by Tommaso Dorigo

Lubos Motl published the other day in his crazily active blog a very nice new review of “Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab“. The review is authored by Tristan du Pree, a colleague of mine who has worked in CMS until very recently – now he moved to a new job and changed to ATLAS! (BTW  thanks Lubos, and thanks Tristan!)

I liked a lot Tristan’s commentary of my work, and since he mentions with quite appreciative terms the slow-motion description of a peculiar collision I offer in my book, I figured I’d paste that below. But before I do that, let me mention a few things that have been going on around me lately, and on which I will post more in the next few days.

– I am back from my honeymoon in the Dominican Republic. The picture below shows I’ve even made friends there!


– I survived an emergency landing in JFK as I was flying back from Atlanta to Paris the other day. Well, maybe that’s putting it a bit more dramatically than needed, but the B767-400 I was on suffered a hydraulic system failure and had to U-turn on the Atlantic to head back to New York. As we approached JFK I was not very concerned (the closest airport to the U-turn point had been Boston, so I figured the situation was not terribly bad) but OTOH the pilot’s report had scared me – he was speaking with a broken voice and sounded on the verge of a nervous breakdown. Seriously, WTF? Don’t they train pilots to sound confident and in control these days?

– In the past few days, together with five of the students from the AMVA4NewPhysics network, I gave public lectures in three high schools, to inspire students to take on a challenge: we are daring them to produce artwork inspired by particle physics, along the project of the EU CREATIONS network (see here). The best works will be on display during the EPS 2017 conference in Venice this July, and the three best will receive consumer electronics prizes.

– From today onwards, and for a full month, I will be twittering from the @CMSvoices account – a one-month shift on twitter. Follow me there! (My regular account is @dorigo).

So, as I promised, below please find a description of “the impossible event”, the in medias res start of Chapter 8 of my book. Enjoy!

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Suddenly an uncommon reaction takes place in the detector. It happens on April 28th 1995, in the middle of an otherwise anonymous store. CDF is collecting good data, and the shift crew in the control room takes care of the usual business: keeping an eye on the colorful monitors that plaster the walls, checking trigger rates, logging the warnings issued by the data acquisition system, and answering e-mails.

An improbable chain of events

As a proton and an antiproton run into each other, one red down-quark in the proton carries for an immeasurably small instant a large fraction of the total energy of its parent. The red down quark gets on a collision course with an anti-up quark from the antiproton which is also endowed with large energy. The anti-up quark’s color is anti-blue: in total the quark-antiquark pair has a net amount of color charge. Yet before the two bodies get close enough to interact, the antiquark chances to emit an energetic gluon. The gluon carries away the anti-blueness of the anti-up quark, transmuting it into anti-redness. This allows the now colorless quark-antiquark pair to turn into a W boson, endowed by its parents with a negative unit of electric charge and with energy far exceeding its rest mass. The boson instantly shrugs off some of that extra energy by emitting an energetic photon. Then it disintegrates, yielding an electron-antineutrino pair. The electron immediately emits a second energetic photon.

The proton and the antiproton generating the collision have both been deprived of one of their quarks and are now colored. As they leave the interaction point they break apart, creating two streams of low-energy hadrons that fly off along the beam pipe. The energetic gluon emitted by the anti-up quark extends the color string that still connects it to the antiproton remnants until the string breaks, yielding two charged pions. One of the two pions receives only a very small share of the energy, and ends up spiraling within the beam pipe. The other pion is conversely quite energetic, and it heads straight into the plug calorimeter after leaving a trace of its passage in the SVX.

Once it reaches the calorimeter the pion withstands a peculiar reaction: it impinges on a lead nucleus where it transfers its up quark to a neutron, receiving a down quark in exchange. This turns it into a neutral pion, while the neutron becomes a proton. The former lead nucleus, now turned into bismuth, immediately breaks apart into lighter nuclear fragments. The neutral pion only manages to tread five microns or so in the lead slab and then decays into two photons; these in turn produce an electromagnetic cascade and further light flashes in the scintillator of the calorimeter. The charged pion has performed an illusionist’s trick, one dreaded by experimentalists: a reaction called charge exchange. What is observable in the detector is a track in the silicon layers, pointing to an electromagnetic energy deposit in the calorimeter. Such a combination is indistinguishable from the signal that would be expected from an energetic electron.

Let us now return to the other three energetic particles produced by the W boson: the real electron, the antineutrino, and the two photons. They move out of the interaction point in different directions, heading toward the tracking system. The neutrino zips unhindered through the sensitive material. The electron leaves a stream of ionization in the gas of the central tracking chamber, and once it enters the calorimeter it produces a well-localized energy deposit. As for the energetic photons, they meet a similar end: they traverse the CTC unseen, but as soon as they enter the calorimeter they initiate additional electromagnetic showers. From the amount of released light in the scintillators at the locations where showers have taken place, experimentalists will be able to estimate the energy of the electron and photons.

After the above confusing chronicle, it is useful to take stock. What remains of the hard collision is the signal of one real electron and two energetic photons, plus a further spurious electron signal originated by the charged pion. In addition to that, the W decay neutrino has left the detector carrying away a significant amount of momentum unseen, so the momenta of observed particles do not add up to zero in the plane transverse to the beams direction. There is thus a momentum imbalance, a significant amount of missing transverse energy which betrays the neutrino escape. All in all, what experimentalists have in their hands is a spectacularly improbable event: one with two electrons, two photons, and a significant amount of missing transverse energy. It is going to be dubbed e-e-gamma-gamma-met event by CDF physicists, but a better name for it would be “the impossible event.”