Photon Transverse Momentum Research Articles

First results from RHIC-PHENIX

The PHENIX experiment consists of a large detector system located at the newly commissioned relativistic heavy ion collider (RHIC) at the Brookhaven National Laboratory. The primary goal of the PHENIX experiment is to look for signatures of the QCD prediction of a deconfined high-energy-density phase of nuclear matter quark gluon plasma. PHENIX started data taking for Au+Au collisions at √sNN=130 GeV in June 2000. The signals from the beam-beam counter (BBC) and zero degree calorimeter (ZDC) are used to determine the centrality of the collision. A Glauber model reproduces the ZDC spectrum reasonably well to determine the participants in a collision. Charged particle multiplicity distribution from the first PHENIX paper is compared with the other RHIC experiment and the CERN, SPS results. Transverse momentum of photons are measured in the electro-magnetic calorimeter (EMCal) and preliminary results are presented. Particle identification is made by a time of flight (TOF) detector and the results show clear separation of the charged hadrons from each other.

Scaling of particle and transverse energy production in $^{208}$ Pb+ $^{208}$ Pb collisions at 158 $\cdot A$ GeV

Transverse energy, charged particle pseudorapidity distributions and photon transverse momentum spectra have been studied as a function of the number of participants (N_{part}) and the number of binary nucleon-nucleon collisions (N_{coll}) in 158 A GeV Pb+Pb collisions over a wide impact parameter range. A scaling of the transverse energy pseudorapidity density at midrapidity as N_{part}^{1.08 \pm 0.06} and N_{coll}^{0.83 \pm 0.05} is observed. For the charged particle pseudorapidity density at midrapidity we find a scaling as N_{part}^{1.07 \pm 0.04} and N_{coll}^{0.82 \pm 0.03}. This faster than linear scaling with N_{part} indicates a violation of the naive Wounded Nucleon Model.

Search for charginos with a small mass difference to the lightest supersymmetric particle at [formula omitted] = 189 GeV

A search for charginos nearly mass-degenerate with the lightest supersymmetric particle is performed using the 176 pb −1 of data collected at 189 GeV in 1998 with the L3 detector. Mass differences between the chargino and the lightest supersymmetric particle below 4 GeV are considered. The presence of a high transverse momentum photon is required to single out the signal from the photon-photon interaction background. No evidence for charginos is found and upper limits on the cross section for chargino pair production are set. For the first time at LEP2, in the case of heavy scalar leptons, chargino mass limits are obtained for any χ ̃ ± 1− χ ̃ 0 1 mass difference.

Nuclear shadowing effects on prompt photons at ultrarelativistic energies

The transverse momentum distribution of prompt photons coming from the very early phase of ultrarelativistic heavy ion collisions for the RHIC and LHC energies is calculated by means of perturbative QCD. We calculate the single photon cross section (A + B → + X) by taking into account the partonic sub processes q + q → + g and q + g → + q as well as the Bremsstrahlung corrections to those processes. We choose a lower momentum cut-off k0 = 2 GeV separating the soft physics from perturbative QCD. We compare the results for those primary collisions with the photons produced in reactions of the thermalized secondary particles, which are calculated within scaling hydrodynamics. The QCD processes are taken in leading order. Nuclear shadowing corrections, which alter the involved nuclear structure functions are explicitly taken into account and compared to unshadowed results. Employing the GRV parton distribution parametrizations we find that at RHIC prompt QCD-photons dominate over the thermal radiation down to transverse momenta kT ≈ 2 GeV. At LHC, however, thermal radiation from the QGP dominates for photon transverse momenta kT ≤ 5 GeV, if nuclear shadowing effects on prompt photon production are taken into account.

Mesurement of (pT) for photons in Pb+Pb collisions at the SPS

The mean transverse momentum of photons has been determined on an event by event basis in Pb+Pb collisions at 158 A GeV. We measure photon multiplicity using a preshower Photon Multiplicity Detector (PMD) and transverse electromagnetic energy by Mid Rapidity Calorimeter (MIRAC) in the overlap region of pseudorapidity 3.5≤η≤4.2. Using ratio of the transverse electromagnetic energy (ETem) to the photon multiplicity (Nγ) we have extracted the (pt) for photons.

Event by event measurement of 〈 pT〉 of photons in S + Au collisions at 200 A·GeV

The mean transverse momentum of photons has been determined on an event by event basis in S + Au collisions at 200 A·GeV from the ratio of the measured electromagnetic transverse energy ( E T em) to the photon multiplicity ( N γ ). The average value obtained is similar to that determined for the same system using spectroscopic techniques. The centrality dependence of the measured values are in agreement with the predictions of the VENUS event generator. 4 Now at: SUBATECH, Ecole des Mines, Nantes, France.

Low transverse momentum photon production in proton-nucleus collisions at 18 GeV/c.

Inclusive single photon spectra have been measured in the transverse momentum (${\mathit{p}}_{\mathit{T}}$) range from a few MeV/c up to approximately 1 GeV/c at various nucleon-nucleon center of momentum rapidities (${\mathit{y}}_{\mathit{NN}}$) in the interval from -2.4${\mathit{y}}_{\mathit{NN}}$+0.5 in 18 GeV/c p-Be and p-W collisions. These distributions were used to estimate the inclusive ${\mathit{p}}_{\mathit{T}}$ and rapidity distributions for ${\mathrm{\ensuremath{\pi}}}^{0}$ production over approximately the same kinematic range. The measured photon spectra are compared with the expected distribution of photons from ${\mathrm{\ensuremath{\pi}}}^{0}$ decays and hadronic bremsstrahlung in order to search for new sources of ``soft'' photons at low transverse momentum. The results indicate no anomalous sources of soft photons in either p-Be or p-W interactions at these energies. The p-W data at forward angles set a limit on any excess of low ${\mathit{p}}_{\mathit{T}}$ photons at 2.65 times hadronic bremsstrahlung at the 99% C.L. \textcopyright{} 1996 The American Physical Society.

Measuring tc with dileptons and photons

Resonant dileptons were first recognized by Siemens and Chin as an important probe of the properties of hot hadronic matter created in ultra-relativistic nuclear collisions. Recently, dileptons from resonance decay have been proposed as a tool to measure the hadronic transition temperature, T c . The technique is simple: the ϱ-meson transverse mass, m T , distribution is measured with dileptons and fitted to a thermal shape, and the fitting temperature is approximately equal to T c This proposal has been tested for the cases of deconfined and chirally-symmetric high temperature phases in the absence of transverse expansion. Assuming that hadronic matter undergoes a strong first-order phase transition, and in the absence of transverse expansion, T c can also be obtained from the photon transverse momentum, p T , distribution for 1 < p T < 2 GeV. Further work is underway to test the effects of transverse expansion on the dilepton and photon measurements. I discuss the status of the proposed measurements, and possible variations to produce new analysis techniques for ultra-relativistic nuclear collisions.

Ratios of W+/- gamma and Z gamma cross sections: New tools in probing the weak boson sector at the Fermilab Tevatron.

The ratios ${\cal R}_{\gamma ,\ell}=B(Z\to\ell^+\ell^-)\cdot\sigma(Z\gamma) /\allowbreak B(W\to\ell\nu)\cdot\sigma(W^\pm\gamma)$, ${\cal R}_{\gamma , \nu}= B(Z\to\bar\nu\nu)\cdot\sigma(Z\gamma)/\allowbreak B(W\to\ell\nu)\cdot\sigma(W^ \pm\gamma)$, ${\cal R}_{W\gamma}=\sigma(W^\pm\gamma)/\allowbreak\sigma(W^\pm)$, and ${\cal R}_{Z\gamma}=\sigma(Z\gamma)/\allowbreak\sigma(Z)$ are studied as tools to probe the electroweak boson self-interactions. As a function of the minimum photon transverse momentum, ${\cal R}_{\gamma ,\ell}$ and ${\cal R}_{ \gamma ,\nu}$ are found to directly reflect the radiation zero present in $W^ \pm\gamma$ production in the Standard Model. All four ratios are sensitive to anomalous $WW\gamma$ and/or $ZZ\gamma/Z\gamma\gamma$ couplings. The sensitivity of the cross section ratios to the cuts imposed on the final state particles, as well as the systematic uncertainties resulting from different parametrizations of parton distribution functions, the choice of the factorization scale $Q^2$, and from higher order QCD corrections are explored. Taking into account these uncertainties, sensitivity limits for anomalous three gauge boson couplings, based on a measurement of the cross section ratios with an integrated luminosity of 25~pb$^{-1}$ at the Tevatron, are estimated.

Photon emission from very high energy nuclear collisions.

The number of high transverse momentum photons emitted by a free-streaming quark-gluon plasma created in a collision between heavy nuclei at energies reached at the CERN SPS, BNL RHIC, or CERN LHC is computed assuming a production-time momentum distribution that is exponential. Comparison to a hydrodynamically expanding plasma with the same initial conditions indicates that the photon yield is relatively insensitive to parton rescattering. The expected background coming from the decay of neutral pions is also computed and compared to the direct yield. Direct photons generally dominate decay photons for transverse momenta greater than 5 GeV.

Inclusive photon production from pp-bar collisions at sqrt s =1.8 TeV.

We report a measurement of the inclusive production of low-energy photons from [ital p[bar p]] collisions at [radical][ital s] =1.8 TeV. A NaI calorimeter located near [eta]=0 was used to measure the photon transverse momentum spectrum over the range 30[le][ital p][sub [ital t]][le]5000 MeV/[ital c]. The data are compared to the photon production expected from known particle decays in an attempt to isolate a low-energy direct photon signal. We find no evidence for excess photons above those expected from hadron decays and place an upper limit of 16% (90% C.L.) on any excess direct photons in the [ital p][sub [ital t]] range from 30 to 100 MeV/[ital c].

History of quark-gluon plasma evolution from photon interferometry.

Two-photon intensity interferometry is used to probe the dynamics of quarks and gluons in a high-energy nucleus-nucleus collision. A (1+1)-dimensional expansion of the plasma according to Bjorken hydrodynamics as well as a (3+1)-dimensional expansion, both with a first-order phase transition, are considered. The correlation of high transverse-momentum photons is sensitive to the details of the space-time evolution of the high density QCD plasma. The so-called ``longitudinal'' and ``outward'' correlations are found to be dramatically affected by the transverse expansion of the system.

A measurement of the direct photon production cross section at the CERN p̄p collider

A measurement of the inclusive cross-section for production of direct photons in p̄p collisions at a centre of mass energy of 630 GeV is presented as a function of the photon transverse momentum. The data correspond to a total integrated luminosity of 7.4 pb −1. The results support predictions from QCD theory.

Intrinsic transverse momentum in the π−p→ γγX reaction at 280 GeV/ c

The correlation between the two high transverse momentum ( p t2.75 GeV/ c) photons of the reaction π − p→ γγX is studied at the beam momentum of 280 GeV/ c. QCD calculations beyond leading logarithms are not sufficient to explain the width of the distributions and the effects due to intrinsic transverse momentum of the incident partons are discussed.

Finite W-Boson Mass Effect and Background in Neutrino Counting Reaction

The neutrino counting reaction e+e- →νν γ is analyzed by calculating the W-exchange diagrams exactly. Around the Z-boson peak the difference between the exact calculation and the calculation in the point interaction approximition (PIA) of the W-boson contribution is less than a few percent in the total cross section. By Making use of the Fierz transformation it is shown that the QED correction to the W-contribution and the interference in PIA is the same form as that to the Z-contribution. The distributions of Z-boson and W-boson contribution in PIA with the QED correction are compared with those of the main background e+e- →e+e- γ under various kinematical cuts. When the energy is fairly above the Z-boson peak and some reasonaable cuts are made, the neutrino counting is really feasible by using any of energy, transverse momentum and angular distributions. At the energy slihtly above the Z-boson peak the neutrino counting can be made possible only by photon transverse momentum distribution.

Validity of the equivalent photon approximation in high-energy electron-proton collisions

Some of the most important physics processes which can be studied at high-energy electron-proton colliders such as HERA involve the hard scattering of quasi-real photons emitted from the incoming electron. Several of these processes have been calculated to next-to-leading order in QCD perturbation theory and can therefore, at least in principle, be used for precision standard model tests. A common feature of such analyses is the use of the equivalent photon approximation, which simplifies the calculations significantly. The authors examines the validity of this approximation in the context of a typical hard scattering process-the production of large transverse momentum photons. By comparing the exact and approximate cross sections they show that, with an appropriate choice of parameter in the electron structure function, the approximation is generally very good in the relevant kinematic regions, and any residual errors are typically less than other theoretical uncertainties in the calculation.

Photoproduction of large transverse momentum photons at HERA

The production of large transverse momentum photons in quasireal photon proton collisions at HERA is investigated. The predictions of a complete next-to-leading-order QCD calculation are presented. Particular attention is paid to the theoretical uncertainties arising from the choice of parton distributions.

Direct photon production in $$\bar pp$$ in collisions at $$\sqrt s = 630$$ GeV

A measurement of the direct production of photons with high transverse momentum from\(\bar pp\) collisions at\(\sqrt s = 630\) GeV is presented. The structure of events containing a high transverse momentum photon is studied. The results support predictions from QCD theory.

Radiative Correction to e+e--&gt;e+e- in the Electroweak Theory. I: Cross Sections for Hard Photon Emission

Various cross sections for radiative Bhabha scattering, e+e-→e+e-γ, are calculated in the standard electroweak theory. They contain distributions on photon energy dσ/dk, acollinearity angle dσ/dζ, acoplanarity angle dσ/dψ, photon transverse momentum dσ/dkT and invariant mass of final e-γ system dσ/dMeγ. In the calculation some realistic experimental cuts are imposed on the configuration of final particles and the energies are chosen to be 70, 93 and 150 GeV in accordance with TRISTAN, SLC and LEP. From the results we can see the effect of Z0-boson exchanged in the s- and t-channel and estimate backgrounds to such interesting processes as e+e-→ννγ, \tildeγ\tildeγγ and e+e-→e*e→e+e-γ.

An analysis of intermediate transverse momentum pion and photon production inpp collisions through a QCD resummation mechanism

We propose a resummation of QCD radiative corrections in large transverse momentum hadronic processes, based on the introduction of an infrared cut-off, linked to the confinement radius. An analysis of the inclusive reactionspp → π0X andpp → γX and of γ/π0 ratio is presented.