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## Archive for 2016

### Enough data to explore the unknown

Friday, June 17th, 2016

The Large Hadron Collider (LHC) at CERN has already delivered more high energy data than it had in 2015. To put this in numbers, the LHC has produced 4.8 fb-1, compared to 4.2 fb-1 last year, where fb-1 represents one inverse femtobarn, the unit used to evaluate the data sample size. This was achieved in just one and a half month compared to five months of operation last year.

With this data at hand, and the projected 20-30 fb-1 until November, both the ATLAS and CMS experiments can now explore new territories and, among other things, cross-check on the intriguing events they reported having found at the end of 2015. If this particular effect is confirmed, it would reveal the presence of a new particle with a mass of 750 GeV, six times the mass of the Higgs boson. Unfortunately, there was not enough data in 2015 to get a clear answer. The LHC had a slow restart last year following two years of major improvements to raise its energy reach. But if the current performance continues, the discovery potential will increase tremendously. All this to say that everyone is keeping their fingers crossed.

If any new particle were found, it would open the doors to bright new horizons in particle physics. Unlike the discovery of the Higgs boson in 2012, if the LHC experiments discover a anomaly or a new particle, it would bring a new understanding of the basic constituents of matter and how they interact. The Higgs boson was the last missing piece of the current theoretical model, called the Standard Model. This model can no longer accommodate new particles. However, it has been known for decades that this model is flawed, but so far, theorists have been unable to predict which theory should replace it and experimentalists have failed to find the slightest concrete signs from a broader theory. We need new experimental evidence to move forward.

Although the new data is already being reconstructed and calibrated, it will remain “blinded” until a few days prior to August 3, the opening date of the International Conference on High Energy Physics. This means that until then, the region where this new particle could be remains masked to prevent biasing the data reconstruction process. The same selection criteria that were used for last year data will then be applied to the new data. If a similar excess is still observed at 750 GeV in the 2016 data, the presence of a new particle will make no doubt.

Even if this particular excess turns out to be just a statistical fluctuation, the bane of physicists’ existence, there will still be enough data to explore a wealth of possibilities. Meanwhile, you can follow the LHC activities live or watch CMS and ATLAS data samples grow. I will not be available to report on the news from the conference in August due to hiking duties, but if anything new is announced, even I expect to hear its echo reverberating in the Alps.

Pauline Gagnon

To find out more about particle physics, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN », which can already be ordered from Oxford University Press. In bookstores after 21 July. Easy to read: I understood everything!

The total amount of data delivered in 2016 at an energy of 13 TeV to the experiments by the LHC (blue graph) and recorded by CMS (yellow graph) as of 17 June. One fb-1 of data is equivalent to 1000 pb-1.

### Assez de données pour explorer l’inconnu

Friday, June 17th, 2016

Le Grand collisionneur de hadrons (LHC) du CERN a déjà produit depuis avril plus de données à haute énergie qu’en 2015. Pour quantifier le tout, le LHC a produit 4.8 fb-1 en 2016, à comparer aux 4.2 fb-1 de l’année dernière. Le symbole fb-1 représente un femtobarn inverse, l’unité utilisée pour évaluer la taille des échantillons de données. Tout cela en à peine un mois et demi au lieu des cinq mois requis en 2015.

Avec ces données en réserve et les 20-30 fb-1 projetés d’ici à novembre, les expériences ATLAS et CMS peuvent déjà repousser la limite du connu et, entre autres, vérifier si les étranges événements rapportés fin 2015 sont toujours observés. Si cet effet était confirmé, il révèlerait la présence d’une nouvelle particule ayant une masse de 750 GeV, soit six fois plus lourde que le boson de Higgs. Malheureusement en 2015, il n’y avait pas suffisamment de données pour obtenir une réponse claire. Après deux ans de travaux majeurs visant à accroître sa portée en énergie, le LHC a repris ses opérations l’an dernier mais à faible régime. Si sa performance actuelle se maintient, les chances de faire de nouvelles découvertes seront décuplées. Tout le monde garde donc les doigts croisés.

Toute nouvelle particule ouvrirait la porte sur de nouveaux horizons en physique des particules. Contrairement à la découverte du boson de Higgs en 2012, si les expériences du LHC révèlent une anomalie ou l’existence d’une nouvelle particule, cela modifierait notre compréhension des constituants de base de la matière et des forces qui les régissent. Le boson de Higgs constituait la pièce manquante du Modèle standard, le modèle théorique actuel. Ce modèle ne peut plus accommoder de nouvelles particules. On sait pourtant depuis des décennies qu’il est limité, bien qu’à ce jour, les théoriciens et théoriciennes n’aient pu prédire quelle théorie devrait le remplacer et les expérimentalistes ont échoué à trouver le moindre signe révélant cette nouvelle théorie. Une évidence expérimentale est donc absolument nécessaire pour avancer.

Bien que les nouvelles données soient déjà en cours de reconstruction et de calibration, elles resteront “masquées” jusqu’à quelques jours avant le 3 août, date d’ouverture de la principale conférence de physique cet été. D’ici là, la région où la nouvelle particule pourrait se trouver est masquée afin de ne pas biaiser le processus de reconstruction des données. A la dernière minute, on appliquera aux nouvelles données les mêmes critères de sélection que ceux utilisés l’an dernier. Si ces évènements sont toujours observés à 750 GeV dans les données de 2016, la présence d’une nouvelle particule ne fera alors plus aucun doute.

Mais même si cela s’avérait n’être qu’une simple fluctuation statistique, ce qui arrive souvent en physique de par sa nature, la quantité de données accumulée permettra d’explorer une foule d’autres possibilités. En attendant, vous pouvez suivre les activités du LHC en direct ou voir grandir les échantillons de données de CMS et d’ATLAS. Je ne pourrai malheureusement pas vous rapporter ce qui sera présenté à la conférence en août, marche en montagne oblige, mais si une découverte quelconque est annoncée, même moi je m’attends à entendre son écho résonner dans les Alpes.

Pauline Gagnon

Pour en apprendre plus sur la physique des particules, ne manquez pas mon livre « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels » disponible en librairie au Québec et en Europe, de meme qu’aux Editions MultiMondes. Facile à lire : moi, j’ai tout compris!

Graphe cumulatif montrant la quantité de données produites à 13 TeV en 2016 par le LHC (en bleu) et récoltées par l’expérience CMS (en jaune) en date du 17 juin.

### Lab news

Friday, April 1st, 2016

Get the latest news from the world’s biggest science lab! All the facts, all the truth, totally verified and true beyond all reasonable doubt. 85% official news. Brought to you by the team that revealed Elvis landing on the moon.

### ATLAS to install neutrino calorimeters

The ATLAS detector is currently the largest experiment on the CERN site, weighing over 7,000 tonnes, spanning 50 m across and almost 50 m long. It can detect nearly all particles produced in the record breaking high energy collisions provided by the LHC. These particles have strange names like the electron, proton, pion, Ξ(1530)3/2+, photon, friton, demi-semi-lepton and Boris. But there is a big problem, which becomes more pressing as we reach higher and higher energies, and that is the neutrino. This is a tiny, neutral, almost massless particle that was predicted in 1930, and it comes in different flavours (the most popular being mint.) The ATLAS Collaboration has an ambitious plan to extend the capabilities of its detector by being the first such general purpose detector to install neutrino calorimeters. At the moment a neutrino is seen as “missing transverse energy”, and that makes it really hard to find new particles.

ATLAS Spokesperson, Dave Charlton, said “Look I really don’t have time for this, I have to go to a meeting!”. After reporters blocked his path and stole his CERN card he added “Fine, how about ‘This is a very exciting time for ATLAS and we are happy to be leading the field in this area. Detecting neutrinos will open up new parameter space and allow to perform searches never seen before.’ Now give me my CERN card, the Weekly meeting cannot start without me.” By seeing neutrinos directly, physicists would be able to observe the annoying neutrino backgrounds that get in the way of dark matter searches. They could count the neutrinos directly to see if they agree with long standing predictions.

Proposals for the new ATLAS neutrino calorimeters

But not everyone is happy with the proposal. We spoke to a neutrino expert, and after she closed the door on us, we went to Wikipedia. Apparently neutrinos are so bad at interacting that they need about one light years of lead before they can be seen. This would have some impact on the local (and not so local) area. We spoke with a representative from Geneva Airport. He said “If the proposed plans are succesful this would mean moving Geneva Airport. The people and businesses of Geneva rely on the airport for connections with the rest of the world. It would be very inconvenient and not very efficient to commute a light year to reach the airport. Most rental car contracts will not allow you travel that far.”

It’s not yet clear where the supply of lead will come from. A sphere of solid lead would contain more than the global supply, even if every atom was liberated from the Earth’s crust. We would need 38 orders of magnitude more than there is on the planet. That’s more than a million million million. It’s lots. There is also a problem with the sheer size of the proposal. “There are problems we still have to solve”, said an ATLAS physicist “We have a Solar Passage Working Group, and NASA is helping us deal with other local astronomical bodies that might pose impact challenges. Trigger is an issue. Right now it takes about 100 milliseconds to trigger an event. With the new neutrino calorimeters it could take up to 3 years.”

The proposals, if approved, will be implemented by 2600.

### CMS developing “truth matching” for data

For decades the CMS Collaboration has used a common tool known as “truth matching” with its simulation studies. Every particle in a simulation has information associated with it, including its mass, energy, charge, momentum, spin, and favourite movies. All these quantities have to be estimated using measurements from the simulated detector, so they are never perfectly known. However with a simulation you can match up the particles to what really happened with the so-called “truth record”, and this is what we call truth matching. If you have a particle travelling with a certain momentum in a certain direction you can compare it to the truth record and find out exactly what kind of particle it is. That means you no longer need those tricky identification algorithms, and you can remove background processes trivially.

“This makes my analysis super easy!” said one CMS student. “I might even graduate next week.” Truth matching has been applied to simulations for several decades, and it it is unique in being the only method that has not also been applied to data. Everything else, from machine learning to Bayesian analysis, have been developed using simulation before being moved over to real data. By employing ouija boards, dowsing techniques, and Feng Shui, CMS psychics have reported initial success. “There are definitely a lot of protons in the LHC beam.” one said. The LHC beam does indeed contain about a million million protons per bunch, and this has been seen by some as a sign of confirmation of the method. Others are more skeptical. “Those protons could have come from the magnets or the pipes. There’s a lot of matter in these tunnels. The results prove nothing.”

One of the first complete data events to be truth matched, a diphoton Higgs decay

If the truth matching of data is successful, it could lead to a revolution in particle physics. Detectors could be slimmed down, time could be saved in the analysis process, and the peer review process would be streamlined. “Rather than having to measure the levels of signal and background, a process that can take months, we can simply count how many electrons bosons we have.” The initial findings are only the first step, and there are plans to extend the data truth matching to more complex final states. It’s expected that by 2019 the CMS Collaboration will be able to truthmatch Higgs bosons, top quarks, and even new particles we’ve never seen before.

A tearful Polish professor, who pioneered the use of the famous ‘pseudorapidity’ variable said “I have been waiting for this breakthrough my entire career. This will make the lives of so many scientists so much simpler.”

### LHCb made a big blunder, and you won’t believe what it is!

Senior LHCb physicists were left red faced today when they discovered a terrible blunder. “How could we not have seen this?” Spokesperson Guy Wilkinson said. “It’s been staring us in the face for years” blurted Operations Coordinator Barbara Storaci.

LHCb, a huge science machine that lives underground on the Franco-Swiss border, is hiding a huge secret. Sources on reddit tell us “This kind of hting happens al the time. The Eiffel Tower was bilt up-side-down for the frist few weeks.” and “OMG! WTF? ORLY?”

Can you see what’s wrong with this picture? 98% of people can’t!

The LHCb schematic, with the approved geometry

It turns out that when LHCb was made, the engineers only built half a detector. “Now I see it I can’t unsee it!” exclaimed a postdoc, spilling crepe on the table as he spoke.

“It may be true that we only built half a detector”, an anonymous researcher said “but at least it was the forward half.” So far there are no plans to correct the problem, and the Collaboration has already produced hundreds of world class papers with the current detector and shows no signs of stopping.

### ALICE alchemists quit after years of research

A team of alchemists working on the ALICE Collaboration have today announced that their research program will end today. The collection of six pesudoscientists, a small minority of the total Collaboration, are hanging up their lab coats after declaring their research “unworkable” and “a total abysmal failure”. The ALICE Collaboration investigates the collisions of Lead ions with other particles in the LHC. The Collaboration has been responsible for a wide range of discoveries concerning the quark-gluon plasma, which is a form of primordial matter from the early universe.

The STAR experiment contained real Gold atoms

However it is not the quark-gluon plasma that the small band of alchemists are studying. Instead they want to turn the Lead into Gold, and they want to use the LHC to do it. Most of them came from the previous generation of ion collider experiments, based in Brookhaven, New York. At those facilities there was an abundance of Gold in the experimental apparatus, and it the alchemists looked to replicate this success.

“I just don’t understand” said Bob Bobbatrop, the Master Mage “we had so much success with the RHIC accelerator! The LHC must be producing negative energy fields and the crystals in our detector must be misaligned.” ALICE Spokesperson, Paolo Giubellino, was quick to distance himself from the misfit alchemists. “They are not representative of the Collaboration as a whole, and frankly, I don’t know how they got in here in the first place. The RHIC facility in Brookhaven collided Gold ions, so of course these so-called alchemists found Gold. They’d have to be even stupider not to find it there! This is why we have a peer review process. We’ve even started to arrange psuedomeetings in a local coffee shop where they present their results, and they haven’t yet noticed that most of the people listening are tourists. Even the local barista rolls her eyes when they talk. Meanwhile we can get on with the real research.”

But like a gauge violating wavefunction, Bob Bobbatrop is not phased. “We have vastly superior software! When we need a random number we don’t rely on a C++ library, we use a 20 sided die. You can’t get more serious than that.”

### Cryogenics team start charity drive

Do you have any old, unwanted fridge magnets? You can send them to CERN! Last year the cryogenics team at CERN faced problems that lead to the failure of some magnets. Now, a charity drive is starting where you can donate your old magnets, and these will be attached to the outside of failing magnets to give them a boost. “We accept any magnets! That magnet you purchased on vacation? Yes, we’ll take it. Do you have magnetic letters? We will take those too.”

Donated magnets in the staging and testing area

Some magnets are more useful than others. Magnets with mini thermometers can help engineers keep track of the state of the supercooled LHC magnets. The resident artists at CERN have expressed an interest in the magnetic “fridge poetry” packs. Magnets that feature cats will be used in the RF cavity sector. So please, take a look at your fridge, and see if you really need that snow globe magnet from Oslo, or that hula girl magnet from Hawai’i. Why leave it sitting in your kitchen when it can be helping research on the world’s largest machine?

### Creative solution to poster defacement row

In recent weeks the media has reported on defacement of the LGBT CERN posters at the lab, with many being removed or subject to grafitti. CERN Director General, Fabiola Gianotti, has taken these incidents very seriously. “The targeting of a single group of posters for abuse like this unacceptable” she said, “and so I have made the decision that from now on, all types posters at CERN will be removed or defaced. CERN is a lab of equal opportunities, and it must be free from discrimination.”

Teams of administrators, including Gianotta herself, have been seen walking the corridors of CERN and instituting this new policy. Posters announcing a SUSY conference have had “NO MORE SYMMTRY BRAKING HERE!!1!” scrawled across them, and a poster advertising a symposium on solar neutrinos was subjected to “Go back to where you came from. The sun.” written on it. Even parking signs are not immune, with slogans such as “Parking? More like… splarking!” and a fire exit sign was seen with a neatly written note underneath saying “They had fire in Hitler’s Germany too, you know”.

One of the many posters subject to the new policy

By attacking all signs and posters at the lab, the aim is to make nobody feel victimised or isolated. Staff are encouraged to use their own initiative and are recommended to mutter incoherently under their breath as they do so. “If nothing else” one technician said “it’s made the lab more surreal. I don’t even know how much a coffee is anymore. Apparently it’s now one ‘WHY ARE YOU READING THIS?!’, but it used to be 1.60 CHF.”

### LIGO result explained

In February 2016, the LIGO experiment announced it had observed gravitational waves, predicted over a century ago by Albert Einstein’s theory of general relativity. The discovery is thought to have come from the merging of two massive black holes, from over a billion light years away. However, two students have come forward to say that they created the waves in their apartment, using a waffle iron, an iPhone, and the cluck of a chicken. “We’ve been working on this prank for weeks” said the first student, “and we had no idea it would be taken seriously!” The second student added “We had to eat so many Pringles to get enough tubes for the wave generator.”

Captain McNuggets, relaxing in the garden

The real hero of the story is their chicken, Captain McNuggets, who made the characteristic “chirp” sound. So did LIGO really detect gravitational waves? “Oh, absolutely!” the pair of students replied. The machine they made could produce gravitational waves of any frequency and amplitude desired, but it was only made “for a bit of a laugh” and is unlikely to see further research. The machine itself was dismantled in October to make space for their latest project, the “ballistic taco-launcher”.

### Live Blogging of the LHCb Bs pi result

Monday, March 21st, 2016

There has been a lot of press about the recent DØ result on the possible $$B_s \pi$$ state. This was also covered on Ricky Nathvani’s blog. At Moriond QCD, Jeroen Van Tilburg showed a few plots from LHCb which showed no signal in the same mass regions as explored by D∅. Tomorrow, there will be a special LHC seminar on the LHCb search for purported tetraquark, where we will get the full story from LHCb. I will be live blogging the seminar here! It kicks off at 11:50 CET, so tune in to this post for live updates.

Mar 22, 2016 -12:23. Final answer. LHCb does not confirm the tetraquark. Waiting for CMS, ATLAS, CDF.

Mar 22, 2016 – 12:24. How did you get the result out so fast? A lot of work by the collaboration to get MC produced and to expedite the process.

Mar 22, 2016 – 12:21. Is the $$p_T$$ cut on the pion too tight? The fact that you haven’t seen anything anywhere else gives you confidence that the cut is safe. Also, cut is not relative to $$B_s$$.

Mar 22, 2016 – 12:18. Question: What are the fractions of multiple candidates which enter? Not larger than 1.2. If you go back to the cuts. What selection killed the combinatoric background the most? Requirement that the $$\pi$$ comes from the PV, and the $$p_T$$ cut on the pion kill the most. How strong the PV cut? $$\chi^2$$ less than 3.5 for the pion at the PV, you force the $$B_s$$ and the pion to come from the PV, and constrain the mass of $$B_s$$ mass.

Mar 22, 2016 – 12:17: Can you go above the threshold? Yes.

Mar 22, 2016 – 12:16. Slide 9: Did you fit with a floating mass? Plan to do this for the paper.

Mar 22, 2016 – 12:15. Wouldn’t $$F_S$$ be underestimated by 8%? Maybe maybe not.

Mar 22, 2016 – 12:13. Question: Will LHCb publish? Most likely yes, but a bit of politics. Shape of the background in the $$B_s\pi$$ is different in LHCb and DØ. At some level, you expect a peak from the turn over. Also CMS is looking.

Mar 22, 2016 – 12:08-12:12. Question: did you try the cone cut to try to generate a peak? Answer: Afraid that the cut can give a biased estimate of the significance. From DØ seminar, seems like this is the case. For DØ to answer. Vincenzo Vagnoni says that DØ estimation of significance is incorrect. We also don’t know if there’s something that’s different between $$pp$$ and $$p \bar{p}$$.

Mar 22, 2016 – 12:08. No evidence of $$X(5568)$$ state, set upper limit. “We look forward to hearing from ATLAS, CMS and CDF about $$X(5568)$$”

Mar 22, 2016 – 12:07. What if the production of the X was the same at LHCb? Should have seen a very large signal. Also, in many other spectroscopy plots, e.g. $$B*$$, look at “wrong sign” plots for B and meson. All results LHCb already searched for would have been sensitive to such a state.

Mar 22, 2016 -12:04. Redo the analysis in bins of rapidity. No significant signal seen in any result. Do for all pt ranges of the Bs.

Mar 22, 2016 – 12:03. Look at $$B^0\pi^+$$ as a sanity check. If X(5568) is similar to B**, then the we expect order 1000 events.

Mar 22, 2016 – 12:02.Upper limits on production given.

Mar 22, 2016 – 12:02. Check for systematics: changing mass and width of DØ range, and effect of efficiency dependence on signal shape are the dominant sources of systematics. All measurements dominated by statistics.

Mar 22, 2016 – 12:00. Result of the fits all consistent with zero. The relative production is also consistent with zero.

Mar 22, 2016 – 11:59. 2 fits with and without signal components, no difference in pulls. Do again with tighter cut on the transverse momentum of the $$B_s$$. Same story, no significant signal seen.

Mar 22, 2016 – 11:58. Fit model: S-wave Breit-Wigner, mass and width fixed to DØ result. Backgrounds: 2 sources. True $$B_s^0$$ with random track, and fake $$B_s$$.

Mar 22, 2016 – 11:56.  No “cone cut” applied because it is highly correlated with reconstructed mass.

Mar 22, 2016 – 11:55. LHCb strategy: Perform 3 independent searches, confirm a qualitative approach, move forward with single approach with Run 1 dataset. Cut based selection to match D∅ strategy. Take home point. Statistics is 20x larger and much cleaner.

Mar 22, 2016 – 11:52. Review of DØ result. What could it be? Molecular model is disfavored. Diquark-Antidiquark models are popular. But could not fit into any model. Could also be feed down of  radiative decays. All predictions have large uncertainties

Mar 22, 2016 –  11:49. LHCb-CONF-2016-004 posted at cds.cern.ch/record/2140095/

Mar 22, 2016 – 11:47. The speaker is transitioning to Marco Pappagallo .

Mar 22, 2016 – 11:44. People have begun entering the auditorium for the talk, at the end of Basem Khanji’s seminar on $$\Delta m_d$$

### Two steps closer to a possible discovery

Friday, March 18th, 2016

Has CERN discovered a new particle or not? Nobody knows yet, although we are now two steps closer than in December when the first signs of a possible discovery were first revealed.

First step: both the ATLAS and CMS experiments showed yesterday at the Moriond conference that the signal remains after re-analyzing the 2015 data with improved calibrations and reconstruction techniques. The faint signal still stands, even slightly stronger (see the Table). CMS has added new data not included earlier and collected during a magnet malfunction. Thanks to much effort and ingenuity, the reanalysis now includes 20% more data. Meanwhile, ATLAS showed that all data collected at lower energy up to 2012 were also compatible with the presence of a new particle.

The table below shows the results presented by CMS and ATLAS in December 2015 and February 2016. Two hypotheses were tested, assuming different characteristics for the hypothetical new particle: the “spin 0” case corresponds to a new type of Higgs boson, while “spin 2” denotes a graviton.

The label “local” means how strong the new signal appears locally at a mass of 750 or 760 GeV, while “global” refers to the probability of finding a small excess over a broad range of mass values. In physics, statistical fluctuations come and go. One is bound to find a small anomaly when looking all over the place, which is why it is wise to look at the bigger picture. So globally, the excess of events observed so far is still very mild, far from the 5σ criterion required to claim a discovery. The fact that both experiments found it independently is what is so compelling.

But mostly, the second step, we are closer to potentially confirming the presence of a new particle simply because the restart of the Large Hadron Collider is now imminent. New data are expected for the first week of May. Within 2-3 months, both experiments will then know.

We need more data to confirm or refute the existence of a new particle beyond any possible doubt. And that’s what experimental physicists are paid to do: state what is known about Nature’s laws when there is not even the shadow of a doubt.

That does not mean than in the meantime, we are not dreaming since if this were confirmed, it would be the biggest breakthrough in particle physics in decades. Already, there is a frenzy among theorists. As of 1 March, 263 theoretical papers have been written on the subject since everybody is trying to find out what this could be.

Why is this so exciting? If this turns out to be true, it would be the first particle to be discovered outside the Standard Model, the current theoretical framework. The discovery of the Higgs boson in 2012 had been predicted and simply completed an existing theory. This was a feat in itself but a new, unpredicted particle would at long last reveal the nature of a more encompassing theory that everybody suspects exists but that nobody has found yet. Yesterday at the Moriond conference, Alessandro Strumia, a theorist from CERN, also predicted that this particle would probably come with a string of companions.

Theorists have spent years trying to imagine what the new theory could be while experimentalists have deployed heroic efforts, sifting through huge amounts of data looking for the smallest anomaly. No need to say then that the excitement is tangible at CERN right now as everybody is holding their breath, waiting for new data.

Pauline Gagnon

To learn more about particle physics and what might be discovered at the LHC, don’t miss my upcoming book : « Who cares about particle physics : Making sense of the Higgs boson, Large Hadron Collider and CERN »

To be alerted of new postings, follow me on Twitter: @GagnonPauline  or sign-up on this mailing list to receive an e-mail notification.

### Deux pas de plus vers une découverte

Friday, March 18th, 2016

Le CERN a-t-il découvert une nouvelle particule ou pas? Personne ne le sait encore, bien que nous ayons maintenant fait deux pas de plus depuis le dévoilement des premiers signes d’une possible découverte en décembre.

Premier pas : les expériences ATLAS et CMS ont montré hier à la conférence de Moriond que les signes d’un signal persistent après la réanalyse des données de 2015 à l’aide de calibrations et de techniques de reconstruction améliorées. Le faible signal est même légèrement renforci (voir tableau). CMS a ajouté de nouvelles données recueillies durant une défaillance de leur aimant. Après beaucoup d’efforts et d’ingéniosité, ceci ajoute 20 % de données supplémentaires. De son côté, ATLAS a montré que toutes les données accumulées à moindre énergie jusqu’à 2012 étaient aussi compatibles avec la présence d’une nouvelle particule.

Le tableau ci-dessous montre les résultats présentés par CMS et ATLAS en décembre 2015 et février 2016. Deux hypothèses ont été testées, chacune correspondant à des caractéristiques différentes pour cette hypothétique particule : “spin 0” correspond à un nouveau type de boson de Higgs, tandis que “spin 2” dénote un graviton.

Local” se réfère à l’intensité du signal lorsque mesuré pour une particule ayant une masse de 750 ou 760 GeV, tandis que “global” indique la probabilité de trouver un petit excès sur une large gamme de valeurs de masse. En physique, les fluctuations statistiques sont monnaies courantes. On trouve toujours une petite anomalie lorsqu’on cherche dans tous les coins. Il est donc sage de prendre en compte un intervalle élargi. Globalement donc, l’excédent d’événements observé est toujours très limité. On est encore bien loin de la barre des 5σ, le critère utilisé pour une découverte. Ce qui est très fort par contre, c’est que les deux expériences l’ont trouvé indépendamment.

Le deuxième et bien plus grand pas franchi, c’est que la confirmation possible de la présence d’une nouvelle particule se rapproche simplement parce que la reprise du Grand Collisionneur de Hadrons est imminente. On attend les nouvelles données début mai. Dans 2 ou 3 mois, les deux expériences connaîtront enfin la réponse

Mais sans plus de données, impossible de confirmer ou réfuter l’existence d’une nouvelle particule avec certitude. Et c’est justement pour cela qu’on paie les physiciens et physiciennes: déterminer les lois de la Nature sans qu’il ne subsiste l’ombre d’un doute.

Cela n’empêche personne de rêver en attendant, car si ceci était confirmé, ce serait la plus grande percée en physique des particules depuis des décennies. Déjà, la frénésie s’est emparée des théoriciens et théoriciennes. On comptait en date du premier mars 263 articles théoriques sur le sujet. Tout le monde essaye de déterminer ce que cela pourrait être.

Pourquoi est-ce si passionnant ? Si elle existe, ce serait la première particule à être découverte à l’extérieur du Modèle Standard, la théorie actuelle. La découverte du boson de Higgs en 2012 avait été prévue et avait simplement complété une théorie existante. Un exploit, bien sûr, mais la découverte d’une particule imprévue révèlerait enfin la nature d’une théorie plus vaste dont tout le monde soupçonne l’existence, mais qui n’a pas encore été trouvée. Hier à la conférence de Moriond, Alessandro Strumia, un théoricien du CERN, a prédit que cette particule s’accompagnerait probablement d’une kyrielle de nouvelles particules.

Les théoriciens et théoriciennes ont passé des années à essayer d’imaginer cette nouvelle théorie tandis que du côté expérimental, on a déployé des efforts héroïques à trier des quantités faramineuses de données à la recherche de la moindre anomalie. Nul besoin de dire que l’atmosphère est fébrile en ce moment au CERN; tout le monde retient son souffle en attendant les nouvelles données.

Pauline Gagnon

Pour en savoir plus sur la physique des particules et les enjeux du LHC, consultez mon livre : « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels», en librairie en France et en Suisse dès le 1er mai.

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution.

### The Four-Leafed Clovers of Subatomic Particles: Tetraquarks

Monday, February 29th, 2016

Hadrons, the particles made of quarks, are almost unanimously produced in the two or three quark varieties in particle colliders. However, in the last decade or so, a new frontier has opened up in subatomic physics. Four-quark particles have begun to be observed, the most recent being announced last Thursday by a collaboration at Fermilab. These rare, fleetingly lived particles have the potential to shed some light on the Strong nuclear force and how it shapes our world.

The discovery of a new subatomic particle was announced last Thursday by the DØ (DZero) collaboration at Fermilab in Chicago. DØ researchers analysed data from the Tevatron, a proton-antiproton collider based at Fermilab. The new found particle sports the catchy name “X(5568)” (It’s labelled by the observed mass of 5,568 Megaelectron-volts or MeV. That’s about six times heavier than a proton.) X(5568) is a form of “tetraquark”, a rarer variety of the particles known as hadrons. Tetraquarks consist of two quarks and two antiquarks (rather than the usual three quarks or quark-antiquark pairs that make up hadrons particle physicists are familiar with). While similar tetraquark particles have been observed before, the new addition breaks the mould by consisting of four quarks of totally different flavours: bottom, strange, up and down.

[Regular readers and those familiar with the theory of QCD may wish to skip to the section marked ——]

a) An example of a quark-antiquark pair, known as Mesons. b) An example of a three-quark particle, known as Baryons. c) An example of a tetraquark (four quarks) Source: APS/Alan Stonebraker, via Physics Viewpoint, DOI: 10.1103/Physics.6.69

The particle’s decay is best explained Strong force, aptly named since it’s the strongest known force in the universe[1], which also acts to hold quarks together in more stable configurations such as inside the proton. The Strong force is described by a theory known as Quantum Chromodynamics (QCD for short), a crucial part of the Standard Model of particle physics. The properties of X(5568) will provide precision tests of the Standard Model, as well as improving our understanding of the nature of Confinement. This is a dimly understood process by which quarks are bound up together to form the particles (such as protons) that make up most of the visible matter in the universe.

Quarks are defined by the strong force, being the only particles known to physics that interact via QCD. They were originally conceived of in 1964 by two of the early pioneers of particle physics Murray Gell-Mann and George Zweig, who posited the idea of “quarks” to explain the properties of a plethora of particles that were discovered in the mid-twentieth century. After a series of experiments in the late ‘60s and ‘70s, the evidence in favour of the quark hypothesis grew much stronger[2] and it was accepted that many of the particles that interacted and decayed very quickly (due to the magnitude of the strong force) in detectors were in fact made up of these quarks, which are now known to come in six different varieties known as “flavours”. A more precise model of the strong force, which came to be known as QCD, was also verified in such experiments.

QCD is a very difficult theory to draw predictions from because unlike electromagnetism (the force responsible for holding atoms together and transmitting light between objects), the “force carriers” of QCD known as gluons are self-interacting. Whereas light, or photons, simply pass through one another, gluons pull on one another and quarks in complex ways that give rise to the phenomenon of confinement: quarks are never observed in isolation, only as part of a group of other quarks/antiquarks. These groups of quarks and anti-quarks are what we call Hadrons (hence the name Large Hadron Collider). This self interaction arises from the fact that, unlike light which simply couples to positive or negative charges, QCD has a more complicated structure based on three charges labelled as Red, Green and Blue (which confusingly, have nothing to do with real colours, but are instead based on a mathematical symmetry known as SU(3)).

The hadrons discovered in the twentieth century tended to come in pairs of three quarks or quark-antiquark pairs. Although we now know there is nothing in the theory of QCD that suggests you can’t have particles consisting of four, or even five quarks/antiquarks, such particles were never observed, and in fact even some of the finest minds in theoretical physics (Edward Witten and Sidney Coleman) once thought that QCD would not permit such particles to exist. Like clovers, however, although the fourfold or even fivefold variety would be much rarer to come by it turns out such states did, in fact, exist and could be observed.

——

A visualisation of the production and decay of X(5568) to mesons in the Tevatron collider. Source: Fermilab http://news.fnal.gov/

The first hints of the existence of tetraquarks were at the Belle experiment, Japan in 2003, with the observation of a state called X(3872) (again, labelled by its mass of 3872 MeV). One of the most plausible explanations for this anomalous resonance[3] was a tetraquark model, which in 2013, an analysis by the LHCb experiment at CERN found to be a compatible explanation of the same resonance found in their detector. The same year, Belle and the BESIII experiment in China both found a resonance with the same characteristics, labelled Zc(3900), which is now believed to be the first independently, experimentally observed tetraquark. The most recent evidence for the existence of tetraquarks, prior to last Thursday’s announcement, was found by the LHCb experiment in 2014, the Z(4430). This verified an earlier result from Belle in 2007, with an astonishingly high statistical significance of 13.9σ (for comparison, one typically claims a discovery with a significance of 5σ). LHCb would also go on, unexpectedly, to find a pentaquark (four quarks and an antiquark) state in 2015, which could provide a greater understanding of QCD and even a window into the study of neutron stars.

Z(4430) was discovered from the analysis of its decay into mesons (hadrons consisting of quark-antiquark pairs), specifically the ψ’ and π mesons from the decay B0 → K + ψ’  π. In the analysis of the B0 decay, it was found that the Z(4430) was needed as an intermediate particle state to explain the resonant behaviour of the ψ’ and π. The LHCb detector, whose asymmetric design and high resolution makes it particularly well suited for the job, reconstructs these mesons and looks at their kinematic properties to determine the shape and properties of the resonance, which were found to be consistent with a tetraquark model. The recent discovery of X(5568) by the DØ collaboration involved a similar reconstruction from Bs and π mesons, which was used to infer its quark flavour structure (b, s, u, d, though which two are the particles and which two are the antiparticles remains to be determined).

X(5568) is found to have a large width (22 MeV) in the distribution of its decays, implying that it decays very quickly, best explained by QCD. Since quarks cannot change flavours in QCD interactions (while they can do so in weak nuclear interactions), this is what allowed DØ to determine its quark content. The other properties of this anomalous particle, such as its mass and its lack of spin (i.e. S = 0) are measured from the kinematics of the mesons it produces, and can help increase our understanding of how QCD combines the quarks in such an unfamiliar arrangement.

The two models for tetraquarks: Left, a single bound state of four quarks. Right, a pair of mesons bound to one another in orbit, resembing a four quark state. Source: Fermilab http://news.fnal.gov (Particle Physicists have a strange relationship with Comic Sans)

One of the long-standing controversies surrounding tetraquark states is whether the states are truly a joint four particle state or in fact a sort of molecule of two strongly bound mesons, which although they form a bound state of four particles in total, is actually analogous to two separate atoms in a molecule rather than a single, heavy atom. The analysis from DØ, based on X(5568)’s mass seems to imply that it’s the former, a single particle of four quarks tightly bound in an exotic hadron, though the jury is still out on the matter.

DØ’s discovery is based on an analysis of the historic data collected from the Tevatron from the 28 years it was operating, since the collider itself ceased operation 2011. Despite LHCb having found tetraquark candidates in the past and being suited to finding such a particle again, it has not yet independently verified the existence of X(5568). LHCb will now review their own data as well as future data that will recommence being collected later this year, to see if they too observe this unprecedented result and hopefully improve our understanding of its properties and whether they are consistent with the Standard Model. This is definitely a result to look out for later this year and should shed some light on one of the fundamental forces of nature and how it acts to create the particles, such as protons, that make up the world around us.

[1] That is, the dimensionless coupling of the force carrier particle interactions is greater than electromagnetism and the weak nuclear force, both of which in turn are stronger than gravity (consider how a tiny magnet can lift a paper clip against the gravity of the entire Earth). Many theories of Beyond the Standard Model physics predict new forces, and it may turn out that all the forces are unified into a single entity at high energies.

[2] For an excellent summary of the history of quarks and some of the motivations behind the quark model, check out this fantastic documentary featuring none other than the Nobel Prize wining physicists, Richard Feynman and Murray Gell-Mann themselves.

[3] Particles are discovered by the bumps or resonances they leave in the statistical distributions of particle decays/scattering events. See for example, one of the excesses of events that led to the discovery of the Higgs Boson.

### A peine une brise, mais elle secoue le monde entier

Thursday, February 11th, 2016

Aujourd’hui, les scientifiques du Laser Interferometer Gravitational-Wave Observatory ou LIGO ont fièrement annoncé avoir détecté les toutes premières ondes gravitationnelles. Décrites il y a exactement cent ans dans la Théorie de la Relativité Générale par Albert Einstein, ces ondes, qu’on a longtemps crues être beaucoup trop faibles pour être captées, ont enfin été détectées.

En 1916, Einstein décrit la gravitation comme une déformation de l’espace et du temps, comme si l’espace n’était qu’un tissu qui s’étire en présence d’objets massifs. Un espace vide serait semblable à un drap tendu. Un objet se déplaçant dans cet espace, comme par exemple une balle de ping-pong, suivrait simplement la surface du drap. Laissez tomber un objet lourd sur ce drap et le tissu sera déformé. La balle de ping-pong ne roulera plus en ligne droite, mais suivra naturellement la courbe de l’espace déformé.

En tombant sur le drap, l’objet lourd créera de petites ondulations qui se propageront autour de lui, comme des vaguelettes à la surface de l’eau. De même, le Big Bang ou une collision entre deux trous noirs peut aussi créer des ondulations qui atteindraient éventuellement la Terre.

C’est ce type d’ondulations que LIGO a enfin détectées, comme l’explique cette excellente vidéo (mais en anglais). Les scientifiques de LIGO ont utilisé un interféromètre, un appareil muni de deux branches identiques tel qu’indiqué sur l’image ci-dessous. Un laser (en bas à gauche) émet un faisceau de lumière qui vient frapper un morceau de verre (au centre). La moitié du faisceau est réfléchie, l’autre poursuit son chemin. Les deux faisceaux parcourent exactement la même distance (4 km) avant d’être réfléchis par un miroir.

Un faisceau de lumière, telle une vague à la surface de l’eau, possède des crêtes et des creux. Au retour, les deux faisceaux se chevauchent à nouveau, mais la longueur des branches est telle que la position des crêtes du premier faisceau est décalée par rapport à celle de l’autre, de telle sorte qu’ils se neutralisent. Par conséquent, un détecteur situé à droite ne décèlerait aucune lumière.

Imaginez maintenant qu’une vague gravitationnelle, produite par exemple par une collision entre deux trous noirs, se propage à travers l’interféromètre. Le « tissus » de l’espace serait étiré puis comprimé sous le passage de cette onde. La longueur des branches de l’interféromètre serait modifiée, décalant ainsi les crêtes et les creux. Les deux faisceaux ne s’annuleraient plus. Un détecteur détecterait une lumière oscillante durant le passage d’une onde gravitationnelle à travers l’appareil.

Le défi de cette expérience consiste à éliminer toutes sources de vibrations, qu’elles proviennent des vagues de l’océan, d’un tremblement de terre, ou même du trafic car elles produiraient des effets semblables. Les faisceaux laser voyagent donc dans des tuyaux à vide et les miroirs sont montés sur des ressorts et suspendus à de fins fils. On amortit ainsi les vibrations externes par un facteur de 10 milliards.

Pour s’assurer qu’un signal provient réellement d’une onde gravitationnelle et non pas d’une autre perturbation, LIGO utilisent deux interféromètres identiques et distants de plus de 3000 km. L’un se trouve en Louisiane, l’autre dans l’état de Washington.

Et voici ce signal, produit lors de la fusion de deux trous noirs d’environ 50 km mais trente fois plus massifs que le soleil. Cette collision a généré une onde gravitationnelle qui s’est propagé pendant un milliard d’années avant d’atteindre la Terre le 14 septembre dernier. L’onde a modifié la longueur des branches de l’interféromètre de 4 km d’à peine un millième de la taille d’un proton. Une petite oscillation durant seulement 20 millisecondes, accélérant rapidement puis disparaissant, exactement tel que prédit par les équations de la relativité générale.

Donc quand les deux instruments ont détecté simultanément ce signal, leur coïncidence n’a laissé aucun doute. Il ne pouvait s’agir que d’ondes gravitationnelles. LIGO n’a détecté que la partie classique de ces ondes. On ne sait toujours pas si les ondes gravitationnelles sont quantifiées ou pas, et si elles s’accompagnent d’une particule appelée le graviton.

Pendant des siècles, les astronomes ont utilisé des ondes électromagnétiques comme la lumière pour explorer l’Univers. Les ondes gravitationnelles fourniront un nouvel outil pour pousser l’exploration de l’Univers encore plus loin. Ce que ces ondes nous apprendrons vaudra bien d’avoir attendu cent longues années pour les découvrir.

Pauline Gagnon

Pour en savoir plus sur la physique des particules et les enjeux du LHC, consultez mon livre : « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels».

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution.

L’interféromètre de LIGO sur le site de Hanford dans l’état de Washington avec ses branches de 4 km de longueur. ©NASA

### A faint ripple shakes the World

Thursday, February 11th, 2016

Today, scientists from the Laser Interferometer Gravitational-Wave Observatory or LIGO have proudly announced having detected the first faint ripples caused by gravitational waves. First predicted exactly one hundred years ago by Albert Einstein in the Theory of General Relativity, these gravitational waves, long believed to be too small to be seen, have at long last been detected.

In 1916, Einstein explained that gravitation is a distortion of space and time, as if it was a fabric that could be distorted by the presence of massive objects. An empty space would be like a taut sheet. Any object, like a ping-pong ball travelling in that space, would simply follow the surface of the sheet. Drop a heavy object on the sheet, and the fabric will be distorted. The ping-pong ball would no longer roll along a straight line but would naturally follow the curve of the distorted space.

A heavy object falling on that sheet would generate small ripples around it. Likewise, the Big Bang or collisions between black holes would also create ripples that would eventually reach the Earth.

These were the small disturbances LIGO was set to find. As explained in this excellent video, the scientists used an interferometer, that is, an apparatus with two identical arms as shown below. A laser (bottom left corner) emits a beam of light that hits a piece of glass (center). Half of the beam is reflected, half of it keeps going on. The two beams travel exactly the same distance (4 km), hit a mirror and bounce back.

A light beam is a wave, and just like waves at the surface of water, it has crests and troughs. The arms length is such that when the beams return and overlap again, the two sets of waves are shifted with respect to each other, such that they cancel each other out. Hence, a detector placed at the bottom right corner would see no light at all.

Now imagine that a gravitational wave, produced by the collisions of two black holes for example, sweeps across the interferometer. The fabric of space would be stretched then compressed as the wave passes through. And so the length of the arms would change, shifting the pattern of crests and troughs. The two beams would no longer cancel each other. A light-sensitive detector would now detect some light that would pulsate as the gravitational wave sweeps across the apparatus.

The challenge is that any vibration caused by waves crashing on the shore, earthquakes, or even heavy traffic would disturb such an experiment by producing similar effects. So the laser beams travel in vacuum and the mirrors are mounted on shock-absorbing springs and suspended on fine wires to dampen any vibration by a factor of 10 billion.

To ensure a signal really comes from a gravitational wave and not from some other disturbance, LIGO used two identical laboratories located more than 3000 km apart in the USA, one in Louisiana, one in Washington State.

And here is the signal generated when two black holes, 50 km in diameter but 30 times more massive than the Sun, merged. This collision sent a gravitational wave that traveled for about a billion year before reaching the Earth on 14 September 2015. This wave changed the length of the 4-km arms by one thousandth of the size of a proton. A tiny ripple that lasted a mere 20 milliseconds, accelerating quickly before disappearing, exactly as General Relativity predicted.

So when both instruments detected the same signal, the coincidence between the two left no doubt. It really was from gravitational waves. So far, the LIGO experiment only detected the classical part of these waves. We still do not know if gravitational waves are quantized or not, that is, if they come with a particle called the graviton.

For centuries, astronomers have used electromagnetic waves such as light to explore the Universe. Gravitational waves will provide a new tool to study it even further. Other experiments such as BICEP2 are already looking for the ripples left over from the Big Bang. What we will learn from these waves will be well worth the hundred-year long wait from their prediction to their discovery.

Pauline Gagnon

To learn more on particle physics, don’t miss my book, out this July.

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The LIGO interferometer in Hanford, Washington State, USA, with its 4km-long arms. ©NASA

### “Ladies and gentlemen, we have detected gravitational waves.”

Thursday, February 11th, 2016

The title says it all. Today, The Light Interferometer Gravitational-Wave Observatory  (or simply LIGO) collaboration announced the detection of gravitational waves coming from the merger of two black holes located somewhere in the Southern sky, in the direction of the Magellanic  Clouds.  In the presentation, organized by the National Science Foundation, David Reitze (Caltech), Gabriela Gonzales (Louisiana State), Rainer Weiss (MIT), and Kip Thorn (Caltech), announced to the room full of reporters — and thousand of scientists worldwide via the video feeds — that they have seen a gravitational wave event. Their paper, along with a nice explanation of the result, can be seen here.

The data that they have is rather remarkable. The event, which occurred on 14 September 2015, has been seen by two sites (Livingston and Hanford) of the experiment, as can be seen in the picture taken from their presentation. It likely happened over a billion years ago (1.3B light years away) and is consistent with the merger of two black holes, of 29 and 46 solar masses. The resulting larger black hole’s mass is about 62 solar masses, which means that about 3 solar masses of energy (29+36-62=3) has been radiated in the form of gravitational waves. This is a huge amount of energy! The shape of the signal is exactly what one should expect from the merging of two black holes, with 5.1 sigma significance.

It is interesting to note that the information presented today totally confirms the rumors that have been floating around for a couple of months. Physicists like to spread rumors, as it seems.

Since the gravitational waves are quadrupole, the most straightforward way to see the gravitational waves is to measure the relative stretches of the its two arms (see another picture from the MIT LIGO site) that are perpendicular to each other. Gravitational wave from black holes falling onto each other and then merging. The LIGO device is a marble of engineering — one needs to detect a signal that is very small — approximately of the size of the nucleus on the length scale of the experiment. This is done with the help of interferometry, where the laser beams bounce through the arms of the experiment and then are compared to each other. The small change of phase of the beams can be related to the change of the relative distance traveled by each beam. This difference is induced by the passing gravitational wave, which contracts one of the arms and extends the other. The way noise that can mimic gravitational wave signal is eliminated should be a subject of another blog post.

This is really a remarkable result, even though it was widely expected since the (indirect) discovery of Hulse and Taylor of binary pulsar in 1974! It seems that now we have another way to study the Universe.