Searches at the LHC and the discovery of theHiggs Boson

Christos Leonidopoulos, University of Edinburgh

March 18, 2014

Producing the Higgs boson at a hadron collider

One of the most characteristic features of a hadron collider (such as the proton-proton collider at the LHC) is the extremely high interaction rates. In theanimation accompanying the lecture, you will see the collision of two individualprotons inside the detector, producing a Higgs boson candidate1. In reality, thetwo colliding protons are never in isolation. Protons at the LHC are grouped inbunches containing more than 100 billions protons each. Moving practically atthe speed of light, the protons collide as the bunches transverse each other.

1The reason that we talk about a “Higgs boson candidate”, and not simply a “Higgsboson” is that we can never know with 100% certainty if a particular “Higgs-looking”collision corresponds to the decay of a true Higgs particle or a background “look-alike”.Physicists carry out a statistical analysis of the full set of “Higgs boson candidates” anddetermine the probability that any given collision could have generated a Higgs boson ora “Higgs-looking”, background event.

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Hadron colliders: a harsh experimental environmentWhen we have two such beams of highly concentrated and highly energetic protonscolliding against each other, there are several complications for the experimentalenvironment in which we hope to detect the Higgs boson:

• We observe, on average, the staggering rate of approximately one billionproton-proton (pp) collisions per second. If we are lucky, maybe one ofthese collisions will produce a Higgs boson. A more accurate statement isthat we expect on average one Higgs boson to be created for every fourbillion “background” collisions (i.e. collisions not producing a Higgs boson).

• The fragments from these collisions typically leave traces (e.g. ionisingtracks or scintillation light) in dedicated subdetectors. This digitisedactivity inside the experimental apparatus captured within a time window of25 ns2 is defined as a collision “event”. More often than not, the detectorrecords events that contain multiple collisions, with the average number ofcollisions per event depending on the running conditions. Obviously, thelarger the number of multiple collisions per event, the harder the task ofidentifying a Higgs boson decay, as its decay will overlap with several othercollisions’ fragments. At the end of the 2012 pp LHC run, the number ofmultiple collisions per event was greater than 20. When the LHC resumesthe pp runs in 2015, this number will be closer to 50.

2One ns is a billionth of a sec.3 / 20

Physics production rates at hadron colliders

One of the challenges here is that we are looking for evidence for a very rareevent, the Higgs production and decay. Let’s look at some concrete numbers.

The table on the left shows the productionrates of various processes at the LHC fornominal running conditions. The total pro-duction rate, representing the full activity in-side the detector, is approximately 1 GHz(109 Hz). Production of W (Z) bosons thatdecay to leptons occurs at approximately 150(20) Hz. The truly “exciting” physics is evenmore rare: New Physics that would mani-fest as new, exotic (i.e. hypothetical) particles(e.g. heavy “cousins” of the known Z bosonand the gluino, notated as Z ′ or g) , or amuch heavier (hypothetical) Higgs boson at500 GeV would only be created at a fractionof a Hz.

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Hadron colliders and the TriggerThe natural question to ask is how we manage to “fish out” the interesting physicsout of the massive rate of less-exciting collisions. The answer is with the trigger :a very sophisticated hardware and software system which one can think of as a100 megapixel digital camera that takes snapshots of the detector activity every25 ns.

Based on these detector signals, the trigger algorithms choose the collisions thatlook most “interesting”. This is a highly calculated selection of the most promisingtiny slice of data for capturing a Higgs boson, or New Physics for that matter. Theoutput of this filtering process is a few hundred events per second. The reasonfor keeping only a relatively small number of events is simply limited resources.

Taking into account that the total collision rate is one billion Hz, we see that we

are talking about a one-in-10-million selection mechanism. In other words, the

trigger throws away 99.9999% of all physics interactions that we have

managed to create with pp collisions inside the detector. The collaboration will

only be able to analyse and search for the Higgs boson in the tiny fraction of

collisions surviving the trigger. When a trigger decision is made to reject a

particular collision, it is made for ever. If a mistake is made in this trigger

filtering process, there is no “undo” button. The collision and the physics

interactions that it contained are lost forever.5 / 20

Hadron colliders and the TriggerThis explains why these experiments, these highly sophisticated digital cameras,have to be so massive and complex, with tens of millions of channels, reading outsignals from state-of-the-art detectors. Before we ever get a chance to look forthe Higgs boson, the experiments have to make some very hard decisions aboutwhich collisions should be kept. These decisions have be to made extremely fast,leaving practically no room for errors.

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The ATLAS ExperimentThe two experiments at the LHC that participated in the discovery of the Higgsboson are the ATLAS and CMS collaborations.

The ATLAS detector is 45 metres long, 25 metres in diameter, and weighs about7,000 tons. The collaboration consists of more than 3,000 physicists from all theover the world.

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The CMS Experiment

The CMS detector is more compact in size. It is 25 metres long, 15 metres indiameter, but weighs even more than the ATLAS detector, about 12,500 tons.Similarly, the CMS collaboration has more than 3,000 physicists from all over theworld.

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The ATLAS and CMS Experiments

At a particle collider, it is not at all unusual to build two experiments looking fordiscoveries, instead of one. This co-existence of experiments, besides promotinga healthy (and mostly friendly) competition among physicists, it serves anothervery important purporse: the need for independent confirmation of any claim ofa scientific discovery.

It is difficult, but not entirely impossible for a collaboration to make a false discov-ery claim. It is much more unlikely to have two independent collaborations makethe same mistake. This mechanism of independent confirmation or rejection ofan experimental observation is the cornerstone of the scientific method.

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The Higgs decay channels

When the search for the experimental discovery of the Higgs boson started, physi-cists did not have much of a clue about its mass value. But the theoretical toolsthat we have allow us to calculate the probability that a Higgs will decay to agiven set of particles as a function of the hypothetical mass. This is demonstratedin this plot3.

One can see that the probability thatthe Higgs decays to a b-quark and itsantiparticle (b), or two W bosons,or two Z bosons, or two τ leptonsor two photons is very different fordifferent hypothetical values of theHiggs mass. Since we don’t knowwhat the Higgs mass is, we have tolook at several different channels tomake sure that we will be able toobserve the Higgs decay no matterwhat.

3x-axis: hypothetical mass values for the Higgs boson in GeV (or GeV/c2); y -axis:probability that the Higgs boson will decay to a partiular final state.

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A Higgs boson candidate decaying into two photonsThe animation shown in the video lectureis an example of a Higgs boson candidatedecaying into two photons. The two pho-tons are represented by the two solid greentowers. The very energetic photons getfully absorbed by a specialised detector:the calorimeter. The calorimeter mea-sures the deposited energy of the photons.When combined, the energy of the two-photon system can give us the mass ofthe Higgs boson candidate.

This animation depicts a single collision giving us a single measurement for themass of a potential Higgs boson. We still do not know if this individual collisionwas a Higgs boson or not4. We collect thousands upon thousands of similarcollisions in order to better quantify the hypothesis that Higgs bosons are indeedcreated and observed inside the detector. The values for the reconstructed massof all these Higgs candidates decaying into pairs of photons are then put into asingle plot.

4And we never will!11 / 20

Di-photon mass distributions

The diphoton mass distribution made by the ATLAS (CMS) collaboration can beseen in the plot on the left (right). A statistical analysis is needed to evaluate anypotential deviation that stands out from the background that looks like a Higgs.The goal is to quantify the probability, in the event that there is no Higgs, thatwe could have a fluctuation caused exclusively by the background that looks somuch like a Higgs peak; and, if there really is a Higgs boson, to determine itsmass. This is typically at the place of the observed deviation. We can see veryclear deviations in both plots, appearing around 125 GeV for both experiments.

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A Higgs boson candidate decaying into two Z bosons

The second animation included in the videolecture shows a Higgs boson candidate de-caying to two Z bosons. One of theZ bosons decays to an electron and apositron. These are again depicted as solidgreen towers. They look very similar tophotons, but these particles are charged, sothey leave ionisation traces in a dedicateddetector made of semiconductor material.

The second Z boson decays to a muon and an anti-muon pair. Muons interactvery weakly with the detector. They are able to travel very far without beingabsorbed by the calorimeter, like the electrons or the photons. They leave hits inanother dedicated detector that can be used to reconstruct their tracks (picturedhere as blue lines) and evaluate their momentum. When we combine the energyof the electron and the positron, and the momenta of the reconstructed muons,we can calculate the mass of the Higgs candidate.

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Di-Z mass distribution by ATLAS

This plot combines all the potential Higgs decays to two Z bosons for ATLAS.The plot looks very different than the diphoton distribution shown earlier. Thisis a cleaner channel (i.e. with smaller background), with fewer events and a verydifferent background shape. The three coloured histograms (light blue, orangeand grey) are hypothetical, simulated Higgs signals with different mass values.The black points correspond to the experimental collision data and they agreewell with a simulated Higgs signal with a mass of 125 GeV.

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Di-Z mass distribution by CMS

A similar distribution produced by CMS can be seen in this plot. CMS has chosenhere to zoom in at the place of the observed deviation. The red-line histogramis again a hypothetical, simulated Higgs distribution that matches very well theexperimental data (black points) around 126 GeV. The value of 126 GeV is com-patible with the value reported by ATLAS. The two measurements are consistentwith each other within the experimental resolution.

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Deviations observed. Now what?

By now, we have observed deviations from the background in different channelsin the Higgs boson searches. But if there is a Higgs boson that decays to multiplechannels, the individual deviations that we observe must give a coherent picture.This means two things: not only must they provide cumulative evidence of a Higgsboson, but they must also give mass measurements that are consistent with eachother.

Furthermore, we have two independent experiments with very similar sensitivity. Ifone experiment claims a discovery, and the discovery is real, it would be extremelyunlikely that the other experiment does not observe anything. If both experimentsclaim a discovery, they also have to agree on the Higgs mass.

Can we combine this information from the individual channels to quantify howunlikely it is to see these deviations if there were no Higgs? This is an extremelydifficult problem that requires very complex calculations and tedious cross checks.Describing the method used for calculating these probabilities is beyond the scopeof this course. However, we can take a closer look at the results.

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Evaluation of deviations by CMS

The results presented by CMS are shownin this plot. A channel with very high sen-sitivity is the one with the two Z bosonsdiscussed earlier and appears with the redcolour. The red curve shows that the prob-ability to observe such a deviation withouta real Higgs behind it is smaller than 1 in a1000 (as determined by checking that theminimum of the red curve reaches belowthe value 10−3, as read off the y -axis onthe left).

Even more sensitive is the diphoton channel depicted with the green colour. Thecorresponding probability is almost 1 in 100,000 (or 10−5). The combination,with the black colour, shows that the probability that we would observe thesedeviations in all channels together if there were no Higgs is smaller than 1 in amillion (or 10−6). What is important here is that the incorporation of multiplechannels contributes constructively in the evidence for the existence of the Higgs,and that the individual deviations appear at the same mass value.

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Evaluation of deviations by ATLAS

This is, similarly, the combined plot for ATLAS extended over a wide mass range.When looking at the full mass spectrum, we can observe smaller or larger deviationsfor several mass values. The deviations between 200 and 300 GeV do not suggestthat there is a Higgs, as they are perfectly consistent with a fluctuation causedexclusively by background. There is a very large deviation, however, around 125GeV (the deep inverse spike on the left). The calculations show that, if therewere no Higgs, the combined probability to observe all these deviations in thedifferent channels is almost as low as one in 10 million (or 10−7). The experimentalobservation crosses a standard, conventional threshold for claiming the discoveryof a new particle that is referred to as a 5-sigma deviation.

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Announcement of the discovery of the Higgs boson

The experimental evidence for the discovery of the Higgs boson was presentedon July 4, 2012 in a joint seminar by ATLAS and CMS at CERN. The audienceincluded Francois Englert, Peter Higgs and the other prominent theorists (Gu-ralnik, Hagen and Kibble) who worked on the theory almost 50 years earlier 5.The announcement of the discovery was met with standing ovation, somethingextremely unusual in particle physics seminars.

5With the exception of deceased Robert Brout.19 / 20

After the discovery, what?

The Higgs discovery was the last missing piece of the Standard Model, a systemof sophisticated equations that can predict a very large range of phenomena withvery high accuracy. The Standard Model is now considered a completed theory.

But here’s the great paradox. It turns out that the powerful equations thathave been introduced in this course describe extremely well just 5% of the knownUniverse. We now know that dark matter and dark energy make up the vastmajority of the cosmos, namely 95% of it. What exactly is dark matter and darkenergy? We don’t know! How is it possible that we understand 5% of the Universeand everything we see so extremely well, but know absolutely nothing about whatmakes up most of the Universe? We don’t know!

Are there yet more particles that we have not discovered yet? Is there a connectionbetween the Higgs boson, the potentially new particles and the dark matter? Theseare some of the questions that we are trying to answer at the LHC. We think thatmeasuring the properties of the Higgs particle could give some clues that wouldhelp us begin to address these questions.

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