## Posts Tagged ‘LHCb’

### 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 $$\Delta m_d$$

### 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.

### Finding a five-leafed clover

Wednesday, July 15th, 2015

Photo Credit: Cathy Händel, Published on http://www.suttonelms.org.uk/olla12.html

Sometimes when you’re looking for something else, you happen across an even more exciting result. That’s what’s happened at LHCb, illustrated in the paper “Observation of $$J/\psi p$$ resonances consistent with pentaquark states in $$\Lambda_b^0\to J/\psi K^-p$$ decays”, released on the arXiv on the 14th of July.

I say this is lucky because the analysts found these states while they were busy looking at another channel; they were measuring the branching fraction of $$B^0\to J/\psi K^+ K^-$$. As one of the analysts, Sheldon Stone, recalled to me, during the review of the $$B^0$$ analysis, one reviewer asked if there could be a background from the decay $$\Lambda_b^0\to J/\psi K^- p$$, where the proton was misidentified as a kaon. As this was a viable option, they looked at the PDG to see if the mode had been measured, and found that it had not. Without a certain knowledge of how large this contribution would be, the analysts looked. To their surprise, they found a rather large rate of the decay, allowing for a measurement of the lifetime of the $$\Lambda_b^0$$. At the same time, they noticed a peak in the $$J/\psi p$$ spectrum. After completing the above mentioned analysis of the $$B^0$$, they returned to the channel.

It’s nice to put yourself in the analysts shoes and see the result for yourself. Let’s start by looking at the decay $$\Lambda_b^0\to J/\psi p K^-$$. As this is a three body decay, we can look at the Dalitz Plots.

Dalitz plots from the decay $$\Lambda_b^0\to J/\psi K^- p$$. Compiled from http://arxiv.org/abs/1507.03414

The above Dalitz plots show all combinations of possible axes to test. In the one on the left, around $$m^2=2.3$$ GeV$$^2$$, running vertically, we see the $$\Lambda(1520)$$ resonance, which decays into a proton and a kaon. Running horizontally is a band which does not seem to correspond to a known resonance, but which would decay into a $$J/\psi$$ and a proton. If this is a strong decay, then the only option is to have a hadron whose minimum quark content is $$uud\bar{c}c$$. The same band is seen on the middle plot as a vertical band, and on the far right as the sloping diagonal band. To know for sure, one must perform a complete amplitude analysis of the system.

You might be saying to yourself “Who ordered that?” and think that something with five quarks hadn’t been postulated. This is not the case. Hadrons with quark content beyond the minimum were already thought about by Gell-Mann and Zweig in 1964 and quantitatively modeled by Jaffe in 1977  to 4 quarks and 5 quarks by Strottman in 1979. I urge you to go look at the articles if you haven’t before.

It appears as though a resonance has been found, and in order to be sure, a full amplitude analysis of the decay was performed. The distribution is first modeled without any such state, shown in the figures below.

Projections of the fits of the$$\Lambda_b^0\to J/\psi K^- p$$ spectrum without any additional components. Black is the data, and red is the fit. From http://arxiv.org/abs/1507.03414

Try as you might, the models are unable to explain the invariant mass distribution of the $$J/\psi p$$. Without going into too much jargon, they wrote down from a theoretical standpoint what type of effect a five quark particle would have on the Dalitz plot, then put this into their model. As it turns out, they were unable to successfully model the distribution without the addition of two such pentaquark states. By adding these states, the fits look much better, as shown below.

Mass projection onto the $$J/\psi p$$ axis of the total fit to the Dalitz plot. Again, Black is data, red is the fit. The inset image is for the kinematic range $$m(K p)>2 GeV$$.
From http://arxiv.org/abs/1507.03414

The states  are called the $$P_c$$ states. Now, as this is a full amplitude analysis, the fit also covers all angular information. This allows for determination of the total angular momentum and parity of the states. These are defined by the quantity $$J^P$$, with $$J$$ being the total angular momentum and $$P$$ being the parity. All values for both resonances are tried from 1/2 to 7/2, and the best fit values are found to be with one resonance having $$J=3/2$$ and the other with $$J=5/2$$, with each having the opposite parity as the other. No concrete distinction can be made between which state has which value.

Finally, the significance of the signal is described by under the assumption $$J^P=3/2^-,5/2^+$$ for the lower and higher mass states; the significances are 9 and 12 standard deviations, respectively.

The masses and widths turn out to be

$$m(P_c^+(4380))=4380\pm 8\pm 29 MeV$$

$$m(P_c^+(4450))=4449.8\pm 1.7\pm 2.5 MeV$$

With corresponding widths

Width$$(P_c^+(4380))=205\pm 18\pm 86 MeV$$

Width$$(P_c^+(4450))=39\pm 5\pm 19 MeV$$

Finally, we’ll look at the Argand Diagrams for the two resonances.

Argand diagrams for the two $$P_c$$ states.
From http://arxiv.org/abs/1507.03414

Now you may be saying “hold your horses, that Argand diagram on the right doesn’t look so great”, and you’re right. I’m not going to defend the plot, but only point out that the phase motion is in the correct direction, indicated by the arrows.

As pointed out on the LHCb public page, one of the next steps will be to try to understand whether the states shown are tightly bound 5 quark objects or rather loosely bound meson baryon molecule. Even before that, though, we’ll see if any of the other experiments have something to say about these states.

### Looking Forward to 2015: Analysis Techniques

Tuesday, January 27th, 2015

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.

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

Or perhaps seen as

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.

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 $$A\to BC$$, $$C\to DF$$

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

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).

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.

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

### Grosse moisson de trèfles à quatre feuilles

Wednesday, April 9th, 2014

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

### Moriond 2014: New Results, New Explorations… but no New Physics

Friday, April 4th, 2014

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.

This 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.

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.

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 $$b\to s\gamma$$. 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.

### The Standard Model checked to the ninth decimal

Tuesday, July 30th, 2013

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.

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