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Notes on the new Higgs boson search by DZERO March 2, 2009

Posted by dorigo in news, physics, science.
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Three weeks ago the DZERO collaboration published new results of their low-mass Higgs boson search. This is about the production of Higgs bosons in association with a W boson, with the subsequent decay of the Higgs particle to a pair of b-quark jets, and the decay of the W to an electron-electron neutrino or muon- muon neutrino pair: in symbols, what I mean is p \bar p \to WH \to e \nu_e b \bar b, or p \bar p \to WH \to \mu \nu_{\mu} b \bar b. I wish to describe this important new analysis today, but first let me make a point about the reaction above.

In order to make this blog more accessible than it would otherwise be, I frequently write things inaccurately: precision is usually pedantic and distracting. But here I beg you to please note a detail I will not gloss over for once: to be accurate, one should write p \bar p \to WH + X…, because what we care for is inclusive production of the boson pair. If we omit the X, strictly speaking we are implying that the two protons annihilated into the two bosons, with exactly nothing else coming out of the collision. While that reaction is possible, it is ridiculously rare -actually, the annihilation into ZH is possible, while the one into WH does not conserve electric charge and is strictly forbidden. Anyway, bringing along a symbol to remind ourselves of the fact that our projectiles are like garbage bags, which fill our detectors with debris when we throw them at one another, is cumbersome and annoying, while accurate. I hope, however, you realize that this is an important detail: Higgs bosons at a hadron collider are always accompanied by debris from the dissociating projectiles.

Two words on associated WH production and its merits

The associated production of the Higgs together with a W boson is the “golden” signature for low-mass Higgs hunters at the Tevatron collider. While producing the Higgs together with another heavy object is not effortless (you are required to produce the collision with more energetic quarks in the two colliding protons, and this makes the production less frequent), the W boson pays back with extra dividends by producing a very clean signature in its leptonic decay, and by allowing the event to be spotted easily by the online triggering system, and collected with high efficiency by the data acquisition.

If you compare the collection of WH events to the collection of directly produced Higgs bosons (p \bar p \to H +X, where again I prefer accuracy by specifying the X), you immediately see the advantage of the former: while their production rate is four times smaller and the leptonic W decay only occurs 20% of the times, this 0.25 x 0.2=0.05=1/20 reduction factor is a small price to pay, given the trouble one would have triggering on direct H \to b \bar b events: the decay to a pair of b quarks is the dominant one for low Higgs boson masses, but the common nature of b-jets makes it unobservable.

Higgs decays to b-quark pairs produced alone simply cannot be triggered in hadronic collisions, because they are immersed in a background which is six orders of magnitude higher in rate, namely the production p \bar p \to g \to b \bar b of bottom-antibottom quark pairs by strong interactions. Even assuming that the online triggering system of DZERO were capable of spotting b-quark jet pairs with 100% purity (which is already a steep hypothesis), the trigger would have to accept a million background events in order to collect just one fine signal event !

Yes, life is tough for hadronic signatures at a hadron collider. Even finding the Z \to b \bar b signal, which is a thousand times more frequent, is a tough business -it took CDF years to find a reasonable sample of those decays, while DZERO has not yet published anything on the matter. But the Tevatron experiments cannot ignore the fact that, if a low Higgs mass is hypothesized, the H \to b \bar b decay is the most frequent: the Higgs boson likes to decay into the heaviest pair of particles it can produce. If the total mass of a pair of W bosons or Z bosons is too heavy, the next-heaviest pair of decay products is b-quarks. This dictates the need to search for H \to b \bar b, and the trouble of triggering on such a process in turn makes the associated WH (or ZH) production the most viable signal.

The DZERO analysis

The new analysis by DZERO studies a total integrated luminosity of 2.7 inverse femtobarns. This corresponds to 150 trillion proton-antiproton collisions, but DZERO has netted almost twice as much data already by now, and it is only a matter of time before those too get included in this search: so one has to bear in mind that the statistical power of the data is soon going to increase by about 40%: the data increase corresponds to an increase in precision by the square root of two, or a factor of 1.41.

DZERO selects events which have an electron or a muon with high energy -the tag of a leptonic decay of the W boson-, missing transverse energy, and two or three hadronic jets. The presence of a energy imbalance in the plane transverse to the beam direction is a comparatively clean signature of the escape of the energetic neutrino produced together with the charged lepton by the W decay, and two jets are expected from the decay of the Higgs boson to a pair of b quarks. However, you might well ask, quid opus fuit tertium ?

No, I bet you would not ask it that way -for some reason, a reminescence of Latin sprung up in my mind. Quid opus fuit tertium – What is the matter with the third one ? The third jet is not specifically a signature of any one of the decay products of the WH pair we are after. However, if you remember what I mentioned above, we are searching for inclusive production of a WH pair: that means we accept the fact that the two projectiles also produced an additional energetic stream of hadrons in the final state. That possibility is by no means rare, and in fact it amounts to about 20% of the Higgs production events. By selecting events with two or three jets, DZERO increases its acceptance of signal events sizably.

A technique which has become commonplace in the hunt of elusive subnuclear particles is to slice and dice the data: categorizing events in disjunct classes is a powerful analysis strategy. By taking two-jet events on one side, and three-jet events on the other, DZERO can study them separately, and appreciate the different nuisances of each class. In fact, they further divide the data into subsets where one jet was tagged as a b-quark-originated one, or two of them were.

And they also keep separated the electron+jets and the muon+jets events: this also does make sense, since the experimental signatures of electrons and muons are slightly different, as are the resulting energy resolutions. In total, one has eight disjunct classes, depending on the number of jets, the number of b-tags, and the lepton species.

In order to decide whether there is a hint of Higgs bosons in any of the classes, backgrounds are studied using Monte Carlo simulations of all the Standard Model processes which could contribute to the eight selected signatures. These include the production of a W boson plus hadronic jets (“W+jets“) as well as the production of top quark pairs: both these processes produce energetic leptons in the final state; but another background is due to events which do not actually contain a lepton, and where a hadronic jet was mistook for one. The latter is called “QCD background” highlighting its origin in strong interaction processes yielding just hadronic jets: despite the rarity of a jet faking a energetic lepton, the huge rate of QCD events makes this background sizable.

Among the characteristics that can separate the WH signal from the above backgrounds, the identity of the parton originating the hadronic jets is a powerful one: b-jets are more rare than light-quark ones, but there must be two of them in a H \to b \bar b decay. DZERO uses a neural network which employs seven discriminating variables to select jets with a likely b-quark content.

The good thing with a neural-network b-tagger is that the output of the network can be dialed to decide its purity. And in fact, DZERO does exactly that. They start with a loose selection which has a rate of “false positives” of 1.5% (light-quark jets that are classified as b-tagged). If two jets have such a loose b-tag, the event is classified as a “double b-tag”; otherwise, the NN output requirement is made tighter, and “single-b-tag” events are collected by requiring that the b-tag has a better purity, with a “false positive” rate of 0.5%. These cuts have been optimized for their combined sensitivity to the Higgs signal.

Apart from b-tags, the signal displays a different kinematics than all backgrounds. Again, seven variables are used, which now describe the event kinematics: the transverse energy of the second-leading jet, the angle between jets, the dijet invariant mass, and a matrix-element discriminant, which is computed by comparing the probability density of the quadrimomenta of the objects produced in the decay in a WH event to that of backgrounds. In the figure above, the matrix element discriminant is shown for all the processes contributing to the class of W+2jet events with two b-tags. The output of the neural network shows that Higgs events fall in the right-side of the distribution, while backgrounds pile up mostly on the left, as can be seen in the figure below.

Results of the search

Since no signal is observed in the NN output distribution seen in the data, DZERO proceeds to set upper limits on the signal cross-section. For 2-jet events they use the NN output is used, while they use the dijet mass distribution for the 3-jet event classes. No justification is provided in their paper for this choice, which looks slightly odd to me, but I imagine they have done some optimization studies before taking this decision. However, I would imagine that the NN output is in principle always more discriminant than just one of the variables on which the network is constructed… Maybe somebody from DZERO could clarify this point in the comments thread, to the benefit of the other readers ?

At the end of the day, DZERO obtains limits on the cross section of the searched signal, which are still above the standard model predictions whatever the Higgs mass: therefore, they do not provide an exclusion of mass values, yet. These results, however, once combined with other results from CDF and DZERO, will one day directly imply that a SM Higgs cannot exist, if its mass is in a specified range. In the graph below you can see the limit set by this analysis on the WH production cross-section as a function of Higgs mass.

The black curve shows the 95% exclusion, while the hatched red curve shows the result that DZERO was expecting to find, based on pseudoexperiments. The comparison of the two curves is not terribly informative, but it does show that there were not surprises from the data.

The result can also be shown in the standard “LLR plot” above, which is showing, again as a function of the Higgs boson mass, the log-likelihood ratio of two hypotheses: the “background only” and the “signal+background” one. Let me explain what that is. For each mass value on the x-axis, imagine the Higgs is there. Then, with large statistics, the data would show a propension for the “signal plus background” hypothesis, and the LLR would be large and negative. If, instead, the Higgs did not exist at any mass value, the LLR would be large and positive. The two hypotheses can be run on pseudo-data of the same statistical power as the data really collected, thus producing the red and black hatched lines in the plot below. The two curves are different, but the red one does not manage to depart from the green band constructed around the black hatched one: that means that the data size and the algorithms used in the analysis do not have enough power to discriminate the two hypotheses, not even at 1-sigma level (which is the meaning of the width of the green band, while the yellow one shows two-sigma contours). The full black line shows the behavior of real data: they have a propension of confirming the background-only hypothesis at low mass, and a slight penchant for the signal+background one at about 130 GeV. But this is a really, really small fluctuation, well within the one-sigma band!

I think the LLR plot is a great way to describe the results of the search visually. It at once tells you the power of the analysis and the available data, and the outcome on the real events collected. Now, it takes twenty thick lines of text to explain it, but once you’ve grabbed its meaning…

Some posts you might have missed in 2008 January 5, 2009

Posted by dorigo in cosmology, personal, physics, science.
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To start 2009 with a tidy desk, I wish to put some order in the posts about particle physics I wrote in 2008. By collecting a few links here, I save from oblivion the most meaningful of them -or at least I make them just a bit more accessible. In due time, I will update the “physics made easy” page, but that is work for another free day.

The list below collects in reverse chronological order the posts from the first six months of 2008; tomorrow I will complete the list with the second half of the year. The list does not include guest posts nor conference reports, which may be valuable but belong to a different list (and are linked from permanent pages above).

June 17: A description of a general search performed by CDF for events featuring photons and missing transverse energy along with b-quark jets – a signature which may arise from new physics processes.

June 6: This post reports on the observation of the decay of J/Psi mesons to three photons, a rare and beautiful signature found by CLEO-c.

June 4 and June 5 offer a riddle from a simple measurement of the muon lifetime. Readers are given a description of the experimental apparatus, and they have to figure out what they should expect as the result of the experiment.

May 29: A detailed discussion of the search performed by CDF for a MSSM Higgs boson in the two-tau-lepton decay. Since this final state provided a 2.1-sigma excess in 2007, the topic deserved a careful look, which is provided in the post.

May 20: Commented slides of my talk at PPC 2008, on new results from the CDF experiment.

May 17: A description of the search for dimuon decays of the B mesons in CDF, which provides exclusion limits for a chunk of SUSY parameter space.

May 02 : A description of the search for Higgs bosons in the 4-jet final state, which is dear to me because I worked at that signature in the past.

Apr 29: This post describes the method I am working on to correct the measurement of charged track momenta by the CMS detector.

Apr 23, Apr 28, and May 6: This is a lengthy but simple, general discussion of dark matter searches with hadron colliders, based on a seminar I gave to undergraduate students in Padova. In three parts.

Apr 6 and Apr 11: a detailed two-part description of the detectors of electromagnetic and hadronic showers, and the related physics.

Apr 05: a general discussion of the detectors for LHC and the reasons they are built the way they are.

Mar 29: A discussion of the recent Tevatron results on Higgs boson searches, with some considerations on the chances for the consistence of a light Higgs boson with the available data.

Mar 25: A detailed discussion on the possibility that more than three families of elementary fermions exist, and a description of the latest search by CDF for a fourth-generation quark.

Mar 17: A discussion of the excess of events featuring leptons of the same electric charge, seen by CDF and evidenced by a global search for new physics. Can be read alone or in combination with the former post on the same subject.

Mar 10: This is a discussion of the many measurements obtained by CDF and D0 on the top-quark mass, and their combination, which involves a few subtleties.

Mar 5: This is a discussion of the CDMS dark matter search results, and the implications for Supersymmetry and its parameter space.

Feb 19: This is a divulgative description of the ways by which the proton structure can be studied in hadron collisions, studying the parton distribution functions and how these affect the scattering measurements in proton-antiproton collisions.

Feb 13: A discussion of luminosity, cross sections, and rate of collisions at the LHC, with some easy calculations of the rate of multiple hard interactions.

Jan 31: A summary of the enlightening review talk on the standard model that Guido Altarelli gave in Perugia at a meeting of the italian LHC community.

Jan 13: commented slides of the paper seminar gave by Julien Donini on the measurement of the b-jet energy scale and the p \bar p \to Z X \to b \bar b X cross section, the latter measured for the first time ever at a hadron machine. This is the culmination of a twelve-year effort by me and my group.

Jan 4: An account of the CDF search for Randall-Sundrum gravitons in the ZZ \to eeee final state.

Scientific wishes for 2009 December 31, 2008

Posted by dorigo in astronomy, Blogroll, cosmology, personal, physics, science.
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I wish 2009 will bring an answer to a few important questions:

  • Can LHC run ?
  • Can LHC run at 14 TeV ?
  • Will I get tenure ?
  • Are multi-muons a background ?
  • Are the Pamela/ATIC signals a prologue of a new scientific revolution ?
  • Will England allow a NZ scientist to work on Category Theory on its soil ?
  • Is the Standard Model still alive and kicking in the face of several recent attempts at its demise ?

I believe the answer to all the above questions is yes. However, I am by no means sure all of them will be answered next year.

CDF publishes multi-muons!!!! October 31, 2008

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NB: This post is aimed at physicists.. However if you are not one, but you are really curious, you might find out that for once the annoying feeling of reading cryptic jargon is paid back by some real news!

I guess the most important message of the post you are about to read is: Do not check the arxiv today if you really cannot spend a couple of hours reading. Make it three. The  paper just released by CDF, titled “Study of multi-muon events produced in ppbar collisions at sqrt(s)=1.96 TeV” is guaranteed to have you fastened to the chair until you are done with its 70 pages.

The article reports on a very careful investigation produced by CDF, using Run II data collected by a trigger selecting events with two (or more) muons of low transverse energy. The study addresses two or three long-standing inconsistencies in measurements of bottom quark production and phenomenology at the Tevatron:

  • the cross section for b \bar b production appears in good agreement with next-to-leading order QCD predictions when b-quarks are tagged by a reconstruction of their decay vertex, while it is found to be significantly larger when the cross section is measured by identifying b-quarks through their semileptonic decay;
  • the invariant mass spectrum of pairs of leptons produced in sequential semileptonic decays (b \to l X \to l l' Y) is not well modeled by the simulation of b-flavored hadrons in CDF;
  • the value of the time-integrated mixing probability of b flavoured hadrons is measured at the Tevatron to be significantly larger than that measured by LEP experiments.

The source of these apparently unrelated inconsistencies is traced back by the study to a sample of events where muons are originated several centimeters away from the primary interaction point (the proton-antiproton collision vertex), which makes b-quark decay as implausible a source as any other Standard Model process, no better than the other backgrounds which the study shows to be insufficient explanation for the observed events: punch-through from pions and kaons, or secondary hadronic interactions in the detector material.

Once a large sample of such weird events are statistically isolated -better say evidenced- in the sample, a further anomaly is found in the number of additional muons contained in narrow cones around the original ones, something which cannot be easily explained with conventional physics. The paper discusses the characteristics of these events, without falling in the trap of putting together an exotic explanation. Instead, what is made clear in the paper is that those measurements quoted above -lepton-based cross sections and phenomenology of b-quarks studied in high-energy hadron collisions- are affected by the findings described in this paper.

Below I offer two plots extracted from the preprint. The first one shows the impact parameter distribution of muons in the events constituting the anomalous signal (black points), compared to the impact parameter of muons attributable to QCD sources (in red). The impact parameter resolution for these tracks is 2.5 times smaller than the bin size. One observes a abnormal tail of muons with very large impact parameter. I recall that the impact parameter, which is measured in the plane transverse to the beam direction, is the distance of closest approach of the backward extrapolation of the track to the primary interaction vertex. A impact parameter of one centimeter is huge, given that the typical decay length of a B meson is of the order of a pair of millimeters.

On the right you see an exponential fit to the impact parameter distribution of the trigger muons for the anomalous events, for events with just two (top) or more than two (bottom) muons inside two narrow cones around the trigger muons. The distribution agrees with the decay of a particle with a lifetime in the 20 picosecond range.

To quote the paper, the first lines of the Introduction offer a quite clear picture of the situation:

“This article presents the study of events, acquired with a dedicated dimuon trigger, that we are currently unable to fully explain with our understanding of the CDF II detector, trigger, and event reconstruction. We are continuing detailed studies with a longer timescale for completion, but we present here our current findings.

The conclusions are also clear, but I will leave them to those of you who want to read a paper which might, just might, constitute the first evidence of physics beyond the Standard Model, ever.

That said, if you have read this blog long enough, you know me for a tough sceptic. I of course would be simply delighted if the CDF signal of multi-muons really were a first evidence for new physics; but I have to play the devil’s advocate, and so one word of caution, make it five paragraphs, is mandatory. Of course, despite the evidence is pretty solid from a statistical standpoint, one must lean back and take a breath. We are discussing the possibility that something really spectacular has just lurked out of CDF data. Extraordinary claims require extraordinary evidence, and once statistical evidence is plain, one must delve with systematics. CDF did, and they have not found any significant source which might account for the effect. But investigations should and will continue.

Is CDF sure about the impossibility of explaining this effect away ? No, CDF does not exclude that possibility, although it is my opinion that the collaboration has reviewed the paper with more care and detail than most of the other papers it has published in its illustrious, 25-year-long life. That means nothing in terms of the likelihood that this result is indeed new physics. It just says we are as sure as we can be that we cannot presently explain it with known sources. Also worth mentioning is that CDF is a really disciplined collaboration, which has really been careful with their claims so far. And the present paper is no exception.

Is there a simple New Physics explanation of the observed effect ? No, as far as I understand no existing model of new physics predicted such a signature in advance, although one must acknowledge that a few ideas exist in the literature which might have a connection with the effect, if proven real. However, there is a paper discussing a similar signature, which probably benefitted from knowing the CDF result in advance from an internal source. I will leave this issue to another time and another place.

Can CDF find more evidence in the near future ? Yes, the analysis of electron events may shed more light on the matter, and although electrons are harder to isolate than muons when they have a low energy, the analysis will be carried out.

Can D0 find a similar signal ? Surely. D0 is a similar detector to CDF, and although their charged particle tracking is slightly inferior to CDF’s, their muon system is more extended, and their silicon detector is also at least as good as that of CDF (ok, even slightly better). The problem with D0, I think, is the time it will take to perform such a complicated analysis. One must not forget that before focusing on these anomalous events, CDF produced a lengthy investigation of the correlated b \bar b cross section, which is the back-bone of the multi-muon analysis, since it demonstrates the understanding of heavy flavors in low-transverse-momentum lepton samples in CDF when particles with large impact parameter are excluded. So, it may take a while to D0 to confirm or disprove the effect CDF is now publishing.

Does the signal hint at other anomalies in different analyses ? That, I am sorry to say, is anybody’s guess. If the multi-muon events are a signal of new physics, then I am sure there is something else to be found, somewhere. The problem is: what is that ? One might be tempted to speculate that data samples collected in past experiments could in principle contain a similar signature: charged tracks with very large impact parameter have been seldom studied at colliders, and tracking algorithms might have purposely discarded those tracks, or could be proven inefficient in their collection. For instance, CDF does collect, with its fantastic SVT trigger, events containing tracks showing a significant impact parameter. However, the efficiency with which the SVT collects those events, if studied as a function of impact parameter, dies out much too soon. Hell, nobody designs a detector aimed at collecting a new physics signature no theorists have thought about!

I imagine hordes of theoretical physicists canceling flights, conferences, and courses today, making room for some serious thinking in their agendas. Good luck!

UPDATE: see the interesting discussion developing at Peter Woit’s site, where he points out a paper by Arkani-Hamed and collaborators which appears quite extraordinarily to have foreseen the above signature of new physics, in a very timely fashion!

UPDATE: there are other bloggers who’ve discussed this. Lubos, Carl, Matti (happy birthday Matti). Others to be added soon…

UPDATE: other excellent, entertaining bloggers have added their own comments to the story: Jester, Chad, theorema egregium. In italian: Marco. In dutch: astroblogs.

UPDATE: John, a fellow collaborator in CDF and one very skilled physicist, explains the result at cosmic variance.

No Z’ below 1 TeV September 7, 2008

Posted by dorigo in news, physics, science.
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New heavy bosons are predicted by several models of physics beyond the standard model. In particular, heavy versions of the Z° boson, called generically Z’ but sometimes distinguished by greek subscripts (Z_\eta, Z_\chi, Z_\psi etc.), might constitute the quickest route to a discovery of superstring-inspired E6 models; and also Kaluza-Klein spin-2 gravitons may appear as a Z’. I won’t describe what those models are about (and besides, I am not the best person to do that), but just mention that many of my colleagues pin their hopes of finding new physics on just such a signature: the production of a new Z’ boson, with its decay to a pair of charged leptons. A pair of muons of several hundred GeV, for instance, is a great discovery channel, because muons cannot easily be mistaken with other final state particles -all collider detectors have an outer shell of drift chambers specifically designed to detect muons, in fact, exploiting the high penetrating power of these particles.

With only a few days separating us from the official start of LHC operations, it is now as good a time as any to take stock with respect to the experimental situation with the search for Z’ bosons. A recent result by CDF, based on 2.3 inverse femtobarns of proton-antiproton collisions produced by the Tevatron accelerator, has pushed the lower limit for the mass of these particles above the TeV. Interestingly, one TeV was a reference point good enough for CMS and ATLAS to produce expectation plots in their technical design reports. Take CMS, for instance: the expected dimuon mass spectrum after just 100 inverse picobarns (about ten times the data that LHC will collect this fall) would present a very narrow, distinctive peak of about 18 events, as shown by the empty histogram over the sharply falling background (shown by the green histogram).

The graph above would be a unmistakable evidence for the production of a new massive neutral particle. Unfortunately, we now know it’s just not going to happen. The CDF result excludes a Z’ boson with mass below 1030 GeV. The analysis is straightforward: having noticed that the mass is measured with the momenta of the two muons, which are obtained from their curvature in the 1.4 Tesla magnetic field, one finds that the mass resolution degrades significantly with dimuon mass, but if one plots the inverse of the mass, this has a fixed relative resolution, making it much easier to search for a signal of unknown mass in a wide range. The data (blue points) is shown in the plot below.

From the very good agreement of all data points with the expectation -which is due to the sum of electroweak production of muon pairs through the so-called Drell-Yan mechanism (yes, that includes the regular Z° boson decay) and background processes due to QCD- it is not too hard to extract direct lower limits on the cross-section times branching-ratio of Z' \to \mu \mu. These are shown below (the red curve) as a function of the hypothetical Z’ mass.

The plot is busy as much as it is colorful. First off, ignore the stretched Brazil flag, and only look at the red curve. That is the upper limit on the cross section, at 95% Confidence Level. That is the result of trying to fit a signal in the histogram of inverse masses, which does not seem to contain any. At 1 TeV, the limit is set at 3.5 femtobarns. Since a SM-like Z’ would have a 4.5 femtobarn cross section, such a particle is excluded. All mass values above 1030 GeV are instead still possible.

The “brazil flag” is then just a prediction of the cross-section limit that CDF could set, a priori computed using the analysis methodology, before looking at the data. The red line wiggles around but stays within the 1-sigma band (yellow).

The phase space of new physics continues to shrink, without any real hint from collider data of the SM becoming inadequate…