Top-quark and Higgs boson perspectives at heavy-ion colliders

0.54 MB·

by  David d'Enterria

Nucl.&Part.Phys.
Proceeds
NuclearandParticlePhysicsProceedings00(2017)1–4
Top-quark and Higgs boson perspectives at heavy-ion colliders
Davidd’Enterria
CERN,EPDepartment,1211Geneva,Switzerland
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2 Abstract

n
The perspectives for measuring the top quark and the Higgs boson in nuclear collisions at the LHC and Future
a
Circular Collider (FCC) are summarized. Perturbative QCD calculations at (N)NLO accuracy, including nuclear
J
partondistributionfunctions,areusedtodeterminetheircrosssectionsandvisibleyieldsafterstandardanalysiscuts
7 √ √
2 inPbPbandpPbcollisionsattheLHC( s =5.5,8.8TeV)andFCC( s =39,63TeV).Intheir“cleanest”decay
NN NN
channels,t¯t→bb¯2(cid:96)2νandH→γγ; 4(cid:96),about103(105)top-quarkand10(103)Higgs-bosoneventsareexpectedat

] theLHC(FCC)fortheirtotalnominalintegratedluminosities.Whereasthet¯tobservationisclearcutatbothcolliders,
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evidenceforHiggsproduction,perfectlypossibleattheFCC,requiresintegrating×30moreluminositiesattheLHC.
e
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p Keywords: Topquark,Higgsboson,heavy-ions,LHC,FCC
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h
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1. Introduction tions) from t¯t → bb¯2(cid:96)2ν t¯t decays provide accurate

1 information [3] on the underlying nuclear gluon distri-
v ThetopquarkandtheHiggsboson(togetherwiththe bution function in the unexplored high-x region where
7 τlepton)aretheonlyelementaryStandardModel(SM) “antishadowing” and “EMC” effects are supposed to
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particlesthatremainunobservedsofarinnuclearcolli- modify its shape compared to the free proton case [4].
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8 sions. Their production cross sections in hadronic col- On the other hand, the Higgs boson has a lifetime of
0 lisions are dominated by gluon-gluon fusion processes τ ≈50fm/cand,onceproduced,traversestheproduced
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1. (gg → t¯t+X;H+X), computable today at the high- medium and scatters with the surrounding partons, re-
0 est degree of theoretical accuracy: NNLO+NNLL for sultinginapotentialdepletionofitsyieldscomparedto
7 the top quark [1], and N3LO for the H boson [2]. The the pp case [5]. The amount of Higgs boson suppres-
1 study of their yield modifications in heavy-ion com- sion can be thereby used to accurately determine the
:
v pared to pp collisions would provide novel extremely final-state density of the produced QGP. The perspec-
i wellcalibratedprobesoftheinitialandfinalpAandAA tivesoft-quark[3]andHiggsboson[6]measurements
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states. Both elementary particles, the heaviest known, in nuclear collisions at current and future colliders are
ar have very different decay channels and lifetimes, and summarizedhere.
can thereby be used to uniquely probe various aspects
ofstrongly-interactingmatterinnuclearcollisions. On The theoretical cross sections and yields in pPb
theonehand, thetop-quarkdecaysveryrapidlybefore and PbPb are obtained with mcfm at NLO (v.6.7) for
hadronizing (τ = (cid:126)/Γ ≈ 0.1 fm/c, much shorter than top-quarks [7], and at NNLO (v.8.0) for the Higgs
0 t
typical O(1 fm/c) QGP formation times) into t → Wb boson [8], using CT10 proton PDFs [9] and EPS09
with∼100%branchingratio,withtheWthemselvesde- nPDFs (including its 30 eigenvalues sets) for the Pb
caying either leptonically (t → Wb → (cid:96)ν,b, 1/3 of ion [4]. The following mcfm processes are run:
the times) or hadronically (t → Wb → qq¯b, 2/3 of 141 for t¯t, 161,166,171,176,181,186 for single-top
thetimes). Thekinematicaldistributionsofthecharged in the t-,s-channels and associated with W [3]; and
leptons ((cid:96) = e,µ unaffected by any final-state interac- 119,116 for gluon-fusion H → γγ, 4(cid:96) (plus 285,90
Davidd’Enterria/NuclearandParticlePhysicsProceedings00(2017)1–4 2
b)106 MCFM (v.6.7) NLO: b) MCFM v8.0
s (n105 nm =PmDFt=EPS09,PDF=CT10 s (n104 PbPb
103
104
Pb-Pb
102
103 pPb
10
102
p-Pb 1
10
pp
10- 1
1
10- 1 p-p 10- 2 NNLO, pp,pPb,PbPbfi H+X
10- 2 top pair (tt + X) 10- 3 PDF=CT10,nPDF=EPS09
single top (t-ch.,s-ch.,t+W) m =m,m/2
H H
10- 3 10- 4
1 2 3 456 10 20 30 100 3 4 567 10 20 30 100
s (TeV) s (TeV)
NN NN
Figure1:Totalproductioncrosssectionsfortop-pairandsingle-topatNLO(left)[3]andfortheHiggsbosonatNNLO(right)[6]inpp,pPband
√
PbPbcollisionsasafunctionof s (theshadedboxesindicatethenominalLHCandFCCenergies).
NN
Table1:Crosssections,andexpectednumberofcountsafterstandardacceptanceandefficiencycuts,fort¯tandsingle-top(s-,t-andtWchannels
combined)atNLO(total,andintheirrespectiveleptonicdecays)[3]andHiggsboson(total,anddi-γand4-leptonchannels,atNNLO)[6]inpPb
andPbPbcollisionsatLHCandFCC.Maximumtheoretical(scaleandPDF)uncertainties(notquoted)arearound10%.
√
System s L t¯t t¯t→bb¯(cid:96)(cid:96)νν single-t tW→b(cid:96)(cid:96)νν H H→γγ H→ZZ∗(4(cid:96))
NN int
(TeV) σ yields σ yields σ yields yields
tot tot tot
PbPb 5.5 10nb−1 3.4µb 450 2.0µb 30 500nb 6 0.3
pPb 8.8 1pb−1 59nb 750 27nb 50 6.0nb 7 0.4
PbPb 39 33nb−1 300µb 1.5×105 80µb 8000 11.5µb 450 25
pPb 63 8pb−1 3.2µb 4×105 775nb 2.1×104 115nb 950 50
for the corresponding γγ, 4(cid:96) backgrounds), as well as LHC and FCC energies are listed in Table 1. Com-
91,101,215,540correspondingtotherestofHiggspro- pared to the corresponding pp results at each c.m. en-
duction channels (vector-boson-fusion, and associated ergy, antishadowing nPDF modifications increase the
with W, Z and top) to obtain the total H cross sec- totaltop-quarkyieldsby2–8%,whereastheHiggscross
tions[6]. Allnumericalresultshavebeenobtainedwith sections are just slightly enhanced (depleted) by ∼3%
the latest SM parameters [10], and fixing the default at the LHC (FCC). The PDF uncertainties, obtained
renormalization and factorization scales at µ = µ = addinginquadraturetheCT10andEPS09uncertainties,
F R
m fort¯tandsingle-top,µ = µ = p = 50GeV are around (below) 10% for the top (Higgs) case. The
t F R T,min;b−jet
for tW, and µ = µ = m /2 for Higgs. These cal- O(5−10%) theoretical µ ,µ scale uncertainties, not
F R H F R
culations reproduce very well the top and Higgs cross quotedeither,canceloutintheratiosof(pPb,PbPb)/(pp)
√
sectionsmeasuredinppcollisionsat s=7,8,13TeV crosssectionsatthesamec.m.energy.
attheLHC.Thecollisionenergydependenceofthetotal
top-quarkandHiggsbosoncrosssectionsareshownin 2. Topquarkmeasurement
Fig.1. Thecrosssectionsincreasebyafactorof×55–
90 for t¯t and ×20 for the Higgs boson between LHC Top quarks decay almost exclusively to a b quark
and FCC energies. The cross sections at the nominal and a W boson and, in a heavy-ion environment, it
is the W leptonic decays that can be best resolved
Davidd’Enterria/NuclearandParticlePhysicsProceedings00(2017)1–4 3
p+Pb→tt+X→bb+(cid:96)+(cid:96)−+νν+X
s
√
√
s
√ √
s s=8.8TeV
Pb+Pb→tt+X→bb+(cid:96)+(cid:96)−+νν+X
s
√
√
s
√ √
s s=39TeV
Figure2: Left: Pseudodatafornuclearmodificationfactorsasafunctionofrapidityofchargedleptonsproducedint¯tdecaysattheLHC(pPb,
top)[3]andFCC(PbPb,bottom)[11].Right:OriginalEPS09gluonnuclearmodificationatQ=mtop(reddashedcurves)andestimatedimprove-
mentsobtainedbyHessianreweighting(greyarea)usingtheLHC(pPb,top)[3]andFCC(PbPb,bottom)[11]pseudodata.
fromthebackgrounds.Theestimatedmeasurableyields EPS09 global fit of the nuclear PDFs is quantified via
using realistic luminosities and analysis cuts (b-jets: the Hessian reweighting technique [12]. The expected
anti-k algorithm with R = 0.5, p > 30 GeV/c, improvements of the gluon PDF nuclear modification
T T
|η| < 3, 5(LHC,FCC), 50% b-tagging efficiency; factor RPb(x,Q2) = gPb(x,Q2)/gp(x,Q2) at Q2 = m2
g top
charged leptons: R = 0.3, p > 20 GeV/c, |η| < areshowninFig.2(right). Theuncertaintiesonthenu-
isol T
3, 5(LHC,FCC),m >20GeV,|m −m |>15GeV; cleargluonPDFarereducedbymorethan10%(50%)at
(cid:96)(cid:96) (cid:96)(cid:96) Z
neutrinos: missingtransverseenergyE/ >40GeV)are theLHC(FCC)mostlyintheantishadowingandEMC
T
listedinTable1.Afterbranchingratios(BR≈5%fort¯t, xregions[3,11].
22% for single-t) and acceptance and efficiency losses
(40–50%fort¯t,20–30%forsingle-t),weexpectabout 3. Higgsbosonmeasurement
500and800leptonictop-quarkeventsinPbPbandpPb
collisionsattheLHC(withcontrollablebackgroundsin The discovery of the Higgs boson in pp collisions
thet¯tcase). AtFCC,thehigherc.m.energyandlumi- at the LHC was carried out in the clean diphoton and
nositieswillyieldverylargedatasampleswith150-and four-leptonchannelswithverylowbranchingratiosbut
400-thousandpairsinPbPbandpPb. Theresultingnu- smalland/orcontrollablebackgrounds[13,14]. Afirst
clearmodificationratiosoft¯tdecayleptonsyieldsasa measurement in heavy-ion collisions will certainly ex-
function of rapidity, RpPb(y(cid:96)) = dσpPb(y(cid:96))/(Adσpp(y(cid:96))) ploit the same final states. Table 1 lists the estimated
and RPbPb(y(cid:96)) = dσPbPb(y(cid:96))/(A2dσpp(y(cid:96))), are shown measurable yields for nominal luminosities after ac-
in Fig. 2 (left) for pPb at the LHC (top) and PbPb counting for typical ATLAS/CMS analysis cuts (pho-
at the FCC (bottom). The effect of antishadowing tons: p > 30,40 GeV/c, |η| < 2.5, 5(LHC,FCC),
T
(shadowing)inthenPDFresultsinsmallenhancements R =0.3;chargedleptons: p >20,15,10,10GeV/c,
isol T
(deficits)atcentral(forward)rapiditiesy(cid:96) ≈0(|y(cid:96)|(cid:38)3). |η| < 2.5, 5(LHC,FCC), Risol = 0.3). After branch-
The impact that these pseudodata would have in an ing ratios (BR = 0.23% for γγ, 0.012% for 4(cid:96)) and
Davidd’Enterria/NuclearandParticlePhysicsProceedings00(2017)1–4 4
V V
e2000 pPb fi H fi g g , 8.8 TeV, L =30 pb-1 e3000 PbPb fi H fi g g , 39 TeV, L =33 nb-1
G int G int
s/ s/2800
nt1800 Pseudodata nt Pseudodata
e e2600
v Fitted S+B v Fitted S+B
E E
1600 g g background 2400 g g background
2200
1400
2000
1200 1800
1600
1000
MCFM, NNLO MCFM, NNLO
1400
PDF: CT10, nPDF=EPS09 PDF: CT10, nPDF=EPS09
800
1200
100 105 110 115 120 125 130 135 100 105 110 115 120 125 130 135
m (GeV) m (GeV)
g g g g
√
Figure3:ExpectedinvariantmassdiphotondistributionsinpPbat s =8.8TeV(withenhancedLHCintegratedluminosities,L =30pb−1,
left)andinPbPbat √s =39TeV(nominalFCCluminosities,right)NfNoraHiggsbosonsignalinjectedontopoftheγγbackgroundinst[6].
NN
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