## Archive for March, 2016

### 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 »

### 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.

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