- Email: [email protected]
Recent results of soft QCD with the ATLAS experiment
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22 www.elsevier.com/locate/nppp
Recent results of soft QCD with the ATLAS experiment ∗ S. Monzani on behalf of the ATLAS collaborationa a Sapienza
Universit`a di Roma and INFN Sezione di Roma 1, Roma, Italy
Abstract We report on recent results of soft QCD with the ATLAS experiment. In particular results on Bose-Einstein correlations are presented as well as measurements on transverse polarization of Λ and Λ, on distributions sensitive to the underlying events in inclusive Z-boson production and of the total cross section from elastic scattering in pp collisions. Keywords: Soft QCD, Bose-Einstein correlations, polarization, underlying events, total cross section
1. Introduction Soft QCD processes are based on interactions at low momentum transfer and at hadron colliders they are the dominant processes. Soft QCD may also occur in the same proton-proton interaction as a hard interaction, this is the case of Pile-up, Multiple Parton Interactions (MPI) or underlying events. These processes are important to be studied as they cannot be calculated from first principles. In fact the strong coupling blows up at low scales and perturbative calculations are not possible. Experimental measurements are made on Minimum bias (where typically kinematics is measured as multiplicity, transverse momentum or eta spectra) and underlying events, on total and diffractive cross-section and on particle correlations. We review in this note recent results from the ATLAS experiment [1] √ based on data collected in proton-proton collision at s = 7 TeV. 2. Bose-Einstein correlations Bose-Einstein correlations (BEC) is a sensitive probe of the space-time geometry of the hadronization region, it allows the determination of the size and the ∗ Talk given at 18th International Conference in Quantum Chromodynamics (QCD 15, 30th anniversary), 29 june - 3 july 2015, Montpellier - FR Email address: [email protected] (S. Monzani on behalf of the ATLAS collaboration)
http://dx.doi.org/10.1016/j.nuclphysbps.2016.02.005 2405-6014/© 2016 Published by Elsevier B.V.
shape of the source from which particles are emitted [2]. BEC is studied through correlations between two identical bosons (consequence of the symmetry of identical bosons wave function), as BEC effect corresponds to an enhancement in two identical boson correlation function when the two particles are near in momentum space [3]: C2 (p1 , p2 ) =
ρ(p1 , p2 ) = C0 [1+Ω(λ, QR)](1+Q) (1) ρ0 p1 , p2 )
where C0 is a normalization factor, the long-range momentum correlations, λ the strength parameter, which is equal to 0 (1) for purely coherent (chaotic) sources and R the effective radius of the source size. Finally ρ is a like-sign two-particle density function taking into account BEC effects, while ρ0 refers only on unlike-sign charged particle pairs. This latter has the same topology and global properties as the like-sign sample but it is free of any BEC effect and it contains hadrons pairs from the decay of resonances such as ρ, η, η , ω, φ and K∗. For this reason, a double-ratio function has been introduced in order to account for the effects of these resonances correlated with non-BEC processes, so that the two-particle correlation function is corrected using MC simulation without BEC effects: C2 (Q) (2) R2 (Q) = MC C2 (Q) Q2 = −(−p1 − p2 )2
(3)
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
2.1. Results Λ and R parameters has been measured at a center-ofmass energy equal to 0.9 and 7 TeV (this latter also at high multiplicity). At least one vertex from at least two tracks (like-sign charged particles) with pT > 100 GeV and |η| < 2.5 has been selected, while at least 120 tracks for high multiplicity events. The R2 (Q) shows higher BEC activity at low-Q and it has been fitted with an exponential function (more precise at low-Q) √ from which s = 0.9 TeV Λ and R has been obtained respectively at √ (particle multiplicity n ≥ 2), s = 7 TeV (nch ≥ 2), ch √ s = 0.9 TeV (nch ≥ 150): λ = 0.74 ± 0.11, R = 1.83 ± 0.25, λ = 0.71 ± 0.07, R = 2.06 ± 0.22, λ = 0.52 ± 0.06, R = 2.36 ± 0.30. These values of fitted parameters are close to the ones obtained by the CMS [4] and ALICE [5] experiments. Λ and R parameters are sensitive to the averaged transverse momentum of the pair (kT = |pT1 + pT2 |/2) and to nch , results cover wider regions in kT and nch and they are in agreement with previous experiments. In particular, R is independent to multiplicity for nch > 50 and shows a saturation from nch > 240 observed for the first time that can be predicted by Pomeron-based models (due to overlapping of colliding protons) [6, 7]. 3. Polarization of Λ and Λ Measurement of the polarization vector transverse to the momentum of Λ can range between 0 (no polarization) and 1. In previous experiments it was measured only with fixed target up to 40 GeV [8], where it was seen that the Λ magnitude polarization increases with pT until it saturates at about 1 GeV, decreases with decreasing |xF | (xF = pZ /pbeam where pZ is the longitudinal momentum and pbeam the momentum of the beam), while it does not depend strongly on the center of mass energy. For Λ decaying to a proton and a π− , polarization P can be obtained through the method of moments: E(P) = gdet (t ; P)dt = E(0) + [E(1) − E(0)]P(4)
19
3. The transverse decay distance L xy is defined as a projection of the vector connecting the primary and secondary vertices onto the Λ direction and the Λ impact parameter a0 is the defined as a shortest distance between the primary vertex and the line aligned with the Λ momentum. Uncertainties on L xy and a0 are denoted σLxy and σa0 respectively. Then a reduction of physics background is applied, that is selecting events with 480 MeV < mππ < 515 MeV and mee < 75 MeV. Main background processes are related to K0S decays and γ decaying in a couple of electrons. 3.1. Results From eq. 4 results of P as function of xF and pT relative to Λ are shown in Fig. 1 and 2, from the fiducial phase space where 0.8 GeV < pT < 15 GeV, 5 · 10−5 < xF < 0.01, |η| < 2.5 and 1100 MeV < mpπ < 1127 MeV [10]. With this selection criteria 423498 Λ and 378237 Λ candidates are selected in the full mass range data (760 μb−1 ), the difference between the 2 numbers is caused by different production cross section for Λ and Λ, different absorption cross sections with the detector material and differences in the reconstruction efficiencies at low pT . The average transverse polarizations of Λ and Λ are
Figure 1: Polarization of Λ as function of xF .
here resumed: where gdet (t ; P) = 1/2(1 + αPt ) is the probability distribution of θ∗ (the angle between the polarization vector transverse to the momentum of Λ and the proton direction) modified by detector efficiency and resolution effects, α the World average of the P-violating decay asymmetry for Λ [9] and t = cosθ∗ . Selection criteria requires a vertex fit probability greater than 0.05, Λ and Λ transverse decay distance significance (L xy /σLxy ) and impact parameter significance (a0 /σa0 ) respectively higher than 15 and lower than
PΛ = 0.010 ± 0.005(stat) ± 0.004(syst)
(5)
PΛ = 0.002 ± 0.006(stat) ± 0.004(syst)
(6)
Both results are consistent with 0 within the estimated uncertainties. Λ polarization is consistent with an extrapolation of the results of the M2 beam line experiment at Fermilab [11], suggesting that the magnitude of polarization should decrease with xF . Unlike for the Λ, the polarization of Λ was measured to be consistent
20
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
4.1. Results UE activity can be studied in terms of normalized and differential distributions of pT of charged particles and their multiplicity nch (see Fig. 3 and 4) with a dataset corresponding to an integrated luminosity of 4.64 f b−1 . Towards, trans-max and trans-min show similar behaviors [16] and only the transverse region results are shown. In Fig. 3 and 4, distributions are
Figure 2: Polarization of Λ as function of pT .
with 0 by all previous experiments. ATLAS also measured the same value for Λ polarization, as well as in bins of xF and pT . 4. Measurement of distributions sensitive to the underlying events in inclusive Z-boson production Underlying events (UE) mainly happens when partons does not participate in the hard-scattering process. Anyway they may also occur in additional semi-hard scatters in pp collisions, multiple partons interactions (MPI) and initial and final state gluon radiation contribute to UE. In this analysis [12] events with a Z-boson candidate decaying into an electron or muon pair, are selected. The phi space around the Z-boson direction has been taken into consideration and divided in 4 sub-regions [13]: the ”toward”, closest to the Z-boson (|Δφ| < 60o where Δφ = 0o is on the Z-boson direction), the ”away”, opposite to the Z-boson direction (|Δφ| > 120) and 2 transverse regions (60o < |Δφ| < 120o ). Transverse and toward regions are the most sensitive to UE. The 2 transverse regions are not equally sensitive, the most active one is the trans-max and less active one is the trans-min. Assuming a flat distributions of UE activity, differences between the 2 transverse regions (Transdiff) [14] are due to hard scatter processes. Selected events with a Z decay have at least 2 primary vertex associated tracks with pT > 400 MeV with electrons or muons with pT > 20 GeV and z0 sinθ < 10 mm in |η| < 2.4 and a dileptons invariant mass of oppositely charged leptons between 66 and 116 MeV. In these events, UE activity is selected respecting the following conditions [15]: pT > 0.5 GeV, |η| < 2.5, a minimum of 1 pixel and 6 in the semiconductor tracker, 1 hit in the innermost pixel layer, impact parameters |d0 | < 1.5 mm and z0 sinθ < 1.5 mm and, for tracks with pT > 10 GeV, goodness of fit greater than 0.01.
Figure 3: Normalized differential number of charged particle distribution in the transverse region, compared with different MC generators.
Figure 4: Normalized differential pT of charged particles in the transverse region, compared with different MC generators.
compared with different MC samples, in particular in Fig. 3 only Pythia [17, 18] well describes the number of charged particles, while in Fig. 4 Powheg [19] shows a best agreement at low values of pT while Alpgen and SHERPA [20, 21] at higher. Mean values of pT and nch show an increase as function of the pT of the Z-boson and of the number of charged particles. Sherpa and Alpgen are usually consistent with data at higher values of pT and nch , while no model is in agreement in the whole ranges (due mainly by the presence of additional jets). NLO/multileg generators give
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
a better description of it. Finally a comparison with other experiments with a jet instead of a Z-boson as leading particle has been made. In particular again differential distributions of pT and nch are compared, where the case with a leading jet shows a more restricted distribution, as no jet with a pT higher than the leading one can be considered. 5. Measurement of the total and elastic cross sections from elastic scattering in pp collisions The total hadronic cross section is a fundamental parameter of strong interactions. Large distances are involved in the collision process and thus perturbation theory is not applicable. Constraints are obtained through the Froissart-Martin bound, which states that the cross section cannot grow asymptotically faster than [ln(s)]2 [22, 23], or the optical theorem. A measurement of the total cross section from elastic scattering in pp collisions has been made in a special run with high − β∗ beam optics with an integrated luminosity of 80 μb−1 by the ALFA detector [1], designed to detect small-angle proton scattering. It is housed in the 2 roman pot of ATLAS at 238 and 241 m from the interaction point (IP) [24]. It covers a pseudorapidity higher than 8.5 and it is made of 2 tracking stations (2 arms), each one composed by 4 detectors. Trigger conditions for elastic-scattering events (coincidence of the main detector trigger scintillators between either of the 2 upper/lower detectors on side A and either of the 2 lower/upper detectors on side C) and 1 reconstructed track in all 4 detectors of the arm which fired the trigger are required.
21
low t fN (t) is dominant with respect to the fC (t) parameter and dσ dt becomes function only of a single exponential form [27]. For this reason and in order to keep the ALFA acceptance higher than 0.1, for the fit of the elastic cross section, a low-t range (-0.01 GeV2 < t < 0.1 GeV2 ) has been selected. Then the fit has been extended to the whole range of t keeping the value of B constant. B = 19.73 ± 0.14(stat) ± 0.26(syst) GeV−2
(8)
Integrating this parameterization of the elastic cross section on the whole range of t: σel = 24.00 ± 0.19(stat) ± 0.57(syst) mb
(9)
5.2. Total and inelastic cross section measurement The total cross section has been measured through the optical theorem: σ2tot =
16π(c)2 dσel |t→0 1 + ρ2 dt
(10)
It has been extrapolated at low t from the elastic cross section and ρ represents a small correction arising from the ratio of the real to the imaginary part of the elasticscattering amplitude in the forward direction and it is taken from theory. From the fit in the considered range of the differential cross section and from eq. 10 the measured total cross section is: σtot = 95.35 ± 1.30mb
(11)
Results on the total and elastic cross section are in agreement with previous experiments and this is visible in Fig. 5 (red dots). Previous measurements show
5.1. Elastic cross section measurement The elastic cross section is calculated through the following equation: dσ 1 = | fN (t) + fC (t)eiαφ(t) |2 dt 16π
(7)
where fN (t) is the purely strongly interacting amplitude (function of the total cross section), fC (t) is the Coulomb amplitude (function of 1/t), −t = (θ∗ × p)2 where p is the nominal beam momentum of the LHC of 3.5 TeV, θ∗ is the scattering angle at IP that is measured from the proton scattering trajectory in ALFA and a phase φ is induced by long-range Coulomb interactions [25, 26]. fN (t) can be written as an exponential relation: e−Bt/2 , where B is the slope parameter of the exponential. At
Figure 5: Cross section as function of the center-of-mass energy, red dots refers to ATLAS results (total and elastic cross section) and they are compared with previous results from the COMPETE [28] and lower energy pp experiments and cosmic rays.
a correlation with respect to the center-of-mass energy,
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
22
well fitted by the [ln(s)]2 relation. The total cross section obtained by ATLAS confirms this behavior, as it is 2σ far from model even if other models have less increase. Measurements of total and elastic cross sections allow to calculate also the inelastic cross section, simply subtracting the latter from the former obtaining the following result: σinel = 71.34 ± 0.36(stat) ± 0.84(syst) mb
Finally, the total cross section is a fundamental parameter of strong interactions, it has been measured at 7 TeV center-of-mass energy confirming its rise proportional to [ln(s)]2 , however the asymptotic energy dependence is yet to be determined. All these studies repeated at 13 TeV will be important in order to confirm the behaviors of actual results and extend them to wider ranges of energy.
(12)
This result is comparable with the measurement obtained from the TOTEM experiment (difference of 1.3 σ) [29]. ATLAS already obtained a value for the inelastic cross section, but this more recent measurement avoids the previous extrapolation to low diffractive masses outside the fiducial region. This lead to significantly reduce uncertainty also with respect to the previous ATLAS measurement through the Minimum Bias, as it is visible in Fig. 6.
Figure 6: Inelastic cross sections measured by TOTEM [29], ALICE [30] and ATLAS, the blue dot represents the ATLAS measurement from elastic scattering in pp collisions.
6. Conclusions Soft QCD processes at hadron colliders are the dominant processes. Study of like-sign hadrons for Bose-Einstein correlation (BEC) has been performed, clear signal BEC has been found at low-Q, studied through the BEC parameters characterizing the correlation strength and the correlation source size. Transverse polarization of Λ and Λ hyperons is consistent with 0 at low xF confirming the behavior of previous experiments showing a decrease of polarization as function of xF . Measurement of UE is strongly affected by jets radiated from hard scatter, NLO/multileg generators give a better description of it and the Trans-min region is the least affected by extra-jets.
References [1] ATLAS Collaboration, 2008 JINST 3 S08003 [2] W.A. Zajc in Hadronic Multiparticle Production (Advanced Series on Direction in High Energy Physics, Vol.2) p. 235, Ed. P. Carruthers (World Scientific, Singapore, 1988). [3] ATLAS Collaboration, submitted to Eur. Phys. J. C., ArXiv:1502.07947. [4] CMS Collaboration, J. High Energy Phys. 05, 029 (2011), ArXiv 1101.3518. [5] K. Aamodt et al., ALICE Collaboration, Phys. Rev. D 82, 052001 (2010), ArXiv 1007.0516. [6] V.A. Schegelsky, S.V. Akkelin, Iu.A. Karpenko, V.M. Shapoval, Adv. High Energy Phys. 2013, 198928 (2013). [7] M.G. Ryskin and V.A. Schegelsky, Nucl. Phys. Proc. Suppl. 219-220, 10 (2011). [8] G. Bunce et al., Phys. Rev. Lett. 36, 1113 (1976); K. Heller et al., Phys. Rev. Lett. 41, 607 (1978); L. Schachinger et al., Phys. Rev. Lett. 41, 1348 (1978); K. Heller et al., Phys. Rev. Lett. 51, 2025 (1983); B.S. Yuldashev et al., Phys. Rev. D 43, 2792 (1991). [9] J. Beringer et al. (Particle Data Group), Phys. Rev. D 86, 010001 (2012). [10] ATLAS Collaboration, Phys. Rev. D 91, 032004 (2015), ArXiv:1412.1692. [11] B. Lundberg et al., Phys. Rev. D 40, 3557 (1989). [12] ATLAS Collaboration, Eur. Phys. J. C (2014) 74:3195. [13] R. Field for the CDF Collaboration, eConf. C010630 (2001) P501. [14] G. Marchesini and B.R. Webber, Phys. Rev. D 57 (1998) 5787. [15] ATLAS Collaboration, New J. Phys. 13 (2011) 053033. [16] ATLAS Collaboration, JHEP 1307 (2013) 032. [17] T. Sj¨ostrand, S. Mrenna, P. Skands, Comput. Phys. Commun. 178 (2008). [18] ATLAS Collaboration, Eur. Phys. J. C 72 (2012) 1849. [19] S. Alioli, P. Nason, C. Oleari and E. Re, JEHP 07 (2008) 060. [20] M.L. Mangano, M. Moretti, F. Piccinini, R. Pittau, A.D. Polosa, JHEP 07 (2003) 001. [21] T. Gleisberg, S. Hoeche, F. Krauss, M. Schonherr, S. Schumann et al., JHEP 02 (2009) 007. [22] M. Froissart, Phys. Rev. 123 (1961) 1053. [23] A. Martin, Il Nuovo Cimento A Series 10 42 (1966) 930. [24] ATLAS Collaboration, Nuclear physics B (2014), 486-548 [25] H.A. Bethe, Ann. Phys. 3 (1958) 190. [26] G.B. West, D.R. Yennie, Phys. Rev. 172 (1968) 1413. [27] V.A. Khoze, A.D. Martin, M.G. Ryskin, Eur. Phys. J. C 18 (2000) 167. [28] J. Cudell et al., COMPETE Collaboration, Phys. Rev. Lett. 89 (2002) 201809. [29] G. Antchev et al., TOTEM Collaboration, EuroPhys. Lett. 101 (2013) 21003. [30] B. Abelev et al., ALICE Collaboration, Eur. Phys. J. C 73 (2013) 2456.
Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22 www.elsevier.com/locate/nppp
Recent results of soft QCD with the ATLAS experiment ∗ S. Monzani on behalf of the ATLAS collaborationa a Sapienza
Universit`a di Roma and INFN Sezione di Roma 1, Roma, Italy
Abstract We report on recent results of soft QCD with the ATLAS experiment. In particular results on Bose-Einstein correlations are presented as well as measurements on transverse polarization of Λ and Λ, on distributions sensitive to the underlying events in inclusive Z-boson production and of the total cross section from elastic scattering in pp collisions. Keywords: Soft QCD, Bose-Einstein correlations, polarization, underlying events, total cross section
1. Introduction Soft QCD processes are based on interactions at low momentum transfer and at hadron colliders they are the dominant processes. Soft QCD may also occur in the same proton-proton interaction as a hard interaction, this is the case of Pile-up, Multiple Parton Interactions (MPI) or underlying events. These processes are important to be studied as they cannot be calculated from first principles. In fact the strong coupling blows up at low scales and perturbative calculations are not possible. Experimental measurements are made on Minimum bias (where typically kinematics is measured as multiplicity, transverse momentum or eta spectra) and underlying events, on total and diffractive cross-section and on particle correlations. We review in this note recent results from the ATLAS experiment [1] √ based on data collected in proton-proton collision at s = 7 TeV. 2. Bose-Einstein correlations Bose-Einstein correlations (BEC) is a sensitive probe of the space-time geometry of the hadronization region, it allows the determination of the size and the ∗ Talk given at 18th International Conference in Quantum Chromodynamics (QCD 15, 30th anniversary), 29 june - 3 july 2015, Montpellier - FR Email address: [email protected] (S. Monzani on behalf of the ATLAS collaboration)
http://dx.doi.org/10.1016/j.nuclphysbps.2016.02.005 2405-6014/© 2016 Published by Elsevier B.V.
shape of the source from which particles are emitted [2]. BEC is studied through correlations between two identical bosons (consequence of the symmetry of identical bosons wave function), as BEC effect corresponds to an enhancement in two identical boson correlation function when the two particles are near in momentum space [3]: C2 (p1 , p2 ) =
ρ(p1 , p2 ) = C0 [1+Ω(λ, QR)](1+Q) (1) ρ0 p1 , p2 )
where C0 is a normalization factor, the long-range momentum correlations, λ the strength parameter, which is equal to 0 (1) for purely coherent (chaotic) sources and R the effective radius of the source size. Finally ρ is a like-sign two-particle density function taking into account BEC effects, while ρ0 refers only on unlike-sign charged particle pairs. This latter has the same topology and global properties as the like-sign sample but it is free of any BEC effect and it contains hadrons pairs from the decay of resonances such as ρ, η, η , ω, φ and K∗. For this reason, a double-ratio function has been introduced in order to account for the effects of these resonances correlated with non-BEC processes, so that the two-particle correlation function is corrected using MC simulation without BEC effects: C2 (Q) (2) R2 (Q) = MC C2 (Q) Q2 = −(−p1 − p2 )2
(3)
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
2.1. Results Λ and R parameters has been measured at a center-ofmass energy equal to 0.9 and 7 TeV (this latter also at high multiplicity). At least one vertex from at least two tracks (like-sign charged particles) with pT > 100 GeV and |η| < 2.5 has been selected, while at least 120 tracks for high multiplicity events. The R2 (Q) shows higher BEC activity at low-Q and it has been fitted with an exponential function (more precise at low-Q) √ from which s = 0.9 TeV Λ and R has been obtained respectively at √ (particle multiplicity n ≥ 2), s = 7 TeV (nch ≥ 2), ch √ s = 0.9 TeV (nch ≥ 150): λ = 0.74 ± 0.11, R = 1.83 ± 0.25, λ = 0.71 ± 0.07, R = 2.06 ± 0.22, λ = 0.52 ± 0.06, R = 2.36 ± 0.30. These values of fitted parameters are close to the ones obtained by the CMS [4] and ALICE [5] experiments. Λ and R parameters are sensitive to the averaged transverse momentum of the pair (kT = |pT1 + pT2 |/2) and to nch , results cover wider regions in kT and nch and they are in agreement with previous experiments. In particular, R is independent to multiplicity for nch > 50 and shows a saturation from nch > 240 observed for the first time that can be predicted by Pomeron-based models (due to overlapping of colliding protons) [6, 7]. 3. Polarization of Λ and Λ Measurement of the polarization vector transverse to the momentum of Λ can range between 0 (no polarization) and 1. In previous experiments it was measured only with fixed target up to 40 GeV [8], where it was seen that the Λ magnitude polarization increases with pT until it saturates at about 1 GeV, decreases with decreasing |xF | (xF = pZ /pbeam where pZ is the longitudinal momentum and pbeam the momentum of the beam), while it does not depend strongly on the center of mass energy. For Λ decaying to a proton and a π− , polarization P can be obtained through the method of moments: E(P) = gdet (t ; P)dt = E(0) + [E(1) − E(0)]P(4)
19
3. The transverse decay distance L xy is defined as a projection of the vector connecting the primary and secondary vertices onto the Λ direction and the Λ impact parameter a0 is the defined as a shortest distance between the primary vertex and the line aligned with the Λ momentum. Uncertainties on L xy and a0 are denoted σLxy and σa0 respectively. Then a reduction of physics background is applied, that is selecting events with 480 MeV < mππ < 515 MeV and mee < 75 MeV. Main background processes are related to K0S decays and γ decaying in a couple of electrons. 3.1. Results From eq. 4 results of P as function of xF and pT relative to Λ are shown in Fig. 1 and 2, from the fiducial phase space where 0.8 GeV < pT < 15 GeV, 5 · 10−5 < xF < 0.01, |η| < 2.5 and 1100 MeV < mpπ < 1127 MeV [10]. With this selection criteria 423498 Λ and 378237 Λ candidates are selected in the full mass range data (760 μb−1 ), the difference between the 2 numbers is caused by different production cross section for Λ and Λ, different absorption cross sections with the detector material and differences in the reconstruction efficiencies at low pT . The average transverse polarizations of Λ and Λ are
Figure 1: Polarization of Λ as function of xF .
here resumed: where gdet (t ; P) = 1/2(1 + αPt ) is the probability distribution of θ∗ (the angle between the polarization vector transverse to the momentum of Λ and the proton direction) modified by detector efficiency and resolution effects, α the World average of the P-violating decay asymmetry for Λ [9] and t = cosθ∗ . Selection criteria requires a vertex fit probability greater than 0.05, Λ and Λ transverse decay distance significance (L xy /σLxy ) and impact parameter significance (a0 /σa0 ) respectively higher than 15 and lower than
PΛ = 0.010 ± 0.005(stat) ± 0.004(syst)
(5)
PΛ = 0.002 ± 0.006(stat) ± 0.004(syst)
(6)
Both results are consistent with 0 within the estimated uncertainties. Λ polarization is consistent with an extrapolation of the results of the M2 beam line experiment at Fermilab [11], suggesting that the magnitude of polarization should decrease with xF . Unlike for the Λ, the polarization of Λ was measured to be consistent
20
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
4.1. Results UE activity can be studied in terms of normalized and differential distributions of pT of charged particles and their multiplicity nch (see Fig. 3 and 4) with a dataset corresponding to an integrated luminosity of 4.64 f b−1 . Towards, trans-max and trans-min show similar behaviors [16] and only the transverse region results are shown. In Fig. 3 and 4, distributions are
Figure 2: Polarization of Λ as function of pT .
with 0 by all previous experiments. ATLAS also measured the same value for Λ polarization, as well as in bins of xF and pT . 4. Measurement of distributions sensitive to the underlying events in inclusive Z-boson production Underlying events (UE) mainly happens when partons does not participate in the hard-scattering process. Anyway they may also occur in additional semi-hard scatters in pp collisions, multiple partons interactions (MPI) and initial and final state gluon radiation contribute to UE. In this analysis [12] events with a Z-boson candidate decaying into an electron or muon pair, are selected. The phi space around the Z-boson direction has been taken into consideration and divided in 4 sub-regions [13]: the ”toward”, closest to the Z-boson (|Δφ| < 60o where Δφ = 0o is on the Z-boson direction), the ”away”, opposite to the Z-boson direction (|Δφ| > 120) and 2 transverse regions (60o < |Δφ| < 120o ). Transverse and toward regions are the most sensitive to UE. The 2 transverse regions are not equally sensitive, the most active one is the trans-max and less active one is the trans-min. Assuming a flat distributions of UE activity, differences between the 2 transverse regions (Transdiff) [14] are due to hard scatter processes. Selected events with a Z decay have at least 2 primary vertex associated tracks with pT > 400 MeV with electrons or muons with pT > 20 GeV and z0 sinθ < 10 mm in |η| < 2.4 and a dileptons invariant mass of oppositely charged leptons between 66 and 116 MeV. In these events, UE activity is selected respecting the following conditions [15]: pT > 0.5 GeV, |η| < 2.5, a minimum of 1 pixel and 6 in the semiconductor tracker, 1 hit in the innermost pixel layer, impact parameters |d0 | < 1.5 mm and z0 sinθ < 1.5 mm and, for tracks with pT > 10 GeV, goodness of fit greater than 0.01.
Figure 3: Normalized differential number of charged particle distribution in the transverse region, compared with different MC generators.
Figure 4: Normalized differential pT of charged particles in the transverse region, compared with different MC generators.
compared with different MC samples, in particular in Fig. 3 only Pythia [17, 18] well describes the number of charged particles, while in Fig. 4 Powheg [19] shows a best agreement at low values of pT while Alpgen and SHERPA [20, 21] at higher. Mean values of pT and nch show an increase as function of the pT of the Z-boson and of the number of charged particles. Sherpa and Alpgen are usually consistent with data at higher values of pT and nch , while no model is in agreement in the whole ranges (due mainly by the presence of additional jets). NLO/multileg generators give
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
a better description of it. Finally a comparison with other experiments with a jet instead of a Z-boson as leading particle has been made. In particular again differential distributions of pT and nch are compared, where the case with a leading jet shows a more restricted distribution, as no jet with a pT higher than the leading one can be considered. 5. Measurement of the total and elastic cross sections from elastic scattering in pp collisions The total hadronic cross section is a fundamental parameter of strong interactions. Large distances are involved in the collision process and thus perturbation theory is not applicable. Constraints are obtained through the Froissart-Martin bound, which states that the cross section cannot grow asymptotically faster than [ln(s)]2 [22, 23], or the optical theorem. A measurement of the total cross section from elastic scattering in pp collisions has been made in a special run with high − β∗ beam optics with an integrated luminosity of 80 μb−1 by the ALFA detector [1], designed to detect small-angle proton scattering. It is housed in the 2 roman pot of ATLAS at 238 and 241 m from the interaction point (IP) [24]. It covers a pseudorapidity higher than 8.5 and it is made of 2 tracking stations (2 arms), each one composed by 4 detectors. Trigger conditions for elastic-scattering events (coincidence of the main detector trigger scintillators between either of the 2 upper/lower detectors on side A and either of the 2 lower/upper detectors on side C) and 1 reconstructed track in all 4 detectors of the arm which fired the trigger are required.
21
low t fN (t) is dominant with respect to the fC (t) parameter and dσ dt becomes function only of a single exponential form [27]. For this reason and in order to keep the ALFA acceptance higher than 0.1, for the fit of the elastic cross section, a low-t range (-0.01 GeV2 < t < 0.1 GeV2 ) has been selected. Then the fit has been extended to the whole range of t keeping the value of B constant. B = 19.73 ± 0.14(stat) ± 0.26(syst) GeV−2
(8)
Integrating this parameterization of the elastic cross section on the whole range of t: σel = 24.00 ± 0.19(stat) ± 0.57(syst) mb
(9)
5.2. Total and inelastic cross section measurement The total cross section has been measured through the optical theorem: σ2tot =
16π(c)2 dσel |t→0 1 + ρ2 dt
(10)
It has been extrapolated at low t from the elastic cross section and ρ represents a small correction arising from the ratio of the real to the imaginary part of the elasticscattering amplitude in the forward direction and it is taken from theory. From the fit in the considered range of the differential cross section and from eq. 10 the measured total cross section is: σtot = 95.35 ± 1.30mb
(11)
Results on the total and elastic cross section are in agreement with previous experiments and this is visible in Fig. 5 (red dots). Previous measurements show
5.1. Elastic cross section measurement The elastic cross section is calculated through the following equation: dσ 1 = | fN (t) + fC (t)eiαφ(t) |2 dt 16π
(7)
where fN (t) is the purely strongly interacting amplitude (function of the total cross section), fC (t) is the Coulomb amplitude (function of 1/t), −t = (θ∗ × p)2 where p is the nominal beam momentum of the LHC of 3.5 TeV, θ∗ is the scattering angle at IP that is measured from the proton scattering trajectory in ALFA and a phase φ is induced by long-range Coulomb interactions [25, 26]. fN (t) can be written as an exponential relation: e−Bt/2 , where B is the slope parameter of the exponential. At
Figure 5: Cross section as function of the center-of-mass energy, red dots refers to ATLAS results (total and elastic cross section) and they are compared with previous results from the COMPETE [28] and lower energy pp experiments and cosmic rays.
a correlation with respect to the center-of-mass energy,
S. Monzani / Nuclear and Particle Physics Proceedings 270–272 (2016) 18–22
22
well fitted by the [ln(s)]2 relation. The total cross section obtained by ATLAS confirms this behavior, as it is 2σ far from model even if other models have less increase. Measurements of total and elastic cross sections allow to calculate also the inelastic cross section, simply subtracting the latter from the former obtaining the following result: σinel = 71.34 ± 0.36(stat) ± 0.84(syst) mb
Finally, the total cross section is a fundamental parameter of strong interactions, it has been measured at 7 TeV center-of-mass energy confirming its rise proportional to [ln(s)]2 , however the asymptotic energy dependence is yet to be determined. All these studies repeated at 13 TeV will be important in order to confirm the behaviors of actual results and extend them to wider ranges of energy.
(12)
This result is comparable with the measurement obtained from the TOTEM experiment (difference of 1.3 σ) [29]. ATLAS already obtained a value for the inelastic cross section, but this more recent measurement avoids the previous extrapolation to low diffractive masses outside the fiducial region. This lead to significantly reduce uncertainty also with respect to the previous ATLAS measurement through the Minimum Bias, as it is visible in Fig. 6.
Figure 6: Inelastic cross sections measured by TOTEM [29], ALICE [30] and ATLAS, the blue dot represents the ATLAS measurement from elastic scattering in pp collisions.
6. Conclusions Soft QCD processes at hadron colliders are the dominant processes. Study of like-sign hadrons for Bose-Einstein correlation (BEC) has been performed, clear signal BEC has been found at low-Q, studied through the BEC parameters characterizing the correlation strength and the correlation source size. Transverse polarization of Λ and Λ hyperons is consistent with 0 at low xF confirming the behavior of previous experiments showing a decrease of polarization as function of xF . Measurement of UE is strongly affected by jets radiated from hard scatter, NLO/multileg generators give a better description of it and the Trans-min region is the least affected by extra-jets.
References [1] ATLAS Collaboration, 2008 JINST 3 S08003 [2] W.A. Zajc in Hadronic Multiparticle Production (Advanced Series on Direction in High Energy Physics, Vol.2) p. 235, Ed. P. Carruthers (World Scientific, Singapore, 1988). [3] ATLAS Collaboration, submitted to Eur. Phys. J. C., ArXiv:1502.07947. [4] CMS Collaboration, J. High Energy Phys. 05, 029 (2011), ArXiv 1101.3518. [5] K. Aamodt et al., ALICE Collaboration, Phys. Rev. D 82, 052001 (2010), ArXiv 1007.0516. [6] V.A. Schegelsky, S.V. Akkelin, Iu.A. Karpenko, V.M. Shapoval, Adv. High Energy Phys. 2013, 198928 (2013). [7] M.G. Ryskin and V.A. Schegelsky, Nucl. Phys. Proc. Suppl. 219-220, 10 (2011). [8] G. Bunce et al., Phys. Rev. Lett. 36, 1113 (1976); K. Heller et al., Phys. Rev. Lett. 41, 607 (1978); L. Schachinger et al., Phys. Rev. Lett. 41, 1348 (1978); K. Heller et al., Phys. Rev. Lett. 51, 2025 (1983); B.S. Yuldashev et al., Phys. Rev. D 43, 2792 (1991). [9] J. Beringer et al. (Particle Data Group), Phys. Rev. D 86, 010001 (2012). [10] ATLAS Collaboration, Phys. Rev. D 91, 032004 (2015), ArXiv:1412.1692. [11] B. Lundberg et al., Phys. Rev. D 40, 3557 (1989). [12] ATLAS Collaboration, Eur. Phys. J. C (2014) 74:3195. [13] R. Field for the CDF Collaboration, eConf. C010630 (2001) P501. [14] G. Marchesini and B.R. Webber, Phys. Rev. D 57 (1998) 5787. [15] ATLAS Collaboration, New J. Phys. 13 (2011) 053033. [16] ATLAS Collaboration, JHEP 1307 (2013) 032. [17] T. Sj¨ostrand, S. Mrenna, P. Skands, Comput. Phys. Commun. 178 (2008). [18] ATLAS Collaboration, Eur. Phys. J. C 72 (2012) 1849. [19] S. Alioli, P. Nason, C. Oleari and E. Re, JEHP 07 (2008) 060. [20] M.L. Mangano, M. Moretti, F. Piccinini, R. Pittau, A.D. Polosa, JHEP 07 (2003) 001. [21] T. Gleisberg, S. Hoeche, F. Krauss, M. Schonherr, S. Schumann et al., JHEP 02 (2009) 007. [22] M. Froissart, Phys. Rev. 123 (1961) 1053. [23] A. Martin, Il Nuovo Cimento A Series 10 42 (1966) 930. [24] ATLAS Collaboration, Nuclear physics B (2014), 486-548 [25] H.A. Bethe, Ann. Phys. 3 (1958) 190. [26] G.B. West, D.R. Yennie, Phys. Rev. 172 (1968) 1413. [27] V.A. Khoze, A.D. Martin, M.G. Ryskin, Eur. Phys. J. C 18 (2000) 167. [28] J. Cudell et al., COMPETE Collaboration, Phys. Rev. Lett. 89 (2002) 201809. [29] G. Antchev et al., TOTEM Collaboration, EuroPhys. Lett. 101 (2013) 21003. [30] B. Abelev et al., ALICE Collaboration, Eur. Phys. J. C 73 (2013) 2456.