Searching for the A boson with CMS November 22, 2014Posted by dorigo in physics.
I am quite happy to report today that the CMS experiment at the CERN Large Hadron Collider has just published a new search which fills a gap in studies of extended Higgs boson sectors. It is a search for the decay of the A boson into Zh pairs, where the Z in turn decays to an electron-positron or a muon-antimuon pair, and the h is assumed to be the 125 GeV Higgs and is sought for in its decay to b-quark pairs.
If you are short of time, this is the bottomline: no A boson is found in Run 1 CMS data, and limits are set in the parameter space of the relevant theories. But if you have a bit more time to spend here, let’s start with the beginning – What’s the A boson, you might wonder for a start.
The A particle is one of the five physical states resulting from the breaking of the electroweak symmetry by the Higgs mechanism, when instead of the minimal insertion in the standard model Lagrangian of a single complex doublet of scalar fields, one inserts two such doublets. The Higgs mechanism works quite similarly: instead of having plugged in four degrees of freedom with the complex doublet, three of which get absorbed by the positive and negative charged W and the Z boson, which all get mass terms in the Lagrangian, and one remains as The higgs boson, we have inserted eight degrees of freedom, so we expect five higgs-like new states to appear.
Of the five higgses of two-doublet models, there are one positively and one negatively-charged one, two more scalars – the h and the H – and the pseudoscalar A. In the analysis it is assumed that h is the particle we found at 125 GeV two years ago. This typically makes the A heavy, although the space of parameters is quite complex and the phenomenology quite varied. As past searches of the A at lower energy have failed, the analysis concentrates on a mass range where the Zh final state is a possibility for the A disintegration: so the A is supposed to be heavier than 216 GeV -the total of Z and h masses.
CMS has collected 20 inverse femtobarns of proton-proton collisions at 8 TeV, and in that data sample there are tens of millions of Z boson decays to ee or μμ pairs. It is exactly there that the search starts. Then, two b-quark-tagged jets are sought in addition; the mass of the two b-jets is required to be close to 125 GeV; and a multi-variate algorithm is used to distinguish the selected data from backgrounds.
Data selection starts from millions of collision events and ends up with a sample of few thousands of them, where a possible A signal would be more easily seen. The initial selection includes two leptons from Z decay, then two jets events are kept, then only ones where the jets are b-tagged, and finally a cut is placed on the output of a multi-variate algorithm (a BDT, boosted-decision trees) which distinguishes data from backgrounds using the distinctive features of the A decay kinematics.
As the two leptons and the two b-jets are supposed to come from the decay of a single particle, the four-body mass is the most distinguishing variable to look at. However, if you just combine the mass of the two leptons and two jets your mass resolution will be good but not great: jet energies, in particular, suffer from a 10% relative resolution which smears a bit the resulting peak from a resonance decay. What is done in the analysis is to fit the four 4-momenta to the hypothesis that the leptons come from the Z and the b’s come from the decay of the 125 GeV higgs. This “pulls” measured energies and momenta in the right direction and the final result is that the resolution on the four-body mass shrinks quite spectacularly, as you may check in the graph below.
You can see several mass peaks, relative to different mass hypotheses for the A boson, before (dashed curves) and after (full curves) the application of the kinematical fit. It is clear that this strongly improves the chance to observe a signal in the data, especially if there is a large background -mostly due to Z+bb production and top production.
One of the nice things about the analysis is that in the final data sample all contributing backgrounds are tightly constrained in normalization from a global fit to a number of control regions simultaneously. There are control regions that specifically select top-rich events, Z+bb-rich events, Z+b-rich events, e.g.; so the fit is capable of correcting the simulation prediction for the yield of each of them. Below, for instance, is the missing ET distribution of the control region rich in top-pair decays: you can see that the yellow top contribution dominates the data, so a match of data and simulation in this control region strongly fixes the top contribution.
In the end, the search for the signal is performed in the two-dimensional plane of 4-body mass and BDT output. This makes less easy to display the fit result, but one can produce projections, e.g. in the mass distribution. As the search is performed in a wide mass range, and the kinematics of the decay is strongly dependent on the A mass, three different BDTs are trained to select the signal. For the central mass region this is the mass projection:
If you are wondering about the few high bins near 320 GeV, well, that’s a 2-sigmaish fluctuation, an effect which is entirely expected given the number of mass points that are independently investigated by the search. The result of the search is that there is no A boson in the data. Upper limits are derived in the context of two-doublet higgs models, and a series of such limits are obtained in the parameter space of the models. I refrain from showing those, as you can only appreciate them if you are an expert, and in that case I strongly suggest that you download the public CMS document describing the analysis, by paying a visit to the public web page of the analysis.
Standard Model or Minimal SUSY ? April 6, 2014Posted by dorigo in physics.
(I posted the text below in my current blog, which is at this link; I post it here as well as I would like to keep this blog active by writing something here now and then – TD).
If I look back at the first times I discussed the important graph of the top quark versus W boson mass, nine years ago, I am amazed at observing how much progress we have made since then. The top quark mass in 2005 was known with 2-3 GeV precision, the W boson mass with 35 MeV precision, and we did not know where the Higgs boson was, or if there was one.
Today, the top quark mass is measured with a 770 MeV uncertainty and the W boson mass with a 15 MeV uncertainty. That alone is a reduction of a factor of 10 in the allowed phase space of those two parameters; but crucially, we no also know the Higgs mass with a 0.5% accuracy. This leaves very little space for the true parameters of the standard model. On the other hand, if the SM were to be enlarged to a minimal version of Supersymmetry, then the theory predictions would blow up considerably, as the MSSM allows much more freedom to those parameters as others (like squark masses) are varied.
The summary of the experimental situation is shown in the graph below, which Sven Heinemeyer produced today for this blog (thanks Sven). The graph summarizes calculations produced by Heinemeyer and his colleagues Hollik, Stockinger, Weiglein and Zeune. In the graph the horizontal axis shows possible values of the top quark mass, in the very restricted range allowed by the latest world’s best CMS measurement; the vertical axis shows values of the W boson mass, in an even narrower range in absolute terms, thanks to precise measurements of that quantity performed by LEP2 and the Tevatron experiments. The experimental determination of those two parameters is symbolized by a grey ellipse which encompasses 68% of their probable values.
Then if we stay within the standard model, the Higgs boson mass measurements by the CERN experiments (+-0.7 GeV) force the two parameters to be bound to lie within the very narrow red line; if instead we take the MSSM as the true underlying theory, the whole green area is possible; different points of this area correspond to different value of other parameters (here a more liberal variation of the Higgs mass is taken, to cover more possibilities). The downward arrow symbolizes that as one increases the “mass scale” of the MSSM the allowed region moves closer to the SM line.
Note that in this graph the grey ellipse and the red line are the only experimental inputs; there is no “LEP indirect” oval here, as this would be too wide for the graph. In other words, the precision electroweak information from the Z boson studies of the nineties has become largely irrelevant in this particular view (it remains a formidable input to verify the general agreement of SM and data, if one studies other parameters).
So, what should we carry home from this graph ? I believe at least two things. One, that the SM likes the W mass to be a bit lower than what is currently measured, and the top quark to be a bit higher; the tension is however only mild -we are talking about just a bit more than one standard deviation for the disagreement. Two, that the MSSM is not killed by these measurements – it would live on regardless of the precise values of W and top masses, as the breadth of the green area shows.
Oh, and a third thing – the experimental measurements of these quantities rock!
Other considerations can be made, but I will stop here for tonight. Tomorrow I will be on a train at 6 in the morning, to participate in a 2-day open discussion organized by INFN in Rome, called “WHAT NEXT”. A very interesting discussion on the long term plans of italian research based on the current status of particle physics, astrophysics, cosmology, and other fundamental investigations. I will have something to report on that later on…
Devotion to the Tevatron October 2, 2011Posted by dorigo in physics.
(This post first appeared on my current site, www.science20.com/quantum_diaries_survivor . Please note that I post on this site quite rarely. Please visit me on the science20 site for updates on experimental particle physics and more!).
I’m nostalgic tonight. The reason ? The Tevatron has finally stopped running, for good.
It’s strange to find out one can mourn the shutdown of a synchrotron just as the passing away of an old friend, but that’s more or less how I feel like tonight. And I am not even among the ones who can claim to have been around for the full duration of the machine’s lifetime, like Giorgio Chiarelli – as Giorgio recounted here, he was there in the CDF control room when the first proton-antiproton beams collided the first time, in 1985.
I started working in CDF in June 1992. In the course of these 19 years I have learned all I know about particle physics, and I have met a large number of extraordinary people. Not only ones from CDF: the Fermi laboratories are of course a place where you interact with the “competitors” from the other experiment, DZERO -you work elbow to elbow, go to the same parties, seminars, and events, and you share joys and frustrations as the machine which feeds data to both experiments outperforms or suffers technical stops. Plus of course technicians, machinists, administrative staff: a number of people who simply did their job there, but who all shared the pride of doing their part for the success of this remarkable human adventure.
As Gary Taubes explains in juicy details in his book “Nobel Dreams” (1987), it was Carlo Rubbia in the late seventies who first launched the idea of a proton-antiproton collider at Fermilab. Back then, he got severely beaten up by the lab director -he had a bad record of changing horses mid-race and keeping proposing new projects. But Rubbia was right: the technology was just getting mature enough for such a machine to be built. In the course of six months Rubbia learned all there was to know, and then some, about making antiprotons; and then CERN accepted his project. The W and Z bosons were discovered by the SppS experiments in 1983. By then, the Tevatron was already in place at Fermilab. Too late to challenge the discovery of the vector bosons, but timely to provide a precise measurement of their mass, and to search for the sixth quark, the top.
The history of the Tevatron and its many successes will no doubt be told by people who have participated more actively and deeply in it than myself, so there is no point for me to try and do that here tonight. I only choose to tell a personal story here. One where I take the part of the moron, incidentally, but that’s beside the point (and not that uncommon after all).
I was in charge as Scientific Coordinator in the CDF control room a few years ago, leading a crew of physicists in the task of taking data as smoothly as possible during my seven-night shift. It was not the first time on that job, but I was eager to see data coming in -you get that kind of feeling when you sit during long nights waiting for something to happen. For a few times in a row during the past nights the sequence of injection of beams in the Tevatron had been started, and then aborted, for a string of reasons which were not immediately clear, and in some cases possibly caused by human errors. In a moment of scorn, I let go with a sarcastic sentence in the CDF E-log: “Injecting protons again. Let’s see where they screw up this time”.
Now I should explain that, since English is not my native language, I have often trouble gauging how strong words are in a sentence I say or write. To me it sounded a bit like saying “let’s see where they find the trouble this time”, or not too much worse than that. But it was by far too careless.
So I was not intending to insult anybody’s competence, but the sentence was indeed inflamatory, plus of course unfit to an E-log. Worse than that, and not considered by me at the moment, the CDF E-log was readable by anybody, Main Control Room Machinists included. Heck, you too could read it in real time.
The following morning I was not even reproached too much for my stupid sentence, but from the feedback I got in a number of ways, that one time I learned quite something. I learned that out there, reading the CDF E-log, there were not just us in the control room, plus the other people on shift in the Main Control Room and in the DZERO CR, plus a handful of people on call for any problem the hardware or software could be facing. There was a whole community of colleagues who read the E-log as frequently as an addicted user reads his or her Facebook homepage. People who cared for the machine, for the data taking, for the success of the experiment. People who had devoted their life to make this as good Science as it could possible be. People who were ready to provide help to solve problems even when they were not on call, and who woke up in the middle of the night just to see whether a new store was in. All of these people had felt outraged by the lack of respect I had shown to the Tevatron machinists.
I have always said I appreciate men and women who regardless how serious their job is do not neglect to take it with a grain of irony; and yet I saw a flaw in that line of reasoning, confronted with the devotion of so many brilliant minds to the common good -the advancement of Science at the hands of the machine they had contributed building and operating for many years.
So the Tevatron has been turned off today, and many people are sad, each in their own private way. To many, the Tevatron was their life. To all, it was the machine that created the opportunity of getting together to work in a friendly, stimulating environment, growing professionally and intellectually. It was a fascinating machine, which deserves a whole chapter in a history of particle physics. 28 years old, with its glitchs and hiccups, by now old and patched up, the Tevatron was still incredibly performant until the very end. Farewell, Tevatron!
Greek Blog November 1, 2010Posted by dorigo in Blogroll, internet, language, news, personal, physics, science.
I had forgotten to link it from here, but the internet always provides a chance for redemption. So here I go. A couple of months ago I have opened another wordpress blog, where I write on particle physics – in Greek. This is a rather extravagant choice, and I think I need to spend a few words explaining it.
First of all, there is my love of the language, which I have been studying for two years. It is a difficult language to master, due to the interplay of several factors: the different alphabet, the enormous wealth of words, and the rather quick evolution of rules and uses. Maybe because of these challenges, I have found it quite entertaining to get on top of it.
The second reason for writing in Greek is, in fact, that I have yet a lot to learn, and I think that writing about science is a very good exercise, allowing me to find a solution to the translation problems I may encounter if I discuss about my job – physics – in that language.
The third reason is that I think there is no offer whatsoever in the web for a blog about particle physics in Greek (if you know any, let me know). So I might just try to fill that hole myself.
In short, the new blog is an experiment. I do not know, nor can predict, how long it will last; for now, if you know modern Greek please stop by. Below is a list of my recent efforts:
ICHEP blog July 12, 2010Posted by dorigo in astronomy, Blogroll, cosmology, internet, news, physics, science.
Just one line here to mention that since May there is a new blog out there – a temporary blog that will cover the end of July event in Paris – the International Conference on High Energy Physics -, how we get there, and the aftermath. The effort includes several well-known bloggers in high-energy physics, and is definitely worth following.
You can visit it here.
Some recent posts you might want to read March 6, 2010Posted by dorigo in Blogroll, internet, news, physics, science.
Tags: B decays, CDF, CMS, Higgs boson, particle physics, quark, top quark, W boson, weak interactions
As the less distracted among you know, I have moved my blogging activities to scientific blogging last April. I wish to report here a list of interesting posts I have produced there in the course of the last few months (precisely, since the start of 2010). They are given in reverse chronological order and with zero commentary – come see if you are curious.
- Understanding muon decay
- CDF on Higgs decays to diphotons
- Bose-Einstein interferences: the collider view
- Are quarks and leptons elementary or composite?
- Constraints on the Higgs mass from the muon anomaly
- Tevatron Higgs searches: past and future
- Exotic hadrons: there is the rub
- The fascinating search for rare W decays
- Three papers on the muon anomaly
- Particle physics in 2020
- Triggering: the subtle art of being picky
- New rare B decays nailed by CDF: a door to new physics?
- The approved CMS Phi signal with 900 GeV data
- Three top quarks: a door to new physics ?
- Luminosity, Michel Parameter, Phase space: what a lousy title for a great post
A reminder for the distracted October 5, 2009Posted by dorigo in physics.
This blog is now inactive since April 15th, 2009, and although I will try to keep it active, by posting links every once in a while to my most relevant articles on the Scientific Blogging site which now hosts my main activities, you should update your bookmarks if you have not done so yet. I keep getting about 500 daily hits here, mostly from google searches of the few good posts among the thousand and more that I have put together in over three years of activity.
So please visit my other blog at Scientific Blogging! You will not be disappointed.
One million hits June 29, 2009Posted by dorigo in Blogroll, internet, news, personal, physics.
While this site has been basically inactive for over two months, it still draws some residual traffic due to google searches and links; so the hit counter has continued to click after April 15th, although at a rate of roughly a third of what it did before.
Today’s news is that we got past the millionth click. Thanks to everybody for your interest in particle physics and in my reports. Please visit www.scientificblogging.com/quantum_diaries_survivor to keep up-to-date with particle physics!
Physics Highlights – May 2009 June 2, 2009Posted by dorigo in news, physics, science.
Tags: CDF, DZERO, Fermi, heavy quarks, Hess, QCD, Randall, standard model
Here is a list of noteworthy pieces I published on my new blog site in May. Those of you who have not yet updated their links to point there might benefit from it…
Four things about four generations -the three families of fermions in the Standard Model could be complemented by a fourth: a recent preprint discusses the possibility.
Fermi and Hess do not confirm a dark matter signal: a discussion of recent measurements of the electron and positron cosmic ray fluxes.
Nit-picking on the Omega_b Discovery: A discussion of the significance of the signal found by DZERO, attributed to a Omega_b particle.
Nit-picking on the Omega_b Baryon -part II: A pseudoexperiments approach to the assessment of the significance of the signal found by DZERO.
The real discovery of the Omega_b released by CDF today: Announcing the observation of the Omega_b by CDF.
CDF versus DZERO: and the winner is…: A comparison of the two “discoveries” of the Omega_b particle.
The Tevatron Higgs limits strenghtened by a new theoretical study: a discussion of a new calculation of Higgs cross sections, showing an increase in the predictions with respect to numbers used by Tevatron experiments.
Citizen Randall: a report of the giving of honorary citizenship in Padova to Lisa Randall.
Hadronic Dibosons seen -next stop: the Higgs: A report of the new observation of WW/WZ/ZZ decays where one of the bosons decays to jet pairs.