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The XXII International Workshop High Energy Physics and Quantum Field Theory

Single top quark production in heavy ion collisions at the LHC.

Alexey Baskakov, PhD student, MSU
Scientific adviser Prof. E.E. Boos

26.06.2015


Heavy ion collisions and top quark
172.2 ± 0.1 . ± 0.7 . (CMS) Higgs boson coupling 1
A.M. Baldin ,"Heavy Ion Interactions at High Energies", report at AIP Conf. Proc. 26, 621 (1975)

Life time 10-25 is less then hadronization time (10-24 )

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Heavy ion collisions and top quark What is already done?
Smearing and decreasing mean and maximum values of invariant mass distributions of three jets from top quark decay (one b-jet and two jets from W-boson) and dijets from W-boson decay in PbPb collisions as compared with pp interactions are predicted for top anti-top pair production[*].
* L.Bhattacharya, K.Ghosh, K.Huitu, arXiv:1210.0116 [hep-ph], (2012).

Single and pair top-quark production cross sections in p-Pb and Pb-Pb collisions at the energies of LHC and Future Circular Collider (FCC) have been estimated with next-to-leading-order perturbative QCD calculations including nuclear parton distribution functions[**].
**D.d'Enterria, K.Krajczar, H.Paukkunen, Phys. Lett. B 746, 64 (2015).

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Monte Carlo modelling, General scheme
Signal events for single top quark production and decay ­ CompHEP v4.5 + MCFM v6.8

+
Hadronisation and fragmentation ­ Pythia v6.4

MCFM

Nuclear medium effects ­ PYQUEN v1.5

PYQUEN
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Monte Carlo modelling scenarios
General modelling CompHEP v4.5 + MCFM v6.8 MSTW2008 NNLO PDF MC event of single top quark production with lepton decay

Well-studied process Initial state effects Final state effects

* EPS 09 NLO ­ nuclear parton distribution functions (PDF)
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Test of the Monte Carlo modelling

Agreement with experimental data on W-production cross section and rapidity dependence of W+,W- charge asymmetry (NW+ - NW-)/(NW+ + NW-) pp(blue open squares) and PbPb(red-filled circles) s = 2.76 [*].
* S. Chatrchyan, et al. (CMS Collaboration), Phys. Lett. B 715, 66 (2012)
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Modelling of the single top quark production
Realistic geometrical acceptance and kinematic cuts for CMS and ATLAS Light quark jet b-quark jet Leptons (e, mu) Pseudorapidity

Lepton transverse momentum
Jet transverse momentum Efficiency of kinematic acceptance

b-tagging efficiency Total efficiency
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Results
Cross section and event rate estimation

Single top quark production cross section

Event rate

130 events rate in a one month PbPb run, up to 1300 events per month at HL-LHC?
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Kinematic distributions. pT distributions of b-jet and light quark jet

b-jet (left column) light quark jet (right column) Linear scale (top row) logarithmic scale (bottom row)
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Kinematic distributions. Invariant top mass, cos(lep&light jet)
The invariant mass distributions of W-boson and b-jet from top quark decays

Cosine of angle between lepton from top decay and jet from light quark associated with the top in top quark rest frame.

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Top anti-top asymmetries
Top/anti-top ratio

Charge asymmetry

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Conclusions
Single top quark production has a large enough cross section and visible event rate for the nominal LHC luminosities and is open to study in PbPb collisions. Smearing and decreasing mean and maximum values of the invariant mass distribution of W-boson and b-jet from top quark decays Significant softening the transverse momentum spectrum of jets associated with top quark and shifting its peak from the half mass of W-boson.

Disappearance of charge asymmetry of single top quark production
Cosine of angle taken in the top rest frame between the lepton from the top decay and the momentum of jet from light quark produced in association with the top is only slightly affected and may be used for separation single top signal from backgrounds in future

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Bibliography
1. CompHEP Collaboration (E. Boos et al.). CompHEP 4.4: Automatic computations from Lagrangians to events. // Nucl.Instrum.Meth. A534 (2004) 250-259 2. E.E. Boos, V.E. Bunichev, L.V. Dudko, V.I. Savrin, A.V. Sherstnev. Method for simulating electroweak top-quark production events in the NLO approximation: SingleTop event generator. // Phys.Atom.Nucl. 69 (2006) 1317-1329 3. Torbjorn Sjostrand, Stephen Mrenna, Peter Z. Skands. PYTHIA 6.4 Physics and Manual. //JHEP 0605 (2006) 026

4. I.P. Lokhtin, A.M. Snigirev. A Model of jet quenching in ultrarelativistic heavy ion collisions and high-p(T) hadron spectra at RHIC. // Eur.Phys.J. C45 (2006) 211-217
5. A.V. Baskakov, E.E. Boos, L.V. Dudko, I.P. Lokhtin, A.M. Snigirev. Single top quark production in heavy ion collisions at the LHC. // arXiv:1502.04875 [hep-ph] (2015)

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Acknowledgments

I would like to thank collaborators E.E. Boos, L.V. Dudko, I.P. Lokhtin, A.M. Snigirev for fruitful discussions, constructive comments and assistance in the work!

This work was supported by Russian Foundation for Basic Research (grants 12-02-91505, 13-02-01050) and Grant of President of Russian Federation for Scientific Schools Supporting No. 3042.2014.2.

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Thank you for your attention!

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