Small-x QCD and forward physics results from CMS

Transcription

1 Small-x QCD and forward physics results from CERN he Collaboration has a comprehensive program of small-x QCD and forward physics measurements, which is supported by an excellent experimental coverage into the very forward phase space. Some of the highlights in terms of testing QCD at low transverse momenta (p ) and at high pseudorapidities (η) with jets and charged particles are summarized. Also extremely rare processes, as the measurement of exclusive W-pair production in photon-photon collisions in proton-proton (pp) data are discussed. he range of physics results is complemented by studies of diffractive collisions, as well as of multi-parton interactions and soft-qcd phenomena. he measurement of the underlying event at different center-of-mass energies is another fundamental result presented. First results based on the data collected at ev are also discussed. PoS(EPS-HEP)9 he European Physical Society Conference on High Energy Physics -9 July Vienna, Austria Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives. International License (CC BY-NC-ND.).

2 Small-x QCD and forward physics results from he Standard Model of particle physics is a remarkably successful theory to describe and predict the properties of interactions at small length scales. he large coupling and self-interacting nature of Quantum Chromodynamics (QCD), representing the theory of the strong interaction, encompasses various areas and phenomena, where first-principles calculations are exceedingly difficult or not even possible. A large variety of phenomenological models and a number of experimental measurements are aiming at a better understanding of these non-perturbative processes. hese are not only profoundly important because they are responsible for a very large fraction of the total cross section of hadron-hadron collisions at high energies, but they are also indispensable ingredients of improving background estimates in case of precise searches for new phenomena beyond the Standard Model. Examples of such measurements to be presented here are single- and double-diffraction phenomena and cross sections, searches for signs of double parton scattering (in four-jet events with two b-jets), features of the underlying event and its dependence on the hard scale present in the collision and on the center-of-mass energy, and particle production at low p. he emission source size of the latter is studied in terms of Bose-Einstein correlations. In addition, a search for signs of anomalous quartic gauge couplings are presented using exclusive WW events. he experiment is ideally suited to carry out the above mentioned studies. he central feature of the apparatus is a superconducting solenoid of m internal diameter. Within the magnet volume are the silicon pixel and strip tracker, the crystal electromagnetic calorimeter, and the brass and scintillator hadron calorimeter. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke of the magnet. he tracker measures charged particles within η <.. It has silicon pixel and silicon strip detector modules, arranged in tracking layers. he barrel region of the pixel detector consists of three layers very close to the beamline. In addition to the barrel and endcap detectors, has extensive forward calorimetry. he forward component of the hadron calorimeter, HF (.9 < η <.), consists of R l [fm] preliminary η < PbPb, s NN =.7 ev ppb, s NN =. ev pp, s = 7 ev pp, s =.7 ev pp, s =.9 ev π. < k <. GeV/c N tracks (fm) R inv... Preliminary s =.7 ev s =.7 ev s = 7 ev s = 7 ev JHEP()9 s =.9 ev s =.9 ev N ch s = 7 ev s = 7 ev PoS(EPS-HEP)9 Figure : Left: N tracks -dependence of the three-dimensional pion radii (the R l component), shown here for. < k <. GeV/c, for all studied reactions. Lines are drawn to guide the eye. Right: R inv is shown as a function of the charged particle multiplicity, N ch, for pp collisions at.7 ev and 7 ev. Outer error bars the statistical and systematic uncertainties added in quadrature.

3 Small-x QCD and forward physics results from (mb) σ SD (a) (extrapolated with PYHIA-MBR) ALICE PYHIA-MBR (ε=.) CDF PYHIA-MBR (ε=.) E7 GLM UA KP CHLM (ISR) Armitage et al. (ISR) -. µb (7 ev) (mb) σ DD (b) -. µb (7 ev) (extrapolated with PYHIA-MBR) ALICE NSD (DD+ND) CDF PYHIA-MBR (ε=.) PYHIA-MBR (ε=.) GLM KP ξ <. η > s (GeV) s (GeV) Figure : Diffractive cross sections as a function of collision energy measured in pp and pp collisions compared to model predictions. Left: total single diffractive (SD) cross section for ξ <.. Right: total double diffractive (DD) cross section for η >. he inner (outer) error bars of the data points correspond to the statistical and systematic (and the additional extrapolation) uncertainties added in quadrature. steel absorbers with embedded radiation-hard quartz fibers, providing fast collection of Cherenkov light. he very forward angles are covered at one end of (. < η <.) by the CASOR calorimeter, made of quartz plates embedded in tungsten absorbers, segmented in φ-sectors and z-modules. A detailed description of the detector can be found in []. Low momentum particles detected after each pp collision appear to be emitted from a source with a finite size. Correlations between identical bosons with a small relative momentum can be used to investigate the dependence of the emission source size on various quantities, e.g. the charged particle multiplicity in the collision. hese studies were carried out both with identified [] and non-identified particles [], with slightly different, complementary methods. As an example, in case of identified particles, the following function was fit to the three-dimensional two-particle correlation, where q l,o,s are relative momentum components in the longitudinally co-moving system of the particle pair: + λ exp[ q l R l + q or o + q s R s ]. he left panel of Fig. shows R l as a function of multiplicity measured in pp, ppb and PbPb collisions at various s values. In case of the non-identified particles, the fit function to the one-dimensional correlation, where Q inv is the invariant relative momentum of the particle pair, was: C[ + λ exp (Q invr inv ) a ]( + δq inv ). he behavior R inv in pp collisions at.7 ev and 7 ev is investigated as a function of multiplicity as shown on the right panel of Fig.. he increase of R inv with N ch shows a scaling property of the lengths of homogeneity with increasing collision center-of-mass energy. Diffractive dissociation processes in pp collisions at 7 ev were studied in events with a large rapidity gap in the detector. he single- (SD) and double- (DD) diffractive processes have been separated utilizing the CASOR calorimeter. he cross sections were measured as a function of the masses M X and M Y of the hadronic systems on the two sides of the rapidity gap. he differential cross sections are integrated over the measurable M X and M Y regions, and extrapolated to the full phase space to obtain the total SD and DD cross sections. his extrapolation, which amounts to approximately a factor of, was carried out using the PHYIA MBR event generator tune, after an extensive set of comparisons between this model and the experimental data. he extrapolated SD and DD cross sections thus obtained are σ SD =. ±.(stat) +.9. (syst)+.7.7 (extrap) mb and σ DD =.7 ±.(stat) +..7 (syst)+.. (extrap) mb, respectively [], and shown on Fig. PoS(EPS-HEP)9

4 Small-x QCD and forward physics results from (/σ) dσ/d S [/rad] Preliminary b-j: p > GeV, η <. j: p > GeV, η <.7 - pb (7 ev), pp b + j + X MADGRAPH+P Z* POWHEG+PYHIA Z' CUEPS-CEQL HERWIG++ UE-EE--CEQL CUEPS-CEQL MPI off DAA otal Uncertainty /[ η ( φ)] ch N ransverse density. s = 7 ev s =.7 ev. MC/data S (rad)... s =.9 ev Leading jet: η <, p > GeV Charged particles: η <, p >. GeV Data PYHIA Z* PYHIA CUEPS HERWIG++ UE-EE-C jet p [GeV] Figure : Left: Normalized cross sections unfolded to the stable particle level as a function of S, compared to predictions of the POWHEG, MADGRAPH, PYHIA, PYHIA and HERWIG++ models. A comparison with the PYHIA CUEPS-CEQL predictions without the simulation of multi-parton interactions is also shown. he lower panel shows the ratios of the predictions to the data. he yellow band represents the total uncertainty. Right: Comparison of the particle density in the underlying event in the transverse region at s =.9,.7, and 7 ev, as a function of p jet. he data (symbols) are compared to various MC simulations (curves). compared to other measurements and models as a function of s. hese results are consistent with a weakly rising trend as a function of center-of-mass energy, as also predicted by the models. At high center-of-mass energies the gluon densities become large at low x values. Hence, the probability to have more than one partonic interaction becomes non-negligible, leading to the production of pairs of different flavored jets via double parton scattering (DPS). he cross section of the production of two jets and two b-jets, as well as a production of three jets in addition to an energetic photon in pp collisions at 7 ev has been measured to gain experimental access to this phenomena. In case of the study of four jet events, a variable S was defined as the azimuthal angle between the light jet pair and the b-jet pair, where the direction of a jet pair is defined by the vector sum of their transverse momenta. he measured cross section as a function of S is shown on the left panel of Fig.. he data are not well described by the models, underestimate the region at values of S <, and do not follow well the decreasing shape towards lower values, showing the need for MPI contributions in the simulation []. A measurement of the underlying event (UE) activity in proton-proton collisions at.7 ev center-of-mass energy was also performed recently using events with charged-particle jets produced in the central pseudorapidity region ( η jet < ). he UE activity is measured in the azimuthal region transverse to the direction of the highest p jet. he center-of-mass energy dependence of the UE activity in the transverse region is presented on the right panel of Fig. as a function of p jet for s =.9,.7, and 7 ev. A fast rise with increasing center-of-mass energy of the activity in the transverse region is observed for the same value of the leading charged-particle. All presented model tunes predict a center-of-mass energy dependence consistent with that of p jet PoS(EPS-HEP)9

5 Small-x QCD and forward physics results from Events / GeV 9 Data 7 - preliminary 9.7 fb ( ev) SM γγ WW Diffractive WW elastic γγ ττ single-inelastic γγ ττ EWK WWqq Inclusive Diboson Drell-Yan stat. error on simulation sum ] - [GeV a W C/Λ.. - preliminary 9.7 fb ( ev) = GeV Λ cutoff Data / MC p (µe) [GeV] Standard Model ev, 9% confidence region 7 ev, 9% confidence region ev -D limit, 9% confidence region -.. W a /Λ [GeV Figure : Left: muon-electron transverse momentum distribution for events with zero associated tracks. Right: excluded values of the anomalous coupling parameters a W /Λ and a W C /Λ with Λ cutoff = GeV. he area outside the solid contour is excluded by this measurement at 9% CL. he predicted cross sections are rescaled to include the contribution from proton dissociation. the data []. A search for (quasi)exclusive γγ W + W processes in pp collisions at ev was carried out using data corresponding to 9.7 fb integrated luminosity. Events were selected by requiring an electron-muon pair with large transverse momentum p (µ ± e ) > GeV/c and no other charged particles detected from the same collision point. After all event selection cuts, events are observed over an expected background of. ±. events, corresponding to an excess of.σ over the background-only hypothesis. his event yield is compatible with the Standard Model prediction. he dilepton transverse momentum spectrum, shown on the left panel of Fig., is studied for deviations from the Standard Model, and the resulting upper limits are compared to predictions assuming genuine anomalous quartic gauge couplings of dimension- with two parameters, a W /Λ and a W C /Λ, where Λ is the scale of new physics. wo-dimensional limits were obtained in the a W /Λ, a W C /Λ parameter space using a dipole form factor with a cutoff scale Λ cutoff = GeV to avoid unitarity violation. he resulting two dimensional 9% confidence region is shown on the right panel of Fig. and compared to the earlier 7 ev result from [7]. Finally, the first physics results obtained at the LHC at ev are presented, namely the pseudorapidity distribution of charged hadrons in pp collisions. he detector operated at zero magnetic field. he yield of charged hadrons produced in inelastic pp collisions was determined in the central region of the pixel detector ( η < ) using both hit pairs and reconstructed tracks. he η distribution of charged hadrons can be seen on the left panel of Fig. []. For central pseudorapidities ( η <.), the charged-hadron multiplicity density is.9 ±.(stat) ±.7(syst). he result is compared to predictions from Monte Carlo event generators and to similar measurements made at lower collision energies on the right panel of Fig.. In the central region, the measured dn ch /dη distribution is consistent with predictions of the PYHIA (tunes CUEPS and - ] PoS(EPS-HEP)9

6 Small-x QCD and forward physics results from dn ch /dη 7 pp s = ev inelastic data PYHIA CUEPS EPOS LHC η dn ch /dη η <. 7 ALICE PHOBOS UA ISR PYHIA CUEPS EPOS LHC parabolic fit in ln(s) s [GeV] pp inelastic Figure : Left: Distributions of the pseudorapidity density of charged hadrons in the region η < in inelastic pp collisions at ev compared to the PYHIA CUEPS and the EPOS LHC models. Right: Center-of-mass energy dependence of dn ch /dη including measurements at lower center-of-mass energy. he solid curve is a second-order polynomial fit in ln(s). CUEPM) and EPOS LHC event generators, while those in a wider η range are better described by the latter model. hese results provide new constraints for the improvement of perturbative and nonperturbative QCD aspects implemented in hadronic event generators. In summary and outlook, the Collaboration has a versatile and active forward, diffractive, exclusive, soft QCD, MPI and low-x physics program, which is facilitated by the unique forward instrumentation. In addition, a joint physics program and data collection is underway with the OEM experiment, with an outlook to tagging diffractive and central-exclusive processes at high luminosity with a new Precision Proton Spectrometer. In the present talk, various recent results concerning Bose-Einstein correlations, diffractive cross sections, four-jet cross sections, underlying event properties, new limits on anomalous quartic gauge couplings have been reviewed, together with the first physics results at the highest-ever LHC energy on charge particle pseudorapidity distributions. References [] S. Chatrchyan et al. [ Collaboration], JINS () S. [] Collaboration, -PAS-HIN-- (). [] Collaboration, -PAS-FSQ-- () [] V. Khachatryan et al. [ Collaboration], Phys. Rev. D 9, no., () [] Collaboration, -PAS-FSQ-- () [] V. Khachatryan et al. [ Collaboration], JHEP 9, 7 () [7] S. Chatrchyan et al. [ Collaboration], JHEP 7, () [] V. Khachatryan et al. [ Collaboration], arxiv:7.9 [hep-ex]. PoS(EPS-HEP)9