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Prompt photon and Drell-Yan pair production in the kT-factorization approach at modern colliders
M.A. Malyshev in collaboration with A.V. Lipatov N.P. Zotov
M.V. Lomonosov Moscow State University D.V. Skobeltsyn Institute of Nuclear Physics

Phys. Lett. B 699 (2011) 93


Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Outline
1. Motivation 2. kT-factorization approach - unintegrated parton distributions - off-shell matrix elements 3. Parameters 4. Numerical results 5. Conclusion

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Motivation
Prompt photon and Drell-Yan pair production are highly sensitive to parton distribution in the hadron. So they provide a test of hard subprocess dynamics. Also these processes contribute largely into the background of the 'new physics'. At LHC energies the studied processes are small-x physics processes. At small x one has to use the kT-factorization approach. For prompt photon production this approach was used in [S.P. Baranov, A.V. Lipatov, N.P. Zotov, 2010]. Recently new experimental data on inclusive prompt photon and Drell-Yan pair production have been obtained at LHC. Here we present the theoretical description of the LHC results for prompt photon production. For Drell-Yan process we present the description of various published Tevatron data.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

kT-factorization approach
1. Unintegrated parton distributions 2. Matrix elements which depend on the transverse momenta of incoming partons.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Unintegrated parton distributions
1. KMR approach(Kimber, Martin, Ryskin) [M.A. Kimber, A.D. Martin, M.G. Ryskin, 2001; G. Watt, A.D. Martin, M.G. Ryskin, 2003]. Weakening of the DGLAP strong ordering:

2. CCFM unintegrated distributions [H. Jung, 2004; M. Deak, H. Jung, K. Kutak, 2008]. Numerical solutions of CCFM equation. The starting distribution is chosen to satisfy proton structure function F2(x,µ2).

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Off-shell matrix elements
I. Prompt photons
1. Compton subprocess q*g* q 2. Annihilation subprocess q*q* g 3. Gluon fusion subprocess g*g* qq [S.P. Baranov, A.V. Lipatov, N.P. Zotov, 2008]. The taking into account of this higher order subprocess is not trivial in our approach.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Off-shell matrix elements
II. Drell-Yan

1. q*q*e+e2. q*q*e+e-g 3. q*g*e+e-q

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Parameters






Significant theoretical uncertainties are connected with the choice of the factorization and renormalization scales. We took R = F = = |pT| for prompt photons production and R = F = = Me+e- for Drell-Yan production. We varied the scale parameter between 1/2 and 2 about the default value = 1. We neglected the quarks masses. For completeness, we use LO formula for the strong coupling constant s(2) with nf = 4 active quark flavours at QCD = 200 MeV.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Parameters
In order to reduce the huge background from the secondary photons produced by the decays of 0 and mesons the isolation criterion is introduced in the experimental analyses. This criterion is the following. A photon is isolated if the amount of hadronic transverse energy EThad deposited inside a cone with aperture R centered around the photon direction in the pseudo-rapidity and azimuthal angle plane, is smaller than some value Emax. EThadEmax - )2 + (had - )2 R2.

(

had

The isolation not only reduces the background but also significantly reduces the so called fragmentation components, connected with collinear photon radiation (10%). Both CMS and ATLAS collaborations take R0.4 and Emax1 GeV.
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
1. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV). The experimental data are of CMS (|y|<1,45).

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
2. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV).The experimental data are of ATLAS (|y|<0,6).

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
2. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV).The experimental data are of ATLAS (0,6<|y|<1,37).

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
2. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV).The experimental data are of ATLAS (1,52<|y|<1,81).

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
3. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV). Contribution of different subprocesses to the cross section, evaluated with KMR distributions. The experimental data are of CMS. The ggqq contribution is significant at the LHC energy, especially at low ET

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
4. Differential cross section of Drell-Yan pair production ppe+e-X at Tevatron energies (S=1.8 TeV) in the invariant masses range 11
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
5. Differential cross section of Drell-Yan pair production ppe+e-X at Tevatron energies (S=1.8 TeV) in the invariant masses range 120
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
6. Differential cross section of Drell-Yan pair production ppe+e-X at Tevatron energies (S=1.8 TeV) versus the transverse momentum of the lepton pair. The experimental data are of CDF.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
6. Differential cross section of Drell-Yan pair production ppe+e-X at Tevatron energies (S=1.8 TeV) versus the transverse momentum of the lepton pair (in the range 0
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Conclusion
In the presented work processes of the inclusive prompt photon and DrellYan pair production in the kT-factorization QCD approach at Tevatron and LHC energies have been studied. The off-shell LO matrix elements for q*g* q, q*q* g, q*q*e+e- and HO matrix elements for g*g* qq, q*q*e+e-g, q*g*e+e-q subprocesses have been evaluated. A reasonably good description of CMS and ATLAS experimental data for the inclusive prompt photon production at LHC and CDF and D0 data for the lepton pair production at Tevatron has been obtained. A theoretical uncertainties investigation has been studied and a predictive power of the used approach has been shown. We are ready to start the description of the Drell-Yan pair production at LHC. It will be interesting to see new LHCb experimental data at large rapidity, since the lepton pair production in forward region corresponds to very small x, up to 10-6.
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Back up

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results
B1. Differential cross section of inclusive prompt photon production ppX at LHC energies (S=7 TeV). Comparison of the results obtained by CCFM A0 distributions with and without the 'Reduced sea' contribution (calculated with KMR distributions) added. The experimental data are of CMS.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Numerical results


5. Different contributions to the inclusive prompt photon production at the Tevatron (left panel) and LHC (right panel). The solid, dashed and dotted histograms correspond to g+g+q+q, qv+g+q and qv+qv +g subprocesses. The dash-dotted histograms the "reduced sea" component. The thick solid histograms - the sum of all contributions.
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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Off-shell quarks
In the presented work we article [S.P. Baranov, A.V. 81, 094034 (2010)]. Acco quark spin density matrix mas s es ): used a method, described in the Lipatov, N.P. Zotov, Phys. Rev D rding to this method, the off-shell has the form (in the limit of zero


s

s k = x P u k u

s

Here P is the momentum of the incoming proton. This prescription gives us the correct on-shell limit.

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Maxim Malyshev, QFTHEP'11

Sochi, September, 25, 2011

Divergencies
We do not use the concept of fragmentation functions obviously. In our approach the effect of final state radiation is already included in calculations at the level of partonic subprocess matrix elements (we have a 2 3 rather than 2 2 subprocesses). But as in the traditional approach the calculated cross sections can be split into two pieces: the direct and fragmentation contributions. They depend from fragmentation scale 2. In our calculations is the invariant mass of the produced photon and any final quark and we restrict direct contribution to M = 1GeV in order to eliminate the collinear divergences in the direct cross section. Then the mass of light quark mq can be safely to zero. The numerical effects of M is really small. It is less important than other theoretical uncertainties (connected with choice of renormalization and factorization scales).

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