PoS(EPS-HEP 2009)306 http://pos.sissa.it/ † ∗ [email protected] Speaker. On behalf of the LHCb collaboration We report on studies of WLHCb and can Z probe production with a the regimetribution LHCb of functions experiment. low are Due Bjorken not to x its welland electroweak acceptance, background constrained. rejection are production, discussed, In and where this studiesmeasurements parton of to review the dis- the strategies sensitivity underlying of parton for differential density cross-section triggering, functions presented. selection ∗ † Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

European Physical Society Europhysics Conference onJuly High 16-22, Energy 2009 Physics Krakow, Poland Tara Shears , UK E-mail: Studies of W and Z productionexperiment with the LHCb PoS(EPS-HEP 2009)306 1(a) Tara Shears above 1.5 GeV. T p 111 GeV. Figure < of 20 (15) GeV, be co- T µµ p m ], one of the four main ex- 1 < 5 are therefore complementary to . 2 ≤ ) above 0.8 GeV, and dimuon invariant T η p ≤ 9 . ] and high x. Production typically involves a 2 must lie within 71 2 µµ m above 10 GeV, having a dimuon invariant mass above 50 GeV. This trigger T p 9. Inside this acceptance LHCb possesses the full range of tracking, identifi- . 4 ≤ η LHCb employs a two level trigger system. The first, L0, takes place in hardware. Events Backgrounds from top, bottom and charm decays (“QCD”), Z, W and WW decay (“elec- Studies of W and Z boson production, here isolated by selecting final states, at the Large In this review we present studies by the LHCb experiment [ This particular combination of trigger thresholds and angular acceptance ensures that LHCb ≤ 9 . containing Z are triggered by requiring two muon objects, of summed troweak”) and and kaonspress misidentified these, as candidate Z (“ bosons bkg”),highest must have (second satisfy been highest) additional studied. transverse criteria momentum in To sup- the muons offline must event exceed selection. a The valence and a sea . Itdensity is the functions low are x sea the quark dominant thatproduction is uncertainty inside less in the well the known. LHCb The predicted acceptance uncertaintythe differential the on potential cross-sections. uncertainty parton sensitivity varies For Z between Z 2% andparton and W density 10%. functions. differential cross-section We discuss measurements have to the underlying 2. Z and W boson selection 2.1 Z selection The second, HLT, takes place in software.of Here two Z muons bosons with are triggered byis confirming just the over existence 90% efficient for Z events containing two muons inside the LHCb acceptance. incident with having originated athadronic the energy. primary The vertex, dimuon invariant and mass possessshows less the than invariant 50 mass GeV of selected ofis candidates associated after pure this (about selection 97%), has and been efficient applied. (91% of The events selection passing the trigger are selected). Collider are of importancelows for samples a to number of be reasons. isolatedperformance in The and clean early potentially experimental data, provide signature a providingtroweak al- luminosity events boson measurement. that production can are Theoretical known be predictions toby for used next-to-next-to-leading elec- the to order, and uncertainty calibrate are in detector limited partonfore in density also precision provide functions. precise Standard Measurements Modelknown, of tests, and W data in and to regions Z constrain where the parton production parton density can density functions there- functions are themselves well where they are not. Studies of W and Z production with the LHCb experiment 1. Introduction periments sited at theis Large Hadron instrumented Collider. only on Unlike1 one the side ATLAS and of CMS the experiments interaction LHCb point, and covers the pseudorapidity region cation, calorimetry and muon detection, andple, can muons trigger can on be objects triggered with with low transverse thresholds. momentum For ( exam- can probe W and Z production for partons of low [ masses above 2.5 GeV. Measurements made withinATLAS and 1 CMS, and beyond are unique to LHCb with this full detector complement. PoS(EPS-HEP 2009)306 ]. 3 for ) 9 T . p 4 of data. 1 exceeding Tara Shears < − T p µµ η < 9 . 0.8 0.2 0.3 0.1 1-5 ;1 µµ → Z ( σ above 1.1 GeV at L0, and a confirmed ) 9 T . p 4 shows the muon transverse momentum of < . Note that statistical errors are below 1% on µ 1 η 1(b) < 3 9 (b) Differential cross-section as aW function candidates. of muon The arrowplaced. shows where the requirement is . 0.5 0.3 0.2 0.1 1-5 ;1 µν → W ( are shown in table σ 1 − above 20 GeV. This criterion results in a trigger efficiency of about 74%. T p Error estimates, as percentages of the W and Z measured cross-sections with 100 pb Estimated measurement uncertainties for W and Z production, inside the LHCb acceptance Here backgrounds from semi-leptonic decays of bottom (“bb”), Z bosons where one W bosons are triggered by requiring a single muon of Reconstruction efficiency Trigger efficiency Luminosity Error source Statistical Background (a) Differential cross-section as ainvariant mass function for of Z dimuon candidates. Table 1: Although these are still underby study, knowledge it of seems the likely integrated that luminosity. the total cross-section will be dominated and using a dataset of 100 pb muon is located outside thehave LHCb been acceptance studied. (“Z”), To and suppress misidentified these, pions candidate and W bosons (“K/pi”), mustselected possess candidates. a This muon selection of has athe lower more efficiency restrictive (35% transverse of momentum triggered criteria, and events pass), a owing purity to of2.3 about 90%. Measurement uncertainties the total cross-section in both cases.timation, Systematic reconstruction errors efficiency and have been acceptance, estimated trigger efficiency for and the luminosity background following es- [ muon at HLT with 30 GeV. The muon must also be isolated. Figure 2.2 W selection Studies of W and Z production with the LHCb experiment PoS(EPS-HEP 2009)306 . + 4 − Tara Shears , luminosity can be extracted with a 1 − and Z differential cross-sections in data − ,W + 4 2008. . Measurements of the total cross-section will quickly 4 − ratio, which is sensitive to the u/d quark ratio, or the W of combined W and Z data for example, the u valence, d − 1 − /W 3:S08005 + JINST ], allowing the shape and normalisation of the prediction to vary. The 4 et al. 5% in this case. Parton density functions are constrained in the low and higher x regions . Inelastic Scattering conference 2009. dimuon final states”, CERN-THESIS-2009-020. production asymmetry which is sensitive to the difference between u valence and d valence Studies of Z and W production with the LHCb experiment have been presented. LHCb can Studies show that even in datasets as small as 100 pb The integrated luminosity limits the precision of experimental measurements of both the total, − [4] See F. de Lorenzi, “Indirect precise luminosity measurement with LHCb”, proceedings of Deep [2] For a fuller description of the[3] LHCb low J. x Anderson, reach “Testing see the S. electroweak Ochesanu, sector these and proceedings. determining the absolute luminosity at LHCb using [1] A. Augusto Alves trigger and reconstruct samples ofdata both which bosons with can reasonable probe efficiency partons and of purity, x and at provide 10 precision of about 2%.density The shapes are precision not depends known on athan priori the 3 the parton choice density of a modelwhich particular adopted. model LHCb can data As lead probes. to parton valence a bias and In - 1 no distribution more fb uncertaintiesMeasurements of could Z be and reduced W byparton production density up at functions to LHCb even in a can the therefore third absence help at of to a an good constrain x luminosity knowledge of measure. of4. the 10 Conclusions be dominated by the uncertainty onsection integrated can luminosity. help constrain Measurements the of parton the density differential functions cross- even if the luminosityReferences is not well known. fitted normalisation corresponds to the integrated luminosityfor of an the external dataset, thus measure. removing thewhose The need uncertainty fitted allows the shape shape constrains to vary. the uncertainty in parton density functions, to theoretical prediction [ and differential, cross-sections (where itratios enters of as Z a and W correlatedratios production error). can can be It be chosen has studied that beenmaximise instead. either minimise sensitivity suggested Besides sensitivity (eg. that cancelling to luminosity the parton dependance, density W functions (such as Z/W), or Studies of W and Z production with the LHCb experiment 3. PDF sensitivity studies W distributions). However, it is also possible to fit W