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Special reportXxxxx xxx

ANIL ANANTHASWAMY

ON 9 December last year, as JohnConway looked at the results of hisexperiment, a chill ran down his neck.For 20 years he has been searching forone of the most elusive things in theuniverse, the Higgs boson aka theGod particle which gives everythingin the cosmos its mass. And here,buried in the debris generated by theworlds largest particle smasher, werea few tantalising hints of its existence.

Conway first revealed the news ofhis experiment earlier this year in ablog. Experimental particle physicistsare sceptics by nature, loath to claimthe discovery of any new particle, letalone a particle of the Higgss stature,and in his blog Conway dismissed hintsof its existence as an aberration, just asmany other supposed signs of theelusive particle have proved to be aftercloser examination. The tiny blips inConways data have so far simplyrefused to go away.

Whats more, using data madepublic last week in a second blog,another team of researchers hasindependently seen hints of a newparticle with similar mass. Both resultsmay yet be dismissed, but thecoincidence is striking, and is one thatis getting physicists excited. If theyhave found evidence of a Higgs particle,

then it points towards the existenceof a universe in which each and everyparticle we know of has a heaviersuper-partner, an arrangement of thecosmos known as supersymmetry.

The Higgs boson is infamous as theonly particle predicted by the standardmodel of physics that remainsundetected. In theory, every otherparticle in the universe gets its mass byinteracting with an all-pervading fieldcreated by Higgs bosons. If the Higgs isdiscovered, the standard model couldjustifiably claim to be the theory thatunifies everything except gravity.

But the model is creaking. Take theHiggs itself. The standard model tightlylinks the masses of the Higgs, the Wboson (the carrier of the weak nuclearforce), and the top quark (one of thefundamental constituents of matter).Experiments at the Large Electron-Positron (LEP) collider at CERN, nearGeneva, in the late 1990s, and at theTevatron, Fermilabs 6.3-kilometre-longparticle accelerator at Batavia, Illinois,

where Conway detected his blips, havehomed in on the mass of the W bosonand the top quark. If you use thesemeasurements to calculate the massrange of the Higgs, and compare it withthe standard models predictions, yourun into trouble. The bestmeasurements of the W and top quarkmass dont agree well with the

standard model, says Conway, who isbased at the University of California,Davis (see Diagram, page 11).

Physicists such as John March-Russell of the University of Oxfordgo further. If you ask most theoristsabout the Higgs, they will say it is veryunlikely that well see just the standardmodel Higgs, he says. And that is whatmakes the hints of new particles seenby Conway and others so intriguing.

Super-partnersWith the help of the Collision Detectorat Fermilab (CDF) Conways team hasbeen searching for a more complexversion of the Higgs than the standardmodel predicts one that mightsupport the supersymmetry modelof the universe.

In supersymmetry, an electron hasa heavier partner called the selectron,while quarks have squarks, and so on.Although none has yet been found,supersymmetry solves some nigglingquestions raised by the standard

model. For instance, when particlephysicists take the measured strengthsof the electromagnetic and the weakand strong nuclear forces, andextrapolate them to the ultra-highenergies of the early universe, they aresupposed to unify. The idea is that inthe early universe these forces were thesame. To get the forces to unify at

If the blips in the debris of the Tevatron particle sasher really are signs ofthe Higgs boson then its not what we expected. It ight ean that its tie toreplace the standard odel with a ore coplex picture of the universe

Conways teamhas beensearching for amore complex

version of Higgs,one thatmight supportsupersymmetry

fermIlab/cdf

Glimpses of the God particle?

What lurks in thedebris of the worldslargest particle smasher?

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this grand unified theory (GUT) scale,the parameters of the standard modelhave to be tuned to an astoundingprecision of 1 part in 1032.

This extreme fine-tuning makesmany theorists uneasy. Why should theproperties of the early universe have tobe so exact to give rise to the universewe have today? It is like creating in a

straitjacket, says March-Russell.Supersymmetry, specifically aversion called the minimalsupersymmetric model, achieves thisgrand unification more naturally, withfar less fine-tuning. The theory predictsfive Higgs bosons of different masses,which makes the process by which theuniverse gets its mass morecomplicated than that laid out by thestandard model with its single Higgs.But very often, in the history ofscience, nature likes simple concepts,but it has quite complicatedrealisations, says March-Russell.

Its a manifestation of this complexreality that Conways team has beenprobing. They are after one of the fiveHiggs predicted by minimalsupersymmetry. Such a Higgs could beproduced by the collision of protonsand antiprotons at the Tevatron andsome would decay into two tau leptons,which are heavier cousins of theelectron. The taus decay immediatelyinto other particles, and it is this debristhe team was sifting through.Essentially, they were creating a plotwhich showed the mass of the particlesthat could give rise to two tau leptonson the x-axis, and the number of suchparticles on the y-axis.

Conway admits they only expectedto see known particles decaying intotau leptons. But then, on that Saturdaymorning before Christmas, the CDFteam saw the blip in their plot: signsthat the Tevatron had produced a smallnumber of some unknown particlewith a mass of 160 gigaelectronvolts(GeV), which had promptly decayed to

two tau leptons. I thought maybe, justmaybe, this could be the beginning ofsomething, says Conway.

Convinced by their analysis, theentire CDF experiment team approvedthe data on 4 January and Conwaypresented it at a conference in Aspen,Colorado, a few days later. The teamhad found a signal which, in particlephysics lingo, had a 2-sigmasignificance a 1 in 50 chance of being

a random fluctuation. Normally, tomerit new particle status a signal mustbe significant to 5-sigma wheretheres only a 1 in 10 million chanceof it being a fluctuation.

People were excited to see this,says Conway. But why was there somuch excitement if the signal wasstatistically insignificant? Thatsbecause a supersymmetric Higgs at thismass is extremely plausible. This kindof [Higgs] mass of 160 GeV is on thelower end of what we were expecting,but we are comfortable with it, in thecontext of supersymmetric models,says Jack Gunion, a theoretical physicistat the University of California, Davis.

He has been advocating anotherversion of supersymmetry called next-to-minimal supersymmetry. When

Gunion saw Conways graph showing apossible Higgs with a mass of 160 GeV,he realised he only had to tune theparameters of his theory by about1 part in 10 to explain it an amountmost physicists are willing to accept.You can only do that in next-to-minimal supersymmetry, saysGunion. To make the minimalsupersymmetry model of the universefit, you would have to tune it to levels

that would make many physicistsuncomfortable, he says.

This is not the first time Gunion hasused next-to-minimal supersymmetryto explain an anomalous signal. In thelate 1990s, the LEP collider at CERN,which smashed electrons andpositrons head-on, saw what seemedto be a new particle with a mass of

100 GeV. Again, the significance of thesignal was about 2-sigma, not enoughto claim a discovery. Because the signaldid not sit well with a standard modelHiggs, it was mostly ignored, and theLEP shut down in 2000, making itimpossible to check the signal further.It is still a big deal, says Gunion,because nobody could explain it.

But Gunions next-to-minimalmodel could and does. I claim that themodel provides a simple explanation,namely that there is a Higgs at 100 GeV,and that it decayed in some extra waysthat werent expected.

That means the LEP data from the1990s and Conways latest findingsfrom the CDF experiment could pointto two of the five supersymmetricHiggs particles, one with a mass of 100GeV and the other with a mass of 160GeV. Gunion, for one, says that it is notsuch a stretch to think so. These arevery naturally explained in next-to-minimal supersymmetry.

First find the leptonThe story doesnt end there, however.Conways initial analysis had giventhem an approximate mass for theHiggs, but there was a more accurateway to determine it.

Conway looked specifically for thosetau leptons that were moving in theso-called transverse plane, which isperpendicular to the Tevatrons beamof protons and antiprotons. In particleinteractions in a collider, energyshould be conserved, but some energycan be emitted as neutrinos whichcannot be detected directly. In the

transverse plane, the detector canindirectly account for the missingenergy of neutrinos with greatprecision. So by limiting themselvesto interactions in the transverse plane,the researchers were able to accuratelycalculate the mass of the heavyparticles that gave rise to the tau pairs,and put those heavy particles into binsof different masses. In each bin, theycould explain, from known physics,

what gave rise to the tau pairs. Exceptin one bin, says Gunion. And guess

where that one bin is?It turns out that the bin is at about

160 GeV. It shows the merest hint of anew particle. There are few events outthere, right at the place where wemight expect a bump, says Conway.It is so preliminary, but it is there.

Conways team is intrigued enoughto pursue their signal. We have gotdata pouring in now, says Conway.We are going to take it to the next

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Special report Higgs boson

step. This involves doubling thestatistics, increasing the sensitivithe instruments, and even searchin other channels besides lookingtau-lepton pairs.

While increasing statistics couhelp verify the veracity of the signone particular analysis could nailidentity of the mystery particle.

A supersymmetric Higgs should up with b-quarks, also known as bquarks, one of the six types of quaIf we see a Higgs being producedassociation with b-quarks, thats agiveaway, says Conway. Thats thanalysis we have been working tofor six to seven years now.

Meanwhile, another team led Tomasso Dorigo of the UniversityPadua, Italy, has been independenanalysing an entirely different setparticle interactions seen by the Cexperiment and it too has found of some unknown particle at 160 GWhile the team is far from convinthat the signal is real, the coincideare intriguing (see Sticking with standard model).

Markus Schumacher of theUniversity of Siegen in Germany highly sceptical that the Tevatronseen anything new. If you look bthe history of particle physics, wehad a lot of 2-sigma effects, saysSchumacher. You have to wait unthe Fermilab experiment analysemore of the data. Dorigo agrees tany claims of supersymmetry, baon the CDF data so far, are premaI have seen hints of new physicsbeyond the standard model comiand going, coming and going, he

Conway also remains cautiousexpecting his teams own 2-sigmasignal to be a fluctuation andevaporate. If that is the case, theleast he has proved that the Tevatcollider is sensitive enough to catglimpses of a host of other theoreparticles (see Race you to the glu

But if the two teams have glima supersymmetric Higgs, then thedoors to the unknown are wide opIts like the first few pages of athriller, says March-Russell. Youthe first little hint that somethingstrange is happening and that thiare not quite what they seem.Then the evidence accumulates.We are turning the first few pagesthis very interesting story. l

Sticking with the standard modelTomasso Dorigo of the Universityof Padua in Italy has put his moneywhere his mouth is. A believer inthe standard model of particlephysics, Dorigo has bet his theoristfriends a cool $1000 that its the rightdescription of reality. Theres a smallchance, however, that his ownexperiment will lose him that bet.

Last week, Dorigos teamannounced the results from theCDF experiment looking at how Zbosons decay to b-quarks, a processdescribed by the standard modelof the universe. However, his teamhas seen, just as John Conwaysteam did last month, a fewanomalous events at a mass ofabout 160 gigaelectronvolts.

If this is indeed asupersymmetric Higgs boson, thentheory predicts the researchers

should have recorded 100 suchevents based on the amount ofdata they have collected. Accordingto Dorigo, the possibility that theyhave already done so cannot beruled out. There is an upwardfluctuation of the data right atabout that mass value, of the sizeone would expect from minimalsupersymmetry, he says.

However, he still firmlybelieves that the signals his teamhas picked up are just noise in thedata, and hes far from concedinghis bet. Extraordinary claimsneed extraordinary evidence,he says. After thirty years ofincredibly precise confirmationsof the standard model we needa huge signal of new physicsbefore I get convinced there issomething beyond.

Tevatron hunts forthe super-partners of thequark and the gluon

It shows themerest hintof a newparticle, rightat the placewhere wemight expect

a bump. It is sopreliminary,but it is there

Race you to the gluino

The chill felt by John Conway inDecember could be a foretaste ofthings to come. The 160-gigaelectronvolt (GeV) signal seenat the Tevatron particle collidersuggests that it is capable of testingthe supersymmetry model of theuniverse by searching for thesuper-partners of some of theknown particles, and means thatthe race to find new particlesbetween the Tevatron and CERNs 27-kilometre-long Large Hadron Collider(LHC), which is due to start up laterthis year, enters new territory.

The Tevatron is scheduled to

run at full throttle until 2009,collecting data faster than everbefore. By 2009, the LHC is expectedto have enough data to startsearching for supersymmetry.If we were to make a discoverybefore the LHC after all these yearsand billions of dollars, that wouldbe really amazing, says Conway.

Markus Schumacher of the

University of Siegen in Germany,who works on the ATLAS detectorfor the LHC, knows only too wellthat the Tevatron could find newparticles with undisputed certaintybefore the LHC. There was alwaysa race between the Tevatron andthe LHC, he says. It might well bethat the Tevatron will be the firstcollider to see something.

That something could be notjust Higgs particles, but othersupersymmetric partners as well.Of course, it depends on whethernext-to-minimal supersymmetry ,with its modest fine-tuning,

is the right description of reality.In that model, the masses of someof the super-partners should be inthe range of about 300 to 400 GeV.That puts such particles in thesights of both the Tevatron andthe LHC. Specifically, partners forthe top quark and the gluon,namely the stop and the gluino,would be up for grabs.

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