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Nothing new from the dielectron spectrum February 20, 2007

Posted by dorigo in news, physics, science.

CDF released a few days ago a new result in the search of high-mass resonances decaying to electron-positron pairs, based on 1.3/fb of Run II data. There are quite a few models of new physics that may yield the searched signature: lots of Z’ models, and Randall-Sundrum gravitons.

The search is quite straightforward from an experimental standpoint, and indeed you can see in the plot below that the mass spectrum is simply understood as the sum of the dominant Drell-Yan production – with the outstanding Z boson at 91 GeV- and fake electron processes, which contaminate the sample at a minimum level.

The agreement of data with Standard Model processes is quite nice, or disturbing, depending on what side you are rooting for. Indeed, no fluctuation in the data appears anywhere in the spectrum, as quantitatively described by the following plot:

In it, you can see that the minimum agreement of the data with expectation occurs at 370 GeV, where the probability of observing as many events as the data is of the order of 1%. Strange ? Absolutely not: quite in line, in fact, with the expected range of minimum observed probability (the region boxed in blue), given that we are checking a wide spectrum.

So from the agreement Z’ models – which allow the computation of an expected cross section for these resonances, as a function of their unknown mass – can be ruled out: you can see it in the following plot.

You can see that the black curve, which is the limit on the cross section of a Z’ decay to a electron-positron pair (the y axis) obtained from the data, is lower than the expected cross section for all the Z’ models up to 730 GeV (for the model which predicts the smallest cross section).

Also interesting is to compare the prediction for a Randall-Sundrum graviton with the cross section limit. It is done in the following plot:

Here, to plot the data several choices were made on the ratio k/Mp, a parameter of the model. K describes the “scale of warping of the extra dimension”, and Mp is the effective Planck scale. You can see that more warping means a less observable graviton – so we can exclude a RS graviton up to 820 GeV if k/Mp=0.1, but much less so if k is smaller.

All in all, just another nice agreement with SM and Tevatron data. The 1000 dollars I bet on the lack of new Physics at the LHC feel comfortable in my safe.



1. island - February 20, 2007

Wow, that plot is nearly identical to the trail of sweat beads on Jacques’ forehead… 😉

2. dorigo - February 20, 2007

I think it will take a long time to settle our bet, but I really admire those who still cling to the hope that new physics is just a couple of bins farther out in a plot, or at a parameter value just below what has been probed so far…


3. island - February 20, 2007

Unfortunately, Tommaso, I have also come to a similar conclusion, having frequently dealt with the dogma that surrounds many assumptions that, by rights, should always be subject to highly suspect review, given new physics, but are not.

I predict that all sides will hold onto their belief systems until the funding is completely gone.

Only then will they be willing to consider going all the way “Back to 1917”… to get the correct vacuum solution for the negative energy states and a valid theory of quamtum gravity in a constantly changing, yet finite, background.

I’ll be long-dead, so somebody say I told you so for me, please?

4. andrew - February 20, 2007

So if there’s no new physics to be found at the LHC, and (therefore) no ILC, do we all pack up and go home?

5. dorigo - February 20, 2007

Well Andrew,

I guess the ball will stay steady in the theorists’ court then. As for HEP experimentalists, they’ll be likely to switch to astroparticle physics – or head for an early retirement.

Jokes aside, I think there’s a whole lot to learn about QCD at low energy, neutrinos, and high energy nuclear physics to keep us busy.


6. a - February 21, 2007

dear Tommaso,

Z’ are also indirectly constrained by global fits of precision data: for a “typical” Z’ these constraints are significantly stronger than the sensitivity of CDF direct searches. In other words CDF (and D0) are mainly exploring regions already indirectly disfavored by previous experiments.

7. dorigo - February 21, 2007

Hi A,

That is a good point which I failed to explain in the post, just as much as the fact that the limits shown are only marginally stricter than previous ones from other searches including different final states.

It does not, in my opinion, change the picture I drew at the end much, though. Direct searches, as you know, in the past did stumble into something unpredicted – by the model you compared the data to, or by indirect information. The search for a mass bump is one of the simplest things one can do, and so it is the most convincing proof that all is well.


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