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Posts Tagged ‘LHCb’

With 2015 a few weeks old, it seems like a fine time to review what happened in 2014 and to look forward to the new year and the restart of data taking. Along with many interesting physics results, just to name a few, LHCb saw its 200th publication, a test of lepton universality. With protons about to enter the LHC, and the ALICE and LHCb detectors recording muon data from transfer line tests between the SPS and LHC (see also here), the start of data-taking is almost upon us. For some implications, see Ken Bloom’s post here. Will we find supersymmetry? Split Higgs? Nothing at all? I’m not going to speculate on that, but I would like to review two techniques which played a key role in two results from LHCb and a few analysis techniques which enabled them.

The first result I want to discuss is the \(Z(4430)^{-}\). The first evidence for this state came from the Belle Collaboration in 2007, with subsequent studies in 2009 and in 2013. BaBar also searched for the state, and while they did not see it, they did not rule it out.

The LHCb collaboration searched for this state, using the specific decay mode \(B^0\to \psi’ K^{+} \pi^{-} \), with \(\psi’\) decaying to two muons. For more reading, see the nice writeup from earlier in 2014. As in the Belle analyses, which looked using muons or electrons in the final \(\psi’\) state, the trick here is to look for bumps in the \(\psi’ \pi^{-}\) mass distribution. If a peak appears which is not described  by the conventional 2 and 3 quark states, mesons and baryons, we know and love, it must be from a state involving a \(c \overline{c}d\overline{u}\) quark combination. The search is performed in two ways: a model-dependent search, which looks at the \(K\pi\) and \(\psi’\pi\) invariant mass and decay angle distributions, and a “model independent” search which looks for structure induced in the \(K\pi\) system induced by a resonance in the \(\psi’\pi\) system and does not invoke any exotic resonances.

At the end of the day, it is found in both cases that the data are not described without including a resonance for the \(Z(4430)^-\).

Now, it appears that we have a resonance on our hands, but how can we be sure? In the context of the aforementioned model dependent analysis, the amplitude for the \(Z(4430)^{-}\) is modeled as a Breit-Wigner amplitude, which is a complex number. If this amplitude is plotted in the imaginary plane as a function of the invariant mass of the resonance, a circular shape is traced out. This is characteristic of a resonance. Therefore, by fitting the real and imaginary parts of the amplitude in six bins of \(\psi’\pi\) invariant mass, the shape can be directly compared to that of an exected resonance. That’s exactly what’s done in the plot below:

The argand plane for the Z(4430)- search. Units are arbitrary.

The argand plane for the Z(4430)- search. Units are arbitrary.

What is seen is that the data (black points) roughly follow the outlined circular shape given by the Breit-Wigner resonance (red). The outliers are pulled due to detector effects. The shape quite clearly follows the circular characteristic of a resonance. This diagram is called an Argand Diagram.


Another analysis technique to identify resonances was used to find the two newest particles by LHCb:

Depiction of the two Xi_b resonances found by the LHCb Collaboration. Credit to Italic Pig (http://italicpig.com/blog/)

Depiction of the two Xi_b resonances found by the LHCb Collaboration. Credit to Italic Pig

Or perhaps seen as


Xi_b resonances, depicted by Lison Bernet.

Xi_b resonances, depicted by Lison Bernet.

Any way that you draw them, the two new particles, the \(\Xi_b’^-\) and \(\Xi_b^{*-}\) were seen by the LHCb collaboration a few months ago. Notably, the paper was released almost 40 years to the day that the discovery of the \(J/\psi\) was announced, sparking the November Revolution, and the understanding that mesons and baryons are composed of quarks. The \(\Xi_b’^-\) and \(\Xi_b^{*-}\) baryons are yet another example of the quark model at work. The two particles are shown in \(\delta m \equiv m_{candidate}(\Xi_b^0\pi_s^-)-m_{candidate}(\Xi_b^0)-m(\pi)\) space below.

Xi_b'^- and Xi_b^{*-} mass peaks shown in delta(m_candidate) space.

\(\Xi_b’^-\) and \(\Xi_b^{*-}\) mass peaks shown in \(\delta(m_{candidate})\) space.

Here, the search is performed by reconstructing \(\Xi_b^0 \pi^-_s\) decays, where the \(\Xi_b^0\) decays to \(\Xi_c^+\pi^-\), and \(\Xi_c^+\to p K^- \pi^+\). The terminology \(\pi_s\) is only used to distinguish between that pion and the other pions. The peaks are clearly visible. Now, we know that there are two resonances, but how do we determine whether or not the particles are the \(\Xi_b’^-\) and \(\Xi_b^{*-}\)? The answer is to fit what is called the helicity distributions of the two particles.

To understand the concept, let’s consider a toy example. First, let’s say that particle A decays to B and C, as \(A\to B C\). Now, let’s let particle C also decay, to particles D and F, as \(C\to D F\). In the frame where A decays at rest, the decay looks something like the following picture.

Simple Model of A->BC, C->DF

Simple Model of \(A\to BC\), \(C\to DF\)

There should be no preferential direction for B and C to decay if A is at rest, and they will decay back to back from conservation of momentum. Likewise, the same would be true if we jump to the frame where C is at rest; D and F would have no preferential decay direction. Therefore, we can play a trick. Let’s take the picture above, and exactly at the point where C decays, jump to its rest frame. We can then measure the directions of the outgoing particles. We can then define a helicity angle \(\theta_h\) as the angle between the C flight in A’s rest frame and D’s flight in C’s rest frame. I’ve shown this in the picture below.

Helicity Angle Definition for a simple model

Helicity Angle Definition for a simple model

If there is no preferential direction of the decay, we would expect a flat distribution of \(\theta_h\). The important caveat here is that I’m not including anything about angular momentum, spin or otherwise, in this argument. We’ll come back to that later. Now, we can identify A as the \(\Xi_b’\) or \(\Xi_b^*\) candidate, C as the \(\Xi_b^0\) and D as the \(\Xi_C\) candidates used in the analysis. The actual data are shown below.

Helicity angle distributions for the Xi_b' and Xi_b* candidates (upper and lower, respectively).

Helicity angle distributions for the \(\Xi_b’ \)and \(\Xi_b*\) candidates (upper and lower, respectively).

While it appears that the lower mass may have variations, it is statistically consistent with being a flat line. Now the extra power of such an analysis is that if we now consider angular momentum of the particles themselves, there are implied selection rules which will alter the distributions above, and which allow for exclusion or validation of particle spin hypotheses simply by the distribution shape. This is the rationale for having the extra fit in the plot above. As it turns out, both distributions being flat allows for the identification of  the \(\Xi ‘_b^-\) and the \(\Xi_b^{*-}\), but do not allow for conclusive ruling out of other spins.

With the restart of data taking at the LHC almost upon us (go look on Twitter for #restartLHC), if you see a claim for a new resonance, keep an eye out for Argand Diagrams or Helicity Distributions.


Let there be beam!

Wednesday, October 15th, 2014

It’s been a little while since I’ve posted anything, but I wanted to write a bit about some of the testbeam efforts at CERN right now. In the middle of July this year, the Proton Synchrotron, or PS, the second ring of boosters/colliders which are used to get protons up to speed to collide in the LHC, saw its first beam since the shutdown at the end Run I of the LHC. In addition to providing beam to experiments like CLOUD, the beam can also be used to create secondary particles of up to 15 GeV/c momentum, which are then used for studies of future detector technology. Such a beam is called a testbeam, and all I can say is WOOT, BEAM! I must say that being able to take accelerator data is amazing!

The next biggest milestone is the testbeams from the SPS, which started on the 6th of October. This is the last ring before the LHC. If you’re unfamiliar with the process used to get protons up to the energies of the LHC, a great video can be found at the bottom of the page.

Just to be clear, test beams aren’t limited to CERN. Keep your eyes out for a post by my friend Rebecca Carney in the near future.

I was lucky enough to be part of the test beam effort of LHCb, which was testing both new technology for the VELO and for the upgrade of the TT station, called the Upstream Tracker, or UT. I worked mainly with the UT group, testing a sensor technology which will be used in the 2019 upgraded detector. I won’t go too much into the technology of the upgrade right now, but if you are interested in the nitty-gritty of it all, I will instead point you to the Technical Design Report itself.

I just wanted to take a bit to talk about my experience with the test beam in July, starting with walking into the experimental area itself. The first sight you see upon entering the building is a picture reminding you that you are entering a radiation zone.


The Entrance!!

Then, as you enter, you see a large wall of radioactive concrete.


Don’t lick those!

This is where the beam is dumped. Following along here, you get to the control room, which is where all the data taking stuff is set up outside the experimental area itself. Lots of people are always working in the control room, focused and making sure to take as much data as possible. I didn’t take their picture since they were working so hard.

Then there’s the experimental area itself.


The Setup! To find the hardhat, look for the orange and green racks, then follow them towards the top right of the picture.

Ah, beautiful. :)

There are actually 4 setups here, but I think only three were being used at this time (click on the picture for a larger view). We occupied the area where the guy with the hardhat is.

Now the idea behind a tracker testbeam is pretty straight forward. A charged particle flies by, and many very sensitive detector planes record where the charged particle passed. These planes together form what’s called a “telescope.” The setup is completed when you add a detector to be tested either in the middle of the telescope or at one end.

Cartoon of a test beam setup. The blue indicates the "telescope", the orange is the detector under test, and the red is the trajectory of a charged particle.

Cartoon of a test beam setup. The blue indicates the “telescope”, the orange is the detector under test, and the red is the trajectory of a charged particle.


From timing information and from signals from these detectors, a trajectory of the particle can be determined. Now, you compare the position which your telescope gives you to the position you record in the detector you want to test, and voila, you have a way to understand the resolution and abilities of your tested detector. After that, the game is statistics. Ideally, you want to be in the middle of the telescope, so you have the information on where the charged particle passed on either side of your detector as this information gives the best resolution, but it can work if you’re on one side or the other, too.

This is the setup which we have been using for the testbeam at the PS.  We’ll be using a similar setup for the testbeam at the SPS next week! I’ll try to write a follow up post on that when we finish!

And finally, here is the promised video.



Major harvest of four-leaf clover

Wednesday, April 9th, 2014

The LHCb Collaboration at CERN has just confirmed the unambiguous observation of a very exotic state, something that looks strangely like a particle being made of four quarks. As exotic as it might be, this particle is sternly called Z(4430)-, which gives its mass at 4430 MeV, roughly four times heavier than a proton, and indicates it is has a negative electric charge. The letter Z shows that it belongs to a strange series of particles that are referred to as XYZ states.

So what’s so special about this state? The conventional and simple quark model states that there are six different quarks, each quark coming with its antiparticle.  All these particles form bound states by either combining two or three of them. Protons and neutrons for example are made of three quarks. All states made of three quarks are called baryons. Other particles like pions and kaons, which are often found in the decays of heavier particles, are made of one quark and one antiquark. These form the mesons category. Until 2003, the hundreds of particles observed were classified either as mesons or baryons.

And then came the big surprise: in 2003, the BELLE experiment found a state that looked like a bound state of four quarks. Many other exotic states have been observed since. These states often look like charmonium or bottomonium states, which contain a charm quark and a charm antiquark, or a bottom and antibottom quarks. Last spring, the BESIII collaboration from Beijing confirmed the observation of the Zc(3900)+ state also seen by BELLE.

On April 8, the LHCb collaboration reported having found the Z(4430)- with ten times more events than all other groups before. The data sample is so large that it enabled LHCb to measure some of its properties unambiguously. Determining the exact quantum numbers of a particle is like getting its fingerprints: it allows physicists to find out exactly what kind of particle it is. Hence, the Z(4430)- state appears to be made of a charm, an anti-charm, a down and an anti up quarks. Their measurement rules out several other possibilities.


The squared mass distribution for the 25,200 B meson decays to ψ’ π- found by LHCb in their entire data set. The black points represent the data, the red curve the result of the simulation when including the presence of the Z(4430)- state. The dashed light brown curve below shows that the simulation fails to reproduce the data if no contribution from Z(4430)- is included, establishing the clear presence of this particle with 13.9σ (that is, the signal is 13.9 times stronger than all possible combined statistical fluctuations. These are the error bars represented by the small vertical line attached to each point).

Theorists are hard at work now trying to come up with a model to describe these new states. Is this a completely new tetraquark, a bound state of four quarks, or some strange combination of two charmed mesons (mesons containing at least one charm quark)? The question is still open.

Pauline Gagnon

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For more information, see the LHCb website


La collaboration LHCb du CERN vient de confirmer hors de tout doute l’existence d’un état très exotique, quelque chose qui ressemble étrangement à une particule formée de quatre quarks. Aussi exotique qu’elle puisse paraître, cette particule porte le nom très pragmatique de Z(4430)-. Ce nom indique sa masse à 4430 MeV, soit  environ quatre fois celle d’un proton, et signale qu’elle a une charge électrique négative. La lettre Z montre qu’elle appartient à une étrange série de particules communément regroupées sous l’appellation d’états XYZ.

Mais qu’est-ce que cet état a donc de si spécial? Le modèle conventionnel des quark est tout simple: il existe six quarks différents, chacun venant avec son antiparticule. Ces douze particules peuvent se combiner pour former des états liés en regroupant deux ou trois d’entre eux. Par exemple, les protons et des neutrons sont composés de trois quarks. Tous les états faits de trois quarks sont appelés baryons. D’autres particules comme les pions et les kaons, qu’on retrouve souvent dans les désintégrations de particules plus lourdes, sont formées d’un quark et d’un antiquark. Elles appartiennent à la catégorie des mésons. Les centaines de particules observées jusqu’en 2003 étaient toutes classifiées soit comme mésons, soit comme baryons.

Puis vint la grande surprise: en 2003, l’expérience BELLE trouva le premier état lié fait en apparence de quatre quarks. Beaucoup d’autres états exotiques similaires ont été observés depuis. Ces états ressemblent souvent à des états de charmonium ou de bottomonium, des particules qui contiennent respectivement un quark charmé et un antiquark charmé, ou un quark bottom et un anti-bottom. Au printemps dernier, la collaboration BESIII de Beijing a confirmé l’observation du Zc(3900)+, un état aussi détecté par BELLE.

Le 8 avril, la collaboration LHCb a rapporté avoir trouvé l’état Z(4430)- avec dix fois plus d’événements que tous les autres groupes précédents. Leur échantillon de données est si grand qu’il a permis à LHCb de mesurer certaines de ses propriétés sans équivoque. La détermination des nombres quantiques exacts d’une particule équivaut à l’obtention de ses empreintes digitales: cela permet aux physicien-ne-s de cerner plus exactement à quelle particule on a affaire. Il en ressort que l’état Z(4430)- serait formé d’un quark charmé, d’un antiquark charmé, d’un quark d et un antiquark u. Leur mesure exclut toutes autres possibilités.


La distribution de la masse (au carré) des 25200 mésons B se désintégrant en paires de ψ’ π- trouvés par LHCb dans l’ensemble de leurs données. Les points noirs représentent les données expérimentales et la courbe en rouge, le résultat de la simulation lorsqu’on inclut la présence du Z(4430)-. La courbe en pointillés juste en dessous en brun clair montre que la simulation ne peut reproduire les données si on supprime la contribution du Z(4430)-. Ceci établit clairement la présence de cette particule avec 13.9σ (c’est-à-dire le signal est 13.9 fois plus fort que toutes les fluctuations statistiques combinées possible. La fluctuation de chaque point est représentée par la petite ligne verticale qui lui est attachée).

Les théoricien-ne-s sont à pied d’oeuvre pour essayer d’imaginer un modèle pouvant décrire ces nouveaux états. S’agit-il d’états complètement nouveaux faits de quatre quarks liés ensemble, des tétraquarks, ou est-ce une étrange combinaison de deux mésons charmés (des mésons contenant au moins un quark charmé)? La question est toujours ouverte.

Pauline Gagnon

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Pour plus de tails (en anglais) voir le site de l’expérience LHCb



Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen


B Decays Get More Interesting

Friday, February 28th, 2014

While flavor physics often offers a multitude of witty jokes (read as bad puns), I think I’ll skip one just this time and let the analysis speak for itself. Just recently, at the Lake Louise Winter Institute, a new result was released for the analysis looking for \( b\to s\gamma\) transitions. Now this is a flavor changing neutral current, which cannot occur at tree level in the standard model. Therefore, the the lowest order diagram which this decay can proceed by is the one loop penguin shown below to the right.

\(b\to s\gamma \\)

One loop penguin diagram representing the transition \(b \to s \gamma \).

From quantum mechanics, photons can have either left handed or right handed circular polarization. In the standard model, the photon in the decay \(b\to s\gamma\) is primarily left handed, due to spin and angular momentum conservation. However, models beyond the standard model, including some minimally super symmetric models (MSSM) predict a larger than standard model right handed component to the photon polarization. So even though the decay rates observed for \(b\to s\gamma\) agree with those predicted by the standard model, the photon polarization itself is sensitive to new physics scenarios.

As it turns out, the decays \(B^\pm \to K^\pm \pi^\mp \pi^\pm \gamma \) are well suited to explore photon polarizations after playing a few tricks. In order to understand why, the easies way is to consider a picture.

Definition of \(\theta\)

Picture defining the angle \(\theta\) in the analysis of \(B^\pm\to K^\pm \pi^\mp \pi^\pm \gamma\). From the Lake Louise Conference Talk

In the picture to the left, we consider the rest frame of a possible resonance which decays into \(K^\pm \pi^\mp \pi^\pm\). It is then possible to form the triple product of \(p_\gamma\cdot(p_{\pi,slow}\times p_{\pi,fast})\). Effectively, this defines the angle \(\theta\) defined in the picture to the left.

Now for the trick: Photon polarization is odd under parity transformation, and so is the triple product defined above. Defining the decay rate as a function of this angle, we find:

\(\frac{d\Gamma}{d \cos(\theta)}\propto \sum_{i=0,2,4}a_i cos^i\theta + \lambda_i\sum_{j=1,3} a_j \cos^j \theta\)

This is an expansion in Legendre Polynomials up to the 4th order. The odd moments are those which would contribute to photon polarization effects. The lambda is the photon polarization. Therefore, by looking at the decay rate as a function of this angle, we can directly access the photon polarization. However, another way to access the same information is by taking the asymmetry between the decay rate for events where theta is above the plane and those where theta is below the plane. This is then proportional to the photon polarization as well and allows for direct statistical calculation. We will call this the up-down asymmetry, or \(A_{ud}\). For more information, a useful theory paper is found here.

Enter LHCb. With the 3 fb\(^{-1}\) collected over 2011 and 2012 containing ~14,000 signal events, the up-down asymmetry was measured.

Up-down asymmetry for the analysis of \(b\to s\gamma\).

Up-down asymmetry for the analysis of \(b\to s\gamma\). From the Lake Louise Conference Talk

In bins of invariant mass of the \(K \pi \pi\) system, we see the asymmetry is clearly non-zero, and varies across the mass range given. As seen in the note posted to the arXiv, the shapes of the fit of the Legendre moments are not the same in differing mass bins, either. This corresponds to a 5.2\(\sigma\) observation of photon polarization in this channel. What this means for new physics models, however, is not interpreted, though I’m sure that the arXiv will be full of explanations given about a week.


At the European Physics Society conference in Stockholm, two experiments operating at the Large Hadron Collider (LHC) at CERN, LHCb and CMS reported on July 19 solid evidence that the Standard Model of particle physics still shows no sign of wear and tear by checking a prediction of the model to the ninth decimal place.

The Standard Model makes very accurate predictions but theorists know this theory has its limits. At higher energy, its equations start breaking down. Theorists are convinced that despite all the success of this model, it is not giving us the big picture. Hence, scientists have been trying to find a “secret passage” to the next level, a more encompassing and more robust theory.

One way to achieve this is to look for a small deviation in a measured quantity from the value predicted by the Standard Model and a good place to find such a deviation is in an extremely rare process. It is much easier to hear a faint noise in a quiet place than in the middle of traffic during rush hour.

Specifically, the scientists measured how often composite particles denoted Bs and Bd (pronounced “b sub s and b sub d)” mesons decay into a pair of muons (particles similar to electrons but about 200 times heavier). A Bs meson is a composite particle containing b and s quarks while Bd mesons are made of b and d quarks. These heavy particles are unstable and quickly break apart into lighter particles.

The Standard Model predicts that Bs mesons decay into a pair of muons about three times in a billion while for Bd mesons, it occurs thirty times less often. This gives two excellent places to look for small deviations that could reveal the existence of new phenomena not foreseen within the Standard Model.

All theories going beyond the Standard Model come with new particles that would affect how other particles decay, i.e. how they break apart. Decays are very much like making change for a big coin. Imagine a coin of one euro. It can be broken into pieces of 1, 5, 10, 20 or 50 cents Now, say a new 25-cent coin is introduced. An automatic teller would not give change for one euro in a particular way (say with coins of 50, 20, 20 and 10 cents) as often as before just because new possibilities now exist.

By measuring how often a Bs meson decays into muons, scientists were hoping to see the first deviations from the predictions of the Standard Model. On the contrary, the two experiments confirmed this prediction within experimental errors.

CMS, whose name stands for Compact Muon Spectrometer, and LHCb, an experiment designed specifically to study particles containing b quarks, are particularly suited for these types of measurements. CMS got (3.0 +1.0-0.9) x 10-9 and LHCb obtained (2.9 +1.1-1.0) x 10-9, while the Standard Model prediction stands at (3.5 ± 0.3) x 10-9. The significances of the CMS and LHCb signals correspond to 4.3σ and 4.0σ, respectively, which means, the excesses of events that are seen most likely come from signal and not from background. Two other experiments presented new results based on smaller data samples. ATLAS (using a partial data sample) and D0 (final result with their full data sample) and they obtained the same upper limit at 15 x 10-9.Bs-mumu-combo

The results obtained by LHCb and CMS, as well as their combined value, is compared to the prediction from the Standard Model shown by the vertical black line and its theoretical uncertainty (green band).

For Bd decays, 95% confidence level upper limits were set at 7.4 x 10-10 for LHCb while CMS obtained 11 x 10-10. The Standard Model predicts this to be less than 1 x 10-10.

All these values are consistent with the Standard Model predictions but they do not yet rule out new physics. After the LHC resumes operation at higher energy in 2015, the LHC experiments will continue improving their Bs measurements. In particular, they will aim to get a first measurement for Bd mesons instead of an upper limit, and then evaluate the ratio for the Bs and Bd mesons, such that some of the experimental and theoretical uncertainties will cancel out, to obtain an even more precise measurement. Since no deviations were found in the ninth decimal position, it means the experiments need to check the tenth decimal position.

More details can be found on the CMS and LHCb websites.

Pauline Gagnon

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Lors de la conférence de la Société européenne de physique à Stockholm, deux expériences du Grand collisionneur de hadrons (LHC) du CERN, LHCb and CMS ont apporté des preuves solides que le Modèle standard de la physique des particules ne montre toujours aucun signe de fatigue en poussant la vérification de l’une des prédictions du modèle jusqu’à la neuvième décimale.

Le modèle standard permet des prédictions très précises, mais les théoricien-ne-s savent que cette théorie a ses limites. À plus haute énergie, ses équations commencent à flancher. Les théoricien-ne-s sont donc convaincu-e-s que malgré tout le succès de ce modèle, il ne nous donne qu’une image incomplète du monde matériel. Par conséquent, les scientifiques cherchent l’entrée du “passage secret” vers un niveau supérieur, révélant une théorie plus globale et plus robuste.

Une façon d’y parvenir est de rechercher le moindre petit écart par rapport aux prévisions théoriques. Et un bon endroit pour trouver une petite déviation est en regardant parmi les procédés extrêmement rares. Il est beaucoup plus facile de déceler un léger murmure dans un endroit calme qu’au beau milieu de la circulation aux heures de pointe.

Plus précisément, les scientifiques ont mesuré la fréquence de désintégrations de particules composites appelées mésons Bs et Bd en une paire de muons (particules similaires aux électrons mais 200 fois plus lourdes). Un méson Bs est une particule composite contenant un quark b et un quark s alors que les mésons Bd sont faits de quarks b et d. Ces particules lourdes sont instables et se désintègrent rapidement en particules plus légères.

Le modèle standard prédit que les mésons Bs se brisent et donnent une paire de muons environ trois fois sur un milliard de désintégrations tandis que pour les mésons Bd, cela devrait se produire environ trente fois moins souvent. Voilà donc deux excellents endroits où l’existence de phénomènes nouveaux non prévus dans le Modèle standard pourrait créer de petites déviations par rapport aux prédictions.

Toutes les théories allant au-delà du Modèle standard s’accompagnent de nouvelles particules. Ces particules affecteraient les possibilités de désintégrations des autres particules, c’est à dire comment elles se brisent. Une désintégration est très semblable à la façon de faire la monnaie pour une grosse pièce. Imaginez une pièce d’un euro. Elle peut être échangée pour des pièces de 1, 5, 10, 20 ou 50 centimes. Mais si on introduit des pièces de 25 centimes, un distributeur automatique ne donnerait plus la monnaie d’un euro en pièces de 50, 20, 20 et 10 centimes aussi souvent qu’avant parce que de nouvelles possibilités existeraient.

En mesurant combien de fois les mésons Bs et Bd se désintègrent en muons, les scientifiques espéraient voir pour la première fois un écart par rapport aux prédictions du Modèle standard. Au contraire, les deux expériences ont confirmé cette prédiction, du moins à l’intérieur des marges d’erreur.

CMS, qui signifie Spectromètre Compact pour Muons, et LHCb, une expérience conçue spécifiquement pour étudier les quarks b, sont tout particulièrement désignées pour ce genre de mesures. CMS a obtenu (3,0 +1,0-0,9) x 10-9 et LHCb (2,9 +1,1-1,0) x 10-9 alors que la prédiction du Modèle standard s’établit à (3,5 ±  0,3)  x 10-9. Cela correspond à des mesures à 4,3σ et 4,0σ, donc venant beaucoup plus probablement du signal plutôt que d’une fluctuation du bruit de fond. Deux autres expériences ont présenté de nouveaux résultats mais basés sur de plus petits échantillons de données. ATLAS (données partielles) et D0 (données finales) mesurent toutes les deux la même limite supérieure, soit 15 x 10-9.


Les résultats obtenus par LHCb et CMS pour les mésons Bs, ainsi que la prédiction théorique du Modèle standard (ligne verticale en noir) avec la marge d’incertitude théorique (bande verte).

Pour les désintégrations de mésons Bd, les collaborations LHCb et CMS ont toutes les deux pu placer  la limite supérieure à 7,4 x 10-10 pour LHCb et 11 x 10-10 pour CMS avec un indice de confiance de 95%.  La prédiction du Modèle standard se situe à moins de 1 x 10-10.

Tous ces résultats sont en accord avec les prédictions du Modèle standard. Après le redémarrage du LHC à plus haute énergie en 2015, les expériences du LHC raffineront leurs mesures pour les mésons Bs et tenteront d’obtenir une première mesure pour les mésons Bd (et non pas seulement une limite). Eventuellement, elles pourront mesurer le rapport entre les mésons Bs et Bd. Ceci permettra à certaines incertitudes expérimentales et théoriques de s’annuler, ce qui donnera une mesure encore plus précise. Puisqu’aucun écart n’a été décelé à la neuvième décimale, nous devrons aller voir ce qui se passe à la dixième décimale.

Tous les détails se trouvent sur les sites de CMS et LHCb (en anglais seulement).

Pauline Gagnon

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Oh what a beautiful day

Tuesday, July 23rd, 2013

In case you hadn’t heard, the past few days have been big days for B physics, i.e. particle physics involving a b quark. On the 18th and 19th, there were three results released in particular, two by LHCb and one by CMS. Specifically, on the 18th LHCb released their analysis of \( B_{(s)}\to\mu\mu\) using the full 3 fb\(^{-1}\) dataset, corresponding to 1 fb\(^{-1}\) of 2011 data at 7 TeVand 2 fb\(^{-1}\) of 2012 data at 8 TeV. Additionally, CMS also released their result using 5 fb\(^{-1}\) of 7 TeV and 30 fb\(^{-1}\) of 8 TeV data.


The decay \(B_{(s)}\to\mu\mu\) cannot decay via tree-level processes, and must proceed by higher level processes ( shown below)

These analyses have huge implications for SUSY. The decay \( B_{(s)}\to\mu\mu\) cannot proceed via tree-level processes, as they would involve flavor changing neutral currents which are not seen in the Standard Model (picture to the right). Therefore, the process must proceed at a higher order than tree level. In the language of Feynman Diagrams, the decay must proceed by either loop or penguin diagrams, show in the diagrams below. However, the corresponding decay rates are then extremely small, about \(3\times10^{-9}\). Any deviation from this extremely small rate, however, could therefore be New Physics, and many SUSY models are strongly constrained by these branching fractions.

The results reported are:

Experiment    \(\mathcal{B}(B_{s}\to\mu\mu)\) Significance \(\mathcal{B}(B\to\mu\mu)\)
LHCb \( 2.9^{+1.1}_{-1.0} \times 10^{-9}\) 4.0\(\sigma\) \(<7.4\times 10^{-10}(95\% CL) \)
CMS \(3.0^{+1.0}_{-0.9}\times 10^{-9}\) 4.3 \(\sigma\) \(< 1.1\times 10^{-9} (95\% CL)\)

Higher order diagrams

Both experiments saw an excess of events events for the \(B_{s}\to\mu\mu)\) channel, corresponding to \(4.o\sigma\) for LHCb (updated from \(3.5 \sigma\) of last year), and \(4.3\sigma\) for CMS. The combined results will, no doubt, be out very soon. Regardless, as tends to happen with standard model results, SUSY parameter space has continued to be squeezed. Just to get a feel of what’s happening, I’ve made a cartoon of the new results overlaid onto an older picture from D. Straub to see what the effect of the new result would be. SUSY parameter space is not necessarily looking so huge. The dashed line in the figure represents the old result. Anything shaded in was therefore excluded. By adding the largest error on the branching fraction of \(B_s\to\mu\mu\), I get the purple boundary, which moves in quite a bit. Additionally, I overlay the new boundary for \(B\to\mu\mu\) from CMS in orange and from LHCb in green. An interesting observation is that if you take the lower error for LHCb, the result almost hugs the SM result. I won’t go into speculation, but it is interesting.

Cartoon of updated limits

Cartoon of Updated Limits on SUSY from \(B\to\mu\mu\) and \(B_s\to\mu\mu\). Orange Represents the CMS results and green represents LHCb results for \(B_s\to\mu\mu\) . Purple is the shared observed upper limit on \(B\to\mu\mu\). Dashed line is the old limit. Everything outside the box on the bottom left is excluded. Updated from D. Straub (http://arxiv.org/pdf/1205.6094v1.pdf)


Additionally, for a bit more perspective, see Ken Bloom’s Quantum Diaries post.

As for the third result, stay tuned and I’ll write about that this weekend!


Vieille énigme résolue

Saturday, July 20th, 2013

Ce matin, lors de la conférence de la Société européenne de physique à Stockholm, l’expérience LHCb du Grand collisionneur de hadrons (LHC)

du CERNa présenté un élément de plus pour clore le chapitre sur une étrange situation qui gardait les théoricien-ne-s perplexes depuis une vingtaine d’années.

LHCb a présenté la mesure la plus précise à ce jour de la durée de vie du baryon b. Les baryons sont des particules formées de trois quarks. Par exemple, les protons et les neutrons sont constitués d’une combinaison de quarks u et d. Ce qui rend les baryons b spéciaux, c’est qu’ils contiennent un quark b, un type de quark beaucoup plus lourd. Toutes les particules composites contenant des quarks b comme les mésons B (faits d’un quark b et soit d’un quark u ou d) et les baryons b sont instables, ce qui signifie qu’ils ont une courte durée de vie. Environ une picoseconde après avoir été créés, ils se décomposent en particules plus légères.

En théorie, les mésons B et les baryons b devraient avoir à peu près la même durée de vie. Mais dans les années 1990, quand le CERN fonctionnait avec le précurseur du LHC, l’accélérateur appelé le LEP (Large Electron Positron Collider), toutes les expériences mesuraient une durée de vie systématiquement plus courte pour les baryons b que pour les mésons B comme on peut le voir sur le graphique ci-dessous. Bien que les marges d’erreurs étaient grandes, la tendance générale vers des valeurs plus basses était d’autant plus surprenante puisque les quatre expériences (ALEPH, DELPHI, OPAL et L3) travaillaient indépendamment.


Les différentes valeurs mesurées pour la durée de vie des baryons b au fil du temps avec les plus anciennes en bas et les toutes dernières venant du LHC tout en haut. La durée de vie mesurée est maintenant très proche de 1.5 picoseconde soit celle mesurée pour les mésons B.

Cette situation avait incité plusieurs théoricien-ne-s à réexaminer leurs calculs et à chercher un effet négligé qui aurait pu expliquer la différence. Malgré tous leurs efforts, il était pratiquement impossible de réconcilier la durée de vie mesurée pour les baryons b (quelque part entre 1,1 à 1,3 picoseconde) avec celle des mésons B qui était à environ 1,5 picosecondes.

Une décennie plus tard, D0 et CDF, deux expériences d’un autre accélérateur, le Tevatron près de Chicago, ont commencé à combler l’écart. Mais il a fallu attendre une autre décennie pour que les expériences du LHC démontrent qu’en fait, il n’y a pas grand différence entre la durée de vie des baryons b et celle des mésons B.

Plus tôt cette année, ATLAS et CMS avaient toutes les deux mesuré des valeurs plus en accord avec la durée de vie des mésons B. Avec ce dernier résultat de haute précision de l’expérience LHCb, il y a maintenant suffisamment d’évidence pour clore l’affaire après une vingtaine d’années de questionnement. LHCb a mesuré la durée de vie du baryon b à 1,482 ± 0,018 ± 0,012 picoseconde. Le rapport à la durée de vie des mésons B est de 0,976 ± 0,012 ± 0,006, une valeur très proche de un, tel que prévu théoriquement.

Une explication possible serait que toutes les expériences du LEP étaient affectées par une erreur systématique commune mais toujours inconnue. Ou simplement une fluctuation statistique (i.e. malchance !). La cause exacte ne sera peut être jamais identifiée, mais au moins, le problème est résolu. C’est une grande réussite pour les théoricien-ne-s qui savent désormais que leurs calculs étaient justes, et ce depuis le début.

Pauline Gagnon

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