years via the CKM unitary triangle or quark flavor physics in general? Its not clear to me if your apparent pessimism about prospects of discovering new physics also includes that arena.

Posted by: amused on June 29, 2013 6:29 AM | Permalink | Reply to this

Re: I lost

Just to clarify, I do understand that the LHC is intended mostly as a discovery machine rather than for doing high precision measurements, which I guess is the point you were making.

And when I wrote about “precision measurements” in my earlier comment I had in mind not just the measurements relevant for studying CKM and flavor physics, but also the high energy scattering calculations and corresponding experimental measurements of the LHC as a discovery machine.

But I did screw up by emphasizing “precision” whereas what the LHC is mostly about is discovery. You’re right I kind of misread that part.

Anyway, the way I interpreted your original post is that, based on what the LHC as a discovery machine has found so far, you are pessimistic about BSM physics being discovered in the coming years. So the main thing I meant to say with my comment was that, despite the situation looking bleak in that regard, the prospects of new physics being discovered in quark flavor physics (“precision measurement physics”, as opposed to high energy “discovery physics”) in the coming years actually look pretty good as far as I can tell.

Posted by: amused on June 29, 2013 8:43 AM | Permalink | Reply to this

Re: I lost

And I didn’t just have in mind the LHC but also the other relevant experiments over the last many years, in particular the “b factories”, which have been producing increasingly precise experimental constraints on various CKM matrix elements.

Belle shut down in 2010; Babar shut down in 2008. Belle II is under construction, but is still several years away (AFAIK).

LHCb, being at a hadron collider, has the advantage of a huge increase in the production cross section while, at the same time, having a similarly huge increase in backgrounds. I don’t really understand the tradeoffs of that experiment well enough to comment.

Anyway, the way I interpreted your original post is that, based on what the LHC as a discovery machine has found so far, you are pessimistic about BSM physics being discovered in the coming years.

I think it’s no longer very likely that BSM physics will just leap out at you, when the LHC hits its design energy of 14 TeV. Instead, discovering new physics will be a long, hard, high-statistics slog.

I think that’s also a pretty good description of the flavour physics experiments, that you are particularly keen on.

Posted by: Jacques Distler on June 29, 2013 1:08 PM | Permalink | PGP Sig | Reply to this

Re: I lost

“I think that’s also a pretty good description of the flavour physics experiments, that you are particularly keen on.”

No doubt it will be a hard slog for that as well, although as far as time scale goes it looks like it could be the shortest path at the moment (due to advances in precision on the theory side; I don’t know what the situation is for experiments).

In any case, I’m genuinely puzzled why people aren’t more excited about the 3sigma discrepancy between SM theory and experiment already found in the CKM unitary triangle…
(and I don’t have anything personal invested in that)

Posted by: amused on July 1, 2013 2:53 PM | Permalink | Reply to this

Unitarity Triangle

In any case, I’m genuinely puzzled why people aren’t more excited about the 3sigma discrepancy between SM theory and experiment already found in the CKM unitary triangle…

I’m sorry. I guess I am behind the times.

Is there a paper more recent than this one, which I thought represented the state-of-the-art with respect to the CKM unitarity triangle?

They, of course, see no such discrepancy.

Posted by: Jacques Distler on July 1, 2013 11:31 PM | Permalink | PGP Sig | Reply to this

Re: Unitarity Triangle

If you bothered to check the paper I cited above - this one - you would see they report approx 3sigma tension in the unitary triangle. There is also a website here where the results will supposedly be updated if there are any updates. Apparantly the situation remains unchanged since end of 2011. (If it has changed significantly since then I’d have to say it’s irresponsible of those people not to have updated it.)

As for why the paper you cited does not seem to find tension in the unitary triangle, I can only speculate that they are not using all the available constraints, in particular the theoretical constraints.

Regarding the latter, the hadronic weak matrix elements that enter the analysis must be computed nonperturbatively, i.e. via lattice QCD. But reference to such computations is amusingly absent in the bibiligraphy of that paper as far as I can see.

And I noticed they write on page 3, regarding their procedure for extracting the gamma-angle, “These processes are theoretically clean provided that hadronic unknowns are determined from experiment”. Perhaps lattice QCD computations of the hadronic unknows are too theoretically dirty for them? ;-)

But I’m really not an expert or well-informed about this stuff, so there could be some later works I’m unaware of where the unitary triangle situation was found to change.

Posted by: amused on July 3, 2013 3:04 PM | Permalink | Reply to this

Re: Unitarity Triangle

Here is a recent overview paper from March 2013. The first paragraph of the introduction mentions 2-3sigma tension with the SM in a bunch of flavor physics quantities, including unitary triangle fits…

Posted by: amused on July 3, 2013 4:02 PM | Permalink | Reply to this

Re: Unitarity Triangle

Sorry, that should have been this paper

Posted by: amused on July 3, 2013 4:34 PM | Permalink | Reply to this

Re: I lost

If your “new physics” is about SUSY (with s-particles) of some sort, you are a loser. If the new boson (125 Gev.) is a Higgs of some types, you are a loser.


Yet, a comment at Matt Strassler blog (http://profmattstrassler.com/2013/07/04/happy-independhiggs-day/#comment-67689 ) might be able to save you on your $750 bet.

Posted by: Tienzen (Jeh-Tween) Gong on July 7, 2013 11:32 AM | Permalink | Reply to this

Re: I lost

What do you think of this paper? Is it a breakthrough?

http://arxiv.org/pdf/1307.3228.pdf

Posted by: Eddy on July 11, 2013 10:20 PM | Permalink | Reply to this

Re: I lost

On a previous post, under a variety of names, you asked Jacques this question about ten times, mostly about loop-quantum-gravity papers like this one. Are you trying to punish him for briefly engaging with that school of research back in the mid-00s? What do you actually want from him - do you want him to make a technical judgment regarding every new paper that comes out from that direction? Does he need to formally state his general disinterest in the subject? Surely his silence indicates this.

Posted by: TiredOfTheTroll on July 11, 2013 11:12 PM | Permalink | Reply to this

Re: I lost

I’m very sorry. I didn’t know this. I was just wondering what his opinion was. Not necessarily on the entire paper, just the abstract. My apologies. I wasn’t trying to troll.

Posted by: Eddy on July 12, 2013 1:10 PM | Permalink | Reply to this

Re: I lost

Hi there

Can you explain “loop-quantum-gravity” in plain-English?

Cheers

Peter Goudge

Posted by: Peter Goudge on July 30, 2014 10:05 AM | Permalink | Reply to this

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