T-odd effects in photon-jet production at the Tevatron
The angular distribution in photon-jet production in is studied within a generalized factorization scheme taking into account the transverse momentum of the partons in the initial hadrons. Within this scheme an anomalously large asymmetry observed in the Drell-Yan process could be attributed to the T-odd, spin and transverse momentum dependent parton distribution function . This same function is expected to produce a asymmetry in the photon-jet production cross section. We give the expression for this particular azimuthal asymmetry, which is estimated to be smaller than the Drell-Yan asymmetry but still of considerable size for Tevatron kinematics. This offers a new possibility to study T-odd effects at the Tevatron.
pacs:12.38.-t; 13.85.Ni; 13.88.+e
It is well-known that the angular distribution of Drell-Yan lepton pairs displays an anomalously large asymmetry. This was experimentally investigated using beams scattering off deuterium and tungsten targets at center of mass energies of order 20 GeV Falciano (); Guanziroli (); Conway (). A next-to-leading order (NLO) analysis in perturbative QCD (pQCD) within the standard framework of collinear factorization failed to describe the data Brandenburg-93 (). More specifically, the observed violation of the so-called Lam-Tung relation Lam-78 (); Lam-80 (); AL-82 (), a relation between two angular asymmetry terms, could not be described. The NLO pQCD result is an order of magnitude too small and of opposite sign. This has prompted much theoretical work Brandenburg-93 (); Brandenburg-94 (); Eskola-94 (); Boer:1999mm (); Boer:2002ju (); Lu:2004hu (); Boer-04 (); Lu:2005rq (); Gamberg:2005ip (); Brandenburg:2006xu (), offering explanations that go beyond the framework of collinear factorization and/or leading twist perturbative QCD.
More recently, Drell-Yan scattering was studied in a fixed target experiment ( GeV) at Fermilab Zhu:2006gx (). The angular distribution does not display a large asymmetry, indicating that the effect that causes the large asymmetry in scattering is probably small for nonvalence partons. For this reason one would like to investigate scattering, which is expected to be similar to the case (an expectation supported by model calculations Boer:2002ju (); Gamberg:2005ip (); Barone:2006ws ()). It is an experiment that could be done at the planned GSI-FAIR facility. It can in principle also be done at Fermilab, although the energy of the collisions is so much higher ( TeV) that the Drell-Yan asymmetry may be quite different in magnitude, possibly much smaller at very high invariant mass of the lepton pair. Nevertheless, it would be interesting to see if NLO pQCD expectations hold at those energies. Recently, such a study of the angular distribution was done for -boson production at the Tevatron and a nonzero result compatible with NLO pQCD Mirkes:1992hu (); Mirkes:1994dp () was obtained Acosta:2005dn (). This may likely be due to the fact that chirality flip effects, such as the T-odd effect to be discussed here, do not contribute to the angular distribution Bourrely:1994sc (); Boer:2000er (). For neutral boson production they do contribute however and therefore could lead to quite a different result. This remains to be investigated.
In this paper we consider an asymmetry in the process that potentially probes the same underlying mechanism and
could have certain advantages over Drell-Yan. This photon-jet
production process has already been studied experimentally in the
angular integrated case at the Tevatron
Kumar:2007mf (). Here we
will calculate the angular dependence within the framework as employed
in Ref. Boer:1999mm (), where transverse momentum and spin
dependence of partons inside hadrons is included
In Section II we discuss the theoretical framework and the expected contributions to the new asymmetry. In Section III we study the phenomenology of this asymmetry, using typical Tevatron kinematics and cuts. We end with a summary of the results and the required measurement.
Ii Theoretical Framework: Calculation of the cross section
We consider the process
where the four-momenta of the particles are given within brackets, and the photon-jet pair in the final state is almost back-to-back in the plane perpendicular to the direction of the incoming hadrons. To lowest order in pQCD the reaction is described in terms of the partonic two-to-two subprocesses
We make a lightcone decomposition of the hadronic momenta in terms of two light-like Sudakov vectors and , satisfying and :
In general and will define the lightcone components of every vector as , while perpendicular vectors will always refer to the components of orthogonal to both incoming hadronic momenta, and . Hence the partonic momenta (, ) can be expressed in terms of the lightcone momentum fractions (, ) and the intrinsic transverse momenta (, ), as follows
We denote with the total energy squared in the hadronic center-of-mass (c.m.) frame, , and with the pseudo-rapidities of the outgoing particles, i.e. , being the polar angles of the outgoing particles in the same frame. Finally, we introduce the partonic Mandelstam variables
which satisfy the relations
Following Ref. Bacchetta:2007sz () we assume that at sufficiently high energies the hadronic cross section factorizes in a soft parton correlator for each observed hadron and a hard part:
where the sum runs over all the incoming and outgoing partons taking part in the subprocesses in (2). The convolutions indicate the appropriate traces over Dirac indices and is the hard partonic squared amplitude, obtained from the cut diagrams in Figs. 2 and 2 Bacchetta:2007sz (). The parton correlators are defined on the lightfront LF (, with for parton 1 and for parton 2); they describe the hadron parton transitions and can be parameterized in terms of transverse momentum dependent (TMD) distribution functions. The quark content of an unpolarized hadron is described, in the lightcone gauge and at leading twist, by the correlator Boer:1997nt ()
where is the unpolarized quark distribution, which integrated over gives the familiar lightcone momentum distribution . The time-reversal (T) odd function is interpreted as the quark transverse spin distribution in an unpolarized hadron Boer:1997nt (). Below we will discuss the T-odd nature of this function and its consequences in more detail.
Analogously, for an antiquark,
The gluon correlator in the lightcone gauge is given by Mulders:2000sh ()
with being a transverse tensor defined as
The function represents the usual unpolarized gluon distribution, while the T-even function is the distribution of linearly polarized gluons in an unpolarized hadron. We include it here because it potentially contributes to the observable of interest. However, it will turn out to yield a power-suppressed contribution.
In the above expressions appears as the T-odd part in the parametrization of the correlator . For distribution functions, however, such a T-odd part is intimately connected to the gauge link that appears in the correlator connecting the quark fields. This gauge link is process-dependent and as a result, T-odd functions can appear with different factors in different processes. The specific factor can be traced back to the color flow in the cut diagrams of the partonic hard scattering. In situations in which only one T-odd function contributes this has been analyzed in detail for the process of interest and related processes, cf. e.g. Refs. Bomhof:2006ra (); Bomhof:2007xt (). It leads to specific color factors multiplying the T-odd distribution function. In the case of single spin asymmetries (SSA) one T-odd function appears, which could be the distribution function . Taking the appearance in the case of leptoproduction as reference (having a factor for the subprocess), one finds appearance with a different factor in Drell-Yan scattering (factor for subprocess Collins:2002kn (); Belitsky:2002sm (); Boer:2003cm ()) or yet another factor for the appearance in the SSA in photon-jet production (factor for all contributions shown in Figs 2 and 2 Bacchetta:2007sz ()). In the asymmetry in Drell-Yan and in photon-jet production the situation is different because a product of two T-odd functions arises (). As we will discuss the factors are in both cases simply , which fixes the relative sign of the asymmetries in the two processes to be .
The factor in photon-jet production arises in the following way. All diagrams have only two color flow contributions with relative strength and . Color averaging in the initial state gives the usual color factors for the process and for the process. Including gauge links in the TMD correlators one schematically has
where the correlators now include gauge links, or , etc. The gauge links are the future and past-pointing ones in which the path runs via lightcone , respectively. The contributions indicate the presence of a term. Following Ref. Bomhof:2007xt (), the correlator (and similarly ) can be split into two parts, , where the non-universal part vanishes upon -integration or -weighting. In what follows we will omit these non-universal parts, but in principle they could contribute (and potentially spoil factorization as recently discussed in Refs Collins:2007nk (); Vogelsang:2007jk (); Collins:2007jp ()) when one considers cross sections that are differential in measured transverse momenta. Ideally one should consider performing the appropriate weighting to remove their possible contributions altogether. We, however, keep the expressions for the cross sections differential because the weighting is often difficult in experimental data analyses and the relation with the Drell-Yan expressions is more straightforward. After weighting one would be left only with the TMD functions and , in which the gauge link determines the sign with which T-odd functions are multiplied, giving in scattering for a positive sign from and a negative sign from . If no factors arise from the other correlators one sees that in SSA involving only one T-odd function , this function appears in photon-jet production with the abovementioned factor but otherwise the normal partonic cross section. This factor and similar ones for other T-odd functions were used in Ref. Bacchetta:2007sz (). In the present case of a product of two T-odd functions, we obtain from both combinations of correlators and in Eq. (12) now a positive sign. Hence, the contribution appears with just a factor , justifying the use of the parametrizations in Eqs (8) and (9) in combination with the normal partonic hard scattering amplitudes squared. For the contribution involving the T-even gluon function no process dependent color factors need to be considered.
The issue of factorization is left as an open question and the same applies to resummation. In Refs Boer:2006eq (); Berger:2007jw () the resummation for the asymmetry is addressed and found to be unclear beyond the leading logarithmic approximation. A similar situation could apply to the asymmetry in the photon-jet production case, but we will not address this here. Our present goal is to point out how the contribution of enters the asymmetry expression in leading order and to give an estimate of its expected magnitude.
In order to derive an expression for the cross section in terms of parton distributions, we insert the parametrizations (8)-(10) of the TMD quark, antiquark and gluon correlators into (7). Furthermore, utilizing the decompositions of the parton momenta in (4), the -function in (7) can be rewritten as
with corrections of order . After integration over and , which fixes the parton momentum fractions by the first two -functions on the r.h.s. of (14), the resulting hadronic cross section consists of two contributions, i.e.
with . We are interested in events in which the photon and jet are approximately back-to-back in the transverse plane, therefore . The cross section depends on the unpolarized (anti)quark and gluon distributions , while depends on the (anti)quark function . More explicitly,
where the sums always run over quarks and antiquarks. Power-suppressed terms of the order , such as the ones proportional to the gluon distribution function , are neglected throughout this paper. If we define the function
with , and denote with , and the azimuthal angles, in the hadronic center-of-mass frame, of the outgoing photon, jet and vector respectively, then (17) can be rewritten as
The approximation has been used in the derivation of the second equation in (19). Furthermore, the same approximation allows us to derive the following relations, starting from the definition of in terms of and ,
Since , (20) implies and , so that
and (19) takes the form
where, in analogy to (18), the following convolutions of distribution functions have been utilized:
Alternatively, equation (LABEL:eq:cross) can be rewritten as
and the azimuthal asymmetry
contains the dependence on and is identical to the azimuthal asymmetry expression that appears in the Drell-Yan process Boer:1999mm (), with the scale equal to . The ratio
only depends on the T-even distribution functions .
Iii Phenomenology: the azimuthal asymmetry
The process is currently being analyzed by the DØ Collaboration at the Tevatron collider Kumar:2007mf (); D0:Atr (). Data on the cross section, differential in , and , have been taken at TeV, and were considered in a preliminary study D0:Atr () with the following kinematic cuts:
the transverse momentum imbalance between the photon and the jet being constrained by the relation
Such angular integrated measurements are only sensitive to the transverse momentum integrated parton distributions
as can be seen from the leading order expression of the cross section,