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Posts Tagged ‘new particle’

Les grandes percées sont rares en physique. La recherche est plutôt jalonnée d’innombrables petites avancées et c’est ce qui ressortira de la Conférence Internationale de la Physique des Hautes Énergies (ICHEP) qui s’est ouverte hier à Chicago. On y espérait un pas de géant mais aujourd’hui les expériences CMS et ATLAS ont toutes deux rapporté que l’effet prometteur observé à 750 GeV dans les données de 2015 avait disparu. Il est vrai que ce genre de choses n’est pas rare en physique des particules étant donné la nature statistique de tous les phénomènes que nous observons.

CMS-2016-750GeV

Sur chaque figure, l’axe vertical indique le nombre d’évènements trouvés contenant une paire de photons dont la masse combinée apparaît sur l’axe horizontal en unités de GeV. (À gauche) Les points en noir représentent les données expérimentales recueillies et analysées jusqu’à présent par la Collaboration CMS, soit 12.9 fb-1, à comparer aux 2.7 fb-1 disponibles en 2015. Le trait vertical associé à chaque point représente la marge d’erreur expérimentale. En tenant compte de ces erreurs, les données sont compatibles avec ce à quoi on s’attend pour le bruit de fond, tel qu’indiqué par la courbe en vert. (À droite) Une nouvelle particule se serait manifestée sous forme d’un pic tel que celui en rouge si elle avait eu les mêmes propriétés que celles pressenties dans les données de 2015 à 750 GeV. Visiblement, les données expérimentales (points noirs) reproduisent simplement le bruit de fond. Il faut donc conclure que ce qui avait été aperçu dans les données de 2015 n’était que le fruit d’une variation statistique.

Mais dans ce cas, c’était particulièrement convainquant car le même effet avait été observé indépendamment par deux équipes qui travaillent sans se consulter et utilisent des méthodes d’analyse et des détecteurs différents. Cela avait déclenché beaucoup d’activités et d’optimisme : à ce jour, 540 articles scientifiques ont été écrits sur cette particule hypothétique qui n’a jamais existé, tant l’implication de son existence serait profonde.

Mais les théoriciens et théoriciennes ne furent pas les seuls à nourrir autant d’espoir. Beaucoup d’expérimentalistes y ont cru et ont parié sur son existence, un de mes collègues allant jusqu’à mettre en jeu une caisse d’excellent vin.

Si beaucoup de physiciens et physiciennes avaient bon espoir ou étaient même convaincus de la présence d’une nouvelle particule, les deux expériences ont néanmoins affiché la plus grande prudence. En l’absence de preuves irréfutables de sa présence, aucune des deux collaborations, ATLAS et CMS, n’a revendiqué quoi que ce soit. Ceci est caractéristique des scientifiques : on parle de découvertes seulement lorsqu’il ne subsiste plus aucun doute.

Mais beaucoup de physiciens et physiciennes, moi y compris, ont délaissé un peu leurs réserves, non seulement parce que les chances que cet effet disparaisse étaient très minces, mais aussi parce que cela aurait été une découverte beaucoup plus grande que celle du boson de Higgs, générant du coup beaucoup d’enthousiasme. Tout le monde soupçonne qu’il doit exister d’autres particules au-delà de celles déjà connues et décrites par le Modèle standard de la physique des particules. Mais malgré des années passées à leur recherche, nous n’avons toujours rien à nous mettre sous la dent.

Depuis que le Grand collisionneur de hadrons (LHC) du CERN opère à plus haute énergie, ayant passé de 8 TeV à 13 TeV en 2015, les chances d’une découverte majeure sont plus fortes que jamais. Disposer de plus d’énergie donne accès à des territoires jamais explorés auparavant.

Jusqu’ici, les données de 2015 n’ont pas révélé la présence de particules ou phénomènes nouveaux mais la quantité de données recueillies était vraiment limitée. Au contraire, cette année le LHC se surpasse, ayant déjà produit cinq fois plus de données que l’année dernière. On espère y découvrir éventuellement les premiers signes d’un effet révolutionnaire. Des dizaines de nouvelles analyses basées sur ces données récentes seront présentées à la conférence ICHEP jusqu’au 10 août et j’en reparlerai sous peu.

Il a fallu 48 ans pour découvrir le boson de Higgs après qu’il fut postulé théoriquement alors qu’on savait ce que l’on voulait trouver. Mais aujourd’hui, nous ne savons même pas ce que nous cherchons. Cela pourrait donc prendre encore un peu de temps. Il y a autre chose, tout le monde le sait. Mais quand le trouverons nous, ça, c’est une autre histoire.

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.

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Giant leaps are rare in physics. Scientific research is rather a long process made of countless small steps and this is what will be presented throughout the week at the International Conference on High Energy Physics (ICHEP) in Chicago. While many hoped for a major breakthrough, today, both the CMS and ATLAS experiments reported that the promising effect observed at 750 GeV in last year’s data has vanished. True, this is not uncommon in particle physics given the statistical nature of all phenomena we observe.

CMS-2016-750GeV

On both plots, the vertical axis gives the number of events found containing a pair of photons with a combined mass given in units of GeV (horizontal axis) (Left plot) The black dots represent all data collected in 2016 and analysed so far by the CMS Collaboration, namely 12.9 fb-1, compared to the 2.7 fb-1 available in 2015. The vertical line associated with each data point represents the experimental error margin. Taking these errors into account, the data are compatible with what is expected from various backgrounds, as indicated by the green curve. (Right) A new particle would have manifested itself as a peak as big as the red one shown here if it had the same features as what had been seen in the 2015 data around 750 GeV. Clearly, the black data points pretty much reproduce the background. Hence, we must conclude that what was seen in the 2015 data was simply due to a statistical fluctuation.

What was particularly compelling in this case was that the very same effect had been observed by two independent teams, who worked without consulting each other and used different detectors and analysis methods. This triggered frantic activity and much expectation: to date, 540 scientific theory papers have been written on a hypothetical particle that never was, so profound the implications of the existence of such a new particle would be.

But theorists were not the only ones to be so hopeful. Many experimentalists had taken strong bets, one of my colleagues going as far as putting a case of very expensive wine on it.

If many physicists were hopeful or even convinced of the presence of a new particle, both experiments nevertheless had been very cautious. Without unambiguous signs of its presence, neither the ATLAS nor the CMS Collaborations had made claims. This is very typical of scientists: one should not claim anything until it has been established beyond any conceivable doubt.

But many theorists and experimentalists, including myself, threw some of our caution to the air, not only because the chances it would vanish were so small but also because it would have been a much bigger discovery than that of the Higgs boson, generating much enthusiasm. As it stands, we all suspect that there are other particles out there, beyond the known ones, those described by the Standard Model of particle physics. But despite years spent looking for them, we still have nothing to chew on. In 2015, the Large Hadron Collider at CERN raised its operating energy, going from 8 TeV to the current 13 TeV, making the odds for a discovery stronger than ever since higher energy means access to territories never explored before.

So far, the 2015 data has not revealed any new particle or phenomena but the amount of data collected was really small. On the contrary, this year, the LHC is outperforming itself, having already delivered five times more data than last year. The hope is that these data will eventually reveal the first signs of something revolutionary. Dozens of new analyses based on the recent data will be presented until August 10 at the ICHEP conference and I’ll present some of them later on.

It took 48 years to discover the Higgs boson after it was first theoretically predicted when we knew what to expect. This time, we don’t even know what we are looking for. So it could still take a little longer. There is more to be found, we all know it. But when will we find it, is another story.

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 ».

To be notified of new blogs, follow me on Twitter : @GagnonPauline or sign up on this distribution list

 

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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!

CMS-lumi-17juin

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.

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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.

table-750GeV

 

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.

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